Spin Proceedings-cement Concrete for Africa

Spearhead Network for Innovative, Clean and Safe Cement

and Concrete Technologies

WorkshopCement and Concrete for Africa

Proceedings

17th August 2011BAM Federal Institute for Materials Research

and Testing

Berlin, Germany

Editorial information

WorkshopCement and Concrete for AfricaProceedings

2011

Published byBAM Federal Institute for Materials Research and TestingUnter den Eichen 8712205 Berlin, GermanyPhone: +49 30 8104-0Fax: +49 30 8112039Email: [email protected]: www.bam.de

Copyright © 2011 by BAM Federal Institute for Materials Research and Testing

ISBN 978-3-9814281-4-8

Contents

The SPIN Project 5N. S. Msinjili, W. Schmidt, H.- C. Kühne

Sustainable Concrete for Developing Countries 10B. Piscaer

State of the Cement and Steel Industry in Uganda 15R. Nassingwa, N. M. Nangoku

Use of Pozzolans as a Binder in the Building Materilas Industry in Uganda 23W. Balu-Tabaaro

Inorganic Binder Systems for Innovative Panel Technology in East Africa – Possible Ways to Produce Building Materials from Local Raw Materials 32G. J. G. Gluth, W. Z. Taffese, G. S. Kumaran, H. C. Uzoegbo, H.-C. Kühne

Challenges of the African Environmental Conditions for Concrete Mixture Composition 37W. Schmidt

From Prescriptive Towards Performance-based Durability Design of Concrete 50D. Bjegovic, I. S. Oslakovic, Marijana Serdar

Environmental Friendly Low Cost Housing Technology 59J. K. Makunza

Low-Cost Shell Structures: Thermal Loading 74M. Gohnert

5

Introduction

Introduction

The SPIN Project

N. S. Msinjili, W. Schmidt, H.- C. KühneBAM Federal Institute for Materials Research and Testing, Germany

Introduction1.

Globally, cement and concrete experts are at the cutting-edge to sustainable, green, healthy but nonetheless high-performance concrete. In the present age, concrete is not yet well established in Africa, which offers the unique opportunity to build up a cement and concrete market based on the highest available state of technology. As this industry needs high level expertise, a central issue in implementation of skilled technology is cross-linking research institutions and laboratories. It should not be neglected that concrete is a product with comparably low transport ranges, which means that an improved concrete market will mainly support the local economy without exceeding fi nan-cial drains to the international market, thus fostering the fi ght against poverty, which is an urgent need in most African countries.

The SPIN project highlights recent developments in the fi eld of cement and concrete research with impact on the local and global economy. Challenges, future developments and opportunities for the African construction industry are in the focus.

The SPIN project is funded by the European Commission (EC) and supported by the African, Carib-bean and Pacifi c (ACP) Group of States under the project body of the ACP Science and Technology Programme. SPIN is acronym for “Spearhead network for Innovative, Clean and Safe Cement and Concrete Technologies”. The project aims to cross-link experts with industry and policy making bodies, aiming to establish sustainable cement and concrete construction in Africa.

The Network2.

The SPIN network consists of a group of scientists, researchers and consultants from seven African countries and three European countries involved in different fi elds of engineering such as cement chemistry, structural engineering, geology engineering, environmental engineering and construc-

Figure 1.0: Participants to the SPIN kick-off meeting (left) and the SPIN Consortium in Kigali (right)

6

Cement and Concrete for Africa

tion materials. The fi rst idea of the project network was established in Spain 2007, it then grew over the years to form a large consortium consisting of the institutions shown in table 1.

The SPIN project’s main objective is to increase the existing network by involving other European and African institutions and public bodies who are in the interest of sustainable development for concrete construction.

Table 1: SPIN Consortium

Institution City, Country

BAM Federal Institute for Materials Research and Testing

Berlin, Germany

University of Witwatersrand, Advanced Cement Training and Projects cc

Johannesburg, South Africa

University of Burundi Bujumbura, Burundi

Kigali Institute of Science and Technology Kigali, Rwanda

University of Dar es Salaam Dar es Salaam, Tanzania

Department for Geological Survey and Mines Entebbe, Uganda

University of Lubumbashi Lubumbashi, Democratic Republic of Congo

Eduardo Mondlane University Maputo, Mozambique

Eindhoven Technical University Eindhoven, The Netherlands

Institut IGH d.d. Zagreb, Croatia

The Action3.

SPIN aims to target certain groups such as educational organisations, research institutions, small and medium scale enterprises and members of the general public to educate and implement prop-er working conditions when dealing with concrete construction and cement as a material. It aims at implementing rules for accident prevention during the application of concrete.

The tasks of the SPIN Project will base on the following items:

Networking between European and African Research Institutions •

It is an undisputed fact that a number of cement and concrete research institutions or industries in Africa are not integrated in international research and standardisation in the fi eld of cement and concrete technology. There is little or no information about cement and concrete for Africa as most of the standards used are based on European conditions.

SPIN has targeted various institutions in Africa to exchange information about worldwide devel-opment of cement and concrete while focusing on sustainable development for Africa. The con-sortium mentioned in table 1 has visited institutions in East and Southern Africa such as Rwanda, Burundi, Tanzania and Mozambique to give lectures and presentations to students, staff and the public about experiences and innovative technology with cement and concrete.

7

Introduction

Fostering of Construction Technology •

Cement is the most widely used material in the world after water. The fi eld of cement and concrete technology is rapidly growing especially in the production of ‘green’ cement. It is important to have a reduction of CO2 emissions in the atmosphere – as currently cement manufacturing proc-ess produces 5 % of the world’s CO2 emissions [1]. This CO2 reduction technology is in the high-est effect being implemented in the developed countries using various scientifi c solutions.

There are a number of other relevant scientifi c solutions that can be accessible for the African construction sector by making use of the resources locally available, however, lack of knowledge and expertise is the major constraint. SPIN will foster the knowledge transfer by developing strategies for proper construction technology taking into consideration the economically cost-effective and ecological aspects.

Strengthening the Cement and Concrete Industry in East and Central Africa •

Cement technology emerged late in the 18th century and continued to mature throughout the 19th and 20th centuries in Europe, Japan, North America and other developed nations with new ma-terials and methods of preparation. This technology developed and spread rather slowly, in order to meet new construction needs. Some of these needs included; military fortifi cations, bridges, dams, piers, tunnels and other vital infrastructure [2].

Cement technology is by far less developed in the eastern and central regions of Africa com-pared to Europe – production costs being a contributing factor to development, which leads to high cement prices. In addition to high cement prices in Africa, the demand as well is higher than production (as shown in fi gure 3), therefore cement needs to be imported from Asia – mainly China, India and Pakistan. South Africa shows a signifi cant development of cement and con-crete technology with a great number of cement and ready-mix concrete plants, however, lack of infrastructure development still exists in many areas.

Figure 2.0: Networking in Kigali

Figure 2.1: Lectures to students

8


Figure 3.0: Cement demand and production in Africa [3]

Training of Experts in Relevant Multidisciplinary Fields •

In most developing countries, there is a lack of advanced technology in researching about ce-ment and concrete. SPIN will offer such opportunities for African researchers and scientists from table 1 to get expert consulting on modern cement and concrete technology which can be im-plemented in the respective countries. The opportunities will involve work placements in the developed laboratory institutions, including fi eld training and workshops.

Generation of a Handbook for Clean and Safe Concrete Technology •

The regular standards adopted for concrete construction in Africa are European standards which realistically does not favour the specifi c climatic and geographical conditions of Africa.

The SPIN consortium will generate a handbook that will include guidelines for enhancing cement and concrete technology in Africa with relative consideration of the specifi c African boundary conditions.

The Result4.

The socio-economic components for a more sustainable development in Africa are viewed to be the most challenging taking into consideration the impact of the construction industry on the envi-ronment which is probably more important in developing countries than it is in developed countries. This is due to the fact that the developing countries are virtually still under construction and that they have a relatively low degree of industrialisation, making the construction industry one of the biggest factors impacting on the environment. The required challenge is to fi nd new approaches to development capable of preventing environmental degradation and excessive social costs, rather than focusing on palliative measures.

It is a necessary requirement to have an initial investment in order to support the development and production of appropriate technologies and building materials, however, these are costs that can later be recovered. Such costs can be substantially reduced if the construction sector works to-gether to share the responsibilities with the government, universities and other private sector in related industries and institutions.

In summary, with proper information and dissemination of knowledge, SPIN aims to help the public become more aware of the benefi ts that such practices represent for them and the environment. In many cases the issue is not the lack of resources, but the lack of coordination to manage them in a more effi cient way. SPIN aims to be the start of future networks between Africa and Europe. The involved institutions mentioned in table 1 and the increased network will be called ‘Spearhead In-stitutions’.

9

Introduction

References5.

[1] Green Cement is Carbon Neutral, Sequesters CO2 from Power Plants. http://cleantechnica.com/2008/09/02/green-cement-is-carbon-neutral-sequesters-co2-from-power-plants/

[2] P J Krumnacher. Lime and Cement Technology: Transition from Traditional to Standardized Treatment Methods.

[3] http://www.africa-confi dential.com/news

10


Sustainable Concrete for Developing Countries

B. Piscaer UNIVERDE, The Netherlands

Introduction1.

For the good order, concrete is made out of aggregates such as sand and gravel, the binder mostly called cement, water and often chemical admixtures. Depending on the mix design consistency or workability can be earth moist to highly fl uid self compacting, develop low to very high strength and range in natural colors from dark grey to white. Mortars is basically concrete made without coarse aggregate.

Concrete is the most used but also the most abused construction material in the world. Thru the use of the key ingredient, Portland Cement, over 2.6 Billion tons of CO2 are emitted every year and this fi gure is increasing at an alarming rate. 15 Billion tons of raw materials are needed per year for con-crete aggregates and cement making. While well build constructions in concrete can last almost 2000 years such as the Pantheon in Rome, many poor applications have demonstrated to last less then 20 years.

What is the difference between an earthquake in Chili of 8.8 magnitude earthquake in Chili that killed around 480 people while a 7.1 earthquake in Haiti killed between 100.000 and 300.000 people? The difference is poor application of mainly concrete. While big infra-structure projects such as a hydro dam, special bridge or port extension will draw the involvement of global operating engineering com-panies who master a better educated knowledge of concrete as a high tech product, it is of the inter-est of market dominant suppliers to continue the application of concrete as a commodity product.

The title in the brochure is “Sustainable Concrete FOR Developing Nations”, my presentation is “Sus-tainable Concrete AND Developing Nations”. The shortcomings and objectives are the same in all countries but the road to achieve it is more complex in developing countries. My personal working experience in the Caribbean countries and those of respected colleges with experience in the Pa-cifi c and Africa has motivated me to present this paper at the occasion of the SPIN Workshop in Berlin. Attention will be given not to special concrete for special purpose but regular day to day con-crete for regular use that effects the purse of the consumer. The below is valid for ALL concrete in all countries, it is presented with the obligation to combine millennium with sustainability objectives, poverty with CO2 reduction. The big difference is that the personal buying power of a bag of cement is very different in Berlin then in Burkina Faso. While the cost of concrete materials in a precast plant in the Netherlands may be only 1/6th of the total cost of a piece, in developing countries it will be well over half.

The other big difference is that industrial by-products that are Portland cement replacing are not easily available in developing countries and those that are from agricultural origin need an industrial approach.

Concrete for developing countries co-insides with a European Eco-Innovation project SUSTCON EPV that I initiated and advise to. It addresses the sustainability thus also social aspects of concrete based on its performance, not on outdated prescriptive aspects that have been imposed by market dominant forces. The below is a combination of previous technical sales market development to the Caribbean cement producers for a German multi national and recent gained knowledge working on sustainable concrete.

As with the fi xed telephone lines, that are not necessary anymore in developing nations since the mobile phones are available, let us leap frog now towards sustainable concrete. Industrialized coun-tries have to free themselves from many bad habits, changing the course of a super tanker. Develop-ing countries should not copy fi xed lines but jump towards sustainability faster, changing the course of a sailing boat.

11


Technical aspects2.

2.1 Particle Size Engineering

Some suppliers like to make you believe but strength of concrete does not only come from cement that hardens. The mechanical packing of all ingredients of all sizes, from the coarse aggregates to the fi ne sand and especially the powders play an important role. Adhesion of all particles is another aspect of binding that will not be dealt with here.

Adding in a proper way so-called inert fi nes will reduce and not increase the use of expensive Port-land cement. A multi-fractional packing of many different sized aggregates is possible, also in a non-industrialized setting by using simple affordable hand sieves or classifi ers from re-bars as seen recently in Portugal. Since labor cost are inferior to material cost a lot of attention should be given to the preparation of many more different fractions then being done in the developing countries where time is of more essence. Using at least 3 fractions of sand and 2 for gravel is not increasing but decreasing the cost. I am confi dent that big industrial suppliers will assist in developing easy functioning equipment for developing nations.

Action: Simple demonstration of packing in a glass can change the awareness of the importance of Particle Size Engineering followed by special education. Contact with leading classifi er equipment producer for hand operated equipment.

2.2 Portland Cement

Cement is all over the world the most expensive ingredient in concrete having an environmental impact 2,5 times bigger then the airlines. One ton Portland cement is one ton CO2 and needs 1,6 tons of raw materials. In several countries more then half of the hard earned hard currency is used to buy fuel for the cement plant. So we have to be very careful with the precious product made by the cement producer. If the cement producers understand that they can make more money with less volume of a higher quality cement, we will get their cooperation in pursuing one general and global objective, reduce the Portland clinker content of concrete.

It is better to be clear and honest in the interest of the well being of the consumer and the intelligent suppliers. The average cement industry is in general not interested in supplying a product that is very steady in quality so that you will use less of it. Technical sales, telling the user that he is better off using a more expensive product of which less is needed in concrete, is hardly practiced in this fi eld, everywhere, so also in developing countries. The more the products varies, the more you will need to reach the minimum standard. Variation in the so-called water demand of cement is com-mon everywhere but especially in countries where their monopolistic position is obvious such as developing countries, it is disastrous for fi ghting poverty. I was present in Jamaica when the whole island that depends on one cement plant, could not make good concrete anymore.

Action: Popularize the “ball in bowl method” by which water demand of powders can cheaply and easily be verifi ed without expensive laser de-fraction equipment. If variations are noted, a report with the cement supplier and an independent verifi cation organization will discover the truth about changing qualities of the Portland cement.

2.3 Supplementary Cementing Materials (SCM’s)

Especially in Europe the use of Portland cement replacement materials is on a good track of devel-opment and will be accelerated by our European Eco-Innovation projects on which we will report in November 2011.

12


SCM’s can be separated in

those recognized by the European cement producers to make so-called blended cements, such –as ground granulated blast furnace slag, industrial and natural puzzolans, ground calcium car-bonates, oil shale ash and silica fume.

those not recognized but scientifi cally sound SCM’s such as Reactive Rice Husk Ash, Activated –Paper Recycling Minerals, Sugar Cane Ash etc.

The problems with SCM’s are

Common knowledge of all the different types and their particular technological and aesthetic –effects,

Availabilities that depends on the natural and industrial resources that varies from country to –country,

Regulations that even in Europe vary from country to country in the freedom of use but which –has nothing to do with technology.

I was personally very encouraged by the popular use of not that easy to use chalk powder in Ja-maica where people discovered that it made the concrete much stronger with less Portland ce-ment.

Action: Inform the new of concrete producers of the possibilities of SMC’s. Assist in an industrialized approach in transforming agricultural by-products such as Rice Husk and Sugar Cane Bagasse into reliable binders.

2.4 Admixtures

It is shocking to see so little use of chemical admixtures in developing countries. Especially these water and cement reducing plus often set-time retarding products can contribute to a much better knowledge of concrete and in many cases lower costs. The problem lies probably again in general knowledge and in complicated technical sales demanding technical sales distribution.

Action: Check presence in countries of concrete admixture distributions with the knowledge that the cement companies might use grinding aids often from the same suppliers as concrete admix-tures.

2.5 Mixing Equipment

A lot can be gained in better mixing. Most common is the “1, 2, 3” method, meaning one shovel of cement, two shovels of sand and 3 of gravel, then mix water. If you are lucky, this is done on a hard plastic plate so no clay gets in. It should be clear that this mixing causes a lot of waste of expensive cement as one can notice about poor total hydration.

The next method is, as you see also all over Europe on small sites, are poor performing free fall mixers instead of more effi cient radial pan mixers.

Realizing that ready mix companies are only located near cities and are often still poorly equiped dry mixing plants, I wonder if there is a future for so-called volumetric mobile mixers you see a lot of in remote area’s in North America. A multi fraction compartment mobile mixer capable of high intensity mixing should have a future in sustainable concrete.

Action: Demonstrate the difference in qualities of mixing techniques in a practical way and propose and develop better but practical equipment.

13


2.6 Quality Control

Independent verifi cation of incoming and outgoing products should become part of the techno-logical framework of a developing country that can choose from worlds best sustainable concrete practice and leapfrog by not making the same mistakes the industrialized countries keep on do-ing.

Action: Investigate the concrete competence per country and means of QC.

Social Aspects3.

3.1 The corporate approach.

Many multi national companies have a Corporate Social Responsibility policy. Many global operat-ing cement companies and construction chemical companies are members of the World Business Council for Sustainable Development based in Geneva. It is possible to make them accountable for their policy in developing countries in particular.

Action: The ACP should draft a chart for the improved use of the most used and most abused construction material in the world, concrete, and make at least the members of the WBCSD ac-countable.

3.2 Education

Also in developed countries the education on concrete is poor but the effect of this on the daily lives is very different. Richer companies in Europe will pay for additional education after regular schools and hire specialist that take care of high volumes of concrete in ready mix plants. Since cement is also here the most expensive ingredient, there is a strong motivation for training.

Action: The European organization for vocational education, CEDEFOP, and again the ACP should look into the possibilities of addressing school partnerships between European and ACP countries. The formula of train-work-vacations should be considered and ideas for this have been considered in countries that are very attractive for young Europeans.

3.3 Verifi cation

Independent verifi cation of incoming and outgoing products, combined with general help on con-crete technology, should become a part of the Corporate Social Responsibility of institutes. It is possible that the BAM in Berlin can become a world leader in such new order. For institutes control-ling 5 grams of questionable binder powder send by mail from different parts of the world or regular Skype training sessions will not be a heavy burden.

3.4 Pricing

Although the purchasing power in developing countries is in no comparison dramatically lower then that in economically developed nations, the price of cement even per ton is often more expensive as we have seen between the US and Mexico. When due to a shortage in 2006 app. 30 million tons were imported in the US from overseas, the Mexicans were not allowed to export facing dumping charges since the price in Mexico was still higher then at the peak in the US! For a recent hydro dam in Panama a more expensive better performing cement from the US was imported since the poor quality of the cement made in Panama by 2 companies demands more volume.

Action: A price study in the ACP countries would provide a global view of the situation and report anomalies and stimulate fair trade.

14


Conclusion4.

Poverty and Intelligence are in no relation! Poverty and Education plus dominant market positions are! Countries with the lowest difference between income have the highest economical strength. Several millennium objectives, poverty and environmental impact reduction, capacity building can all be integrated in the pursuit to transform the most used construction product, concrete, into the most sustainable construction product world-wide when used in the right manner in the right place.

Other then fi nancial objectives should play a role in organizing this. On behalf of SUSTCON EPV and their Eco-Innovative concrete partners I invite the best APC people to dialogue and actions.

15

State of the Cement and Steel Industry in Uganda


Nassingwa Ruth1 and Nangoku Mumoita Naomi 2

1 Senior Laboratory Technician, Geological Survey and Mines Department, Uganda2 Mineral Processor, Geological Survey and Mines Department, Uganda

Abstract

The use of cement and steel has been in existence right from the time before independence though on a small scale since the demand was very low at that time. However, during the time of the con-struction of Owen Falls Dam, the demand of the cement and steel products increased. This led to the establishment of the fi rst cement factory in 1954 in the eastern part of the country. However, these industries suffered severe set backs from 1970-1985 during the dictatorship era in Uganda. The operation of the industries almost came to a stand still since the environment was not condu-cive for sustaining them.

