Fire behaviour of thermally insulated RC beams strengthened with NSM-CFRP strips: Experimental study

This paper presents the results of fire resistance tests on reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) strips installed according to the near surface mounted (NSM) technique using two different adhesives. The beams were simultaneously subjected to a service load and the ISO 834 standard fire. Different fire protection schemes were studied, comprising a thinner insulation layer along the bottom soffit of the beams and a thicker one at the CFRP anchorage zones. The main objectives of this paper were (i) to understand in further depth the fire behaviour of NSM-strengthened RC beams, in particular the structural effectiveness of the strengthening system during fire, (ii) to evaluate the efficiency of the above-mentioned fire protection strategy in extending the CFRP mechanical contribution during fire, and (iii) to compare the fire performance of the NSM-strengthening system with that of the alternative externally bonded reinforcement (EBR) technique, recently investigated under similar test conditions. The results obtained showed that using the adopted insulation schemes (i.e., thicker insulation at the anchorage zone and thinner insulation in the current zone), even after the CFRP-concrete bond is highly damaged in the central zone of the beams, the strengthening system is able to retain its structural effectiveness through a cable mechanism: for insulation thicknesses of 25 mm (current zone) and 50 mm (anchorage zones), the fire resistance of the strengthening system was extended up to 114 min. The loss of effectiveness of the CFRP system occurred when the average temperature in the adhesive at the CFRP anchorage zones attained values ranging from 2.2 to 5.6 times its glass transition temperature (T_g). The comparison with the EBR-strengthened beams confirmed the much better performance of the NSM strengthening.     

Repair of shear-deficient normal weight concrete beams damaged by thermal shock using advanced composite materials

The use of advanced composite materials such as Fiber Reinforced Polymers (FRPs) in repairing and strengthening reinforced concrete structural elements has been increased in the last two decades. Repairing and strengthening damage structures is a relatively new technique. The aims of this study was to investigate the efficiency and effectiveness of using Carbon Fiber Reinforced Polymer (CFRP) to regain shear capacity of shear-deficient normal weight high strength RC beams after being damaged by thermal shock. Sixteen high strength normal weight RC beams (100 × 150 × 1400 mm) were cast, heated at 500 °C for 2 h and then cooled rapidly by immersion in water, repaired, and then tested under four-point loading until failure. The composite materials used are carbon fiber reinforced polymer plates and sheets. The experimental results indicated that upon heating then cooling rapidly, the reinforced concrete (RC) beams exhibited extensive map cracking without spalling. Load carrying capacity and stiffness of RC beams decreased about 68% and 64%, respectively, as compared with reference beams. Repairing the thermal damaged RC beams allowed recovering the original load carrying without achieving the original stiffness. Repaired beams with CFRP plates with 90° and 45° regained from 90% to 99% of the original load capacity with a corresponding stiffness from 79% to 95%, whereas those repaired with CFRP sheet on the web sides and a combination of CFRP plates and sheet regained from 102% to 107% of the original load capacity with a corresponding stiffness from 81% to 93%, respectively. Finally, finite element analysis model is developed and validated with the experimental results. The finite element analysis showed good agreement as compared with the experimental results in terms of load–deflection and load–CFRP strain curves.     

Fatigue behavior of RC T-beams strengthened in shear with EB CFRP L-shaped laminates

The use of externally bonded carbon fiber-reinforced polymer (EB-CFRP) to strengthen deficient reinforced concrete (RC) beams has gained in popularity and has become a viable and cost-effective method. Fatigue behavior of RC beams strengthened with FRP is a complex issue due to the multiple variables that affect it (applied load range, frequency, number of cycles). Very few research studies have been conducted in shear under cyclic loading. The use of prefabricated CFRP L-shaped laminates (plates) for strengthening RC beams under static loading has proven to be technically feasible and very efficient. This study aimed to examine the fatigue performance of RC T-beams strengthened in shear for increased service load using prefabricated CFRP L-shaped laminates. The investigation involved six laboratory tests performed on full-size 4520 mm-long T-beams. The specimens were subjected to fatigue loading up to six million load cycles at a rate of 3 Hz. Two categories of specimens (unstrengthened and strengthened) and three different transverse-steel reinforcement ratios (Series S0, S1, and S3) were considered. Test results were compared with the upper fatigue limits specified by codes and standards. The specimens that did not fail in fatigue were then subjected to static loading up to failure. The test results confirmed the feasibility of using CFRP L-shaped laminates to extend the service life of RC T-beams subjected to fatigue loading. The overall response was characterized by an accelerated rate of damage accumulation during the early cycles, followed by a stable phase in which the rate slowed significantly. In addition, the strains in the stirrups decreased after the specimens were strengthened with CFRP, despite the higher applied fatigue loading. Moreover, the addition of L-shaped laminates enhanced the shear capacity of the specimens and changed the failure mode from brittle to ductile under static loading. Finally, the presence of transverse steel in strengthened beams resulted in a substantially reduced gain in shear resistance due to CFRP, confirming the existence of an interaction between the transverse steel and the CFRP.     

