Highway Research Bulletin

YEAR 2004-2005
Bulletin No. 73

Assessment of suitability of coralline sand-cement mixes for base/subbase courses
By R.K. Dutta* & G. Venkatappa Rao**


ABSTRACT

A laboratory study has been carried out to assess the suitability of coralline sand-cement mixes for their use as a base or sub-base courses in road and air-field pavements. Compaction tests were conducted on different sand-cement mixes and these results were used for CBR determination and for casting the cylindrical specimens for strength evaluation. Wetting and drying tests were also carried out to study the durability of the soil-cement mixes. It has been concluded that 7 per cent cement content as the optimum for its use as a base course in highway and airport pavements.
Key Words: Coralline sand, Unconfined compressive strength^ CBR, Wet-dry Test.
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* Senior Lecturer Department of Civil Engineering, National Institute of Technology, Hamirpur - 177005, Himachal Pradesh, India.
** Professor Department of Civil Engineering, Indian Institute of Technology, New Delhi - 110016, India.
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1. INTRODUCTION
Soil cement is used for a variety of constructions ranging from sub-base and base courses of highways and airport pavements. An increasingly popular use of soil cement is for the bases of rigid pavements. Ramana Sastry and Srinivas (1995) reported that the stabilized base reduces the deflection at the joints such that load transfer devices are not necessary. Superior strength and durability, and its resistance to water, hot weather and frost are some of the factors in favour of soil-cements.

India has an estimated 18,000 square km of coral, reefs. Indian coral reefs are mainly located in six regions in the country. These are Lakshwadeep, the Andaman and Nicobar Islands, Gulf of Kutch, Gulf of Mannar, Palk Bay and most recently, the table reefs along the Ratnagiri Coast of Maharashtra. Coral rock is used for building purposes in coastal India. In Lakshwadeep, which has one of the highest densities of population, islanders used coralline rocks to build homes as it works out less expensive than transporting cement, and gravel 300km from the mainland. Exploitation of corals, coral debris and coral sands is widespread in the Gulf of Mannar and the Gulf of Kutch reefs. With the increase in construction activity in these regions, the conventional civil engineering materials, like, soil, aggregates or rocks are becoming scarce and are difficult to find. The only materials available are coralline limestone and coralline debris or coralline sand. There is hardly any data on these materials and as such their suitability as base or sub base courses is yet to be assessed systematically. Coralline sand is obtained by natural degradation of parent coral reef rock. It has two basic characteristics that make it totally different from other sand having similar gradation. Firstly, it is nearly 100 per cent calcium carbonate. Secondly the particles are hollow within. Also the coralline sand is usually of uniform texture, but with some shell fragments. In view of this they are highly crushable and amenable to stabilization with cement. The Paper presents a detailed laboratory study to assess the suitability of coralline sand-cement mixes for their use as base or sub-base courses in highway and airport pavement.

2. BACHGROUND
Sands, including desert sands, have been successfully stabilized with Portland cement. Studies of Seshagiri Rao (1992) have indicated that beach sand stabilized with cement and rice-husk-ash may be used as base course for roads along the coast in place of conventional granular sub-base and base courses. This type of construction is found to be not only economical but also convenient as granular materials required for sub-base and base courses are not available within economical leads in the coastal areas.

Soil cement is a hardened material obtained by mechanically compacting a mixture of soil, water and a quantity of Portland cement, which, ensures that the mixture meets certain durability requirements. The cement requirement for low plasticity soils and sand varies from 3 to 11 per cent 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). The soil-cement losses for 12 cycles of wet-dry test allowed by AASHTO for sandy and gravelly soil are 14 per cent and 7-day unconfined compressive strength to be 2072.9 kPa to 4144.7 kPa. The 7-day unconfined compressive strength for chemically stabilized bases/subbases as per MoRT&H/(IRC:50, 1973) is 1716.75 kPa (17.5 kg/cm2). 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 Indian 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 a 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.

Coralline sand is mainly consisting of calcium carbonate and very small quantities of calcium magnesium carbonate. Solid calcium carbonate exists in three forms: aragonite, calcite and vaterite. Mineralogical studies reported by Taft (1967) have shown that calcium carbonate exists in the form of aragonite, high magnesium calcite (12 to 17 percent magnesium carbonate) and low magnesium calcite (2 to 3 percent magnesium carbonate). Vaterite has not been encountered. However, isolated occurrences of calcium magnesium carbonate (referred as dolomite) have been reported from marine environment offshore Australia (Skinner et al 1963) and from the Persian Gulf (Wells 1962).