From 1986 the production started picking up as the industries were revamped with the help of the government and other entrepreneurs. Currently, the demand of high quality cement is high due to the booming construction industry that requires specifi c quality and strength of cement. This has made entrepreneurs to import high quality cement from other countries like Kenya, Egypt, Pakistan and Dubai among others since the volumes on the local market can’t meet the demand. On the other hand, there is an increased number of fabricators in Uganda and this has increased the de-mand of Steel. However, a lot of emphasis is required in the research and development at enterprise level to enhance productivity, product diversifi cation and capacity utilization. For quality related is-sues, a Materials laboratory was set up near Kampala city to boost smaller ones localized at fac-tory levels. Therefore, there is need for exploitation of raw materials and development of new tech-nologies that will enhance production of high quality products in Uganda.

Key words: cement and steel, concrete, housing strategies, environment and laboratories.

Introduction1.

The cement and steel industries have played a major role in the national development due to their importance and need in the construction industry. Cement and steel are the most actively traded commodities in any developing country where infrastructure is lacking and has to be put up. From the time these industries were established in Uganda, there has been a noticeable change in the country’s infrastruture.

Uganda has two cement factories; one located in the eastern part of the country over 200km east of the capital Kampala called Tororo cement limited and the other to the west about 500 km away near Kasese town called Hima Cement Industries. On the other hand, the steel industry is donned with over ten companies operating steel mills in Uganda and they are widely spread in the country. The steel industry has registered great success as shown by the rate at which steel mills have been established. From 1960 to 1988, there were only two companies i.e. the East African steel rolling which is the oldest and the steel rolling mill both established in the East at Jinja. Later on BM tech-nical services was set up in the west at Mbarara. From 2002, several other companies have been set majorly near Kampala city. This is ascribed to the increased number of fabricators who make various items like doors, windows and smaller machinery at lower prices and also the increased infrastructure development in Uganda.

This current sudden increase in infrastructure development, as a result of the improved economic climate by the current government requires large amounts of cement and steel of high quality which needs to be addressed.

16


Cement Production in Uganda2.

2.1 Cement Factories

There are two cement factories in Uganda and these include;

2.1.1 Tororo Cement Limited

This is the largest cement producing company in Uganda. It is located in the Eastern part of the country and it was established in 1954. Its development was due to the increased demand of ce-ment for the construction of the Owen Falls Dam in Jinja.

The company produces both cement and steel products. For cement, the company used to pro-duce two types; Ordinary Portland cement and Portland pozzolana cement but for the last fi ve years, it has been producing only Portland Pozzolana cement. This was due to the fact that the cost of production of Ordinary Portland cement was high and he retturns were very low. Recently the company installed a new cement grinding mill and rotary packers with modern state of art of tech-nology. This has enabled the company to expand its cement manufacturing capacity from 692,828 to 919,229 metric tons in 2007 and 2010 respectively.

And for steel, the company produces a wide range of products from barbed wires, corrugate iron sheets to construction steel in all types.

The cement and other products from Tororo cement limited expert are exported to the neighbour-ing countries like Rwanda, DRC and Southern Sudan among others.

2.1.2 Hima Cement Industries

This is the other cement producing factory in Uganda. It is a subsidiary company of Bambuli Ce-ment Ltd of Kenya and a member of the French grant Lafarge Group.

It was established in 1967 but Bambuli acquired a signifi cant state in Hima cement ltd in 1999.

The company produced about 365,000 tons per year of cement until they acquired necessary ap-proval to launch the construction of a new production line in 2006. Currently it produces about 780,000Metric tons about double its former production. It supplies the Ugandan market especially in western Uganda and also in the regional market to Rwanda, southern Sudan, eastern Congo and Burundi. In Uganda, for a plant to be permitted to produce cement, it must have enough reserves of limestone that can sustain the plant for about 25 years. It for this purpose that Hima cement ac-quired dura limestone to sustain his operation for the next 30 years.

2.2 Types of Cement

In Uganda, there are mainly two types of cement that are manufactured and these include:

Ordinary Portland cement –Portland pozzolana cement –

2.2.1 Ordinary Portland Cement (OPC)

The basic raw materials for OPC are clay and limestone. After these materials are quarried they are ground and intermixed. The mixture is burnt in a kiln under extreme temperatures about 1450-16000 °C.

Once it leaves the kiln, the cement is ground and mixed with 4-7 gypsum which helps to inhabit settling while cement in being worked.

To generate and maintain the required kiln temperature, fuel has to be used whichover the years has seen its prices increase. This increased fuel price has led to an extra cost in the production of OPC which has made it more expensive than other types. Due to the extra cost in production, the manu-facturers have abandoned its production.

17


2.2.2 Portland Pozzolans Cement (PPC)

Pozzolana are materials which posses little or no hydraulic value which can only be when fi nely di-vided and in the present of moisture. In this state they can chemically react with alkalis. Pozzolanas can either be natural or of industrial origin.

The natural pozzolanas are often related to volcanic activities and typical materials are volcanic ash and pumicite. Pozzolana are usually introduced in the mix during alkaline activation.A particular process of alkali activation of pozzolana is the mixing of slag with ordinary portland cement where-by the slag is activated due to the solubility of calcium hydroxide resulting from the reaction of portland cement

This type of cement is widely available in Uganda since its cost of production is much cheaper as the raw materials are readily available at cheaper rates.

2.3 Cement Raw Materials

The availability of cement as a building material largely depends upon the availability of its raw ma-terials. The two types of cement use different types of raw materials for their manufacture.

This type of Ordinary Portland cement requires;

Limestone –Clay and mudstone –Gypsum –

The raw materials for limestone are of varying geological ages and widely distributed in the country and these include;

Carbonatites, marble, travertines, tufas, lake limestone and secondary limestone.

In areas where suitable sedimentary deposits are scarce, metaphorphic deposits and carbonatites can be used for cement production. According to existing literature, the remaining materials of suf-fi cient quantity for major cement plant are the marbles and carbonatities.

Resources of clay and mudstone suitable for cement production do exist in several areas country wide. Clay and mudstone are very essential in the cement production as they are the main source of silica, alumina and iron oxide. Clay and mudstone are readily available in Uganda and are of a low cost.

There are few deposits of gypsum and there are dominate in the western part of the country. The deposits occur within the rifts sediments at Kibuku, Muhokya, Kanyantete and at lake Mburo. More exploration work is required at Lake Mburo as the gypsum deposits are expected to extend in the near by valleys. Due to the few deposits of gypsum, it is imported from Kenya, Egypt and Oman.

Portland Pozzolana cement requires;

clinker –pozzolana materials –gypsum –

The clinker is mainly imported from Kenya while pozzolana are obtained form Uganda. The poz-zolanas used are of natural occurrence as they are products of volcanism. These natural poz-zolanas exist in the eastern and the north eastern part of the country.

Carbonatites lava and tufas exist in the western and southern parts of Uganda.

Due to the existence of large volumes of pozzolanas in Uganda which are cheap, Portaland Poz-zolana cement is the largely produced cement in Uganda.

18


2.4 Cement Market

The demand for cement in Uganda currently has out stripped its supply and this is as a result of demographic growth, urbanization and economic growth. The demand of cement in Uganda has drastically increased over the years. The annual growth is about 5-6 % currently compared to the 3-4 % from 1980-1990.

The total output of the two cement factories is 1,162,241 metric tons while the imported cement ac-counts for 566,082 Metric tons. 390,476 Metric tones are exported into regional market (UBOS 2010). Although the demand of cement is said to over ridden the production, the consumption rate is signifi cantly low at only 35 kg per capita per year. This is refl ected in the large number of people still living in unsafe shelters that are found in both urban and rural areas. This is because of the skyrocketing prices of cement which are relatively high for an ordinary man for a product that is manufactured within the country and whose raw materials are locally sourced. Despite the low con-sumption, prospects are very good with the strong demographic growth.

2.5 Use of Cement

Cement is basically used for the construction purposes in Uganda. Cement can be used;

as a binder in the erection of walls of either fi red clay bricks or concrete blocks. –for trowel fi nish for both walls and fl oors. – in concrete making –

Concrete has become the most popular and widely used construction material in the World. In Uganda, concrete is prepared and fabricated in all sorts of conceivable shapes and structural sys-tem in the realms of infrastructure, habitation, transportation, work and play. The common products are;

concrete blocks in different sizes for house and wall construction –Facing bricks for decoration purposed of walls and adding strength to the walls –Pavers –Roofi ng tiles –

Raw Materials for the Manufacture of Concrete

Cement •It should be of high quality and strength to enhance the durability of the product. Most concrete industries in Uganda use Power Plus and Power max from Hima cement while others use Toro-ro’s Portland Pozzolana. These industries import cement from Kenya and Dubai when its out of stock in Uganda.

Aggregates •Granite is the rock that is quarried for this purpose. This rock is found in various part of the coun-try.

Sand •For concrete making, lake sand is prepared since it is usually clean. It is mainly obtained from Lake Victoria

Sica •It is always used in the already mixed concrete as a binder. It improves strength and also retards setting of the concrete. It is imported from South Africa.

Dusty Stone •This is used as a binder and it is readily available.

Since concrete is a brittle material and is strong in compression, it weakens in tension. To improve the strength of concrete, steel is used inside the concrete. This reinforced concrete is usually used for pillar erections in storied buildings.

19


The Steel Production in Uganda3.

The Ugandan steel industry has been growing at amazing rates averaging from 20 % to 30 % per annum from imports and exports respectively from 2002-2006 due to the booming housing and construction sector in the region (URA, 2010). The industry is dominated by local small scale indus-trialists and a few medium to large scale producers.

The East African Steel Rolling mill at Jinja ran by the Madhivan group was the fi rst steel milling com-pany in Uganda since 1960s followed up by Steel rolling mills under the Alarm group of companies. But over the years especially from 2002, the number of steel milling companies has increased to over ten companies. This is due to the increased demand of steel by the many fabricators on the Ugandan market as a result of the booming housing and construction sector.

Initially the steel milling companies relied on the imported billets and later predominantly using scrap iron as a raw materials. Although scrap iron is the major raw materials, there is an outstanding shortage of scrap steel. People have nowadays resorted to stealing manhole covers in the night in the bid to collect scrap. By year 2000, the national scrap deposits were estimated at 150,000 to 200,000 MT while the local steel production capacity stood at 72,000 MT per annum. This refl ected that steel scrap inputs was mainly imported. However, recent studies by DGSM show that substan-tial iron ore deposits of relatively good quality exists in Uganda

In the early 1980’s – 1990’s the main steel products made were agricultural machinery like hoes, ox ploughs, pangas and their spare parts but currently, there is a wide range of products like barbed wires in different gauges, wire nails of various sizes, galvanized corrugated iron sheets and twisted /round bars in all size among others.

Impacts of Cement and Concrete4.

Cement is one of the raw materials used in concrete making and Cement on the other hand, is a product of limestone which is a naturally occurring mineral. Therefore, there is a great potential of altering the normal functioning of the environment through human endeavors to harness this min-eral resource in different parts of the country. These impacts of cement and concrete are divided into socio-economic, political and environmental divisions. These impacts are both positive and negative in nature.

4.1 Environmental Impacts

The environment acts as a source of raw materials for the mining industry and a sink for wastes generated during the mining process (MEMD, 2005). Below are some of the environmental impacts of cement and concrete.

Noise Pollution: – The people who stay in the areas around mineral deposits and cement plants suffer noise pollu-tion which comes from blasting and quarrying of raw materials and also from heavy trucks that ferry materials from the quarry to the cement plant and back.

Vegetation Clearance: –Since most of the mining is carried out by artisanal and small scale miners who employ crude and inappropriate methods of mining, this usually calls for clearance of surface vegetation and excavation to locate the mineral ore. Even after the mining of activity there is little effort to have such areas reclaimed (MEMD, 2003).

Air Pollution –Each tonne of Portland cement produced releases approximately one tonne of carbondioxide in to the environment. The production of cement is a signifi cant contributor to atmospheric pollu-tion and the green house effect (Swamy R.N, 1999).

20


Waste Disposal –The waste water from ready mixed concrete plants can be a big problem to the environment. This is because it contains chemical that are not environmental friendly. Usually this water is not given proper attention and is let to fl ow in water channels thus contamination fresh water bod-ies.

4.2 Socio-economic Impact

Cultural Distortion –Since there isn’t enough technical support in this sector in Uganda, expatriates are hired to do the work. Since they come from different cultures and traditions, they are mingled with the local culture and tradition hence distorting it.

Development of Infrastructure –Due to the presence of cement and concrete high class structures and roads are built which durable.

Employment Opportunity –Many people have been employed in the cement and concrete plants hence improving on their standards of living.

Economic Development –The money obtained from the plants is spent within the community thereby boosting economy.

Loss of Social Networks –During mining, many families are relocated thereby losing contacts from their friends and rela-tives.

4.3 Political Aspects

Through exports and imports of cement in the regional market and beyond, political ties be- –tween the countries is strengthened.

Due to the strong and long lasting structure, the political leader stays in power longer since peo- –ple consider infrastructure development as they vote for their new leaders.

With the friendly mining act 2003 which become operational in 2004, there are several wrangles –over the ownership of the license of mineral deposits in various parts of the country. This brings about long-term hatred which may hinder further exploration of the mine.

Laboratories in Uganda5.

Uganda has one Materials laboratory that is located about 10km from Kampala city. This labora-tory carries out tests on all materials used for construction. This ranges from construction of com-mercial buildings to roads. But the main purpose of this laboratory is to test materials used in road construction.

The materials laboratory also boosts smaller laboratories located at plant sites. Each plant, either cement or concrete must have qualifi ed personnel who must run the laboratory

These laboratories however, suffer several challenges such as:-

lack of high technology equipments –little facilitation from the government –maintenance of the available equipments is very costly –lack of enough qualifi ed personnel –

21


Low Cost Housing Development6.

The quality of housing Ugandans live in has continued to improve over the years and at the same time, there has been a decline in the use of mud and poles for walls easing pressure on both native forests and woodlands. In 1991 over 85 per cent of houses in both urban and rural areas had rammed earth for fl oor but by 2002 only 29 percent urban and 77 percent rural houses had the same.

One of the major reasons as to why most people are living in such unsafe shelters is the increased price of cement. Since most people are living below the poverty line, they can not afford to buy a 50kg bag of cement costs 12 US$ in many retail outlets around Kampala.

Below are some of the strategic available in Uganda that require more emphasis and support so that every family can live in safe shelters.

Introduction of New and Improved Technology •Hydraform building system is one of the new technology which uses a mixture of soil and line to make inter locking blocks. There blocks don’t need cement to join them hence reducing on costs.

Other local initiatives include:

Rammed soil walls –The soil is bonded with molasses in order to increase its strength.

Timber houses:- –Some people in Uganda have resorted to the use of timer instead of soil since it is much safer.

Forming of Organizations •The government encourages the formation of community based groups such as associations, cooperatives and societies.

Through these groups, the people provide labor in construction as a way of self help initiative.

Savings and Credit Mobilization •The savings and credit groups increase access of resources to local people. It is because of this that the government encourages people to at least join a saving and credit group.

The government also intervenes incase the interest rates are high so that they are affordable to everyone.

Skills Development/Training •The government through the Ministry of Education has a drive to equip the communities with appropriate technical skills for construction. The government has advocated free education in technical colleges as a way of equipping young people with skills.

Assess to Land •To promote low cost housing in Uganda, the government has provided land under affordable terms (leasing).e.g Namuwongo slum up grading project in Kampala.

Through the above strategies, people have started accessing safe shelters through the different projects. These strategies require more in put in terms of fi nancing for their success.

22


References7.

[1] UBOS (2010) Statistical Abstract, Uganda National Bureau of Statistics.

[2] URA (2010) Annual Report, Uganda Revenue Authority.

[3] MEMD(2005) Annual Report, Ministry of Energy and Mineral Development.

[4] NEMA (2006/2007) state of the environment report, National Environment Management Authority.

[5] Gabriel Data (2009) Unpublished report on iron ore deposits in Uganda.

[6] Kato Vincent (2006) limestone deposits in Uganda.

[7] UPS (2007) state of Uganda population report 2007; planned urbanization for Uganda’s growing population, Population Secretariat, Kampala, Uganda.

[8] Fredrick Bjork; key issues from the 1999 Vancouver symposium on concrete technology for sustainable development.

[9] Swamy, R. N A; Designing concrete and concrete structures for sustainable development: 1999 CNMET/ACI International Symposium on Concrete Technology for Sustainable Devel-opment.

[10] John Baptist Kirabira: - Options for improvement of the Ugandan Iron and Steel industry; 2nd international conference on advances in engineering and technology pp 228 – 234.

23

Use of Pozzolans as a Binder in the Building Materilas Industry in Uganda


William Balu-Tabaaro Department of Geological Survey and Mines, Republic of Uganda

Abstract

Due to the rapid increase in population in Uganda demand for housing has outstripped housing availability. This is largely due to the high cost of building materials. The majority of the population of Uganda many of whom live in rural areas cannot afford these high cost materials and hence can-not build durable and decent houses. Traditional building materials like burnt bricks are getting more expensive due to shrinking availability of fuel energy resources, especially fi rewood. The cut-ting down of large chunks of forests to generate fi rewood is creating a lot of environmental prob-lems, such as degradation, soil erosion and weather uncertainties. Hope therefore lies in the devel-opment of alternative building materials that are cheaper and that have little impact on the environment.

One such alternative building material is the abundant volcanic ashes (Pozzolans) in the Kisoro and Kabale areas. The Kisoro, and Kabale, Volcanic Ashes (Pozzolans) have been extensively studied and found to be cementitious when activated with cement or lime. Once converted into pozzolan cement, they can be used to manufacture produce binders, blocks, wall panels, etc. to provide a cheap alternative building material that will assist in increasing low cost housing in Kisoro and Kabale. The objectives of this study are to carry out an assessment of how low cost housing tech-nology can be used to convert the pozzolanic materials to use in low cost housing construction. The data generated will be used by potential investors to establish facilities to produce building materi-als for low cost housing, using pozzolanic materials

The successful commercialization and popularization of the pozzolanic materials will create em-ployment for the majority poor in these areas. Provision of cheaper building materials will also en-able them to get affordable and decent housing.

Research work carried out on Kabale-Kisoro Pozzolans indicated good materials that could be used to produce alternative binders.

Introduction – The Housing Situation1.

1.1 An Overview of the Existing Housing Situation in Uganda

According to results of the 2002 Population and Housing Census, Uganda is presently estimated to have a population of about 24.7 people with an average household size of 5.7. The same results gave an occupancy density of 1.05 and hence an estimated housing stock of 2,690,900 units and a backlog of 235,914 units in the country.

Uganda also has a lot of pozzolanic materials based on volcanic ashes found in Kisoro and Kabale districts that could be used elsewhere to produce low cost building materials. It is known that tech-nologies based on these materials have been developed and commercialized in other countries. Uganda has the potential to develop similar technologies locally and get them commercialized in order to provide low cost effective building materials to solve the housing problem.

1.2 Building Materials

One of the ways to improve both the quality and quantity of housing is to increase the availability of low cost effective building materials. Building materials and construction are very important inputs

24


to the housing sector, but suffer from dependence (60 %, 1992) on imports, poor distribution, short-age, lack of local skills and equipment, lack of standardization of both locally manufactured and imported materials and equipment, and low production capacities by the factories. Shortage and importation of materials is the cause of their high prices and high construction costs. In general the building materials industry in Uganda suffers from:

High cost of materials and construction due to the unfavorable economic factors and perform- –ance, and overall shortage of the materials, tools, equipment and skills.Lack of standardization of materials and their quality control –Local building materials are usually in short supply due to the fact that the factories have low –production and cater for a high demand.The traditional building materials and building techniques are not allowed to be used in urban –areas. There is too much dependence on imports, there is poor distribution and high transporta-tion costs add to the problem.Related services such as consultancies are also in short supply and unevenly distributed. –

1.3 Materials in Uganda

The majority of Uganda’s population lives in poor and non-durable housing. In most cases, there is barely anything called housing as some live in mud walled huts. The main problem to access to decent housing is due to high cost of building materials, which are not affordable by the majority poor. Lack of appropriate technology to harness some abundant local raw materials also hinders access to low cost housing. Although there are abundant local reserves of pozzolanic materials that could be developed into building materials at lesser cost than other traditional materials, there is need to use low cost technologies to develop cheap building materials that will lead to enhance-ment of low cost housing in Uganda.

