Experimental behaviour of existing precast prestressed reinforced concrete elements strengthened with cementitious composites

Strengthening of reinforced concrete (RC) members by means of fibre reinforced polymers (FRP) has gained increasing importance in the last few decades. On the other hand the necessity of skilled labour, high costs and particularly the weak response under high temperature conditions represent critical issues for the effective application of this technique. The use of fibre reinforced cementitious matrix (FRCM) composites applied to RC members seems to be a promising technique since it combines cost economy and high performance. Despite the fact that a number of experimental investigations on strengthening of RC elements by means of fibre reinforced polymers (FRP) composites are available in the literature, very little information is available about fibre reinforced cementitious matrix composite (FRCM). Hence, the use of cementitious composites in strengthening of RC structures is strongly limited by the lack of design models, guidelines, and recommendations and by the few available experimental investigations.This work aims to better understand the behaviour of FRCM strengthened RC full-scale elements through experimental tests on precast prestressed double-T beams. In addition to investigating the experimental behaviour of an innovative and promising strengthening system, a further element of novelty of the work is that the tested beams belong to an actual existing industrial building, since the few experimental tests available in the literature are mostly related to small-scale and cast-in-place RC elements.     

Thermomechanical properties of bio-based composites made from a lactic acid thermoset resin and flax and flax/basalt fibre reinforcements

Low viscosity thermoset bio-based resin was synthesised from lactic acid, allyl alcohol and pentaerythritol. The resin was impregnated into cellulosic fibre reinforcement from flax and basalt and then compression moulded at elevated temperature to produce thermoset composites. The mechanical properties of composites were characterised by flexural, tensile and Charpy impact testing whereas the thermal properties were analysed by dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). The results showed a decrease in mechanical properties with increase in fibre load after 40 wt.% for the neat flax composite due to insufficient fibre wetting and an increase in mechanical properties with increase fibre load up to 60 wt.% for the flax/basalt composite. The results of the ageing test showed that the mechanical properties of the composites deteriorate with ageing; however, the flax/basalt composite had better mechanical properties after ageing than the flax composite before ageing.     

Fabrication and mechanical behaviors of cementitious composites reinforced by 3D woven lattice sandwich fabrics

Glass fibers were firstly woven to form three-dimensional (3D) woven lattice sandwich fabrics (WLSFs) which then were applied to reinforce cementitious foams and mortars to fabricate novel ductile cementitious composites. Failure behaviors of WLSF reinforced cementitious composite structures were studied through compression and three-point bending experiments. The WLSF greatly enhances the strength of cementitious foams at a level of four times. For cementitious mortars, compression strength of WLSF reinforced blocks is a little greater for the fraction of the textile is small as well as the compression strength of the textile pillars is not strong. But in flexure, excellent stretching ability of the glass fiber textiles greatly improves the flexural behavior of WLSF reinforced cementitious composite panels. Load capacity and ultimate deflection of these composite panels were greatly enhanced. Flexural capacity of the WLSF reinforced beam is four times greater. Reinforced by WLSF, failure of the cementitious composite is ductile.     

Structural behaviour of fabric reinforced cementitious matrix (FRCM) strengthened concrete columns under eccentric loading

The structural behaviour of eccentrically loaded reinforced concrete columns with rectangular cross sections strengthened with a cement based composite materials wrapping system, is analysed in the paper, both experimentally and analytically.The main issues focussed in the paper were: i) the effectiveness of the cement based wrapping systems to improve the strength of the reinforced concrete columns, ii) the influence of the load eccentricity and the reinforcement ratio on the structural response of wrapped columns, iii) the prediction, by an analytical procedure, of the structural behaviour of wrapped columns.A total of 8 reinforced concrete columns with end corbels, wrapped with fabric meshes of PBO (short of Polypara-phenylene-benzo-bisthiazole) fibers embedded into a cement based matrix (PBO-FRCM system), were tested varying both the reinforcement ratio, ρ_f, and the eccentricity-to-section height ratio (e/h). The influence of mechanical and geometrical parameters on the structural response of wrapped columns was analysed in terms of failure modes, strength and ductility.To predict the structural response of wrapped columns, a non linear second-order analysis that takes into account the changes in geometry caused by lateral deformations is, also, developed. Theoretical results were compared with experimental ones to validate the effectiveness of the proposed procedure.     