Variation of strength and stiffness of fibre reinforced polymer reinforcing bars with temperature

This paper presents the results of tensile mechanical properties of FRP reinforcement bars, used as internal reinforcement in concrete structures, at elevated temperatures. Detailed experimental studies were conducted to determine the strength and stiffness properties of FRP bars at elevated temperatures. Two types of FRP bars namely: carbon fibre reinforced polyester bars of 9.5 mm diameter and glass fibre reinforced polyester bars of 9.5 mm and 12.7 mm diameter were considered. For comparison, conventional steel reinforcement bars of 10 mm and 15 mm diameter were also tested. Data from the experiments was used to illustrate the comparative variation of tensile strength and stiffness of different types of FRP reinforcing bars with traditional steel reinforcing bars. Also, results from the strength tests were used to show that temperatures of about 325 °C and 250 °C appear to be critical (in terms of strength) for GFRP and CFRP reinforcing bars, respectively. A case study is presented to illustrate the application of critical temperatures for evaluating the fire performance of FRP-reinforced concrete slabs.     

Strengthening steel bridge sections using CFRP laminates

This paper presents results from an experimental investigation to determine the feasibility of using carbon fiber reinforced polymer (CFRP) epoxy laminates to repair steel composite bridge members. Six specimens, each consisting of a 6.1 m long W8×24 wide flange A36 steel beam acting compositely with a 0.114 m thick by 0.71 m wide reinforced concrete slab, were first loaded past yield of the tension flange to simulate severe service distress. The damaged specimens were then repaired using 3.65 m lengths of 2 or 5 mm thick CFRP laminates bonded to the tension flange and tested to failure. The results indicated significant ultimate strength gains but more modest improvement in the elastic response. Non-linear finite element analyses were in good agreement with the experimental results. The study suggests that it is feasible to strengthen steel composite members using CFRP laminates.     

Structural performance of shear-critical RC deep beams with corroded longitudinal steel reinforcement

The effect of corrosion of longitudinal reinforcement on the structural performance of shear-critical reinforced concrete (RC) deep beams was experimentally investigated. A total of eight medium-scale reinforced concrete beams were constructed. The beams measured 150 mm wide, 350 mm deep and 1400 mm in length. The test variables included: corrosion levels (0%, 5%, and 7.5%), existence of stirrups and FRP repair. Six beams were subjected to artificial corrosion whereas two beams acted as control un-corroded. Following the corrosion phase, all beams were tested to failure in three point bending. The test results revealed that corrosion of properly anchored longitudinal steel reinforcement does not have any adverse effect on the behaviour of shear critical RC deep beams. Corrosion changed the load transfer mechanism to a pure arch action and as a result the load carrying capacity was improved. A strut and tie model was proposed to predict the failure loads of shear-critical RC deep beams with corroded longitudinal steel reinforcement. The predicted results correlated well with the experimental results.     

Fire protection systems for building floors made of pultruded GFRP profiles: Part 1: Experimental investigations

This paper presents results of experimental investigations on the behaviour of GFRP pultruded profiles exposed to fire, in order to study the viability of their structural use in floors of buildings, taking into account the fulfilment of fire resistance requirements. The feasibility and efficacy of using three different protective coatings/layers, often used to protect structural steel, and a water cooling system to provide fire protection to GFRP pultruded profiles were investigated. The experimental programme included dynamic mechanical analyses (DMA), thermogravimetric and differential scanning calorimetry (TGA/DSC) experiments and fire resistance tests on GFRP tubular loaded beams. The unprotected GFRP beam failed after about 38 min, the three different passive protection systems provided a fire resistance between 65–76 min and the water cooling system provided a fire resistance of at least 120 min. Failure occurred in the upper part of the beams, due to compression and shear stresses. Results of these experiments allowed defining the field of application of each investigated solution, according to building code requirements.     

