Abstract: “Interfacial Transition Zone Composition and Bonding in Cementitous Materials with Asphalt-Coated Particles” by Alexander Sebastian Brand—Advisor, Dr. Jeff Roesler, University of Illinois at Urbana-Champaign. DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Civil Engineering in the Graduate College of the University of Illinois at Urbana-Champaign.
Abstract:
The use of reclaimed asphalt pavement (RAP) as a partial replacement of coarse aggregate in concrete paving mixtures has seen recent interest as a result of significant availability of stockpiles of RAP materials. While previous research has primarily focused on the mechanical and durability properties of concrete with RAP aggregates, this research investigates the microstructural changes that 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, such as through densification and pozzolanic reactions with silica fume and with asphalt chemical treatments, 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 multi-phase 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. This is consistent with the microstructural observations that failure cracks propagate around aggregates and primarily through the asphalt coating. Therefore, a certain replacement percentage of virgin aggregates with RAP can be used in concrete pavement applications without detriment to the flexural load capacity along with application in a two-lift pavement construction.
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. The proposed necessary suite of tests includes complexometric titration for free CaO determination, x-ray diffraction, and the autoclave expansion test. When used as a coarse aggregate in concrete, the SFS RAP and virgin SFS aggregates had suitable and favorable mechanical properties.
The findings from these investigations lead to the conclusion that, at this time, RAP can perform suitably as an aggregate replacement in concrete up to certain volume fractions. The failure mechanism of concrete with RAP occurs primarily by asphalt coating cohesion failure with the larger, more porous ITZ allowing for easier crack initiation. This failure mechanism provides similar to greater fracture properties (i.e. total fracture energy), which results in similar to greater slab flexural load capacities relative to virgin aggregate concrete. Additionally, not all RAP materials will perform or behave the same, so each source needs to be characterized, trial mix tested, and possibly tested for expansion potential (as is the case with SFS RAP), prior to usage as an aggregate in concrete.
To read the dissertation, and for more information, please contact: Dr. Jeff Roesler: E-mail: jroesler@illinois.edu.