Abstract
The paper presents experiments involving punching of RC slabs strengthened using externally bonded carbon fiber reinforced polymer (CFRP) sheet and textile reinforced mortar (TRM). Twelve RC slab specimens of two concrete grades (39.9 and 63.2 MPa) and employing two strengthening schemes (CFRP and TRM) were tested. Specimens were supported on two opposite edges. Experimental load-displacement variations show two peak loads in strengthened slabs and one peak followed by a plateau in control. Second peak or the plateau corresponds to the combined action of aggregate interlock and the dowel action of back face rebars and strengthening layers. The dowel action of back face rebars and strengthening layers had no role in ultimate punching load (i.e. first peak). Strengthened slabs showed 9-18% increase in ultimate punching load (i.e. first peak) whereas there was significant increase in the second peak load (190-276% for CFRP; 55-136% for TRM) and energy absorption (~66% for CFRP and 22-56% for TRM). An analytical model was also developed for predicting the punching shear strength (first and second peaks) of strengthened slabs showing good comparison with experiments.
Highlights
Punching shear failure of reinforced concrete (RC) slabs is a major concern for the structural designers of buildings and bridges
An analytical model was developed for predicting the punching shear strength of strengthened RC slabs
The second peak load is the algebraic sum of the resistance offered by concrete after the formation of shear cone through aggregate interlocking and dowel action provided by steel rebars and carbon fiber reinforced polymer (CFRP) /textile reinforced mortar (TRM) layers: Pu2 Pa Pd where, Pd is the shear resistance provided by dowel action of steel rebars and CFRP / TRM layers; Pa is the shear resistance offered by aggregate interlock in concrete which can be estimated from:
Summary
Punching shear failure of reinforced concrete (RC) slabs is a major concern for the structural designers of buildings and bridges. Ebead and Marzouk (2004) tested two-way slabcolumn connections to investigate the effect of using CFRP strips as an external strengthening technique against punching shear failure. El-Salakawy et al, (2004) used a combination of FRP sheets and steel bolts for shear strengthening of RC slabs and found increased connection ductility and ultimate strength along with a change in the failure pattern from the punching to the flexural mode. Chen and Li (2005) used Glass Fiber Reinforced Polymer (GFRP) laminates for the shear strengthening of slabs They showed that the flexural strengthening of slabs by external bonding of GFRP laminates can increase the punching strength, significantly. The paper studies the effect of CFRP and TRM strengtheneing of RC slabs, commonly adopted for their flexural strengthening, on the punching shear resistance of slabs and provides an analytical method for its prediction
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