Kwang W. Kim

Variation of fracture toughness of asphalt concrete under low temperatures

Abstract This study presents the results of experimental evaluation on fracture toughness of asphalt concrete at various low temperatures (from −5°C to −30°C in 5°C steps). An asphalt cement, penetration grade of 85/100 and two aggregates, a granite and a limestone, were used to prepare asphalt concrete beam specimens which were conditioned using two different procedures and tested under three-point bending setup. The first procedure dealt with evaluation of fracture toughness of the asphalt concrete at a control temperature, −5°C, following conditioning at the specified temperatures. The second procedure dealt with evaluation of fracture toughness at the temperatures at which the samples were conditioned. The results showed that fracture toughness (KIC) for both aggregate mixtures in both procedures changed in a manner that it increased by lowering temperature from −5°C to −15°C, and then decreased thereunder. An improved mechanical adhesion due to the strengthened grip of asphalt matrix resulted from differential thermal contraction (DTC) is responsible for increased resistance to the applied loads. The reduction of fracture toughness below −15°C is explained as the effect of internal damage due to DTC that is a consequence of the large difference in coefficients of thermal contraction between aggregate and asphalt cement. Granite aggregate mixture showed a slightly better resistance to fracture throughout the temperatures. Relatively good linear relations between average values of σf and KIC were found from the regression analysis. Increasing flexural strength resulted in an increased fracture toughness for all mixtures. KIC of granite mix showed more critical to the change of σf.

Mode I reflection cracking resistance of strengthened asphalt concretes

Abstract This study was devised to evaluate the performance of polymer-modified asphalt mixtures and specially designed reinforcement techniques against reflection cracking for the typical asphalt pavement overlays. Selected polymers from previous studies were used as asphalt modifiers along with fibre; a polypropylene film (vinyl) and a grid were used as reinforcing materials. Using the reinforced asphalt mixtures with the optimum asphalt content from mix design, an asphalt mixture slab was made and cut into two specimens. A layer of grid or polypropylene film was placed at the bottom of each specimen to strengthen the pavement layer against cracking. Simulated-repeated loading was applied to the asphalt mixture specimens using a hydraulic dynamic loading frame. The asphalt mixture specimens were bonded to a cement concrete base which was cut to simulate a crack. Crack propagation under repeated loading was monitored and the effectiveness of the devised crack preventing techniques was evaluated. From the test results, a significant retardation of crack progress was monitored from some of the modified and reinforced asphalt mixtures.

Correlation Analyses for Implementation of Binder Properties for Rut Potential Estimation of Asphalt Mixtures

This study examined the relationship of three binder properties with three permanent deformation (rut) test data. The three binder properties include the stiffness (G*/sin δ) measured by dynamic shear rheometer, the large molecular size (LMS) ratio, measured by gel-permeation chromatography, and the absolute viscosity. The three rut tests at 60°C include the wheel tracking (WT), asphalt pavement analyzer (APA), and deformation strength (SD) tests on 28 hot-mix asphalt mixtures (two aggregates, two gradations and seven binders). The regression analyses showed the two binder properties (G*/sin δ and LMS) had very high correlation (R2>0.81) with the rut depth values of WT and APA, and with SD values. This is an indication that G*/sin δ and LMS are reliable properties that provide a reasonable estimation of the rut potential of asphalt mixtures. Prediction models for rut estimation were developed by multiple regression using binder properties and aggregate information. The predicted rut data were well fit (R2>0.92) to the measured rut data. Therefore, it was found that the two binder properties could be reliably used for the estimation of the rut potential of asphalt mixtures.

