Experimental investigation on characterizations of gas permeability for ultra-high performance concrete

The aim of this study was to characterize the gas permeability of ultra-high performance concrete (UHPC) and establish the corresponding estimation methods to guide counterpart permeability control and durability design. This paper presents the findings of an experimental study on 33 groups of specimens parameterized by the water -binder ratio, curing regimes, and inclusions of silica fume, steel fibers, polypropylene fibers, coarse aggregates, and nanomaterials using a nitrogen-based CEMBUREAU methodology. Mercury intrusion porosimetry, scanning electron microscopy, and backscattering electron microscopy were conducted to scrutinize the pore structure and microscopic morphology of UHPC to reveal the corresponding mechanism. Regression models for the intrinsic gas permeability coefficients of UHPC were developed, considering the effects of the experimental parameters. The results reveal that the intrinsic gas permeability coefficient of UHPC does not exceed 1.32 x 10 -19 m 2 at a standard curing age of 90 d. A lower water -binder ratio and the incorporation of silica fume can decrease the intrinsic gas permeability coefficient of the UHPC matrix, owing to a decrease in the internal capillary pore volume and a denser microstructure. The inclusion of steel fibers, polypropylene fibers, and coarse aggregates can significantly increase the gas permeability of UHPC. This is caused by the creation of interfacial transition zones with pores and microcracks, facilitating the formation of gas pathways in the UHPC matrix. The incorporation of nanomaterials, including nano-SiO 2 and nano-CaCO 3 , can decrease the intrinsic gas permeability coefficient to lower than 10 -22 m 2 , owing to the role of the fine particles in improving the density of the microstructure. Compared to standard curing at a temperature of 20 +/- 2 degrees C and a relative humidity higher than 95 %, heat treatment at 90 degrees C exhibits superior improvement of the gas permeability by accelerating the hydration process. The nitrogen-based CEMBUREAU methodology is considered feasible for the development of a standard test protocol, and the proposed regression models well reflecting the influences of various test parameters can be used in the gas permeability control of UHPC.