Mechanical Properties of Scrap Tire Crumbs-Clayey Soil Mixtures Determined by Laboratory Tests

Some laboratory tests, such as Proctor compaction test, direct shear and cyclic direct shear tests, consolidation test, and unconfined compression test, were performed on scrap tire crumbs-clayey soil mixtures to study the mechanical properties of the mixtures. *e results show that (1) the maximum dry unit weight and the corresponding optimum moisture content of the mixtures decrease rapidly with the increase of scrap tire crumbs content (CSTC), showing good potential for using the mixtures as lightweight fill material; (2) it is not possible to prepare the mixture when CSTC exceeds 30% due to the occurrence of cracks in the mixture after removing from a mould; and (3) the shear strength of mixtures approximately increases by 20% when CSTC is up to 30%, while the residual strength decreases by 15%, compared with that of pure clayey soil. During shearing, the dilation of the mixtures occurs, particularly under the condition of a high CSTC and a low vertical pressure. Besides, the compressive strength and consolidation settlement of the mixtures decrease with CSTC increasing.*e results indicate that it is possible for scrap tire crumbs used to improve clayey soil, which is suitable to act as a fill material.


Introduction
Six hundred million scrap tires were generated in China by 2015, while two hundred million scrap tires still remained in stockpiles [1].Huge quantities of scrap tires were generated in other countries as well.For example, nearly three hundred million scrap tires were generated in the European Union by 2015 [2]; two hundred million scrap tires were generated in the US [3]; and one hundred and fifty million scrap tires were estimated to be generated per year in India [4].Land filling or stockpiling of scrap tires is prone to cause environmental problems, such as (a) largely occupied spaces, (b) health hazards (scrap tires will provide a natural breeding place for diseases caused by insects and rodents), and (c) air pollution (stockpiling of scrap tires could pose a major fire risk and then cause air pollution).erefore, there exists an urgent need to explore new and beneficial ways to recycle or reuse scrap tires.
Scrap tire pieces have been applied in civil engineering, such as using as embankment fill material [5][6][7][8], drainage material to collect leachate landfill [9], retaining wall backfill material [10,11], and bridge abutment fill material [12].Laboratory testing and field demonstration of the application of sands mixed with scrap tire pieces with various shapes and sizes have shown that the ductility and shear strength of sands could be significantly enhanced [13].Furthermore, the mixture has lower unit weight and higher compressibility, compared with clean sands [14].
It is concluded that scrap tire pieces with various shapes and sizes can be successfully used for modification of sand.However, it is necessary to study effects of amounts of scrap tire pieces on the mechanical performance for clayey soil.Hasan et al. [15] and Sellaf et al. [16] investigated the mechanical properties of scrap tire chips-cohesive clayey soil mixtures.e results revealed that it is possible to use clayey soil mixed with tire chips as a fill material.Attom et al. [17] found that increasing the content of scrap tire shreds can improve the shear strength and permeability of clayey soil but can cause a decrease in the plasticity index, the expansion pressure, and expansion potential, compared with that of pure clayey soil.Kalkan [18] concluded that the silica fume-scrap tire rubber ber mixture materials can be used to improve clayey soils.
e mechanical properties of scrap tire crumbs-clayey soil mixtures, such as shear strength, residual shear strength, uncon ned compressive strength, and consolidation settlement, were explored by a series of laboratory tests.ree scrap tire crumbs contents (C STC ), 10, 20 and 30%, were used in this study.
e e ects of the C STC on the mechanical properties of the mixtures were discussed.

Materials
2.1.Soil Specimens.Figure 1 shows the materials used in the experiments.e components of clayey soil are shown in Table 1.Besides, the engineering parameters of clayey soil were measured according to the Standard for Soil Test Method (GB/T50123) [19].Such parameters are summarized in Table 2.

Scrap Tire
Crumbs.Scrap tire crumbs were obtained from a local rubber processing factory.It should be noted that the steel and u in the scrap tire crumbs have been removed.e scrap tire crumbs vary from 0.5 to 4.5 mm in diameter.e speci c gravity of scrap tire crumbs was tested to 1.15 according to ASTM C127 (2007) [20].
e sieve analysis of scrap tire crumbs was determined according to ASTM D422-63 (1998) [21].e result of the sieve analysis is shown in Figure 2.