3. CLIMATE OF THE AREA
Andaman and Nicobar Islands are endowed with a maritime climate year round with least variation between maximum (30.1°C) and minimum (23.1"C) temperatures. The average relative humidity in the area varies from 68 to 86 per cent. The maximum temperatures are experienced during the dry season, when evapo-transpiration losses are found to be highest. Since, these islands are under the influence of both the south-west and north-east monsoons, they receive rain from April to December. The mean annual precipitation is around 3100 mm unevenly distributed throughout the year.

4. EXPERIMENTAL WORK
Test Materials: A brief description of the material used in this investigation is as follow:
Sand: The coralline sand used in the present study was collected from Andaman and Nicobar Islands, India. The sand consists of carbonate shells, shell fragments and coralline fragments. The grain size distribution curve of the coralline sand is shown in Fig. 1. The sand contains a gravel content of 6 per cent, sand content 93 per cent and silt and clay content as 1 per cent. The other properties of the sand are given in Table 1. As per IS classification, it is classified as fine sand. The X-ray diffractogram for coralline sand is shown in Fig. 2. It is evident that the sand is essentially calcitic with some aragonite. Treatment with acid also indicated that the sand is nearly 100 per cent carbonate. The SEM image shown in Fig. 3 shows the voids present in the particles and the rounded surface.


Cement: Commercially available ordinary Portland cement has been used in the study.
Preparation of Mixes: Soil is sieved through 4.75 mm sieve after air drying. The percentage of cement is varied from 3 to 11 per cent with increments of 2 per cent.



Tests Conducted: To make an assessment of the suitability of coralline sand-cement mixes, the various tests conducted are listed below:

Grain size analysis and classification: IS 1498-1970.
Compaction Test:
IS 4332, Pt. Ill, 1967.
Unconfined compressive strength test: IS 4332, Pt. V, 1970.
Wetting and Drying Tests: IS 4332. Pt. V, 1968.
CBR Tests: IS 2720, Pt. XVI, 1979.



Test Procedure
Preparation of Specimens: The coralline sand-cement mixes are compacted in a Proctor’s mould by the standard procedure for light compaction by giving 25 blows to each layer.

Curing:
Immediately after preparation of the specimens at various cement contents for unconfined compressive strength and wetting and drying tests, each specimen after extraction from the mould was tightly packed in a polythene bag for 24 h in order to avoid any moisture loss. The specimens moulded were then cured in a dessicator for 7 days before unconfined compressive strength and wet-dry test.

Wetting and Drying Tests:
The wetting and drying tests on sand-cement mixtures were conducted on a 38 mm diameter and 76 mm high specimens. The specimens were then 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.

Unconfined Compression Tests:
The unconfined compressive strength tests were conducted at a deformation rate of 1.2 mm/min. Unconfined compressive strength test specimens of size 38 mm diameter and 76 mm height are prepared as follows. The quantity of cement and sand to get the desired unit weight is computed and dry mixed, after, which, the required amount of water is added and mixed together. The resulting mix is then placed in a mould and compacted statically. The samples were then taken out carefully. It is established that this compaction procedure resulted in dry unit weights in the sands that are approximately of the same order as those obtained from proctor’s standard procedure.

CBR Tests: The coralline sand-cement mixes were compacted in the CBR mould by the standard procedure by giving 56 blows to each layer. The CBR of sand-cement mixes compacted at optimum moisture content have been determined after 96 hours soaking.

5. RESULTS AND DISCUSSION

Compaction Tests: The results of Proctor compaction tests are presented in Fig. 4. The variation of maximum dry unit weight and optimum moisture contents with cement content are presented in Figs. 5 & 6; respectively. It is evident from these figures that the behaviour of sand-cement mixes and sand is similar. The behaviour observed is similar to that observed for dune sand (IIT Delhi, 1992).

