Optimization of concrete mix designs toward the bond properties of steel reinforcement in self-healing concrete by Taguchi method

Abstract

The bond of steel reinforcement in normal concrete has been extensively studied, while its bond properties in self-healing concrete has rarely been investigated because the main focus of developing self-healing concrete relies on the improved healing performance such water tightness and strength regain. In fact, the mix design of concrete will greatly affect the bonding. This study elaborates on the optimisation of mix designs for self-healing concrete toward the bond properties of steel reinforcement. The proposed healing agents were bacterial healing agent (BAC) and crystalline admixtures (CA). Mix design parameters include water-cement ratio (w/c) (0.40, 0.50 and 0.60), fine ratio (FR) (0.34, 0.44 and 0.54), and healing agent (HA) (BAC, CA and REF, without healing agent). The Taguchi method was used to formulate nine self-healing concrete mixes by considering three factors and three levels in the design. The compressive strengths of all different concrete mixtures were initially assessed and the pull-out tests on a single steel rebar embedded in concrete were performed in three different states of concrete (uncracked, cracked and healed). The sizes of the cracks were varied and three groups were identified: cracks with a width of 200-300, 300–400 and 400-500 μm. Water immersion was selected as the healing regime with two healing periods lasting 28 and 112 days. The Taguchi analysis and in-depth statistical evaluations were performed for optimisation purpose. Results showed that the compressive strength considerably increased as lowering the w/c, increasing the FR and incorporating the healing agents. The bond of steel reinforcement with the concrete matrix in the uncracked state improved after the BAC or CA was added into the concrete. The formation of cracks in the concrete significantly reduced the bond strength. After being subjected to healing, the bond strength slightly recovered. A longer healing time induced a better bond strength recovery. The phenomenon of crack closure was found at all healed specimens and the healing products were confirmed to be calcium carbonates.