Moisture damage susceptibility of asphalt mixtures: Experimental characterization and modelling

A well-functioning, long-lasting and safe highway infrastructure network ensures the mobility of people and facilitates the transport of goods, promoting thus environmental, economic, and social sustainability. The development of sustainable highway infrastructure requires, among other activities, the construction of pavement systems with enhanced durability. Moisture damage in asphalt pavements is associated with inferior performance, unexpected failures and reduced service life. All of these contribute to the increase of operational and maintenance costs in order to fulfill the intended service life of the pavement system. Moreover, global warming and climate change events such as temperature extremes, high mean precipitation and rainfall intensity may further increase the probability and rate of pavement deterioration.
This dissertation aims to obtain an advanced understanding of the influence of moisture on pavement durability by developing a set of tools, i.e. experimental methods and computational models, which will provide insight into the fundamental moisture damage processes and on their impact on pavement systems. Based on this knowledge, researchers and practitioners will be able not only to design pavements with increased resiliency, thereby providing reliable services to road users, but also to minimize the risks in the face of changing climate conditions.
Moisture diffusion is well-known to degrade the mechanical properties of asphalt mortars, namely bitumen, filler and sand, thus increasing the propensity of pavements to cracking. To determine the changes in the cohesion properties of the mortar, uniaxial tension tests were performed. Mortar samples were prepared and then subjected to five combinations of moisture and thermal conditioning, in an attempt to reproduce the various conditioning states that pavements undergo in the field, before being tested. Tensile strength and fracture energy were used to evaluate the changes in mechanical properties due to the various conditioning protocols. To post-process the experimental data, a new data analysis procedure was suggested in order to obtain a more accurate calculation of fracture energy. The procedure uses nonlinear finite element analysis to specify the unloading response outside the fracture zone, and then utilizes this information to compute the fracture energy of the binders. This methodology yields a framework for the calculation of fracture energy when only force-displacement data are available and therefore the estimation of the true stress-strain curve is not feasible.
The experimental investigation revealed the deteriorating impact of moisture on the fracture characteristics of asphalt mortars, especially as regards to their low temperature properties. These effects were not reversible upon drying. On the contrary, the application of a drying cycle caused embrittlement of the mortars and indicated that continuous wet and drying cycles in the field may result in materials with poor performance characteristics. Also, the application of freeze-thaw cycles was shown to increase the susceptibility of mortars to low temperature cracking. Nevertheless, on the whole, the effect of freeze-thaw on fracture properties was observed to depend on the conditioning state (dry or wet) and composition of the mortars. The use of additives, such as hydrated lime filler and SBS modifiers, were found to improve the wet strength and fracture energy of the mortars. On the basis of moisture uptake measurements, it was confirmed that the chemical composition influences significantly the diffusivity characteristics of the mortars. Also, the maximum moisture uptake was found to be the main parameter that dictates the intensity of mortar damage.
In addition, moisture susceptibility was studied at mixture level. At this level, besides moisture diffusion, excess pore pressure can contribute to the degradation of mixture performance depending on the mixture type, the traffic loading and the environmental conditions. Hence, a moisture conditioning protocol that comprises two conditioning types, namely bath immersion and pore pressure application, was proposed for evaluating susceptibility of asphalt mixtures to moisture. Also, evidence was collected of the effect that dynamic pore pressure has on mixture degradation by means of X-ray computed tomography and image analysis techniques. The two damage mechanisms were found to be relatively independent from each other, suggesting that an asphalt mixture can be more prone to one damage mode than the other, depending on its composition. The proposed protocol captures both processes that occur when water interacts with a pavement and can provide more reliable conclusions with regard to mixture sensitivity.
In order to improve our perception of the influence of material microstructures on moisture sensitivity of the asphalt composite, an energy-based elasto-visco-plastic model with softening was implemented to model damage due to the coupled effects of moisture diffusion and mechanical loading. The model consists of a generalized Maxwell model, with hyperelastic springs and viscous time-dependent components, in series with an inelastic component that accounts for the irreversible processes within the microstructure of the material. Then, a computational scheme was proposed by means of a staggered approach: first a three-dimensional diffusion model was applied to obtain information on the accumulation of moisture within the mixtures and then the elasto-visco-plastic model was used to quantify mortar damage due to moisture diffusion. This method was successfully applied to study the influence of mixture morphology on moisture sensitivity. The results demonstrated that moisture content in a mixture strongly depends on its morphology, whereas the interconnectivity of the voids network controls the rate of damage development. Also, the analysis revealed the positive effect of using binders with high resistivity against moisture and quantified the benefits that would arise due to this choice, especially when designing porous mixtures that have an intrinsic sensitivity to moisture due to their morphological characteristics.
More broadly, frost damage can be classified as part of the moisture damage related mechanisms. In the field, frost damage can be mainly attributed to the expansion of water accumulated in the pores of the pavement at sub-zero temperatures that causes additional stresses to the pavement structure. A numerical scheme to simulate frost damage was proposed. This scheme comprises a model that simulates the volume expansion of water during the water-to-ice phase-change, a thermal conduction model to simulate temperature distribution in the pavement, and the elasto-visco-plastic model to determine critical areas with a propensity to cracking on the basis of the pavement stresses.
In conclusion, this thesis contributes to establishing a relationship of the physico-mechanical properties of the constituent materials and mixture morphology with the moisture susceptibility of pavement structures. The proposed experimental methods and computational models can serve as tools to investigate a great variety of parameters before a pavement structure is actually built. This allows for new materials and mixture designs to be investigated and the risks involved with their use to be minimized.