Unconfined Compressive Strength Test: Typical stress-strain curves obtained are presented in Fig. 7. It is evident from this figure that the UCS increases significantly with cement content from a value 412.0 kPa for 3 per cent cement to 3953.4 kPa for 11 per cent cement content. This increase in strength is nearly linear as shown in Fig. 8. The 7-day UCS for gravelly and sandy soil is adopted by AASHTO is varying between 2072.9 kPa to 4144.7 kPa. 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 7-day UCS of soil-cement mixes as per the requirements of MoRT&H/(IRC:50 1973) for chemically stabilized bases/subbases is 1716.75 kPa (17.5 kg/cm2). The UCS of the present sand at 7 per cent cement content is 1883.5 kPa which falls in between AASHTO & Maclean and Robinson (1953) criteria and satisfy the requirements of MoRT&H/(IRC:50, 1973).





California Bearing Ratio Test: The typical load deformation curves for CBR of sand-cement mixes are presented in Fig. 9 and variation in CBR with cement content is shown in Fig. 10. From these figures it can be inferred that the CBR values for the sand-cement mixes increases significantly with an increase in the cement content upto 9 per cent and after that there is little improvement. For a cement content as low as 3 per cent the CBR value is 81 and for higher cement contents the value is invariably more than 100. Durability of Coralline Sand-Cement Mixes. One of the most important properties that the sand-cement mixes should have is the ability to retain its strength over the year when exposed to the destructive forces of weather. One of the most commonly used durability tests on soil-cement mixes in a non-frost area is wetting and drying test. The results of brush loss with cement content are presented in Fig. 11. From Fig. 11, it can be seen that the loss is 52 per cent for 3 per cent cement content whereas it decreases to 17 per cent for 5 per cent cement content and further to 6.5 per cent for 7 per cent cement content. The soil-cement losses for 12 cycles of wet dry tests as allowed by AASHTO for sandy and gravelly soils are 14 per cent. But for coralline sand, this value is less than 14 per cent with cement content of 7 per cent or higher. Further the results of maximum volume change with cement content are presented in Fig. 12. From this figure, it can be seen that the maximum volume change is 2.47 per cent for 3 per cent cement content whereas it decreases to 1.44 per cent for 5 per cent cement content and further to 0.92 per cent for 7 per cent cement content. The maximum volume change for 12 cycles of wet dry tests for sandy and gravelly soils is 2 per cent. But for coralline sand, this value is less than 2 per cent with cement content of 5 per cent or higher.







6. CONCLUSION
On the basis of the results and discussion presented in this Paper, it can be concluded that coralline sand-cement mixes behave, like, normal sand and satisfy the strength and durability criteria as well as giving higher CBR values. The results reveal that 7 per cent cement content is the optimum for use as a base course in highway and airport pavements.

REFERENCES
1. IIT Delhi (1992),” Cement Stabilization of Subgrade Soil at Jaipur Airport”, Aconsultancy report submitted to National Airport Authority, New Delhi, India.

2.
IRC:50 (1973), Recommended Design Criteria for the Use of Cement-Modified Soil in Road Construction.

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 Robinson, P. J. M. (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 Shen, C.K. (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 Monismith, C.L(1977),” A Thickness Design Procedure for Pavement with Cement Stabilized Bases and Thin Asphalt Surfacings”, Proc. 4th Int. Conf. on SDAP, University of Michigan, Michigan, Vol. 1, pp. 427-451.

7.
Portland Cement Association (1963), “Soil Cement Inspector’s Manual”, PCA, Shokie, III.

8.
Ramana Sastry, M.V.B and Srinivas, K. (1995),” Influence of the Uniformity Coefficient of Soil on the Density and Strength Characteristics of Soil-Cement”, Highway Research Bulletin, No. 52, pp. 37-48.

9. Seshagiri Rao, Y.(1992),” Stability and Economic Aspects of Cement Rice-Husk-Ash Stabilized Beach Sand as Base Course for Beach Roads”, unpublished M.Tech thesis, J.N.T. University, Hyderabad.

10. Skinner, H.C.W., Skinner, B.J and Rubin, M. (1963),” Age and Accumulation Rate of Dolomite Bearing Carbonate Sediments in South Australia”, Science, Vol. 139, pp. 335-336.

11. Taft, W.H. (1967),” Modem Carbonate Sediments “in G.V. Chilingar, H.J. Bissel and R.W. Fair Bridge (Editors), ‘Carbonate Rocks’, Elsevier, Amsterdam, pp. 29-50.

12. Wells, A.J. (1962),” Recent Dolomite in The Persian Gulf, Nature, Vol. 194, pp. 274-275.