Bond strength of short lap splices in RC beams confined with steel stirrups or external CFRP

Abstract

This paperinvestigates thebondbehaviour of lapped steel bars usingfifteen RC beamstested in flexure.Twelveofthe beams were designed to fail by bond splitting at midspan, where the main flexural reinforcement was lapped10 bar diameters.The parameters studied include the amount and type of confinement at midspan (noconfinement, internal steel stirrups or externally bonded carbon FRP), concrete cover and bar size.The results showthatthe CFRP confinement enhanced the bond strength of the lapped bars by up to 49% with reference to unconfined beams, and improved significantly the overall behaviour of the specimens. The experimental resultsare compared with existing modelsto predictthebond strength enhancement provided by CFRP confinement.It is shown that existing modelsoverestimate considerably the CFRP strains and show a large scatterwhen predicting experimental results.Based on the testresults, anew approachtopredictthe bond strength enhancement duetoCFRP confinement isproposed.This can be used during the assessment and strengthening of substandard RC constructions.Keywords: substandard lapsplices, seismic strengthening, RC beams, CFRP confinement, bond-splittingstrength, bar slip 1.IntroductionExtensive damage in recent major earthquakes in Mediterranean and developing countries has highlighted the seismic vulnerability of existingsubstandardRC buildings built with little or no seismic detailing and low quality materials (Kashmir, 2005;China, 2008; Indonesia and Italy, 2009; Haiti,2010 2.Experimental programme Geometry of beam specimensA total offifteenRC beams were tested inflexuralfour-point bending. The Hence, three beams were reinforced internally using two6 mm smoothstirrups at the lapped zone.To replicate old construction practices, the stirrups were closed with 90 degree hooks instead of 135 degree hooks typically required by current seismiccodes.The midspan region of three beams was fully wrapped with 1 layer and three beams with 2 layers of externally bonded CFRP sheets. For comparison, three unconfinedbeams with lapped bars and three benchmark beams with continuous bottom bars were alsocast.

Fibre-reinforced polymer strengthening of substandard lap-spliced reinforced concrete members: A comprehensive survey

“…More recently, Garcia et al (2014Garcia et al ( , 2015 proposed a practical strain control approach (model 12 in Table 1) to calculate the bond strength enhancement due to FRP confinement. The effect of the CFRP confinement is considered through an additional confining pressure f o assumed to act over a split cross sectional area equal to (c min(x,y) +d b )•l b , as shown in Figure 9.…”

“…Dimensions in mm. Beams by Hamad et al (2004a) Beams by Garcia et al (2015) Beams by Garcia et al (2014) Model 12 in Table 1 TABLES (Priestley et al 1992, Seible et al 1997, also Eurocode 8 Garcia et al (2014Garcia et al ( , 2015 40 …”

“…Moreover, it is more convenient to compare the relative improvement that the FRP provides to confined specimens over unconfined specimens in terms of bond strength and ductility for different rib areas, rather than comparing the absolute bond improvement. It has been also argued that FRP-strengthened lap-spliced elements have highly variable properties of concrete in tension, but tests performed by the authors (Garcia et al 2014(Garcia et al , 2015 and by other researchers (Hamad et al 2004a(Hamad et al , 2004b(Hamad et al , 2004c on beam splice specimens indicate that the FRP strengthening reduced the concrete variability in tension, thus providing more consistent bond strengths. In summary, the influence of concrete strength and bar characteristics on require further investigation.…”

“…More recently, Garcia et al (2014Garcia et al ( , 2015 investigated the bond strength enhancement resulting from the confinement provided by CFRP sheets in 24 beam splice specimens tested in flexure, similar to those tested by Harajli et al (2006) and Hamad et al (Hamad et al 2004a, Hamad et al 2004b, Hamad et al 2004c). Lap lengths of 10d b and 25d b were used to examine the mobilised "local" and average bond strengths, respectively.…”

Seismic retrofitting of RC buildings using CFRP and post-tensioned metal straps: shake table tests

“…The article details the adopted retrofitting strategy, and discusses global and local results from the shake table tests on the retrofitted structure. This study is part of a comprehensive research effort focusing on the seismic retrofitting of substandard RC buildings using PTMS (Garcia et al 2014a) and FRP composites (Garcia et al 2014b(Garcia et al , 2015.…”