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An excessive amount of differential settlement was observed during the construction of taxiways at Changi Airport in Singapore. The pavement exhibited severe settlement (up to 200 mm). The first cracks appeared in April 2018, 11 months after the pavement construction was completed. The cracks stayed in the asphalt concrete layer, but 15 months after the emergence of the first cracks, they extended downward and propagated beyond a subgrade layer. From the analyses performed on the pavement structure using the finite element method, it was found that tensile strain occurred at the surface of the pavement. The amount of tensile strain was higher at the joint between the main pavement and the shoulder pavement than in any other location on the pavement. Fracture analyses were conducted and indicated that cracks would propagate from the top of the pavement due to the tensile strain and move to its bottom. To identify an indication of settlement, heavy weight deflect (HWD) tests were conducted over three pavements experiencing three levels of settlement—severe, moderate, and no settlement. Backcalculation analysis was done for the three pavements using backcalculation programs BAKFAA and BAKSIMPLEX, and the closed-form solution. It was observed that the backcalcuated asphalt modulus was highly affected by root mean square (RMS) %. It is hence less reliable when the asphalt concrete modulus was determined at a higher RMS %, whereas the correlation became stronger when it was determined at an RMS % lower than four. Meanwhile, subgrade modulus backcalculated even at higher RMS % values was less affected by the error because most errors occurred and accumulated near the loading area of HWD. From the statistical analysis (t-test) performed on subgrade moduli backcalculated, it was concluded that the means of subgrade moduli obtained from the three different pavements differed from the hypothetical means of interest at a 95% level of confidence. This indicates that if a pavement undergoes settlement, the indication of settlement could be captured from HWD tests and the backcalculation of subgrade moduli. [ABSTRACT FROM AUTHOR]

Because of its efficiency in analyzing complex viscoelastic problems, the finite-element (FE) analysis has been widely used to identify the time--and rate-dependent effects of viscoelastic materials on various structural conditions. When performing the FE analysis on a viscoelastic structure, most FE programs require fundamental material properties, shear and bulk moduli, of the given viscoelastic material as their input. However, the shear and bulk modulus tests are difficult to perform, so they have been commonly estimated from a single material test on the basis of the assumption that the Poisson's ratio of viscoelastic materials is a time- independent constant. Such an assumption, however, might not be suitable because the Poisson's ratio of the viscoelastic materials is also a function of time. Therefore, this study developed computation algorithms for determining the time-dependent Poisson's ratio and shear and bulk moduli of asphalt mixtures, which have been well recognized as a viscoelastic material, by employing the indirect tension testing system. The shear and bulk moduli determined by the developed approach appear to be reasonable and realistic. Their applicability and reliability were also verified by comparing experimental data to the results of the FE analysis performed on the same circular specimen as that used in the indirect tension creep test. DOI: 10.1061/(ASCE)EM.1943-7889.0000151 CE Database subject headings: Viscoelasticity; Poisson ratio; Material properties; Finite element method; Creep. Author keywords: Viscoelasticity; Poisson ratio; Material properties; Finite element method; Creep.

The viscoelastic continuum damage model, developed based on Schapery's correspondence principle and the continuum damage mechanics, has received a great deal of attention because of its mathematical soundness and effectiveness in describing damage growth in viscoelastic media and has been used to make reliable estimations on the fatigue lives of asphalt mixtures. Its applications to field mixtures, however, have been limited because the model requires performing the uniaxial tension test. As an alternative, this study developed an analytical methodology for applying the model to the indirect tension test, which has been successfully used in testing both laboratory-made and field-cored mixtures. From the results of the indirect tension tests conducted on asphalt mixture at three different crosshead-controlled rates, it was found that the stress-pseudostrain curves could be superimposed onto one equality line in the linear viscoelastic range of the given mixture, and its rate dependency was successfully eliminated in the [C.sub.1] and [S.sub.1] plots. This indicates that the methodology developed for the indirect tension test has a capability of evaluating damage development in asphalt mixtures through the viscoelastic continuum damage model. It would be potentially of great benefit to pavement engineers who want to estimate the remaining lives of field mixtures. DOI: 10.1061/(ASCE)EM.1943-7889.0000094 CE Database subject headings: Asphalts; Mixtures; Poisson ratio; Viscoelasticity; Cyclic tests; Biaxial tests; Stiffness; Tensile strength; Damage; Temperature effects. Author keywords: Asphalts; Mixtures; Poisson ratio; Viscoelasticity; Cyclic tests; Biaxial tests; Stiffness; Tensile strength; Damage.

The indirect tension test has received extensive attention due to its capability to test field cores from thin asphalt layers. For that reason, the fundamental viscoelastic material property, creep compliance, or complex modulus has been determined by employing the indirect tension testing system. However, if the creep compliance or complex modulus is to be determined from such a multiaxial testing mode, another crucial material property must be introduced, that is, Poisson's ratio. It is well known that the Poisson's ratio of a viscoelastic material is a function of time or frequency; nevertheless, it has been commonly assumed to be a constant and a time- or frequency-independent property. In this study, computation algorithms for determining the time- and frequency-dependent Poisson's ratio of asphalt mixtures were developed based on the theory of viscoelasticity. From the comparative study, it was found that the Poisson's ratio determined from the frequency domain corresponded well to that determined from the time domain. Further study performed on the sensitivity of Poisson's ratio clearly showed that the time-dependent Poisson's ratio was an important property in determining accurate creep compliance, and an inappropriate assumption for Poisson's ratio overestimated or underestimated the measured creep compliance. DOI: 10.1061/(ASCE)0899-1561 (2009)21:8(416) CE Database subject headings: Asphalt pavements; Mixtures; Poisson ratio; Viscoelasticity; Creep; Cyclic tests.

