Thermal conductivity of reinforced soils:A literature review

Muge Elif Orakoglu ,JianKun Liu

1.School of Civil Engineering,Beijing Jiaotong University,Beijing 100044,China

2.Technical Education Faculty,Department of Construction,Firat University,Elazig,Turkey

1 Introduction

Soil thermal conductivity is defined as key parameter in heat flux in a unit time through a unit cross sectional area of soil under a unit temperature gradient.The constitutive equation for thermal conduction based on the temperature gradient was presented by Fourier in equation(1)(Fourier,1955;Tritt,2004):

where,dT/dxis the temperature gradient,Ais a cross-sectional area,Qis the heat flow,andλis thermal conductivity of soil.

Soil thermal conductivity plays a significant role in heat flux.Several methods are available for determining soil thermal conductivity,and those pertaining to reinforced soils are divided into two categories:steady-state and transient state.The steady-state method is explained by Fourier’s solution based on the heat conduction equation in a known heat flux and a known thermal gradient.There are various available techniques of the steady state method,namely,guarded-hot-plate apparatus,heat flow meter apparatus,guarded comparative-longitudinal heat flow technique,rhometer apparatus,and the Rapid k method (Mitchell and Kao,1978).The transient state method is based on the soil thermal conductivity while the temperature of the reinforced sample changes in a period of heating or cooling.The transient methods include the thermal needle method,which is widely used in soils due to its simplicity and commercial availability (Barry-Macaulayet al.,2013).Table 1 summarizes the literature pertaining to methods used in measuring soil thermal conductivity.

A number of studies suggest that determining soil thermal conductivity through in-situ measurement procedures is expensive and time-consuming.That is why many empirical relations have been developed to estimate the soil thermal conductivity (λ) based on effective parameters such as soil texture,moisture content,temperature,porosity,and saturation (Camillo and Schmugge,1981;Campbell,1985;Sundberg,1988;Beckeret al.,1992,Tarnawski and Leong,2000;C?té and Konrad,2005).

Usowiczet al.(2006) developed a statistical-physical model based on a non-linear regression equation using the parameters of water content,bulk density,organic matter,and content of quartz and other minerals (as determined based on content of sand,silt,and clay).Peiet al.(2013) established a thermal conductivity model of soil-rock media by a multiple linear regression method (Equations(2)and(3)) based on the saturation,temperature,porosity,dry density,and water content.They compared this with the other three primary general models,namely,the BP neural model,the physical model and the CK model,and they developed a general regression model based on the apparent heat capacity theory specifically relevant to in cold regions.The main multiple linear model equations (Zhanget al.,2012b;Zhang,2003) are as follows.

For the unfrozen state:

For the frozen state:

whereλuis thermal conductivity of unfrozen soil,wis water content,ρdis dry density,nis porosity,Sris the saturation ratio,andλfis thermal conductivity of frozen soil.

Table 1 The methods used for measuring soil thermal conductivity

2 The thermal conductivity of reinforced soils

As is known,reinforced soils are solid composites which consist of a combination of two or more different structures in which their identities are preserved.The advantages of newly produced composites are higher specific strength,stiffness,and lower thermal conductivity (Kazemianet al.,2010).Reinforced soil techniques are based on the example of the bricks made from clay reinforced with straw by the Egyptians in the third millennium BC (Mathews and Rawling,1994).

Various soil types blended with waste-created bricks,fibers,geosynthetics,and polymer have been examined to determine the effects on thermal properties.Table 2 shows various reinforcement materials for blending soils.

Polyesters have the widest range of use because of their simple handling and cheap price.Many researchers have focused on determining the different thermal parameters of polyester composites with variable fibers.For example,Sultanaet al.(2013) studied sand polyester composites,and their experiments showed that the thermal conductivity decreased with an increase in sand content from 10% to 60%.

Al-Malah and Abu-Jdayil (2007) studied bentonite(natural clay)-based unsaturated polyester composites.They determined the lowest thermal conductivity of an unsaturated polyester composite with 50% filler content and 40% polyester content in weight.

Table 2 Various soil reinforcement materials

Processed types of agriculture waste/materials have been widely tested by blending them (various types of soils,cement,sand,and water) to create new composites.Rautet al.(2011) reviewed the use of waste materials in different compositions to develop waste-created bricks,and they determined that this approach was economical,environmentally sustainable option.Zhanget al.(2012a) revealed the thermal conductivity of soil in earthen ruins by adding different concentrations (5%,10% and 15%) of potassium silicate (PS).Their test results showed that the larger the PS concentration,the lower the thermal conductivity (Figure 1).

