Interfacial transition zone composition and bonding in cementitious materials with asphalt-coated particles

This research investigates the microstructural changes that reclaimed asphalt pavement (RAP) aggregates produce as well as the large-scale response testing of concrete slabs containing RAP. The main objectives were to examine the fundamental bonding and interaction between asphalt and cement paste in order to explain the properties observed in laboratory-sized specimens and to quantify how the RAP aggregates affected the flexural capacity of large-scale concrete slab despite its known reduction in tensile strength and modulus.;Microstructural characterizations were carried out by principally studying the interfacial transition zone (ITZ) and the interfacial bond energies between asphalt (RAP) and cement paste. Studies of the ITZ were performed using Euclidean distance mapping and image analysis of compositional backscatter electron micrographs of polished epoxy-impregnated samples to examine the porosity, calcium hydroxide (CH), and unhydrated cement distributions. The findings suggested that concrete with RAP aggregates has a larger, more porous ITZ, which explains, in part, the reduced strength observed for concrete with RAP, as the more porous ITZ allows for easier crack initiation. The CH morphology was not greatly affected by the presence of the asphalt, although there was less CH immediately at the asphalt-cement paste interface. Another finding was that silica fume did somewhat improve the ITZ properties of mortar with RAP but did not significantly improve the composition relative to the dolomite ITZ, which explains why silica fume has not been reported to significantly improve the bulk mechanical properties of concrete with RAP aggregates.;The fundamental bonding potential between asphalt and cement was investigated by measuring the surface free energy of the materials. Chemical treatments of the asphalt samples were also performed to determine if this bonding potential could be improved. Using Fourier transform infrared spectroscopy, it was found that certain chemical treatments, such as nitric acid, chromic acid, and maleic anhydride, oxidized the asphalt surface, as measured by increased carbonyl and sulfoxide spectroscopic indexes. Using the sessile drop method to measure surface free energy, these chemical treatments increased the energy of interaction and interfacial energy between asphalt and cement paste. However, the work of adhesion between asphalt and cement paste was found to be higher than the asphalt work of cohesion; this leads to crack propagation occurring preferentially through the asphalt binder and not through the ITZ nor directly at the RAP-cement paste interface. This is the other mechanism for the reduced strength observed in concrete with RAP. Furthermore, this explains why improvements to the microstructure are not effective in improving the bulk strength. Thus, the reduction in concrete strength with RAP aggregates has been attributed to (1) the larger, more porous ITZ and (2) the dominance of asphalt cohesion failure. The reduction in modulus is primarily a function of the larger, more porous ITZ caused by the asphalt on the RAP particles. Using established multiphase elastic models, it was found that accounting for the larger, more porous ITZ did predict composite moduli that were similar to the experimentally-measured values.;The behavior and failure of concrete containing RAP with large-scale concrete slabs was investigated to determine if the trends from the laboratory-sized specimen tests were still valid. Despite the strength and modulus reductions, monotonic slab test results showed that the flexural capacity (by edge loading) of concrete slabs with recycled materials---45% coarse RAP, 100% coarse recycled concrete aggregate (RCA), and a blend of 45% coarse RAP and 55% coarse RCA---is similar to the flexural capacity of concrete slabs with (virgin) dolomite aggregates. This finding was linked to the experimental results that the inclusion of the recycled aggregates did not statistically affect the fracture properties of the concrete; rather the total fracture energy may actually increase relative to virgin aggregate concrete.;Further testing in this study examined the effects of RAP containing steel furnace slag (SFS) on the properties of concrete. It was found that, despite years in service, the SFS in the RAP retains significant contents of expansive free calcium oxide (CaO). When tested for expansion with the asphalt coating intact, it was found that the expansion was negligible to minimal, whereas if the asphalt coating was removed, the expansion was profound, since residual free CaO and MgO existed in the SFS aggregate. Testing of various virgin SFS and SFS RAP indicated that some SFS sources may exhibit little to no expansion (i.e. contain little free CaO), thus concluding that the SFS and SFS RAP source should be chemically and mineralogically characterized and tested for expansion potential prior to being considered for use in any bound application. When used as a coarse aggregate in concrete, the SFS RAP and virgin SFS aggregates had suitable and favorable mechanical properties. (Abstract shortened by ProQuest.).