Experiment Descriptions
3.1.Sample Preparation.Hasan et al. [15] found that the shear strengths increase up to 30% for ne tire chip and 20% for coarse tire chip-cohesive clayey soil mixtures.e diameter of ne tire chips is less 0.425 mm, and the diameter of coarse tire chips is between 2 and 4.75 mm.Moreover, it is not possible to prepare the mixture when C STC exceeds 30% due to the occurrence of cracks in the mixture after removing from a mould.erefore, the content of scrap tire crumbs used in this study is less than 30%.e scrap tire crumbs content, C STC , is de ned as the ratio of weight of scrap tire crumbs to the total dry weight of the mixture [15].e process of preparing the mixtures includes three steps: Step 1: the clayey soil was oven-dried at approximately 65 °C [19], and then the soil mass was ground to particles of less than 0.05 mm in diameter.
Step 2: the mixtures were compacted at the optimum moisture content to obtain the maximum dry unit    Advances in Materials Science and Engineering weight.For the mixtures with various C STC , weights of needed clayey soil, scrap tire crumbs, and water were calculated, respectively.e dry clayey soil particles and dry scrap tire crumbs were mixed uniformly in a laboratory mixer.Water needed was then added in the laboratory mixer until the optimum water content of the mixture is reached.
Step 3: the mixture was compacted in three equalthickness layers with a hammer that delivers 25 blows to each layer.e hammer has a mass of 2.5 kg and has a drop of 30.5 mm. e mixture specimens were obtained using a standard ring sampler, and the specimens were saturated in a vacuum pump.
To determine the maximum dry unit weight and the corresponding optimum moisture of the mixtures, the Proctor compaction tests were carried out according to ASTM D698-78 (1989) [22].e variations of maximum dry unit weight and the corresponding optimal moisture content for the mixtures are given in Figure 3.It is shown that the maximum dry unit weight and the corresponding optimum moisture of mixtures are less than that of pure clayey soil and decrease rapidly with C STC increasing.It is similar to that found by Kalkan [18] and Akbulut et al. [23], who mixed clayey soil with scrap tire rubber ber.e reason of the reduction in maximum dry unit weight of the mixtures is the low density of scrap tire crumbs [24].In addition, bibulous rate of the scrap tire crumbs is very low compared with that of the clayey soil, resulting in the decrease of optimum moisture content for the mixtures.

Direct Shear Tests.
Figure 4 shows the direct/residual shear test apparatus (ShearTrac II) used in this study.is apparatus is capable of performing the consolidation and shearing phases of a standard direct shear test and cyclic   e direct shear tests were carried out according to the procedure described by ASTM D3080-98 (2003) [25].e vertical pressures of 50, 100, 200, and 300 kPa were applied.During shearing, the shear rates were kept 0.1 and 0.8 mm/min.e shear strengths, shear parameters (cohesion and the angle of internal friction), and volume changes of the mixtures were derived from test results.

Cyclic Direct Shear Tests.
e cyclic direct shear tests were carried out to elucidate the in uence of C STC on the residual shear strength of mixtures with the apparatus shown in Figure 4. e mixtures were placed in the shear box; after about 12 hours (to allow for consolidation), shearing was recommenced at a rate of 0.1 mm/min under the vertical pressure of 50, 100, 200, and 300 kPa, respectively.

Consolidation Tests.
One-dimensional consolidation tests for the mixtures using incremental loading were conducted with the apparatus shown in Figure 4. e mixtures were consolidated at 50, 100, 200, and 300 kPa, respectively, and each vertical pressure increment is maintained until excessive pore water pressure completely dissipates.

Uncon ned Compression Tests.
e mixture specimens are 39.1 mm in diameter and are trimmed to 80 mm height.Compressive strengths of the mixtures with the C STC of 10%, 20%, and 30% were obtained using a strain rate of approximately 3%/min under uncon ned compaction conditions.5 shows some typical curves of the shear stress versus shear strain for the test specimens, where the shear stress has a general tendency to increase with the shear strain increasing.It is similar to the researches by Hasan et al. [15] and Tang et al. [26].In addition, it should be noted that the shear strength of the mixture is equal to the shear stress at the shear strain of 10% [15].6 and 7.In Figure 6, each point represents the shear strength of mixtures under a certain vertical pressure, where the shear strengths of mixtures increase with the increase of vertical pressure.Besides, an approximately linear correlation between the shear strength and vertical stress was found in Figure 6.It is also found that C STC has a strong in uence on the shear strength of the mixture.e shear strength approximately enhances by 20% when C STC is up to 30%.However, when C STC is less than 10%, the shear strength of mixtures decreases compared with that of pure clayey soil.