Behavior of full-scale reinforced concrete beams retrofitted for shear and flexural with FRP laminates

Four full-scale reinforced concrete beams were replicated from an existing bridge. The original beams were substantially deficient in shear strength, particularly for projected increase of traffic loads. Of the four replicate beams, one served as a control and the remaining three were implemented with varying configurations of carbon fiber reinforced polymers (CFRP) and glass FRP (GFRP) composites to simulate the retrofit of the existing structure. CFRP unidirectional sheets were placed to increase flexural capacity and GFRP unidirectional sheets were utilized to mitigate shear failure. Four-point bending tests were conducted. Load, deflection and strain data were collected. Fiber optic gauges were utilized in high flexural and shear regions and conventional resistive gauges were placed in eighteen locations to provide behavioral understanding of the composite material strengthening. Fiber optic readings were compared to conventional gauges.Results from this study show that the use of fiber reinforced polymers (FRP) composites for structural strengthening provides significant static capacity increases approximately 150% when compared to unstrengthened sections. Load at first crack and post cracking stiffness of all beams was increased primarily due to flexural CFRP. Test results suggest that beams retrofit with both the designed GFRP and CFRP should well exceed the static demand of 658 kN m sustaining up to 868 kN m applied moment. The addition of GFRP alone for shear was sufficient to offset the lack of steel stirrups and allow conventional RC beam failure by yielding of the tension steel. This allowed ultimate deflections to be 200% higher than the pre-existing shear deficient beam. If bridge beams were retrofit with only the designed CFRP failure would still result from diagonal tension cracks, albeit at a 31% greater load. Beams retrofit with only the designed shear GFRP would fail in flexure at the mid-span at an equivalent 31% gain over the control specimen, failing mechanism in this case being yielding of the tension steel. Successful monitoring of strain using fiber optics was achieved. However, careful planning tempered by engineering judgement is necessary as the location and gauge length of the fiber optic gauge will determine the usefulness of the collected data.     

Strengthening and repair of RC beams with fiber reinforced concrete

The use of a jacket made of fiber reinforced concrete with tensile hardening behavior for strengthening RC beams is investigated by means of full-scale tests on 4.55 m long beams. A 40 mm jacket of this material was directly applied to the beam surface. Both the strengthening and the repair of RC beams were studied. In particular, in the latter case the beam was initially damaged and eventually repaired. A numerical analysis is also performed in order to better understand the reinforcement behavior. The experimental and numerical results show the effectiveness of the proposed technique both at ultimate and serviceability limit states.     

Effect of temperature on low velocity impact damage and post-impact flexural strength of CFRP assessed using ultrasonic C-scan and micro-focus computed tomography

The effect of temperature on the low velocity impact resistance properties and on the post-impact flexural performance of CFRP laminates were studied. With this aim, 150 × 75 mm cross-ply carbon fibre/epoxy laminates with a [0/90/90/0]_2s layup, therefore with a total of sixteen layers, were impacted at ambient temperature (30 °C) and at elevated temperatures (55, 75 and 90 °C) at a velocity of 2 m/s using a drop weight impact tower. This was followed by flexural tests carried out at ambient temperature using a three-point bending rig. Damage assessment of impact and post-impact behaviour were carried out using ultrasonic C-scan and microfocus X-ray computed tomography (μCT). Interrupted flexural tests using μCT allowed delamination propagation to be observed. In general, lower projected damage was observed at elevated temperatures, which resulted also in a possible hindrance to delamination and shear cracks propagation during impact and in a greater amount of retained flexural strength after impact.     

