Suitability of Desert Sand Cement Mixes for Base Courses in Highway Pavements

 

Mohamed Y. Al-Aghbari

Department of Civil Engineering, Sultan Qaboos University
Al-Khod, Sultanate of Oman
aghbari1@squ.edu.om

and

Rakesh Kumar Dutta

Department of Civil Engineering, National Institute of Technology,
Hamirpur, India
rkd@recham.ernet.in

ABSTRACT

Soil-cement stabilization has been used for many years because of the significant improvement in soil properties that may be achieved as a result of cement treatment. The improvement in engineering properties of cement-treated soils is believed to be due mainly to the hardening of cement in the presence of moisture and extension of curing period. Different cement contents and curing period render different reactions for cement-treated soils. The paper presents the laboratory study carried out to investigate the possibility of improving the engineering properties of desert sands in Oman using cement. The properties of the stabilized sand such as compaction characteristics, shear strength parameters, unconfined compressive strength, durability and permeability were evaluated and their variations with the content of cement and curing time of the stabilizing agent are quantified.

Keywords: Desert sand, Compaction tests, Unconfined compressive strength, Direct shear test, Permeability, Durability test.

INTRODUCTION

Desert sands covers vast areas of Oman including Wahiba Sand and Rub Al-Khali. The desert sands are creating major problems for construction of infrastructures in Oman. The desert sands are usually fine- grained and poorly-graded materials with small amounts of silt. These soils are characterized by loose fabric structure and are susceptible to erosion by wind and rain (Al-Khanbashi et al, 2000). Desert sands are not suitable for support of structures and roads because they are loose and vulnerable to collapse upon wetting (Elsharief et al 1999). Desert sands are also not suitable for dam construction because of their relatively high permeability and low shear strength.

The paper investigates the possibility of improving the engineering properties of desert sands in Oman using cement. The properties of the stabilized sand such as compaction characteristics, shear strength parameter, unconfined compressive strength, durability and permeability were evaluated and their variations with the content of cement and curing time of the stabilizing agent are quantified.

BACKGROUND

Solid stabilizing agent such as cement has long been used to improve the handling and engineering characteristic of soils which ensures that the mixture meets certain durability requirements for civil engineering purposes. The cement requirement for low plasticity soils and sand varies from 3% to 11% by dry weight (PCA, 1963). The use of soil cement as a paving material in the construction of low cost roads dates back to 1920, when the State Highway Department, USA built short sections of roads with soil-cement. To-date thousands of kilometers of road bases has been laid down. The performance of soil-cement bases and sub-bases has been adjudged as more than satisfactory by various research workers and agency like AASHTO and Larsen (1967). Apart from the PCA guidelines, design methods for thickness determination of soil-cement base courses have been provided by Mitchell and Shen (1967) and Mitchell and Monismith (1977). But due to the semi-rigid or semi-flexible characteristics of soil-cement, the design procedures for soil-cement pavements are yet to be laid down specifically under Oman conditions. Maclean and Robinson (1953), dealing with the methods of stabilization in relation to airfield pavements, suggested that stabilized soil layers should be regarded as forming part of a flexible pavement. They presented theoretical evidence and examples of crack patterns to show that soil-cement with 7-day compressive strength of 1765.8 kPa and corresponding flexure strength of 343.4 kPa would develop closely spaced fine cracks which divided the material into pieces of the size of crushed stone. They further stated that soil-cements having 7-day strength in excess of 3384.5 kPa should be avoided to restrain rigidity or to maintain its flexible character.

CLIMATE OF THE AREA

The overall climate of the Oman may be described as subtropical, warm and arid. Air temperatures range between 35o and 50oC around midday from May to October and 20o and 35oC during winter months. In the interior of the desert the highest temperatures on the ground during summer reach 70oC, but they may fall to freezing in winter. The average rainfall over the Oman is less than 100 mm. per annum but this is very spasmodic and up to 50% of the annual total may fall in a single day. Some monsoon showers are also received during summer months on the east coast and in the mountain belt which forms the watershed between the Arabian Gulf and the Gulf of Oman. Moisture also condenses in the form of fog and dew especially on coastal belts. Strong winds and sand storms are of common occurrence throughout the Oman, being especially frequent and severe in summer months. Desert sands are a dominant landscape feature.

EXPERIMENTAL WORK AND TEST MATERIALS

Sand

The desert sand used in the present study was collected from Al-Wahiba area in Oman. The grain size distribution of the sand is shown in Fig 1. The physical properties are tabulated in Table 1. Minimum and maximum void ratios of the desert sand were 0.58 and 0.72 respectively. The maximum dry unit weight and the optimum moisture content of the desert sand determined from compaction test was 16.4 kN/m3 and 13% respectively. The sand is classified as poorly-graded sand (SP).

