Distributed cracking mechanisms in micro-polypropylene based textile reinforced concrete

A pultrusion system was developed for manufacturing micro polypropylene textile-reinforced concrete (PP-TRC) composites. Warp-knitted textiles with pillar and tricot knitted patterns were produced from micro multifilament PP yarns and fed into the mortar in a computer-controlled production setup. Instrumented uniaxial tension mechanical tests were conducted on specimens with tricot and pillar knitting patterns of textiles at different volume fractions. Tensile properties of PP-TRC composites were compared to unidirectional fiber composites from a previous study. The strain-hardening mechanism was observed due to the formation of distributed cracking and enhanced tensile response. Digital Image Correlation (DIC) was used to measure the crack spacing and width distributions as a function of applied strain. The nature of crack formation shows the extent of parallel cracking in TRC to be more uniform and densely packed compared to the uniaxial PP fiber composites. The homogeneous distribution of cracks is observed in both textile knitting patterns as measured by the distributed cracking parameters. A significant difference between the average crack width-strain curves of pillar and tricot knits is not observed. The strain measured from DIC was correlated with the damage parameters, the sequence of evolution of cracks, crack spacing, and crack width. A procedure is developed to measure the allowable stress at the ultimate crack width limit. Results show that a limit state of 1 mm crack opening will correspond to the maximum allowable stress of 7.5 MPa that corresponds to a nominal strain of about 7% as the limit state for PP-TRC materials.