International Concrete Abstracts Portal

The punching shear capacity of concrete slabs reinforced with three-dimensional fiber-reinforced polymer (FRP) double-layer reinforcement cagescomposed of glass fiber-reinforced pultruded grating elements has been investigatedusing full-scale experimental tests and a number of different analytical models. Testspecimens were full-scale prototype bridge deck slabs with varying end restraint andsupport conditions, differing dimensions, and two different FRP bar fiber lay-ups. Thetests results were compared with the punching shear models in ACI 318, ACI 440,Eurocode 2, BS 8110, CEB-FIB MC90, JSCE, and a number of models proposed in theliterature specifically for FRP reinforced slabs. Based on this investigation a newempirical model has been developed to predict the punching shear capacity of doublelayer grids having either restrained or simply supported edges and including anoverlapping splice. The model is shown to give reasonably good predictions for bothsimply supported and restrained slabs.

Recently, there has been a rapid increase in using the non-corrodible fiber-reinforced polymers (FRP) reinforcing bars as alternative reinforcements for concretestructures especially those in harsh environments. The elastic stiffness, ultimatestrength, and bond characteristics of FRP reinforcing bars are quite different from thoseof steel, which affect the shear capacity. The recently published FRP design codes andguidelines include equations for shear design of one-way flexural members. However,very little work was done to investigate the punching shear behavior of two-way slabsreinforced with FRP bars. The current design provisions for shear in two-way slabs arebased on testing carried out on steel reinforced slabs. This study presents a new modelto predict shear capacity of two-way concrete slabs that were developed based onextensive experimental work. The accuracy of this prediction model was evaluatedagainst the existing test data. Compared to the available design models, the proposedshear model seems to have very good agreement with test results with betterpredictions for both FRP and steel-reinforced concrete two-way slabs.

The efficacies of the Near Surface Mounted (NSM) and Externally BondedReinforcing (EBR) techniques for the shear strengthening of rectangular cross sectionRC beams are compared. Both techniques are based on the use of carbon fiberreinforced polymer (CFRP) materials. The NSM was the most effective technique, andwas also the easiest and fastest to apply, and assured the lowest fragile failure modes.The performance of the ACI and fib analytical formulations for the EBR shearstrengthening was appraised. In general, the contribution of the CFRP systemspredicted by the analytical formulations was larger than the values registeredexperimentally. The capability of the De Lorenzis formulation of predicting thecontribution of the NSM technique for the shear strengthening of RC beams wasappraised using bond stress and CFRP effective strain values obtained in pulloutbending tests. This formulation provided values 61% lower than the values obtainedexperimentally.

Many concrete bridges related to railways in the U.K. consist of prestressedrectangular concrete beams, post-tensioned together transversely to aid lateraldistribution of load; this bridge type has been repeatedly flagged as having insufficientshear capacity. Sixteen tests on small-scale beams, which are scaled-down replicamodels of the actual bridge beams, are presented. The specimens are tested under afour-point loading system and are both prestressed (PRC) with and without stirrups andnon-prestressed (RC) with and without stirrups, to provide full understanding of theirshear behaviour. Four further tests are then presented on RC beams strengthened inshear with FRP bars inserted from the soffit into pre-drilled holes and fixed in placeusing epoxy resin; this method allows strengthening in cases where the webs areinaccessible. Comparisons are made with current code predictions for the strength ofall specimens. The results show that unstrengthened RC beams behave mostly asexpected and as predicted by codes, while for PRC beams a great variation in shear-carrying capacity following shear cracking is observed for different span-to-depthloading ratios. The proposed FRP strengthening scheme is effective and providessignificant improvement to the shear-carrying load capacity.

To assess the strengthening efficiency of near-surface mounted (NSM)carbon fiber reinforced polymer (CFRP) laminates according to their groove depth anddisposition, 4-point bending tests were performed on 4 specimens strengthened withNSM CFRP. A structural model for the finite element method (FEM) able to simulateaccurately the experimental results was determined to analyze the strengtheningefficiency of the NSM technique analytically. Applying the model, parametric analysiswas performed considering the groove depth and spacing of CFRP laminates. Analyticalstudy on the groove depth revealed the existence of a critical depth beyond which theincrease of the ultimate load becomes imperceptible. In other words, this means thatthere exists a limit of strengthening efficiency where it remains in a definite level evenif the groove depth is increased. Analytical results regard to the spacing of the CFRPlaminates showed that comparatively smooth fluctuations of the ultimate load wereproduced by the variation of the spacing and the presence of an optimal spacing rangefor which relatively better strengthening efficiency can be obtained. Particularly, aspacing preventing the interference between adjacent CFRP laminates and theinfluence of the concrete cover at the edges as well as allowing the CFRP laminates tobehave independently was derived. Using the analytical results, various strengtheningschemes could be established with different numbers of CFRP laminates, groove depthsand dispositions of the reinforcements for a determinate quantity of reinforcements.