Experimental Study on Strength Behaviour of Cement Blended Soil-Fly Ash Mixtures
EXPERIMENTAL STUDY ON STRENGTH BEHAVIOUR OF CEMENT BLENDED SOIL-FLY ASH MIXTURES
Ajanta Kalita
Research Scholar, Department of Civil Engg., Indian Institute of Technology, Guwahati–781039, India.
E-mail:
Baleshwar Singh
Associate Professor, Department of Civil Engg., Indian Institute of Technology, Guwahati–781039, India.
E-mail:
ABSTRACT: An experimental testing program was undertaken for evaluating the effect of a class F fly ash and cement on the strength properties of locally available sand and red soils. Both unconfined compression tests and California bearing ratio tests were carried out. Fly ash and cement were mixed in different proportions ranging from 20–50% and 1–5% by dry weight of soil, respectively. The unconfined compression test results indicate that addition of cement to soil-fly ash mixtures leads to increased peak stress as well as change of behaviour to a brittle one. Effectiveness of cement is more pronounced for soil–35% fly ash mixes. For soil-fly ash mixtures, the soaked CBR value is lesser than the unsoaked value. In contrast, reverse trends with the soaked values being greater take place on addition of cement. The study indicates that the soil-fly ash mixes are suitable for good subgrade layer of road embankments, and when blended with cement is appropriate as road sub-base or base materials.
383
Experimental Study on Strength Behaviour of Cement Blended Soil-Fly Ash Mixtures
1. INTRODUCTION
Soil is the cheapest available material utilized for various construction-related purposes. The scarcity of suitable graded soil at construction sites has forced engineers and scientists to utilize waste products of industries that either degrade the environment or pose problems for their disposal. In this connection, utilization of by-products like fly ash as suitable ingredients for geotechnical construction is necessary.
Kaniraj & Havanagi (1999a) mixed Class F fly ashes with sand and silt soils in different proportions and their geotechnical characteristics were investigated. Kaniraj & Havanagi (1999b) added cement to stabilize the fly ash-soil mixtures and established correlations for unconfined compressive strength and secant modulus as functions of curing time, fly ash content, and cement content.
Baghdadi & Shihata (1999) presented an experience with soil-cement in a research project to investigate alternative pavement systems. The possibility of simplifying the durability test and use of the soil of the site to construct soil-cement courses were also investigated. Lo & Wardani (2002) conducted an experimental work to study the strength and dilatancy of a silt stabilized with the addition of a cement and fly ash mixture in a slurry form. Pandian Krishna (2002) conducted experimental work to study the CBR value of fly ash-soil-cement mixtures.
Kaniraj Gayathri (2003) conducted a laboratory experimental study to investigate the effect of cement content, curing period, and curing conditions on the development of the strength of stabilized Class F fly ashes with reference to their use as pavement base courses. Kolias et al. (2005) conducted a laboratory study to evaluate the effectiveness of using high calcium fly ash and cement in stabilising fine-grained clayey soils.
Schnaid et al. (2007) studied the stress-strain-strength behavior of an artificially cemented sandy soil produced through the addition of cement. Consoli et al. (2007) investigated the influence of the amount of cement, porosity and moisture content on the strength of a sandy soil artificially cemented.
The literature review indicates that several studies have been carried out related to use of fly ash and cement for improving subgrade soils. The addition of fly ash reduced the volume change characteristics and improved the subgrade strength. The major advantages of using fly ash in geotechnical applications are high shear strength, low specific gravity, and pozzolanic nature, which could result in significant engineering benefits in addition to facilitating mass disposal of fly ash.
The aim of the present study is to investigate the strength behaviour of local sand and red soils when mixed with class F fly ash and cement.
2. EXPERIMENTAL PROGRAM
2.1 Materials
Uniformly fine sand was chosen and was procured from a deposit of the nearby Brahmaputra River. The red soil used was collected from IIT Guwahati campus. The fly ash was collected from a thermal power plant located at Farakka in West Bengal. The fly ash obtained from this plant has CaO content in the range of 1.7% to 2.7% and can be classified as Class F fly ash. Ordinary Portland cement (53 grade) was used in the study.
2.2 Preparation of Specimens
For preparing specimens for strength tests, first the required amounts of soil, fly ash and cement were mixed together in a dry state, and then the required amount of water was added equal to the corresponding optimum moisture content of the mix. Specimens of the mixes were prepared to achieve their respective maximum dry densities using static compaction. For curing specimens of unconfined compression tests, they were placed at 100% humidity in desiccators kept at room temperature, and then left for 3, 7, 14, and 28 days. For soaking specimens of CBR tests, they were placed under water in buckets for 4 days. The tests were also carried out on unsoaked specimens. Table 1 shows the testing program. Here, BS, RS and FA indicate Brahmaputra sand, red soil and fly ash, respectively. 20FA means 20% by weight is fly ash.