 


Figure 1. Grain size distribution of desert sand.

 

Table 1. Physical characteristics of desert sand
PropertyValue
D50 (mm)0.21
Gravel (80 - 4.75 mm), %0
Sand (4.75 - 0.075 mm), %100
Silt & Clay, %0
Specific gravity2.6
Uniformity coefficient2.47
Coefficient of curvature1.05
ClassificationPoorly graded sand (SP)

Cement

The cement used in the study was ordinary Portland cement (OPC). It was obtained from Cement company in Oman. This cement is the most widely used in the construction industry in Oman. The physical properties and the chemical analysis of cement are tabulated in Tables 2 & 3.

Table 2. Physical characteristics of ordinary Portland cement
Propertyvalue
Specific Gravity3.05
Fineness (cm2/g)3357
Initial Setting (minutes)110

 

Table 3. Chemical composition of ordinary Portland cement
Compound%
SiO2 21.94
Al2O34.95
Fe2O33.74
CaO62.33
MgO2.08
SO32.22
K2O0.56
Na2O0.32
TiO20.17
Mn2O30.05
Cl-0.01
Loss-on-Ignition (LOI)1.78

Tests Conducted and Experimental Procedure

Compaction tests

The compaction tests on desert sand-cement mixes were conducted in accordance with British Standard BS 1377: Part 4: 1990. The percentage of the stabilizer added to desert sand were 2, 4, 8, 10 and 12%, by dry weight of the sand. The dry sand passing sieve No. 4 (4.75 mm) and the required amount of stabilizer were mixed thoroughly to a uniform colour. Water was added as needed to facilitate the mixing and compaction process.

Unconfined compression tests

The dry sand and the stabilizer were mixed together to make a uniform colour and the required amount of optimum water content corresponding to different percentage of stabilizer was added to it. The resulting mixture was then compacted in the compaction mould. Specimens from the compaction mould were then extracted carefully and were placed in sealed plastic bags. Specimens were then cured at room temperature for 7, 14 and 28 days. Two specimens were prepared for each percentage of stabilizer and curing time respectively. Unconfined compressive strength tests were then conducted in accordance with ASTM D1633-84 using 100 and 600 kN DARTEC machines at a loading rate of 0.5 kN/sec.

Direct shear tests

The shear box tests were performed on cured samples according to British Standard BS 1377: Part 7: 1990. The shear box size used in the study was 60 mm square and 25 mm high. Based on the results of compaction tests the required amount of soil-cement mixture to fill the shear box was determined. The soil-cement mixture was then compacted at the optimum moisture content in the shear box in layers and tamped properly to achieve the required maximum dry density. The shear box along with compacted soil was then placed in sealed plastic bag and allowed to cure at room temperature for a specified curing time (i.e. 7, 14 and 28 days). For each curing time four specimens were prepared.

Durability Tests

The durability tests on desert sand-cement mixtures were conducted as per ASTM D 559-93. The specimens were immersed in water for 5 hours followed by drying was carried out for another 42 hours. This will constitute one wet-dry cycle. After each cycle, the specimens were brushed with a steel wire brush and the loss in the material is recorded as brush loss in percentage. The test was conducted for 12 numbers of cycles.

Permeability Tests

The specimens were compacted at their required optimum moisture content and were cured for 0, 7, 14 and 28 days for conducting the permeability tests. Prior to the permeability test, the specimens were soaked in water for three days.

RESULTS AND DISCUSSION

Moisture Density Relation

The results of proctor compaction tests are presented in Figure 2. The variation of maximum dry unit weight with cement content is presented in Figure 3. It is evident from this figure that there is marginal increase in maximum dry unit weight of the sand-cement mixes with the increase in cement content. The variation of moisture content with cement content is presented in Figure 4. Figure 4 reveals that there is decreasing trend in the optimum moisture content of the sand-cement mixes with the increase in cement content. But for all practical purposes it can generally be concluded that the behaviour of sand-cement mixes and sand is nearly similar.

 


Figure 2. Compaction curves for desert sand at different cement contents.

 


Figure 3. Variation of maximum dry unit weight with cement content for desert sand.

 


Figure 4. Variation of optimum moisture content with cement content for desert sand.

