Internal Reinforcement In Concrete Structures Research Articles

Compressive behaviour and prediction model for short and slender FRP-confined GFRP bars

Glass fibre reinforced polymer (GFRP) composite bars can be used as internal reinforcement for concrete structures built in harsh environmental and service conditions. Their utilisation as longitudinal reinforcement for compression members is not widely recommended though due to the anisotropic nature of FRP, their complex failure modes, and limited research, especially for slender bars. A more recent innovation has resulted in the wrapping (confinement) of GFRP bars with FRP composites oriented in the hoop direction. These confined bars have demonstrated promising improvement in FRP bar compressive behaviour and ductility. In the current study, the experimental compressive behaviour of eighty short and slender FRP-confined GFRP bars is reported and a compressive strength model is proposed. The experimental parameters investigated are (i) wrapping layers (i.e. one, two, and three layers oriented ± 83.3° to the longitudinal axis of the bar), and (ii) slenderness ratios (i.e. 15, 30, 45, and 60). The resulting compressive load capacity and strength, stress–strain behaviour, and failure modes are then reported for each tested bar. Test results indicate that an increase in slenderness results in decreased strength. In addition, failure modes shifted from splitting to buckling of the longitudinal fibres. GFRP bars with at least two winding layers had a significantly higher compressive strength than the control GFRP bar (not including winding) and they also exhibited bi-linear stress–strain behaviour as well as ductile failure modes. A compressive strength prediction model is then developed, which is based on the modification of the Johnson-Euler buckling theory and correlates well with the experimental results. The research presented herein has the potential to contribute to the development of design guidelines for concrete compression members, such as columns, piers and piles, that are reinforced with FRP-confined GFRP bars.

Development and mechanical performance evaluation of a GFRP-reinforced concrete boat-approach slab

The use of glass fiber-reinforced polymer (GFRP) composite bars as internal reinforcement in concrete structures exposed to marine and aggressive environments has gained popularity in recent years. The fact that designers and workers are unfamiliar with the design and handling of GFRP bars contributes to GFRP bars being used less often than traditional steel reinforcement bars. This case study presents the construction and loading test for GFRP-reinforced concrete boat-approach slab. Two types of reinforcement mesh stemmed from a similar design requirement were implemented: 16 mm diameter bars spaced 150 mm apart in both directions and 24 mm diameter bars spaced 300 mm apart in both directions to evaluate the fabrication and installation efficiency with GFRP bars. Loading tests and monitoring were employed to evaluate the field performance of the GFRP-reinforced slab. The results show that a slab reinforced with 16 mm diameter GFRP bars can be durable and reliable, and can increase worker productivity during handing and installation. Moreover, considering the appropriate subgrade modulus for the supporting ground in the design and analysis can lead to a more cost-effective design of GFRP-reinforced concrete approach slabs. Lastly, this case study demonstrates the durability and resilience of GFRP-reinforced concrete as boating infrastructure.

Durability assessment of GFRP rebars in marine environments

Technologies developed over the last two decades have facilitated the use of glass fiber reinforced polymer (GFRP) bars as internal reinforcement for concrete structures, specially in coastal environments, mainly due to their corrosion resistance. To-date, most durability studies have focused on a single mechanical parameter (tensile strength) and a single aging environment (exposure to high alkalinity). However, knowledge gaps exists in understanding how other mechanical parameters and relevant conditioning environments may affect the durability of GFRP bars. To this end, this study assesses the durability for different physio-mechanical properties of GFRP rebars, post exposure to accelerated conditioning in seawater.Six different GFRP rebar types were submerged in seawater tanks, at various temperatures (23°C, 40°C and 60°C) for different time periods (60, 120, 210 and 365 days). In total six different physio-mechanical properties were assessed, including: tensile strength, E-modulus, transverse and horizontal shear strength, micro-structural composition and lastly, bond strength. It was inferred that rebars with high moisture absorption resulted in poor durability, in that it affected mainly the tensile strength. Based on the Arrhenius model, at 23°C all the rebars that met the acceptance criteria by ASTM D7957 are expected to retain 85% of the tensile strength capacity.

