Application of Fly Ash and Blast Furnace Slag Stabilised ...ir.uz.ac.zw/jspui/bitstream/10646/1417/1/Antonio_et_al_Application_of... · characteristics of the project site and to

Application of Fly Ash and Blast Furnace Slag Stabilised Soils in Pavement Design:

Case Study Harare-Mutare Road, Zimbabwe

Antonio A.1, Shumba S.1, Vassileva L.D.1

1University of Zimbabwe, Department of Civil Engineering, P.O. MP167 Mount Pleasant, Harare, Zimbabwe

Corresponding Author: [email protected]

Abstract. This project is based on the incorporation of fly ash and blast furnace slag from industries and mines for the

stabilisation of soils for pavement design and construction. The project aimed at the determination of the subgrade

characteristics of the project site and to designing a pavement structure using fly ash and blast slag in stabilisation of the

soil. Tests were conducted at two sites along the Harare-Mutare Road in Zimbabwe on chainages 30.000 m to 35.000 mm.

The research was carried out from November 2012 to July 2013. A series of laboratory tests were done on the stabilized

and unstabilised soil samples to Zimbabwe Standards (ZWS/SAZ) 185 Part 1 (1998) and part 2 (2001) and British

Standards (BS) 1377 (1990) Part 1standards for the purpose of analyzing the bearing strength of untreated and treated

samples when stabilizing materials are added. These results were later used for designing the pavement layers. The study

proposes the use of waste products, fly ash and slag, as stabilizing materials in place of/in combination with conventional

methods of stabilisation with lime and cement. These waste products can be used in combination with the natural soils and

might make roads durable and combat cracking, depressions and pothole build up on roads. They might reduce volume

imported primary aggregates since site soil properties will be enhanced thus reducing transport cost. Also it will reduce

energy cost compared to conventional road construction such as asphalt construction which requires heating of bitumen.

Only water will be used as a solvent in this project. The pavement layers were designed according to South African

Pavement Engineering Manual and Transport and Road Research Laboratory (TRRL) Manuals after analysis of laboratory

and in-situ test results. The use of fly ash and slag improves the bearing capacity of pavements on low volume roads in

urban areas and saves construction costs. The total project cost is $158,500-00 and use of Portland cement as a

stabiliser costs 150% more as compared to the use of fly ash and slag.

Keywords: Fly Ash; Subgrade; Bitumen; Stabilisation.

1 Introduction

1.1 Background

The use of waste products in soil stabilisation has been practiced for a long time and this has helped reduce the

costs in pavement construction. Due to the increasing industrial and agricultural activities tonnes of waste

materials are deposited in the environment with little effective method of waste management (Raheem, 2013) and

these include rice husks and ground nut shell. Pavement design is aimed at achieving a pavement structure which

is economical, comfortable, yet safe to travel by motorist; and which minimizes development of pavement distress

features during the design life of the pavement (Moloisane and Vissser, 2014; Ferguson and Zey, 1990). Fly ash

has been used in roadways and interstate highways since the early 1950s (American Coal Ash Association, 2003)

and cement has been partially substituted with fly ash in pavement construction. Fly ash is typically finer than

Portland cement and lime. Fly ash consists of silt-sized particles which are generally spherical, typically ranging

in size between 10 and 100 micron. Fly ash and lime can be combined with aggregate to produce a quality

stabilized base course (American Coal Association, 2003) and the advantages include provides a strong, durable

mixture, lower costs and increased energy efficiency. The technique of stabilization with fly ash and slag set out

from a need experienced by road constructors for alternative technologies and materials, chiefly on account of the

poor quality of the natural materials available (Kolodziejczyk., 2012).

The benefits of fly ash in improving soil conditions include eliminates the need for expensive borrow materials,

expedites construction by improving excessively wet or unstable subgrade and reduces or eliminate the need for

more expensive natural aggregates in the pavement cross-section. Brooks (2009) discusses the improvement of

expansive soil as a construction material using rice husk ash (RHA) and fly ash (which are waste materials).

mailto:[email protected]

Warren (1990) conducted a study to analyse soil-lime and soil-ground blast furnace slag using the scanning

electron microscope. The implementation of new technological solutions to reinforce and improve weak subgrade

soils enable the use of natural soils as well as waste materials such as power station wastes and metallurgy wastes

such as blast furnace slag which have been traditionally regarded as unsuitable for pavement construction

(Kolodziejczyk., 2012).

