Verification of Appropriate Temperature for Asphalt Pavement Laying: a Case Study of Kaduna State
CHAPTER ONE
PREAMBLE TO THE STUDY
Asphalt concrete pavements are inherently graded viscoelastic structures. Oxidative aging of asphalt binder and temperature cycling due to climatic conditions represent the major cause of this non-homogeneity. Current pavement analysis and simulation procedures involve the use of a layered approach to account for these non-homogeneities; a common example of such an approach is the recently developed American Association of State Highway and Transportation Officials (AASHTO) Mechanistic-Empirical Design Guide (MEPDG) [1]. Figure 1-1 illustrates the difference between layered and smoothly- graded approaches for a simple geometry consisting of material variation in one direction. In this example the layered approach is shown with sub-division of body into three layers and each modeled using average properties ( Ei ), where as in case of smoothly graded modeling approach the material variation is accounted for through
CHAPTER TWO
REVIEW OF FGMS AND VISCOELASTIC MODELS
FUNCTIONALLYGRADED MATERIALS
Functionally Graded Materials (FGMs) are characterized by spatially varied microstructures created by non-uniform distributions of the reinforcement phase with different properties, sizes and shapes, as well as, by interchanging the role of reinforcement and matrix materials in a continuous manner [25]. They are usually engineered to produce property gradients aimed at optimizing structural response under different types of loading conditions (thermal, mechanical, electrical, optical, etc) [26]. These property gradients are produced in several ways, for example by gradual variation of the content of one phase (ceramic) relative to other (metallic) used in thermal barrier coatings, or by using a sufficiently large number of constituent phases with different properties [27]. Hilton [28, 29] has proposed designer viscoelastic FGMs (VFGMs) that are tailored to meet the design requirements such as viscoelastic columns subjected to axial and thermal loads. Muliana [30] has recently proposed micro mechanical model for thermo-viscoelastic response of FGMs.
Apart from the engineered or tailored FGMs, several engineering materials naturally exhibit graded material properties. Silva et al. [31] have extensively studied and simulated bamboo, which is a naturally occurring graded material. Apart from natural occurrence a variety of materials and structures exhibit non-homogeneous material distribution and constitutive property gradations as an outcome of manufacturing or construction practices, aging, different amount of exposure to deteriorating agents etc.
Asphalt concrete pavements are one such example, whereby aging and temperature variation yield continuously graded non-homogeneous constitutive properties.
ASPHALT CONCRETE PAVEMENTS AS GRADED STRUCTURES
The constituents of asphalt concrete include asphalt binder (bitumen) and mineral aggregates. Asphalt binder is derived from crude oil as a by-product of fractional distillation. Due to its organic nature asphalt binder undergoes oxidative aging as time progresses, the effect of which is most prominent in form of hardening or stiffening. The effect of aging creates graded material properties due to variation in the amount of aging across the depth of pavement. The Strategic Highway Research Program (SHRP) Project A-368 dealt with the chemical composition changes during the aging process of asphalt binders. The final report from this project identifies the process of age hardening as a non-reversible and continuous process that extends throughout the life of a pavement [32]. The aging and temperature induced property gradients have been well documented by several researchers in the field of asphalt pavements [30, 33-35]. The current state-of- the-art in viscoelastic simulation of asphalt pavements is limited to either ignoring non- homogeneous property gradients [22, 36-38] or considering them through a layered
approach, for instance, the model used in the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic Empirical Pavement Design Guide (MEPDG) [1]. Significant loss of accuracy from the use of the layered approach for elastic analysis of asphalt pavements has been demonstrated [39].
CHAPTER THREE
VISCOELASTIC CHARACTERIZATION OF ASPHALT CONCRETE
MOTIVATIONAND BACKGROUND
This is especially the case when crushing failures occur under the narrow loading strips prior to or in exclusion of the desired tensile failure, which is assumed to occur along a vertical plane spanning between the loading strips. The IDT setup developed for HMA uses a 19 mm wide loading strip on the top and bottom of the testing specimen (c.f. Figure 3-2). With the increased use of finer aggregate gradations and polymer modified asphalt binders in HMA mixtures, the IDT results can be suspect, particularly at testing temperatures above 0°C because of excessive deformations and crushing under the narrow loading heads during the creep and strength testing
CHAPTER FOUR
ASPHALT CONCRETE PAVEMENT SYSTEMS
PRACTICALIMPLICATIONS OF THE PRESENT RESEARCH
The asphalt pavement continuously undergoes property variations due to effects of aging and climatic cycling. A pavement section constructed at the University of Kaduna’ Advance Testing and Research Engineering Laboratory (ATREL) for study of reflective cracking is shown in Figure 6-1. Notice that on left the pavement is shown immediately following the construction, whereas picture on right shows the pavement after nineteen months.
CHAPTER FIVE
CONCLUSIONS AND EXTENSIONS
SUMMARYAND FINDINGS
This dissertation describes the development, implementation, verification and application of viscoelastic FGM finite-element analysis procedures. Two formulations have been presented, (1) correspondence principle based, and (2) recursive time integration procedure, respectively.
A viscoelastic characterization procedure using the indirect tensile test is presented and comparisons are made between regular and flattened test geometries. In depth verification is performed for the formulations developed and implemented in this thesis. The verification examples demonstrate the accuracy and efficiency of the proposed procedures, when compared to conventional approaches. Three types of asphalt pavement systems have been simulated using the procedure developed herein. The results are compared against the conventional simulation approaches. The application examples further demonstrate the superiority of the proposed approaches over the conventional methods.
The key findings identified on the basis of research conducted in this study are summarized as follows:
- The indirect tensile creep test (IDT) can be utilized for determination of viscoelastic properties of asphalt concrete at low and intermediate test
- The flattened IDT geometry is a viable alternative to regular IDT for viscoelastic characterization of asphalt concrete at low and intermediate
- Non-homogeneous form of generalized Maxwell model is selected as the constitutive model of choice for the formulations developed herein.
CONCLUSIONS
Based on the findings from this study following conclusions can be drawn:
- The non-homogeneous viscoelastic analyses procedures presented in this dissertation are suitable and preferred for simulation of asphalt pavement
- The proposed procedures yield greater accuracy and efficiency over conventional approaches for simulation of non-homogeneous viscoelastic
- The layered approach for simulation of aging and temperature induced property gradients in asphalt concrete can yield significant errors; the most pronounced errors are at layer interfaces in the stress and strain
- The interface between asphalt concrete layers can be realistically simulate dusing the procedures developed in this thesis.
- In a limited study of aged full-depth asphalt pavements, the shear strains at thetire edge were found to be the most critical
- In case of layered approaches, at the layer interfaces the average of results fromeach material can be significantly different when compared to continuous FGM results at the same location. This difference is usually exaggerated with time when the time dependent (viscous) portion of material has spatial
FUTURE EXTENSIONS
Based on the finding and conclusions from this study following extensions are recommended:
- Further verification and validations are required for the flattened indirect tensile test for viscoelastic characterization and strength determination of asphalt concrete. Verifications should include numerical simulation of the creep tests for different test geometries. Comparisons should be made between numerically determined viscoelastic properties with inputted properties to the analysis model. Analysis procedure similar to that utilized by Buttlar and Roque [43] could be used for development of correction factors if necessary.
- In the current study generalized Maxwell model was used for constitutive representation of non-homogeneous viscoeasltic material, the formulations should be extended to other commonly used models for asphalt concrete such as 2P2S1D model proposed by Olard et al. [49, 50].
REFERENCES
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