اثر چرخه انجماد – ذوب بر خواص مکانیکی رس با آهک تثبیتی

ABSTRACT Effect of freeze–thaw cycling on the mechanical properties of lime-stabilized expansive clays

In many cases, the quality of the materials available for civil engineering works needs to be improved. An established technique developed for reusing in situ non classified soils for construction work is to improve the soil properties. Such improvements are also motivated by economic and environmental reasons. The stabilization of clayey soils with hydrated lime, quicklime, cement or other binders leads to a material with improved mechanical and physical properties.

Quicklime, which is non-hydrated calcium oxide, absorbs water from the moist soil, thereby changing into calcium hydroxide that hardens the links between soil particles and improves the soil strength (NLA, 2004). The immediate reaction that takes place between lime and clay is cationic exchange between the ions associated with the surfaces of the particles and the calcium ions in the lime, leading to flocculation (Bell, 1996). This reaction is succeeded by a long-term one which can take months or years to complete, depending on the rate of chemical breakdown and the hydration of the silicates and aluminates in the clay. This reaction is known as the pozzolanic reaction and results in the formation of a cementitious material. A number of studies have been performed in order to analyze the behavior of lime-treated clays, but few have been carried out on the durability of the material, in particular the effect of freeze–thaw (F–T) cycles (Al-Kiki et al., 2011; Al-Mukhtar et al., 2012; Ingles and Metcalf, 1973; Locat et al., 1996). The behavior of clay materials under F–T cycles is a complex phenomenon as it depends on several parameters such as the type of clay mineral (kaolinite, illite or montmorillonite), their degree of saturation, the duration and the amplitude of the F–T cycles (Cui et al., 2014; Svensson and Hansen, 2010; Wang et al., 2007). The effect of the freeze may be irreversible for some materials that change texture completely or reversible as in the case of montmorillonite (Svensson and Hansen, 2010). Heller-Kallai (2013) showed that due to the effect of temperature, some minerals become amorphous, while others recrystallize to new phases. The applied gradient of temperature alters the structure and the properties of clay materials. The degree of saturation of the material itself plays an important role in this change (Birgersson et al., 2008).

In cold regions, the mechanical properties of soils are severely affected by the ice lenses created between particles during freezing and the excess of water during thawing (Konrad, 1989). These effects can substantially reduce the strength and bearing capacity of the foundation soil (Aldaood et al., 2014; Kamei et al., 2012; Wang et al., 2007). According to Lee et al. (1995) cohesive soils with a uniaxial compression strength (UCS) b55 kPa exhibit negligible freezing–thawing effects while soils with a UCS N103 kPa exhibit a decrease of more than 50% in resilient modulus due to the freezing–thawing process. Parsons and Milburn (2003) showed that weight loss in the soil samples ranged from 2 to 41% after 12 F–T cycles. Soil weight loss is induced by the soil-particle separation on a microscopical scale due to ice pressure within the soil pores. The volume change during the freezing of water increases the pressure within the soil porosities and reduces particle cohesion and soil strength, making the soil more erodible. Cement-treated soils experienced the least weight loss, with reductions in mass of 2 to 7%. Lime-treated samples had the greatest losses, particularly for lean clay. Fly ash-treated soils had intermediate losses that ranged from 7 to 19%. Wang et al. (2007) noted that by increasing the number of F–T cycles, there was a slight and continuous decline in the resilient modulus and the failure strength. For a soil under a constant confinement pressure, subjected to different numbers of F–T cycles, the failure strength decreased slightly at the beginning then increased to a certain level and remained constant without being influenced by the number of F–T cycles. The particle rearrangement caused by the applied pressures closed the cracks produced by the F–T action and improved the soil strength. Khoury and Zaman (2007) investigated the effect of freeze– thaw durability of aggregate stabilized with various additives, including cement kiln dust, fly ash and fluidized bed ash. Results showed that the resilient modulus (modulus of elasticity under cyclic loads) decreased as the number of F–T cycles increased. This behavior was attributed to the fact that pozzolanic reactions are prevented due to F–T cycles. BinShafique et al. (2010) found that F–T cycling did not change the plasticity of fly ash stabilized soils. However, the unconfined compressive strength decreased by about 20% for stabilized soft soils and by about 40% for stabilized expansive soils. Even after losing strength due to F– T cycling, the strength of stabilized soils was still at least three times higher than that of the unstabilized soils. Al-Kiki et al. (2011) showed that natural clayey soil did not sustain the effects of environmental conditions, but that after stabilization with lime, the mechanical characteristics improved. Similar observations were documented by Aldaood et al. (2014).

In view of the lack of references reporting on the durability of lime treated clayey soils subjected to F–T cycles, this paper analyzes the behavior of two clays with two different plasticities: kaolinite and bentonite. These materials are studied in a non-saturated condition at the optimum water content. The objective of this research is to investigate the effect of the curing time and the percentage of lime treatment on frost resistance and swelling pressure. In addition, the intensity and the kinetics of degradation versus the number of F–T cycles were analyzed. Moreover, the porosity and the pore size distribution of some samples were investigated by a mercury porosimetry test in order to analyze the effect of lime treatment and F–T cycles.