1.2 Material Source

There is a need to ascertain whether the proposed road materials, in this case as stabilizers, are able to meet the

demand for completing a project. This will be confirmed upon testing and designing of the road pavement layers

comprised of the optimum mixes of fly ash and slag soil samples (TRL Project Report, 1993). Most road

construction and maintenance activities demand a lot of natural and financial resources. However, current

roadway design and construction practice does not always systematically or holistically address environmental

impacts or environmental impacts (Muench et al, 2011). This gives a hard time for designers to conceptualize the

environmental impacts likely to occur when materials are proposed for road construction work. The Ministry of

Transport in Zimbabwe has been traditionally using cement and lime for stabilisation despite the abundance of fly

ash in the country. Most of the areas in Harare, Zimbabwe have low bearing and expansive soils which need to be

removed before constructing the pavements or stabilised. Low bearing capacity, low strength, low stiffness and

high porosity of the soils results in excessive settlement and severe damages to the roads (Rabbani et al., 2012).

Emery (1992) takes note of the environmental, economical and technical feasibility of the use of by-products and

waste as construction material based on the quantity available necessary for the completion of projects in adequate

proportions which will justify considerable savings through reduced landfill costs. Distances from processing

locations to construction sites should be reasonable in terms of competition with conventional materials, the

processed waste or by-products should be incapable of harming the environment, given proper handling and use,

and their physical, mechanical and chemical characteristics should meet reasonable, applicable specifications for

the contemplated bulk or cementitious application (Marko, 2004). The factors affecting the life cycle cost analysis

of a road building project and total costs are shown in Figure 1.

Figure 1 Factors affecting the Life Cycle Cost Analysis of a road building project and total costs as a function of

the quality of the road (Kennedy, 2006)

1.2 Types of Fly Ash

Fly ash is defined as a heterogeneous mixture of amorphous and crystalline phases and is generally fine powdered

ferroaluminosilicate material with aluminium (Al), calcium (Ca), iron (Fe), sodium (Na) and silicon (Si) as

predominant elements. Certain elements like boron (B), molybdenum (Mo), sulphur (S) and selenium (Se) are

characteristically enriched in fly ash particles (Kosmatra and Kerkhoff, 2002). The emission of fly ash from the

stack into the atmosphere is controlled by particulate devices such as scrubbers, mechanical and electrostatic

precipitators (ESP) (Kolodziejczyki, 2012). The typical composition of fly ash, granulated blast furnace slag and

cement are shown in Table 1.

Table 1 Chemical Analysis of Fly Ash and Portland Cement and Ground Granulated Blast Furnace (GGBF) Slag

(Kosmatra and Kerkhoff, 2002).

Compound Cement Content Class F Fly Ash Class C Fly Ash GGBF Slag

Silica (Sio2) % 22.00 52.00 35.00 35.00

Alumina (Al2o3) % 5.00 23.00 18.00 12.00

Iron Oxide (Fe2o3) % 3.50 11.00 6.00 1.00

Calcium Oxide (CaO) % 65.00 5.00 21.00 40.00

Sulphate (SO4) % 1.00 0.80 4.10 9.00

Sodium Oxide (Na2O) % 0.20 1.00 5.80 0.30

Potassium Oxide (K2O) % 1.00 2.00 0.70 0.40

Total eq. Alkali (as Na2O) % 0.77 2.20 6.30 0.60

Loss of Ignition, % 0.20 2.80 0.50 1.00

Blaine Fineness, m2/kg 350.00 420.00 420.00 400.00

Relative Density 3.15 2.38 2.65 2.96

2 Materials and Methods

2.1 Study Area

The study was carried out in Goromonzi District of Zimbabwe as shown in Figure 2 showing the positions of the

trial pits for geotechnical investigations.

Figure 2 Map of the study area showing an extract of Goromonzi 1731 C4 map, grid Number: Eastings 315-320,

Northings 8019-8022 (Source: Surveyor General’s Office, Zimbabwe).

2.1.1 Description of the Proposed Works

The purpose of this project is to design a pavement section comprising of fly ash and slag stabilized road layers

and note how these materials enhance certain soil properties. A site investigation is done to ascertain necessary

parameters of the site such as California Bearing Ratio (CBR) and dry density of the untreated and treated site

samples mixed with fly ash which will be used in the design process. Soil samples were taken along Harare-

Mutare Road from chainage 30.000 m to 35.000 m.

