FRP-reinforced Concrete Research Articles

Experimental study of immediate and time-dependent deflections of GFRP reinforced concrete beams

Due to the mechanical properties of fibre reinforced polymer (FRP) bars, deflections often drive the design of FRP reinforced concrete (RC) flexural members. This has led to an increasing number of studies focused on the analysis of short-term deflections of FRP RC beams. However, investigations and experimental data focused on long-term deflections are scarce. The time-dependent deflection in RC beams is a function of member geometry, material properties and loading characteristics. Maximum service loads, as well as repeated loading, affect the deflections under sustained loads. This paper presents the results and discussion of an experimental programme concerning eight glass FRP RC beams tested at service load, and subsequently subjected to sustained loading for 250days. Two reinforcement ratios and two levels of sustained load were considered. The experimental results revealed an effect of the loading–unloading processes and the reinforcement ratio on short and time-dependent deflections. No significant influence of the sustained load level was observed. Moreover, the theoretical predictions obtained with different models have been compared with the experimental results. The modified time-dependent factor presented in ACI 440.1R-06, together with the modified Bischoff’s equation to compute the immediate deflections due to sustained load, give the best agreement with experimental deflections.

Analysis of cracking behaviour and tension stiffening in FRP reinforced concrete tensile elements

The differences in mechanical and bond properties of FRP bars when compared to those of traditional steel reinforcement for reinforced concrete (RC) structures may affect the cracking and deformability behaviour of FRP RC members. To analyse this cracking behaviour, the reinforcement strain profile along a reinforcing bar has been experimentally recorded by using a specially manufactured reinforced concrete element in tension, in which the FRP reinforcement was internally strain gauged. Furthermore, a general procedure derived from a cracking analysis based on slip and bond stresses has been used to study the deformability of FRP reinforced concrete elements under tension. The tension stiffening effect is included via the experimental nonlinear bond–slip law obtained from pull-out test of an experimental programme previously published. The comparison between numerical predictions of the reinforcement strain profile along the reinforcing bar during a tensile test and experimental data confirms that the bond-based model adequately reproduces the redistribution of stresses after crack formation. To further prove the model reliability, numerical predictions of the load–mean strain relationship are compared to previously reported experimental data on FRP RC tensile elements and predictions furnished by the models usually adopted for the analysis of FRP reinforced concrete structures.

Flexural Behavior of Innovative Hybrid GFRP-Reinforced Concrete Beams

Fiber-reinforced polymer (FRP) bars are emerging as a competitive option for replacing steel bars as reinforcement in various concrete structures exposed to aggressive environments. However, the low elastic modulus and brittleness of FRP bars significantly reduce the stiffness and the ductility of FRP-reinforced concrete (FRP-RC) members. In order to improve the flexural behavior of FRP-RC members and meanwhile ensure their satisfactory corrosion-resistant performance, an innovative FRP-reinforced concrete encased steel composite (FRP-RCS) member, which consists of ductile structural steel shapes in combination with corrosion-resistant FRP-reinforced concrete, was conceived and studied. An experimental investigation on the flexural behavior of the proposed FRP-RCS beams was conducted by testing a total of five large-scale simply supported beam specimens subjected to four-point bending loads. The test specimens included one FRP-RC beam reinforced with GFRP bars only and four FRP-RCS beams reinforced with both GFRP bars and encased structural steel shapes. The main parameters considered in this study were concrete compressive strength and amounts of GFRP reinforcement. The test results indicated that using encased steel shapes provided a significant enhancement in load carrying capacity, stiffness, ductility and energy absorption capacity of test beams. The tested FRP-RC beam suffered a brittle failure caused by sudden fracture of tensile GFRP bars whereas the proposed FRP-RCS beams behaved in a ductile manner due to the beneficial residual strength of encased steel shapes following concrete crushing. In addition, the experimental results also demonstrated that the concrete compressive strength had little effect on load carrying capacity of FRP-RCS beams whereas the load carrying capacity can be enhanced by increasing the reinforcement ratio. Analytical methods were also constructed using OpenSEES2.2.2 to simulate the load-deflection response of tested beams..

Exposure Test of FRP-Reinforced Concrete Structure in Temperate Marine Tide Zone

Through exposure test of FRP-reinforced concrete member in littoral test area of temperate marine zone, the durability and constructional measures of FRP-reinforced concrete structure are investigated. The test results show that FRP shell on the surface of FRP-reinforced concrete member is effective in avoiding wave flush, freezing and thawing damage, preventing chloride ion corrosion and improving durability. On the other hand, reliable measures should be taken to protect concrete structures with no FRP shell on the surface. Based on above, the provided reference for evaluation durability of FRP-reinforced concrete structure, and proper constructional measures for FRP-reinforced concrete structure are presented.

