BEHAVIOUR OF PRESTRESSED HOLLOWCORE SLABS HYBRID STRENGTHENED USING BONDED OVERLAY AND FRP LAMINATES UNDER FLEXURE AND SHEAR

Pradeep, Pradeep and S, Suriya Prakash (2018) BEHAVIOUR OF PRESTRESSED HOLLOWCORE SLABS HYBRID STRENGTHENED USING BONDED OVERLAY AND FRP LAMINATES UNDER FLEXURE AND SHEAR. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Precast Prestressed Hollow Core Slabs (PPHCS) are used in many civil engineering structures including residential and commercial buildings. A hollow core slab consists of 40% to 50% voids running along the length of slab to reduce its self-weight and thereby leading to economy in construction. Strengthening of these precast slabs is required for various reasons that include (i) increase in dead or live load, (ii) structural degradation due to corrosion and (iii) provisions of cut-outs in the slabs for utility purposes. Structural deficiencies of hollow core slabs can be overcome by adopting a suitable strengthening scheme to ensure the structural integrity of PPHCS. This research work evaluates the efficiency of different strengthening techniques on the un-cracked and pre-cracked prestressed hollow core slabs tested at low (a/d= 3.75) and high (a/d = 7.50) shear span to depth ratios. The different strengthening schemes considered in this study include i) slabs strengthened with only bonded overlay, ii) slabs strengthened with only Near Surface Mounted (NSM) Carbon Fibre Reinforced Polymer (CFRP) laminates, iii) slabs strengthened with only Externally Bonded (EB) CFRP laminates and iv) hybrid strengthening using combination of bonded overlay in compression and NSM/EB CFRP laminates in tension region. To realize the objectives, twenty-two full-scale hollow core slabs were tested. The dimensions of the hollow core slabs used in this investigation are 600 mm wide, 150 mm deep, and 3500 mm in length. The bonded overlay strengthened specimen was able to increase the peak strength of 59.2% and 55.6%, respectively, at low and high shear span to depth ratios. Similarly, externally bonded strengthening solely increased the peak strength by 16.9% and 58.4% for specimens tested at low and high a/d ratios, respectively. Only NSM strengthening resulted in increase of the peak strength by 49.4% and 42.6% at low and high a/d ratios, respectively. Hybrid strengthening xiv resulted in the best performance with highest increase in peak strength at both a/d ratios without a significant reduction in the ultimate displacement. The NSM and hybrid NSM strengthening techniques were able to completely restore the initial stiffness and the peak strength of pre-cracked specimens. In the numerical studies, three dimensional Finite Element (FE) models of hollow core slabs are developed considering material and geometrical nonlinearities, and a phased nonlinear analysis was carried out by using FE software ANSYS V.14.5. The analytical calculations are carried out using Response-2000 program which is based on Modified Compression Field Theory (MCFT). Both the numerical and analytical models predicted the behaviour of PPHCS with and without different strengthening techniques and are in agreement with experimental results. Parametric studies were carried out using the calibrated models to evaluate the effect of bonded overlay thickness, FRP laminates ratio, and their hybrid strengthening schemes on the behaviour of prestressed hollow core slabs. The parametric study indicates that there is an optimum combination of bonded overlay thickness and NSM FRP reinforcement ratio at which the increases in peak strength by more than 100% can be achieved without much compromise in ductility.

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