A Study on the Strength Behavior of Stabilized Clayey Sands Using Lime-Polyamide Fiber   Mahyar Arabani Akbar K. Haghi and Mehdi Veis Karami Dept. of Civil Engineering, the University of Guilan, Rasht, Iran

ABSTRACT

This paper presents experimental investigations for strength improvement of clayey sands obtained from northern Iran and reinforced with lime-polyamide fiber. In this method, Polyamide 66 strips were placed between compacted layers. The results of reinforced and unreinforced samples are compared with several lime-stabilized soils among the middle-east countries. The comparison shows the superiority of this new composite in improving the strength.

Keywords: Lime, Material Composite, Mechanical Properties, Polymers

INTRODUCTION

A wide range of investigations has been performed on soil lime-stabilization in previous years by several authors [1, 2]. The most mentionable effect of lime on soil is to improve workability and compatibility and reduce swelling and shrinkage potentials by saturating the clay particles with calcium ions. This leads to strength increase by pozzolanic and carbonation cementation processes. Cation exchange and pozzolanic reactions result in strength increase. The level of reactivity and hence strength gained in soil-lime mixtures depends on the level of pozzolanic product created [3].

It is possible that the reactions affecting strength and stiffness improvement of soils are the pozzolanic and carbonation reactions. The pozzolanic reaction occurs where water and lime in soil mass produce a high pH system, which is sufficient for the solution of the clay mineral structure. If a pH higher than 12 is obtained, a properly lime-water-soil system would be designed at a normal paving temperature. This level of alkalinity is high enough for reactions between most of clay minerals. The degree of soil-lime reactivity varies among different soil types [3].

In our previous studies the modification of clayey sand to improve their engineering properties is well described [4, 5]. It is shown that among the various stabilizing agents, the most prominent is lime. Guilan has a humid and rainy climate placed near Caspian Sea in the north of Iran. Most of soil layers were formed by the sediments of Quaternary, transported by “Sefid-Roud” River, which is the biggest river in north of Iran. In Guilan, the groundwater table is high and the natural moisture content is ranged between 11 to 56%. The general soil texture in this area consists of sand and clay. Kaolinite and Illite clay are the general clay minerals in Guilan, but Kaolinite is dominant clay mineral, due to heavy rainfall in the region. Because of this, lime-stabilization and cement-stabilization are the most common techniques for soil improvement in this province. Through stabilization, the plasticity of clayey sand is reduced, it becomes more workable, and its compressive strength and load-bearing properties are improved. However, the benefits of polyamide additions did not reported previously and can be included as the primary ingredient necessary for stabilizing the clayey sand. We reported the interaction of polyamide 66 yarns with component of cement in our recent work [6]. In this study, a good adhesion between the component of cements and polyamide obtained. In one of our recent studies [7], we presented a new polymer concrete using polyamide yarns. In view of the above works, we studied the effect of polyamide strips on engineering properties of lime stabilized clayey sands in this paper.

It worth to mention that there are a number of specializes stabilizers for which there is little technical information or test data. These stabilizers are sold under trade names. These products are only useful in stabilizing fine-grained soils [8]. The objective of our study is to present a new polymeric stabilizer, which has never been reported before.

EXPERIMENTAL STUDY

Samples were prepared using natural aggregates founded in a province located in north of Iran, classified under clayey sands. Laboratory samples were re-aggregated to obtain a standard form of aggregations for experiments. A mix of coarse aggregate with five different fine materials was prepared. The fine content of mixes, provided from dominant clay of the area, were 5%, 15%, 22%, 30% and 36% in order to stabilize the samples, the hydrated lime, produced in cement plant of Guilan province with more than 80% purity (passed No. 200). Materials specifications are presented in Table 1, in which ?d is dry density, wopt is optimum moisture content.

Table 1. Properties of the Materials used in Experimental Study

Sample Name	Clay Content (%)	wopt (%)	rd (gr/cm3)*
S1-C5	5	6	2.08
S2-C15	15	6.5	2.12
S3-C22	22	8	2.15
S4-C30	30	9.5	2.14
S5-C36	36	11	2.11
(*) Dry density; to find the unit weight g in kN/m3 multiply by g=9.8 m/sec2—Editor's note.