Bederinaet al.(2007) studied the effect of the addition of wood shavings (wastes of woodworking activities) on thermal conductivity of sand concretes made from dune sand and river sand.They also compared their experimental results with a theoretical approach based on the auto-coherent method.They found that an increase in waste contents decreased the thermal conductivity of sand concrete.Additionally,Xu and Chung (2000) determined the thermal conductivity of cement pastes and mortars decreases with the addition of sand,in contrast to opposite effects of silica fume addition.Rimet al.(1999) studied the thermal and mechanical properties of a clay-cement-wood composite with different proportions of wood aggregates.Their test data showed that an increase wood aggregates (from 10% to 50%) decreased the thermal conductivity (from 0.24 to 0.08 W/(m·K)) (Figure 2).

The use of natural cellulose fibers such as flax,palm,sisal,kenaf,straw,and jute in polymeric matrices in soil reinforcement methods has gained a great of attention in civil engineering applications due to their desirable properties (Alamriet al.,2012).Millogoet al.(2014) determined the engineering and thermal properties of pressed adobe blocks (PABs) fabricated withHibiscus cannabinusfibers.Their test results showed that thermal conductivity decreased with increasing amounts and lengths of the fibers.There was a greater decrease in the thermal conductivity with long fibers(60 mm in length) than with 30 mm long fibers (Figure 3).Khedariet al.(2005) produced soil-cement blocks reinforced with coconut fibers in 18 different mix ratios.Their results showed that the thermal conductivity decreased when the fiber content increased(Figure 4).

Khedariet al.(2001) figured out the effects on thermal conductivity of 18 coconut fiber specimens and 12 durian specimens prepared with different fiber lengths,grain sizes of sand,and mixing proportions(cement,sand,fiber).They found that packing fibers in composites creates voids in the composites,and short-length fibers are more difficult to coordinate and pack than longer ones.Thus,the short fiber lengths create many voids in composites,resulting in lower thermal conductivity (Figures 5,6).

Figure 1 Change of thermal conductivity with addition of potassium silicate to the soil of earthen ruins (Zhang et al.,2012a)

Figure 2 Influence of weight percentage of wood aggregates in a clay-cement-wood composite on thermal conductivity (Rim et al., 1999)

Figure 3 Change of thermal conductivity with adding coconut fibers to soil-cement block(Khedari et al., 2005)

Figure 4 Change of thermal conductivity with adding Hibiscus cannabinus fibers to pressed adobe blocks (Millogo et al., 2014)

Figure 5 Change of thermal conductivity in composites with adding young coconut fibers of different lengths and different sand sizes (Khedari et al., 2001)

Figure 6 Change of thermal conductivity in composites with adding durian fibers of different lengths and different sand sizes (Khedari et al., 2001)

Gorhan and Simsek (2013) investigated the thermal conductivity of clay bricks made with rice husks.Their results showed that the thermal conductivity coefficients increased in samples with added rice husks and increasing firing temperatures as compared to reference samples.Also,the thermal conductivities of the samples with added coarse rice husks were lower than those of the samples with added ground rice husks.Alami (2013) determined the thermal conductivity of two composites different filler materials and different dispersed-phase materials.Clay-fronds-crushed olive pits and clay-straw-olive husks were also tested and the results revealed that the clay-straw-husks specimen had a great reduction in thermal conductivity.Balet al.(2013) revealed a decline in the thermal conductivity of laterite samples (10cm×10cm×3cm) with added millet waste.

Baniassadiet al.(2011) investigated the mechanical and thermal properties of polymer (polyamide resin)/nanoclay (organo-modified montmorillonite) composites with added nanoclay.Their experimental test results showed that the addition of nanoclay increased the thermal conductivity of polyamide resin,and the higher the amount of nanoclay,the higher the thermal conductivity.Additionally,Hostleret al.(2009) studied the thermal conductivity of clay-based aerogels.The thermal conductivity of samples was determined with and without added poly (vinyl alcohol).The test data revealed that the polymer decreased thermal conductivity,and this reduction was related to a more complex interaction between the polymer and the clay.

3 Conclusions

This paper provides a review of thermal conductivity in soils reinforced with different materials.Fibers,polyester composites,agricultural wastes/materials and clay nanoparticles have been successfully used in various soils to reduce the thermal conductivity which enables them to be used as a thermal insulator.

Clearly the thermal conductivity of soils reinforced with various natural and synthetic fibers and agricultural waste/materials decreases with increased added material contents,added material surface frictions,and existing chemical binders.Conversely,the thermal conductivity increases with addition of nanoclay to polymer/nanoclay composites,increased fiber contents and higher firing temperature.

Our literature review shows that,both experimental and modeling,adding various fibers and materials produces lower soil thermal conductivity.Future research should assess reinforced soils with more waste materials and various fiber matrices (two or more) and polymer composites to address problems arising from clumping effects,fiber aspect ratios and weight fractions,gradation,and particle shapes and sizes.

This research is supported by the National Natural Science Foundation of China (Nos.51378057,41371081,and 41171064) and the National 973 Project of China (No.2012CB026104).

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