When employing linear elastic layer analysis, which is widely used for the structural analysis of flexible pavements, it is commonly observed that most critical tensile strains occur at the bottom of an asphalt layer. It is well known that an asphalt mixture is a viscoelastic material, so its response is time and rate dependent; therefore, the results from the elastic assumption may not be borne out in the response of the asphalt layer. This study utilized viscoelastic analysis to identify critical tensile strains and their time-dependent responses. It was found that tensile strains occurred at the bottom and at the top of an asphalt layer and grew because of the theological behavior of an asphalt mixture. As load repetitions continued, the strain at the top showed a higher strain value than the strain value at the bottom. A sensitivity analysis performed on various design parameters indicated that the development of strains at the top and at the bottom is a function of the structural characteristics of pavement structures. Expected trends were observed and findings compared favorably to results of field-calibrated top-down cracking model predictions. DOI: 10.1061/(ASCE)0899-1561(2009)21:7(324) CE Database subject headings: Viscoelasticity; Finite element method; Tensile strain; Cracking; Flexible pavements.

Creep compliance (or relaxation modulus), which is a fundamental property that determines the strain (or stress) development in flexible pavements or damage evolution in asphalt mixtures, can be determined from either a creep compliance test using static loading or a complex modulus test using cyclic loading. Since the nature of each test is different, creep compliance determined from the complex modulus test was significantly different compared with that determined from the creep compliance test. From this rigorous experimental and analytical study, it was concluded that the creep compliance or complex modulus test alone is not capable of providing complete information over the typical time or frequency range used in single-temperature tests. In general, the complex modulus test provides accurate creep compliance at short loading time, while the creep compliance test provides accurate creep compliance at longer loading time. An approach to determine accurate creep compliance from the combination of creep and complex modulus tests at a single temperature was developed. The method provided accurate creep compliance values that corresponded to short-term and long-term experimental data. DOI: 10.1061/(ASCE)0899-1561(2008)20:2(147) CE Database subject headings: Viscoelasticity; Creep; Asphalt; Mixtures; Static loads; Cyclic loads; Relaxation, mechanics.

Fatigue cracking is a major problem in asphalt pavement. It would be beneficial for predictions of the fatigue life of asphalt mixtures to be made with minimal effort and time, both at the design stage and the operating stage of pavement. For practicality, the indirect tension (IDT) test has widely been used for testing both laboratory-made and field-cored mixtures; however, a fatigue test using the IDT test needs to be performed in the stress-controlled mode rather than in the strain-controlled mode. Results may not be comparable to those from other mixture tests, such as the flexural beam fatigue or uniaxial tension test, so this study employed the viscoelastic continuum damage (VECD) model. The curve, which is a final product of the VECD model, is known as a single-characteristic curve of an asphalt mixture and is independent of loading types. Three curves obtained from two fatigue tests and one monotonic strength test using the IDT test were similar when superimposed into a single curve, indicating that the VECD model can be applicable both to the IDT fatigue test and the strength test. This also indicates that the fatigue life of a mixture subjected to sinusoidal strain loading can be determined through the model's damage principles using the IDT test results. Consequently, a system for predicting the fatigue life of asphalt mixtures was developed. It is expected that the predictive system will be useful in estimating the fatigue life of asphalt mixtures fabricated in the laboratory and those cored from field pavements. [ABSTRACT FROM AUTHOR]

The fracturing of materials is well known on the basis of the theory of fracture mechanics. An important concept in fracture mechanics theory is that crack propagation is governed by the fundamental material properties of energy dissipation and energy threshold. However, to apply the fracture mechanics approach and measure these properties, a notch needs to be introduced into a continuum body. This requires additional effort to make the notch, which is often sensitive to the formation of materials surrounding the area near the crack tip. These effects become more complex for a material that exhibits time, rate, and temperature dependency. To overcome these complexities and problems regarding fracture testing, this study used the analogy between the material's behaviors with and without a notch. A transfer of the fracture mechanics principle to continuum viscoelastic media was the key to the model developed in this study. For this purpose, the energy release rate (G) which is theoretically derived from a viscoelastic cracked body, was transferred to a continuum body by employing the same energy principles but used in continuum damage mechanics. From the evaluation of the model for various mixtures at multiple temperatures, predictions made were in agreement with the expected results from the well-known fatigue model. Consequently, it is expected that the developed model will bridge the gap between viscoelastic fracture and continuum damage mechanics, and provide better cracking performance predictions of asphalt mixtures. [ABSTRACT FROM AUTHOR]

Since the linear elastic layer solution for the layered systems was developed in the 1940s, the linear elastic layer analysis has been systemized and widely used for the designs of roadway pavements as a tool for evaluating the structural soundness of pavements. The primary assumption made in the analysis is that the layered system consisting of materials that are linear elastic; and hence, an application of the elastic layer analysis to asphalt mixtures, which is a well-known viscoelastic material, has been limited. Therefore, the intention of the study was to derive a viscoelastic solution able to take into account the time- and rate-dependent nature of the viscoelastic materials in the multilayered system. In this paper, a linear viscoelastic solution for the multilayered system subjected to a cylindrical unit step (static) load was derived from the elastic solution by using the principle of elastic-viscoelastic correspondence and the numerical inversion of Laplace transforms. The solution was then extended to simulating pavement responses subjected to a moving load by employing the Boltzmann's superposition principle. The soundness of output from the viscoelastic solution was confirmed by comparing them to those of the finite-element analysis (FEA). Compared to the time and effort required in FEA, the analysis based on the viscoelastic solution was much faster. Therefore, it is expected that the viscoelastic solutions derived in this study will be an effective tool for the design of flexible pavements. [ABSTRACT FROM AUTHOR]