Non-linear analytical model of composites based on basalt textile reinforced mortar under uniaxial tension

The recent development of inorganic based composites as low-cost materials in reinforced concrete structural strengthening and precast thin-walled components, requires the creation of models that predict the mechanical behaviour of these materials.Textile Reinforced Mortar (TRM) shows complex stress–strain behaviour in tension derived from the heterogeneity of its constituent materials. This complexity is mainly caused by the formation of several cracks in the inorganic matrix. The multiple cracking leads to a decrease in structural stiffness. Due to the severe conditions of the serviceability limit state in structural elements, the prediction of the stress–strain curve is essential for design and calculation purposes. After checking other models, an empirical nonlinear approach, which is based on the crack control expression included in the Eurocode 2, is proposed in this paper.Following this scope, this paper presents an experimental campaign focused on 31 TRM specimens reinforced with four different reinforcing ratios. The results are analysed and satisfactorily contrasted with the presented non-linear approach.     

Effects of fabric parameters on the tensile behaviour of sustainable cementitious composites

The mechanical behaviour of fabric-reinforced composites can be affected by several parameters, such as the properties of fabrics and matrix, the fibre content, the bond interphase and the anchorage ability of fabrics. In this study, the effects of the fibre type, the fabric geometry, the physical and mechanical properties of fabrics and the volume fraction of fibres on the tensile stress–strain response and crack propagation of cementitious composites reinforced with natural fabrics were studied. To further examine the properties of the fibres, mineral fibres (glass) were also used to study the tensile behaviour of glass fabric-reinforced composites and contrast the results with those obtained for the natural fabric-reinforced composites. Composite samples were manufactured by the hand lay-up moulding technique using one, two and three layers of flax and sisal fabric strips and a natural hydraulic lime (NHL) grouting mix. Considering fabric geometry and physical properties such as the mass per unit area and the linear density, the flax fabric provided better anchorage development than the sisal and glass fabrics in the cement-based composites. The fabric geometry and the volume fraction of fibres were the parameters that had the greatest effects on the tensile behaviour of these composite systems.     

Guidelines for flexural resistance of FRP reinforced concrete slabs and beams in fire

Several building codes are currently available for the design of concrete structures reinforced with fiber-reinforced polymer (FRP) bars. Nevertheless, there is little information provided about structural behavior in case of fire and no reliable design methods are available for FRP reinforced concrete (RC) members in fire. The goal of this paper is to provide guidelines for the calculation of the resistant bending moment of FRP-RC members exposed to fire in compliance with the provisions of Eurocodes, based on studies recently carried out by the authors. The paper provides a conceptual approach to fire safety checks for bending moment resistance of FRP-RC members. With reference to thermo-mechanical analysis, a simplified design method (for both thermal and mechanical analyses) for sagging bending moment resistance of FRP-RC slabs in fire situations is finally suggested.     

Flax and polyparaphenylene benzobisoxazole cementitious composites for the strengthening of masonry elements subjected to eccentric loading

To address the structural problems caused by eccentric loads in unreinforced masonry, three different types of masonry were prepared based on clay bricks bonded with a natural hydraulic lime mortar combined with a flax or polyparaphenylene benzobisoxazole (PBO) fabric-reinforced cementitious matrix (FRCM) composite. The mechanical behaviour when subjected to concentric and eccentric loads was studied by performing axial compression tests, with eccentric load tests only carried out in instances of large eccentricities. Analysis of the load–displacement and moment–curvature response revealed that both the flax- and PBO-based strengthening systems improve the strength and deformability of masonry. However, compared to the PBO fabric composite, the use of flax fabric provides a greater deformability that helps prevent the composite and substrate debonding.     

In-plane shear performance of masonry panels strengthened with FRP

The opportunities provided by the use of Fiber Reinforced Polymers (FRPs) composites for the shear strengthening of tuff masonry structures were assessed on full-scale panels subjected to in-plane shear-compression tests at the ENEL HYDRO S.p.A. laboratory, ITALY. Tuff masonry specimens have been arranged in order to simulate both mechanical and textural properties typical of buildings located in South-Central Italian historical centres. In this paper, the outcomes of the experimental tests are presented. The monotonic shear-compression tests were performed under displacement control and experimental data have provided information about in-plane behaviour of as-built and FRP strengthened tuff masonry walls. Failure modes, shear strength, displacement capacity and post-peak performance are discussed.     

Preparation and characterization of flax biocomposites made of seed mucilage reinforced by fibers

Natural biocomposites were prepared from flax fibers and mucilage polysaccharides extracted from flax seeds, as a matrix, in two steps: impregnation and compression molding. The ribbons were preimpregnated with water plasticized mucilage. Solid mucilage (30%, w/w) was added to the ribbon impregnated with 20% mucilage, and the composite was compression molded. The solidified mucilage was homogeneous and rigid (2 GPa) with an elastic deformation of approximately 1%. The mechanical properties of the composites were in the ranges of 7–10 GPa, 300–400 MPa and 4–5% for the modulus, maximal strength and strain, respectively. The two latter parameters were larger than the ones for the fiber. The experimental values of the modulus and strength were in accordance with the values computed using the rule of mixture, which indicated a good interface between the fibers and the matrix. This was confirmed visually with scanning electron microscopy. The water sorption behavior of the composites was intermediate between the mucilage and the fiber alone.     

