This material characteristic of pavement materials is very important as the lifespan and durability of the pavement is largely dependent on its deformation characteristics which can be evaluated from its stiffness properties. The stiffness property is also a key parameter in the mechanistic approach to flexible pavement design.
The subbase is a part of the flexible pavement structure and its major structural functions is to support the base and surface layer and also to spread axle loads to the subgrade such that the subgrade is not overstressed. Due to its importance within the pavement structure, its behaviour when subjected to traffic loading is crucial to the design and the overall performance of the pavement. The fines content among other factors are known to influence the stiffness of pavement materials which includes the subbase layer.
The aim of the study is to evaluate the effect of fines content on the stiffness of subbase.
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In order to achieve the above aim, a number of objectives have been set out. These are:
- Characterize a standard and modified subbase material. This is to determine basic material characteristic prior to main testing.
- Determine the engineering properties under both static and dynamic loading (resilient modulus).
- Evaluate the impact of fines content on pavement design.
In the second semester, the following has been achieved:
- A fundamental understanding of the subject of the project was developed through strategic review of resources. This has lead to the development of a preliminary literature review on the project topic as presented in the next chapter.
- A concept of the proposed soil property tests to be done in laboratory and type of materials to be used were determined. Although, the specific laboratory test procedure has not been concluded.
- A concept into the scope of the project was determined. This is evident in the aims and objectives presented in the preceding sections.
Some of the difficulties encountered were in the area of searching and sorting resources for the preliminary studies into the project topic. This was partly due to the large volumes of research work that had been carried out on the related topics and the lack of clear understanding and focus of the project.
Schedule of project activities
The activities during the summer term are shown as a summary in the Gantt chart and are also itemized below:
- Index properties - Particle size distribution, Liquid limit, plastic limit and plasticity index
- Laboratory compaction characteristics - optimum moisture content, maximum dry density, CBR
- Stiffness tests - resilient modulus tests
- Variation of fines and moisture content
- Combination of changes in fines and moisture content
- Laboratory test results
- Inferential result analysis
- Formulation of discussion concepts and report conclusions
- Preparation of draft final report
The surface materials of roads i.e. Asphalt and concrete usually overlies on one or more layers of engineered soil. A typical flexible pavement is made up of base and subbase layer of granular materials and subgrade underlying a surface layer of hot-mix asphalt concrete (Figure 1). Other materials like recycled concrete aggregates (RCA), Fly ash etc, have also in recent times been used as pavement materials.
The overall stiffness of the pavement structure is greatly affected by the composition of the underlying material layers, which are primarily made up of gravel and sand. These material layers perform different functions within the pavement structure and if these functions were missing or inadequate the overall performance of the pavement and road would be compromised. Owing to the critical effect of the underlying material on the performance of the pavement, the behaviour of these materials under traffic loading needs to be understood.
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The pavement structure layers are usually characterized by their resistance to deformation under load. This can be either a measure of their strength i.e. failure stress of the material or stiffness i.e. the relationship between stress and strain in the elastic range. The lifespan and durability of the pavement is largely dependent on its deformation characteristics which can be evaluated from its stiffness properties. Hence, the stiffness property is very important in the characterization of pavement materials.
The subbase is a part of the pavement structure and its major structural functions is to support the base and surface layer and also to distribute wheel loads to the subgrade such that the subgrade is not overstressed. Its position within the pavement structure depends on the type of pavement i.e. in a flexible pavement it is the layer between the base and the subgrade material while in the case of a rigid pavement, it is the layer between the concrete slab and the subgrade. In addition to functioning as part of the pavement structure, it slows down the intrusion of fines from the subgrade into pavement structural layers, minimizes damage due to frost action, serves as a drainage layer by preventing the accumulation of free water within or below the pavement structure and provides a working platform for construction equipment in places where the subgrade is very soft. Due to its importance within the pavement structure, its behaviour when subjected to traffic loading is crucial to the design and the overall performance of the pavement. Typical subbase materials are usually granular consisting of crushed aggregates and sand. Other materials like recycled concrete aggregates (RCA), PFA etc have also been used successfully as subbase.
Some characteristics of typical subbase materials are presented below:
Subbase materials are usually made from unbound compacted granular material or a similar material bounded with cement. They usually consist of crushed rock with appropriate proportion of coarse grained soils. The stability of the soil-aggregate mix is dependent on the particle size distribution, particle shape, relative density, internal friction and cohesion. The particle size distribution seems to be the most important of all the factors stated, particularly the proportion of fines to coarse grained particles. A balance in the proportion of fines to coarse particles needs to be achieved for an optimum performance of a subbase layer. For instance, an aggregate mix having little or no fines to fill the voids has a relatively low density, high permeability and non-frost susceptibility. It however has a low workability due to its non-cohesive nature. On the other hand, aggregate mix containing a greater proportion of fines would posses low density, low permeability and frost susceptible. This highlights the need to achieve a balance in the proportion of fines to granular materials in the aggregate mix.