Mechanical behaviour of corroded RC beams strengthened by NSM CFRP rods

Corrosion of steel in reinforced concrete leads to several major defects. Firstly, a reduction in the cross-sectional area of the reinforcement and in its ductility results in premature bar failure. Secondly, the expansion of the corrosion products causes concrete cracking and steel–concrete bond deterioration and also affects the bending stiffness of the reinforced concrete members, causing a reduction in the overall load-bearing capacity of the reinforced concrete beams. This paper investigates the validity of a repair technique using Near Surface Mounted (NSM) carbon-fibre-reinforced polymer (CFRP) rods to restore the mechanical performance of corrosion-damaged RC beams. In the NSM technique, the CFRP rods are placed inside pre-cut grooves and are bonded to the concrete with epoxy adhesive.Experimental results were obtained on two beams: a corroded beam that had been exposed to natural corrosion for 25 years and a control beam, (both are 3 m long) repaired in bending only. Each beam was repaired with one 6-mm-diameter NSM CFRP rod. The beams were tested in a three-point bending test up to failure. Overall stiffness and crack maps were studied before and after the repair. Ultimate capacity, ductility and failure mode were also reviewed. Finally some comparisons were made between repaired and non-repaired beams in order to assess the effectiveness of the NSM technique. The experimental results showed that the NSM technique improved the overall characteristics (ultimate load capacity and stiffness) of the control and corroded beams and allowed sufficient ductility to be restored to the repaired corroded elements, thus restoring the safety margin, despite the non-classical mode of failure that occurred in the corroded beam, with the separation of the concrete cover due to corrosion products.     

An experimental study on the effect of environmental exposures and corrosion on RC columns with FRP composite jackets

The objective of this study was to evaluate the effects of various environmental conditions on the long-term behavior of reinforced concrete (RC) columns strengthened with carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) sheets. Small-scale RC columns were manufactured in the laboratory and conditioned under accelerated environmental cycling and accelerated corrosion process of reinforcing bars. Then, uni-axial compressive failure tests were conducted in order to evaluate the change of mechanical properties of the test columns due to the environmental effects. The results revealed that the mechanical properties of RC column system (RC + FRP) were altered due to the environmental conditioning and the corrosion of steel reinforcement, and each type of environmental conditions had its unique effects and features.     

Bond analysis of corroded reinforced concrete beams under monotonic and fatigue loads

This study investigated the fatigue bond behaviour of corroded steel reinforced concrete beams. Nine beams (152 × 254 × 2000 mm [6 × 10 × 78.74 in.]) were constructed and tested. Bond failure occurred in all the beams. The variables in this test series were: the type of load applied (monotonic or repeated loading), the repeated load range, whether the reinforcement inside the beam was corroded or not, and whether a carbon fibre reinforced polymer (CFRP) repair method was used or not. The fatigue life of the beams varied linearly with the range of applied load with a very shallow slope. Corroding the beams to a low corrosion level decreased the fatigue bond strength by about 30%. Corrosion caused the concrete in between the lugs of the reinforcing bars to be partially crushed due to the formation of the rust products from the corrosion process. This reduced the strength of the concrete keys and increased the rate of slip in the bar under repeated loading.     

Effect of curing conditions on strength development in an epoxy resin for structural strengthening

Civil structures such as bridges and buildings can be strengthened with prestressed fibre reinforced polymer (FRP) strips to enhance both their stiffness and load-bearing capacity. End anchorage is a crucial issue for prestressed FRP strips. An innovative anchorage procedure, called the “gradient anchorage method” and based on the possible accelerated curing of the epoxy-resin in the end region of the FRP strip, has recently been conceived with the aim of avoiding more invasive mechanical fastening systems. An in-depth knowledge of the actual development of the key mechanical properties of resins under different curing conditions (i.e., in terms of curing temperature) is of paramount importance for employing the above mentioned gradient method in practical applications. This paper presents experimental results and analytical investigations aimed at developing a better understanding of the strength development of a commercial adhesive under different curing times and temperatures. Firstly, direct tensile tests on epoxy specimens were performed at different curing temperatures. It was shown that the necessary curing time to reach the maximum tensile strength can be significantly reduced from several hours at room temperature to approximately 30 min at 90 °C. Furthermore, higher curing temperatures reduced the activation time after which strength starts to increase. The experimental observations are shown graphically with both the activation time and reaction duration at different curing temperatures. Secondly, pull-off bond tests were conducted on 100 mm wide and 1.2 mm thick FRP strips bonded to concrete using epoxy adhesives cured either at 90 °C for different durations or at room temperature. An optical image correlation system (ICS) allowed the load transfer behaviour of the inhomogeneous cured adhesive between the FRP strip(s) and concrete to be studied. Finally, using the experimental measurements, the bond shear stress–slip interface relationships for the different test specimens were identified in order to present the effect of elevated curing temperatures and curing durations.     

