Effect of coarse aggregates and hybrid fibers on mechanical properties of ultra high performance concrete

The development direction of ultra high performance concrete (UHPC) is shifting toward realizing a balance between its comprehensive properties, such as low cost, high mechanical performance, and environmental friendliness, in a bid to facilitate its large-scale application. Accordingly, coarse aggregate UHPC (CA–UHPC) has recently attracted considerable attention from academia and engineering. CA–UHPC has a satisfactory application prospect in long-span lightweight composite bridges, wet joints of bridges, and other fields. Two concepts of the CA–UHPC matrix mix design are proposed based on the modified Andreasen & Andersen (MAA) model to evaluate the accuracy of the mix design method for the CA–UHPC matrix and the effect of fiber hybrid on the mechanical properties of CA–UHPC are explored. Subsequently, direct tensile tests, compressive strength and workability measurements, and microstructural analysis are conducted on CA–UHPC with different CA volume fractions (10%, 12%, 14%, and 17%) and fiber hybrid types (Straight steel fiber, hook-end steel fiber, and copolymeraldehyde (POM) fiber). The influence rules of CA volume fractions and fiber hybrid types on the tensile stress–strain curves, tensile toughness ratios, cubic compressive strength, and slump flow of CA–UHPC are revealed. A fiber image analyzer (microscope) is used for observing microstructures of the CA–UHPC matrix and fibers. The test results demonstrate that a dense particle packing skeleton of CA–UHPC can be realized via the MAA model, irrespective of the inclusion of CAs. The interfacial bond behavior between the CA and UHPC matrix and that between the fiber and UHPC matrix are concluded to be excellent for CA–UHPC designed using the two concepts of the MAA model based on the interfacial microstructural analysis. Additionally, the prepared CA–UHPC with hybrid fibers can achieve the synergistic improvement of workability and tensile toughness. CA–UHPC with the hybrid type of straight steel and hook-end steel fibers exhibits more considerable tensile toughness, while CA–UHPC that with the hybrid type of straight steel and POM fibers shows superior workability. The surface morphology of the POM fiber pulled out from the UHPC matrix shows that some slender flocs exist on the POM fiber surface, corresponding to several curly filaments scraped and torn from the POM fiber to release the tensile stress during the drawing process. This indicates that the action mechanism of POM fibers on the tensile toughness enhancement of CA–UHPC is the effective chemical bond behavior between the POM fiber and the CA–UHPC matrix. Relative research achievements on the mechanical properties of hybrid fiber CA–UHPC in this paper provide data support for the optimal design of the CA–UHPC matrix mix and establish a theoretical foundation for improving the toughness of CA–UHPC.