Pozzolanic Building Materials in Uganda2.

Defi nition of a Pozzolan

Pozzolan is defi ned as siliceous or siliceous and aluminious material which though not cementitious itself, reacts when in a fi nely divided form, with lime or ordinary Poland cement, in the presence of water at ordinary temperature and form stable and insoluble mineralogical phases of cementitious characteristics.

Pozzolans can be natural and artifi cial. Natural pozzolans include volcanic ash, pumice, obsidian, tuffs, etc while artifi cial ones include fl y ash, blast furnace slag, burnt clays, reject bricks, burnt rice husks, ashes, etc.

In Uganda, most of the pozzolans are derived from the abundant volcanic ashes in the Kabale, Kisoro and Kapchorwa areas (See map). These geological materials were formed many years ago and large quantities of these materials have accumulated over the years. Samples from these areas were collected, characterized and some test work on their pozzolanicity , grindability , reactivity, have been carried out. Their corrversion into building materials (blocks, wall panels) have yet to be carried out.

25


Characterisation of Pozzolans in Uganda3.

Determination of Geological, Petrological, Chemical and Physical Properties

3.1. Mineralogical Tests

The Pozzolanicity of the volcanic ashes as indicated by the glass state was determined by Petro-graphic analysis and the results are shown in Table 3.1.1

Table 3.1.1 Glass state of the volcanic ash: Source University of Toronto Cements

Sample No. Location % Glass

1. Kisoro clinic quarry 45

2. Kwasembi – Busanza Road 30

3. Nyagishenyi Chamke Road 00

4. Chamka 1 ½ miles to Rwanda Border 33

5. Hakitembe – Nyakabande county 30

6. Hamugeza – Mulumdwa 35

7. Kasheregyenyi – Mulumdwa 15

9. Junction: Kashenyi – Muko 30

9A. Junction: Kashenyi Road 20

10. To Junction: Kashenyi - Road 20

Figure 1: Map Showing Location of Pozzolans in Uganda

26


3.2 Pozzolanicity Tests

The pozzolanicity tests give indications of the reactivity of a pozzolan with lime. The procedure fol-lowed was in accordance to the European standard EN 196. In the tests, comparison is made of calcium hydroxide present in aqueous solution in contact with the hydrated cement after a period of time, 8 to 15 days, with the quality of calcium hydroxide capable of saturating a solution of the same alkalinity. The test is positive when the concentration in the solution is lower than the satura-tion concentration. Results of the pozzolanicity tests are shown in Table 3.1.2

Table 3.1.2 Pozzolanicity Tests

Pozzolan(moles/litre)

Sample No. Hydroxyl ion conc. (moles/litre)

CaO conc

Bunagana 2 3.96 8.05

Burnt clay - 26.73 6.04

Hakitembe 5 32.67 10.06

Nyagishenyi 3 1.58 8.68

Rubanda 12 3.96 7.47

3.3 Mineralogical Composition

The mineralogical compositions of the Uganda pozzolan were determined by X-ray diffraction.

Table 3.1.3 Uganda Pozzolan Mineralogical Composition

Sample No. Minerals identifi ed by X-ray Diffraction

1 Augite (24-203)* and possibly Halloysite - 10A (9-451

2 Augite, Aluminium (24-202) and possibly Pyrophanite (29-902)

3 Augite (24-203) and possibly quarts (5-490)

4 Diopside, Aluminium (38-466) and Forsterite (4-768)

5 Diopside (11-654) and Forsterite (4-768)

6 Forsterite, Ferroan (33-675) and possibly Augite (24-201)

7 Forsterite, Ferroan (33-657) and possibly Pyrophanite (29-902)

8 Quartz (5-490) and Muscovite - 2M1 (19-814)

9 Augite (24-203), Forsterite (7-74) and possibly Kutnohorite (11-345)

9A Diopside (19-239)

10 Augite (24-203) and Forsterite (4-768)

11 Quartz (33-1161) and Kaolinite - 1MD (6-221)

12 Quartz (5-490) and Calcite (5-586)

13 Calcite (24-27) and Aragonite (5-453)

* Numbers in parenthesis refer to JCPDS powder index fi les. Source: University of Toronto.

3.4 Chemical Analysis

The Chemical compositions of a selection of the pozzolans were analysed. The Chemical composi-tions of the pozzolans were compared with the requirements prescribed by ASTM C618 for Class N material.

27


According to ASTM C618, natural pozzolans shall conform to the chemical requirements presented in Table 3.1.4a to be classifi ed as a Class N material. Class N covers raw or calcined natural poz-zolans for use as mineral admixtures in concrete.

Table 3.1.4 a. Chemical Requirements for class N according to ASTM C618.

Class NSiO2 + Al2O3 + Fe2O3, min % 70.0Loss on Ignition, max % 10.0Alkalis (optional), requirement Na2O-content, max % 1.5Moisture content*, max % 3.0SO3, max % 4.0

* Equivalent Na2O = Na2O + 0.658 K2O

The chemical composition, i.e. the content of the major oxides of the pozzolans was determined by ICP. The following oxides were quantifi ed: Al2O3, CaO, Fe2O3, K2O, MgO, Na2O, and SiO2.

The results of the analysis are summarized in Table 3.1.4 b

Table 3.1.4 b: Chemical Analysis (All values are presented in % by volume)

Pozzolan No. SiO2 Al2O3 Fe2O3 MgO CaO Na2O Ka2O LOI* NC**

1 42.72 11.66 13.97 7.89 12.10 1.22 2.74 2.35 657 45.03 15.02 13.55 5.44 8.32 2.48 3.63 1.10 859 45.75 15.79 12.27 4.28 7.42 2.62 4.01 1.62 65

19 42.22 9.77 12.60 10.34 13.32 1.60 2.78 1.47 6021 45.07 11.16 12.11 11.13 10.51 2.05 2.49 0.24 5522 34.49 5.76 10.81 8.67 14.37 0.18 3.03 15.19 525 47.31 7.15 7.53 6.26 10.57 0.36 3.39 12.14 5

27B 47.33 8.23 7.44 5.70 9.77 0.30 3.56 13.56 5

* Loss on Ignition** Estimated Non crystalline Matter (Content of Glass)

Development of the Pozzolanic Cement – Test Work Programme4.

4.1 Grinding Tests

After the chemical and pozzolanicity tests, samples of pozzolans were subjected to grinding tests, after size analysis. Using past tests, a range of sizes were targeted. For each grind size, the fi neness was determined and mortar cubes made for subsequent tests for compression strength, initial and fi nal setting, water ratios and durability. The grinding tests were also used to determine optimal grinding costs using work indices.

28


Table 4.1 Natural particle size of volcanic ash

Sample Distributionfunction

Sieve size in microns ( )

12.5 mm 6.3 mm 2.8 mm 150 mm

2 Cumm % undersize

74.7 41.4 14.4 0.9

3 Cumm % undersize

74.3 51.9 25.1 5.4

5 Cumm % undersize

99.4 92.9 61.8 11.9

12 Cumm % undersize

95.2 64.3 26.6 3.3

9 Cumm % undersize

99.8 90.1 46.2 5.3

No.2 - Bunagana roadNo.3 - Nyagishenyi (Katarara)No.5 - Hakilembe (Gihinga)No.12 - Kikombe (Rubanda)No.9 - Muko

Table 4.2 Sieve size with 80 % and 50 % passing for volcanic ash in natural state

Sample No. Sieve size in mm

80 % Passing 50 % Passing

2 14,000 76,000

3 15,200 6,000

5 4,700 2,350

12 9,000 4,800

9 5,200 3,300

After size analysis, the volcanic ash had their specifi c gravities determined and the results are shown in Table 4.3.

Table 4.3 Specifi c Gravities of Volcanic ash samples

Sample No. Specifi c Gravity

2 2.22

3 2.30

5 2.02

12 2.40

9 2.38

Average specify gravity 2.9

29


4.2 Work Index

Using the grinding tests, the work index was used to determine power consumption, a factor that would help in evaluating costs of production. The work index was calculated using the formula:

W = Wi (10/P½ − 10/F½)

Where Work Index denoted by Wi, is the amount of work required in Kwh/short ton to reduce a material from infi nite size to 80 percent passing 100 microns and is calculated from the above for-mula where:

F = 80 percent passing size in the feed, expressed in microns P = 80 percent passing size in the product, expressed in microns W = Work required in Kwh/short ton to reduce a material from F to P

The work index (Kw-h/ton) was determined by grinding silica sand whose comminution energies are known, and the same conditions were used for volcanic ash in a 220mm × 200mm ball mill at 45 % ball charge and 72 rpm. The resultant particle size distribution was determined. The experimental variables for work index determination are time of grind and particle size. The calculated work indi-ces for volcanic ash samples are shown in Table 4.4.

Table 4.4 Work Index of Volcanic ash

Sample No. Specifi c Gravity Work Index (Ei (KW-hr/ton)

2 2.22 10.16

3 2.30 12.23

5 2.43 11.72

12 2.40 8.77

9 2.38 9.49

Average 10.49

After grinding, various surface areas were determined and the results are as shown in Table 4.5

Table 4.5 Specifi c surface area of ground volcanic ash

Sample No. Specifi c surface area (Blaine cm2/g)

4 hrs 5 hrs 6 hrs

Hakilembe No. 5 3300 4400 4800

Bunagana No. 2 4000 4400 4900

Rubanda No. 12 3800 4200 4400

4.3 Mixing Trials

After all the grinding tests, mixing trials were carried out. The mixing involved additives of Ordinary Portland Cement and lime. But due to low quality lime from Uganda, a lime from Kenya was used. (see table for chemical and physical properties).

The different sizes of ground pozzolans were activated with Ordinary Portland Cement and lime in various proportions from 10 % to 40 % i.e. ratios of 1:10 to 1:2.5 (cement: pozzolans). The various mix ratios were then subjected to various tests (i.e. water ratios, compression strengths etc.) to determine various characteristics of the cement.

The results of compression tests are shown in the tables 4.6 – 4.9.

30


Table 4.6: Portland-Pozzolan Cement characteristics

Ash Type: “Sample 2” ground for 4 hours in 220 x 200 mm ball millOPC: Twiga BrandSpecimen: 8 x 4 cylinders

Mix ratio OPC/ASH (%)

Standard Consistency

Setting Time (min) Compressive strengths 7 days

water (cured MPa)

100/00 27.6 100 165 56.7290/10 27.2 98 160 55.9380/20 26.6 92 175 56.0070/30 25.2 135 170 49.1660/40 24.3 125 183 46.1850/50 23.8 120 229 38.6140/60 23.9 160 250 37.02

Table 4.7: Compressive strength of OPC- Pozzolan cements

Ash Type: “sample 3” No. 2OPC: Twiga Brand (Tanzania)Specimen: 8 x 4 Cylinders


Compressive Strength (MPa)

7 days 28 days

Curing conditions Curing conditions

Air Water Air Water100/00 46.58 55.93 43.20 75.72

90/10 40.50 50.16 40.20 56.7280/20 39.60 48.77 30.25 69.6770/30 35.20 46.77 40.00 68.6760/40 33.64 45.98 32.44 52.1550/50 26.90 32.64 25.44 44.7940/60 25.10 36.22 23.69 40.2130/70 18.91 21.22 18.90 31.8520/80 11.35 15.13 13.14 24.8810/90 5.90 9.30 5.00 11.74

Table 4.8: Compressive strengths of OPC – Pozzolan cement mortars

Ash Type: “Sample 4”OPC: Twiga Brand (Tanzania)Specimen: 100 mm cubes, cement: sand: 1:3 W/C = 0.53


Compressive strength7 days

(Mpa) water cured28 days

100/00 26.0 38.490/10 22.0 29.380/20 18.0 25.460/40 9.4 13.250/50 6.4 13.540/60 4.1 8.230/70 2.7 5.8

31


Table 4.9: Compressive Strength of OPG-Pozzolan Cement Mortars

Ash Type: “Sample 5” No. 12OPC: Twiga Brand (Tanzania)Specimen: 100 mm cubes, cement: sand, 1:3 W/C = 0.53


Compressive strength7 days

(Mpa) water cured28 days

100/00 26.0 38.490/10 16.3 25.680/20 10.5 24.770/30 10.2 21.960/40 4.8 13.650/50 3.9 10.3

Conclusions and Recommendations5.

Tests carried out identifi ed pozzolanic materials that proved reactive when activated. It was estab-lished that these pozzolanic materials can be used as binders to produce building materials and at a price cheaper than Ordinary Portland Cement.

This shows that low cost buildings can be constructed especially for low income and rural popula-tions. There is need to carry out socio-economic studies.

References6.

[1] Byamugisha, S.S.; Balu-Tabaaro, W. (UGSM; Entebbe, UGA). The western rift valley volcanic fi elds, and their association and role in the lime-pozzolana cement manufacture in Uganda. UGSM unp. Report 1986. – No. SSB/12, WBT/1

[2] Day Robert L. Pozzolans for use in low cost housing: state of the art report. Department of Civil Engineering. Universidad de Calgary. Investigacion reportada No. CE92-1. Enero 1992.

[3] Heikal M. et al, “Limestone fi lled pozzolanic cement’, Cement & Concrete Research Issue No. 11, Vol. 30, 2000.

[4] Malhotra V.M, Mehta P.K. Pozzolanic and cementitious materials. Publicado por Gordon and Breach. Inglaterra. 1996.

[5] Ndawula, G. (UGSM; Entebbe, UGA), The potential for use of volcanic ash pozzolan based cements in Kisoro District. UGSM unp. Report. 1992.– No. GN/1.

[6] Martirena J, Betancourt S.: Notes on a Book for Technology for the manufacture of Lime Pozzolana Binders.

[7] Martirena J.F.: The Development of Pozzolanic Cement in Cuba, Journal of Appropriate Technology, vol. 21, No. 2, September 1994, Intermediate Technology Publications, U.K.

[8] Kabagambe-Kaliisa, F.A. (UGSM; Entebbe, UGA), Possible Sources of pozzolana in Uganda, UGSM unp. Report 1998. – No. FAKK/14.

[9] Groves, A.W. (UGSM; Entebbe, UGA). Report on the prospects of using the volcanic tuff of the Fort Portal District for the manufacture of cement. UGSM unp. Report. 1929. – No. AWG/3.

32


Inorganic Binder Systems for Innovative Panel Technology in East Africa – Possible Ways to Produce Building Materials from Local Raw Materials

Gregor J. G. Gluth1, Woubishet Z. Taffese2, G. Senthil Kumaran3, Herbert C. Uzoegbo4, Hans-Carsten Kühne1

1 BAM Federal Institute for Materials Research and Testing, Germany2 EiABC Ethiopian Institute of Architecture, Building Construction and City Development, Addis Ababa University, Ethiopia3 KIST Kigali Institute of Science and Technology, Rwanda4 University of the Witwatersrand, South Africa

Abstract

Many African countries face serious problems associated with the rapid growth of urban population and the resulting demand for affordable building materials. In search for appropriate solutions to improve the situation, the “LightSHIP” project was initiated, whose aims were to identify the re-quired product specifi cations, to evaluate possible approaches and ultimately to develop new build-ing materials for East Africa. It was concluded that these materials should be produced in Africa mainly from local raw materials; prefabricated, easily transportable construction elements are to be preferred. It is therefore reasonable to focus on artifi cial stones and partition boards. To be inde-pendent of imported cement, it is suggested to make use of volcanic rocks, which are abundant in East African countries, for lime-pozzolan binders and geopolymers in the production of these con-struction elements. Future research activities should thus concentrate on assessment of the ap-plicability of available volcanic rocks, the infl uence of their properties on the resulting binders and the design of appropriate binder-reinforcement-fi ller systems.

Introduction1.

Africa had huge rates of urban growth over the past fi ve decades. Africa’s urban population grew from 33 million in 1950 to 373 million in 2007, which constitute 38.7 % of the total African population in 2007. According to UN projections, the urban population is expected to grow at annual growth rate of 2.8 % to achieve 1234 million in 2050, which is 62 % of the expected total population. Thus, population increase is becoming largely an urban phenomenon in Africa.1 Two cities in Sub-Saharan Africa, affected by these developments, are Kigali (Rwanda) and Addis Ababa (Ethiopia). Both face a large increase in population – e.g., the population of Addis Ababa has doubled nearly every dec-ade – and the associated problem of tremendous pressure on social and physical infra-structures.1b,2

To overcome these infrastructure problems, it is necessary to construct a huge number of housings and public buildings in short time and at acceptable costs. Although a lot of effort has been put into attempts to improve the situation, the cities of Kigali and Addis Ababa are far away from providing suffi cient new living space and infrastructure. The reasons for defi cient construction activities are the lack of suffi cing fi nancial resources on the one hand, and the lack of affordable adequate build-ing materials on the other hand. Although there are deposits of raw materials for cement produc-tion, prices for cement are very high in East Africa, which makes concrete structures too expensive in many cases.3 Furthermore, burning of cement clinker and even bricks is hampered in Rwanda due to energy shortage. Existing approaches, such as the use of magnesium oxychloride cement for partition boards in Addis Ababa,1b are a great progress but have certain limitations. For example the magnesia binder boards can be used only indoors because of poor water resistance.4

33

Inorganic Binder Systems for Innovative Panel Technology in East Africa

For the given reasons, there is an urgent need for the development of new building materials in Af-rican countries. The materials should be produced in Africa with the highest possible fraction of local raw materials to reduce costs and to make the production largely independent of price move-ments in the international market. Furthermore, for economic and ecological reasons, the produc-tion of these building materials should as little as possible consume energy and produce carbon dioxide. The successful development of such materials would not only serve to solve many of the infrastructure problems in African cities, but would also create many new jobs in the building sector, which is traditionally an important employer in Africa,5 and therefore would have an additional pos-itive impact on the society. The present paper describes steps undertaken towards the develop-ment of new building materials in Ethiopia and Rwanda and presents future plans, aiming at the production of new binder materials and lightweight wall boards in Sub-Saharan Africa.

Step 1 – The LightSHIP Project2.

The LightSHIP (Lightweight Construction – Scientifi c Cooperation for Housing with Innovative Pan-el Technology) project is a preparatory measure, funded by the International Bureau of the German BMBF Federal Ministry of Education and Research within the scope of the funding scheme “Part-nerships for sustainable solutions in developing countries”. Project partners were the institutes of the authors as well as the German-African Business Association and a German small sized building company.

In the course of the project duration, several visits by researchers were made at the institutes in Ethiopia, Rwanda, South Africa and Germany. During the visits, workshops with partners from in-dustry and governmental administrations as well as inspections of several laboratories, factories, construction sites and housing areas were conducted. Based on the gathered information, follow-ing conclusions regarding the fabrication and specifi cations of the materials to be designed could be drawn:

Pre-fabricated elements are to be preferred. This will lead to an optimised production process –and will minimize the risk of failures during the fabrication process.The elements should be easily transportable and easy to erect. –The materials have to be produced with the highest possible fraction of local resources. –Already existing experiences in the involved African countries have to be taken into account dur- –ing the development process. These exist, inter alia, in the area of cement and concrete blocks and partition boards.

From these requirements, it is obvious that lightweight partition boards as well as artifi cial blocks could be adequate construction elements, although other solutions are possible, too.