Unconfined Compressive Strength

The variation of average unconfined compressive strength (UCS) with cement content is presented in Figure 5. It is evident from this figure that the 7 days UCS increases significantly with cement content from a value 112.89 kPa for 2% cement to 1802.75 kPa for 12% cement content. This increase in strength is nearly linear as shown in Figure 5. Similar trend is observed at 14 days and 28 days curing time. The variation of UCS with curing time is presented in Figure 6. It is evident from this figure that with 2% cement content the 7 days UCS value is 112.89 kPa. This value has increased to 178.65 kPa for 28 days curing time. Similar trend is observed in respect of other cement content. Maclean and Robinson (1953) has given 7-day UCS of soil-cement mixes to be varying between 1765.8 kPa to 3384.5 kPa. The UCS of the present sand at 12% cement content is 1802.75 kPa which falls in between these two limits.

 


Figure 5. Variation of unconfined compressive strength with cement content for desert sand.

 

 


Figure 6. Variation of unconfined compressive strength with curing time for desert sand.

Mitchell (1981) gave the following relationship between qu and the curing time.

qu(t) = qu(t0) + K log(t/t0)

where

It was thought pertinent to check the validation of the analytical model proposed by Mitchell (1981) on the experimental results obtained in the present study. The results are shown in Figure 7. It is evident from this figure that predicted values of UCS are much higher than the experimental values. Thus it can be concluded that Mitchell (1981) analytical model over estimate the UCS of desert sand cement mixes.

 


Figure 7. Comparison of experimentally obtained unconfined compressive strength of cement stabilized sand and the analytically computed unconfined compressive strength using Mitchell (1981) equation.

Shear Strength Parameters

The specimens have been prepared at their optimum moisture contents. Cement stabilized sand specimens have been tested after 0, 7, 14 and 28 days curing time. In order to compare the effects of amount of cement and curing time on mechanical behaviour of the desert sand, shear strength parameters such as cohesion and angle of friction have been evaluated from the tests. The variation of angle of friction with cement content is shown in Figure 8. A study of this figure reveals that the angle of friction of the pure sand at 0 days was 35.640 which increased to 39o when 12% cement was added to pure sand. Similarly at 7 days, the angle of friction at 2% cement content was 39o which increased to 48o when 10% cement content was added to sand. Similar trend is observed for other cement content. The variation of angle of friction with curing time is shown in Figure 9. A study of this figure reveals that the angle of friction of the sand with 2% cement content at 0 days was 36o which increased to 430 when the curing time was increased to 28 days. Similar trend is observed for other combinations.

 


Figure 8. Variation of angle of friction with cement content.

 


Figure 9. Variation of angle of friction with curing time

Thus it can be concluded that addition of cement in sand or increasing the curing time increases the angle of friction significantly. Mitchell (1981) has reported that the angle of friction for cement stabilized granular soils is generally in between 45o - 50o. The angle of friction of the present sand at 10% cement content and 7 days curing time is 48o.

The variation of cohesion with cement content is shown in Figure 10. A study of this figure reveals that the cohesion of the pure sand at 0 days was 0.0 kPa which increased to 16 kPa when 12% cement was added to pure sand. Similarly at 7 days, the cohesion at 2% cement content was 22.23 kPa which increased to 170 kPa when 12% cement content was added to sand. Similar trend is observed for other cement content. The variation of cohesion with curing time is shown in Figure 11. A study of this figure reveals that the cohesion of the sand with 2% cement content at 0 days was 9.7 kPa which increased to 26.05 kPa when the curing time was increased to 28 days. Similar trend is observed for other combinations. Thus it can be concluded that addition of cement in sand or increasing the curing time increases the cohesion significantly. Mitchell (1981) gave the following empirical relationship between qu and cohesion.

c = 50 + 0.225 qu(t) (1)

where c = cohesion in kPa and qu(t) = unconfined compressive strength at t days in kPa.

 


Figure 10. Variation of cohesion with cement content

 


Figure 11. Variation of cohesion with curing time

 


Figure 12. Comparison of experimentally obtained cohesion of cement stabilized sand and the predicted cohesion using Mitchell (1981) equation.

It was thought pertinent to check the validation of the empirical equation proposed by Mitchell (1981) on the experimental results obtained in the present study. The results are shown in Figure 12. It is evident from this figure that predicted values of cohesion are much higher than the experimental values. Thus it can be concluded that Mitchell (1981) empirical equation over estimate the cohesion of desert sand cement mixes.

Durability of Desert Sand-Cement Mixes

The durability of stabilized soil is primarily evaluated from the degree to which the engineering properties are retained. Although dominating exposure condition(s) may vary geographically but such exposures are common. One of the most commonly used durability tests on sand-cement mixes in a non-frost area is wetting and drying test. The results of brush loss on 7 days ordinary Portland cement stabilized desert sand is presented in Figure 13.

