Effect of emulsified asphalt on engineering properties and toughening effect of ultra-high performance concrete

At present, cracking is the main form of damage to ultra-high performance concrete (UHPC), and increasing flexural toughness is the primary technical approach to addressing this issue. Currently, fiber, polymer, or nanomaterial modification is commonly used to improve UHPC toughness. Emulsified asphalt has also demonstrated the potential to toughen and resist cracking. To investigate the toughening effect of emulsified asphalt in cementitious materials, this study employed a method in which emulsified asphalt and water are added simultaneously during wet mixing to prepare UHPC specimens. The effects of different types and dosages of emulsified asphalt on the workability, mechanical strength, and flexural toughness of UHPC were investigated through extensibility tests, flexural toughness tests, compressive strength tests, flexural strength tests, and tensile strength tests. The results showed that the workability of UHPC mixes with two types of emulsified asphalt gradually decreased as the emulsified asphalt dosage increased. When the dosage of either type of emulsified asphalt exceeded 3%, the extension did not meet the specification requirements. Furthermore, as the dosage of the two types of emulsified asphalt increased, the mechanical strength of the UHPC slightly decreased. Compared with the same dosage of cationic emulsified asphalt, anionic emulsified asphalt had a smaller impact on the mechanical strength of UHPC because anionic emulsified asphalt and cement hydration products both carried the same charge, causing them to repel each other. This mutual repulsion reduced the influence of emulsified asphalt on the cement hydration reaction process, which was conducive to the generation of cement hydration products. Consequently, the negative impact on the formation of UHPC strength was lessened, and the uniformity of the steel fiber distribution within the UHPC was ensured. In terms of flexural toughness, both types of emulsified asphalt enhanced the flexural toughness of the UHPC, with anionic emulsified asphalt providing better results than cationic emulsified asphalt. Compared with cationic emulsified asphalt, anionic emulsified asphalt resulted in better workability and mechanical strength in the UHPC, and its toughening effect was more significant. Considering both mechanical properties and overall performance, the use of anionic emulsified asphalt in UHPC is recommended, with an optimal dosage of 3%. The microstructure of UHPC specimens without emulsified asphalt and those mixed with 3% anionic emulsified asphalt was observed using a scanning electron microscope. The observations revealed that emulsified asphalt could fill the original microcracks in UHPC, improve the internal structure of the matrix, increase the effective contact area between steel fibers and the matrix to a certain extent, and enhance the toughening effect of steel fibers. Additionally, the viscoelastic properties of emulsified asphalt increased the energy required for crack propagation, effectively reducing the generation and development of microcracks and serving as a buffer for the destabilization damage of hydration products.