2.1.2 General Description of the Project Site

Site vegetation and topography drainage

Vegetation comprises of medium dense, vegetation mostly made up of runner grass and a few trees. The ground

slopes generally slope eastwards close to the marshy areas which allows quick drainage of rain water.

Site Geology

The project site falls under part of the granitic formation i.e. igneous rocks in the country.

Water Table

Ground water table levels of the project site ranged from 600 to 800 mm below the ground surface. Therefore, the

road will require a culvert to channel rain water, reducing damages to the pavement structure.

2.2 Trial Pits

The trial pits were used for the purposes of collecting soil samples of different soil types and soil profiling. The

Dynamic Cone Penetration (DCP) tests described in Franki (2008) and SAZ 185 (2001) was used as a rapid in situ

measurement of the structural properties of existing road pavements constructed with unbound materials.

Geotechnical site investigations and field investigations were carried out and the objectives are outlined in

Tomlinson (2001) and Eurocode (EC) 7 (1994). The purpose of classifying the test materials is to arrange the

programme for strength tests (Tomlinson, 2001). Example of methods employed comprise of moisture content

tests, sieve analysis and atterberg limit test of either the soil samples or the fly ash and blast furnace slag.

The California Bearing Ratio (CBR) test was conducted according to procedures outlined in ZWS 185 (2001)

outlines a procedure for the determination of the CBR of disturbed soil samples and for soils treated with an

amount of stabilizer (cement or lime). In this project, a range of 0-40% by mass of mixtures of fly ash and slag

sieved through a 9.5 mm sieve were used to treat the project site soil samples for testing according to mixes

proposed by Kennedy (2006) for hydraulically-bound mixtures like Coal Fly Ash and Pulverised Fuel Ash

(CFA/PFA). Chemical analysis tests are required for the purposes of determining the natural occurring conditions

of three different types of soils (Tomlinson, 2001). This also include testing the fly ash and blast furnace slag for

Sulphate, sulphide and other ions that might affect the quality of water underground if we are to use the material

as stabilizers.

2.3 Materials

Fly ash and slag samples were collected from Dan River Textile Company, Harare, Zimbabwe. The stabilising

material fly ash and blast furnace slag is produced from coal. Soil samples of different soil types were collected

from site shown in Figure 2. These were to be treated with fly ash and slag so as to note the strength

enhamcement done project soil sample and thus leading to the design of the base and subbase layers of pavements

for the three soils.

2.4 Pavement Design

The design part constituted the sizing of the pavement structural elements such as the sub base, base and road

base courses. The cross section design for the proposed road for the Harare-Mutare Road was also done. The

following design manuals were used: SATCC Road Design Manual (1990), Transport Research Laboratory

(TRL), 1988, Road Note 6 and the American Association of State Highway and Transportation Officials (1990)’s

“A policy on Geometric Design of Highways and Streets”.

The design process of the new road pavement comprised of three steps according to the Transport Research

Laboratory report of 2010. These include the estimation of the volume of traffic and the cumulative equivalent

axles that will travel on the road during the design period. The strength of the subgrade is assessed and the most

economical combination of the pavement layer is selected. A 2% cement content addition to the soil was used as

the control during the analysis and design.

3 Results and Discussions

3.1 Borehole Logs

The results of trial pit excavations are shown in the borehole log records for Borehole Number TH100 are shown

in Table 2. Manual methods of boring using a pick and shovel were used and the boring was 1 m square. The

location is Chainage 30,000 m on Harare to Mutare Road, Zimbabwe. Sampling was conducted on 19 April 2013

and completed on 20 April 2013. The Dynamic Cone Penetrometer (DCP) was used for bearing capacity and

California bearing ratio of the soil determination. Sample 1 (S1) and sample 2 (S2) were collected for laboratory

tests.

Table 2 Borehole record for Trial Pit 1.

Changes in strata Samples Water Level In situ Tests

Depth

(m)

Legend Description Depth

(m)

Type

And

No.