Wave propagation in glass fibre-reinforced polymer (GFRP) bars subjected to progressive damage—Experimental and numerical results

Glass fibre reinforced polymer (GFRP) is a material of increasing interest in civil engineering applications. It is used as internal reinforcement for concrete structures (especially in extremely corrosive environments), as well as structural profiles, plates, and gratings. However, estimating the life of GFRP structures represents a challenge for the construction industry. To address this challenge, a numerical model of elastic wave propagation in GFRP bars is presented in this paper so that it can be used to reduce the number of physical testing required to estimate the degradation of GFRP bars in the field.The proposed numerical model is designed to assist the application of ultrasonic non-destructive testing (NDT) techniques and develop a framework for the evaluation of progressive damage of GFRP bars due to a highly alkaline environment. The proposed methodology consists of: (i) development and calibration of a numerical model of elastic wave propagation in GFRP bars in Abaqus, (ii) simulation of deterioration of the bars, and (iii) comparison of experimental, large and low strain tests (shear test and ultrasonic evaluation) with results of numerical simulations.The results show that the model (low strain properties) correctly captures the deterioration rates seen in the shear strength test (large strains). The model is used to study ultrasonic features and demonstrates that low strain measurements (e.g. wave dispersion, wave amplitude) can be used to estimate the effect of progressive damage on large strain properties in GFRP bars, which can result in a decreased number of required destructive tests.

Physical, mechanical and durability properties of GFRP bars with modified acrylic resin (modar) matrix

Glass fiber reinforced polymer (GFRP) bars are currently used as internal reinforcement of concrete structures, as an alternative to traditional steel rebars, to avoid corrosion problems and durability issues.GFRP bars are usually made of continuous glass filaments embedded within a polyester (for non-structural applications) or vinyl ester resin. These resins have been used for many years in several technological fields and widely employed as matrices for composite materials with structural functions. However, since the matrix plays a fundamental role in protecting the fibers from the external environment, the development of new organic matrices able to withstand aggressive environmental conditions while keeping a low cost of production is certainly of great interest.In this study, the possibility of using a modified acrylic resin (ModAR), never employed for such applications, as matrix for GFRP bars has been investigated. Physical, mechanical and durability tests have been carried out, according to international standards, to evaluate the technical feasibility of using ModAR matrices. Experimental results showed that ModAR GFRP bars are able to offer high thermal resistance while keeping physical and mechanical properties within standard limits.

Fracture patterns and mechanical properties of GFRP bars as internal reinforcement in concrete structures

Glass Fiber Reinforced Plastic (GFRP) composites as rolled bars can be used as steel rebar to prevent oxidation or rust which is one of the main reasons concrete structures deteriorate when exposed to chlorides and other harmful chemicals. GFRP is successful alternative for reinforcement with high tensile strength- low strain, corrosion resistance and congenital electromagnetic neutrality in terms of longer service life. The main goal of the study is to investigate the mechanical and bonding properties of GFRP bars and equivalent steel reinforcing bars then compare them. GFRP and steel rebar are embedded in concrete block with three different levels. Mechanical properties of GFRP and steel bars in terms of strength and strains are determined. On the other hand; modulus of elasticity of GFRP and steel bars, modulus of toughness and modulus of resilience were calculated using stress-strain curves, as a result of the experiments. Pull-out tests are conducted on each GFRP and rebar samples which are embedded in concrete for each embedment level and ultimate adherence strengths are determined in terms of bar diameter–development length ratio. Yield strength, strain and modulus of elasticities of GFRP samples are compared to steel rebar. According to the test results reported in this study, GFRP bars are used safely instead of steel bars in terms of mechanical properties.

Effect of macro-synthetic structural fibers on the flexural behavior of concrete beams reinforced with different ratios of GFRP bars

Fiber reinforced polymer (FRP) bars are increasingly used as internal reinforcement in concrete structures for mitigating the corrosion problems associated with steel reinforcement. However, its wide spread use in construction is limited by the parameters such as lack of awareness/usage, brittle failure mode, high initial cost, availability of comprehensive guidelines and test data on its long term effectiveness. The use of macro-synthetic structural fibers is explored in this study for improving the post-cracking, and deformability characteristics of glass FRP (GFRP) reinforced concrete beams. In total, eighteen full-scale RC beams with and without structural synthetic fibers are tested under flexure. The test specimens include: (i) control beams with steel reinforcement, (ii) GFRP bar RC beams without macro-synthetic fibers, and (iii) GFRP RC beams with 0.35%, 0.7%, and 1.0% volume of macro-synthetic fibers. The effect of fiber addition on cracking, stiffness and deformability characteristics of GFRP RC beams with different longitudinal reinforcement ratios (0.7% and 2.0%) is investigated. Experimental results reveal that the addition of fibers significantly improved the post-cracking response. Moreover, the crack widths reduced significantly at service loads with the increase in fiber dosage. Addition of 1% fibers helps in enhancing the deformability of GFRP RC beams and converts the failure from brittle flexure-shear to ductile flexure mode with higher pseudo-ductility. Analytical calculations of deflections showed a close correlation with the test results.