Serviceability Limit State of FRP RC Beams

Owing to the particular mechanical properties of FRPs, the design of Fiber Reinforced Polymer (FRP) Reinforced Concrete (RC) structures is often governed by serviceability requirements, rather than ultimate capacity. The low stiffness of FRPs generally results in large strains being mobilized already at low levels of externally applied load, and in turn can lead to significant crack widths and deflections. This paper reviews and discusses the serviceability limitations inherent in current design codes and guidelines in terms of stress limitation, cracking and deflection control. The predictions obtained in accordance to these code equations, as well as other existing proposals for the design of FRP RC structures, are then compared to the results of an experimental program on 24 GFRP RC beams tested under four-point load.

Numerical modelling of carbon fibre‐reinforced polymer and hybrid reinforced concrete beams in fire

ABSTRACTInvestigation on the fire resistance of fibre‐reinforced polymer (FRP) reinforced concrete (RC) is essential for increased application of FRP bars in the construction industry. Experimental tests for determining the fire resistance of RC elements tend to be expensive and time‐consuming. Although numerical models provide an effective alternative to these tests, their use in case of FRP RC structures is hindered because of the insufficient constitutive laws for FRP bars at elevated temperatures. This paper presents the details of a numerical modelling work that was carried out for simply supported carbon FRP (CFRP) and hybrid (steel‐FRP) bar RC beams at elevated temperatures. Constitutive laws for determining temperature‐dependent strength and stiffness properties of CFRP bars are proposed. Numerical models based on finite element modelling were employed for the rational analysis of beams using the proposed constitutive laws. The behaviour of concrete was simulated by means of a smeared crack model based on the tangent stiffness solution algorithm. The employed numerical models were validated against previous experimental results. The theoretical rebar stresses were calculated in both the FRP and steel bars, and the differences are discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Delamination of FRP-reinforced concrete by means of an extended finite element formulation

The present analysis focusses on the finite element modeling of delamination tests in FRP-reinforced concrete blocks using a continuous–discontinuous XFEM approach, which takes into direct consideration the mechanical properties of concrete, adhesive and FRP. Both FRP and adhesive layers are considered as linear elastic materials, while the constitutive behavior of the concrete is governed by an elasto-damaging constitutive law. As soon as a critical damage threshold is reached in the concrete, additional degrees of freedom, representative of the displacement discontinuity corresponding to the delamination process, are added. The proposed approach only requires the use of material parameters of direct mechanical meaning, which can be obtained by standard testing procedures. Hence the use of one-dimensional bond–stress–slip laws, whose parameters must be identified through a cumbersome procedure, appears to be unnecessary. Validation of the proposed approach is carried out by comparison of the computed results with two sets of experimental results. In both cases, after adoption of a simple damage criterion, structural behavior, slip, strain and stress profiles along the reinforcement are satisfactorily reproduced. Moreover, bond–stress–slip laws, which agree with those assumed in previous works based on the use of interface elements, were obtained retrospectively.

Flexural behavior of GFRP-reinforced concrete encased steel composite beams

To improve the ductility and meanwhile ensure satisfactory corrosion-resistant performance, a new type of FRP-reinforced concrete encased steel (FRP-RCS) composite beams comprised of ductile structural steel shapes in combination with corrosion-resistant FRP-reinforced concrete was proposed and studied. An experimental investigation on flexural behavior of the proposed FRP-RCS beams was conducted by testing a total of seven simply supported beam specimens subjected to four-point bending loads. The test specimens included one FRP-reinforced concrete (FRP-RC) beam reinforced with GFRP bars only and six FRP-RCS beams reinforced with both GFRP bars and encased structural steel shapes. The main parameters considered in this study were concrete compressive strength, amounts of GFRP reinforcement as well as ratio and configuration of encased structural steel shapes. The test results indicate that using encased steel shapes can provide a significant enhancement in load carrying capacity, stiffness, ductility and energy absorption capacity of tested beams. The tested FRP-RC beam suffered a brittle failure caused by the sudden fracture of tensile GFRP bars whereas the proposed FRP-RCS beams behaved in a ductile manner mainly due to the beneficial residual strength of encased steel shapes following concrete crushing. In addition, an analytical method was suggested to predict the load carrying capacity of the proposed FRP-RCS beams.

Experimental Research on Flexural Behavior of FRP-Reinforced Concrete Beams

In order to improve the durability of reinforced concrete member, fiber reinforced polymer (FRP)-reinforced concrete member is introduces. Monotonic loading was applied to a reinforced concrete beam and FRP-reinforced concrete beam, in order to investigate the failure progress and characteristics of FRP-reinforced concrete beam.. The presenting study provide a basis for improving the design method for FRP-reinforced concrete member.

