Effect of Coarse Aggregates and Hybrid Fibers on the Mechanical Properties of Ultra High Performance Concrete (UHPC)

To evaluate the mix design method for coarse aggregate ultra high performance concrete (CA-UHPC) matrix and explore the effect of fiber hybrid on the mechanical properties of CA-UHPC, two concepts for CA-UHPC matrix mix design were proposed based on modified Andreasen & Andersen (MAA) model. Then uniaxial tensile tests, compressive strength and workability measurements, and microstructural analysis were conducted on CA-UHPC with different coarse aggregate volume contents (10%,12%,14%,17%) and different types of fiber hybrid (straight steel fiber, hook-end steel fiber, copolymeraldehyde (POM) fiber). Test results show that a dense particle packing skeleton of CA-UHPC can be designed via MAA model regardless of the inclusion of coarse aggregates. Both the interfacial bond behavior between coarse aggregate and UHPC matrix, and that between fiber and UHPC matrix are excellent for CA-UHPC designed with two concepts of MAA model. It is found that the prepared CA-UHPC with hybrid fibers can achieve synergistic improvement of workability and tensile toughness. The improvement effect of hook-end steel fiber and POM fiber is more significant on tensile toughness and workability, respectively. Some slender flocs existed on the surface of the POM fiber pulled-out from UHPC matrix, indicating that the mechanism of POM fiber on CA-UHPC tensile toughness enhancement is the super chemical bond behavior.