Modified strip model for punching-shear strength of FRP-reinforced concrete edge–column slab connections

The use of fiber-reinforced polymer (FRP) bars as internal reinforcement for concrete structures in aggressive environments has emerged as an innovative solution to eliminate corrosion problems. Glass FRP (GFRP) reinforcing bars have recently gained wide acceptance as a viable construction material for sustainable new constructions due to their costing less than other FRP types. This paper reports the test results for full-sized edge–column slab connections (ECS –connections) reinforced with sand coated GFRP bars. These connections were cast using normal-strength concrete (NSC) and high strength concrete (HSC). The effect of concrete strength (NSC and HSC) and moment-to-shear force ratio (M/V) (300 mm, 600 mm) on the performance of the tested connections were examined. In addition, a new mechanical model, called Modified Strip Model, is presented for punching-shear strength predictions of FRP reinforced concrete (RC) ECS –connections. The test results reveal that the use of HSC directly augmented the punching-shear strength and improved the load–deflection response of the tested connections, However, Increasing the M/V ratio produced significant shear stresses, thus decreasing the punching-shear strength for the NSC and HSC connections, respectively. The mechanical model presented herein takes into account all possible mechanisms of loads transfer in FRP-RC ECS-connections. Hence, a complete shear–moment (V-M) interaction diagram can be plotted to predict the punching strength of such connections. Moreover, the model revealed higher accuracy and narrower scatter for punching-shear strength predictions compared to other available equations in FRP design codes and guides. The EN 1992–1-1–05 punching-shear equation was also examined for FRP-RC ECS–connections and adapted to consider the tensile properties of the FRP bars instead of steel.

Flexural strength and serviceability evaluation of concrete beams reinforced with deformed GFRP bars

This paper presents a study to investigate serviceability, strength, and deformability of normal- and high-strength concrete beams reinforced with deformed glass-fiber-reinforced-polymer (GFRP) bars under flexural loading. Understanding the flexural behavior of concrete beams reinforced with deformed GFRPbars with normal- and high-strength concrete would contribute to the existing literature and would provide information critical to the further use of deformed GFRP bars as internal reinforcement for concrete structures. Eight beams with a cross-sectional width and height of 200 mm and 300 mm, respectively, and a clear span of 2700 mm were tested under two-point flexural loading until failure. Four beams were made with 35 MPa normal-strength concrete (NSC); the other four with 65 MPa high-strength concrete (HSC). The bottom/tensile reinforcement of each beam consisted of two GFRP bars. Four different GFRP bar sizes (12 mm, 16 mm, 20 mm, and 25 mm in diameter) were used with reinforcement ratios ranging from 0.38% to 1.63%. Seven beams failed in concrete compression and the eighth beam failed in tension when the FRP ruptured. The test results showed that increasing the FRP reinforcement ratio had a greater effect on the service moment than the resistance moment. The effects of bar spacing on the behavior of wide beams were also investigated, revealing that the service moment increased when the bar spacing decreased, while the resistance moment increased when the concrete strength increased. In addition, the deformability concept produced significantly higher ductility indices than the energy-based concept. Neither method evidenced a clear trend in the ductility indices of the tested beams. A new approach was proposed to determine the ductility index of concrete beams with FRP bars based on beam curvature. This approach demonstrated a clear trend with the tested beams: the high-strength concrete beams had higher ductility indices than the normal-strength concrete beams.

Seismic performance of building reinforced with CFRP bars

Over the past two decades, laboratory tests have suggested that fibre reinforced polymer (FRP) bars can replace steel bars as internal reinforcement in concrete structures, especially those are subjected to aggressive environments. FRP bars has potential for replacing steel bars as these material are corrosion-resistant and have high tensile strength, make it suitable for use as structural reinforcement. Unfortunately, the application of FRP for reinforcing structures in Indonesia is dubious. This paper aims to study the behaviour of reinforced concrete (RC) building reinforced with Carbon FRP (CFRP) and steel bars. The observed building was designed for Pekanbaru, according to SNI 2847:2013 for design of structural RC with steel bars reinforcement and the ACI 440.1R-15 for design and construction of structural RC with FRP bars. Two reinforcedconcrete moment resisting frames, extracted from 6 storey heights building with rectangular floor plan, were designed for the city of Pekanbaru. The building had the typical storey height of 3.6 m except for the first floor where the column length was 4.0 m. Two frames were investigated, including the frame without (Frame A) and with shear wall (Frame B). Pushover analysis was conducted to establish the design force and the design deformation demands. The results show that the amount of bar areas needed for reinforcing the building with CFRP bars was significantly reduced. Moreover, the maximum forced demand attained for the building reinforced with CFRP bars were enhanced by about 1.9 times compared to that of reinforced with steel bars.

