Material characterization

2.3.2 Fatigue characterization

Bound materials (like asphalt mix) undergo fatigue damage due to repetitive application of load. In the laboratory, the loading may

be applied in a stress or strain controlled manner on samples of various geometries. The repetitive loading may be flexural, axial, or torsional in nature; however, flexural fatigue loading is generally used for pavement engineering applications. Loading can be simple (where stress or strain amplitude level is maintained constant) or compound (where stress or strain amplitude level is varied during the course of testing) in nature.

Figure 2.17 shows typical fatigue characteristics of bound ma- terials due to simple flexural fatigue loading. The fatigue life is

2.3. Asphalt mix

Stress ratio

Figure 2.17: Schematic diagram showing a possible fatigue behav- ior of bound materials in simple flexural fatigue testing.

generally expressed as the number of repetitions at which the elas- tic modulus reaches a predefined fraction of the original elastic modulus value [2, 176]. The stress ratio is defined as the ratio between the applied stress amplitude (for constant stress ampli- tude testing) and the flexural strength (known as the modulus of rupture) of the bound material. From Figure 2.17 it can be seen that if the strain (or stress ratio) level is high, the fatigue life is expected to be low and vice versa. Typically, strain is used as a parameter for fatigue characterization of asphalt material, and the stress ratio is used for cement concrete or cemented material. It has been observed that at a very low level of strain (or stress ra-

tio), the sample does not fail due to such repetitive loading 3 and this is known as the endurance limit. This property later formed the basis of perpetual pavement design [209, 300].

3 That is, the fatigue life virtually becomes infinity.

44 Chapter 2. Material characterization

For compound fatigue loading, the following empirical relationship generally satisfies,

where, n i = the number of repetitions applied at a given strain (or stress ratio) level, and N i is the fatigue life of the material at that strain (or stress ratio) level. Equation 2.55 was originally devel- oped based on experiments conducted on aluminum [200], and sub- sequently adopted in pavement engineering for characterizing the fatigue behavior of asphalt mix, cement concrete, and cemented material [67, 98, 124, 206, 217, 269, 281]. By using Equation 2.55 one assumes that fractional damages caused due to repetitions at various levels of strains (or stresses) are linearly accumulative [82].

A significantly large number of research studies is available on fatigue characterization of asphaltic material, covering the issues related to the mode/process of testing [2, 67, 191, 236, 269], fac- tors affecting the fatigue behavior [70, 176, 236], variability in test results [67, 236, 269], fracture and damage modeling of asphalt mix [57, 95, 150, 235], stiffness reduction [2, 176], asphalt heal- ing [118, 151], endurance limit [309] and so on. One can refer to articles, like [15, 156, 177] for an overview on the fatigue behavior of asphalt mix.

2.4 Cement concrete and cemented material

Cement concrete is made up of aggregates, cement, admixtures (if required) and water. Hydration of cement and the subsequent hardening contribute to the strength of the material. Cement con- crete, in a hardened state, is characterized by its elastic modulus, compressive strength, tensile strength, bending strength (that is, modulus of rupture) etc. Empirical equations are suggested for the interrelationships between these physical properties [5, 207, 208].

2.5. Closure

An elastic modulus of cement concrete can be measured as a tan- gent modulus, a secant modulus or, dynamic modulus [207]. Fa- tigue performance of cement concrete is an important considera- tion in the concrete pavement design [1, 132, 206, 211, 217, 281, 285]. One can, for example, refer to textbooks such as [207, 208] etc. for a detailed study of the properties and characterization of cement concrete.

Cemented materials (generally locally available/marginal materi- als are utilized as the bound form) are bound material, hence these can be characterized in a manner similar to other bound material. Interested readers can refer to, for example [86, 106, 169, 217], for further study on the characterization of cemented material and its application in pavement construction.

2.5 Closure

This chapter has discussed the concept of “elastic modulus” (stiff- ness) of various materials used for road construction. The elastic moduli of materials are used as an essential input during pave- ment analyses discussed in subsequent chapters (Chapters 3 to 6). The issues related to time dependency (for asphaltic material) and stress dependency (for unbound granular material) of elastic modulus have been highlighted. Pavement layers undergo damage (for example, damage due to fatigue for bound materials, perma- nent deformation, etc.). Generally, such damages propagate with the load repetitions. The number of repetitions a pavement can sustain until failure is an important consideration in pavement design. This will be discussed further in Chapter 7. Considerable literature is available on the choice of Poisson’s ratio value of pave- ment materials [1, 32, 141, 201, 207]; there are instances where the Poisson’s ratio value is measured as more than 0.5 [178, 294]. It is generally considered that variation of Poisson’s ratio values does not significantly affect the pavement analysis results [201].

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