Supporting further research activities, test were conducted on different building materials, already in use in African countries. Compressive and bending strength tests were performed on lightweight partition boards, hardened magnesium oxychloride cement pastes and lightweight concretes; fur-thermore, fi re resistance tests were performed on lightweight partition boards and the swelling and shrinkage behaviour of lightweight concrete was investigated. These tests aimed at defi ning speci-fi cations for the new building materials and providing a sound data basis for comparison of these new materials with conventional construction materials. The results of the investigations will be published in future reports.

Step 2 – Development of Binders Produced from Local Raw Materials3.

3.1 General

For the production of artifi cial stones, inorganic binders are required. Likewise, for the fabrication of partition boards, usually inorganic binder materials are used. Since conventional cements are most-ly imported and very expensive in most African countries,1b,3 it is necessary to search for alternative

34


routes to produce the required binders. As stated above, these binders should be mainly made from local resources. Since huge amounts of volcanic rocks (e.g. pumice) are available in East Afri-can countries,1b these could serve as the main raw material to produce natural pozzolan-based binders. According to Gartner,6 pozzolan-based cements are one of only few ways to produce bind-ers in an economical and ecologically feasible way, i.e. with the much lower consumption of energy and with greatly reduced output of carbon dioxide, compared to ordinary Portland cement. There-fore, the use of natural pozzolans as raw material for production of binders would be a contribution to environment protection not only in Africa, but in the entire world.

3.2 Lime-pozzolan Binders

Natural pozzolans are long known to be able to harden by reaction with aqueous calcium hydroxide solutions. In most applications, calcium hydroxide is supplied by Portland cement clinker, i.e. the pozzolans are used as additions to produce blended cements. However, under favourable condi-tions it is suffi cient to activate ground volcanic rock powder by addition of only lime to initiate poz-zolanic reaction.7 This means that lime instead of Portland cement can be used to produce binders from natural pozzolans.

The term “pozzolanic reaction” refers to the reaction of silica with lime, which can be expressed as: S + xCH + yH → CxSHy+x or C-S-H (cement chemistry notation: C = CaO; S = SiO2; A = Al2O3; H = H2O). The C-S-H in hardened lime-pozzolan binders is closely related to the C-S-H gel in hard-ened Portland cement pastes, however, in general it has lower C/S ratio. In addition to C-S-H, AFm phases (C4AH13, C2ASH8 and other) and under certain circumstances C3AH6 (hydrogarnet) might form during reaction of the pozzolans when the latter contain reactive alumina. These phases bridge the gaps between the particles of pozzolan and thus lead to hardening of the paste with compres-sive strengths up to 20 MPa.7a Hardening of lime-pozzolan binders is in general slower than that of Portland cement. However, higher temperatures, as found in most African countries, promote the pozzolanic reaction and thus hardening.

In future research the infl uence of crucial parameters on the reactions of available volcanic rocks and the properties of the hardened pastes have to investigated. These include composition of the pozzolans, grinding, and possible use of small amounts of activators such as NaSO4 or CaCl2.

3.3 Geopolymeric Binders

Another way to utilise aluminosilicates as binders is activation by highly concentrated alkali hydrox-ide or silicate solutions to produce geopolymers.8 Most research on geopolymers was concerned with metakaolin or industrial by-products (e.g. fl y ash) as starting material, however, natural minerals and volcanic rocks have proven to be useful as raw material for geopolymers, as well.9 Usually ge-opolymers are cured at slightly elevated temperatures of 35-85 °C but can also be produced at room temperature (ca. 23 °C).10

The process of geopolymerisation consists of dissolution of the aluminosilicate starting material, coagulation and gelation of the aluminate and silicate species in solution and fi nally in rearrange-ment and partial crystallisation of the gel. Geopolymers thus comprise amorphous and semi- or fully crystalline alkali aluminosilicates, which have been identifi ed as various zeolites or zeolite pre-cursors. The structure of geopolymers (including the amorphous phase) is made up of tetrahe-drally coordinated Si4+ and Al3+ ions, linked by oxygen, with only few non-bridging oxygen atoms; they can thus be described as three-dimensional aluminosilicate framework. The negative charge on the AlO4

- tetrahedrons is charge-balanced by the alkali cations.8,10a

Compared to conventional cement-bonded materials, geopolymers can have superior properties such as rapid strength development, heat and fi re resistance, acid resistance, dimensional stability and improved adhesion.8 Beside use as conventional building material, geopolymers can fi nd many

35

Inorganic Binder Systems for Innovative Panel Technology in East Africa

other applications, especially in mining and in toxic waste immobilisation,11 which could provide further benefi t to the African countries, which possess demand in these areas.

Similar to the case of lime-pozzolan binders, the infl uence of the composition and granulometry of the aluminosilicate sources (volcanic rocks) on the properties of the resulting geopolymers has to be investigated. Further parameters which strongly affect the structure and performance of the geopolymerisation product are the activator species including their concentrations, the curing re-gime (temperature, duration) and the possible use of additional alumina or silica sources; these have to be examined as well.

Step 3 – Future Cooperation and Development of Innovative Lightweight 4. Wall Boards

After identifi cation of the most appropriate natural pozzolans for fabrication of lime-pozzolan bind-ers and geopolymers, their use for specifi ed applications has to be tested and optimized. This comprises in particular assessment of their applicability for the production of artifi cial stones and panels for lightweight partition boards. The latter probably requires the use of fi bre reinforcement to enhance the tensile strength of the panels. As in the case of the binder materials, the fi bres should originate from the producing countries and should be as cost-effective as possible. Al-though there are problems associated with the durability of vegetable fi bres in highly alkaline envi-ronments12 as in geopolymers and lime-pozzolan binders, their use should be considered, since they are cheap and abundant in many African countries. In addition, for both applications the infl u-ence of fi ller materials on the properties of the resulting products has to be evaluated.

Currently, a joint project of the scientifi c partners together with the German-African Business As-sociation and partners from the industry to develop and optimize lime-pozzolan binders and ge-opolymers based on natural pozzolans is in preparation. Furthermore, it is planned by the German-African Business Association to establish several further projects, concerned with different aspects of the production of partition boards in Africa. These efforts are ultimately aimed at producing light-weight partition boards for outdoor use from local raw materials and thus to improve the housing situation in Africa signifi cantly.

References5.

[1] (a) World Urbanization Prospects: The 2007 Revision; United Nations: New York, 2008. (b) Taffese, W. Z. Innovative Low-cost Lightweight Constructions for Ethiopia, unpublished manuscript; EiABC/Addis Ababa University: Addis Ababa, 2011.

[2] Ethiopia: Addis Ababa Urban Profi le; United Nations Human Settlements Programme (UN HABITAT): Nairobi, 2008.

[3] Marinescu, M. V. A.; Schmidt, W.; Msinjili, N. S.; Uzoegbo, H. C.; Stipanovic Oslakovic, I.; Kumaran, G. S.; Brouwers, H. J. H.; Kühne, H.-C.; Rogge, A. In 13th International Congress on the Chemistry of Cement, Madrid, Spain, 2011.

[4] Deng, D. Cem. Concr. Res. 2003, 33, 1311-1317.

[5] Wells, J.; Wall, D. Habitat Int. 2003, 27, 325-337.

[6] Gartner, E. Cem. Concr. Res. 2004, 34, 1489-1498.

[7] (a) Massazza, F. In Structure and Performance of Cements, 2nd ed.; Bensted, J.; Barnes, P., Eds.; Spon Press: London, 2002; pp 326-352. (b) Massazza, F. Cemento 1976, 73, 3-38. (c) Shi, C.; Day, R. L. Cem. Concr. Res. 2001, 31, 813-818. (d) Moropoulou, A.; Bakolas, A.; Aggelakopoulou, E. Thermochim. Acta 2004, 420, 135-140. (e) Ubbríaco, P.; Tasselli, F. J. Therm. Anal. 1998, 52, 1047-1054. (f) Türkmenoglu, A. G.; Tankut, A. Cem. Concr. Res. 2002, 32, 629-637.

36


[8] Duxson, P.; Fernández-Jiménez, A.; Provis, J. L.; Lukey, G. C.; Palomo, A.; van Deventer, J. S. J. J. Mater. Sci. 2007, 42, 2917-2933.

[9] (a) Xu, H.; van Deventer, J. S. J. Int. J. Miner. Process. 2000, 59, 247-266. (b) Xu, H.; van Deventer, J. S. J.; Lukey, G. C. Ind. Eng. Chem. Res. 2001, 40, 3749-3756. (c) Chávez-García, M. L.; García, T. A.; De Pablo, L. In 13th International Congress on the Chemistry of Cement, Madrid, Spain, 2011.

[10] (a) Rahier, H.; van Mele, B.; Biesemanns, M.; Wastiels, J.; Wu, X. J. Mater. Sci. 1996, 31, 71-79. (b) Sagoe-Crentsil, K.; Taylor, A.; Brown, T. In 13th International Congress on the Chemistry of Cement, Madrid, Spain, 2011.

[11] (a) van Jaarsveld, J. G. S.; van Deventer, J. S. J.; Lorenzen, L. Miner. Eng. 1997, 10, 659-669. (b) van Deventer, J. S. J.; Provis, J. L.; Duxson, P.; Lukey, G. C. J. Hazard. Mater. 2007, 139, 506-513.

[12] Vegetable Plants and their Fibres as Building Materials (Proc. 2nd Int. RILEM Symp.); Sobral, H. S., Ed.; Chapman and Hall: London, 1990.

37

Challenges of the African Environmental Conditions for Concrete Mixture Composition


Wolfram SchmidtBAM Federal Institute for Materials Research and Testing, Berlin, Germany

Introduction1.

Concrete technology was exposed to a rapid development during the last three decades. For the longest time in its history, concrete was considered as a three component system consisting of ag-gregates, which are bound by the hardened cement paste consisting of hydrated cement. Tradition-ally, the only way of adjusting the consistency of concrete was using well adjusted aggregates and grading curves and adding excess water to the concrete, accepting that the latter in return reduces strength and durability. During the last three decades, however, concrete has developed further from a three component system towards an (at least) fi ve component system (Figure 1), since the use of mineral additions and chemical admixtures has become state of the art. Both components are able to enhance the workability, the compactability, and the density of the microstructure with effects on strength, ductility and durability, while cement can be saved in parallel. Due to reasona-ble use of admixtures and additions, concrete can be designed to match mechanically high per-formance specifi cations. Traditionally, cement paste was considered the weakest component in concrete. However, in modern concrete a good paste composition can yield highest performance, passing the role of the mechanical bottleneck towards the aggregates.

Figure 1: Traditional versus modern concrete mixture components

Recently, further components are under world-wide investigation, being able to give an added value to cementitious products, such as phase change materials, superabsorbent polymers, air-purifying and self-cleaning agents, or light refl ecting beads. However, strictly speaking these can also be al-located to the admixtures or additions.

General rules for a good mixture composition apply everywhere in the world. Materials should be chosen wisely according to the specifi cations and boundary conditions. Poor quality and quality scattering of the raw materials should be avoided as much as possible. The total grading of all con-crete solid components should be considered, and for the proper processing on the construction site, concrete should provide adequate workability. However, everywhere in the world, different raw materials are available and concrete needs to withstand varying climatic threads. Also prices, avail-ability and infrastructure for concrete components vary greatly. Finally, building history, construc-tion site conditions and national regulations diverge from country to country.

38


In its history, concrete was always a construction material of the northern hemisphere. The prime father of cement, the “Opus Caementitium” was invented by the Romans, the invention of the mod-ern cement took place around 1850 in the UK. After this invention cement and concrete wrote an unmatched success story in the construction history and were matter of intensive research in North-ern America, Europe, Northern and Eastern Asia, all regions above the equator, while in Africa it is a rather new material. It is hence evident that the signifi cant part of the today’s technology and knowledge about concrete is based on the boundary conditions of the northern hemisphere, where concrete development always went hand in hand with the developing industries, particularly heavy industries. Africa is climatically differing, the geography and infrastructure differs strongly and also the concrete related industries and the construction technology are different.

Europe, Japan and Northern America provide a small meshed network of ready-mix plants. Cement is typically delivered to the mixing plants several times per week. Mixing of concrete and fi lling of the truck mixer takes place in computerised and controlled environment, and also the infl uence of the transportation are manageable. E.g. in Germany, the average distance between construction site and mixing plant is only 17 km. This allows a steady and calculable concrete quality, so that mixture composition considerations regarding the robustness of the fresh performance play a sec-ondary role compared to cost and performance considerations. In Africa the major way of prepar-ing concrete is on-site mixing with cement that is stored in bags on the construction site. This process is exposed to numerous sources of errors, so that robustness of workability at differing climatic conditions might play the key role for good concrete.

Concluding the last passages, Africa shows the following specifi cations:

Wide range of climatic boundary conditions –

Low level of regulations with special emphasis on the African environment –

Poor ready-mix infrastructure –

Different concrete operation techniques –

These specifi c African aspects should be considered for the composition of concrete. This applies for each of the fi ve components as well as for the general approach towards mixtures. Particularly from the point of view of the available concrete infrastructure and the future infrastructural needs, the specifi c African boundary conditions might suggest different, new or forgotten approaches.

Mixture Components2.

2.1 Cement

Cement is traditionally considered as the binder to glue the aggregates together. During the last decades many different cement types were developed with tailor-made properties according to specifi c conditions. Sales of blended cements increase steadily, while OPC sales reduce in parallel. An example, showing this for Germany is given in Figure 2. The same trend can be observed for most European countries. This is on the one hand result of the necessity to reduce CO2 emissions and to save energy costs, on the other hand the companies see the chance, to supply cement with specifi c properties. For example, fl y ash and limestone interact in synergy, fostering the formation of monocarboaluminate, while cements blended with slag and limestone can generate similar per-formance as OPC. A very comprehensive report about the present and future of cement is given by Schneider et al [4], according to which already today cements are very versatile, however, future developments point into the direction of even a wider variety and complexity.

In Northern America still today OPC totally dominates the cement market, while the largest market share in Asia is covered by fl y ash blended cement. Cements in Europe are more diversifi ed. OPC, blends with fl y ash, limestone, and mixed components share comparable sales numbers. In Africa mixed blends and pozzolanic cements are the predominating cement types. However, differing from Europe, where during the last decades blends were developed in order to improve or modify

39

the performances according to specifi cations, in Africa, the use of supplementary cementitious materials (SCM) is prior a result of the necessity to save energy and a lack of alternatives. While fl y ash, slag, and silica fume, materials, which typically improve the performance, are easily available in Europe due to heavy industries, Africa’s cement industry is bound to the naturally available re-sources like limestone, or natural pozzolans.

This certainly forces special considerations in the mixture composition. Natural pozzolans can en-counter large quality scatterings. Typically the contribution of their pozzolanic reaction to the late strength is signifi cantly smaller than the effect of fl y ash. Furthermore, they can introduce large al-kali contents [1], which increases the risk of alkali silica reaction (ASR), hence the choice of aggre-gates needs to be considered with special emphasis on ASR. Finally, natural pozzolans often pro-vide large porous surfaces, which increase the water demand of the binder paste, so that the workability can be reduced.

Special considerations also have to be given to the cement storage, when superplasticizers are used. In Africa, cement is typically stored in 50 kg bags. According to Schmidt et al. [2], storage conditions at differing humidity and temperature might affect the set retarder (typically gypsum, hemi-hydrate, and anhydrite) with strong infl uence on the performance of superplasticizers and the setting behaviour.

Figure 2: Development of sales of different cements in Germany during the last 15 years based on the data of bdz [3]

2.2 Water

The water to cement ratio (w/c) or analogously the water to powder ratio (w/p) and the water to ce-mentitious materials ratio (w/cm) are major infl uencing factors for the workability and compactabil-ity, and the mechanical properties of concrete. In the past, however, improved workability due to higher water contents often outweighed the negative effect on the cement paste. Today, due to superplasticizers, the consistency can be modifi ed largely independently of the water content. As a result, today, high performance concrete (HPC) and ultra-high performance concrete (UHPC) is put more and more into the focus of research, since water cement ratios can be reduced to extremely low values. However, the construction market predominantly still requires normal concrete with compressive strengths between 15 and 35 MPa. Although technologically possible, there is no ne-cessity to force low w/c for normal concrete applications. Even with very high w/c good concrete can be designed if a reasonable mixture composition is chosen.

Sub-saharan countries do not possess a network of ready-mix concrete plants, which would allow delivering a uniform concrete quality to the site. Even in South Africa, which has a relatively highly elaborated concrete infrastructure, mixing concrete on the construction site with cement from bags is state of technology.


40


This, and the necessity to adjust the workability by supplementary water, bears a very high risk that different mixing charges contain uncontrolled excess water. Furthermore, Africa’s climate is known to be very hot in many regions, which causes quick and untraceable diffusion of water. This again yields random water contents, which in addition might vary also with the distance to the concrete surface, causing high risk of cracks.

In order to overcome these problems, concrete mix designs should provide components that avoid quick diffusion. Kaolinite, Illite or other clayey materials in small amounts can help to maintain the water in the concrete. Furthermore, modifi ed cellulose or starch can effectively reduce the diffusion of water. Both latter are natural materials, which are cheap and available in Africa. Finally, the only way of uncoupling the workability from the water content is using superplasticizers (SP). Particu-larly at very high temperatures, when cement hydration is accelerated causing rapid loss of work-ability, the use of SP can be considered as the only solution to provide steady concrete quality. The newest generations based on polycarboxylate are very versatile but they are also expensive and their processing requires complex fabrication technology. The very fi rst generation of SP, based on lignosulphonate, which is a waste product of the paper industry, still possess a large market share everywhere in the world, and their use can be assumed to bring great benefi t to the Africa typical on-site mixing process.

2.3 Fillers

Fillers are fi ne powders that help modifying mixture compositions according to particular specifi ca-tions. They can be used to reduce the cement content in concrete while maintaining the workability of concrete with high cement content. As fi llers are inert or their reaction occurs much later than the cement reactions, fi llers are a reasonable option to reduce the infl uence of scattering cement qual-ities. This is particularly valid at high temperatures. Fillers can also be used supplementary to the cement in order to improve specifi c properties. Pozzolanic or hydraulic fi llers like fl y ash, natural pozzolans, or slag can furthermore react with time with the calcium hydroxide generated during the cement hydration yielding calcium silicate hydrates (C-S-H), calcium aluminate hydrates (C-A-H), or calcium aluminate silicate hydrates (C-A-S-H), which densify the microstructure and increase the strength. In most of the countries with long lasting concrete tradition, fl y ash, slag, silica fume, or limestone powder are well available and established. Africa does not inevitably have access to these materials (with the exception of limestone). However, Africa is rich of raw materials that could be used as alternative fi llers. Specifi c African fi llers could be bagasse ash [5], dolomite powder [6] and natural pozzolans. The latter, however, often provide high and porous structures (Figure 3), which demands for higher water contents than e.g. necessary with industrial pozzolans like fl y ash and silica fume, which positively contribute to the packing density and rheology of concrete.

Figure 3: SEM micrographs (BAM) of a natural pozzolan (Rhenish Trass, left) and an industrial pozzolan (fl y ash, right)

41

2.4 Admixtures

Without doubt, admixtures for concrete were the catalyst of the rapid enhancements in concrete technology during the last decades. Improved rheology modifi ers fi rstly allowed developments like self-compacting concrete, UHPC, engineered cementitious composites (ECC) and other break-throughs in concrete technology. However, admixtures should be considered as modifi ers and sup-plementary boosters of already well engineered concrete technology, they should not be consid-ered as means to neglect in return reasonable concrete technology. Comparing the relatively high prices of admixtures to the predominating needs in Africa, namely housing and infrastructure, both at reasonable costs, admixtures should only be considered for mixture composition with care.

