Alireza Zeinali

Comparison of Performance Properties of Terminal Blend Tire Rubber and Polymer Modified Asphalt Mixtures

Incorporation of used tires in asphalt mixtures has been a major advancement in the using recycled materials in asphalt pavements. Tires contain some of the polymeric components that have been used to modify the asphalt binders for decades, but in a solid form. This paper presents the result of a research to examine whether or not the proper application of the tire rubber in asphalt mixtures can enhance the pavement’s mechanical properties to the same degree that the traditional polymeric binder modifiers do. The performance of a terminal blend tire rubber mixture was compared to a polymer-modified mixture through mechanical testing at the Asphalt Institute laboratory. Two mixtures in this study were designed with the same gradation in accordance with the California standard specifications, one with polymer, and the other with terminal blend asphalt. Flexural beam fatigue test, Superpave shear test, and disk-shaped compact tension test were conducted to evaluate the performance of the mixtures with respect to fatigue cracking, rutting, and low temperature cracking, respectively. The results revealed that the terminal blend mixture would have a better rutting performance. Furthermore, the terminal blend mixture exhibited a slightly more ductile behavior at the low temperature of -12°C. The two mixtures showed a similar performance with respect to fatigue cracking at 20°C.

Evaluation of the DC(T) test in discerning the variations in cracking properties of asphalt mixtures

As the temperature of an asphalt pavement drops to below freezing point, the asphalt material starts to lose its ductility, and consequently, the asphalt mixture becomes more susceptible to cracking. The disk-shaped compact tension [DC(T)] fracture test has been used to quantify the fracture properties of asphalt concrete at subzero temperatures for more than a decade. The Asphalt Institute laboratory has successfully utilised the DC(T) test in several research studies. As a result of these studies, a valuable database is gathered which represents the sensitivity of the DC(T) test, and exhibits how the test is capable of capturing the effect of various factors on the performance of asphalt mixtures at low temperatures. This paper briefly presents the findings from some of these studies. The paper presents the response of the DC(T) test to the variations in several factors relative to asphalt pavements including the crude source of the asphalt binder, ageing of the asphalt mixture, deficiency in the in-place density, pavement temperature, using a warm-mix agent, using RAP and RAS materials, and chip sealing as a preservation method. The test showed to be well capable of capturing the variations in these factors; therefore, it can be utilised by the pavement managers to assess the changes in the specimens from the asphalt pavements over time to help determine if any maintenance or rehabilitation action is required. Furthermore, the test can be used to evaluate the laboratory-made mixtures and ascertain if they would perform satisfactorily in their designated environmental conditions.

Effect of Long-Term Ambient Storage of Compacted Asphalt Mixtures on Laboratory-Measured Dynamic Modulus and Flow Number

The asphalt mixture performance tester (AMPT) requires an exhaustive examination of the factors that influence the dynamic modulus and flow number test results. Interlaboratory research conducted in NCHRP Project 9–29, Phase 4, showed that dynamic modulus test results were highly dependent on the testing laboratory and the preparation of the AMPT specimens. Factors could include the oxidative aging and the steric hardening of asphalt in the mixture specimens during storage in a laboratory. The Asphalt Institute, Lexington, Kentucky, performed an experiment in cooperation with FHWA to evaluate the effect of ambient laboratory sample storage on the mechanical properties of asphalt mixture specimens. A half-factorial test matrix with triplicate specimens was developed to evaluate the effect of the following factors on the dynamic modulus and flow number of a laboratory standard mixture: storage duration (1 to 84 days), binder type (PG 64–22 and modified PG 76–22), storage of the specimen in a bag, and the cutting and coring of the specimen after compaction or immediately before it was tested. The results showed that storage duration could affect significantly the properties of the mixture with neat binder. The bagging of specimens did not seem to prevent their stiffening. However, storage of specimens uncut and uncored seemed to delay the stiffening phenomenon.

Effect of Laboratory Mixing and Compaction Temperatures on Asphalt Mixture Volumetrics and Dynamic Modulus

Employment of conventional asphalt testing protocols for characterization of polymer-modified asphalts remains a challenge. NCHRP launched Project 9–39 to identify an approach to determine the mixing and compaction temperatures applicable to unmodified and modified asphalt binders. The Asphalt Institute, Lexington, Kentucky, in cooperation with FHWA, embarked on the research reported here to evaluate how mixture performance is affected by laboratory mixing and compaction temperatures. Samples were mixed and conditioned at various temperatures and conditioning durations with modified and unmodified binders. Volumetric analysis confirmed that slight changes in the mixing temperature did not change the results of volumetric testing. Compacted samples were tested to determine the dynamic modulus as an indication of pavement performance. A comprehensive statistical analysis was performed, and the overall results showed that the impact of the conditioning temperature and duration on the dynamic modulus was more significant than the influence of the mixing temperature. Further, for modified binders the significance of the impact of the mixing temperature seemed to be a function of the crude source of the binders.

