Effect of confinement on the axial load response of ultrahigh-strength concrete columns

Twelve ultrahigh-strength concrete (UHSC) columns confined by rectangular hoops and crossties were constructed and tested under pure axial load. Primary variables of the investigation were the amount, spacing, and configuration of transverse reinforcement, and concrete compressive strength, ranging from 50 to 200 MPa. The amount of transverse reinforcement ranged from 53 to 151% of that required by the American Concrete Institute (ACI) seismic design provisions. Four different configurations of transverse reinforcement were investigated that included hoops as well as hoops and crossties. The behavior of UHSC columns are characterized by sudden spalling of the concrete cover and extremely brittle failure unless the columns are adequately confined with transverse reinforcement. Therefore, more confinement is required in a column with higher concrete strength than in a column with lower concrete strength to achieve a reasonable postpeak response and sufficient deformability. The amount of transverse reinforcement has a pronounced effect on the postpeak behavior of UHSC columns. It is concluded that the ACI provisions overestimate the maximum axial load by 8-21%; however, the seismic design provisions provide for significant postpeak response of the UHSC columns in this test series. A design expression is proposed for the amount of seismic confinement for 200 MPa UHSC columns, based on the effective confinement index.