Research on FRP-Reinforced Concrete Exposure Test

The paper makes an analysis of the selected location and feasibility of the exposure test ground in the real marine environment, and the endurance test of the FRP- reinforced concrete built-up beam has been carried out on the exposure test ground. After 6 month and 12 month’s exposure test, the beams' mechanic performance and the chloride ion content in different depths have been determined. The results show that after the exposure test, the beams' ultimate capacity has been greatly decreased to a certain extent, and the FRP board can effectively prevent chloride from penetrating.

Development of damage-controlled latter cast FRP–RC hybrid girders

A type of fiber reinforced polymer (FRP)–reinforced concrete (RC) hybrid structure as one kind of novel construction with integrated structural performances is addressed. The FRP–RC hybrid structure designed under a damage-controlled methodology is composed of external hybrid FRP shells and a RC core. The bonding of interior walls of the hybrid FRP shell and cast RC core is realized through epoxy resins under wet condition. The external hybrid FRP shell consists of carbon FRP with high strength and high modulus as longitudinal reinforcements and glass FRP with high ductility as both longitudinal and shear reinforcements. In addition, the external hybrid FRP shell provides the concrete core and steel rebars with an effective protection against corrosion. A series of 4-point bending experiments are performed to study the structural performance of FRP–RC hybrid girders designed and fabricated under the concept of damage-controlled methodology. Based on the bending theories, the flexural properties of the developed structures are also evaluated. The investigation reveals that the damage of FRP–RC hybrid structure is controllable and that the wet bonding is effective.

A Theoretical Improved Adhesively Bonded Bi-material Beam Model for FRP–RC Hybrid Beams

In this paper we present an improved bi-material beam theory with adhesive interface, which has been applied to the study of the interfacial behavior in a concrete beam reinforced by an externally bonded fibre reinforced polymer (FRP) plate. The work explicitly considers the interfacial slip effect on the structural performance by including the effect of adherend shear deformations. This new method needs only one differential equation to determine both shear and normal interfacial stress whereas the others solutions in the literature need two differential equations. Compared with previously published analytical results, this one improves the accuracy of predicting the interfacial stresses and the solution is in a closed form. This research is helpful in the understanding of the mechanical behavior of the interface and design of FRP–reinforced concrete (RC) hybrid beams.

Design Approach for Calculating Deflection of FRP-Reinforced Concrete

Deflection of reinforced concrete is typically computed with an effective moment of inertia Ie that accounts for nonlinear behavior after the concrete cracks. Existing expressions for Ie tend to overpredict the member stiffness of concrete reinforced with fiber-reinforced polymer (FRP) bars, and an alternative expression is used as the basis for developing a practical design approach to compute deflection. The proposed expression has a rational basis that incorporates basic concepts of tension stiffening to provide a reasonable estimate of deflection for both steel and FRP-reinforced concrete without the need for empirically derived correction factors. Calculation of deflection with the proposed expression for Ie is recommended using the code value for the elastic modulus Ec of concrete because computed values of deflection are relatively insensitive to variations in Ec, and shrinkage restraint is taken into account by using a reduced cracking moment less than the code-based value of the cracking moment M...

Effects of tensile softening on the cracking resistance of FRP reinforced concrete under thermal loads

Fiber reinforced polymer (FRP) bars have been widely used as reinforcement for concrete structures. However, under elevated temperatures, the difference between the transverse coefficients of thermal expansion of FRP rebars and concrete may cause the splitting cracks of the concrete cover. As a result, the bonding of FRP-reinforced concrete may not sustain its function to transfer load between the FRP rebar and the surrounding concrete. The current study investigates the cracking resistance of FRP reinforced concrete against the thermal expansion based on a mechanical model that accounts for the tensile softening behavior of concrete. To evaluate the efficacy of the proposed model, the critical temperature increments at which the splitting failure of the concrete cover occurs and the internal crack radii estimated are compared with the results obtained from the previous studies. Simplified equations for estimating the critical temperature increments and the minimum concrete cover required to prevent concrete splitting failure for a designated temperature increment are also derived for design purpose.

Experimental study of hybrid FRP reinforced concrete beams

Fiber-reinforced polymer (FRP) has become a practical alternative construction material for replacing steel bars as reinforcement in concrete structures. However, the brittleness of FRP greatly reduces the ductility of FRP-reinforced concrete (FRPRC) beams. In order to improve its flexural ductility, and at the same time retain the high strength feature of the FRP bars, it is proposed that steel longitudinal reinforcement should be added to form a hybrid FRPRC beam. Twelve specimens consisting of plain concrete beams, steel-reinforced concrete (SRC) beams, pure FRPRC beams and hybrid FRPRC beams were fabricated and tested. The test results show the hybrid FRPRC beams behave in a more ductile manner when compared with the pure FRPRC beams. Also, it is observed that a higher degree of over-reinforcement in the beam specimen resulted in a more ductile FRPRC beams. Hence, the addition of steel reinforcement can improve the flexural ductility of FRPRC members, and over-reinforcement is a preferred approach in the design of FRPRC members.