Pull-through performance of carbon fibre epoxy composites

An investigation of the through-thickness properties of carbon fibre prepreg laminates, Non-Crimp Fabric laminates and non-crimp 3D orthogonal woven composites by pull-through testing was performed. Influence of matrix system and curing temperature on the performance of the 3D woven composites was investigated.     

Mechanical behavior of Kevlar/basalt reinforced polypropylene composites

In this study, mechanical behavior of thermoplastic composites reinforced with two-dimensional plain woven homogeneous and hybrid fabrics of Kevlar/basalt yarns was studied. Five types (two homogeneous and three hybrids) of composite laminates were manufactured using compression molding technique with polypropylene (PP) resin. Static tensile and in-plane compression tests were carried out to evaluate the mechanical properties of the laminates. The tension and in-plane compression tests had shown that the composites with the combination of Kevlar and basalt yarns present better tensile and in-plane compressive behavior as compared to their base composites. Improvement in the properties such as elastic modulus, strength and failure strain in both tension and in-plane compression was observed due to the hybridization. Numerical simulations were performed in ABAQUS/Standard by implementing a user-defined material subroutine (VUMAT) based on Chang-Chang criteria. Good agreement between the experimental and numerical simulations was achieved in terms of damage patterns.     

Multi-scale shear properties of flax fibre reinforced polyamide 11 biocomposites

Multiscale analyses are carried out to evaluate and understand the shear properties and behaviour of a flax fibre reinforced polyamide 11 (PA 11) biocomposite. Tensile tests of [±45]_n laminates are performed to evaluate the macroscale in-plane shear properties, while microbond tests are performed to evaluate the apparent interfacial shear strength. Although the shear stiffness of PA 11 biocomposites is lower than the available literature values, the shear strength is higher due to a relatively high interfacial bonding strength. Flax/PA 11 interfacial bonding is controlled by hydrogen bonding rather than adhesive pressure induced by residual thermal stress. A superficial fibre cell-wall layer (primary cell-wall) is observed at different scales, which highlights the contribution of the global structure of flax fibres to the shear properties of biocomposites.     

Processing parameters and characterisation of flax fibre reinforced engineering plastic composites with flame retardant fillers

This work studies the possibility of compounding natural fibres (flax) into engineering plastics (PA6 and PB6) and comparing the results with counterpart glass fibre composites. The problem in compounding is the difficulty to compound the fibres with such polymers of high melting temperatures without decomposing the natural fibre thermally. Preliminary experiments are tried to define the possible processing window using the kneader namely temperature, compounding time and shear rate. Fibre content is tried in range of 0–50 wt.% with 10% step. The mixing temperature covers the range around the melting temperature ‘Tm’ [Tm−20, Tm+20]°C. The use of pre-melting temperature in compounding would utilise the energy evolving by fibres mutual rubbing. Compounding time is optimised at the minimum level. Shearing rate is tried at 25, 50, 75 and 100 rpm. Optimum conditions are defined to be 210–230 °C and 200–210 °C for PBT and PA6 respectively. Shearing rate is also defined to lie within 25–50 rpm.Two different additives of non-organic mineral and organic phosphate flame retardants are tried with the prepared composites either alone or in combination with each other. The loading of flame retardants is limited to 20 wt.% in order to leave a space for natural fibres as well as the polymer and to keep in turn the overall composite mechanical properties. A mix of 1:1 ratio between the both types of retardants is needed to reach V0 flame retardation level. Mechanical properties are even improved 30% in E-modulus and 4% in strength with respect to composites without flame retardants. However, the injection moulding is reported to be difficult because of the high viscosity and the parameters should be optimised regarding the desired flame retardance level and the required mechanical properties as well as keeping the fibres not damaged.     

Fatigue and Izod impact performance of carbon plain weave textile reinforced epoxy modified with cellulose microfibrils and rubber nanoparticles

This work details an experimental investigation on understanding the effects of hybrid epoxy resins, filled with micro-fibrillated cellulose (MFC) and carboxylated nitrile-butadiene rubber nanoparticles (XNBR), on the tensile–tensile fatigue performance of carbon plain weave textile reinforced composites. Twelve combinations of MFC and XNBR weight contents in the epoxy resin (from 0% to 0.5% MFC and from 0% to 3% XNBR) were considered for preliminary quasi-static tests and five of them were selected to study the fatigue behaviour considering different loading levels. Moreover, the effect of the twelve fillers contents was observed on the Izod impact strength. The investigation finds that the best fatigue performance, for the considered weight contents of fillers, is of the composite enhanced with the maximum content of MFC. The SEM observations of the fracture surfaces indicate the extensive “plastic” deformation of the matrix and the improved fibre and matrix adhesion.     

