12TH INTERNATIONAL

BRICK/BLOCK Masonry c O N F E R E N C E

DEVElOPING THE USE OF AAC MASONRY IN CHENNAI, INDIA

Fudge. C A - H+H Celcon Limited, UK Gangwal. K - Hyderabad Industries Limited, India

ABSTRACT

In early 7999, Hyderabad Industries Limited opened a new f/y ash based autoclaved aerated concrete (aae) plant near Chennai in India. The initial aim of this production facility was to manufaeture masonry units of aae. This paper describes how the phy­sical properties of aae are used to the benefit of designers, contraetors and ultima te users. Whilst other lower cost forms of masonry exist, aae units are used to produee eost savings overall in multi-storey framed eonstruetions. Being approximate/y a third to a half of the density of other masonry materiais, aae can be used to reduee the de­ad load on supporting struetures. This enables either eost savings in the sizing of sup­porting f/oors, concrete frames etc or the ability to eonstruct a greater number of sto­reys using existing struetural members.

Key words: Masonry, autoclaved aerated eoncrete, struetures, In dia, bloekwork, con­crete frames.

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INTRODUCTION

Hyderabad Industries Limited is a company of the CK Birla group of indus­tries, the largest manufacturer of fibre cement roofing sheets in India, having a turnover of more than Rs 300 Crores. In 1999, they commenced the pro­duction of aac blocks, in technical collaboration with H+H Celcon Limited, UK, at a state-of-the-art facility, set up with capital cost of Rs 300 million 10-cated near Chennai (Madras). Within India there are many small and locally available competing materiais. Typically these are of locally dug clay brick units which are initially dried in the sun and then baked in a low temperatu­re fire. The units are usually of low compressive strength, that is 5 N/mm2 or less, and have highly variable dimensions. Alternatively, there are low cost concrete units produced predominantly from dense aggregates, generally ho­IIow in format.

After years of extensive research, a decision was made to manufacture aac using fly ash, the resulting residue from the burning of coai to generate elec­tricity, which is abundant in supply and, if not used, is an environmental pro­blem to dispose of. A source of fly ash near to Chennai was found to have go­od properties enabling high quality aac to be produced. It was also agreed to develop a plant with a relatively low initial capital and overhead cost for such a facility, but that could be expanded at a later stage. As a result, a flexible plant layout was used which could be extended with additional autoclaves and mechanised as the business developed. The most up to date equipment was used in the plant to enable the production of units with a high dimensional to­lerance, with consistent material properties. Clearly for any venture such as this to succeed with the comparatively high capital and running costs needed for aac, the full benefits of the material have to be used to sell the product at a premium against other types of similar material. Designers and builders had to be educated to examine the total cost of build and cost saving benefits as­sociated with the material when used in a construction, not just the first cost of the product.

PROPERTIES OF AAC MASONRY

Autoclaved aerated concrete is a lightweight material, which has been used th­roughout the World for decades. Developed in Scandinavia, aac combines the desired physical properties required by a designer in one material (reference 1). To understand the properties of aac it is useful to know how the material is produced.

The materiais used in the production of aac blocks are fly ash, cement, lime and water. The manufacturing process, which is closely monitored at ali times, begins by mixing fly ash and water to form a slurry. The slurry is then heated and mixed with cement and lime, and finally, a small quantity of aluminium powder is evenly dispersed through the mixture before it is poured into a mould.

The aluminium initiates a chemical reaction, generating minute bubbles that form the characteristic aac structure and appearance. When the mixture has partially set, the resultant 'cakes' are wire cut into the desired predetermined size, and the cut cakes are transferred to autoclaves for high-pressure steam curing . During this process, the ingredients combine to form the calcium silicate hydrates, which es­tablish the special properties of the finished product.

In India the masonry units are manufactured to an existing Indian Standard for aac blocks (reference 2) and are used in accordance with acode of practice writ­ten specifically for aac masonry (reference 3) . Aac is lightweight, typically 400 to 700kg/m', that is a quarter to a half the density of other forms of masonry unit. Yet despite this relatively low density it typically has a compressive strength of between 3 and 7 N/mm2. One of the material's major benefits is its high thermal characteristic; the material typically has a design thermal conductivity of betwe­en 0.10 and 0.20 W/mK dependant upon the density (reference 4). Values for ot­her forms of masonry unit can be in the order of ten times these values.

