Zhang, Jizhe
(2016)
Moisture damage of aggregate-bitumen bonds.
PhD thesis, University of Nottingham.
Abstract
Moisture damage of the asphalt mixture is defined as the loss of strength, stiffness and durability due to the presence of moisture (in a liquid or vapour state) leading to adhesive failure at the aggregate-bitumen interface and/or the cohesive failure within the bitumen or bitumen-filler mastic. The presence of moisture can accelerate the distress of asphalt pavement in several different modes, such as rutting, fatigue cracking, thermal cracking and the formation of potholes. In the field, the moisture damage normally happens first at the interface of two pavement layers or at the bottom of pavement layers and develops gradually upward. Once moisture has come into contact and interacted with the asphalt mixture, moisture damage could be developed by the following mechanisms: detachment, displacement, spontaneous emulsification, pore pressure, and hydraulic scour. It should be mentioned that moisture damage is not limited to only one mechanism but is the result of a combination of several mechanisms.
As mentioned previously, the common modes of moisture damage of asphalt mixtures are a loss of adhesion between the aggregate and bitumen and/or a loss of cohesion in the mixture. Among these two failures, the adhesive failure is recognised as the main mode of moisture damage. Hence, the physico-chemical interactions between aggregates and bitumen in the presence of moisture are believed to partially govern the moisture sensitivity of asphalt mixtures, which can also affect the serviceability, performance and durability of the asphalt pavement.
This thesis describes the work that was carried out with regard to the moisture damage evaluation of aggregate-bitumen bonds through different procedures. The fundamental properties of the individual material such as the chemical composition and rheological properties of bitumen, moisture absorption, surface morphology and mineralogical composition of aggregates were first characterised. Two types of equipment, namely the dynamic contact angle (DCA) analyser and dynamic vapour sorption (DVS) system were used for determining the surface energy of the bitumen and aggregates, respectively. The obtained surface energy results were then combined thermodynamically to determine the work of adhesion between aggregate and bitumen, and the reduction in the adhesive properties if water is introduced into the system. Three established mechanical tests consisting of the standard peel test, Pneumatic Adhesion Tensile Testing Instrument (PATTI) test and a pull off test were developed and redesigned to make sure that these tests are practical, reliable and feasible to measure the bonding strength of aggregate-bitumen combined specimens. The composite substrate peel test (CSPT) was developed to prepare composite substrates using crushed coarse aggregates as a more practical replacement for the aggregate substrates prepared from aggregate boulders. Finally, the moisture damage results from mechanical tests and thermodynamic results were compared and correlated with the basic physico-chemical properties of the original materials.
The results showed that in the dry condition, all techniques used in this research, including the mechanical tests and the surface energy tests led to similar results, with bitumen rather than aggregates dominating the bonding properties of aggregate-bitumen systems. After moisture conditioning, the four mechanical tests, including standard peel test, CSPT, PATTI test and pull-off test showed similar moisture sensitivity ranking and failure surface results demonstrating the good correlation between these four tests. In addition, based on the comparison conducted, the four mechanical tests are all considered to be reliable to evaluate the moisture sensitivity of different aggregate-bitumen systems. However, based on the aggregates considered in this research, the moisture sensitivity parameters obtained from the surface energy tests are suggested unreliable to evaluate the moisture sensitivity of aggregate-bitumen systems.
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