Experimental and numerical investigation on layered UHPC beams incorporated with recycled macro fibers

The high cost of production is a major hindrance to the wider use of ultra-high performance concrete (UHPC). Recently, the authors' group has proposed to replace expensive steel fibers in UHPC using macro fibers mechanically recycled from the solid waste of glass-fiber-reinforced polymer (GFRP). Such a concept has been justified by a series of tests, whose results indicate that the recycled macro fibers are effective in mitigating the development of cracks. However, a somewhat negative impact on UHPC strength would be generated by macro fibers. To lower the negative effect on concrete strength at the compression area and further increase the added value of macro fibers, this paper attempted to employ the layered UHPC structure, in which only the tension layer was reinforced with macro fibers. A total of 30 layered UHPC beam specimens were prepared and tested. The test variables comprised of the thickness of the bottom layer and the macro fiber volume fraction within the bottom layer. In addition, the fiber re-arrangement analysis was performed. The results revealed that, under the layer thickness of 50 mm, when the macro fiber volume fraction increased from 2 % to 6 %, the flexural toughness increased from 58.87 to 136.59 J (132.02 % enhancement). The maximum fiber efficiency was reached at the group with a fiber fraction of 2 % and a bottom layer thickness of 50 mm. Furthermore, numerical simulation was conducted to evaluate the feasibility of the tensile constitutive relation of UHPMFRC in the layered structure. The results indicated that it could not be directly used as model input in the numerical simulation of layered UHPMFRC structures when a strong fiber alignment effect was exhibited.