Aac units have excellent resistance to fire, with extensive use and testing carried out in many European countries. This means that they can be used as fire-break walls, but in addition can form a protective cladding for other types of construc­tion, for example, steel frames. Being non-combustible, they do not add to the surface spread of flame. Masonry built with aac is often more slender than other types of masonry for the same fire resistance period.

Aac also has good acoustic properties/insulation and test data (reference 5) has proved that a wall constructed of aac masonry can match the performance of he­avier types of masonry for the same thickness, despite its low density. The rela­tionship, known as the 'mass law', indicates that aac has a different equation compared to other materiais. This is owing to the internai structure of the mate­rial with its non-interconnecting cells .

A wide range of finishes can be applied to the masonry from externai renders or cladding materiais to internai plasters or lining boards such as plasterboard. The smooth, accurate surface allows for thinner finishes, especially if the ma­sonry is constructed using thin joint mortar. The type of externai finish desired is usually chosen to fit with the local environment, but a large variety is avai­lable. Solid, rendered walls of aac masonry are commonly built throughout the countries manufacturing this material. With the closed-cell structure of the ma­terial, aac blocks have excellent resistance to water penetration . Units of aac can easily satisfy national requirements for walls to resist the passage of nor­mal rainwater . The resulting thickness of blockwork required is often more slender than other forms of masonry. In severe conditions of high wind driven rain it is usually prudent to provide some form of cladding, as with ali forms of construction .

Whilst aac has the general characteristics of concrete, it terms of its workability it is akin to timber. As a result, holes can be made with a normal drill, and chasing

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out with a chisel or similar wood working tools. Fixings into the wal ls can be ca­rried out w ith nails, screws or proprietary plugs.

USE IN INDIA

The general use of the product in India is similar to other countries, but there are some variations owing to climate and competing forms of construction. Aac blocks are produced that can be used in both internai and externai walls, and for loadbearing and non-Ioadbearing walls.

It was decided to produce various sizes to meet customer requirements, with a range of thickness from 75 to 300mm. In addition, a standard format of face si­ze to meet general dimensional co-ordination of 600mm in length and 200mm in height was chosen. For practical and production reasons, it was decided to ma­nufacture the material at an average density of 650 kg/m 3

• Thus for a 200mm ma­sonry unit, the block weight will be in the order of 16kg, stil l an easy single per­son lift. At such a density, the design thermal conductivity is specified as 0.16 W/mK, whilst a compressive strength of 4 N/mm2 is achieved.

Being light in weight, the use of aac masonry is extensively used in multi-sto­rey buildings as well as for areas requiring a specific fire rating. With multi-sto­rey structures of reinforced concrete commonly built in India, the lightweight nature of the material provides an overall economic design with significant sa­vings in both cement and reinforcement. The build rates using such a low weight product are improved with aac, beca use the construction operatives become less ti red, particularly with the repetitive nature of brick and block la­ying. Lower weights on the structure results in lower loads on the foundations, thus allowing a more economic designo Alternatively, it enables the structure to be repeated in soils of a lower soil bearing capacity. It is also possible to cre­ate extra storeys to an existing building if the complete structure is designed from aac compared to adense aggregate masonry. Flexibility is also enhanced with the potential to construct unplanned walls at a late stage in the build processo

The high thermal performance, that is low thermal conductivity, is used to great benefit in India. Unlike the original manufacturing Scandinavian countries, where the aim was to provide a barrier to the cold externai temperatures, the require­ments for the Chennai area of India are to provide a cool interior during the hot summer months. Savings in energy costs of electricity for air conditioning are achieved since the load required is less. The thermal mass effect of aac has been documented (reference 6) for high externai temperatures, with lower fuel costs calculated . There also exists a market for cold storage areas, with aac masonry providing an ideal construction solution.

Associated w ith the low weight, but enhanced acoustic properties, the material is being used for internai walls separating different rooms, such as between hotel

bedrooms, offices etc. The thickness of masonry required compares favourably with other forms of construction, but with the added benefit of lightweight on the supporting structure.

Again in common with uses elsewhere in the World, the fire resistance of the ma­terial is being used to great effect. In the region, the masonry wall offers better fi­re integrity compared to traditional brick and reinforced concrete. It is being used in structures where fire rating is needed and is providing a solution where more than 4 hours fire resistance is desired.