The major group of admixtures are superplasticizers. The interaction with cement is typically identi-cal for all types of superplasticizers. They consist of a negatively charged backbone, which adsorbs on cement clinker phases and hydration products with positively charged zeta potentials [7]. The dispersion mechanism of older generations of SP, such as lignosulphonate or poly naphthalene condensates, works predominantly by electrostatic repulsion. The generation of polycarboxylate ether SP (PCE) provides steric hindrance of particles as the major dispersion mechanism. The dif-ferent ways of operation are illustrated in Figure 4. Taking the results of former studies of the author into account [8], the African conditions would favour the use of PCE, and particularly modifi cations with low backbone charge density. Due to their slow adsorption, they can well buffer workability losses due to high temperatures. However, also lignosulphonates are a reasonable option, as they are effective, cheap and well accessible. They became less popular in many European countries due to their quick loss of performance, which made them element of uncertainty during the truck transportation. As in Africa the common method is mixing on the construction site, this aspect can be ignored. For the practical mixture composition, it should be considered to add them to the con-crete at a late stage, as their dispersing force cannot be maintained for long time after addition and as they are known to intercalate at early stage, which would yield signifi cant performance losses or uneconomically high dosages.

Figure 4: Electrostatic repulsion of particles (left) versus steric hindrance (right)

Another important group of relevance are retarders such as tartaric, citric or gluconic acid. Retard-ers allow long transportation distances. They hence might be of high importance in many African countries, where the network of ready mix plants is signifi cantly less dense than e.g. in Europe. Long distance transportation on the truck mixer might become increasingly necessary for large scale concrete construction, where on site mixing is not an option. However, the use of retarders should only be considered, when the cement quality is very homogenous and the performance of SP is not affected.


42


Finally viscosity modifying agents (VMA) are admixtures with specifi c relevance for the African con-ditions. They positively affect the water retention, thus avoiding shrinkage and uncontrolled loss of water. However, in case SP is not available, they also allow modifying the rheology with water, with-out bleeding or segregation. Typical examples of VMA are cellulose ether or starch ether. Simple modifi cations make them cold water soluble and resistant against high pH values. Due to the defor-estation problem in many East African countries, it is likely that instead of cellulose, starch ether should be favoured. Many African countries produce large amounts of cassava. However, starch can also be retrieved from potatoes, corn or other popular plants. According to former works of the author, there is no critical infl uence of high temperatures on the performance of modifi ed starch [9].

Shrinkage compensating agents, typically surfactants, might play a minor role for standard applica-tions. They mainly avoid autogenous shrinkage, which is particularly a thread to high performance concrete. Air entraining agents are mainly used in order to provide frost resistance for concrete. This might only be used for very special applications in some regions in sub-saharan Africa. Other examples of admixtures, which are in use with concrete, are set accelerators, foaming and de-foaming agents, or fi bres. These however, are admixtures, which are in use for very special applica-tions, mainly in the fi eld of grout and repair mortar.

2.5 Aggregates

A good grading of aggregates is the key for good concrete quality. The infl uence of the particle and grain size distribution has been subject to research since the fi rst publications related to the topic by Fuller and Thomson [10]. In 1907 already, Fuller and Thomson found out empirically that the best compaction can be achieved, when the fi nes content follows an elliptic curve [10]. These fi ndings developed to the today well established fuller curve following the equation:

Ai=(Di/Dmax)n

with:

Ai = Sieve screening for a mesh sizeDi = Particle diameterDmax = Maximum particle diameter of the mixn = Allocative function (often n=0.5 is suggested)

The main focus of research during the recent years is put on the maximisation of the mechanical properties and durability for highest possible performance. Considering the specifi c African condi-tions, particularly the typical mixing on the construction site, however, ultimate performance of concrete might be of signifi cantly lower importance than robustness against infl uences of the ad-verse conditions, such as scattering amounts of total water and cement. Varying delivery charges, climatic changes during the day and storage conditions on the construction site yield total water contents, which vary and which can neither be determined nor predicted easily. This, in addition, demands for robust grading. Typically sand-rich grading curves can better absorb negative effects of total water variations, and should be favoured, although a grading with high coarse aggregate content can improve the mechanical properties.

In many regions in sub-saharan Africa the climate is characterised by high temperatures combined with high relative humidity. These conditions and the use of natural pozzolans in cement and as fi ller foster the occurrence of ASR, a chemical reaction from the high alkali content in the paste and glassy phases of the aggregates (Figure 5). Opal and fl int components in the aggregates can thus cause serious damages in concrete constructions in presence of high temperature and humidity. Concrete engineers in Africa should even more than anywhere in the world be aware of this risk and adjust the mixture composition in a reasonable way.

43

Figure 5: Thin section (BAM) of concrete damaged by ASR

Lightweight concrete is getting more and more popular all over the world, as with improved cement quality and enhanced paste mixture compositions, concrete strength and performance are less and less depending upon the mechanical properties of the aggregate. East Africa provides a large ap-pearance of pumice. Such material is not yet well investigated, although it can be estimated to provide numerous benefi ts, particularly considering the African conditions. Due to the porous struc-ture it will absorb high amounts of water counteracting shrinkage and uncontrolled diffusion of water due to high temperatures, when aggregates are pre-wetted. Furthermore it can be assumed to provide pozzolanic properties, which would enhance the bond between paste and aggregate. Finally, lightweight aggregates reduce the heat conductivity, contributing to cooler indoor climate. However, the hot and humid conditions are a thread to porous structures as they are breeding grounds for fungi and vermin. Concrete mixture composition with pumice should thus always put special emphasis into the density of the microstructure of the paste.

Mixture Composition According to Specifi cations3.

A classical approach to composing concrete mixtures mainly based on the system cement, water, and aggregate, is fi rstly detecting the required water demand depending upon the specifi ed con-sistency and the aggregates’ properties. In some countries such curves are part of the national standards or relevant guidelines, or they can be taken from literature (e.g. in Germany [11, 12, 13]). Qualitative examples of such curves are presented in Figure 6.


44


Figure 6: Qualitative examples of relation curves between water demand and aggregates’ properties

Figure 7: Qualitative examples of relation curves between w/c and compressive strength

After identifi cation of the water demand, the water to cement ratio (w/c) can be estimated according to the well known relation between w/c and compressive strength depending upon the particular cement (Figure 7). In Germany, these values can be found in literature [12-14], however, comparable relations are available in other countries and languages. The w/c and the resulting cement content possibly need to be modifi ed according to specifi c regulations. E.g. often exposition to chloride or frost environment demands for minimum thresholds regarding w/c or cement content. After fi xation of the w/c, the air volume needs to be estimated. At good compaction and without air entraining agent, 20 l is a reasonable assumption. Considering the calculation of the volume components for one m³ of fresh concrete:

1000 [dm³] = VCement + VWater + VFillers + VAggregates + VAir = z/ρz + w/ρw + g/ρg + VAir,

the required mass of aggregate [kg/m³] calculates:

g = (1000 - z/ρz + w/ρw + VAir) x ρg

In case, fi llers are part of the mixture concept, their amount needs to be specifi ed. In many coun-tries the total amount of powder components is limited, and sometimes special regulations apply for reactive fi ller, such as fl y ash or silica fume. The required aggregates content calculates accord-ingly as:

g = (1000 - z/ρz + w/ρw + f/ρf + VAir) x ρg

In case the cement quality scattering is low, the applied correlation laws for strength, consistency and w/c are reliable, and the air content estimation fi ts in with the real air volume after compaction, the resulting concrete should provide the specifi ed consistency and strength.

45

This approach, however, is rather static, and furthermore, the database for the correlation curves is typically outdated, so that today’s technological achievements are not well represented. Modern cement is very stable in its performance. Superplasticizers allow controlling the rheology without modifying the water content and reducing the water content without negative infl uence on the rhe-ology, respectively. Furthermore, the versatility of today’s blended cement cannot be implemented adequately. Better understanding of the importance of the packing density and the interparticular forces of the paste, today, theoretically allow high-performance concrete mixtures with only a min-imum of cement. As a result, today, the properties of concrete can be adjusted individually accord-ing to the specifi cations for the application. Some examples are given in Figure 8.

Figure 8: Examples of properties depending on the concrete application

Instead of approaching the concrete mixture composition from the aggregates’ properties, the mix-ture can be generated starting from an optimised paste density and the total solids grading. Ce-ment can for example be replaced stepwise by another fi ne powder component until the optimum packing density is reached. Several methods are available to determine the maximum packing den-sity, which is identical with the minimum water demand. A comprehensive overview of numerous methods to determine this value is given by Hunger and Brouwers [15]. An optimised powder frac-tion combined with a reasonable overall grading allows reducing cement signifi cantly without per-formance losses or even with improved properties. Hence, since systematic mixture composition rules as described above can never cover the large range of different raw materials and their quali-ties, opportunities and drawbacks, concrete mixture composition should rather be considered from a specifi ed performance point of view.

Mixture Composition Approaches for Africa4.

The most urgent structural needs in Africa are housing and infrastructure. Housing projects for two to seven storey buildings typically do not demand for high performance concrete, however, in order to build sustainably, a reliable concrete quality is required to avoid undesired strain in the structure. Road construction also does not demand for high performance concrete but rather for early traffi -cability, low cost but nevertheless long lasting durability. These specifi cations suggest a new ap-proach to concrete technology in Africa and the rediscovery of largely forgotten techniques.


46


4.1 Cementitious Pre-mix Compounds for Concrete for Structural Building

The major difference between African and e.g. European concrete construction is the location, where concrete is mixed. Weighing of all concrete components on the construction site is prone to errors. Typically, mixing on the construction site is not even conducted by help of a scale but only by volumetric units (shovels, buckets, barrows), increasing the standard deviations of the concrete performance. As the implementation of a dense network of ready-mix plants will take time, the situ-ation will not change soon.

Then the hot climatic boundary conditions were identifi ed as thread to a uniform concrete quality and compactability. For robust casting of concrete at high temperatures, superplasticizer improve the robustness of the performance, VMA can be additionally used in order to avoid segregation and uncontrolled diffusion of water. The handling of these on the construction site, however, is delicate and requires high skills and long term experiences with plasticized concrete.

It would thus be a reasonable option to develop binder compounds, which already include cement, well adjusted fi nes and admixtures as required. These only need to be amended by water and ag-gregates. A good adjustment of fi nes could absorb the infl uence of varying aggregate types and size distributions. An example for such a binder compound including cement, fi llers, superplasti-cizer and supplementary admixtures is described by Schmidt et al. [16]. In order to demonstrate the robustness of such a binder pre-mix compound for self-compacting concrete in practice, a so-called “idiot-proof test” was initiated. An extremely wide range of different aggregate gradings was given to fi ve unskilled test persons (Person 1-5) with the task to cast concrete by adding water and the pre-mix compound. The test persons were left alone and they were free to use a scale or volu-metric units for weighing. The grading variations are shown in Figure 9. Although the fresh proper-ties of self-compacting concrete are generally delicate to handle and although each test person mastered the challenge quite differently, the variation of results for both, fresh and hardened con-crete properties are well acceptable (Figure 10).

Figure 9: Grading curves varied in the “idiot-proof test” to prove the effect of a pre-mixed binder compound [16].

47

Figure 10: Variations due to varied grading curves and personnel in slump fl ow (left) and compressive strength after 28 days.

Such a concept could reduce signifi cantly quality fl uctuations due to limited mixing facilities, while it would additionally open a regional market segment, the blending of pre-mixed compounds, for the local industries.

4.2 Roller Compacted Concrete, an Option to Accelerate Africa’s Infrastructural Build-up

The second area of challenge for the African construction industry is the infrastructure. The increas-ing markets in many African countries demand for many new roads to connect the important cities and trade knots. These might be several hundreds of kilometres apart. In other countries, which exhibit comparable infrastructural needs, namely long interurban distances, roller compacted con-crete RCC had been considered as a reasonable choice to either quickly and cost-effectively build roads and dams. In terms of planeness, RCC cannot compete with bituminous road constructions, however, it provides high resistance against static and dynamic loads, abrasion and chemical at-tacks. RCC typically consists of the same components as normal concrete but the mixture compo-sition is rather based on soil-mechanical considerations. Green strength is very high, so that con-structions with RCC can be rapidly put under load after casting. RCC is placed by help of typical equipment used for road construction. Mounting volumes and casting rates are very cost effective. Typical design strengths for RCC may vary depending on the cement content between 15 and 40 MPa.

RCC mixture compositions normally contain low cement contents. The powder content is typically increased by supplementary use of rock powders. The total fi nes content is rather high (>450 kg/m³) in order to provide the required green strength. Due to the low cement contents, optimised compactability due to high water contents often outweighs the negative effect of the high w/c. Studies by Nanni [17] showed that crushed powders and marginal powders can signifi cantly im-prove the properties. It is suggested that grading curves should be dominated by large amounts of coarse aggregates. Tatro and Hinds [18] suggest grading curves with signifi cantly higher coarse contents than the Fuller parabola would suggest, while Marchand et al. [19] propose a Fuller curve with an exponent of 0.45 (Figure 11). In case of dam construction, RCC is often constructed with large aggregates, however, according to Marchand et al. and the German bulletin for RCC for road construction [20], for load-bearing surface layers, the maximum grain size should be limited to 16 mm.


48


Figure 11: Suggested aggregate grading curves for RCC according to [18, 19]

The major strengths of RCC are low costs and very quick construction with high durability and low maintenance costs. These parameters highly suggest RCC for interurban road constructions through a continent featuring the geographical and infrastructural boundary conditions of Africa.

Conclusions 5.

Concrete construction has no long lasting tradition in most African countries, as a result, the infra-structure for building with concrete is no well developed. Typically mixing of concrete in most Afri-can countries is conducted on the construction site, which is prone to human errors. Furthermore, the climatic conditions can cause uncontrolled water losses and poor workability retention. Hence highest priority in mixture composition has to be given to the robustness of a mixture, in order to make sure that even at varying ambient conditions a steady concrete quality is placed. It is there-fore suggested to favour a moderate grading curve of the aggregates over a performance optimised curve. In order to uncouple the workability of concrete from the water content, the use of superplas-ticizer should be taken into consideration. Africa offers a high amount of raw materials, which pro-vide potential to improve concrete quality, such as bagasse ash or natural pozzolans. These need to be subject of intensive future research.

The major future challenges for the concrete construction industry are housing and infrastructure. In order to overcome drawbacks of the unavailability of ready-mix plants, it is suggested to develop binder compounds including cement, fi llers and admixtures for specifi ed concrete properties, which only need to be amended on the construction site by aggregates and water. In order to quickly de-velop interurban infrastructure, the use of roller compacted concrete should be given special con-siderations.

References 6.

[1] Müller, U.; Bürgisser, P.; Weise, F.; Meng, B. (2010): The natural pozzolana ‘Rhenish trass’ and its effect on ASR in concrete. In Sixth International Conference on Concrete under Severe Conditions, Environment & Loading - CONSEC10. Mérida, Mexico, pp. 313-320.

[2] Schmidt, W.; Ramge, P.; Kühne, H.-C. (2009): Effect of the storage conditions of cement onthe processing and hardening properties of concrete, Concrete Plant + Precast Technology, vol. 06, pp. 10-17.

49

[3] Zahlen und Daten 2009-2010, Bundesverband der Deutschen Zementindustrie e. V.

[4] M. Schneider, M. Romer, M. Tschudin, H. Bolio, Sustainable cement production – present and future, Cement and Concrete Research, Volume 41, 7, July 2011, pp. 642-650.

[5] Paula, G.; Souza C. A.; Aguilar, M. T. (2011): Physical-Chemical Characteristics of Cane Bagasse Ashes used in the manufacturing of concretes, 13th International Congress on the Chemistry of Cement, Madrid, Spain.

[6] Schöne, S.; Dienemann, W.; Wagner, E. (2011): Portland Dolomite Cement as Alternative to Portland Dolomite Cement, 13th International Congress on the Chemistry of Cement, Madrid, Spain.

[7] Plank, J.; Hirsch, Ch. (2007): Impact of zeta potential of early cement hydration phases on superplasticizer adsorption, Cement and Concrete Research 37, pp. 537-542

[8] Schmidt, W.; Brouwers, H. J. H.; Kühne, H.-C.; Meng, B. (2010): Effects of Superplasticizer and Viscosity-Modifying Agent on Fresh Concrete Performance of SCC at Varied Ambient Temperatures, in Khayat, K. H. and Feys, D. (Eds.) Design, Production and Placement of Self-Consolidating Concrete - Proceedings of SCC2010, Montreal, Canada, September 26-29, 2010. Springer.

[9] Schmidt, W.; Kühne, H.-C.; Meng, B. (2010): Temperature related effects on self-consolidat-ing concrete due to interactions between superplasticizers, supplemental admixtures and additions, in Malhotra, V. M. (Ed.) 9th ACI International Conference on Superplasticizers and Other Chemical Admixtures – Supplementary Papers, Seville, Spain.

[10] Fuller, W. B. and Thomson, S.E. (1907): The laws of proportioning concrete, Transactions ofthe American society of civil engineers, vol. 59, pp. 67-172.

[11] Bonzel, J. and Dahms, J. (1978): Über den Wasseranspruch des Frischbetons. beton 9/78, S. 331-336; beton 10/78, S. 362-367; beton 11/78, pp. 413-416.

[12] Betontechnische Daten 2009, HeidelbergCement AG.

[13] Grübl, P.; Weigler, H.; Karl, S. (2001): Beton – Arten, Herstellung und Eigenschaften, 2. Aufl age, Ernst & Sohn.

[14] Walz, K.: Anleitung für die Zusammensetzung und Herstellung von Beton mit bestimmten Eigenschaften; Beton- und Stahlbetonbau, Sonderdruck, Jhg. 53, Heft 6, 1958, pp. 163-169.

[15] Hunger, M. And Brouwers, H.J.H. (2009): Flow analysis of water-powder mixtures: Applica-tion to specifi c surface area and shape factor, Cement and Concrete Composites 31, pp. 39-59.

[16] Schmidt, W. ; Meng, B.; Kühne, H.-C.; Rosignoli, D. (2008): Development and application of a novel ready-made compound including additions and admixtures for the easy production of SCC, Betonwerk + Fertigteil-Technik, pp. 4-11.

[17] Nanni, A. (1998): Limestone Crusher-Run and Tailings in Compaction Concrete for Pavement Applications, ACI Materials Journal 5-6, pp. 158-163.

[18] Tatro, S.; Hinds, J.K. (1992): Roller Compacted Concrete Mix Design, Proceedings of the Conference sponsored by the Construction, Geotechnical engineering and Materials Engineering Division of the American Society of Civil engineers, Sand Diego, California, pp. 323-340.

[19] Marchand, J.; Gagné, R.; Ouellet, E.; Lepage, S. (1997): Mixture Proportioning of roller Compacted Concrete – A Review, American Concrete Institute, ACI SP-171, pp. 457-486.

[20] Zement-Merkblatt Straßenbau S6, 9.2001, „Walzbeton für Tragschichten und Tragdeck-schichten“.


50


From Prescriptive Towards Performance-based Durability Design of Concrete

Dubravka Bjegovic1,2, Irina Stipanovic Oslakovic1,3, Marijana Serdar2

1 Institut IGH d.d., Croatia2 University of Zagreb, Faculty of Civil Engineering, Croatia3 University of Twente, Construction Management and Engineering Dept., The Netherlands

Abstract

Durability of a structure is defi ned as its ability to preserve functionality, stability and aesthetic properties under expected environmental infl uences without larger maintenance and repair costs during designed service life. With the requirements of 100-year design service life for major bridge structures and enormous rehabilitation and repair costs associated with inability to satisfy these requirements, durability of civil engineering structures is today one of the key problems of struc-tures worldwide.

In this paper two main levels of durability design are presented: prescriptive and performance-based design. Prescriptive durability design is based on set of empirical rules, which need to be followed to assure required service life. The major diffi culty with prescriptive design lays in the com-pliance procedure, since it is almost impossible to perform effective control of most of the pre-scribed limiting values. Performance-based design is based on durability indicators, which are prescribed in the design phase, tested during prequalifi cation testing, used in service life models and tested during construction as a part of quality control on site.

Introduction1.