Water content of specimens had been determined by modified proctor compaction test according to AASHTO guideline. The samples were prepared for further testing procedures reinforced with polyamide 66. All samples were then placed compacted in 5 layers in standard uniaxial test and indirect tensile test cylindrical molds and compacted to reach to relative density of 90 to 95 percent. Polyamide 66 strips were placed between any these compacted layers. The cylindrical samples for compressive and tensile tests that were prepared had a dimension of 10cm high and 5cm diameter.

After 24 hours curing in laboratory open condition, at 23ºC, the samples were taken out from molds and placed in an oven in constant temperature at 45ºC, in order to increase the rate of curing, in humid condition to allow chemical reactions to take place for a 7-day strength [9]. While a curing of 48 hours was taken, samples were tested. Three samples were prepared and cured for each test in this manner.

The tests were performed on specimens just 15 minutes after they were taken out from the oven. A capping procedure with chemical materials were performed on upper and lower surfaces of specimens to make these surfaces slick and obtain a uniform stress distribution on surfaces and body of the samples. A uniaxial test with a compressive test instrument and a constant rate of strain (about 0.5mm/min) was used for each specimen. The tests were performed according to I. R. Iran Code for highway design based on AASHTO T 220.

By a digital compressive apparatus, with 0.01kN accuracy, Brazilian tests were performed to obtain the tensile strength of specimens, based on ASTM C496 for indirect tensile test. The size and curing time of both tensile and compressive samples were similar.

RESULTS

Compressive Strength

The behavior of clayey sands stabilized with lime was presented in our previous research [4]. In the Figs. 1, 2 and 3 we compared the compressive strength of polyamide 66 reinforced and non-reinforced lime-stabilized materials versus different clay contents stabilized with 3%, 6% and 9% lime content respectively. The results obtained for non-reinforced samples are represented in our previous studies [4,5]. Based on the variation of compressive strength presented in Figs. 1 through 3 it is clear that the higher the clay content, the higher the compressive strength. Meanwhile, the compressive strength of specimens reinforced with polyamide 66 is higher than the others. As shown in these figures, compressive strength can even be raised up to 25% when polyamide 66 strips as a reinforcing agent is used.

Figure 1. Comparison between compressive strength of reinforced and
non-reinforced samples vs. clay content for 3% lime content

Figure 2. Comparison between compressive strength of reinforced and
non-reinforced samples vs. clay content for 6% lime content

Figure 3. Comparison between compressive strength of reinforced and
non-reinforced samples vs. clay content for 9% lime content

In Fig. 4 we are comparing the reinforced and none-reinforced samples in comparison with several lime-stabilized soils among the middle-east countries. As it is shown clearly, the compressive strength of our reinforced sample is relatively higher than the strength reported in literatures [10,11].

Figure 4. Compressive strengths for reinforced and
non-reinforced samples stabilized with 6% lime

Tensile Strength

Figs. 5, 6 and 7 show the results of tensile strength of samples reinforced and non-reinforced with polyamide 66 strips. Two important parameters can be considered and discussed, when polyamide 66 as reinforcing fibers is used in lime stabilized clayey sand samples. First of all, an increase in the tensile strength of specimens can be observed when specimens are reinforced with polyamide 66 strips. Herein, the higher the clay content the higher the tensile strength. Meanwhile, as it is shown in Figs. 5, 6 and 7, increase in tensile strength is more important for reinforced specimens. In view of the above, about 90 percent increase in tensile strength of specimens is obtainable by using polyamide 66 strips as reinforcing fibers in the samples. This is a significant improvement.

Figure 5. Comparison between tensile strength of reinforced and
non-reinforced samples vs. clay content for 3% lime

Figure 6. Comparison between tensile strength of reinforced and
non-reinforced samples vs. clay content for 6% lime

Figure 7. Comparison between tensile strength of reinforced and
non-reinforced samples vs. clay content for 9% lime

In general, most clayey sands can be successfully stabilized with lime [12]. However, the addition of polyamide 66 can significantly improve the engineering properties of stabilized clayey sand and moderates the cracks propagation. A typical cracks distribution in a polymeric reinforced specimen in tension as well as in compression is shown in Figs. 8, 9 and 10 respectively. The overall results of this study indicate that lime/polyamide stabilizer composite is performing adequately.