Subbase aggregate mix is specified in terms of particle size distribution curve within which the proposed material must lie. Fig ... shows the grading requirements of AASHTO and Highway Agency.
The physical property of subbase materials is affected when the inter-grain contact in the form of a binder material is destroyed. The plasticity of the binder material which is the fines in the case of unbound subbase is therefore important. The MOT type 1 specifies that the material passing the 0.425mm sieve should be non-plastic while the AASHTO specification requires that the liquid limit and the plasticity index should not be greater than 25% and 6% respectively.
The permeability of subbase materials depends on the particle size distribution, type of coarse aggregate, type of binder and density of the aggregate mix. The permeability of granular materials decreases when soil binder is added to them due to the filling up of the void spaces between the relatively large particles. Cedergren (1972) suggested a clean and open graded material as criteria for permeability of base materials. The type of coarse aggregates may be considered in terms of physical and engineering properties of the parent rock of the aggregates. Aggregates made from rocks having clay minerals may exhibit low permeability characteristics when inundated. Due to the close packing (densification) of particles created by compaction, the permeability of the material would be influenced.
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There are no established requirements on the permeability of subbase materials. This is due to its dependency on other soil properties (grading, aggregate size, density etc) which are adequately controlled in the design and construction of subbase materials.
Stiffness and strength characterization
The subbase materials like the other pavement materials are characterized by their resistance to deformation under load. This can be either a measure of their strength i.e. failure stress of the material or stiffness i.e. the relationship between stress and strain in the elastic range. The behaviour of the subbase material as a whole under these conditions are necessary as the resistance of the material to deformation implies its capability to withstand traffic loads before reaching a critical point.
The lifespan and durability of the pavement is largely dependent on its deformation characteristics which can be evaluated from its stiffness properties. Hence, the stiffness property is very important in the characterization of pavement materials of which the subbase is a vital component. The California Bearing Ratio (CBR) test, Resistance Value (R-value) and elastic (resilient) modulus (MR) are the three commonly used pavement stiffness/strength characterizations.
Resilient modulus (MR)
The resilient modulus is a stiffness property of a material and it is basically an estimate of its elastic modulus except that the applied load is rapid and repetitive (cyclic). This material property better represents actual loading conditions pavement structures undergo and therefore used in the mechanistic analyses for predicting different distresses such as rutting and roughness. It is not a measure of strength, as strength is the stress needed to cause failure or break a material. The elastic modulus (E) represents a constant ratio of stress and strain i.e. stiffness can be determined for any solid material:
E=stress strain (1)
The concept of resilient modulus (MR) was first introduced by Seed, et al (1962) while researching on the fatigue failure in asphalt pavements. It was initially for characterizing the elastic response of subgrade soils in flexible pavements. Cyclic triaxial testing has been used in other classical geotechnical engineering applications. The peculiarity of the resilient modulus test lies in the fact that the cyclic load simulates transient loads which are well below the failure stresses for the sample of the pavement material. Thus, this material characteristic is useful in defining failure criteria for various distress modes such as repeated load (fatigue) cracking, permanent deformation, and thermal cracking effects. The use of MR is particularly becoming more preferred to other deformation properties due to the reliability of the output in terms of representing actual loading conditions. It being is used as an input parameter in the mechanistic-empirical approach to flexible pavement design.
Where d is the repeated (cyclic) deviator stress, 1 and 2 are the axial and confining stress respectively. The shear stress on the plane of the sample and the dynamic stress component in the horizontal direction are zero during testing. The resilient modulus is measured in a standard triaxial resilient modulus test (AASHTO T 292).
The resilient modulus, Mr is expressed as:
σd - Cyclic deviator stress εa - Axial strain
The factors which are known to influence resilient modulus include the stress state, deviator stress and confining pressure, moisture content, temperature, plasticity index, density and gradation.
Resilient modulus of granular materials
As stated earlier, typical subbase materials are granular materials hence, the stiffness behaviour of granular materials in terms of MR are discussed in this section.
Generally, after each application of load, granular materials experience some non-recoverable deformation as well as some resilient deformation. After the first few load applications, the resilient deformation increases more than the non-recoverable deformation. The deformation can be considered to be elastic when the magnitude of the cyclic load (when repeated for a large number of times) is relatively small as compared to the strength of the material, thereby producing a nearly complete recoverable deformation which is proportional to the load (Huang, 1993). This characteristic behaviour of granular materials is what the MR represents. Due to the unrecovered energy in the material, permanent strain is accumulated in the material due to repeated loading and unloading.