Numerous cases of damaged reinforced concrete structures worldwide evidence the lack of ade-quate guidelines for durability design. Durability problems cover a wide range of degradation mech-anisms, including attack by external aggressive environments such as chlorides or sulphates, and by internal material incompatibilities such as alkali-aggregate reaction. The severity of the environ-mental infl uence on concrete depends on the properties of concrete and its exposure conditions. The crucial part of the concrete is the concrete cover layer, which acts as a physical and chemical barrier, protecting the inner concrete and reinforcement from degradation. This layer is the most exposed to the environmental infl uences, and is, at the same time, the most affected by any poor workmanship during placing, compacting or curing. Durability of concrete is therefore largely con-trolled by the quality of this concrete cover layer. Since the importance of the concrete cover has been recognised, there is a signifi cant effort to highlight this importance through standards, guide-lines and recommendations from Technical committees, all as a part of durability design of con-crete structures.

Now days, durability design of concrete structures could be divided into two main levels:

Prescriptive durability design, covered by EN 206 [1], and –Performance – based durability design as described in Model Code 2010 [2], based on service –life modeling either deterministic, semi-probabilistic or full probabilistic approach.

Two main levels and two sublevels with approaches towards durability design are shown in Figure 1 [3]. Moving from right hand side, levels of durability design are becoming more complex but at the same time more precise in predicting service life of concrete structures. Avoidance of deterioration and deem-to-satisfy method are prescriptive design approaches, mostly based on a set of rules for dimensioning, material and product selection and execution procedures. By following these rules, as a cookbook, one ensures that the structure will achieve a service life of 50 years if concrete

51


structure or component is exposed to the loading considered in the design. Performance based design approach, on the other side, demands understanding of physical deterioration processes in reinforced concrete, and factors affecting and resisting them. Probabilistic methods are used in the civil engineering practice for calculation of bearing capacity and stability of structures and their use is standardised and accepted. Nowadays same methods are being applied to calculation of dura-bility and service life of reinforced concrete structures under aggressive environment.

Prescriptive Durability Design 2.

The prescriptive design approach for durability design presents a practical concept for durability design, given in EN 206-1 [1] and EN 1990 [4]. The improvements comparing to the previous Model Code 1990 [5] are in the classifi cation of the environment in which the structure will be used during the required life with acceptable levels of maintenance. Prescriptive design usually covers the is-sues given in Figure 2, with particular attention to aspects such as: mix design (maximum water to cement ratio and minimum cement content), minimum concrete cover, see Table 1, depending on the environmental conditions. But, the prescriptive approach ignores, to a large extent, the different performance of different cement types and of the minerals addition to the cement or to the con-crete.

The main assumption of the prescriptive durability design approach is that if these rules are met, the structure will achieve a service life of 50 years. These prescribed values are the only framework for decision making during design of durability. In the prequalifi cation phase concrete mixture is decided mainly upon compressive strength, which is during construction controlled on laboratory cured specimens, Figure 2.

Defi ciency of the prescriptive durability design approach is that durability properties are prescribed on the basis of requirements for constitutive materials, construction and curing without prescribing exact property, testing method and limiting values for specifi c material properties. No calculation procedure is defi ned if longer service life is required. This applies especially for structures with an intended long service life like infrastructures. Moreover, the prescriptive approach ignores the dif-ferent performance of different cement types and of the minerals addition to the cement or to the concrete. It also cannot be used for new materials, (e.g. new cement types, new steel types, non-steel reinforcement), new types of structures or new environments [6].

Figure 1: Different levels of durability design [3]

52


Exposure class

Description of classMax. w/c

ratio

Min. strength

class

Min. mass of cement

(kg/m3)

Min. concrete cover, c + Δc

(mm) [6]

No risk of corrosion

X0

Concrete without reinforcement or embed-ded metal (all exposures except freeze/thaw, abrasion or chemical attack) - Concrete inside buildings with very low air humidity.

- C 20/25 - -

Corrosion induced by carbonation

XC 1Dry or permanently wet - Concrete inside buildings with low air humidity, concrete permanently submerged in water

0.65 C 25/30 260 20 + 10

XC 2Wet, rarely dry - Concrete surfaces subject to long-term water contact, many foundations

0.60 C 30/37 280 35 + 15

XC 3Moderate humidity - Concrete inside building with moderate or high air humidity, external concrete sheltered from rain

0.55 C 30/37 280 35 + 15

XC 4Cyclic wet and dry - Concrete surfaces subject to water contact, not within exposure class XC2

0.50 C 30/37 300 40 + 15

Corrosion induced by chlorides other than from see water

XD 1Moderate humidity - Concrete surfaces exposed to airborne chlorides

0.55 C 30/37 300

55 + 15XD 2 Wet, rarely dry – e.g. swimming pools 0.55 C 30/37 300

XD 3Cyclic wet and dry - Parts of bridges expo-sed to spray containing chlorides, e.g. car park slabs

0.45 C 35/45 320

Corrosion induced by chlorides from sea water

XS 1Exposed to airborne salt but not in direct contact with sea water - Structures near to or on the coast

0.50 C 30/37 300

55 + 15XS 2Permanently submerged - Parts of marine structures

0.45 C 35/45 320

XS 3Tidal, splash and spray zones - Parts of marine structures

0.45 C 35/45 340

Freeze/thaw attack

XF 1Moderate water saturation, without de-icing agent

0.55 C 30/37 300

55 + 15XF 2

Moderate water saturation, with de-icing agent

0.55 C 25/30 300

XF 3 High water saturation, without de-icing agent 0.50 C 30/37 320

XF 4High water saturation, with de-icing agent or sea water

0.45 C 30/37 340

Chemical attackXA1 Slightly aggressive chemical environment 0.55 C30/37 300

55 + 15XA2 Moderately aggressive chemical environment 0.50 C30/37 320

XA3 Highly aggressive chemical environment 0.45 C35/45 360

Table 1: Recommended limiting values for composition and properties of concrete for selected environ-ment clasess [1]

53

Premature loss in durability of structures is mainly caused by poor quality of construction. Quality assurance and quality control on site are therefore recognised as a precedent step in achieving designed quality and durability of concrete. The major diffi culty with prescriptive design lays in the compliance procedure, when it is necessary to assure that the quality of prepared concrete is equal to that of the prescribed. All requirements are considering constituting materials in a prescriptive manner and it is impossible to perform effective control of w/c ratio and cement content in practice. The routine control is based on testing the specimens for the compressive strength, which are cured in laboratory conditions and do not represent the real quality of concrete cover. Furthermore, sampling, placing and compacting of the laboratory specimens differs from the concreting method applied on site. Testing laboratory specimens ignores the effect of concrete method on the proper-ties of concrete.

As shown in Figure 2, practical problem can be experienced after the construction has been fi n-ished, when the test results (28 days after pouring) are actually analysed, if the required properties are not met, usually based on compressive strength or some other mechanical properties. Costs of repair works which then usually imply strengthening or removal of concrete cover and adding new concrete, are exponentially increasing comparing to the cost of the initial concrete. Usual problem is also from a practical and legal point of view, whose responsibility is then this failure and who will pay penalties.


Figure 2: Procedure during prescriptive durability design of concrete

54


Performance Based Durability Design3.

Performance-based approach to the durability design of reinforced concrete structures means that it is based on durability indicators like materials parameters, measured on laboratory and on site specimens, and geometrical characteristic of cross section of reinforced concrete element, as shown in Figure 3. These durability parameters (e.g. chloride diffusion coeffi cient, gas permeability, water permeability, capillary absorption coeffi cient, porosity) and geometrical characteristic of cross section (e.g. concrete cover depth) are some of the input parameters for service life design models and for quality assurance model.

Durability indicators of concrete are fundamental in evaluating and predicting the durability of the material and service life of structure. They are prescribed during design of concrete structures, tested during prequalifi cation testing, used in service life models and tested during construction as a part of quality control on site. They must be quantifi able by laboratory tests in a reproducible manner and with clearly defi ned test procedures. Nowadays, many testing procedures for perform-ing permeability properties tests on concrete are standardized or being already used for longer period, and proved to have satisfactory precision. [7-12]

Figure 3: Main concept of performance based quality control in a real structure [8]

During performance based design procedure the designer has to specify required values of durabil-ity indicators for the achievement of design service life of the structure, that the contractor needs to satisfy before the concrete is accepted for the application in the structure. In the specifi cation of requested material the tests procedure should be adopted together with the acceptance criteria and the level of testing expected for the project. This information are recorder and followed during compliance testing. In quality assurance procedure, performance-based quality control provide the means to evaluate the quality of the as-built structure and that the design specifi cations have been met. In order to compare the in-situ concrete quality to the design specifi cation, the same test methods must be used for design and quality control, Figure 4 [10, 13]. The as-built condition of the structure is recorded in the ‘birth certifi cate’ of the structure, as an integral part of testing on the occasion of acceptance of structure.

Performance-based design approaches have the advantage that the infl uence of various mix con-stituents can directly be assessed. Testing the specifi c concrete in the design stage therefore al-lows an optimization between mix design properties and cover depth specifi cations [10]. Quality control during construction is performed on laboratory specimens, but also on site, with the same methods prescribed in the design phase and used during prequalifi cation phase, as shown in Fig-ure 4. This way real in-situ performance of concrete is assessed and aging factor for concrete properties may be determined.

55

The key for successful implementation of performance-based design, are well established and standardized limiting values and methods of testing specifi c concrete properties. It is also neces-sary to establish link between properties required specifi c environmental class and required service life of a structure. Most of the efforts from researchers and designers are nowadays targeted at this issue [8 - 17].

Figure 4: Procedure during performance based design

In 2007 French Association of Civil Engineering (Association Francaise de Genie Civil, AFGC) has published a “Guide for the implementation of a predictive performance approach based upon du-rability indicators”. In Table 2 are presented recommended limiting values for concrete durability indicators (concrete porosity Pwater, water permeability Kliq, gas permeability Kgas and non-steady state chloride migration coeffi cient Dapp(mig)) for different exposure classes [14]. The values indi-cated in the Table 2 correspond to measurements performed in accordance with the standardized testing methods on test specimens water-cured for three months after casting and as mean values of at least 3 test specimens [14, 15].

Similar efforts have resulted in the development of The South African Durability Index (DI) method, which also uses durability indicators prescribed for a given environmental classes [10, 16]. Promo-tion of Durability Index method through researchers and designers, as well as governmental agen-cies, resulted in inclusion of this method in most national infrastructural construction projects. Du-rability indexing is based on the oxygen permeability index (OPI), chloride conductivity test and sorptivity of concrete, with concrete classifi cation presented in Table 3.


56


Table 2. Durability parameters and limit values for specifi c exposure class [14]

Exposure classes

Required service life / Structure category

From 50 to 100 years /Building & civil engineering structures

From 100 to 120 years /Large structures

X0 and XC1 Pwater< 14 % Pwater< 12 %Kgas< 100 × 10-18 m2

XC2 Pwater< 14 % Pwater< 12 %Kgas< 100 × 10-18 m2

XC3 Pwater< 12 %Kgas< 100 × 10-18 m2

Pwater< 9 %Kgas< 10 × 10-18 m2

XC4 Pwater< 12 %Kliq< 0,1 × 10-18 m2

Pwater< 9 %Kgas< 10 × 10-18 m2

Kliq< 0,01 × 10-18 m2

XS1Pwater< 11 %

Dapp(mig)< 2 × 10-12 m2/sKliq< 0,1 × 10-18 m2

Pwater< 9 %Dapp(mig)< 1 × 10-12 m2/s

Kgas< 10 × 10-18 m2

Kliq< 0,01 × 10-18 m2

XS2 Pwater< 13 %Dapp(mig)< 7 × 10-12 m2/s

Pwater< 12 %Dapp(mig)< 5 × 10-12 m2/s

XS3Pwater< 11 %

Dapp(mig)< 3 × 10-12 m2/sKliq< 0,1 × 10-18 m2

Pwater< 10 %Dapp(mig)< 2 × 10-12 m2/sKgas< 100 × 10-18 m2

Kliq< 0,05 × 10-18 m2

Where:Pwater is concrete porosity, acording to EN 12390-7:2001. Kliq is water permeability in laboratory, according to EN 12390-8:2001,Kgas is gas permeability in laboratory, according to EN 993-4:1995, Dapp(mig) is chloride migration test, according to NT BUILD 492:1999

Table 3. Durability classes and limit values of Durability Indexes [12]

Durability class OPI(log scale)

Sorptivity(mm/h 0,5)

Conductivity(mS/cm)

Excellent >10,0 < 6 < 0,75Good 9,5 – 10,0 6 – 10 0,75 – 1,50Poor 9,0 – 9,5 10 – 15 1,50 – 2,50

Very poor < 9,0 >15 >2,50

Most of these limiting values are still given as deterministic values. As concrete is an inherently variable product, it is very important that the criteria nominated are set and assessed on a statisti-cal basis that balances the clients risk of accepting defective concrete, against the suppliers risk of having compliant concrete rejected. Limiting values for durability indicators should ideally be based on a probabilistic approach. The environmental load is also a non-deterministic value, since the values may increase in time (e.g. due to chloride accumulation). The time dependency of the resist-ance is an expression of the degradation of the materials properties and aging factor of concrete. With probabilistic approach service-life design would be based on time-dependent formulation of resistance variables, variables describing the environment and variables describing limiting values [18]. Example of this effort can be found in The Netherlands, where in 2003 was developed the per-formance and probability based guideline for the designing durable civil engineering structures with the service lives up to 200 years [19].

57

Conclusion4.

The concept of reinforced concrete durability started to develop only twenty-fi ve years ago, when it became obvious that reinforced concrete could have serious durability problems, especially when exposed to the actions of aggressive environment. The approach to the design of reinforced con-crete structures that takes into consideration durability issues is still to most extend empirical. In this paper two approaches to durability design are presented: prescriptive and performance-based design. Prescriptive approach is based on the set of rules, and by following these rules one ensures that the structure will achieve a service life of 50 years. Performance-based approach is based on durability indicators like materials properties measured on laboratory specimens and on site and geometrical characteristic of cross section of reinforced concrete element. The key for successful implementation of performance-based design are well established and standardized limiting values and methods of testing specifi c concrete properties. On the basis of the values measured on the ‘real’ structure more accurate calculations can be made of the expected service life of the structure. In addition, protective measures could be also timely prescribed if the measured values were below those designed.

References5.

[1] EN 206-1: 2000: Concrete - Part 1: Specifi cation, performance, production and conformity.

[2] fi b Bulletin 55: Model Code 2010 - First complete draft, Volume 1, 2010.

[3] fi b Bulletin 34: Model Code for Service Life Design, 2006.

[4] EN 1990:2002 Eurocode - Basis of structural design.

[5] CEB-FIB Model Code 1990 (1993): Bulletin d´information 213/214. Lausanne, Switzerland, May 1993.

[6] Siemes, T.: History of Service Life Design of Concrete Structures, Workshop “Design of Durability of Concrete”, Duranet, Berlin, 1999, 19-27.

[7] Romer, M. Recommendation of RILEM TC 189-NEC “Non-destructive evaluation of the concrete cover”: Comparative test - Part I: Comparative test of ‘penetrability’ methods, Materials and Structures Volume 38, Issue 284, 2005, 895 – 906.

[8] RILEM TC 178-NEC, Non-destructive evaluation of the penetrability and thickness of the concrete cover, State-of-the-Art Report, May 2007.

[9] RILEM TC 230-PSC: Performance-based specifi cations and control of concrete durability http://www.rilem.net/gene/main.php?base=8750&gp_id=244.

[10] Beushausen, H, Alexander, M.: The South African performance-based approach for specifi -cation and control of concrete durability, Proceedings of Performance based Specifi cations for Concrete, Editors: Frank Dehn, Hans Beushausen, Leipzig, June 2011, 301 – 311.

[11] Andrade, C.: Types of Models Service Life of Reinforcement: The Case of the Resistivity, Concrete Research letters, Vol. 1[2] – June 2010, 73 – 80.

[12] Polder, R., Andrade, C., Elsener, B., Vennesland, Ø., Gulikers, J., Weidert, R., Raupach, M.: Test methods for on site measurement of resistivity of concrete, Materials and Structures, 2000, Volume 33, Number 10, 603-611.

[13] Mayer, T.F., Schiessl, P. Life cycle management of concrete structures Pat I: Birth certifi cate, Concrete Repair, Rehabilitation and Retrofi tting II / Alexander, M.G. Beushausen, H.D. ; Dehn, F. ; Moyo, P. London, UK : Taylor & Francis Group, 2008.


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[14] Baroghel-Bouny, V. et al. Concrete design for a given structure service life, Scientifi c and technical document, Guide for the implementation of a predictive performance approach based upon durability indicators, AFGC, 2007.

[15] Baroghel-Bouny, V., Wang, X., Thiéry, M.: Performance-based Assessment of Durability and Prediction of RC Structure Service Life by Means of Durability Indicators – The Case Of Chloride Ingress, Proceedings of Performance based Specifi cations for Concrete, Editors: Frank Dehn, Hans Beushausen, Leipzig, June 2011, 330 – 340.

[16] Beushausen, H., Alexander, M.: Performance – Based Service Life Design of Reinforced Concrete Structures Using Durability Indicators, Manuscript number 104, pp 8.

[17] Monteiro, A. V., Gonçalves, A. F. Assessment of Concrete Cover in Structures, Part 1 – Sta-tistical Tolerance Analysis Approach, Proceedings of Performance based Specifi cations for Concrete, Editors: Frank Dehn, Hans Beushausen, Leipzig, June 2011, 220 – 229.

[18] Bjegović, D.; Mikulić, D.; Stipanović Oslaković, I.; Serdar, M. Performance based durability design of coastal reinforced concrete structures // MWWD & IEMES 2008 Proceedings, 2008. 68-69.

[19] Polder, R. B, van der Wegen, G., van Breugel, K.: Guideline for Service Life Design of Struc-tural Concrete – A Performance – based Approach with Regard to Chloride Induced Corro-sion, Proceedings of Performance based Specifi cations for Concrete, Editors: Frank Dehn, Hans Beushausen, Leipzig, June 2011, 25 – 34.

59

Environmental Friendly Low Cost Housing Technology


John K. Makunza University of Dar es Salaam, Department of Structural and Construction Engineering, Tanzania

Abstract

There are different types of shelters which include multi-stories buildings, bungalows and others. An adequate shelter is a basic human need, yet about 80 % of the rural population in developing countries still live in spontaneous low quality settlements, as they cannot afford the high cost of building materials which could produce better shelters. One alternative for the expensive materials is to use low cost housing technology. The technology uses the available soil on site, which is sta-bilized with a small amount of cement with or without lime depending on the characteristics of the soil so as to improve the engineering properties of the produced bricks.

This paper discusses on a study done on low cost housing technology that is environment friendly. The study was done by assessing the engineering properties of soil, as well as the stabilized soil bricks. The bricks were of two categories, namely solid bricks and voided bricks where by the in-ternal voids were made by inclusion of plastic bottles. The study was concentrated on cheap lo-cally available materials which can result into, adequate compressive strength of both the bricks and walls and have low water absorption. The study was done through site visits, sampling of soils, performing various fi eld and laboratory tests on the soil as well as on the bricks.

The test results obtained from both fi eld and laboratory tests have revealed that natural soils with 40 to 55 % silt plus clay content are suitable materials for producing good quality stabilized soil bricks which meet the minimum requirements of BS 5628 Part 1 of 2.8 N/mm2 and are 50 % cheap-er when compared to the cement-sand blocks. Therefore these stabilized soil bricks can be used for the construction of strong low cost houses.

Key words: Less cement, No felling of trees, Disposal of plastic bottles, increases economy and heat insulation.

Introduction1.

From the human rights point of view, every human being has the right to live in a good quality house or shelter. The houses are supposed to meet at least the minimum acceptable quality standards, such as suffi cient air circulation, durability and strength. Houses can be built from varieties of ma-terials like timber, masonry; stones, burnt bricks, cement-sand blocks, concrete, etc. In developing countries, where most people have low income, the common materials such as timber, cement-sand blocks and concrete are not affordable due to the high costs involved with. However, there are few people who can afford to have burnt bricks houses, and very few can afford cement-sand blocks and concrete constructions. Some people if properly guided are able to build better houses by improving mud brick houses such as the one shown in Figure 1.

