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

Development of a composite substrate peel test to assess moisture sensitivity of aggregate–bitumen bonds

This paper presents the development of a suitable procedure to prepare peel test specimens using coarse aggregates and compare the results with the established standard peel test. The newly developed composite substrate peel test (CSPT) was found to be effective in characterising the moisture sensitivity of the aggregate–bitumen bond and the results correlated well with the results from a standard peel test. The results from the CSPT and the standard peel test showed that the fracture energy after moisture damage was found to be aggregate type dependent. Limestone tends to have better resistance to moisture damage than granite when moisture adsorptions are similar. Furthermore, in terms of similar aggregates, lower moisture adsorption results in better moisture resistance. This phenomenon suggests that in a moisture susceptible asphalt mixture, the effect of aggregate may be more influential than the effect of bitumen. Strong correlations were found between the standard peel test and the CSPT in terms of moisture damage evaluation and suggest that the CSPT maybe a more practical procedure to test the aggregate–bitumen bond for actual aggregates used in asphalt mixtures

Influence of aggregate mineralogical composition on water resistance of aggregate–bitumen adhesion

The effects of aggregate mineralogical composition on moisture sensitivity of aggregate–bitumen bonds were investigated using four aggregate types (two limestone and two granite) and two bitumen grades (40/60 pen and 70/100 pen). Moisture sensitivity (or water resistance) of the aggregate–bitumen bonds were characterized using retained strength obtained from three different tensile tests (peel, PATTI and pull-off). The results showed significant differences in the amount of moisture absorbed by a given aggregate which suggested strong correlations between aggregate mineral composition and moisture absorption. For most of the aggregate–bitumen bonds, failure surfaces transformed from cohesive to adhesive with conditioning time thereby confirming the strong influence of moisture on aggregate bonds. The three tensile tests used in this study showed similar rankings in terms of moisture sensitivity but the pull-off test was found to be the most sensitive. The effect of bitumen on moisture sensitivity was found to be lower than the effect of aggregates, with the moisture absorption properties of the aggregates depending strongly on certain key minerals including clay, anorthite and calcite. Strong correlations were also found between mineral compositions and moisture sensitivity with clay and anorthite having strong negative influence while calcite showed positive effect on moisture sensitivity. Previous studies have identified various mineral phases like albite, quartz, and k-feldspar, as detrimental in terms of moisture sensitivity. The results appear to support the extension of the existing list of detrimental aggregate minerals to include anorthite and clay while supporting the case of calcite as a moisture resistant mineral

Moisture sensitivity examination of asphalt mixtures using thermodynamic, direct adhesion peel and compacted mixture mechanical tests

Moisture damage in asphalt mixtures is a complicated mode of pavement distress that results in the loss of stiffness and structural strength of the asphalt pavement layers. This paper evaluated the moisture sensitivity of different aggregate–bitumen combinations through three different approaches: surface energy, peel adhesion and the Saturation Ageing Tensile Stiffness (SATS) tests. In addition, the results obtained from these three tests were compared so as to characterise the relationship between the thermodynamic and the mechanical tests. The surface energy tests showed that the work of adhesion in dry conditions was bitumen type dependent, which is in agreement with the peel test. After moisture damage, all of these three tests found that the moisture sensitivity of aggregate–bitumen combinations were mainly aggregate type dependent. Based on the peel test, the moisture absorption and mineralogical compositions of aggregate were considered as two important factors to moisture sensitivity. This phenomenon suggests that in a susceptible asphalt mixture, the effect of aggregate may be more influential than the effect of bitumen. The SATS test and the peel test showed similar moisture sensitivity results demonstrating the good correlation between these two mechanical tests. However, the surface energy tests and the mechanical tests cannot correlate in terms of moisture sensitivity evaluation

Application of Fickian and non-Fickian diffusion models to study moisture diffusion in asphalt mastics

The objective of this study was to investigate certain aspects of asphalt mastic moisture diffusion characteristics in order to better understand the moisture damage phenomenon in asphalt mixtures. Moisture sorption experiments were conducted on four asphalt mastics using an environmental chamber capable of automatically controlling both relative humidity (85 %) and temperature (23 °C). The four mastics tested were identical in terms of bitumen type (40/60 pen), bitumen amount (25 % by of wt% total mix), mineral filler amount (25 % by wt%) and fine aggregate amount (50 % by wt%). The materials differed in terms of mineral filler type (granite or limestone) and fine aggregate type (granite or limestone). Preliminary data obtained during the early part of the study showed certain anomalous behavior of the materials including geometry (thickness)-dependent diffusion coefficient. It was therefore decided to investigate some aspects related to moisture diffusion in mastics by applying the Fickian and two non-Fickian (anomalous) diffusion models to the moisture sorption data. The two non-Fickian models included a two-phase Langmuir-type model and a two-parameter time-variable model. All three models predicted moisture diffusion in mastics extremely well (R 2 > 0.95). The observed variation of diffusion coefficient with thickness was attributed in part to microstructural changes (settlement of the denser fine aggregates near the bottom of the material) during the rather long-duration diffusion testing. This assertion was supported by X-ray computed tomography imaging of the mastic that showed significant accumulation of aggregate particles near the bottom of the sample with time. The results from the Langmuir-type model support a two-phase (free and bound) model for moisture absorbed by asphalt mastic and suggests about 80 % of absorbed water in the free phase remain bound within the mastic. The results also suggest that moisture diffusion in asphalt mastic may be time-dependent with diffusion decreasing by about four times during a typical diffusion test lasting up to 500 h. The study concludes that both geometry and time-dependent physical characteristics of mastic are important factors to consider with respect to moisture diffusion in asphalt mastics

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

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

Recommendation of RILEM TC237-SIB: protocol for characterization of recycled asphalt (RA) materials for pavement applications

This recommendation proposes an experimental protocol to characterize recycled asphalt materials. The guidelines presented in this document are based on the results of a round robin test organized by the RILEM Technical Committee 237-SIB “Testing and characterization of sustainable innovative bituminous materials and systems” and provide information on the testing procedure, data analysis and indications for the preparation of a test report