Alex K. Apeagyei

Moisture-induced strength degradation of aggregate–asphalt mastic bonds

A common manifestation of moisture-induced damage in asphalt mixtures is the loss of adhesion at the aggregate–asphalt mastic interface and/or cohesion within the bulk mastic. This paper investigates the effects of moisture on the aggregate–mastic interfacial adhesive strength as well as the bulk mastic cohesive strength. Physical adsorption concepts were used to characterise the thermodynamic work of adhesion and debonding of the aggregate–mastic bonds using dynamic vapour sorption and contact angle measurements. Moisture diffusion in the aggregate substrates and in the bulk mastics was determined using gravimetric techniques. Mineral composition of the aggregates was characterised by a technique based on the combination of a scanning electron microscope and multiple energy dispersive X-ray detectors. Aggregate–mastic bond strength was determined using moisture-conditioned butt-jointed tensile test specimens, while mastic cohesive strength was determined using dog bone-shaped tensile specimens. Aggregate–mastic bonds comprising granite mastics performed worse in terms of moisture resistance than limestone mastic bonds. The effect of moisture on the aggregate–mastic interfacial bond appears to be more detrimental than the effect of moisture on the bulk mastic.

Assessing asphalt mixture moisture susceptibility through intrinsic adhesion, bitumen stripping and mechanical damage

Durability is one of the most important properties of an asphalt mixture. A key factor affecting the durability of asphalt pavements is moisture damage. Moisture damage is generally considered to be the result of two main mechanisms; the loss of adhesion between bitumen and the aggregate and the loss of cohesion within the mixture. Conventional test methods for evaluating moisture damage include tests conducted on loose bitumen-coated aggregates and those conducted on compacted asphalt mixtures. This paper looks at results from the rolling bottle and the saturated ageing tensile stiffness (SATS) tests in an attempt to better understand the underlying processes and mechanisms of moisture damage with the help of surface energy measurements on the constituent bitumen and aggregates. Combinations of materials were assessed using both the rolling bottle and SATS tests. The surface energy properties of the binders were measured using a dynamic contact angle analyser and those of the aggregates using a dynamic vapour sorption device. From these surface energy measurements, it was possible to predict the relative performance of both the simple rolling bottle test and the more complicated SATS test. Mineralogical composition of the aggregates determined using a mineral liberation analyser was used to explain the differences in performance of the mixtures considered.

Rutting as a Function of Dynamic Modulus and Gradation

This study was conducted to investigate rutting resistance of asphalt concrete (AC) mixtures as a function of dynamic modulus and gradation. The Flow number (FN) test, the (NCHRP 9-19) recommended procedure for evaluating rutting resistance of AC mixtures, was used to simulate rutting in the laboratory. The FN test involves applying a repeated creep load to AC specimens for 10,000 cycles or until an accumulated strain of five percent. FN tests were conducted at 54°C and accumulated strain was monitored for each load cycle. The results were used to determine the onset of tertiary flow (or FN) for 16 AC mixtures (eight surface mixes, five base mixes, and three stone matrix asphalt) produced in Virginia. First-order multiple regression models were developed to describe the relationship among FN, dynamic modulus, and gradation. The results showed FN was strongly correlated to dynamic modulus values at 38°C, and gradation (percent passing various sieve sizes) for the 16 AC mixtures. Using previously published data, the veracity of the relationship of FN as a function of dynamic modulus and gradation was verified for 12 mixtures. The results suggest dynamic modulus and gradation could be considered as potential rutting specification parameters for QC/QA purposes in the field. The results may also be useful for optimizing the laboratory mix design process.

Examination of moisture sensitivity of aggregate–bitumen bonding strength using loose asphalt mixture and physico-chemical surface energy property tests

In this study, the moisture sensitivity of different kinds of aggregates and bituminous binders is examined by comparing the performance between five empirical test methods for loose mixtures – static immersion test, rolling bottle test (RBT), boiling water test (BWT), total water immersion test and the ultrasonic method – with more fundamental surface energy-based test data. The RBT and BWT results showed that limestone aggregates perform better than granite aggregates and that, for unmodified binders, stiffer binders provide better moisture resistance compared with softer binder. Both tests were sensitive to aggregate type, binder type and anti-stripping agent type. Ranking of the mixtures by RBT and BWT was in general agreement with the surface energy-based tests, especially for mixtures that performed worst or best in RBT and BWT. The magnitude of the work of debonding in the presence of water was found to be aggregate type dependent which suggests the physico-chemical properties of aggregates may play a fundamental and more significant role in the generation of moisture damage, than bitumen properties.

