Influence of interface transition zones (ITZ) and pore structure on the compressive strength of recycled aggregate concrete

This study aims to clarify the influence of interfacial transition zones (ITZs) and pore structure on the compressive strength of the recycled aggregate concrete (RAC). The compressive strength of the RAC are evaluated with the change of five different replacement ratios (10 %, 20 %, 30 %, 40 % and 50 %) and four various of particle sizes (5-10 mm, 10-16 mm, 16-20 mm and 20-25 mm) of the recycled coarse aggregate (RCA), the evolution process of crack propagation behavior have been accurately characterized, and the ITZs and pore structure of the RAC are analyzed. The results showed that the RAC incorporated 30 % RCA with particle size of 10-16 mm exhibited the maximum compressive strength, crack area and fractal dimension. There is a new interfacial transition zone (ITZ2) with an average elastic modulus and microhardness of about 15 GPa and 1 GPa respectively between RCA and new matrix, which deteriorates the homogeneity inside RAC; As the particle size of recycled aggregate increases, the width of ITZ2 first increases and then decreases. When the particle size of aggregate is 10-16 mm, the width of ITZ2 reaches the maximum of 50 mu m. Compared with ITZ2, the old interfacial transition zones (recycled aggregate-bonded mortar, ITZ3, new mortar-bonded mortar, ITZ4) have weaker mechanical properties, larger widths, and more obvious interface effects. Under uniaxial compression load, the old ITZs (ITZ3 and ITZ4) are an important factor affecting the performance and crack development of RAC. Taking the harmful pore structure of adhered mortar phase, new mortar phase, and ITZs phase as independent variables and the compressive strength of RAC as the dependent variable, a multiple linear regression model is established; through the analysis of the regression model, it is found that the ITZs are the main factor affecting the compressive strength of RAC.