The product in Chennai is providing savings since a single aac unit covers the same area as 14 standard brick units, enabling a faster construction time. This results in a saving of time, labour and materiais; this will be covered in more de­tail later. The fewer joints result in considerable savings in mortar as compared to normal brick construction. The truer plain surfaces of the finished wall are being used to good effect by leaving the walls unrendered for buildings situa­ted in low rainfall areas. Even when plaster is used, the quantity required is sig­nificantly less, leading to savings in the cost of finishing. With a generally thin­ner overall wall solution needed compared to other forms of construction, the building has a larger useable floor area for the same externai dimensions. It has also been found that savings with regard to wastage in the aac blocks is signi­ficant compared to other indigenous materiais. It has been found that wastage of approximately 2% can be achieved, owing to the consistency in quality and workability.

The product is being used to great effect where workability is an issue, since they are easy to cut, shape and chase using ordinary wood working tools. This makes them ideal for closing the cavity of externai walls at reveals and for cutting around and over joists, or for special shapes such as infill. This has been found to greatly increase on-site productivity as plumbing and electrical services can be routed after the structure has been erected in a faster and more effective man­ner. As mentioned previously, site wastage is kept to a minimum due to the ea­se of working .

COST ANAL YSIS CASE STUDY

A cost analysis has been carried out for a three storey commercial building in Chennai having a floor area of some1000m 2

• The structure itself is based on a reinforced concrete frame with non-Ioadbearing masonry infill panels forming the perimeter. A full design was carried out considering brickwork as the infill mate­rial and then aac blocks within the frame. The analysis brought out the quantita­tive savings in materiais and labour, but ignored other beneficiai costs. For exam­pie, savings such as a greater floor area due to a thinner wall solution have been ignored, as has the benefits of energy savings on the air conditioning and the quicker financiai returns from investment owing to a speedier construction and earlier completion of the project.

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The building was designed using Indian codes of practice for loading, founda­tions and reinforced concrete. A live load of 300kg/sq m was used and a soil sa­fe loadbearing capacity of 15T /sq m. Ali material and labour rates were based on prevailing market prices. Brick sizes of 225xl15x75mm were compared to aac block sizes of 600x200mm in a thickness of either 100 or 200mm. Savings we­re found in a large number of areas. Firstly, owing to the lower overall dead 10-ad of the structure, there was a reduction in the foundation sizes and quantity of material required . The design of the RC frame was optimised to give lower reinforcing steel requirements reducing the overall quantity by some 10%. In terms of the overall weight of the structure, there was a saving of some 25%, since a 200mm thick aac wall will have a mass of about a third of that of a 225mm thick brick wall.

A direct comparison was carried out for the cost of the masonry alone. There is no doubt that the cost of the aac units is much higher than the brick equivalents; in fact, the differential is some two and a half times of the basic cost. There are, however, savings in the mortar quantities, which lead to significant reductions in the amount of cement required. Labour costs are also significantly lower with the laying time much reduced. As a result the total cost of the masonry is less using the aac option . Another influencing factor is in the cost of plastering the walls. With a more accurate wall finish being achieved with the aac blockwork, there is the opportunity to reduce the thickness of the finishing plaster. In some cases this can reduce the required thickness by a half. In summary, the study showed that the overall the aac blockwork leads to some 9% saving in the cost compared to the brick option. A similar analysis was carried out for a 1 OOmm thick wall and in this case the difference was found to be a 7% saving

When ali of these costs have been taken into account, for the structure as a who­le, there is an overall saving of 5%. This is obviously significant in the financing of such a project

CONCLUSIONS

Aac masonry has been successfully introduced into the Chennai area of India, des­pite its obvious material basic cost disadvantage. The physical characteristics of the material have shown that its benefits can be used to its maximum potential, with the result that the there can be reductions in the overall cost of a building and a better living environment.

REFERENCES

1. RILEM Recommended Practice, Autoclaved Aerated Concrete, Properties, Testing and Design, E & FN 5pon, 1993.

2. Bureau of Indian 5tandards 15: 2185 5pecification for Concrete Masonry Units: Part 3 Auto­claved cellular (aerated) concrete blocks, New Delhi, 1984.

3. Indian 5tandards Institution 15: 6041 Code of Practice for construction of autoclaved cellular concrete block masonry, New Delhi, 1985.

4 . Millard, W R, 'The thermal performance of European autoclaved aerated concrete', Advances in Autoclaved Aerated Concrete, 1992.

5. Luckin . K R, jones. A J, Engledow. G, '50und insulation performance of autoclaved aerated concrete', Proceedings of the British Masonry 50ciety, 2,1989.

6. Aroni. 5 'On energy conservation of autoclaved aerated concrete', Materiais and 5tructures, 23, (January),pp68-77, 1990.

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