Evaluation of moisture sorption and diffusion characteristics of asphalt mastics using manual and automated gravimetric sorption techniques

One of the most important factors influencing the durability of asphalt mixtures is moisture-induced damage resulting from the presence and the transport of moisture in pavements. Moisture-induced damage is an extremely complicated phenomenon that is not completely understood but believed to be governed by the interaction of moisture with asphalt mix components (mastic and aggregates). The objective of this study was, therefore, to characterize the sorption and diffusion characteristics of asphalt mastic using gravimetric vapor sorption techniques. Moisture transport, in the hygroscopic region, in asphalt mastics was studied using both static and dynamic gravimetric vapor sorption techniques to determine equilibrium moisture uptake and diffusion coefficients as a function of aggregate and filler types. For the 25-mm diameter thin asphalt mastic films and the testing conditions (23°C and 85% relative humidity) considered, the kinetics of moisture uptake obtained were characteristic of Fickian diffusion with a concentration-dependent diffusion coefficient. Equilibrium moisture uptake and diffusion coefficient estimated from the static measurements were comparable and of the same order of magnitude as those from dynamic sorption techniques. Both measurement techniques ranked the mixes similarly, which suggest either method could be used to characterize moisture transport in asphalt mastics. Equilibrium moisture uptake was relatively higher in mixtures containing granite aggregates compared with limestone aggregate. In contrast, the diffusion coefficient of limestone aggregate mastics was higher than granite. Thus, an inversely proportional relationship exists between moisture uptake and diffusivity of the asphalt mastics studied. The results suggest moisture transport is a function of aggregate type and that both equilibrium moisture uptake and diffusion coefficient are useful in studying moisture susceptibility in asphalt mixtures. The effect of mineral filler type on diffusion coefficient was minimal in the mastics containing granite aggregate but relatively high in mastic samples containing limestone aggregates. Diffusion coefficient was found to increase with sample thickness, which was unexpected because diffusion coefficient (in an isotropic material) is considered an intrinsic property that is independent of sample size. The results suggested anisotropic diffusivity can occur in asphalt mastics and could be attributed to factors, including mineralogy, microstructure, air voids, and the tendency of the aggregates to settle at the bottom of asphalt mastic with time. In addition to characterizing moisture transport in asphalt mastics, the results presented in this paper will be useful as inputs for numerical simulation of moisture damage in asphalt mixtures.

Rutting Resistance of Asphalt Concrete Mixtures That Contain Recycled Asphalt Pavement

This study evaluated the rutting resistance of plant-produced asphalt concrete (AC) mixtures in the laboratory. Nineteen plant-produced AC mixtures were used; these mixtures contained reclaimed asphalt pavement (RAP) amounts that ranged from 0% to 25%. Tests on the mixtures included the dynamic modulus (|E*|) test at multiple temperatures and the flow number (FN) test at 54°C to characterize stiffness and rutting resistance, respectively. Mixtures that contained no RAP showed |E*| values comparable to those that contained 25% RAP in most cases. For most of the 19 mixtures tested, mixtures with lower FNs either contained no RAP, contained 25% RAP, or had PG 64-22 as the design binder grade. Mixtures that contained moderate amounts of RAP (10% and 15%), regardless of design binder grade, had higher FNs than mixtures with either high or low RAP amounts. Statistical analysis showed that the RAP amount was the most significant factor to affect rutting resistance in the mixtures studied. A linear inverse relationship between RAP and FN appeared to describe the data well. As the RAP amount increased, a downward trend occurred in both effective binder content (Pbe) and rutting parameter (G*/sin δ). The effect of RAP on FN was unexpected, because it showed the rutting resistance to decrease with increased RAP. Possible reasons might have been the use of softer asphalt binder in mixtures with higher RAP and the observed decrease in both Pbe and G*/sin δ with increased RAP amounts. More rutting-related mechanistic studies are needed of AC mixtures that contain RAP.