Mechanical properties improvements in two-phase and three-phase composites using carbon nano-fiber dispersed resin

Static strength tests were carried out for cured carbon nano-fiber (CNF) dispersed resin as tow-phase composites and for CFRP laminates using CNF dispersed resin as three-phase composites. To obtain these CFRP laminates, the CNF dispersed resin was impregnated to CF reinforcement and cured by hot press. The CNF used was a cup-stacked type of nano-fiber, CARBERE^®, made by GSI CREOS Corporation, Japan. Two CNF aspect ratios of 10 and 50 were employed. These fiber lengths of the CNF were controlled about 1000 nm (AR10) and 5000 nm (AR50), respectively. The CNF was dispersed to EPIKOTE 827^® epoxy resin in two values of CNF weight ratios, 5 and 10% to the resin. TORAYCA^® C6343 plain woven fabric was used for reinforcement of the CFRP laminates. The cure condition with the agent of aromatic amine EPIKURE W^® was 100 °C for two hours followed by a post cure of 175 °C for 4 h. The static strength tests led to the conclusion that the dispersion of CNF into epoxy improves mechanical properties of the tow-phase composites, and that CFRP laminates with CNF dispersed resin also exhibit higher compressive strength than CFRP laminates without CNF as control. Possibilities of improvement in mechanical properties were confirmed in the two and three-phase composites. Moreover, a proportional tendency in strength improvements to CNF weight content was found in the two present composites so far in the present test results.     

An investigation of contact resistance between carbon fiber/epoxy composite laminate and printed silver electrode for damage monitoring

An addressable conducting network (ACN) enables the structural condition to be monitored by the electrical resistance between electrodes on surface of CFRP (carbon fiber reinforced polymer) structure. To improve the reliability of ACN for damage detection, the contact resistance between the electrodes and CFRP laminates needs to be minimized. In this paper, the silver nanoparticles electrodes were fabricated via printed electronics techniques on CFRP composite. The contact resistance between the silver electrodes and CFRP was measured with respect to various fabrication conditions such as the sintering temperature of silver nanoink and the surface roughness of CFRP laminates. The interfaces between silver electrode and carbon fibers were observed using scanning electron microscope (SEM). From the study, it was found that the lowest contact resistance of 0.3664 Ω could be achieved when the sintering temperature of the silver nanoink and surface roughness were 120 °C and 230 nm, respectively.     

Contribution to the understanding of the mechanical behaviour of CFRP-strengthened RC beams subjected to fire: Experimental and numerical assessment

Recent experimental tests and numerical simulations about the fire resistance behaviour of CFRP-strengthened RC beams proved that CFRP strengthening systems are able to attain considerable fire endurance, provided that adequate fire protection systems are used. In a fire event, even though a CFRP laminate may rapidly debond from the central part of the beam in which it is installed, if sufficiently thick insulation is applied in the anchorage zones, the laminate transforms into a “cable” fixed at the extremities, thus maintaining a considerable contribution to the mechanical response of the strengthened beam. This paper presents experimental and numerical investigations on CFRP-strengthened RC beams with the objective of understanding in further depth their fire resistance behaviour, namely the influence of the above mentioned “cable” mechanism on the mechanical response of the beams. The experimental campaign, performed at ambient temperature, comprised 4-point bending tests on RC beams strengthened with CFRP laminates according to either the EBR or the NSM techniques, in both cases fully or partially (only at the anchorages, thus simulating the cable mechanism) bonded to the soffit of the beams. For the test conditions used in this study, for both types of strengthening systems, partially bonding the CFRP laminates did not affect the stiffness of the beams and caused only a slight reduction of their strength (6–15%). The numerical study comprised the simulation of the structural response of all beams tested. Non-linear finite element models were developed in Atena commercial package, in which a smeared cracked model was adopted to simulate concrete and appropriate bond-slip constitutive relations were defined for the CFRP-concrete interfaces. A very good agreement was obtained between experimental data and numerical results, providing further validation to the “cable” mechanism and the possibility of taking it into account when designing fire protection systems for CFRP-strengthened RC beams.     