Table 1: Testing Program
Mix proportions / Proportions of cement used (%) / Curing period (days)UC tests / CBR tests
BS
BS+20FA
BS+35FA
BS+50FA
RS
RS+20FA
RS+35FA
RS+50FA / 0,1,2,3,4,5
-do-
-do-
-do-
0,1,2,3,4
-do-
-do-
-do / 3,7,14,28 / 0,4
2.3 Gradation Tests
Grain size distribution indicates if a material is well graded, poorly graded, fine or coarse, etc. The grain size distribution curves of the red soil, Brahmaputra sand, and fly ash are shown in Figure 1. The red soil can be classified as silt of intermediate plasticity. The Brahmaputra sand consists predominantly of fine sand-size fraction with some fine fraction.
Fig. 1: Grain Size Distribution of Sand, Red Soil, and Fly Ash
Figure 2 shows the Scanning Electron Micrograph (SEM) of the fly ash at 2000 times magnification. It can be noted that the particles are spherical in shape and have a porous structure. The degree of fineness has influence on the pozzolanic activity.
Fig. 2: SEM of Farakka Fly Ash
3. UNCONFINED COMPRESSION TESTS
This test is one of the most common tests used to study the strength characteristics of soil mixes. Although the test conditions do not simulate the actual field conditions, the test results can be used for cases of low confinement. To study the effect of pozzolanic reactions on the shear strength of the soils when blended with different fly ash contents, the specimens were cured up to 28 days. Three specimens of each mix and curing time were subjected to the test.
Figures 3 and 4 illustrate the development of the unconfined compressive strength in relation to curing time for BS and RS soils with varying fly ash percent. It can be seen that considerably higher compressive strengths are obtained with RS mixes compared with BS mixes. Further, the strength steadily increases with fly ash content for BS mixes. In contrast, the strength is seen to be the maximum with 35% fly ash content for RS mixes.
Fig. 3: Effect of Fly Ash and Curing Period on Unconfined Compressive Strength of Sand
Fig. 4: Effect of Fly Ash and Curing Period on Unconfined Compressive Strength of Red Soil
When cement was added to any mix of sand-fly ash or red soil-fly ash, the strength gain continued steadily up to 28 days of testing. Stress-strain plots of BS-35% fly ash-cement and RS-35% fly ash-cement mixes for 28 days are shown in Figures 5 and 6, respectively. In case of sand, distinct brittle failure is noticed at an axial strain ranging from 1%–2.5%. In case of red soil, peak axial stress is also reached at the same strain range but failure takes place soon after the peak is crossed.
It can be noted that when cement is added, the gain in strength of sand–35% fly ash mixes is much greater than that of red soil–35% fly ash mixes. For the BS-35FA mixes, the strength increases from 159 kPa at 1% cement to 1384 kPa at 4% cement content. For the RS-35FA mixes, the corresponding strength values are 510 kPa and 818 kPa.
If the y-axis (axial stress) of Figures 5 and 6 are redrawn in the same scale, it will be evident that the initial stiffnesses of the sand-fly ash-cement mixes are significantly higher than those of the red soil-cement mixes. The stiffness was observed to increase at all stages of curing. Further, no ductility is noticeable for the BS-35FA mixes whereas very limited ductility can be seen for the RS-35FA mixes.
Fig. 5: Stress-Strain Plots of Sand + 35FA + Cement Mixes (28 Days Curing)
Fig. 6: Stress-Strain Plots of Red Soil + 35FA + Cement Mixes (28 Days Curing)
4. CALIFORNIA BEARING RATIO TESTS
The strength of the subgrade and sub-base layers of pavements is commonly evaluated using the California Bearing Ratio test. The unsoaked CBR values of BS and RS are found to be 19.1% and 6.7% respectively, whereas the soaked values of BS, RS and FA are 11.1%, 4.7% and 2.9%. Figures 7 and 8 indicate the comparison between sand and red soils with varying fly ash percent, under unsoaked and soaked conditions respectively. When no fly ash is added, the CBR value of the sand is higher in both the conditions.
