Strengthening of flat slabs against punching shear failure with FRP materials

Fibre reinforced-polymers (FRP) have been extensively used over the past two decades in the field of civil/structural engineering. The application of FRP for retrofitting purposes improves the structural response of RC buildings and provides an alternative strengthening option to the current methods. However, the existing design codes do not provide definitive guidance for retrofitting of slab-column connections strengthened with FRP due to insufficient research covering this area. This study investigated the effectiveness of strengthening the slab-column connections with externally bonded (EB) and near surface-mounted (NSM) techniques, using Carbon FRP laminates and bars. The experimental programme is conducted in three stages. In the first stage, the influence of various types of strengthening schemes with EB CFRP laminates was investigated. In the second stage, a non-bolted anchorage system for EB CFRP laminates was developed. The effect of the non-bolted anchorage and the cross-sectional area of CFRP laminates was investigated. In the third stage, the applicability of the NSM retrofitting method for punching shear of flat slabs was experimentally investigated. The effect of the CFRP bar diameter and the strengthening layout was studied in this stage. All slabs were tested against monotonically increasing load until failure. The slabs were horizontally unrestrained, i.e. the edges/corners were free to rotate during the testing. The slabs were instrumented with strain gauges, LVDTs and dial gauges to monitor and record the behaviour of the slabs during the testing. The deflection of the slabs and the strain profile in steel and FRP reinforcement were measured and analysed. The deformability and the ultimate load (key aspect) for the control samples and strengthened samples were also investigated. It was found that the EB and NSM strengthening methods could be used to improve the punching shear capacity of slab-column connections. The EB CFRP laminates increased the punching load by up to 25%. The samples with the non-bolted anchorage system exhibited a relatively gradual mode of destruction with a secondary increase of the load after initial debonding. the NSM strengthened samples displayed up to 44% higher ultimate load compared to the control sample. The higher efficiency of the NSM method over the EB method could be attributed to the stronger bond between the NSM bars and the concrete. The theoretical method presented in this study could be used for calculating the punching shear capacity of flat slabs retrofitted with NSM CFRP reinforcement. This method is based on the sectional analysis of the retrofitted slab, where the CFRP bars are treated as an additional layer of reinforcement. This method can be amalgamated into the design codes (in this case, ACI-318, EC2 and FIB MC 2010) by replacing two terms (reinforcement ratio and effective depth) with their equivalent values into the design equations. The experimental results were in good correspondence with the proposed theoretical method. The predictions of the design codes were slightly more conservative, but this level of safety is always needed when it comes to designing

Behaviour of column constructed with FRP tubes filled with concrete

This paper investigates the behaviour of Concrete-Filled FRP Tubes (CFFT) as columns. The experimental programme consists of preparing and testing one steel column acting as control sample and three columns made with GFRP. The GFRP tubes were produced by filament winding method where the amount and orientation of the fibres was changed. The tubes had dimensions of 1000x100 mm (length x diameter) and were filled with C25/30 concrete. The columns were tested under compression and the load was applied at a pace rate of 0.5 mm/min. It was found that the GFRP tubes can efficiently confine the concrete and could be used as alternative material to steel tubes. The steel and GFRP samples developed a high level of strain throughout the testing. The GFRP sample with fibre orientation of 90° failed by FRP rupture, whereas the remaining samples failed by buckling. The orientation of the fibres at 90° was more efficient than orientation of fibres at 45° in terms of increasing the ultimate capacity. The GFRP samples displayed lower ultimate capacity compared to steel samples with same wall thickness, but increasing the wall thickness of the GFRP columns increased the ultimate load accordingly