Results and Discussion
e shear strength parameters of mixtures, such as the cohesion and the angles of internal friction, were determined to further analyze of the e ect of C STC on the shear strength of mixtures.e cohesion is calculated to be 29.15,28.11, 34.61, and 36.74 kPa for pure clayey soil and the mixture with C STC of 10, 20, and 30%, respectively.Moreover, the angle of internal friction is calculated to be 24.46 °, 23.38 °, 24.18 °, and 25.83 °for pure clayey soil and the mixture with C STC of 10%, 20%, and 30%, respectively.e relationship between shear strength parameters and C STC is illustrated in Figure 7, showing that C STC has a signi cant in uence on the cohesion of the mixture.However, the angle of internal friction of mixtures does not change signi cantly with C STC increasing.With the increase of C STC , there is an increase in the friction between clayey soil particles and scrap tire crumbs, as well as the friction between scrap tire crumbs and scrap tire crumbs.However, the value of C STC is small (≤30%) in this study.Furthermore, the dilation of mixtures during shearing will weaken the friction.erefore, the angle of internal friction does not change signi cantly with the increase of C STC .for mixtures under vertical pressure of 200 kPa.It indicates that there is no peak shear stress observed in the shear stressshear strain curves.erefore, the shear stress at the shear strain of 10% can be used as the shear strength of the mixtures.e shear strengths of mixtures with various C STC are given in Table 3, and the Mohr-Coulomb failure envelops for mixtures are shown in Figure 9. e results show that when the vertical pressure is less than 100 kPa, the shear strength of mixtures does not vary significantly with the decrease of shear rate.However, when the vertical pressure exceeds 100 kPa, the shear strength significantly increases with the shear rate increasing.

Effects of Vertical Pressure and C STC on Volumes of
Mixtures.Figure 10 shows the volume changes of the mixtures (in height) versus shear strain for the mixtures and possible specimen volume compression (with a "−" sign) and dilation (with a "+" sign) shown in this figure.It was found that, when the vertical pressure is 50 kPa, the clayey soil and the mixture with C STC of 10% compress during shearing.However, the mixture with C STC of 20% initially compresses and then begins to dilate after the shear strain exceeds 4.5%.Furthermore, with C STC of 30%, the mixture compressed at first and then began to dilate after the shear strain of 5% to 6%. is shearing dilation for scrap tire rubber shreds-soil mixture was also observed by Hasan et al. [15].However, when the vertical pressure is 300 kPa, during shearing the clayey soil and the mixtures with C STC of 10 and 20% are compressed.It can be concluded that during shearing, the dilation of mixtures occurs, particularly under the condition of a high C STC and a low vertical pressure.

Evaluation of Cyclic Direct Shear Tests
Results. e residual shear strengths of mixtures were obtained from cyclic direct shear tests.In addition, the volume changes of mixtures were measured during cyclic direct shearing.e test results for 48 specimens are presented in Figures 11-13 and Table 4. Figure 11 presents the variations of shear stress

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Advances in Materials Science and Engineering versus shear strain for mixtures under vertical pressure of 200 kPa.Figure 12(a) presents that the shear stress of mixtures increases initially, after reaching a maximum value, it decreases with the shear displacement increasing, indicating a strain-softening phenomenon for the mixtures.It also can be found that the peak strength of mixtures enhances with the increase of C STC .Conversely, the residual shear strengths of mixtures rapidly decrease with C STC increasing.e residual shear strength of the mixture with C STC of 30% decreases by about 15%, compared with that of pure clayey soil.
e reason is that the volume of void in the mixture is larger than that of the pure clayey soil.
Figure 12(b) shows that, during cyclic direct shearing, the vertical displacements decrease initially and then increase with increasing the shear displacement.It can be also observed that the mixtures can e ectively limit the vertical deformation under the same shear rate and the same vertical pressure, compared with that of pure clayey soil.
e envelopes of the residual shear strength for the mixtures are shown in Figure 13.e results reveal that the residual shear strengths of mixtures gradually decrease with C STC increasing.However, when the C STC is larger than 20%, the e ect of the C STC on the residual strength of the mixture can be neglected.
It can be also concluded from Table 4 that the required shear displacement to obtain the residual shear strength of mixture increases with increasing the C STC value.As the C STC increases from 10 to 30%, the corresponding maximum shear displacements increase by 45.8%, 79.2%, and 102%, respectively, compared with that of pure clayey soil.Furthermore, changes in the moisture contents of the mixtures were measured (Table 4).It is concluded that the moisture content increases by an average of 1.3%, compared with the initial moisture content of the shear surface.
e increase of moisture content is caused by the increase of the void ratio of mixture located in shear zone during cyclic shearing.

