Rehabilitation of Reinforced Concrete Slab-column Connections for Two-way Shear

Even though a reinforced concrete flat-plate structural system is an economical structural system, it is prone to brittle shear failure at slab-column connections, which may result in the progressive collapse of a building. For that reason, connections with insufficient two-way shear strength may need to be rehabilitated and rehabilitation can be a cost-effective alternative to replacement. This study focused on the rehabilitation of slab-column connections in existing structures built in the mid 20th century. The main objectives of this study were to develop efficient strengthening methods for deficient connections that do notsatisfy current code requirements and to develop efficient post-earthquake repair methods for connections that experience seismic-damage.

Experimental research on seven 2/3-scale interior slab-column connections was conducted to quantify the effects of low flexural reinforcement ratio and earthquake-damage. The efficiency of various rehabilitation techniques on improving the two-way shear strength of connections was also investigated. The test results show that two-way shear strength was sensitive to the slab top reinforcement ratio within the (c+3h) region, where c is the column dimension and h is the slab thickness. The damage induced by lateral displacement cycles up to 1.25% lateral drift did not affect the two-way shear capacity of the specimens tested.

Three alternatives for repairing and strengthening slab-column connections that were experimentally evaluated are as follows:

The first alternative is the installation of steel collars on the column under the slab. In addition to increasing the deformation capacity, the two-way shear strength and the post-punching capacity under gravity loading, the installation of steel collars increased the lateral load capacity and prevented punching shear failure under reversed cyclic lateral loading.

The second alternative is the installation of external Carbon Fiber Reinforced Polymers (CFRP) stirrups. The installation of externally installed stirrups increased the deformation capacity and the two-way shear strength. In addition to enlarging the failure surface, which increased the number of flexural reinforcement contributing to the residual capacity following a punching shearfailure, a tightly knit array of CFRP stirrups helped to prevent stripping out of the tensile flexural reinforcement. For those reasons, the installation of CFRP stirrups was effective to increase the post-punching capacity.

And the third alternative is the application of well-anchored CFRP sheets on the tension side of the slab. The application of CFRP sheets increased the flexural capacity of the connection, limited the width of flexural cracks, and therefore increased the two-way shear strength. However, the deformation capacity was reduced. After punching shear failure occurred, the well-anchored CFRP sheets acted as tension bands and allowed the slab to carry substantial shear force through larger deformations. CFRP anchors were very effective to prevent delamination of CFRP sheets.

Guidelines for the evaluation and rehabilitation of existing slab-column connections are given. These guidelines are based on the results of the tests conducted in this study and on the synthesis of the literature review conducted as part of this project.