Level

(m)

Depth of

Boring

(m)

Depth

(m)

Type

0

Loose light

sand

0.6

Dense brown

silty sand

0.5 S1 0.8 0.9 0.5 DCP

1.2

Dense brown

sand with

some clay

1.0 S2 1.0 DCP



location is Harare-Mutare Road on chainage 32.500 m. Sampling was conducted on 19 April 2013 and completed

on 20 April 2013. The Dynamic Cone Penetrometer (DCP) was used for bearing capacity and California bearing

ratio of the soil determination. Sample 4 (S4) and sample 5 (S5) were collected for laboratory tests.



Depth

(m)


(m)

Type

And

No.

Level

(m)

Depth of

Boring

(m)

Depth

(m)

Type

0

Loose light

sand

0.7

Loose

brown silty

sand

0.5 S3 0.65 0.75 0.5 DCP

1.0

Dense

brown silty

sand

1.0 S4 1.0 DCP








Depth

(m)


(m)

Type

And

No.

Level

(m)

Depth of

Boring

(m)

Depth

(m)

Type

0

Loose light

sand

0.6

Dense

brown silty

sand

0.5 S5 0.5 DCP

1.1

Dense

brown

clayey sand

1.0 S6 1.1 1.2 1.0 DCP








Depth

(m)


(m)

Type

And

No.

Level

(m)

Depth of

Boring

(m)

Depth

(m)

Type

0

Loose light

sand

0.8

Dense

brown silty

sand

0.5 S7 0.9 1.0 0.5 DCP

1.3

Dense

brown

clayey sand

1.0 S8 1.0 DCP

3.2 Particle Size Analysis of Fly Ash and Slag Samples

Figure. 3 shows sieve analysis results for the fly ash and slag. From the results the waste materials have been

observed to range from the gravelly to clayey size particles. It is recommended that the material retained from the

37.5 mm to 9.5 mm sieves be used as aggregates for treating the subgrade whilst material passing through the 6.7

mm sieve be used as stabilizing agent for specific pavement layers. From the particle size distribution above, the

waste materials have been observed to range from the gravelly to clayey size particles. The recommendations are

that the material retained from the 37.5 mm to 9.5 mm sieves must be used as aggregates for treating the subgrade

whilst material passing thru the 6.7 mm sieve be used as stabilizing agent for specific pavement layers.

Figure 3 Particle size analysis of the fly ash and slag samples.

3.3 Variation of CBR with Fly Ash and Slag Content

Figure 4 shows the variation of the CBR value with fly ash and slag content. From the results it can be observed

that Maximum CBR value recorded was 33.5% for the 10 % fly ash and slag treated soil sample whilst the lowest

was that of 11.8% for the 35% fly ash and slag sample. There is an increase in CBR value from the untreated soil

of sample of 15% to 33.5 % to the 10% treated soil sample. From the 10% fly ash and slag stabilized soil, there is

a sudden drop of the CBR values.

Figure 4 Variation of CBR value with fly ash and slag content

3.4 Values of CBR and Dry Density for Different Fly Ash and Slag Contents

Table 5 shows the results of CBR and dry density for different values of fly ash content.

Table 5 Values of CBR and Dry Density for different Fly Ash and Slag Contents.

Fly Ash & Slag Content (%) Dry Density (Kg/m3) CBR Value (%)

0 1937 15

5 1865 28.5

10 1820 33.5

15 1830 20.2

20 1820 18.6

25 1719 16.8

30 1668 13.6

35 1597 11.8

40 1531 14.6

Figure 4 shows the variation of CBR and dry density for different fly ash contents. From the results, the dry

densities tend to reduce as the fly ash and slag content are increased. However, the 10% fly ash and slag treated

sample has the highest CBR value of 33.5% with a dry density of 1820 kg/m3 and the lowest being that of the

40% treated sample having a CBR value of 11.8% and a dry density of 1531 kg/m3. The optimum fly ash content

was 10%. The 10% fly ash and slag treated samples was observed as the optimum percentage to be used in the

design of the pavement structure.

Figure 5 Variation of CBR and dry density for different fly ash contents.

3.5 Variation of the Coefficient of Permeability for the Various Fly Ash Treated Soils

The falling head permeameter was used to determine the coefficient of permeability of the different fly ash and

slag soil mixtures and the results are shown in Table 6.

Table 6 Values of the coefficient of permeability of the different Fly Ash and Slag Stabilised Mixtures.