Influence of low velocity impact on fatigue behaviour of woven hemp fibre reinforced epoxy composites

The purpose of this work is to study the resistance to low velocity impact of woven hemp/epoxy matrix composites and the influence of impact damage on their residual quasi-static tensile and cyclic fatigue strengths. Impact characteristic parameters were evaluated and critically compared to those found in the literature for other similar composites. Damage mechanisms were analysed by using AE monitoring and microscopic observations. An analytical model is used to predict the fatigue lifetime of impacted specimens. Moreover a damage scenario is proposed, reduced to two phases in post-impacted fatigue behaviour, instead of three phases for non impacted specimens.     

Tests of continuous concrete slabs reinforced with carbon fibre reinforced polymer bars

Although several research studies have been conducted on simply supported concrete elements reinforced with fibre reinforced polymer (FRP) bars, there is little reported work on the behaviour of continuous elements. This paper reports the testing of four continuously supported concrete slabs reinforced with carbon fibre reinforced polymer (CFRP) bars. Different arrangements of CFRP reinforcement at mid-span and over the middle support were considered. Two simply supported concrete slabs reinforced with under and over CFRP reinforcement and a continuous concrete slab reinforced with steel bars were also tested for comparison purposes. All continuous CFRP reinforced concrete slabs exhibited a combined shear–flexure failure mode. It was also shown that increasing the bottom mid-span CFRP reinforcement of continuous slabs is more effective than the top over middle support CFRP reinforcement in improving the load capacity and reducing mid-span deflections. The ACI 440.1R–06 formulas overestimated the experimental moment at failure but better predicted the load capacity of continuous CFRP reinforced concrete slabs tested. The ACI 440.1R–06, ISIS–M03–07 and CSA S806-06 design code equations reasonably predicted the deflections of the CFRP continuously supported slabs having under reinforcement at the bottom layer but underestimated deflections of continuous slabs with over-reinforcement at the bottom layer.     

Flexural stiffness modelling of RC slab strengthened by externally bonded FRP

This paper describes the behaviour of strengthened and unstrengthened reinforced concrete slabs, subjected to a single, specific, local, load at their centres. In the experimental stage, the influence of the reinforcement on the various slabs tested was analysed by studying their behaviour at failure and by making a bending stiffness analysis. In order to predict the mechanical behaviour of the slab, a model was developed to predict the value of the bending stiffness, the mid-span displacements and the strain on each material making up the slabs. The experimental results were compared to those of the model and there was good agreement.     

Characterization of thermoplastic natural fibre composites made from woven hybrid yarn prepregs with different weave pattern

This paper focuses on the effect of weave structure on mechanical behaviour and moisture absorption of the PLA/hemp woven fabric composites made by compression moulding. The unidirectional woven fabric prepregs were made from PLA (warp) and PLA/hemp wrapped-spun hybrid yarn (weft) with two different weave patterns; 8-harness satin and basket. Unidirectional composites with 30 mass% hemp content were fabricated from these prepregs, and compared to winded PLA/hemp hybrid yarn laminates with same composition. The composite from the satin fabric had significantly lowest porosities and best mechanical properties compared to the composite made from the winded hybrid yarn and basket fabric. The tensile, flexural, and impact strength were 88 MPa, 113.64 MPa, and 24.24 kJ/m², respectively. The effect of weave pattern on water absorption is significant. Although the composite from hybrid yarn laminate has larger water absorption than that of the pure PLA, it exhibits lower moisture absorption than both weaves.     

Evaluating the mechanical properties of reinforced LDPE composites made with carbon fibres recovered via solvothermal processing

Carbon fibre was recovered from a thermoset composite via a solvo-thermal process and used as reinforcement in low density polyethylene (LDPE). The oxidized recovered carbon fibres have shown better properties than original non-oxidized fibres. The best interactions between the continuous and dispersed phases were found using 3-aminopropyl-trimetoxysilane and experimentally synthesized polyalkenyl-polymaleic anhydride based polymers. The tensile strength of the prepared composites nearly doubled when 3-aminopropyl-trimetoxysilane was used as compatibilizer, in comparison to the composites prepared without additives. Based on infrared analysis, a chemical reaction has been proposed between –COOH groups of compatibilizers and the –OH groups of the carbon fibre surface for the best composites.