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Figure 1: House constructed of mud-walls and roofed with iron sheets

But most people in developing countries are still using mud and interwoven timber and bamboo daubed with mud to build their houses as indicated in Figure 2. In some cases mud bricks are used to build the walls which sometimes may be plastered with cement-sand plaster. Such houses are sometimes vulnerable to weather as during rain season, the rainwater wets the mud walls, a condi-tion which is not healthy for the occupants. On the other hand, the walling soil material can expand and loose cohesiveness, especially with wooden poles or cement plaster, and crack and the ce-ment plaster can delaminate and fall-down as shown in Figure 3. From observation and interviews, it has been learnt that thieves can easily chop-out some bricks or part of the mud wall and break-in and steal. Therefore the houses are not safe and are less reliable. Some of the said houses are shown in Figures 1, 2, 3 and 4.

This study has been carried out with the objective of assessing the strength prop-erties of stabilized soil bricks. The bricks were of two groups, where by the fi rst one was composed of solid bricks, and the second group had bricks in which each one contained an internal void made by inclusion of a plastic bottle of 0.5 litre. The included bottle was originally used to keep drinking water, and was then supposed to be disposed away. The inclusion of plastic bottles in the bricks accounted for reduc-tion of material to save cost, to enhance thermal insulation and fi nally as one way of disposing the bottles since they are just thrown away without any proper method

of disposing. The study was mainly concerned with testing and evaluating the characteristics of soils, stabilized soil bricks and wall specimens.

Figure 2: Wooden poles and mud walled house

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Figure 3: House built of non-stabilized soil bricks

The methodology adopted in carrying out the study included the following:

Site visits and sampling of soil material, –Laboratory tests on the soil, and results analysis for determining the characteristics and adequa- –cy of the soil. Collecting the plastic bottles –Mixing the materials; soil plus cement plus a reasonable amount of water –Production of the bricks both solids and those which included embedded bottles –

Figure 4: Pure soil brick-wall in which part of the cement plaster has fallen away

Reviews on Soils2.

Soil is a natural product of weathering and mechanical disintegration of rocks that forms the crust of the earth. Different types of soil include;

Residual soil –Sedimentary soil –Volcanic soil –Organic soil –

Soil is made up of varying proportions of four types of materials depending on the grainsize name-ly: gravel, sand, silt and clay. Each of these soils has a different characteristic way. Soil particle

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sizes can approximately be divided into four groups according to the diameter of the grains, thus:

Gravel - these are soil particles bigger than 2mm. –Sand - these are soil particles smaller than 2mm but bigger than 0.06mm. –Silt - these are soil particles smaller than 0.06mm but bigger than 0.002mm. –Clay - these are soil particles smaller than 0.002mm. –

The proportion in which each type of material is present, will determine the behavior and properties of the resulting soil.

2.1 Properties of Soil

Soil properties change from one soil to another depending on the nature of the particles fraction making them up and the complex way in which they mix together. One can distinguish between chemical properties, which are linked to the presence of salts, oxides and sulphates, and physical properties, which are numerous including colour, structural stability, adhesion, apparent dry den-sity, moisture content, absorption capacity, capillary potential and range, permeability, linear shrink-age, dry strength, porosity and many more. Understanding the chemical and physical properties of the soil enables one to defi ne the quality and performance of soil for building purposes. It is impor-tant to have a thorough grasp of at least three fundamental properties, which are the texture or particle size distribution, plasticity, and compressibility of the soil.

2.2 Texture or Particle Size Distribution

This is measured by particle size analysis for the coarse fraction (gravels, sand and silts) and by sedimentation analysis for the fi ne fraction. Gravels and sands give the material strength while clays bind it together and silt fulfi lls a less clear intermediate function.

2.3 Plasticity

Plasticity defi nes the extent to which a soil can be distorted without any signifi cant elastic reaction. The plasticity of soil as well as the limits between different states of consistency is defi ned by meas-uring the “Atterberg limits”. The point at which the material passes from plastic to liquid state is known as liquid limit (LL). The point at which it passes from plastic to solid state is known as plastic limit (PL). At LL, the soil begins to display some resistance to shearing. At PL the soil ceases to be plastic and becomes crumbly. The plasticity index (PI), which is equal to LL- PL, determines the extent of the plastic behavior of the soil. A combination of LL and PL defi nes the sensitivity of the soil to variations in moisture content.

2.4 Compressibility

The compressibility of a soil defi nes its maximum capacity to be compressed for a given amount of compaction energy at given optimum moisture content. When a force is applied to the soil, the soil material is compressed in which the proportion of the voids decreases. The more the density of the soil can be increased the lower its porosity will be. Moisture content must not be too high. “The Proctor test” shows the relationship between optimum moisture content and optimum dry density for a given amount of compaction energy.

2.5 Soil Stabilization

In many developing countries especially in rural areas, soil is used for the construction of houses. Natural compacted soil has good insulation and fi re resistant properties [1]. It is, however, vulnera-ble to moisture and the erosive effects of weather. Walls constructed out of soil, if well compacted

63


have adequate compressive strength under dry conditions; however they will loose their strength under adverse moisture movement. Alternating wetting and drying will erode and deteriorate the walls. Soil durability and strength can also be improved by:

(i) Improving the distribution of grain sizes or grading(ii) Compacting the soil; (iii) Adding stabilizers or chemicals;

Any soil can be improved and used as a building material for various types of structures by adding stabilizing materials, and the product is called stabilized soil. A properly stabilized, consolidated, well-graded soil that is adequately moisturized, mixed, and cured will provide a strong, stable, wa-terproof and long-lasting building bricks. Stabilizer material in the soil will perform the following;

(i) bind the soil particles together making the product stronger [2](ii) water proofi ng – reducing the amount of voids and water which can be absorbed by the soil(iii) reducing the shrinkage and swelling properties of soil(iv) increase the tensile strength of soil.

When a stabilizing material is added to the soil, it increases the engineering properties of the soil and so the bricks and life span of the resulting structure considerably. Different types of soil may require different types of stabilizers. The common stabilizer materials include cement, lime, a com-bination of lime and cement, and a combination of lime and pozzolanas. Sometimes, burning clay bricks is considered as stabilizing the bricks. Following are the most common stabilizing materi-als;

2.6 Cement

Soil stabilization is in most cases achieved through the use of cement as the stabilizing agent. Or-dinary Portland cement (OPC) is mostly used for stabilization purposes and it works best with sandy soils. Stabilization may be diffi cult if the clay content is too high. Generally the combined percentage of silt and clay should not be less than 10 percent and more than 40 percent. The ce-ment content varies depending on the desired strength of bricks and type of soil, although 5 %-10 % by weight of dry soil is often used.

2.7 Lime

Lime makes a good stabilizer for soil with high content of clay (i.e. >40 %). Lime reacts with the clay to form strong bonds between the soil particles. The amount of lime for stabilization purposes is taken to range from 4 % to 8 % of the dry weight of soil.

2.8 Combination of Lime and Cement

When a soil has too much clay (>40 %) it is important to use the combination of lime and cement stabilization because lime will make the soil easier to work with, and cement will help to gain the strength and waterproof the product.

2.9 Combination of Lime and Pozzolana

Pozzolana is a material which contains much silica. Volcanic ash, pulverized blast furnace slag, pumice and silica fl our are examples of pozzolanas. Lime and pozzolana will react and make ce-ment which may be almost as good as Portland cement. This can be used for both clayey (>40 %) and sandy (<10 %) soils. The problem with this combination is that the reaction for strength devel-opment is very slow.

64


2.10 Stabilization of Bricks by Firing or Burning

This is commonly used in rural areas where no any other material is required apart from the soil and fi rewood. The soil is mixed with water and is made in form of bricks, which upon being dry are staked in kilns, then are burnt at high temperatures [5]. This is only possible when the soil is too clayey (>40 %). The bricks produced may have higher strength compared to other types of bricks but it has the disadvantage that it requires a lot of fi rewood for burning which may cause defor-estation also produces a large quantity of smoke which causes air pollution.

Investigation on the Stabilized Soil Bricks3.

Stabilized soil bricks are made from ordinary soil mixed with a specifi ed amount of stabilizer, e.g. cement, lime and water in an appropriate ratio, then highly compacted in a brick press or mould resulting in a solid and dense brick. Researches and experiences have shown that stabilized soil bricks if well and appropriately used, they are up to 50 % cheaper than other conventional walling materials such as concrete blocks while their structural performance is nearly the same [3].

The right type of soil for stabilized soil bricks should contain approximately 30 %-40 % clay+silt and 60 %-70 % sandy soil. The soil used should also be sieved through a 4.76 mm sieve. Also the soil should be friable upon drying, easily compacted and should be able to dry without harmful shrink-age or cracking.

3.1 Sampling of Soils

Soil samples were taken from two different sites in Iringa region and were identifi ed as sample S1 and S2. The soil samples were then taken to the Building Materials Laboratory of the University of Dar es Salaam for testing in order to check their suitability for brick making purposes. The soil was tested in order to determine its suitability and the amount of the stabilizer required. The tests which were performed on the soils are herein detailed, thus:

(a) Bottle TestThis test was performed in order to determine the approximate amount of clay, silt, sand and grav-el present in the soil. The test revealed that the percentage of clay plus silt contents were 52 % for soil sample S1 and 48.5 % for sample S2.

(b) Box (Linear Shrinkage) TestThis test is used to determine the amount of cement or lime to be used for stabilizing a particular soil. The test results for the soil sample S1 was 12 mm while for sample S2 it was 18 mm.

(c) Grain Size DistributionThe grain size distribution expresses the size of the particles in terms of percentage of mass of in-dividual sizes. Determination of particle size distribution is achieved using sieve analysis. Wet and dry sieving for grains with diameter greater than 0.063 mm and sedimentation analysis for grains with diameter less than 0.063 mm.

Prior to sieving, the samples were oven dried at 105°C for 24 hours. The samples remained dry dur-ing the whole process of sieving. The advantage of this method is that the materials retained on each sieve were easily weighed. For particles between 0.25 mm and 0.063 mm wet sieving was necessary unless the whole sample had been washed to remove all particles less than 0.063mm in size before drying. The obtained results are given in Table 1.

65


Table 1: Summary of particle size distribution

Soil description S1 S2Clay (%) 34 25Silt (%) 23 23Sand (%) 40 51.3 Gravel (%) 3 0.7Solid density (?S) 2.65 2.65Soil classifi cation Gravelly silty clayey SAND Gravelly silty clayey Sand

(d) Optimum Moisture Content and Maximum Dry Density The Optimum Moisture Content (OMC) at which the maximum dry density is obtained was deter-mined by the Proctor test. The test results were recorded on a graph showing the dry density, ex-pressed in kg/dm³, on the ordinate axis, and the moisture content (MC), expressed in percentage by weight, on the abscissa. The samples S1 and S2 were taken and tested and then graphs were plotted to obtain the maximum dry density and optimum moisture content. The obtained test re-sults are tabulated in Table 2.

Table 2: Optimum moisture content and maximum dry density

Soil description S1 S2Optimum moisture content (%) 29 22Maximum dry density (kg/dm3 ) 1.55 1.72

(e) Atterberg LimitsSoils can have various states of consistency, liquid, plastic or solid. Atterberg defi ned these various hydrous states of soil and the boundaries separating them as limits and indices, expressed as per-centages by weight of the moisture content. In the same manner, tests for sample soils S1 and S2 were performed, and their respective results are shown in Table 3

Table 3: Summary of Atterberg limit test

Soil sample S1 S2Liquid limit (LL) % 43 37.6Plastic limit (PL) % 21 20Plasticity index (PI) % 22 17.6

66


Production of Bricks4.

The collected soil samples were sieved through a 4.6mm sieve so as to remove bigger particles. Bigger lumps were fi rst crushed before sieving, and particles which were retained on the sieve, were further crushed and sieved again. The main mixing design was cement: lime: soil in an ap-propriate proportion based on the results obtained from linear shrinkage test. Also lime was intro-duced in the mix because the soils had clay+ silt content greater than 40 %. The other mix design was cement: soil only.

The materials were manually mixed using shovels until they acquired a uniform color. Water was easily added by using sprinkler. The mix ratios used for the whole process of pro-ducing the bricks for sample S1 were as follows;

Cement: lime: soil 1:1:15 batch by volume, and water- cement ratio was 1:4 for sample S1 and 1:5 for sample S2

4.1 Machinery and Equipment

There is a wide choice of machinery and equipment available. The quality of the equipment used is im-portant, but the quality of the soil remains of para-mount importance. To make bricks one should have the following tools; buckets, oil can, watering can, 4.6mm sieve, shovels, trowels, hoes, and mortar pans, rammers and brooms. Bricks can be molded by using block press (Cinva ram) or locally made hand mould from timber or steel depending on the availability of materials and fi nancial position of the people concerned. There are also other sophisti-cated electric mould machines, which are used for mass production of stabilized soil bricks.

Production of Voided Bricks5.

Internal voids in a bricks are usually provided for the purpose of enhancing economy for the materi-als, and for heat and sound insulation. For these reasons, a total of 34 bricks were produced, each having a void of around 500 cm3 (500 ml), which was formed by the inclusion of a plastic bottle that was originally used to keep drinking water with a brand known as Kilimanjaro (ref. Fig. 7) or Uhai. The inclusion of a bottle in a brick is a means also of disposing plastics to keep the environment clean.

For the voided bricks, the mix ratios were 1:10, 1:12.5, 1:16 and 1:18 (cement: soil), there was no lime addition. After mixing cement and soil, water was added, and the materials were again mixed till a uniform color was attained. The mixture was then loaded in a steel mould (Fig. 6). A timber element already turned into a bottle shape was set at half depth the mould at centre for the purpose of form-ing a void that would house the plastic bottle (Fig. 7). Then the materials were compressed till the

Figure 5: Stabilized soil bricks

Figure 6: Steel mould for bricks production

67


maximum compaction point was reached, then the top plate of the mould was released to allow for the removal of the bottle-shaped timber element. To the formed recess, a tightly closed empty bottle or full of water was placed in. Then some mixed soil material was added and the top steel plate tightened. Again the soil material with a bottle included was pressed to the compaction effort required, then the top plate was taken away and the brick pushed up using a lever system of the mould. The brick was gently lifted and placed on a fl at surface. The procedure was repeated for all bricks production. Some of the bricks are depicted in Figure 5.

In comparison to the traditional cement-sand blocks which have ra-tios ranging from 1:6 to 1:8, the ratio of 1:1:15 or 1:15 for stabilized soil bricks consumes less cement than traditional approach. The saving in cement is almost 50 %.

5.1 Curing

Curing is the process of watering the cement-stabilized brick so as to achieve the required strength properties. Cement needs water to gain its full strength, therefore, curing was done twice a day and this process continued for at least two weeks. Curing of stabilized soil bricks is very important in order to get good quality soil bricks.

5.2 Testing of Bricks and Wall Specimens

Before using the bricks for any construction work, it was important to test a sample of few bricks to check if they have achieved a minimum strength requirement of BS 5628 Part 1 of 2.80 N/mm² [4]. The tests were carried out after the bricks reached a minimum age of 28 days. Bricks produced from the two samples S1 and S2 were randomly selected and labeled for testing. All bricks were of the same size i.e. 30 cm x 14 cm x 10 cm which implies that the compressive area of each brick was 4200 cm2.

5.3 Compressive Strength

The compressive strength test was done in order to de-termine the compressive strength of the bricks against vertical loading. The procedure for testing each brick was done as follows:

External dimensions of each sampled brick were tak- –en and the brick was marked for identi fi cationThe brick was weighed and recorded in grams –The brick was placed into the machine and all set-up –procedures were properly doneThe brick was gradually loaded at a rate of 1kN/sec –until it failedThe ultimate load was recorded. –

After fi nishing the testing and recording for all the bricks, the obtained results were analyzed, the results of which are summarized in Figures 9 to 12. In Figures 9 and 11, shown are the compressive strengths of each individual brick for soil samples S1 and S2. Also the control minimum compressive strength value obtained from BS 5628 Part 1 of 2.80 N/mm2 is indicated. For soil S1 the strengths are almost above the control value, while for soil S2, three strength values are below the control value.

Figure 7: A plastic bottle for inclusion in a brick

Figure 8: Testing of brick specimens

68


Figure 10 shows the densities of each single stabilized soil brick from the main mix ratio. The densi-ties are slightly lower than the densities for cement-sand blocks of around 2.0 kg/dm3 because the bricks are produced from natural soil material which is stabilized with a very little amount of cement and lime. On the other hand soil bricks offer higher heat or fi re resistance due to the nature of the constituent materials.

Figure 9: Compressive strength results of bricks (Samples S1 and S2)

Figure 10: Densities of soil bricks from soil samples S1 and S2

69


5.4 Strength of Voided Bricks

The compressive strength of voided bricks ranged from approximately 1.0 N/mm2 to 1.5 N/mm2. This strength range is on the lower side when compared with the minimum BS 5628 values. How-ever, for non load bearing walls the strength is suffi cient.

5.5 Water Absorption Test

The test was carried out in order to assess the water absorption of the bricks. The test was done by fi rst marking and measuring the weight of each of the eight bricks. The dry weight of S1 bricks ranged from 6.44 to 6.71kg. After soaking in the water for 24 hours, the bricks were taken out of the water and on attaining surface dry; they were fi nally weighed, where by the weight ranged from 7.35 to 7.65 kg. In the same manner, bricks made from soil S2 weighed from 6.60 to 7.4 kg when dry and 7.4 to 8.16 kg after soaking for 24 hours. The water absorption was obtained by assessing the weight difference. The results are summarized in Figure 12, thus;

Figure 11: Compressive strengths of voided bricks

70


Vertical Load Carrying Capacity of Sample Walls6.

6.1 Building the Wall Specimens

Building the walls is the most challenging part of construction, and therefore it should be done care-fully in order to achieve a stable and plumb wall. A wall should be designed and built to meet various requirements such as stability, accommodation of movements, resistance to rain penetration, du-rability, fi re resistance, thermal properties and other construction details. Some of the stabilized bricks were used to build sample walls which were tested for their vertical load carrying capacity.

6.2 Compressive Strength of Sample Walls

Load bearing walls may be designed to carry in plane horizontal loads induced by wind, bracing effects or earthquake; the loads are transferred to the walls primarily via diaphragms such as fl oors or roofs. The load bearing capacity of the brick work depends upon the thickness and number of joints, type of bond and the brick ex-actness.

The compressive strength test was done (ref. Figure 13) to determine the vertical load carrying capacity of the walls built from the stabilized soil bricks made from the two samples.

The size of the wall in plan was 2½ times the brick length, the thickness was 140 mm and the height was 5 courses as recommended by RILEM. The obtained test results showed that the com-pressive strength for walls constructed from soil sample S1 were 1.23 N/mm2 and 1.32 N/mm2, while from soil sample S2 it was 1.12 N/mm2. The average compressive strength was found to be 1.22 N/mm2.

Figure 13: Wall specimen under test

Figure 12: Water absorption for stabilized soil bricks

71


Discussion on the Results7.

From the bottle test results it was found that the percentage of clay plus silt content in the samples was greater than 40 %. Therefore stabilization with cement and lime had to be done for the main mix, while the other mix only cement was used. The gradation test showed that the amount of clay and silt in both samples was more than 40 % (S1= 57 % and S2= 48 %). The Atterberg limits test revealed that the two samples were of clays of low plasticity and therefore they were good for sta-bilization.

From the results obtained in Linear shrinkage test, both samples were to be mixed in the ratio of 1:15, but since lime had to be applied too, then the batching mix by volume of cement: lime: soil became 1:1:15.