Feasibility of deformation strength for estimation of rut resistance of asphalt concrete

ABSTRACT There has been a need for developing a simple mechanical test method to estimate deformation resistance that can be used for estimation of rut tendency of asphalt mixtures. Some dynamic and triaxial tests are available for this purpose, but have limitations in their use due to complexity and high variation in output. Currently, one of the best ways of judging rut tendency is to run wheel tracking (WT) test. At high temperature, if the load and deformation at the limited area, where load is directly contacted, are properly considered, a strength measured thereby will have better correlation with rut parameters. Based on this assumption, a new approach is introduced for estimation of rut tendency. A static load through a round-edge loading head was applied to a Marshall-type specimen in the direction of compaction at 60°C, and the maximum load (P) and deformation (y) at the P were obtained. A strength value defined as deformation strength (SD) is calculated using , in which D is loading head diameter and r is radius of edge curvature. Regression analyses based on wheel tracking parameters showed that R2 for the depth of rut vs. SD is over 0.95 and R2 for the dynamic stability vs. SD is over 0.81 for data obtained using D=4cm with r=1cm loading head. Total mean R2 was over 0.87 for three loading heads and two aggregate mixes. This level of R2 is much higher than that of other static strengths of asphalt mixture. Therefore, if the SD value is higher, the mixture shows smaller rut depth and requires more number of repetition (cycle) for inducing unit depth of rut in WT test. This is a sign that the mixture with higher SD will perform better against rutting. Therefore, it is found that there is a good possibility of using SD as a relative index of rut estimation of the asphalt mixture.

A New Static Strength Test for Characterization of Rutting of Dense-Graded Asphalt Mixtures

A new static-test protocol was developed for measuring the deformation resistance of asphalt mixtures at high temperature, 60°C. A round edge loading head was used for estimating rut-related performance by statically pressing a specimen at the flat top center. The value measured by this test is considered as a strength against deformation or deformation strength and designated as “SD.” The SD has shown a high correlation with the rut depths of wheel tracking (WT) and the asphalt pavement analyzer (APA) tests. In this study, the feasibility of utilizing this test method to predict the rutting potential of asphalt mixtures is shown. Stiffness (G∗/sin δ) test data for asphalt binders, two rutting tests’ (WT and APA) data, and SD data for various laboratory mixtures were used in this evaluation. More than 50 field mixtures were also used for APA and SD tests. Correlation analyses showed that the SD had very high correlation with the stiffness of the binder (R2=0.9), with rut depth of WT (R2>0.90) and somewhat less with rut depth of APA (R2=0.77). It was concluded that the newly developed test is comparable to WT and APA and that the SD is a property providing a reasonable estimation of rut potential of asphalt mixtures at high temperatures, even though it is a static-test property.

Tensile reinforcement of asphalt concrete using polymer coating

Abstract This study investigates the possibility of utilizing a polyester resin for reinforcing flexible pavements. The application of a thin-layer coating with a polymer, unsaturated polyester resin (UPR) on the surface of a laboratory-prepared unmodified asphalt concrete mixture was studied as a tensile reinforcement method for such a material. Selected laboratory performance tests were conducted and the results are compared with those of a normal (uncoated) asphalt concrete mixture and a modified asphalt mixture, both mixtures being widely used in Korea. The polymer coating was found to be effective in improving Marshall stability, tensile strength and flexural strength of asphalt concrete. These improvements can be explained as the effect of reinforcement by a thin polymer layer which is fully bonded to the specimen faces. The reinforcement was also effective in reducing the stiffness of the mixture whilst improving load-carrying capacity. This improvement in strength and reduction in stiffness resulted in a retardation of crack initiation resulting from cyclic load application and a significantly improved resistance to crack propagation. The study has shown that there is a possibility of using the polymer coating as a method of tensile reinforcement with flexible pavements.

Estimation of service-life reduction of asphalt pavement due to short-term ageing measured by GPC from asphalt mixture

If the aggregate is heated higher than the specified level, the asphalt binder in the mixture will be aged at a much higher level during the short-term ageing (STA) period. The asphalt binder in that mixture will be oxidised (aged) more than expected during STA time due to the highly elevated temperature. If the binder in the mixture is severely oxidised, the asphalt pavement will have to lose its service life, or the expected service life will be reduced. Therefore, the objective of this study was to evaluate the ageing level of asphalt in the mixture at different STA temperatures and to estimate the service-life reduction (SLR) of asphalt mixture due to the STA. In this study, various temperatures were used for STA to evaluate ageing levels of the binder in the mixture before and after STA. A gel-permeation chromatography technique was used on the mixture particles without binder recovery to estimate the significance of ageing for each case of STA. Statistical analyses were carried out to examine the difference in ageing levels among STA temperatures. Statistical test results found that the ageing level of the binder after STA was significantly higher than that of binders before STA at α = 0.05 level. It was also found that the ageing level of binders in the warm-mix asphalt mixture was significantly lower than that of binders in hot-mix asphalt (HMA) after STA at the α = 0.05 level. It was also observed that if an HMA mixture was short-term aged at the higher temperature, a significant SLR could be induced due to the higher temperature during STA time.