Bond behaviour of sand coated GFRP bars to concrete at elevated temperature – Definition of bond vs. slip relations

The use of glass fibre reinforced polymer (GFRP) bars as internal reinforcement of concrete structures has been growing, mainly due to the advantages they present over steel reinforcement, namely their low weight, high strength and corrosion resistance. However, at moderately elevated temperatures, especially when approaching the glass transition temperature (Tg) of the polymeric matrix (usually between 65 and 150 °C), the stiffness, strength and bond properties of these rebars are known to be significantly degraded. The first part of this paper presents an experimental investigation comprising tensile and pull-out tests on sand coated GFRP rebars at elevated temperatures under steady-state conditions; the tensile tests were carried out up to 300 °C, whereas the pull-out tests were performed up to 140 °C (measured at the GFRP-concrete interface); two embedment lengths of the rebars were considered. The obtained results confirmed that the stiffness and strength of the GFRP-concrete interface are significantly reduced with temperature increase, especially when the Tg of the GFRP rebars is approached and exceeded. In the second part of the paper, analytical bond vs. slip relations for the GFRP-concrete interface are proposed for each of the tested temperatures; the defining parameters of these local laws were calibrated with the experimental data from the pull-out tests. Moreover, the accuracy of two empirical (relaxation) models in predicting the GFRP-concrete bond strength reduction with temperature was also assessed.

Effects of Diameter on the Durability of Glass Fiber–Reinforced Polymer Bars Conditioned in Alkaline Solution

AbstractCurrent standards do not consider the diameter of glass fiber-reinforced polymer (GFRP) bars used as internal reinforcement in concrete structures to be a factor influencing bar durability....

Evaluation of Physical and Durability Characteristics of New Headed Glass Fiber–Reinforced Polymer Bars for Concrete Structures

This paper presents the results of a collaborative research project between Quebec’s Ministry of Transportation and Ontario’s Ministry of Transportation, which aimed at characterizing a new type of headed glass fiber–reinforced polymer (GFRP) reinforcing bar and evaluating its suitability as internal reinforcement for concrete structures. To achieve these objectives, the project was implemented in three stages: (1) evaluation of the physical and mechanical properties; (2) determination of the pullout behavior in concrete; and (3) characterization of the long-term durability of the headed GFRP bars. A total of 57 specimens embedded in a 200-mm concrete cube were tested with the direct pullout test to investigate the effect of confinement, bar size, concrete compressive strength, and exposure conditions on the pullout behavior of the headed GFRP bars. Simultaneously, microstructural analyses and measurements of the physicochemical and mechanical properties were carried out on conditioned and unconditioned headed GFRP bars. The results show that the materials, geometry, and interface configuration of the head provided very good mechanical interlocking to the GFRP bars. Up to 63 and 53% of the guaranteed tensile strength of the straight GFRP bars were achieved for 15.9- and 19-mm diameter bars with headed ends, respectively. Scanning electron microscopy and differential scanning calorimetry showed no material changes in the head and bars after exposure to alkaline solution and freeze–thaw cycling. Exposure to the alkaline solution under sustained loading had the most detrimental effect, with the bar retaining 79.4% of its pullout strength. The results indicate that the tested headed GFRP bar has suitable mechanical and durability properties for use as reinforcement in concrete bridge components.

High temperature effects on concrete members reinforced with GFRP rebars

GFRP rebars are often used for the internal reinforcement of concrete structures, such as bridge deck slabs, to improve the corrosion resistance. Several studies were conducted to evaluate the static and fatigue behaviour of these elements but the fire resistance still needs further investigation. This paper presents an experimental investigation aimed at understanding the static behaviour of concrete beams reinforced with GFRP rebars exposed to localized elevated temperatures. Two parameters were varied: the maximum temperature imposed on the bottom side of the specimens (230°C and 550°C) and the lapping scheme of the rebars, including rebars with hooks and laps of different lengths. The mechanical response was investigated by quasi-static three-points bending tests at room temperature and after heating. The results show that the geometry of the reinforcement has a more relevant influence on the ultimate load than on the initial stiffness of the specimens. The localized heating temperature generates damage in concrete and partial evaporation of the matrix in the GFRP rebars without causing the collapse of the element. The reduction of the load carrying capacity mainly depends on the reinforcement geometry in the overlapping areas.