Under unsoaked condition, as fly ash is added, the CBR value of the sand-fly ash mixes does not improve much. In contrast, improved CBR values are obtained for all the red soil-fly ash mixes. For RS mixes, the CBR increases from 6.7% with no fly ash to a value of 37.2% at 50% fly ash content.
Under soaked condition, for BS mixes, the maximum CBR value of 17.0% is obtained at 35% fly ash content. For RS mixes, the CBR values reduce drastically upon soaking, and they remain almost constant for the different fly ash contents, ranging from 11.7% to 10.3%. It is to be noted that only soaked CBR values are used in the design of pavements.
Fig. 7: Comparison of Unsoaked CBR Values of
Soil-fly Ash Mixtures
Fig. 8: Comparison of Soaked CBR Values of
Soil-fly Ash Mixtures
When cement up to 3% was added to any mix of sand-fly ash or red soil-fly ash, gain in CBR was noted with increasing cement content. Figures 9 and 10 respectively highlight the comparison of CBR values between BS-35% fly ash-cement and RS-35% fly ash-cement mixes for unsoaked and soaked conditions. The gain in CBR values of sand-35% fly ash mixes is found to be generally greater than that of red soil-35% fly ash mixes.
Under unsoaked condition, for the BS-35FA mixes, the CBR value increases from 31.5% at 1% cement to 39.0% at 3% cement content. For the RS-35FA mixes, the corresponding CBR values are 27.3% and 32.7%.
Under soaked condition, for the BS-35FA mixes, the CBR increases from 35.2% at 1% cement to 50.9% at 3% cement content. The corresponding CBR values for the RS-35FA mixes are 28.4% and 52.2%.
A minimum CBR of 6% and 20% are recommended for use in the subgrade and sub-base layers of strong and durable road pavements. From Figure 10, it can be seen that the red soil-35% fly ash mix is suitable for subgrade layer. When this mix is blended with even only 1% cement content, it becomes suitable for sub-base layer. The sand-35% fly ash mix with minimum 1% cement content can also be used in sub-base layer.
Fig. 9: Comparison of Unsoaked CBR Values of
Soil-35FA-Cement Mixtures
Fig. 10: Comparison of Soaked CBR Values of
Soil-35FA- Cement Mixtures
5. CONCLUSIONS
The results reveal that certain proportions of fly ash added to the soils can improve unconfined compressive strength. The optimum contents of fly ash are 50% and 35% for the sand and red soils respectively. When soaked CBR values are compared, the fly ash contents of 35% and 20% are found to be the optimum for the two respective soils. The strength and CBR of the mixes generally improved on further addition of small amounts of cement. Such large proportions of fly ash in actual field applications will prove to be very advantageous and helpful for mass utilization.
REFERENCES
Baghdadi, Z.A. and Shihata, S.A. (1999). “On the Durability and Strength of Soil-Cement”, Ground Improvement, 3:1–6.
Consoli, N.C., Montardo, J. P., Prietto, P.D.M. and Pasa, G.S. (2007). “Engineering Behavior of a Sand Reinforced with Plastic Waste”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 128(6): 462–472.
Kaniraj, S.R. and Havanagi, V.G. (1999a). “Geotechnical Characteristics of Fly Ash-Soil Mixtures”, Geotechnical Engineering Journal, 30(2):129–146.
Kaniraj, S.R. and Havanagi, V.G. (1999b). “Compressive Strength of Cement Stabilized Fly Ash-Soil Mixtures”, Cement and Concrete Research, 29: 673–677.
Kaniraj, S.R. and Gayathri, V. (2003). “Factors Influencing the Strength of Cement Fly Ash Base Courses”, Journal of Transportation Engineering, ASCE, 129(5): 538–548.
Kolias, S., Kasselouri-Rigopoulou, V. and Karahalios, A. (2005). “Stabilisation of Clayey Soils with High Calcium Fly Ash and Cement”, Cement & Concrete Composition, 27: 301–313.
Lo, S.R. and Wardani, S.P.R. (2002). “Strength and Dilatancy of a Silt Stabilized by a Cement and Fly ash Mixture”, Canadian Geotechnical Journal, 39:77–89.
Pandian, N.S. and Krishna, K.C. (2002). “California Bearing Ratio Behavior of Cement-Stabilized Fly Ash-Soil Mixes”, Journal of Testing and Evaluation, 30(6): 1–5.
Schnaid, F., Prietto, P.D.M. and Consoli, N.C. (2001). “Characterization of Cemented Sand in Triaxial Compression”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 127 (10): 857–868.
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Experimental Study on Strength Behaviour of Cement Blended Soil-Fly Ash Mixtures
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