Parameters Influencing Fluidity of UHPC and Their Effect on Mechanical and Durability Properties

A number of parameters can influence the fluidity of an ultrahigh-performance concrete (UHPC) mix and, hence, its viability for use in construction. These parameters include the carbon content of silica fume, high-range water reducer (HRWR) dosage, water-cement (w/c) ratio, and proportion of fine sands used in the mix. However, the relationship between those variables and their effect on the fresh and hardened properties of UHPC is not yet well understood. Using spread as a measure of fluidity, the effects of these variables on the compression strength, direct tensile strength, air voids, chloride ion penetration resistance, and freeze-thaw (F-T) resistance are experimentally investigated. The test results indicate that changing the ratio of the silica sands used can lead to an enhancement in the packing density and a corresponding substantial increase in compressive strength. Increasing the HRWR dose results in a mild reduction in the compressive strength of the mixture but does not adversely affect durability. However, increasing the dosage beyond a limit can lead to fiber segregation, which can adversely affect the mechanical properties. A response surface that ties the water-cement ratio and HRWR dosage to the compressive strength and spread is proposed. The response surface can be used as a design aid to properly design or optimize a UHPC mix.