Microstructural Characterization Of Portland Cement Concrete Containing Reclaimed Asphalt Pavement Aggregates Using Conventional And Advanced Petrographic Techniques PDF Download

The Alkali-Silica Reaction (ASR) is one main detrimental factor to affect the durability of concrete. The research comprises two parts, i.e. microstructural characterization of ASR products (3 phases), and modeling of concrete damage due to ASR. The experimental results will provide new findings on the microstructure properties of ASR-damaged concrete. The work in the first phase of the research aims at characterizing the micromechanical properties of ASR products by new techniques of nanoindentation and micro-indentation, with emphasis on their viscous behavior. The concrete samples were extracted from a heavily ASR-affected concrete pavement in Bécancour (Québec). The concrete is characterized by numerous fine-grained limestone aggregate particles with microcracks filled with secondary reaction products that extend into the cement into a network from one aggregate particle to another. After careful sample preparation (polishing), the surface of the aggregate particle and of the veinlets (i.e. cracks filled with crystalline ASR product within the aggregate particles) was examined by Atomic Force Microscopy (AFM) before nanoindentation testing. Both nanoscale and microscale indentation modulus and hardness of ASR products were measured. The test results show that ASR crystalline products exhibit important relaxation behavior of about 40%. Then, a simplified rheological model was proposed to fit the load relaxation curves and their asymptotic values. These results suggest that ASR product relaxation is significant and mostly irreversible. The second research phase explored the use of the novel micro-scratch technique to characterize the fracture energy (i.e., toughness) of the ASR-affected limestone aggregate particles within a core specimen extracted from a heavily ASR-affected concrete bridge from the Québec City area. The ASR-affected aggregate particles were typically showing "zoning" (i.e. light grey and dark grey) surrounding white veinlets within reacted limestone aggregate particles. As a reference, an undamaged/virgin quarried limestone specimen from a local quarry was selected and subjected to similar testing. Besides the elastic properties, the toughness of the reactive aggregate particles was statistically measured to be around 1.5 MPam1/2. The fracture toughness of reactive aggregate particles was affected neither by the bedding line directions nor by the "zoning" that was first thought to correspond to "reacted" portions of the particles. Besides the major cracks filled by ASR products, the results indicated that the surrounding reactive aggregate was not characterized by any significant internal damage distribution. In the later phase of the experimental program, our research focused on characterizing the creep and stress relaxation properties of the ASR crystalline products typically filling microcracks within reactive limestone particles (specimen from the heavily ASR-affected concrete pavements in Bécancour (Québec) used in phase 1). The testing carried out was micro-indentation under controlled relative humidity. It was found that an increase in relative humidity strongly reduces the irreversible creep deformation of ASR crystalline products, which act a greater characteristic time. That is, the water content seems to favor irreversible sliding mechanisms along/between the ASR crystals under constant load. Finally, the implications the research findings are discussed with respect to the stress build-up process within reactive aggregate particles. The rheological property of ASR products may play a critical role to releasing the internal stress induced by the ASR product expansion. Finally, a "1D thought model" is proposed as a new research avenue to account for the major results of this work into ASR-damaged concrete modeling, i.e., the visco-elastic property of ASR products and the damage toughness of reactive aggregates.