Experimental study on bond performance of GFRP bars in self-compacting steel fiber reinforced concrete

Reinforcing bars made of Glass-Fiber-Reinforced Polymers (GFRPs) are more and more common as internal reinforcement of concrete structures and infrastructures. Since the design of GFRP reinforced concrete members is often controlled by serviceability limit state criteria (i.e. deflection or crack width control), an accurate knowledge of the GFRP-concrete bond behavior is needed to formulate sound design equations. Furthermore, bond laws currently available and widely accepted for conventional steel rebars cannot be straightforwardly applied for GFRP ones. Hence, an experimental program consisting of 36 pullout bending tests was carried out to evaluate the bond performance between GFRP bars and steel fiber reinforced self-compacting concrete (SFRSCC) by analyzing the influence of the following parameters: GFRP bar diameter, surface characteristics of the GFRP bars, bond length, and SFRSCC cover thickness. Based on the results obtained in this study, pullout failure was occurred for almost all the specimens. SFRSCC cover thickness and bond length plaid important role on the ultimate value of bond stress of GFRP bars. Moreover, the GFRP bars with ribbed and sand-coated surface treatment showed different interfacial bond behaviors.

Innovative Design of GFRP Bars for Concrete Structures

This part analyzed GFRP rebar’s research situation, summarized its behaviuor as internal reinforcement for concrete structures. The research has been developed over recent years, however the rules and standard codes for design RC concrete structures with these rebars not consider that the internal reinforcement can work in compression. This paper presents the development of the research to get a new kind GFRP rebar for work as internal reinforcement of concrete structures, with an innovative design for work both in tension and compression, and their mechanical properties: strength, bond,…

Innovative Design of GFRP Bars for Concrete Structures

This part analyzed GFRP rebar’s research situation, summarized its behaviuor as internal reinforcement for concrete structures. The research has been developed over recent years, however the rules and standard codes for design RC concrete structures with these rebars not consider that the internal reinforcement can work in compression. This paper presents the development of the research to get a new kind GFRP rebar for work as internal reinforcement of concrete structures, with an innovative design for work both in tension and compression, and their mechanical properties: strength, bond,…

Behavior of GFRP Reinforcing Bars Subjected to Extreme Temperatures

Corrosion of steel reinforced concrete members has stimulated the research on fiber-reinforced polymers (FRP) to be used as an internal reinforcement for concrete structures. The behavior of glass fiber-reinforced polymer (GFRP) reinforcing bars subjected to extreme temperatures is very critical for applications in North America, especially in Canada. There is a high demand for experimental studies to investigate the thermal stability of strength, along with the ultimate elongation, and modulus of GFRP bars. This paper evaluates the variation of mechanical properties of sand-coated GFRP reinforcing bars subjected to low temperatures (ranging from 0 to −100°C ) and elevated temperatures (ranging from 23 to 315°C ). Tensile, shear and flexural properties are investigated to get an overview of the thermal stability of mechanical properties of GFRP bars subjected to large variations of temperatures. Microstructural analyzes using scanning electronic microscopy (SEM), physical measurements by thermogravimetric...

An experimental study of the mechanical properties of fibre reinforced polymer (FRP) and steel reinforcing bars at elevated temperatures

This paper presents the results of an experimental study of the mechanical properties of FRP reinforcement bars, used as internal reinforcement in concrete structures, at elevated temperatures. Two types of FRP bars namely: carbon fibre reinforced polyester (CFRP) bars of 9.5 mm diameter and glass fibre reinforced polyester (GFRP) bars of 9.5 mm and 12.7 mm diameter were used in the study. For comparison, conventional steel reinforcement bars of 10 mm and 15 mm diameter were also tested. Results from the experimental study show that the stress–strain relationships of FRP bars remained almost linear at elevated temperatures until failure. However, there was a gradual reduction in the failure strength of FRP bars at elevated temperatures, at an almost linear rate to zero at about 500 °C. Their elastic modulus remained almost unchanged until 300–400 °C. After this temperature, there was a sharp drop in the elastic modulus. These properties can be used as input in computer programs for modelling the fire behaviour of concrete structural members reinforced with FRP bars.

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.