Moisture damage assessment using surface energy, bitumen stripping and the SATS moisture conditioning procedure

Durability is one of the most important properties of an asphalt mixture. A key factor affecting the durability of asphalt pavements is moisture damage. Moisture damage generally results in the loss of strength of the mixture due to two main mechanisms; the loss of adhesion between bitumen and aggregate and the loss of cohesion within the mixture. Conventional test methods for evaluating moisture damage include tests conducted on loose bitumen-coated aggregates and those conducted on compacted asphalt mixtures. The former test methods are simpler and less expensive to conduct but are qualitative/subjective in nature and do not consider cohesive failure while the latter, though more quantitative, are based on bulky mechanical test set-ups and therefore require expensive equipment. Both test methods are, however, empirical in nature thus requiring extensive experience to interpret/use their results. The rolling bottle test (RBT) (EN 12697-11) for loose aggregate mixtures and the saturation ageing tensile stiffness (SATS) test (EN 12697-45) for compacted asphalt mixtures are two such methods, which experience suggests, could clearly discriminate between ‘good’ and ‘poor’ performing mixtures in the laboratory. A more fundamental approach based on surface energy (SE) measurements offers promise to better understand moisture damage. This article looks at results from the rolling bottle and the SATS tests in an attempt to better understand the underlying processes and mechanisms of moisture damage with the help of SE measurements on the constituent bitumen and aggregates. For this work, a set of bitumens and typical acidic and basic aggregate types (granite and limestone) were selected. Combinations of these materials were assessed using both the rolling bottle and SATS tests. The SE properties of the binders were measured using a dynamic contact angle Analyser and those of the aggregates using a dynamic vapour sorption device. From these SE measurements it was possible to predict the relative performance of both the simple RBT and the more complicated SATS test. Mineralogical composition of the aggregates determined using a mineral liberation analyser was used to explain the differences in performance of the mixtures considered.

In-Place Pavement Recycling on I-81 in Virginia

During the 2011 construction season, the Virginia Department of Transportation completed an in-place pavement recycling project to rehabilitate a 3.66-mi section of pavement on southbound I-81 in Augusta County near Staunton. In 2008, the directional traffic volume was 23,000 vehicles per day with 28% being trucks (85% of the truck traffic consisted of five- and six-axle tractor trailer combination vehicles). This section of the Interstate showed structurally related deterioration at the pavement surface, had a low structural capacity, and had a history of frequently recurring maintenance. The construction project used three in-place pavement recycling techniques (full-depth reclamation, cold central-plant recycling, and cold in-place recycling) and a unique lane-closure schedule to accomplish the work. A construction contract with a value of

Influence of aggregate absorption and diffusion properties on moisture damage in asphalt mixtures

7.64 million and a time frame of approximately 8 months was awarded in December 2010. The in-place recycling portion of the work was completed in fewer than 20 workdays spanning 6 weeks. This construction work represents the first time in the United States that those three recycling techniques were combined in one project on the Interstate system, and the project showed that the construction techniques can be used on higher-volume facilities. The construction techniques and the traffic management plan are described, and the results of acceptance testing and initial field testing are discussed. Suggestions for future study based on lessons learned during this project are offered.

Stiffness of High-RAP Asphalt Mixtures: Virginia’s Experience

An experimental study was undertaken to characterise moisture sensitivity of asphalt mixtures by comparing certain physico-chemical properties of selected aggregates of different mineralogies to the moisture-induced strength degradation of the aggregate–mastic bonds. The aim of the study was to evaluate the effect of using different aggregate types (as substrates) with a single mastic type that had shown severe moisture sensitivity in the past when combined with a susceptible aggregate substrate. Four different aggregate types and an asphalt mastic (made with a 40/60 pen base bitumen) were used. Aggregate moisture sorption at ambient temperature was characterised using gravimetric techniques. Aggregate specific surface area was determined by octane adsorption using a dynamic vapour sorption device. Dynamic mechanical analysis techniques based on data from a dynamic shear rheometer were used to characterise the rheological properties of the asphalt mastic. Aggregate–mastic bond strength as a function of moisture conditioning time was determined using a tensile pull-off test set-up. The results were used to estimate equilibrium moisture uptake, diffusion coefficient, characteristic diffusion time, and aggregate ‘porosity’. The effect of moisture on bond strength was aggregate substrate-type-dependent with three out of the four aggregates performing well and the fourth performing poorly. The moisture absorption and diffusion properties of the poorly performing aggregates were worse than the ‘good’ performing aggregates. Susceptible aggregate–mastic bonds had high porosity, high moisture absorption, high diffusion coefficient and contained granite as substrates. Results of statistical analyses suggested that the differences in moisture sensitivity of the other three aggregates were not significant. Therefore, two unique damage models, one for ‘good’ performing and another for ‘poor’ performing were proposed to characterise moisture damage sensitivity of asphalt. The influence of aggregate moisture absorption and diffusion on asphalt mixture moisture damage was found to be aggregate-type-dependent. The results also suggested that in a susceptible mixture, the effect of the substrate aggregate may be more influential than the effect of mastic. The results have important implications for the selection of coarse aggregate for asphalt mix design.