This paper investigates the mechanical characteristics (encompassing compressive strength, flexural strength, toughness, and impact resistance) of ultra-high performance fiber-reinforced concrete (UHPFRC) incorporating polypropylene (PP) and polyvinyl alcohol (PVA) fibers. An experimental program was conducted, based on which the polymer and metallic fibers were utilized at the same fiber content, and different sets of single and hybrid fiber-reinforced composites were fabricated and tested. Despite the fact it has been exhibited through previous research that the hybridized PVA-PP fibers do not result in the development of the mechanical characteristics of engineered cementitious composites (ECC), the UHPC composites incorporating such hybrid fibers show augmented levels of toughness, flexural strength, and resistance to impact loads. A comparison was also made to assess the potentiality of the used fibers in terms of environmental impact and cost. Based on the results, hybridization with PVA and PP fibers leads to remarkable improvement in technical performance and mitigation of the economic and environmental impact of UHPFRC composites.

This paper is aimed at investigating the effects of different fiber inclusion on the mechanical properties of ultra-high-performance concrete (UHPC) by adding mineral admixtures as cement replacement materials to reduce production costs and CO2 emissions of UHPC. Throughout this research, 21 mixture designs containing four cement substitution materials (silica fume, slag cement, limestone powder, and quartz powder) and three fibers (steel, synthetic macrofibers, and polypropylene) under wet and combined (autoclave, oven, and water) curing were developed. To investigate the mechanical properties in this research, a total of 336 specimens were cast to evaluate compressive strength, the modulus of rupture (MOR), and the toughness index. The findings revealed that at the combined curing, regarded as a new procedure, all levels of cement replacement recorded a compressive strength higher than 150 MPa (21.76 ksi). Furthermore, the mechanical properties of the mixture design containing microsilica and slag (up to 15%) were found to be higher than other cement substitutes. Also, it was shown that all levels of the fiber presented the MOR significantly close together, and samples made of synthetic macrofibers and steel fibers exhibited deflection-hardening behavior after cracking. The mixture design containing microsilica, slag, limestone powder, and quartzpowder, despite the significant replacement of cement (approximately 50%) by substitution materials, experienced a slight drop in strength. Therefore, the development of this mixture is optimal both economically and environmentally.

An experimental campaign, performed on different types of ultra-high-performance fiber-reinforced cementitious composite (UHP-FRCC)—made with four replacement rates (0, 20, 50, and 70%) of cement with fly ash and cured for 1, 4, and 13 weeks—is described in this paper. Specifically, 72 cylinders were tested to measure the compressive strength and Young’s modulus of elasticity; stress-strain relationships were obtained from 72 dumbbell-type specimens subjected to uniaxial tension, and 12 beams, tested in four-point bending, provided the moment-curvature diagrams. The best UHP-FRCC was selected through an eco-mechanical analysis, capable of combining the mechanical performance with the environmental impact of concrete. When the ultimate bending moment of a beam is the functional unit of this analysis, the higher the replacement rate of cement, the better the beam performance, although material properties and structural ductility show opposite trends.

Ultra-high-performance concrete (UHPC) is a relatively new class of cementitious material that exhibits superior durability and long-term stability, as well as compressive and tensile strength properties when compared to conventional concrete. Currently, several testing methods exist in the literature to evaluate the tensile strength of fiber-reinforced concrete, but there is still a lack of agreement on which method is most suitable for UHPC. This study compares three different test methods to estimate and evaluate the tensile response of a standard UHPC mixture based on experimental testing. The tests include the direct tension test, the doubleedge wedge-splitting test, and the four-point bending test. The test specimens were deconstructed to determine fiber distribution through the depth and along the length of specimens to explain the experimental observations and understand the correlation between fiber distribution and measured tensile strength.

The maximum post-cracking tensile strength (σpc) recorded in numerous investigations of ultra-high-performance fiber-reinforced concrete (UHP-FRC) remains mostly below 15 MPa, and the corresponding strain (εpc) below 4/1000. Both values are significantly reduced when the specimen size increases, as is needed for real structural applications. Test data on σpc and εpc from close to 100 series of direct tensile tests carried out in more than 20 investigations are analyzed. Factors influencing the strain capacity are identified. However, independently of the numerous parameters encountered, two observations emerged beyond all others: 1) the higher the post-cracking tensile strength (whichever way it is achieved), the higher the corresponding tensile strain; and 2) fibers mechanically deformed and/or with slip-hardening bond characteristics lead to an increase in strain capacity. A rational explanation for these observations is provided. The authors believe that achieving a large strain (εpc) at maximum stress is paramount for the successful applications of ultra-high-performance concrete in concrete structures not only for strength but, more critically, for ductility and energy absorption capacity improvements.