Retrofitting of footings

Abstract

Footing is onc of thc most important structural e1cmcnts in the field of construction. Punching shear is a govel11ing factor to be considered in the dcsign of footings. Depth of footing is a major concern of punching strcngth calculation. Hence the increase of footing depth is expectcd to contributc to thc punching strength of existing footings. A total of twelve model footings were cast and tested in the laboratory in an effort to ascertain the effect of concrete overlay on the punching shear capacity of footings. In this regard model footings wcrc prepared with different combinations of concrete overlay, compressive reinforcement, dowel bars. Test program was carried out to gather basic infol111ation on real punching behavior of footings subjected to conccntrated loading. The punching strength of footings with ovcrlay has been found to be significantly higher than that of footings without ovcrlay and having lesser thickness. It has been observed that footings having concrcte overlay could attain punching capacity very near to the capacity of footings having similar thickness, but cast monolithically. The code specified strengths of thc specimens wcre calculated in accordance with ACI 318-95, BS 8110-85, CAN3-A23.3-M84 and CEB-FIB. It appears that most of the codes do not recognize the role of percentage of longitudinal stcel on the punching shear strength. Overlay on old concrete provide additional punching strength; dowel bars and compressive rcinforccmcnt did not increasc the punching strength, but these contributed significantly in other ways. Dowels provided bonding between old concrete and overlay. Hence, the samples of overlay with dowel behaved monolithic-ally at the time of failure. On the other hand, overlay without dowel bars was separated during failure. Compressive reinforcement showed some significance for the samples without overlay to reduce the deflection. But it was not very significant for the samples with overlay.