Development of double-helix macro BFRP fibers for concrete reinforcement

Basalt fiber is more cost-effective than carbon fiber and has superior mechanical and chemical properties than glass fiber. Most existing studies considered the use of micro basalt fibers in concrete matrix to reduce shrinkage and control micro cracks. However, macro fibers are often required to reinforce concrete material when large deformation and macro cracks in structural elements are of concern. Very few studies have been reported for macro Basalt Fiber Reinforced Polymer (BFRP) fibers to improve mechanical properties of concrete. The geometric characteristics of macro fibers are critical for concrete reinforcement. It is known that macro fibers with smooth surface possess poor bond strength. To enhance the bonding properties between macro BFRP fibers and concrete, in this experimental study, pullout tests on macro BFRP fibers with various geometric forms from different grades of mortar matrix were carried out. Smooth fiber, etched fiber and double-helix fiber were manufactured and evaluated. The effects of embedded length and matrix strength are also investigated through pullout behavior of single BFRP fiber. The variation of etched intervals of etched fibers on bond strength was also taken into account. It was found that the double-helix fiber outperformed other types of fibers in terms of strength utilization ratio and energy absorption capability. An empirical model for interfacial bond-slip relation was proposed based on the test results. Based on results of pullout tests, the outstanding performance of double-helix BFRP fiber was further verified by examining the mechanical properties of normal-strength concrete specimens reinforced with different types of macro BFRP fibers.