 


Figure 13. Variation of brush loss with cement content for desert sand

From this figure, it can be seen that the loss is 7.48% for 4% cement content whereas it decreases to 4.39% for 8% cement content and further to 2.00% for 12% cement content. The soil-cement losses for 12 cycles of wet dry tests as allowed by AASHTO for sandy and gravelly soils are 14%. But for desert sand, this value is less than 14% with a cement content of 4%.

Further the results of maximum volume change with cement content are presented in Figure 14. From this figure, it can be seen that the maximum volume change is 5.43% for 4% cement content whereas it decreases to 3.75% for 8% cement content and further to 1.87% for 12% cement content. The maximum volume change for 12 cycles of wet dry tests for sandy and gravelly soils is 2%. But for desert sand, this value is less than 2% with a cement content of 12%.

 


Figure 14. Variation of maximum volume change with cement content for desert sand

 

Permeability of Desert Sand-Cement Mixes

The specimens have been prepared at their optimum moisture contents. Permeability tests on cement stabilized sand specimens were conducted after 0, 7, 14 and 28 days curing time. The effect of amount of cement and curing time on the permeability of the desert sand is shown in Figure 15. A study of this figure reveals that the permeability of the pure sand at 0 days was 0.06 cm/sec which decreased to 3.10x10-05 cm/sec when 12% cement was added to desert sand. Similarly at 7 days, the permeability at 2% cement content was 5.84x10-03 cm/sec which decreased to 7.81x10-06 cm/sec when 12% cement content was added to sand. Similar trend is observed for other curing period. Further the permeability decreases with the increase in curing period also as evident from Figure 15.

 


Figure 15. Variation of permeability with cement content and curing time for desert sand

CONCLUSION

On the basis of the results and discussion presented in this paper, the following conclusion can be drawn:

  1. There is no significant change in the OMC and maximum dry densities of the desert sand-cement mixes with the increase in cement content.
  2. Cement stabilization imparts substantial strength to desert sand making it suitable as a good layer in a base course in highway pavement.
  3. The Mitchell (1981) analytical model over estimate the unconfined compressive strength and cohesion of desert sand-cement mixes.
  4. Addition of cement in desert sand or increasing the curing time increases the angle of friction and cohesion significantly.
  5. A desert sand-cement mix is durable as it does not suffer brush loss and maximum volume change beyond the stipulated values during durability test.
  6. The permeability decreases with the increase in cement content as well as the curing period.
  7. 12% cement content is the optimum for use as a base course in highway pavements.

REFERENCES

  1. Al-Khanbashi, A., A. M. O. Mohamed, A. Moet, and B. Hadi (2000) Stabilization of desert sand using water-born polymers. Proceedings of the First International Conference on Geotechnical, Geoenvironmental Engineering and Management in arid Lands, Al-Ain, United Arab Emirates, pp. 143-148.
  2. Elsharief, A. M., Y. E-A. Mohamedzein, and Y. Hussein (1999) Geotechnical properties of Qoz soil. Proceedings of the Twelfth Regional Conference for Africa on Soil Mechanics and Geotechnical Engineering, Durban, South Africa, pp. 317-320.
  3. Larsen, T.J. (1967) Tests on soil-cement and cement modified basis in Minesota. Journal of PCA, R & D Lab., Vol. 9, No. 1, pp. 25-47.
  4. Maclean, D.J and P. J. M. Robinson (1953) Methods of soil stabilization and their application to the construction of airfield pavement. Proc. Institution of Civil Engineers, London.
  5. Mitchell, J.K. and C.K. Shen (1967) Soil-cement properties determined by repeated loading in relation to bases for flexible pavements. Proc. 2nd Int. Conf. on SDAP, University of Michigan, Michigan, Vol. 1, pp. 427-451.
  6. Mitchell, J.K. and C.L Monismith (1977) A thickness design procedure for pavement with cement stabilized bases and thin asphalt surfacing. Proc. 4th Int. Conf. on SDAP, University of Michigan, Michigan, Vol. 1, pp. 427-451.
  7. Mitchell, J.K. (1981) Soil improvement - State-of-the-art-report. Session 12, Proc. Tenth Int. Conf. on Soil Mech. and Foundation Engg., Stockholm, Vol. 4, pp. 509-565.
  8. Portland Cement Association (1963) Soil cement inspector’s manual. PCA, Shokie

 

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