Improved shear strength model for FRP-strengthened RC beams accounting for adverse FRP-steel interaction

RC beams shear-strengthened with externally-bonded FRP side strips or U-strips usually fail by debonding. As such debonding occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups may not have reached yielding at beam shear failure. Consequently, the internal steel stirrups cannot be fully utilized. This adverse shear interaction between internal steel stirrups and external FRP strips may significantly reduce the benefit of shear-strengthening FRP but has not been considered by any of the existing FRP strengthening design guidelines. In this paper, an improved shear strength model capable of accounting for the effect of the above shear interaction is first presented, in which the unfavorable effect of shear interaction is reflected through a reduction factor (i.e. shear interaction factor). Using a large test database established in the present study, the performance of the proposed model as well as that of three other shear strength models is then assessed. This assessment shows that the proposed shear strength model performs better than the three existing models. The assessment also shows that the inclusion of the proposed shear interaction factor in the existing models can significantly improve their performance.

Novel manually made NSM FRP (MMFRP) bars for shear strengthening of RC beams

This paper presents an experimental study evaluating the effectiveness of the Near Surface Mounted (NSM) technique with innovative manually made FRP bars (MMFRP) for shear strengthening of RC beams. RC beams designed to fail in shear were tested in three-point bending. To delay the onset of MMFRP bar debonding, a new anchorage is also developed and tested. This paper reports the results of a series of tests on simply supported rectangular RC beams, strengthened in shear with MMFRP bar either with or without the proposed anchorage. The load-deflection responses of all test beams are plotted, in addition to selected strain results. Performance and the failure modes of the test beams are presented and discussed in this paper. The proposed MMFRP bars and end anchorage enhanced the shear capacity between 25 to 48% over the control specimen. Furthermore, the adoption of the proposed end anchorage of MMFRP bars significantly enhanced the ductility of the test specimens.

Shear Strength and Durability Testing of Adhesive Bonds in Cross-Laminated Timber

The paper addresses the quality of the interface and edge bonded joints in layers of cross-laminated timber (CLT) panels. The shear performance was studied to assess the suitability of two different adhesives, Polyurethane (PUR) and Phenol-Resorcinol-Formaldehyde (PRF), and to determine the optimum clamping pressure. Since there is no established testing procedure to determine the shear strength of the surface bonds between layers in a CLT panel, block shear tests of specimens in two different configurations were carried out, and further shear tests of edge bonded specimen in two configurations were performed. Delamination tests were performed on samples which were subjected to accelerated aging to assess the durability of bonds in severe environmental conditions. Both tested adhesives produced boards with shear strength values within the edge bonding requirements of prEN 16351 for all manufacturing pressures. While the PUR specimens had higher shear strength values, the PRF specimens demonstrated superior durability characteristics in the delamination tests. It seems that the test protocol introduced in this study for crosslam bonded specimens, cut from a CLT panel, and placed in the shearing tool horizontally, accurately reflects the shearing strength of glue lines in CLT.

Influence of service loading and the resulting micro-cracks on chloride resistance of concrete

Chloride-induced corrosion of steel in reinforced concrete structures is one of the main problems affecting their durability, but most previous research projects and case studies have focused on concretes without cracks or not subjected to any structural load. Although it has been recognised that structural cracks do influence the chloride transport and chloride induced corrosion in reinforced concrete structures, there is little published work on the influence of micro-cracks due to service loads on these properties. Therefore the effect of micro-cracks caused by loading on chloride transport into concrete was studied. Four different stress levels (0%, 25%, 50% and 75% of the stress at ultimate load – fu) were applied to 100 mm diameter concrete discs and chloride migration was measured using a bespoke test setup based on the NT BUILD 492 test. The effects of replacing Portland cement CEMI by ground granulated blast-furnace slag (GGBS), pulverised fuel ash (PFA) and silica fume (SF) on chloride transport in concrete under sustained loading were studied. The results have indicated that chloride migration coefficients changed little when the stress level was below 50% of the fu; however, it is desirable to keep concrete stress less than 25% fu if this is practical. The effect of removing the load on the change of chloride migration coefficient was also studied. A recovery of around 50% of the increased chloride migration coefficient was found in the case of concretes subjected to 75% of the fu when the load was removed.

Modelling the nonlinear behaviour and fracture process of AS4/PEKK thermoplastic composite under shear loading

The accurate determination of non-linear shear behaviour and fracture toughness of continuous carbon-fibre/polymer composites remains a considerable challenge. These measurements are often necessary to generate material parameters for advanced computational damage models. In particular, there is a dearth of detailed shear fracture toughness characterisation for thermoplastic composites which are increasingly generating renewed interest within the aerospace and automotive sectors. In this work, carbon fibre (AS4)/ thermoplastic Polyetherketoneketone (PEKK) composite V-notched cross-ply specimens were manufactured to investigate their non-linear response under pure shear loading. Both monotonic and cyclic loading were applied to study the shear modulus degradation and progressive failure. For the first time in the reported literature, we use the essential work of fracture approach to measure the shear fracture toughness of continuous fibre reinforced composite laminates. Excellent geometric similarity in the load-displacement curves was observed for ligament-scaled specimens. The laminate fracture toughness was determined by linear regression, of the specific work of fracture values, to zero ligament thickness, and verified with computational models. The matrix intralaminar fracture toughness (ply level fracture toughness), associated with shear loading was determined by the area method. This paper also details the numerical implementation of a new three-dimensional phenomenological model for carbon fibre thermoplastic composites using the measured values, which is able to accurately represent the full non-linear mechanical response and fracture process. The constitutive model includes a new non-linear shear profile, shear modulus degradation and load reversal. It is combined with a smeared crack model for representing ply-level damage initiation and propagation. The model is shown to accurately predict the constitutive response in terms of permanent plastic strain, degraded modulus as well as load reversal. Predictions are also shown to compare favourably with the evolution of damage leading to final fracture.