Spatial distribution of steel fibers and air bubbles in UHPC cylinder determined by X-ray CT method

•Steel fiber and air bubble distribution in UHPC were evaluated by X-ray CT method.•The lower yield stress of fresh mixture aggravates the homogeneity of UHPC along depth.•Relationship between splitting tensile strength and steel fiber content is presented. This paper investigated the spatial distribution of steel fibers and air bubbles in UHPC cylinder specimen by using X-ray CT system. Four levels of superplasticizer dosage were adopted to obtain cement mortar mixtures with different rheological properties and then steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cylindrical specimens with diameter of 100 mm and height of 200 mm were prepared under the same condition for every mixture and the content of steel fibers and air bubbles at different casting depth was determined by X-ray CT measurement. On the other hand, the cylinder specimen was cut into four pieces for evaluating the splitting tensile strength variation with the casting depth. The results reveal that steel fibers and air bubbles are uniformly distributed in UHPC specimen when the fresh mixture is prepared by adding a lower dosage of superplasticizer. This homogeneity is also proved by the splitting tensile strength for samples at different depths. When the yield stress of fresh mortar was reduced by the higher dosage of superplasticizer, the steel fiber sedimentation and air bubble rise occurred and the splitting tensile strength was significantly decreased at the top portion of cylinder specimen. The influence of steel fiber content on splitting tensile strength can be regressed into a good quadratic equation. Therefore, the balance between homogeneity and flowability should be fully considered for manufacturing UHPC elements with a high depth.