When a pavement surface becomes heavily deteriorated, the pavement designer can choose from a number of maintenance and rehabilitation alternatives spanning from thin surface-type treatments to more substantial surface renewal programs involving milling an d the placement of a hot-mix asphalt overlay. In the past, thin surface treatments were generally limited to lower volume roads, as ride quality and the existence of loose aggregate prevented its use on higher volume, higher speed facilities. However, th is is no longer the case with the advent of bonded overlay systems, where a spray paver application system enables heavier tack coat application rates to be used in conjunction with aggregates already coated with asphalt. Thin bonded overlay systems, or TBOs, appear to have performance benefits over traditional surface treatments and overlay systems. However, the assessment and, moreover, the quantification of the crack resistance of these thin pavement surface renewal systems presents a significant scientific challenge. The development of such a system is the focus of this paper. Nineteen pavement sections representing five pavement projects, various tack coat application rates and types, asphalt concrete gradations (dense and gap), construction techniques (conventional and spray paver) are evaluated in this study. Core samples from each of the pavement projects were procured and fracture characterization tests were performed using the disk-shaped compact tension fracture test (ASTM D7313-07b) and/or a variation of the test. The test variation, termed the compact tension test, or C(T), is a specifically tailored version of the DC(T) test designed for testing TBOs. The laboratory fracture results obtained from both of these tests demonstrate significantly higher fracture resistance characteristics for gap-graded TBOs as compared to conventionally placed asphalt overlays. This appears to be due to the significant upwards wicking of the heavy tack coat material into the gap graded mix. The dense graded TBOs, which are a relatively new concept, showed fracture energies that were slightly higher than conventionally placed overlays but significantly lower than the gap graded TBOs, probably as a result of the lower amount of upwards wicking of the tack c oat binder as compared to the gap graded system. In order to further explore the cracking resistance of TBOs, a series of numerical simulations were performed to evaluate the relative thermal cracking potential of dense and gap graded TBOs as compared to conventional HMA overlays. The simulation results further reinforce the finding obtained from laboratory fracture testing.
Asphalt concrete material selection procedures rely mainly on specifying low temperature binder properties as criteria for thermal cracking prevention. However, the effects of cooling rate are not rigorously considered in these methods. This study examines the effects of cooling rate on accumulation of thermal stresses in asphalt pavements. Enhanced integrated climatic model (EICM) simulations have indicated that the lowest pavement temperature and highest cooling rate events do not usually occur simultaneously. In more severe climates, such as the Midwest USA, there are frequent occurrences of high cooling rate events in the range of 1 to 3°C/hour. Using fundamental viscoelasticity formulations and Boltzmann's superposition principle thermally induced stresses can be estimated with relative ease. This formulation utilizes low temperature viscoelastic properties such as creep compliance or relaxation modulus as an input and can be readily used as a tool to identify materials with high susceptibility to thermal cracking as a result of severe cooling rate. To verify the dependence of asphalt concrete's thermal cracking performance on cooling rates; five mixtures from the SHRP General Pavement Study (GPS) sections were studied. Thermally induced stresses at different cooling rates were computed for a pavement section by means of viscoelastic finite-element simulations. In addition, thermal cracking predictions were made using TCMODEL, which is the thermal cracking software used in the AASHTO Mechanistic Empirical Pavement Design Guide. For different cooling rates, the amount of induced thermal stresses was found to vary significantly for asphalt concrete mixtures produced with same Superpave PG binder grades. Mixtures having high stress accumulations according to the analytical solution were found to perform poorly in the field and were predicted to have poor performance in the finite-element and TCMODEL simulations. The results of this study indicate that when evaluating low temperature cracking performance, both cooling rate and lowest pavement temperature may need to be considered.
The Indirect Tension Test (IDT) is frequently used in civil engineering because of its benefits over direct tension testing. During the Strategic Highway Research Program (SHRP), in the mid-1990's, an IDT protocol was developed for evaluating tensile strength of Hot Mix Asphalt (HMA) mixtures. However, with the increased use of finer aggregate gradations and polymer modified asphalt binders in HMA mixtures, the IDT results can be misleading because of crushing failure under the narrow loading heads. For such mixtures the 150-mm diameter, 50-mm thick, cylindrical specimens tends to fail in crushing beneath the loading heads versus the desired indirect tension at the center of the specimen. Therefore, a new specimen configuration is proposed for strength testing of HMA. In place of the loading heads at the top and bottom, the specimen is trimmed to produce flat planes with parallel faces, creating a "flattened-IDT." A viscoelastic finite element analysis of the flattened configuration was performed to evaluate the optimal trimming width. In addition, the numerically determined geometry was verified by means of laboratory testing of 3 different HMA mixtures. This integrated modeling and testing study shows that for the HMA mixtures with fine aggregate gradations and compliant asphalt binders used in this study, the flattened IDT eliminates the severe crushing observed in the regular IDT. It is recommended that further testing and analysis be performed on the flattened IDT arrangement, leading to a revision of the current AASHTO standard for IDT testing as asphalt mixtures.
The economical use of pavement milling in recent years has resulted in the availability of significant amounts of recycled asphalt pavement (RAP). The use of RAP in new asphalt concrete mixtures can result in cost savings for both aggregate and asphalt binder. When properly utilized in mix design and production, lower costs can be realized without a sacrifice in the serviceability of the pavement. However, cost savings may not be realized by the owner and there may be a loss in serviceability of the pavement when unauthorized use occurs. This study was conducted to modify and/or develop a test to determine the presence and amount of RAP in post-production mixtures as a practical quality assurance tool. A testing and analysis procedure was developed to identify the presence and amount of RAP in asphalt mixtures. The development of the procedure was carried out by testing of laboratory as well as plant mixtures with known RAP amounts. To validate the most promising method two sets of blind samples were evaluated. An extensive testing of RAP materials from various sources was also carried out to determine the effect of RAP variability on newly developed procedures. From a literature review it was identified that asphalt binder properties change most dramatically with aging and therefore a procedure based on testing of asphalt binders was developed. An extensive procedure coupled with an analysis technique based on micromechanics was developed for determining amount of RAP in the asphalt concrete.