Aggregate Type and Particle Shape
Hicks and Monismith, (1971) reported a higher resilient modulus and better load distribution properties when crushed aggregate having angular to sub-angular shaped particles are used as compared to uncrushed gravel with sub-rounded or rounded particles. Lekarp et al (2000) showed that a higher MR was observed for rough particles. Janoo et al (2004) concluded that increase in resilient modulus with increasing bulk stress for large-scale tests at higher bulk stress was due to the sufficient confinement to hold the particles in place provided by high bulk stress, thereby amplifying the effect of aggregate angularity.
Zehgal (2009) while studying the impact of grain crushing on road performance, concluded that particle breakage resulted in a decrease of almost 50% in the resilient modulus of the material and an increase of 113-291% (depending on the state of density and stresses considered) in its permanent deformations. Hicks and Monismith (1971) reported a decrease in resilient modulus with increasing fines content when partially crushed aggregates were tested. On the other hand, the opposite effect was observed in the case of fully crushed aggregates. When the fines content was increased from 0 to10%, Barksdale and Itani (1989) noted a significant drop in MR of about 60%. However, Hicks (1970) reported that 2-10% variation in fines content resulted in a minor influence on MR.
Gradation and Grain Size
Thom and Brown (1988) observed that well-graded aggregates were a little less stiff than uniformly graded aggregates. Kolisoja (1997) also showed that the resilient modulus of aggregates having similar grain size distribution and the same fines content, increased with increasing maximum particle size. The particle to particle contact decreases as the size of the particle increases, thereby producing less total deformation and consequently higher stiffness.
The resilient modulus of granular materials both measured from laboratory and in situ tests have been found to be influenced by moisture content. A remarkable dependence on moisture content has been observed in previous researches, with increasing level of saturation producing decreasing MR (Lekarp et al, 2000).
Lekarp et al, (2000) while studying a range of well-graded unbound aggregates discovered that the stiffness tends to increase with increasing moisture content when the material is below the optimum moisture content. Evidently, this is due to the development of suction pressure within the material. However, a contrary effect was observed when the moisture content of the material is above the optimum moisture content. In addition, the combination of high degree of saturation and low permeability leads to high pore pressure, low effective stress, and therefore low stiffness and low deformation resistance (Barksdale, 2001). As saturation is approached, development of positive pore pressure due to rapid applied loads may occur.
Hicks and Monismith (1971) noted that the resilient modulus remained almost unaffected when the aggregate tested was completely crushed but it increased with relative density for the partially crushed aggregate. They found out that the effect of density on MR was lesser for fully crushed than for partially crushed aggregates. In addition, they reported that increment in the fines content of the granular material decreased the significance of changes in density on the resilient modulus.
A distinct increase in MR with increasing density was reported by Barksdale and Itani (1989) only at low values of mean normal stress while the effect of density was less obvious at high stress levels.
As the specimen density of a crushed limestone and gravel was increased from Proctor to modified Proctor density, an 80% reduction in total plastic strain in crushed limestone and a 22% reduction in gravel (Allen, 1973). Similarly, an average of 185% more permanent axial strain was observed from the behaviour of several granular materials when the material was compacted at 95% instead of 100% of maximum compaction density (Barksdale, 1972). Also, Thom and Brown (1988) found that the permanent strain varies with the compaction level. Hence, as the density of granular materials increases under cyclic loading, its resistance to permanent deformation appears to improve.
Kim et al. (2001) concluded that the variations in the modulus values with number of loading cycles and loading frequency were almost negligible in a practical sense. They also noted that the modulus value was not influenced by specimen size at the same density but decreased as the maximum particle size increased.
Resilient Behaviour of Subbase Materials
The subbase material referred to in this study is an unbound granular material comprising of crushed aggregate and sand in a predetermined proportion.
The prediction of the failure or distress parameter is one of the key considerations for a rational design for any pavement design (Yoder and Witczak, 1975). In order to achieve this, the stress distribution within the pavement sub-layers needs to be understood. There is no widely accepted fundamental design procedure in the pavement design industry hence, no single stress distribution theory accepted. The multilayered elastic theory system enjoys a great deal of engineering reliance and it is being used widely.
The analytical solution to the state of stress or strain of the multi-layered elastic system concept has several assumptions. They are: (1) the material properties of each layer are homogeneous; (2) each layer has a finite thickness except for the lower layer (subgrade layer), and all are infinite in the lateral directions. Other assumptions are, (3) each layer is isotropic; that is, the property at a specific point is the same in every direction or orientation, (4) full friction is developed between layers at each interface. Finally, (5) surface shearing forces are not present at the surface, and (6) the stress solutions are characterized by two material properties for each layer which are Poisson's ratio, µ and the elastic modulus E.