Figure 8. Pore size distribution within the polyamide strip and
tensile failure mode in an indirect tensile test

Figure 9. A typical crack pattern in a polymeric reinforced
specimen in the indirect tensile test

Figure 10. Optical micrograph of a cut-polyamide strips

The pore size distribution in a fibrous material has significant impact on the mixture transport process. These distributions are clearly shown in Fig. 8. Pore size distribution can influence the spontaneous uptake of liquids (i.e., moisture content) and therefore the strength keeps on increasing. It should be noted that the amount of pores within the strips can have significant effects on the strength improvement as well. The greater the pores, the more moist aggregates the strip can hold.

In Fig. 10 we can see the crack distribution adjacent to the polyamide strips. The cracks did not spread out in the direction where the strip is located.

DISCUSSIONS

There is no doubt that lime migration is usually occurs in the context of diffusion of clayey sand in the presence of stabilizers. Conceptually, the parameters that control this phenomenon should be those associated with diffusion process. It is also clear that lime migration refer primarily to the migration of Ca⁺² into the layer structure of clay, it will be equally applicable to any stabilizer that supplies calcium. The extent to which this takes place depends, primarily, on the chemical potential (concentration gradient of calcium) between the core (reaction front) and the outer surface (pore). The amount of stabilizer determines the supply of calcium, the most necessary component for stabilization, and calcium may be supplied from various sources. Therefore, lime migration, in principle, will not be associated solely with addition of lime as stabilizer. In view of the above, in order to be able to improve the stabilization criteria, we are suggesting the application of polyamide in our mixture, which seems to be more reliable than any other stabilizers that have been reported before [13]. That means, the presence of polyamide fiber as an admixture, can greatly influence the engineering properties and stabilization of clayey sand. This can be achieved in normal construction operations in the field.

CONCLUSIONS

The results of this study lead us to the following conclusions:

• Stripping of polyamide can improve the cohesion properties. So we can increase the bearing capacity of foundations.
• The strength and stiffness of the clayey sand is well improved using the lime/polymeric stabilizer. Furthermore, the cracking patterns observed were minimal. This revealed that the stiffness of the mixture is significantly improved. Thus the use of these materials in pavement layers can decrease the deflection and increase the strengths under traffic loads.
• The variation of stress-strain in the mix design indicates that the lime/polymeric stabilizer can be used in heavy-duty pavements.
• Based on the variations of tensile strengths obtained, the design life of the lime/polymeric stabilizer can be improved significantly. The results indicate that the lime/polyamide can be a good candidate for the stabilization of clayey sands as base, sub-base, stabilized sub-grade for highways and airport pavement as well as for any construction practices.
• Stability of slopes can be achieved using such materials as clayey sand in natural and/or artificial soils.

REFERENCES

1. Diamond, S. and E. B. Kinter (1965) “Mechanism of Soil-Lime Stabilization,” An Interpretive Review, Highway Research Record, HRB, 92, National Research Council, Washington D.C., pp 83-102.
2. Dallas, N. Little (1995) “Stabilization of Pavement Subgrades and Base Courses with Lime,” Kendall/Hunt Publishing Company, Dubuque, Iowa.
3. Dallas, N. Little (1996) “Assessment of In Situ Structural Properties of Lime-Stabilized Clay Subgrades,” Transportation Research Record, TRB 1546, pp 13-23.
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5. Arabani, M. and M. Veis Karami (2005) “A Study on the Geomechanical Characteristics of Clayey Sands Treated with Lime,” Abstract Accepted for Publication in the 2nd National Congress on Civil Engineering, 2NCCE, University of Science and Technology, Iran.
6. Haghi, A. K. and M. Arabani (2005) “Interaction of Expanded Polystyrene Beads and Polyamide 66 Yarns with Components of Cement–A New Composite,” Accepted for Publication, Asian Journal of Chemistry.
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10. Tuncer, E. R. and A. A. Basma (1991) “Strength and Stress-Strain Characteristics of a Lime-Treated Cohesive Soils,” Transportation Research Board (TRB), 1295, pp. 70-79.
11. Nagih, M., M. El-Rawi, and M. Y. Al-Samadi (1995) “Optimization of Cement-Lime-Chemical Adhditives to Stabilize Jordanian Soils,” Journal of Islamic Academy of Sciences, Vol. 8, No. 4.
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13. American Society for Testing and Materials (1994) “Standard Guide for Evaluating Effectiveness of Chemicals for Soil Stabilization,” D 4609-94 Annual Book of Standards, Vol. 04 (08) PA, USA.

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