FRP-Reinforced Concrete Beams: Unified Approach Based on IC Theory

In general, steel-reinforced concrete involves a ductile steel material and a very strong and ductile bond between the steel reinforcement and concrete, so that debonding rarely governs the design. In contrast, fiber-reinforced polymer (FRP) reinforcement is a brittle material with a weak and brittle bond, making debonding a major issue. Consequently, there has been an extensive amount of research on FRP debonding and in particular intermediate crack (IC) debonding. This paper shows that the very good research by the FRP research community on the mechanics of IC debonding can be applied to a wide range of apparently disparate reinforced concrete behaviors to produce a unified approach. Hence, a single mechanism, or unified approach, based on IC debonding is proposed in this paper for dealing with moment rotation, tension stiffening and deflections, member ductility and moment redistribution, shear capacity, confinement, and fiber concrete for FRP RC beams.

Analytical Behaviors of Steel and CFRP Reinforced Concrete Beams in Fire

Abstract The study of thermal behaviors of concrete structures has been an area of great interest to researchers over a long period of time. This paper presents the details of analytical studies, which were carried out in order to investigate high temperature performance of steel or carbon fiber reinforced polymer (CFRP) bar reinforced concrete (RC) beams. This performance is considered to be highly dependent on elevated temperature properties of employed materials (i.e., concrete, steel, and FRP), concrete cracking model, and temperature related strain components. The effects of these parameters in relation to the experimental data were studied in order to establish those that are critical for the prediction of fire resistance of steel or FRP RC beams. Constitutive models for temperature dependent thermal expansion coefficient of concrete and steel have been proposed and validated against the recorded beam behaviors. The effects of transient creep strain were found insignificant in the studies presented in this paper. Non-orthogonal and orthogonal crack formations were found to be suitable for steel and CFRP reinforced beams, respectively. A highly non-linear rebar stress distribution was observed at the mid-span of the beam. Compressive stresses were produced in the steel bars, owing to their restrained thermal dilatation during heating.

Equivalent Moment of Inertia Based on Integration of Curvature

This paper evaluates the benefits of computing deflection with an equivalent moment of inertia based on integration of curvature to account for changes in member stiffness along the span. Results are evaluated for steel and fiber-reinforced polymer reinforced (FRP-reinforced) concrete flexural members with different loading arrangements and support conditions. Closed-form solutions of integrated expressions for deflection are expressed in terms of an equivalent moment of inertia Ie′ and compared to deflection computed with an effective moment of inertia Ie based on the stiffness at the critical section. Results from this comparison are validated with measured deflections from an experimental database for FRP-reinforced concrete. Current code-related approaches are also compared to the experimental database. It is shown herein that the use of an integration-based expression for the moment of inertia can lead to improved prediction of deflection, though the use of an effective moment of inertia based on member stiffness at the critical section gives a reasonably conservative estimate of deflection in many cases. The benefits of taking account of changes in stiffness along the member span are more evident when low reinforcing ratios are used in combination with FRP reinforcement, and use of the integration-based expression Ie′ may be warranted when deflection control is critical in such cases.

Flexural Behavior of Continuous FRP-Reinforced Concrete Beams

Continuous concrete beams are commonly used elements in structures such as parking garages and overpasses, which might be exposed to extreme weather conditions and the application of deicing salts. The use of the fiber-reinforced polymers (FRP) bars having no expansive corrosion product in these types of structures has become a viable alternative to steel bars to overcome the steel-corrosion problems. However, the ability of FRP materials to redistribute loads and moments in continuous beams is questionable due to the linear-elastic behavior of such materials up to failure. This paper presents the experimental results of four reinforced concrete beams with rectangular cross section of 200×300 mm continuous over two spans of 2,800 mm each. The material and the amount of longitudinal reinforcement were the main investigated parameters in this study. Two beams were reinforced with glass FRP (GFRP) bars in to different configurations while one beam was reinforced with carbon FRP bars. A steel-reinforced conti...

Experimental Behaviour of Carbon FRP Reinforced Concrete Beams at Ambient and Elevated Temperatures

The applications of fibre reinforced polymer (FRP) bars in buildings can provide a potential market for their use. Since buildings may be exposed to elevated temperatures, the behaviour of carbon FRP (CFRP) reinforced concrete (RC) beams was studied at high temperatures and compared with normal temperature behaviour. Steel RC beams were used as control specimens. The failure modes of beams at normal and elevated temperatures were found to be the same. The stiffness of cracked FRP RC was less than steel reinforced concrete at normal temperature whereas FRP reinforced beams were stiffer than steel RC beams at elevated temperatures.