Strengthening of concrete columns using in situ concrete jacketing

Abstract

This research aims at investigating the ability of cast in situ reinforced grout jackets to improve an existing reinforced concrete column's axial compressive strength and ductility. Current codes of practice give little guidance on the design of such jackets. Various studies were consulted and it appeared that the performance of such jackets was dictated by their ability to confine the reinforced concrete column from expanding laterally as additional load was imposed. The testing process was broken down into two main parts. Firstly, a series of scaled down control column specimens were subjected to an axial compressive load until failure so that their behaviour could be better understood. Similar control specimens were then jacketed using one of the two types of steel reinforcement configurations and tested in a similar manner to the control columns. The main difference between both configurations was mainly that Type 1 specimens had a greater cross sectional area of steel stirrups than Type 2 specimens provided at each stirrup level so that different degrees of confinement were created. Control Column test results consisting in load/ displacement graphs suggested that specimens made from weaker grade concrete generally were subjected to a greater deflection up to peak load together with a significant amount of cracking prior to failure. On the other hand, specimens made from a higher grade concrete were subjected to a sudden explosive failure followed by an immediate drop in the column's load carrying capacity. Comparisons between control column and jacketed column load/displacement test results have shown a significant increase in strength and ductility, particularly in the case of configuration type 1 where a greater degree of confinement was provided through the use of two types of stirrups per level. Strength increments of up to 187% percent were recorded in this case. Furthermore, it was seen that all jacketed specimens had more ductile post peak behaviour given that the decrease in load carrying capacity was seen to occur at a much slower rate. Recorded steel and concrete strain gauge results for two of the jacketed specimens have suggested that besides the confinement effects mentioned, some degree of composite action did happen in between the control column and jacket surface. This fact was also illustrated in the calculation carried out by three of the theoretical models considered which estimated the degree of composite action to lie somewhere between 14 and 28% for the tested specimens.