The stress states of sub-layers within the pavement changes when loaded, and therefore values of MR vary as it is a stress-dependent variable (Park and Lytton, 2004). Figure 4 shows a simplified stress distribution within a flexible pavement and the changes some elements of the inner pavement structure experiences in terms of both confining and deviator stress. The response of the layers under repeated wheel loads would depend on the MR characteristics of the sub-layers.
From previous researches, a general consensus on the main governing factor of resilient modulus of unbound subbase materials is the confining stress while the effect of the deviator stress is considered relatively small.
Based on this assumption, several confining stress models for MR of subbase materials have been proposed to account for its stress dependency. The k-θ model is a representative example, and is given as the following relationship (Hicks and Monismith, 1971):
Where θ = (σ1+ σ2+ σ3) = bulk stress; and K1 and K2 = model parameters. It assumes the Poisson's ratio to be a constant, but previous research has proved that Poisson's ratio varies with applied stresses. Another limitation of the model is that the effect of stress on the resilient modulus for is represented only by the sum of the principal stresses.
Conversely, a universal model that considers the effect of both confining and deviatoric stress on MR was proposed by Uzan (1992):
Where σd = deviatoric stress; PA = reference pressure to normalize stresses = 100 kPa; and Ku1, Ku2, and Ku3 = model parameters.
Also, another model putting into consideration the deviator stress was proposed by Nataatmadja (1992) and has been found to be very useful for ranking granular materials.
Where qr = repeated deviator stress; A and B = experimental coefficients.
Resilient Behaviour of Recycled Concrete Aggregates (RCA)
Nataatmadja and Tan (2001) concluded that the resilient response of a subbase material made with recycled aggregates was comparable to that made with natural aggregates. They also observed that the resilient behaviour of RCA depends on the strength of the original concrete, the amount of softer material in the recycled aggregates and the flakiness index RCA. In addition, they suggest that a well-graded RCA may produce a higher resilient modulus under low deviator stresses, as compared with other materials.
The stress-strain response of granular materials is strongly non-linear as they exhibit volume dilatancy in both resilient and total deformation under shear, thereby making their characterization complex (Uzan, 1999). Their response also depends on the stress level and stress history of the material.
From the literature review, it is evident that a general trend of decrease in MR with increasing fines content is observed (Hicks and Monismith, 1971; Zehgal, 2009). However, the magnitude of the impact of fines content on MR particularly, appears unclear (Barksdale and Itani, 1989; Hicks, 1970). Also, the significance of the effects of changes in density on MR decreases as the fines content in a granular material is increased (Hicks and Monismith, 1971). This observation was based on tests performed on aggregate mix in which the fines content increase was due to particle breakage i.e. simulating crushing due to field compaction. Thus, the likelihood of observing a similar outcome on aggregates in which the increase in fines content is based on the direct addition of fines to the aggregate mix is uncertain.
Resilient modulus decreases with general increases in the level of saturation. However, Lekarp et al, (2000) revealed that stiffness tends to increase with increasing moisture level when the material is below the optimum moisture content and it decreases when the moisture content is beyond the optimum moisture content. This may not be the case when the gradation of the material is altered by the addition of fines as the conclusion was based on tests performed on well-graded unbound aggregates.
With respect to unbound subbase materials, the confining stress is considered to be the main governing factor of its MR while the effect of the deviator stress is relatively small. In order to achieve a representative stress condition of the material, the specimen must be prepared in the laboratory in such a way that its behaviour under load is similar to that in the pavement structure. This is however difficult to achieve as undisturbed samples cannot be obtained from the field hence, complicating the characterization of subbase materials.
In the UK, significant quantities of fines of about 15-20% of rocks crushed are produced by rock quarries. This material does not lie within the subbase grading requirements as described in the preceding sections; hence they are left behind in quarries and are not being used. The possibility of using these fines by pushing the limit of the subbase grading requirement envelope while maintain the structural function of the subbase is of significant interest. This would eventually reduce the cost pavement materials and also facilitate sustainability of natural aggregates.
Also, Haynes and Yoder (1963) reported instability of granular materials when the degree of saturation is higher than 80% while being subjected to repeated loading. The combined effect of the increase in moisture and fines content on the MR of granular materials has been adequately investigated except for the observation made by Lekarp et al, (2000). They observed that increase in moisture content increases resilient modulus until it reaches a certain point where the effect reverses, and additional increases in the moisture content lower the resilient modulus. They also observed that this effect is pronounced when the fines content of the granular material is increased. Hence, In addition to the increase in fines content, the variation of moisture content to simulate field inundation conditions of typical subbase materials would better improve the applicability of the proposed grading requirement extension.
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