Experimental Study to Improve Performance of Two-Stage Concrete Without Injection Focusing on the Interfacial Transition Zone

Abstract

Two-stage concrete (TSC), also known as preplaced aggregate concrete, prepacked concrete, and rock-filled concrete, is a non-conventional concrete with an unusual construction method. It is produced by firstly placing the coarse aggregate into the formwork and after that, the voids are filled with a high-flow mortar mixture. This type of concrete has been applied in mass concrete, underwater concrete, and repair and strengthening of existing structures with economical and technical benefits. Previous studies showed that the interfacial transition zone between the coarse aggregate and the cementitious material has a primary influence on TSC, affecting the performance of hardened concrete. Also, more than the mechanical resistance of the coarse aggregate, other factors such as shape, good particle size distribution, combined with a mortar with non-shrinkage, non-segregation characteristics, and good flowability are important to achieve satisfactory performance of TSC. In this experimental study, several types of admixtures (C–S–H type hardening accelerator and three types of expansive mineral admixtures) were added to a premixed high flow mortar to improve the interfacial transition zone between aggregate and mortar of TSC blocks without injection. Measurements of porosity, air permeability coefficient, and compressive strength were conducted for TSC cores and conventional concrete specimens. Experimental results showed that the use of Calcium Oxide (CaO) expansive admixture was the most effective method evaluated and that there is a high potential to expand applications of the TSC produced without injection, as it was possible to improve significantly both mechanical performance and mass transfer resistance, reaching similar values when compared to conventional concrete.