Evaluation of Consolidation Tests Results.
e estimation of consolidation settlement is one of the most important  procedures in the design of soft ground improvement projects [17].Figure 14 shows the changes of consolidation settlement for pure clayey soil and the mixtures under various vertical pressures.When the vertical pressure is less than 100 kPa, the consolidation settlement of mixtures does not vary signi cantly with C STC increasing.However, as the vertical pressure is greater than 100 kPa, the settlements of the mixtures with C STC of 10, 20, and 30% decreased approximately 25%, 12%, and 8%, respectively, compared with that of pure clayey soil.
e reason of the reduction in consolidation settlement for mixtures is that, with the increase of C STC , the mixtures tend to be more elastic and resilience.

E ect of C STC on Compressive Strength of Mixtures.
For each compression test under the undisturbed, remolded, and compacted conditions using a strain-controlled axial load, the compressive strength of mixtures, which is taken as the load per unit area at 15% axial strain, can be obtained.e variations of axial stress for the mixtures with axial strain increasing are plotted in Figure 15.e results from Figure 15 present that the uncon ned compressive strength of these specimens was decreased from 111 to 90 kPa, from 111 to 69 kPa, and from 111 to 66 kPa for pure clayey soil and mixtures with C STC of 10%, 20%, and 30%, respectively.It is pointed out that, when C STC is 20%, there is an approximately 40% decrease in compressive strength of the mixture compared with that of pure clayey soil.However, when C STC exceeds 20%, the e ect of C STC on the compressive strength is weak for the mixtures.
e decrease in the compressive strength of mixtures with C STC increasing is attributed to the presence of scrap tire crumbs, which causes the increase of contact between scrap tire crumbs, resulting in higher resilience, higher deformation, and less strength.

E ects of C STC on the Failure Modes of Mixtures.
e failure modes of pure clayey soil and the mixture with C STC of 10% by uncon ned compression tests are presented in Figures 16(a) and 16(b), respectively.Figure 17 shows the failure modes of the mixture with C STC of 20%; this failure mode is similar to that of the mixture with C STC of 30%.As shown in Figures 16(a

Conclusions
A series of laboratory tests were performed on scrap tire crumbs-clayey soil to study the mechanical properties.e following conclusions can be obtained: (1) Maximum dry unit weight and the corresponding optimum moisture content of the mixtures decrease with increasing the C STC value.Scrap tire crumbs-clayey soil mixture has the advantages of higher shear strength, and lower density and settlement compared with that of pure clayey soil. is mixture can be used in many geotechnical applications, such as backfills behind retaining structures and embankments on soft compressible soil.

Figure 3 :
Figure 3: Variations of dry unit weight and the corresponding optimal moisture content for mixtures.

Figure 5 :
Figure 5: Variations of shear stress versus shear strain for mixtures.
4.1.Direct Shear Behavior of the Mixture 4.1.1.E ects of Vertical Pressure and C STC on Shear Strength.Figures 5-7 compare the results of direct shear tests of pure clayey soil with those of the mixtures. Figure
Figure 8  shows the variations of shear stress with the increase of shear strain

Figure 9 :Figure 10 :
Figure 9:e shear strength of mixtures under di erent shear rates.

Figure 12 :
Figure 12: Plots of shear stress and vertical deformation of mixtures against shear displacement for mixtures under vertical pressure of 200 kPa.(a) Shear stress versus shear displacement.(b) Vertical displacement versus shear displacement.

( 2 )
For a given C STC , the direct shear strength of the mixtures increases with the increase in vertical pressure and decreases with the decrease in shear rate.Moreover, the dilation of the mixtures occurs during shearing, particularly under the condition of a high C STC and a low vertical pressure.(3)Residual shear strength, compressive strength, and consolidation settlement decrease markedly with C STC increasing, while start to decrease slightly when the C STC exceeds 20%.

Figure 16 :
Figure 16: Inclined plane failure of (a) clayey soil and (b) mixture with C STC of 10%.

Figure 17 :
Figure 17: Bulging failure of mixture with C STC of 20%.

Table 1 :
Components of clayey soil.

Table 2 :
Some properties of clayey soil.

Table 3 :
E ects of shear rates on shear strength of mixtures.

Table 4 :
e results of repeated direct shear tests for mixtures.