Enhancing the performance of UHSC columns intersected by weaker slabs

This paper presents strategies for enhancing the transmission of ultra-high-strength concrete (UHSC) column loads through normal-strength concrete (NSC) slabs in the joint of slab-column connections. Parameters of the investigation were the ratio of column concrete strength to slab concrete strength (f_cc ^′/f_cs ^′), as well as the placement of steel fiber-reinforced concrete, and the installations of high-strength dowel bars and special structural steel inserts in the weak slab joint. The beneficial effects of using high-strength dowel bars and high-strength structural steel inserts on the ability to transmit axial loads from the UHSC columns through the weaker slabs were demonstrated. The benefits of placing steel fiber-reinforced concrete in the joint are not as significant as the additional high-strength steel reinforcement. Predictions of the effective concrete strengths using current code approaches and using equations proposed in the literature were compared to the experimental results. The results indicate that the design approach in the CSA Standard (CSA A23.3-14) and the method proposed by Kayani (1992), which is based on the a data fitting approach, provide reasonable lower bound predictions of the effective strength for the UHSC columns with weaker slab layers.