Experimental study on the fire resistance of GFRP pultruded tubular columns

This paper presents experimental investigations about the fire behaviour of GFRP pultruded columns with square tubular cross-section. The objectives of the study were to evaluate (i) the efficacy of passive (calcium silicate (CS) boards) and active (water-cooling) systems in providing fire protection to GFRP pultruded columns; and the effects of (ii) the number of sides exposed to fire; and (iii) the service load level on the GFRP columns’ fire response. To this end, fire resistance tests were performed on 1.5 m long GFRP columns, subjected to two different load levels corresponding to axial shortenings of L/1500 and L/750 (L being the span), and simultaneously exposed to fire, in either one or three sides, according to the time–temperature curve defined in ISO 834. Results obtained were analysed regarding the thermal and mechanical responses of the GFRP columns (unprotected and protected), namely in terms of the evolution of axial and transverse displacements, the failure modes and the fire resistance. It was possible to conclude that for one-side exposure, water-cooling is the most effective protection, particularly with flowing water, providing more than 120 min of fire resistance. For three-sides exposure, the fire resistance of the different solutions tested was severely reduced, namely that of the water-cooling systems, which provided fire resistances of only about 20 min, regardless of using flowing water; for this type of exposure, the best performance was provided by the CS board protection, with about 40 min of fire resistance. The results obtained in this study draw the attention to the technical advantages of adopting a building architecture in which the GFRP columns are integrated in the façades and embedded in partition walls, preventing the columns from being exposed to fire in three sides. As expected, the load level increase caused a reduction of fire resistance, due to the higher stresses developed in the sections’ walls and to the GFRP strength decrease with temperature.     

Punching shear behavior of concrete flat plate slab reinforced with carbon fiber reinforced polymer rods

Flat plate slab system is widely adopted by engineers as it provides many advantages . The system can reduce the height of the building, provide more flexible spatial planning due to no beams present, and further reduce the material cost. However, the main problem in practice is the brittle failure of flat plate slab under punching shear. In this paper, the punching shear behavior has been studied and an experimental work using carbon fiber reinforced polymer (CFRP) rods as shear reinforcement has been conducted in flat plate slab system.This exploratory research is to study the behavior of the flat plate slab with CFRP-rods reinforced in punching shear zone under constant gravity load and lateral displacements in a reversed cyclic manner. Three specimens of interior column-slab connection specimens were tested including one standard specimen without any shear reinforcement, the second one reinforced with CFRP-rods and the third one reinforced with stud rails as the reference to the second one. The slabs were 3000 mm long × 2800 mm wide × 150 mm deep, and were simply supported at four corners. Punching shear failure occurred for the standard specimens at a lateral drift-ratio, lateral drift divided by the length of vertical member, of approximately 5%. The specimen reinforced by CFRP-rods had significant flexural yielding and sustained deformations up to a drift ratio of approximately 9% without significant losses of strength, and punching shear was not observed in this specimen. The displacements increased up to 1.79 times larger than that of the standard specimen. And this specimen showed 42% superior ductile performance than the standard specimen and even the same capability with the stud-rail reinforced specimen. The results of the experiment indicate that CFRP-rods using in the flat slab has a better foreground.     

High temperature and fire behaviour of continuous glass fibre/polypropylene laminates

This paper reports elevated temperature mechanical property measurements on woven glass fibre/polypropylene composites. Tensile and compressive stress rupture measurements were made on 12 mm thick laminate exposed to 50 kW m^?2 heat flux. Behaviour was qualitatively similar to that of thermosetting laminates, but compressive behaviour was significantly inferior, due to a poorer resin–matrix bond, and to the loss of compressive properties at temperatures above the melting point.COM-FIRE, a finite difference implementation of the Henderson Equation, was able to model the thermal and residual resin profiles in the laminate during fire exposure. The thermal predictions were used, in conjunction with the measured mechanical property data, to model changes in elastic properties and stress rupture behaviour in fire. Because of the non-linearity of the tensile stress–strain curves, a 3-parameter model was needed to describe behaviour. In contrast the compressive response could be modelled by a simpler 2-parameter or saw-tooth model.