Curvature-Deflection Relations for Polypropylene Fiber-Reinforced Deflection-Hardening Alkali- Activated Slag Composite

To obtain a tensile strain-hardening behavior by using the ordinary polypropylene (PP) fiber, the micromechanical model recommends the increase of the fiber low interfacial bond with an ordinary cementitious matrix. It was predicted that the alkali-activated slag (AAS) composite high drying shrinkage performance would affect the fiber interface frictional bond during the pullout. Generally, twenty different PP fiber reinforced AAS composites with drastically different material properties were obtained in this research. In order to alter the drying shrinkage performance of the composites, two different curing methods, namely, heat-treatment and laboratory temperature, were applied to the developed specimens. All examined composites showed a deflection-hardening response in flexure; for most of them, the flexural strength was significantly increased from the cracking strength and the responses were similar to flexural response of a strain-hardening cementitious composite (SHCC). However, for some of the mixing proportions, the composites did not display a tensile strain-hardening behavior; indeed, given slight increase in flexural strength. Among the experimented composites, the two mixing proportions were different only in the curing method, altering the drying shrinkage performance of the AAS composites. Practically, the AAS composite with the lower shrinkage performance showed a considerable increase in the flexural strength and a high deflection capacity; however, the deflection capacity for the AAS composite with the higher shrinkage performance was much further improved. As the first stage of the research, the flexural behavior of all twenty different composites was studied in detail by using the Digital Image Correlation (DIC) technique. The DIC inspections considered the strain data at the beam mid-span, which was experimentally distributed linearly across the beam depth; also, the curvature distribution data along the beam during the test had a good agreement with the expectations. According to the structural mechanics and the changes occurring during the four-point bending test (FPBT) in the curvature distribution along the deflection-hardening cementitious composites, the initial and ultimate state boundaries were also set theoretically for the mid-span curvature in relation to the load-point deflection. Basically, the mid-span curvature was calculable by either the strain or curvature distributions; however, due to the cracking nature of the composites, a couple of issues were influential on the strain distribution calculation method for some of the bending specimens examined. According to the theory of the curvature-deflection relation and, the experimental data, the mid-span curvature was estimated by a polynomial which would serve as a simple calculation for different material properties at the ultimate flexural strength.

Keywords: Flexural Deflection-Hardening; Strain-Hardening Cementitious Composites (SHCC); Polypropylene (PP) fiber, Alkali-Activated Slag (AAS); Strain Distribution; Curvature Distribution; Digital Image Correlation (DIC)