Bonded overlay strengthening of hollow core slab with and without interface shearkeys connection

Precast Prestressed Hollow Core Slabs (PPHCS) are most commonly used as flooring and roofing elements. Usually, a new layer of concrete is placed on the top of hollow core slabs to create a continuous and levelled surface. The common thickness of this bonded overlay will be around 50 mm to 75 mm deep. The provision of Bonded Overlay (BO) will increase the cracking load and flexural strength of hollow core slab after the full composite action is developed. In the present study, the effect of shear keys at the interface of bonded overlay and hollow core slab is studied. The hollow core slab and bonded overlay is expected to have a full composite action until failure without any interface separation. The dimension of hollow core used in this investigation is 600mm wide, 150mm depth and 3500mm length. In total, three full-scale hollow core slabs were tested under shear span (a) to depth (d) ratio of 7.5. The three specimens which include un-strengthened slab denoted as control slab, slab strengthened with bonded overlay without any shear keys at the interface and bonded overlay with shear keys. Bonded overlay specimens without shear keys resulted in interfacial failure and it was able to increase the peak load by 38.4% compared to the control specimen. However, the bonded overlay with shear keys resulted in full composite action till the final failure and it was able to increase the peak load by 59.6% compared to the control specimen. The provision of shear keys at the interface of hollow core slabs and bonded overlay resulted in full composite action

Efficient Hybrid Strengthening for Precast Hollow Core Slabs at Low and High Shear Span to Depth Ratios

Prestressed hollow core slabs are widely used in precast construction. The objective of this study is to understand the efficiency of hybrid strengthening technique at different levels of flexure to shear or shear span (a) to depth (d) ratios. Hybrid strengthening technique includes bonded overlay on the compression side and carbon fiber reinforced polymer (CFRP) composites on the tension side. Two different techniques are employed for the application of CFRP composites namely Near Surface Mounted (NSM) or Externally Bonded (EB). Fourteen full scale prestressed precast hollow core slabs were strengthened using different combinations of these techniques and tested at low (a/d= 3.75) and high (a/d = 7.50) shear span to depth ratio. The bonded overlay strengthening increased the peak strength by 59.2% and 89.0% respectively at low and high shear span to depth ratios. The externally bonded strengthening solely increased the peak strength by 16.9% and 87.6% at low and high a/d ratios, respectively. The NSM strengthening increased the peak strength by 49.4% and 68.9% at low and high a/d ratios respectively. Hybrid strengthening resulted in the best performance with highest increase in peak strength at both a/d ratios without a significant reduction in the ultimate displacement

Experimental evaluation of bonded overlay and NSM GFRP bar strengthening on flexural behavior of precast prestressed hollow core slabs

A hollow core slab is a precast prestressed concrete member with longitudinal cores that reduce its self-weight. Strengthening of prestressed hollow core slabs is required for several reasons to maintain their structural integrity. The main focus of this research is to understand the flexural behavior of hollow core slabs and failure modes with hybrid strengthening techniques including bonded overlay and near surface mounted (NSM) glass fiber reinforced polymer (GFRP) bars. The test variables include different strengthening techniques such as (i) bonded overlay with and without shear keys, (ii) NSM GFRP rebar and (iii) hybrid strengthening combination which includes a thin bonded overlay in the compression region and NSM GFRP bars in the tension region. A total of seven full scale slabs were cast, strengthened and tested until failure. Test results show that bonded overlay increases the flexural strength by 89% without much compromise on the ductility when compared to control slab, whereas NSM GFRP bar strengthening increases the strength by about 100% but with reduced ductility. Hybrid strengthening led to a highest increase in the ultimate strength (about 200%) and ductility when compared to only NSM and only bonded overlay strengthening

Experimental and Numerical Studies on Efficiency of Hybrid Overlay and Near Surface Mounted FRP Strengthening of Pre-cracked Hollow Core Slabs

