Moisture transport and drying shrinkage properties of steel-fibre-reinforced-concrete.

Drying shrinkage has a serious impact on the structural and durability performance of concrete pavements. Shrinkage strain development and distress can only be fully understood by knowing the moisture transport and free shrinkage properties of concrete. This paper uses experiments and FE inverse analysis to determine these properties for conventional concrete (CC) and RCC reinforced with recycled-steel-fibres from tyres. Moisture diffusivity versus moisture content and a relationship between free shrinkage and moisture loss are derived. These values can be used to predict shrinkage strains and stresses in road pavements and other ground restrained slabs

Steel-fibre-reinforcement and increasing the load-bearing capacity of concrete pavements.

Elastic theories form the basic concept used in most of the existing design codes for industrial or transportation plain and conventionally reinforced concrete ground slabs. The post-cracking load bearing capacity of slabs-on-ground is not taken into account in most of these codes. Therefore, these codes (e.g. PCA, ACI) cannot be used directly for steel fibre reinforced concrete (SFRC). Guidelines for SFRC (e.g. Concrete Society) use the ultimate limit state concept for fibre reinforced ground floors, but only partially, since cracking is only allowed to occur on the bottom surface of the slab. The highly repetitive nature of the loads which may cause considerable degradation in the mechanical properties of the pavement and foundation also is not considered in this method. The aim of this paper is to evaluate the load-bearing capacity of SFRC pavements through numerical simulations, and to assess the accuracy of the analytical methods used in different design codes

Seismic behaviour of deficient RC frames strengthened with CFRP composites

A full-scale two-storey RC building with poor detailing in the beam–column joints was tested on a shake table as part of the European research project ECOLEADER. After the initial tests which damaged the structure, the frame was strengthened using carbon fibre reinforced materials (CFRPs) and re-tested. This paper investigates analytically the efficiency of the strengthening technique at improving the seismic behaviour of this frame structure. The experimental data from the initial shake table tests are used to calibrate analytical models. To simulate deficient beam–column joints, models of steel–concrete bond-slip and bond-strength degradation under cyclic loading are considered. The analytical models are used to assess the efficiency of the CFRP rehabilitation using a set of medium to strong seismic records. The CFRP strengthening intervention enhanced the behaviour of the substandard beam–column joints, and resulted in substantial improvement of the seismic performance of the damaged RC frame. It is shown that, after the CFRP intervention, the damaged building would experience on average 65% less global damage compared to the original structure if it was subjected to real earthquake excitations

Bond behavior of fiber reinforced polymer bars under direct pullout conditions

This paper examines the behavior of Eurocrete fiber-reinforced polymer (FRP) bars (glass, carbon, aramid, and hybrid) in concrete under direct pullout conditions. More than 130 cube specimens were tested in direct pullout where no splitting was allowed to develop. In normal concrete, the mode of bond failure of FRP bars was found to differ substantially from that of deformed steel bars because of damage to the resin rich surface of the bar when pullout takes place. Bond strengths developed by carbon fiber-reinforced polymer and glass fiber-reinforced polymer bars appear to be very similar and just below what is expected from deformed steel bars under similar experimental conditions. The load slip curves highlight some of the fundamental differences between steel and FRP materials. This paper reports in detail on the influence of various parameters that affect bond strength and development such as the embedment length, type, shape, surface characteristics, and diameter of the bar as well as concrete strength. The testing arrangement is also shown to influence bond strength because of the “wedging effect” of the bars

Effect of section geometry on development of shrinkage-induced deformations in box girder bridges

Non-uniform shrinkage strains can lead to significant additional deflections in large box girder bridges, leading to serviceability problems. This article examines experimentally and analytically the effect of different cross-section geometries on the shrinkage camber of bridge box girders. Small-scale beams were tested to determine the development of shrinkage strains across the beams of depth. Parameters investigated include cross section thickness, drying conditions, and type of concrete mix. Based on the experimental results, inverse analysis is utilised to obtain a surface factor and a hydro-shrinkage coefficient. In this study, such vales are used to determine, for the first time, shrinkage-induced bending deformations of long-span bridges using a hydro-mechanical approach. The results are then used to examine numerically the effect of different section geometries on the development of shrinkage camber. It is shown that the analytical predictions match the experimental results with an accuracy of 85%. A further parametric study is carried out to investigate the effects of specimen geometry and ambient relative humidity. The hydro-mechanical approach is further validated using shrinkage field data from the 230 m two-span box girder Yiju River Bridge (China). The approach proposed in this study is expected to contribute towards improving the predictions of the long term behaviour of box girder bridges and towards better bridge management

Strength degradation in curved fiber-reinforced polymer (FRP) bars used as concrete reinforcements