Performance of Modified WMA mixtures prepared using the same class PG Binders of HMA Mixtures

When the same class performance graded (PG) binders are used, the asphalt mixtures using the same aggregate and gradation are expected to perform similarly whether they are the hot-mix asphalt (HMA) or warm-mix asphalt (WMA). In this study, HMA and WMA mixtures with the same class PG binders were used to compare their performance. Two high temperature (60°C) rut-related properties were evaluated, the wheel tracking (WT) and the strength against deformation (SD). One ambient-temperature (25°C) property, the indirect tensile strength (St), was also evaluated. The performance grades of normal (unmodified) and modified binders for all WMA and HMA mixtures were 64-22 and 76-22, respectively. According to study results, the modified WMA mixtures, which were short-term aged at 135°C, did not show high enough SD and WT resistance at 60°C than the HMA mixtures prepared using the same class PG binders, 76-22. However, the same WMA mixtures were found to be similar to or better than HMA mixtures in St. Therefore, it was found that tensile strength of WMA mixtures was not inferior to HMA at ambient temperature. However, rut related properties of modified WMA mixtures were observed to be inferior to the same PG HMA mixtures at high service temperature.

Estimation of Optimum Compaction Temperature for PMA and WMA Mixtures by Volumetric Property Evaluation

The optimum compaction temperature (OCT) is defined as the temperature at which asphalt provides proper viscosity in an asphalt mixture. For normal asphalt, the proper viscosity level has been suggested as 280 ± 30 mm2/s by the American Association of State Highway and Transportation Officials (AASHTO). This suggestion has been effectively utilized in selecting the OCT of normal hot-mix asphalt (HMA). However, because polymer-modified asphalt (PMA) and warm-mix asphalt (WMA) have been used recently, it is difficult to apply this method in general. This study evaluates important volumetric properties (air void ratio and void filled with asphalt) of HMA and WMA mixes. The three binders evaluated included 64-22 and 76-22 for HMA and 70-22 for WMA for dense-graded asphalt mixtures compacted using a Superpave gyratory compactor. The OCT at which the optimum asphalt content could be obtained was determined for the three binders. The OCTs obtained via volumetric property evaluation were 20 °C and 38 °C lower than the values determined via the AASHTO method for PMA and WMA. In mechanical performance comparisons, the mixture compacted at the lower OCT was not inferior to the mixture compacted at a higher OCT. It is therefore concluded that the OCT should be determined based on the volumetric property of mixtures, because compaction is not solely a function of viscosity, especially for PMA and WMA mixtures.

Estimation of Rutting Characteristics of Waste Tire Rubber-Modified Asphalt Binder Using GPC

This study investigated laboratory evaluation of rubber-modified binder using DSR and GPC. The asphalt binder is modified using the crumb rubber from scrap tires to improve its performance in the pavement on street and highways in an urban area setting. While improving high-temperature grade of binders by increasing stiffness using rubber is not a new idea, it is not well known whether or not this rheology change is reflected in molecular size distribution or chromatogram of the binder. In this study, the rutting resistance index and chemical composition of rubber-modified binders (RMB) were evaluated using a dynamic shear rheometer and a gel-permeation chromatography system respectively. A source of PG 64-22 asphalt was used for preparing a total of 12 RMB binders. Routine Superpave binder testing was carried out; RMB as the original binder and then RTFO-aged binder as RTFO in DSR test at high temperatures. From chromatogram data, the correlation of large molecular size was evaluated by service temperatures, 70 and 76 degrees. A significant correlation was found between molecular size distribution and the rutting resistant property of the binder. The size and type of CRM were significant factors affecting the rheology of the binders.