Behavior of Strengthened Isolated Reinforced Concrete Footings

Abstract

The main purpose of the current research is to investigate the load-carrying capacity and stress redistribution below rigid footings resting on sandy soil and subjected to concentric and eccentric loading. Previous research regarding strengthening footing by concrete jacketing had focused on the punching capacity, after strengthening the footing and by testing on spring bed or resting on four supports and neglecting the interaction between footing and soil. However, the current study assess strengthening including the interaction between real bed soil and the concrete jacket for strengthened footings subjected to concentric and eccentric loading. Therefore, this study introduces a detailed experimental and numerical investigation to assess the effect of strengthened RC square footing resting on sand with different bonding techniques to connect the existing and new surfaces. A set of eight footings subjected to concentric and eccentric loading were experimentally tested. A full concrete jacket was applied to four footings, and two footings were strengthened with a strip concrete jacket around column faces only, while two footings were left without any form of strengthening to act as the control footing, one subjected to concentric loading and the other one subjected to eccentric loading. The footings were designed to fail in flexural to show footing behavior before and after cracking up to failure. Moreover, a comparison was held between six footings. Three of them have shear dowels and a bonding agent, and the others have a bonding agent only to connect the existing and new surfaces. Furthermore, good compaction was applied to the bed sand soil to increase the soil's bearing capacity and avoid any shear failure before footing flexural failure. The mechanical properties of the concrete, steel reinforcement bars, and bed soil were experimentally tested and characterized in this research, while the properties of the bonding agent and the epoxy adhesive were used considering the manufacturer’s data sheets. A numerical model was developed using a non-linear FE program (Abaqus 6.14). All the test footings were modeled with the same material properties used in the experimental program. The results showed good agreement between the experimental tests and the finite element analysis, and the numerical model was found capable of stimulating the behavior of strengthened footings with concrete jacket resting on cohesionless soil up to failure. The model was also applicable for predicting the different failure modes and compared them with failure modes conducted from the experimental footings. Moreover, the adhesive bond and shear dowels at the contact surface between the new concrete jacket and the existing concrete were represented in the model to consider the slip movement between surfaces and its effect on the stress distribution. The contact surface interaction between footing and bed soil was represented to consider the effect of strengthening on the soil-footing interaction.