Steel reinforcements in concrete tend to corrode and this process can lead to structural damage. Fiber-reinforced polymer (FRP) reinforcements represent a viable alternative for structures exposed to aggressive environments and have many possible applications where superior corrosion resistance properties are required. The use of FRP rebars as internal reinforcements for concrete, however, is limited to specific structural elements and does not yet extend to the whole structure. The reason for this relates to the limited availability of curved or shaped reinforcing FRP elements on the market, as well as their reduced structural performance. This article presents a state-of-the art review on the strength degradation of curved FRP composites, and also assesses the performance of existing predictive models for the bend capacity of FRP reinforcements. Previous research has shown that the mechanical performance of bent portions of FRP bars significantly reduces under a multiaxial combination of stresses. Indeed, the tensile strength of bent FRP bars can be as low as 25% of the maximum tensile strength developed in a straight counterpart. In a significant number of cases, the current design recommendations for concrete structures reinforced with FRP were found to overestimate the bend capacity of FRP bars. A more accurate and practical predictive model based on the Tsai−Hill failure criteria is also discussed. This review article also identifies potential challenges and future directions of research for exploring the use of curved/shaped FRP composites in civil engineering applications

A practical method for determining shear crack induced deformation in FRP RC beams

This article proposes a practical semi-empirical method for determining shear crack-induced deformations in Glass Fibre Reinforced Polymer (GFRP) Reinforced Concrete (RC) beams. Current design guidelines neglect shear and shear crack-induced deformations in the calculation of deflections of GFRP RC beams. However, shear-induced deformations can be up to 30% of the total beam deflection due to the lower stiffness of GFRP bars compared to steel. To calculate the component of deflection due to shear action and crack opening, the proposed model uses a ‘single fictitious inclined crack’ with a width equal to the sum of the individual effective shear crack widths. Twelve shear tests were conducted on six RC beams reinforced internally with GFRP bars considering different reinforcement types and test parameters. The additional deformation due to shear cracks calculated by the proposed model is then used to predict the overall deformations of such beams up to failure. It is shown that, in comparison to current design guidelines, the proposed model predicts more accurately the total deflection of FRP RC beams at both service and ultimate loads. This article contributes towards the development of more accurate models to assess the overall shear deflection behaviour of FRP RC beams so as to balance the performance, serviceability and economic viability of structures

Novel Cold-formed Steel Elements for Seismic Applications

Novel cold-formed steel (CFS) elements are investigated in this paper for seismic resistant multi-storey moment frames. Premature local buckling and low out-of-plane stiffness are known as the main structural deficiencies of CFS sections with thin-walled elements. These lead to low energy dissipation capacity of the structures made up of CFS sections as the main load bearing members in seismic events. In order to improve the energy dissipation capacity of CFS members, an innovative CFS beam section with curved flanges is developed by numerical FE analysis and experimental work. A web bolted through plate CFS beam-column connection is used to limit out-of-plane actions in transferring the beam forces to column faces. This type of connection, however, produces premature web buckling and needs to be strengthened by a combination of vertical and horizontal out-of-plane stiffeners. Six beam-column connection assemblies including different stiffener configurations were tested. It is shown that the ductility factor and the moment strength are increased by up to ~75% and ~35% respectively relative to the specimen without stiffener. Correspondingly, activation of connection slip leads to a highly stable hysteretic behaviour and a significant increase (up to ~240%) in the hysteretic energy dissipation capacity

REUSED TYRE POLYMER FIBRE FOR FIRE-SPALLING MITIGATION

Polypropylene fibres (PPF) are used in concrete principally to reduce plastic shrinkage cracking, but also to prevent explosive spalling of concrete exposed to fire. In the EU alone, an estimated 75,000 tonnes of virgin PPF are used each year. At the same time an estimated 63,000 tonnes of polymer fibres are recovered from end-of-life tyres, which are agglomerated and too contaminated with rubber to find any alternative use; currently these are mainly disposed of by incineration. The authors have initiated a study on the feasibility of reusing tyre polymer fibres in fresh concrete to mitigate fire-induced spalling. If successful, this will permit replacement of the virgin PPF currently used with a reused product of equal or superior performance. A preliminary experimental investigation is presented in this paper. High-strength concrete cubes/slabs have been tested under thermo-mechanical loading. This study has shown promising results; the specimens with the tyre polymer fibres have shown lower vulnerability to spalling than those of plain concrete

Constitutive model for rubberized concrete passively confined with FRP laminates

This article develops an analysis-oriented stress-strain model for rubberized concrete (RuC) passively confined with fiber reinforced polymer (FRP) composites. The model was calibrated using highly instrumented experiments on 38 cylinders with high rubber contents (60% replacement of the total aggregate volume) tested under uniaxial compression. Parameters investigated include cylinder size (100×200mm or 150×300mm; diameter×height), as well as amount (two, three, four or six layers) and type of external confinement (Carbon or Aramid FRP sheets). FRP-confined rubberized concrete (FRP CRuC) develops high confinement effectiveness (fcc/fco up to 11) and extremely high deformability (axial strains up to 6%). It is shown that existing stress-strain models for FRP-confined conventional concrete do not predict the behavior of such highly deformable FRP CRuC. Based on the results, this study develops a new analysis-oriented model that predicts accurately the behavior of such concrete. This article contributes towards developing advanced constitutive models for analysis/design of sustainable high-value FRP CRuC components that can develop high deformability