From the Compressive strength test results, some of the compressive strength values met the minimum requirements of 2.8N/mm2 according to BS 5628 Part 1 for bricks [4]. It seems that for the solid bricks which didn’t meet the requirements, the reason behind was that they didn’t get proper compaction or the volume of the mix (soil, cement and lime) placed in the mould was not enough for proper compaction.

7.1 Voided Bricks

The average compressive strength of voided bricks was 1.2 N/mm2, being lower than the minimum require-ment of BS 5628 Part 1. For a framed structure, these bricks can be used for external walls too. The use of these bricks will help to reduce heat and sound insulation. On the other hand the bricks lead to reduced cost because there is saving of materials, especially cement and soil. The use of stabilized soil bricks is environ-mentally friendly as there is no need of deforestation to get wood for fi ring the bricks.

From the results of Water absorption test, it was found that the water absorption of the bricks made from soil sample S1 was 12.3 % while that of bricks from soil sample S2 was 9.86 %. This means that bricks from all samples were within the required range as the maximum water absorp-tion allowed is 20 %, therefore they are durable.

From the test results for Vertical load carrying capacity of the wall specimens, the strength values for the walls constructed with bricks made from samples S1 were 1.23N/mm2 and 1.32N/mm2, while for walls from sample S2 it was 1.12N/mm2. The results agree with the fact that the strength of the brick work in compression is much smaller than the compressive strength of the bricks from which it is built.

Conclusions and Recommendations8.

8.1 Conclusions

In this study, it has been revealed that bricks made from stabilized soil with a mix ratio of 1:1:15 for cement: lime: soil has compressive strengths greater than the minimum value given in the British Standard (BS 5628 Part 1) of 2.80 N/mm2 [4]. The average compressive strength of the bricks made from soil sample S1 is 3.53 N/mm2 while from sample S2 it was 2.28 N/mm2. The utilized cement is almost 50 % less when compared with cement consumption of traditional sand blocks.

The technology is more environmental friendly because it uses less cement; it further reduces the construction costs when internal voids are introduced inside the brick body. An added advantage is that the inclusion of a 500 ml plastic bottle in a brick keeps the environment clean as it is a way of disposing them bearing in mind that there is no any proper policy in Tanzania of disposing plastic bottles. The other advantage is that it is environmentally friendly for there is no need of felling trees for fi ring bricks. Building with stabilized soil bricks is a technology which offers a good possibility for enabling low income groups to build their own houses at low cost. The technology stands to

72


contribute to the national goal of providing housing for the majority of Tanzanians. Buildings con-structed by stabilized soil bricks will have long life if all production and construction procedures are followed, hence minimum maintenance will be required.

8.2 Recommendations

For effective use of stabilized soil technology, the management of the soil resources must be stud-ied before digging to keep the balance between the soil requirement for the building and environ-mental use (ref. Fig. A1). To achieve a successful dissemination of this technology, the following recommendations should be implemented.

Promoting stabilized soil bricks through advertising and pilot housing, so that many people will –get appropriate knowledge about this technology.There should be prepared operation manual for the soil preparation, use of the brick press mould –and the building process. A study on the effects of dynamic loading to the sample walls is necessary so as to be able to –assess how the walls may behave.Another study on heat absorption and transfer through the specimen walls or model building is –neededAPPENDIX

Appendix9.

(a) Clay soil preparation for producing bricks

(b) Laying of clay bricks on “rough surface”

(c) Burnt clay bricks

Figure A1: Burnt bricks production

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References10.

[1] Chavez, J.R.G., “Innovative Building Materials For Sustainable Applications in Low Cost Housing”, Universidad Autonoma Metropolitana – Azcapottzalco, Departmento de Medio Ambiente, Laboratorio de Investigaciones en Arqitectura Bioclimatica, San Pablo 180, Colo-nia Reynosa Tamaulipas, C.P. 02200, Mexico D. F., 2003.

[2] Humberto, C. et al, “Structural Behaviour of load bearing brick walls of soil-cement with the additional of ground ceramic waste”, Revista Brasileira de Enginharia e Ambinental, Vol 7, No.3, pp 552-558, October 2003.

[3] Makunza, J.K., “Suitability of Crusher Dust as a Masonry Construction Material”, Tanzania Journal of Engineering and Technology (Accepted for publication), Vol 1, No.1, College of Engineering and technology, University of Dar es Salaam, 2006.

[4] BS 5628 Part 1, 1985, “Structural use of plain masonry”, BSI, London.

[5] Kaywanga A., “Investigation of Structural Qualities of Locally Produced Burnt Clay Bricks in Iringa-Region at Ilula and Ilole Villages”, Final Year Project, Department of Structural and Construction Engineering, University of Dar es Salaam, July 2011.

Detail drawing for the steel mould for stabilized soil bricks production. The mould exerts high com-pressive pressure to the stabilized soil mixture and result in high strength bricks.

Figure A2: Details of Steel mould for bricks production

74


Low-Cost Shell Structures: Thermal Loading

M. GohnertSchool of Civil and Environmental Engineering, University of the Witwatersrand, South Africa

Abstract

Thin shell masonry structures are ideal for low-cost housing. Curved structures are ideally suited to resist external forces and are the most effi cient structures. In shell structures, the forces are prima-rily in-plane, referred to as membrane forces. Shells, dominated by compressive stresses, are an absolute requirement in masonry structures, to minimize cracking. However, shell structures which are designed for pure compression still exhibit cracking on the external surface. Cracked shells are unsightly and a precursor to durability problems. For this reason, several studies have been under-taken to determine the cause of cracking in shells. This study is an investigation into the effects of thermal loading. The research includes the mapping of thermal loads by experimentation, an as-sessment of the effects of thermal loading and possible solutions to prevent cracking in the shell.

Introduction1.

Masonry homes have been constructed for many centuries and are, perhaps, the most common type of home. Shell structures, constructed of masonry, are less common despite the fact that shell structures are the most effi cient structural forms known to man. These types of structures are in-spired by nature, which possess optimal shapes developed through evolution. The effi ciency of shell structures is due to the in-plane distribution of forces, called membrane forces [1]. Membrane forces are either compressive or tensile. However, in masonry, tensile forces should be avoided. Although tensile forces are acceptable in reinforced concrete and steel structures [2],they are not acceptable in masonry due to its low tensile capacity. The shape of the structure is also a signifi cant factor, which infl uences how stresses are distributed.

This paper is focused on thermal loading of shell structures. Thermal loading changes the shape of the structure and causes tensile strains, which may cause cracking. This research therefore consid-ers how thermal load affects shell structures. A masonry shell structure is instrumented for this purpose—to collect thermal strain data and to determine the distribution of strain on the surface of the shell. The results are analyzed to determine the effects and the crack potential.

General Layout of the Masonry Dome2.

A cross-section of the test dome is given in Fig. 1. The lower portion of the dome comprises a dou-ble brick cylindrical shell with a single brick hemisphere roof. The inside diameter is 5310 mm, tota-ling 22.15 m2 of fl oor space. At the apex of the dome, a skylight is installed to admit natural light. At quarter points, a door and three windows were installed.

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Fig. 1: Cross-section of the dome structure

The shape of the dome determines the distribution of stresses (i.e., hoop and meridian). The merid-ian stresses are compressive, but the hoop stresses vary depending on the ratio of the height to diameter of the dome. In hemispherical brick structures, the hoop stresses in the middle third of the dome are typically in tension. As the dome fl attens, the hoop tensions are reduced and eventually eliminated as the shape approximates a centenary form [3]. At a height/diameter ratio of approxi-mately 0.28 or less, the shell is in complete compression [4]. A masonry shell in compression is ideal to minimize cracking, but not practical for several reasons: Flat masonry shells require a ring beam to resist the larger kicking out forces (horizontal radial forces at the base of the shell)—this will affect the cost of the structure. In a hemisphere, the kicking out force is less and a ring beam may not be required. Furthermore, fl at masonry shells are diffi cult to construct. For these reasons, the shell was designed as a hemisphere for constructability and to eliminate the need for a ring beam at eaves height. The disadvantage, however, is that the structure has a propensity to crack due to hoop tensions.

Loads are primarily carried in the plane of the shell, which enables economy of materials. These forces are called membrane forces and occur in the hoop and meridian directions. Bending and shear forces are expensive forces and therefore the desigers of masonry shells attempt to avoid these types of forces. Although shells are primarily membrane structures, some bending and shear forces are likely to occur at the support (refereed to as boundary conditions), which complicates the analysis.

Stress Analysis3.

The shell was analyzed with four types of loads – dead, live, wind and temperature. The shell roof is inaccessible, thus only a nomimal live load (i.e., a maintenance load) is required. The round shape of the shell is useful in reducing the effects of wind. The assumed pressures are distributed accord-ing to a trignomic function [1] given as Eq. 1:

fq sincosPPz = (1)

Where θ is the horizontal angle (zero degrees in the direction of the wind) and φ is the vertical angle (zero degrees at the apex).

The effects of dead, live and wind are distributed in the shell as membrane forces (in-plane com-pressive or tensile forces) in accordance to thin shell theory.

Fig. 2 is an ABAQUS model illustrating the Von Mises stress patterns in the shell for the combina-tions of loadings.

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Fig. 2: Stress distribution in the shell

The analysis illustrates the tension stress concentrations that occur around the window and door openings (expressed as warm colors). These stress concentrations are tensile stresses. A vertical crack is therefore expected at the apex of the arch as well as 45° cracks about halfway down the arch. The analysis is confi rmed by a crack pattern study of existing unreinforced masonry shells. In Fig. 3, the discoloration lines on the surface of the shells are the location of cracks. A comparison of the stress pattern of Fig. 2 and the crack pattern in Fig. 3 demonstrates that the location of the cracks coincide exactly with what was predicted with the fi nite element analysis. These cracks are therefore unrelated to cracks due to thermal loading, but a direct consequence of dead, live and wind loads.

Fig. 3: Crack patterns in an existing unreinforced shell

From the ABAQUS analysis, the distribution of hoop and meridian moments through a vertical slice of the shell is illustrated in Figs. 4 and 5.

Two studies of stress distribution in the shell were performed to determine the infl uence of arches incorporated in the shell around openings (i.e., around doors and windows). The solid lines repre-sent the hoop stresses (Fig. 4) and meridian moments (Fig. 5) through a section of the shell with arches incorporated. The dashed line represents the structure without arches. As illustrated, it was found that by incorporating arches around the door and window openings, the stress is reduced signifi cantly. These stiffened arches have proven useful in reducing the hoop tensions and transi-tional stresses between the hemisphere and cylindrical shells.

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The effects of temperature are often ignored in the analysis and how it infl uences cracking in the shell is an enigma to many researchers and designers. However, temperature loading is a signifi -cant contributor of stress in the shell. A fi nite element ABAQUS model was used to model and predict the effects of temperature loading. A temperature differential of 20 °C was applied to the thickness of the shell 1, applied linearly over the inner and outer surfaces. In addition, to improve accuracy, the temperature load was varied to simulate the effects of the sun (i.e., maximum load where the sun strikes the shell normal to the tangent). The results of the analysis are given in Fig. 6.

Temperture Loading Hoop Forces

0 0.5

1 1.5

2 2.5

3 3.5

4

-4 -2 0 2 4 6 8Hoop forces (kN/m)

Height (m)

Variation of temperature applied to the shell surfaceEven distribution of temperature over the shell surface

Fig. 6: Hoop forces due to temperature loading

The fi nite element analysis indicates that a variation of thermal loading to the surface of the shell (to simulate the sun) is not signifi cantly different to the analysis of a shell subjected to an even distribu-tion of temperature loading. The temperature model, however, was found to be sensitive to the degree of fi xity at the base. The model assumed a pinned support, located at the damp-proof course. In masonry structures, just above ground level, a plastic sheet is placed on a course of bricks, which is referred to as DPC (damp proof course). The DPC prevents the capillary fl ow of moisture through the bricks. Although the DPC prevents the ingress of moisture, the plastic mate-rial does not bind well to the mortar – a small moment or shear may cause the bond to crack and create a pin or sliding support. The effect is often seen as negative, but was found to be benefi cial

Hoop Forces-vertical section

0

1

2

3

4

-10 -5 0 5 10

Hoop forces (kN/m)

Hei

ght (

m)

With arches around opennings Without arches around openings

Meridian Moments

0

0.5

1

1.5

2

2.5

3

3.5

4

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

Moments (kN.m/m)

Hei

ght (

m)

With arches around opennings Without arches around openings

Fig. 4: Hoop forces through a vertical section Fig. 5: Meridian moment through a vertical section


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to relieve stress induced by temperature loading. The structure is therefore able to expand and contract, without the restraining infl uence of a fi xed support.

Stiff arches are placed around the window and door openings. The restraining effect of the arches was found to increase the temperature stress in these regions. Fig. 7 illustrates the bending mo-ments through an arch and through the full height of the shell. As seen in the fi gure, the moments are signifi cantly higher around door and window openings. The DPC reduces the stress at the base, but the arches attract stress.

Temperature Induced Meridian Bending Moments

0

1

2

3

4

-0.5 0 0.5 1 1.5

Moment (kN.m/m)

Heig

ht (m

)

Through vertical section of shell Though vertical section of an arch

Fig. 7: The temperature infl uence of the arches on the bending moments in the shell

The dome structure was instrumented with Demec targets to determine the temperature effects on the dome during day light hours. The location of the targets are shown in Fig. 8. The meridian and hoop strain readings are given in Figs. 9 and 10, respectively.

12345678

N

Fig. 8: Location of the Demec targets for temperature strain readings (viewed from top)

Temperature strains were recorded twice daily. In Figs. 9 and 10, the dashed lines represent the morning readings (07:00) and the solid lines represent the afternoon (15:30) readings. In the south-ern hemisphere, structures have greater exposure to the sun on the Northern face. As illustrated in the graphs, the strains are larger on the side of the shell which is exposed to the sun. The meridian strains are the largest and peaks at the apex of the shell. The hoop strains appear less than the meridian strains.

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The exterior face of the shell is composed of a 10 mm sand/cement plaster. The tensile cracking strain of the plaster is predicted as being less than 0.001 [5]. As seen from the graphs, the meridian strains signifi cantly exceed these strains; therefore, thermal cracking is expected, especially in the upper regions of the shell. Cracking in the upper region of the shell is therefore most likely caused by thermal loading.

Temperature Distribution in Groin Valuts4.

The most current research on temperature loading is studying the distribution of temperature on the surface of groin vaulted structures. The shell in Fig. 11 is a 3 m x 3 m x 1.5 m high groin vault con-structed of thin soil tiles, 20 mm thick – two layers of tiles are placed in the upper regions, which increases to four layers at the supports. The shape of the arches of the shell is catenary, to eliminate tensile forces.

The shell was instrumented with a 16 thermal couples, placed on the exterior surface of the shell and data collected by means of a data-logger. A series of smaller shells were also constructed to study the effects of various surfaces (painted and volcanic rocks applied to the exterior to dispell heat). The results for an exposed soil tile surface is given in Fig. 12.

Meridian Temperature Strains on Surface of Dome

-0.0035-0.003

-0.0025-0.002

-0.0015-0.001

-0.00050

0.00050.001

0.0015

0 2 4 6 8 10

Target Points

Stra

in

Hoop Temperature Strains on Surface of Dome

-0.004-0.003-0.002-0.001

00.0010.0020.003

0 2 4 6 8 10

Targets

Stra

ins

Fig. 9: Meridian strains on the surface of the dome

Fig. 10: Hoop Temperature strains on the surface of the dome


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Fig. 11: Groin vaulted shell

Fig. 12: Temperature distrubution on the surface of the shell

As seen from Fig. 12, the maximum temperature on the surface of the shell tends towards the apex of the shell and the Northern face. The average approximate variation in surface temperature is 18° C (readings were taken over the summer months). Differing from the dome, the catenary groin vaults did not crack when subjected to thermal loading. The groin vault, however, is a pure com-pression structure.

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Resisting Temperature Strains5.

5.1 Counteracting Temperature Stresses by Self Weight

The self weight of the structure will, to a certain degree, counteract the temperature stresses. How-ever, the shell is composed of a single brick and the dead load strains are minor, which range from 0.0000028 at the apex to 0.0000056 at the base of the dome. The measured temperature strains peak at 0.003, which is signifi cantly higher than the dead load strains. Thus, the compressive strains from the dead load are not suffi cient to counteract the temperature strains. Thus, no benefi t is derived from increasing the thickness of the shell.

Increasing the weight of the shell does not necessarily mean that the strains are increased. If the shell walls are constructed with a double brick wall the weight would be doubled but the area of the wall is also doubled.

5.2 Prestressing

Prestressing shell structures is rare, but has been done on a few structures. Goudi [6] documented a barrel vaulted structure which was presstressed along its length to counteract the tension stress-es in the shell caused by end support walls. The prestressing was composed of longitudinal strands laid along the length of the exterior of the shell. The strands were prestressed by twisting the strands together to create a tension in the cable. Likewise, a spider web shaped net may be laid over the shell and stressed to induce a compression in the shell (see Fig. 13). The prestressing will increase the compressive stresses and therefore the shell is more resistant to temperature loads. A similar effect could be created by additional weight placed on the apex of the shell. Architects/en-gineers in antiquity placed lanterns on the apex to prestress domes. Examples are the St. Paul‘s and Florence cathedrals [7].

Fig. 13: Prestressing a dome structure

5.3 Insulation

Insulating the exterior of the shell is a common method of improving the thermal comfort of the in-terior space and has been used in shell design for many years. However, the exterior insulation is generally not considered as a means of preventing thermal cracking in the exterior walls, since thermal stresses are not considered as adversely affecting the integrity of the shell [2]. This is based on the premise that as the shell is heated, the entire structure expands without inducing thermal stresses. This is not entirely correct since the ring beam/foundations are usually buried below the soil surface and not exposed to the sun. Shells constructed of reinforced concrete are capable of resisting tension stresses and therefore thermal stresses do not adversely affect the shell. Masonry domes are generally not reinforced and therefore susceptible to thermal stresses.


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Conclusions6.

Masonry shells, particularly shapes that are not catenary, are prone to surface cracking. Some cracks are the result of boundary conditions (fi xity at supports), openings for windows and doors, asymmetrical loading (wind) and stiffeners (arches). These types of cracks are predictable through analytical methods such as fi nite elements. Thermal loading, however, is frequently not considered as a source of cracking. Through experimentation, it was found that thermal strains are signifi cant. An existing domed shaped shell was instrumented with Demec targets and strain measurements were taken twice daily. The temperature strains were found to exceed the tensile strength of the surface plaster and therefore was identifi ed as a major contributor of cracking. The greatest poten-tial is near the apex of the shell, which correlates with the position of the mid-day sun (where the sun rays are normal to the tangent of the shell surface).

The possibility of thermal cracking is also increased by the shape of the shell and the magnitude of compressive forces that exists in the shell prior to thermal loading. Shells that are not catenary in shape, will have tensile stresses in the hoop direction or at the boundary. These tensile stresses compound, increasing the possibility of cracking in the shell. However, the effects of thermal stress-es may be reduced by increasing the compressive stresses in the shell – prestressing or by apply-ing weight at the apex are optional methods, which counteract thermal strains.

References7.

[1] D.P. Billington, Thin shell concrete structures, 2nd ed., McGraw-Hill, New York, 1982.

[2] A. Wilson, Practical design of concrete shells, Monolithic Dome Institute, Texas, 2005.

[3] W. Zalewski, E. Allen, Shaping structures: statics, Wiley, New York, 1998.

[4] A. Spottiswood, Stress distribution in shell structures, Research Report, University of the Witwatersrand, 2002.

[5] R. Fernandes, Fibre reinforced soilcrete blocks for the construction of low cost housing, Research Report, Lausanne Federal Institute of Technology, 2004.

[6] G.R. Collins, The designs and drawings of Antonio Gaudi, Princeton University Press, Princeton, 1983.

[7] F. Escrig, Geometry and Structures: historical impressions about architecture, J. of IASSS. 52 (1), (March n. 167, 2011), 25-38.

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