Prestressed precast hollow core slabs (PPHCS) are commonly used as floor elements in the precast buildings. These slabs could crack due to various reasons and can affect the overall integrity of the structure. Also, tension stiffening of concrete in PPHCS is not as effective due to the absence of additional reinforcement other than prestressing strands and the transition between a flexurally ‘uncracked’ and flexurally ‘cracked’ section is rapid. Therefore, strengthening of such cracked slabs is essential for ensuring its adequate performance. Effect of FRP strengthening of hollow core slabs is relatively well established. However, the effect of hybrid strengthening on the behavior of pre-cracked hollow core slabs is not fully understood yet. The performance of pre-cracked hollow core slabs strengthened with near surface mounting (NSM) of carbon fiber reinforced polymer (CFRP) laminates, and hybrid strengthening is investigated. Hybrid strengthening includes a combination of concrete bonded overlay in the compression zone and NSM CFRP laminates in the tension zone. A total of eight full-scale hollow core slabs are tested at two different shear span (a) to the depth (d) ratios of 3.75 and 7.50. Before strengthening of the hollow core slabs, the slabs were pre-cracked by applying the load equal to 65% of its ultimate capacity. Strengthening by NSM technique increased the ultimate capacity of the slabs by 50% whereas hybrid strengthening increased the strength by 130% when compared to pre-cracked control specimens. Finite element (FE) models were developed and calibrated to predict the behavior of tested hollow core slabs. Peak load predictions obtained from the finite element analysis had good correlation with the test results. © 2018 Institution of Structural Engineer

Analytical and numerical studies on hollow core slabs strengthened with hybrid FRP and overlay techniques

The objective of this study is to understand the behaviour of hollow core slabs strengthened with FRP and hybrid techniques through numerical and analytical studies. Different strengthening techniques considered in this study are (i) External Bonding (EB) of Carbon Fiber Reinforced Polymer (CFRP) laminates, (ii) Near Surface Mounting (NSM) of CFRP laminates, (iii) Bonded Overlay (BO) using concrete layer, and (iv) hybrid strengthening which is a combination of bonded overlay and NSM or EB. In the numerical studies, three-dimensional Finite Element (FE) models of hollow core slabs were developed considering material and geometrical nonlinearities, and a phased nonlinear analysis was carried out. The analytical calculations were carried out using Response-2000 program which is based on Modified Compression Field Theory (MCFT). Both the numerical and analytical models predicted the behaviour in agreement with experimental results. Parametric studies indicated that increase in the bonded overlay thickness increases the peak load capacity without reducing the displacement ductility. The increase in FRP strengthening ratio increased the capacity but reduced the displacement ductility. The hybrid strengthening technique was found to increase the capacity of the hollow core slabs by more than 100% without compromise in ductility when compared to their individual strengthening schemes

Experimental study on behavior of GFRP stiffened panels under compression

Glass Fiber Reinforced Polymer (GFRP) materials are extensively used in the aerospace and marine industries because of their high strength and stiffness to weight ratio and excellent corrosion resistance. Stiffened panels are commonly used in aircraft wing and fuselage parts. The present study focuses on the behavior of composite stiffened panels under compressive loading. With the introduction of stiffeners to unstiffened composite plates, the structural stiffness of the panel increases resulting in higher strength and stiffness. Studies in the past have shown that the critical structural failure mode under compressive loading of a stiffened composite panel is by local buckling. The present study attempts to evaluate the mechanical behavior of composite stiffened panels under compression using blade stiffener configuration and in particular on the behavior of the skin- stiffener interface through experimental testing. A novel test fixture is developed for experimental testing of GFRP stiffened panels. A non-contact whole field strain analysis technique called digital image correlation (DIC) is used for capturing the strain and damage mechanisms. Blade stiffeners increased the strength, stiffness and reduced the out-of plane displacement at failure. The failure of both the unstiffened and stiffened panels was through local buckling rather than through material failure. DIC was able to capture the strain localization and buckling failure modes