Carl Thodesen

Improved Resistance of Long-Term Aged Warm-Mix Asphalt to Moisture Damage Containing Moist Aggregates

Due to the use of many warm-mix-asphalt (WMA) additives in the market, the effect of long-term aging on these WMA mixtures is generally unclear; hence, it is necessary to simulate their performance in the laboratory. The objective of this study was to investigate the influence of various WMA additives on the moisture susceptibility of mixtures containing moist aggregates following a long-term aging process. Mass loss (percentage), indirect tensile strength (ITS) of dry and conditioned specimens, deformation (flow), and dissipated energy values were measured. The experimental design included two aggregate moisture contents (0% and approximately 0.5% by weight of the dry mass of the aggregate), two lime contents (1 and 2% lime by weight of dry aggregate), one liquid antistripping agent (ASA), five WMA additives, and two aggregate sources. A common long-term aging procedure was used in this study. The test results indicated that long-term aging improved the moisture resistance of WMA mixtures regardless of WMA additive, ASA, and moisture content. In addition, aggregate source affected the moisture resistance regardless of WMA additive, ASA, and aggregate moisture content. The effects of various WMA additives on the dissipated fracture energy of unaged mixtures are generally similar except that mixtures with Asphamin have relatively lower dry dissipated fracture energy. In addition, the aggregate type affected the dissipated energy value of WMA mixtures regardless of aging states. DOI: 10.1061/(ASCE)MT.1943-5533.0000567.

Evaluation of Statistical Validity of Some Asphalt Binder Grade Systems

The performance grade (PG) and the penetration (pen) grade systems were examined for their statistical validity. It is found from this limited study that the pass/fail high-temperature of the PG binder follows a normal distribution. Since PG system has no gap between each grade, the binder falling out of limit trespasses into another binder grade. The Korean pen grading system has the same problem. Therefore, many products can have similar property levels with an adjacent-grade even though the mean value is within the specification limit. The PG binder will have a high chance of passing the rheological requirements at above the upper limit but the least chance of falling below the lower limit because the means were above median and variations were higher than they should be. Since PG is characterized without gap between each grade, the producer must maintain the coefficient of variation below the current level to satisfy the quality control limit of the product. Similarly, in other grading systems such as the Korean pen specifications, due to its lack of a gap between grades, higher percentage of products will be duplicated even if the mean is at the median of the specification limits due to material’s quality uncertainty. Especially if the mean moves toward the upper limit, one-half of the binder grade will be overlapped with the higher binder grade. Therefore, these specifications should be statistically examined for further modification.

Laboratory Testing Methods for Evaluating the Moisture Damage on the Aggregate-Asphalt System

The durability of the bitumen-aggregate system is a critical factor affecting the performance of asphalt pavements. It is achieved by a careful selection of the materials based on the analysis of their compatibility and their water sensitivity. Currently, a variety of analytical test methods are used to evaluate the power of the binder to adhere to various aggregates and their susceptibility to moisture. Many of these methods are time consuming and/or require sophisticated and expensive instrumentation. The most common procedure, the indirect tensile strength (ITS) test, has been questioned by many researchers and simpler testing procedures, such as the rolling bottle test, are considered to be an indicative measure. Several studies have introduced a new test procedure, the binder bond strength (BBS) test. The test, based on the pull-off strength of the bond between asphalt and aggregate measured before after water conditioning, has shown good repeatability, reliability and the ability to determine the effects of different aggregate type, conditioning time and moisture on the aggregate-asphalt system. This paper explores the potential of the BBS test by correlating the results of test methods presently used to evaluate the strength of the bond of the asphalt-aggregate system measured with the BBS test. Four different aggregate types and a traditional bitumen were tested according to the respective European standard, including type, length and temperature of conditioning. However, the different aspects of the mixture considered in each test method determine a poor agreement of the results. Comparable levels of moisture resistance are observable only in the long term.

Effect of Wheel Track Sample Geometry on Results

The wheel track test can be used as a means of comparing the permanent deformation behaviour of different types of asphalt mixtures. One advantage of this test method lies in the fact that it can be applied to laboratory compacted specimens and specimens extracted from pavements in the field. However, differences in results have been noted between core samples obtained in the field and laboratory prepared samples with similar mix designs. It is thought that potential reasons for such deviations may be due to differences in the confining pressure of the sample during wheel track loading. One possible reason for such differences may lie in the fact that laboratory prepared samples are composed of rectangular asphalt slab, while field samples are circular and are encased in plaster of Paris to provide testing stability.