Fly ash & Slag (%) Coefficient of Permeability, k (m/s) (x10-6)

0 10.66

5 7.604

10 3.181

15 2.491

20 1.841

25 1.598

30 2.323

35 1.407

40 1.261

There is a gradual decrease in the coefficient of permeability from the untreated soil sample to the 10% fly ash

and slag treated soil sample. Generally, there is a decrease in the coefficient of permeability as the amount of fly

ash and slag is increased.

3.6 Chemical Analysis Test Results for Different Fly Ash and Slag Stabilised Soils

A set of chemical analysis tests were conducted to ascertain the feasibility of the proposed stabilizing materials

and how they would impact the environment. These test will be used to carry out an environmental impact

assessment for the project site. The pH, nitrate content and temperature of the different fly ash and slag soil

mixtures were determined and the results are shown in Table 7.

Table 7 Values of the pH, Nitrate Content and Temperature of the different Fly Ash and Slag Stabilised

Mixtures.

Fly ash & Slag (%) pH Nitrate Content (mg/l) Temperature (oC)

5 10.2 0 24.6

10 10.3 0 25.0

15 10.3 0.25 25.4

20 10.6 0.15 26.1

25 10.8 0.13 25.6

30 10.9 0.14 25.6

35 10.9 0.15 25.7

40 11.1 0.16 25.8

There is no nitrate ions in the 5 and 10% mixtures of fly ash and slag. The graph increased to 0.25 mg/l for the 15

% mixture but decreased at the 20% to a nitrate concetration of 0.15 mg/l. Nitrate content gradually increased

from the 25 % mixture of fly ash and slag to the 40 % mixture with a concetration of 0.16 mg/l. There is an

increase in the pH value as the fly ash content is being increased from a value of 10.2 for the 5% mixture of of fly

ash and slag to 11.1 for the 40% mixture. There is in an increase in temperature from 5% mixture to the 20% fly

ash mixture. The graphs then decrease to a temperature of 25.6 °C for the 25% mixture. Temperature then

increases steadily from 25.6 °C to 25.8 °C for the 40% mixture of flyash and slag.

4 Design Work

4.1 Preliminary Design

The preliminary design work involved determining the design traffic and the California bearing ratios and the

bearing capacity of the soil. The natural ground and finished levels of the road were calculated and plotted. The

different base thicknesses were then determined for the 2% cement content control and for the other different fly

ash and slag contents. The pavement was then drawn and the bill of quantities prepared.

4.2 Design Drawings

The design drawing for the Pavement Structure for a 10% fly ash and slag stabilized sub grade is shown in Figure

6.

Figure 6 Pavement Structure for a 10% fly ash and slag stabilized sub grade.

The design drawing for the Pavement Structure for a 2% cement stabilized sub grade is shown in Figure 7.

Figure 7 Pavement Structure for a 2% cement stabilized sub grade (Control).

The design drawing for the Pavement Structure for an untreated subgrade is shown in Figure 8.

Figure 8 Pavement Structure for untreated subgrade.

5 Conclusions and Recommendations

5.1 Conclusions

Fly ash and slag can be used as stabilising materials for pavements with a significant reduction in cost. The small

difference in the project costs indicate that the material can be primarily used in the place of cement. The volume

of fly ash will be adequate since Dan River Textiles produces more than eight tonnes of fly ash per week and

more is produced in the thermal power stations. The use of fly ash and slag stabilisation can be ideal for low

volume urban roads. The total project cost is $158,500-00 and there is a 150% saving when we use the fly ash and

slag for stabilising the soil as compared to the use of Portland cement.

5.2 Recommendations

The study needs to be conducted at different sites with different soil properties to evaluate the effectiveness of the

product. Chemical analysis tests need also be carried out to evaluate the effect of the fly ash properties. The

results from this study will provide the background to the other studies being conducted in the Department of

Civil Engineering.

6 Acknowledgement

Remember to thank those that have supported you and your work. Use the singular heading even if you have

many acknowledgements.

6.1 Department of Civil Engineering, University of Zimbabwe, Harare, Zimbabwe.

6.2 Ministry of Transport and Infrastructure Development, Zimbabwe.

6.3 Dan River Textiles, Zimbabwe.

6 References

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Ferguson, G. and Zey, J., 1990. Stabilisation of Pavement Subgrade with Class C Fly Ash, 9th

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Application of Fly Ash and Blast Furnace Slag Stabilised ...ir.uz.ac.zw/jspui/bitstream/10646/1417/1/Antonio_et_al_Application_of... · characteristics of the project site and to