There are too many reinforced concrete (RC) columns constructed before the mid-1970s without sufficient transverse reinforcement. By now, we understand quite well the critical role of transverse reinforcement in enabling a column to maintain its integrity under large displacement reversals in the nonlinear range of response. Poorly confined RC columns undergo a fast resistance decay due to the formation of criss-crossing inclined cracks, which can cause an abrupt failure or gradual disintegration and trigger collapse of the structure. Those columns need to be strengthened to increase their drift capacity. Although there are several alternatives to retrofit RC columns, they often require specialised workmanship and equipment, and involved installation procedures. This research examines an easy-to-design and easy-to implement retrofit and repair technique involving external post-tensioned clamps fastened around the column.

Ten large-scale RC columns and six beams were strengthened with the proposed clamps. Test results suggest that the lateral prestress applied by the clamps 𝜎𝐿 increases the shear at inclined cracking and drift capacity. The unit shear stress at inclined cracking was observed to be nearly proportional to √(1 + 𝜎𝐿/𝑓𝑡 ), where 𝑓𝑡 is the tensile strength of the concrete. Test results from RC beams with clamps suggest that such increase may lead to similar increases in the shear stress at failure 𝑣𝑢. This observation implies that the fraction of shear strength attributed to the concrete (𝑣𝑐) is also proportional to √(1 + 𝜎𝐿/𝑓𝑡 ). Regarding column drift capacity, measured drift capacities of columns strengthened with clamps were compared with drift capacity estimates of similar columns with rectilinear ties. Comparisons were made using four closed-form equations and a Machine-Learning (ML) algorithm calibrated to estimate drift capacity of tied columns. The ratio of measured drift capacity to estimated drift capacity exceeded 1.0 when using the closed-form equations and approximately 1.3 when using the ML algorithm.

Four other RC columns underwent testing with the proposed clamps applied as a repair measure. Repairs resulting in yielding of the longitudinal reinforcement and drift capacities exceeding what would be expected for a column with rectilinear ties met two criteria: a) the ratio of nominal shear resistance of the clamps 𝑣𝑠 to shear demand 𝑣𝑚𝑎𝑥 was larger than 0.6, and b) the measured lateral expansion of the concrete column core was smaller than 1%. Although measuring lateral expansion in the field can be challenging, a correlation between lateral expansion and maximum crack width was observed.

Overall, the experimental tests showed that the proposed post-tensioned clamps can be effective as a retrofit and repair technique for non-ductile RC columns.