Development of the indirect ring tension fracture test for hot-mix asphalt

Almost all of the current hot-mix asphalt (HMA) fracture tests are considered to be research tools. This paper describes the development of the indirect ring tension (IRT) fracture test for HMA, which was designed to be an effective and user-friendly test that could be used at the Department of Transportation level. Numerical modelling was utilised to calibrate the stress intensity factor formula of the IRT fracture test for various specimen dimensions. The results of this extensive analysis were encapsulated in a single equation. An experimental plan was developed to optimise the test parameters for HMA specimens. The experiment results revealed that the test is highly repeatable, and capable of capturing the variations in the fracture properties of HMA. Moreover, the data from laboratory tests were utilised to estimate the maximum allowable crack lengths for the pavements based on a viscoelastic model.

Laboratory Investigation of Asphalt Pavements with Low Density and Recommendations to Prevent Density Deficiency

In most USA asphalt construction projects the goal of compacting a hot-mix asphalt (HMA) layer is to achieve the optimum density which is 92% of the maximum specific gravity (Gmm) of the asphalt mixture. However, this level of density is not always achieved. This paper evaluates the effect of field compaction deficiencies on the HMA durability through laboratory testing. HMA samples were collected from construction sites in the United States. A series of laboratory tests were conducted to compare the performance of HMA mixtures at their actual in-place density as well as the desired density of 92% of Gmm. The statistical analysis on the results showed that the performance of the pavements could significantly improve by eliminating the deficiencies in their in-place densities. Moreover, the compactibility of the mixtures was investigated using the compaction data from the Superpave gyratory compactor. Compaction characteristics of the mixtures were compared to a control mixture, and the results showed that the shortage in the binder content of the mixtures could be a major factor which may have caused the density deficiencies. Furthermore, the effect of higher-than-optimum binder content was evaluated on the compactibility of the control mixture.

Employment of Mechanical Testing to Evaluate the Effect of Density on Asphalt Pavement Performance

Considering the budget allotted to construct and maintain roadway surfaces, a 5 to 10 % increase in their performance could potentially yield annual savings of billions of dollars. Considering this, the Kentucky Transportation Cabinet (KYTC) launched a project to better understand the effect of in-place density on the durability of their hot-mix asphalt (HMA) pavements. In-place density is known to be a crucial factor in HMA performance. The Asphalt Institute in cooperation with the KYTC embarked upon the research and collected numerous HMA samples from five pavement construction sites in Kentucky. An experimental laboratory plan was developed to examine potential improvement in the mixtures’ performance if they were compacted to a desired higher density. The laboratory test plan consisted of flow number, flexural beam fatigue, and disk-shaped compact tension tests to evaluate the mixtures’ performance with respect to rutting, fatigue cracking, and low-temperature cracking. A statistical analysis of the results was performed to evaluate the effect of density on the durability of each of the pavements. The results showed that the service life of the pavements could improve considerably by achieving a target density of 92 %. The compactibility of the HMA mixtures was also investigated through their compaction characteristic curves by using the Superpave gyratory compactor. Based upon the compactibility analysis, the mixtures could be better compacted in the field by increasing their lift thickness.

Application of the Hinged Dowel System for Increasing the Durability of Concrete Pavement Joints

Concrete joint failure remains to be one of the main distress and maintenance issues in jointed concrete pavements. This article introduces a new load transfer system for placement in concrete pavement joints. The new dowel bar system was approved by the United States Patent Office as a new invention. Finite element modeling was used to quantify the benefits of using the new load transfer system in terms of reduced levels of deformation and stress in concrete slabs. The analysis for various loading conditions showed that the shear stress is reduced by 15%, which could result in a significant reduction in shear-induced cracking near the joints. The new invention has the capability of addressing curling and warping induced stresses as well as horizontal movement due to contraction and expansion. Furthermore, the new system prevents the dowel bars misalignment during new construction, as well as expediting the process of retrofitting damaged joints. BACKGROUND Portland cement concrete pavements (PCCP) are capable of spreading the traffic loads over a large area of the subgrade due to the rigidity of their surface layer. However, the concrete slabs are susceptible to thermally induced stresses and they are constructed with joints to prevent the slabs from shattering and cracking as they expand or shrink. Jointed plain concrete pavements (JPCP) need special mechanisms to transfer the traffic loads across the joints. Poor load transfer results in large displacements of the joints and pumping of fine materials. Poor subgrade support and large deflections are major causes behind the joint failure in concrete pavements. Interlock of aggregates at the joint face used to be the sole means of transferring traffic loads across JPCP joints (Nowlen 1968). In this mechanism the load is transferred by shear interaction of aggregates. However, the load transfer