Fiber Reinforced Polymer Concrete Research Articles

Bond Stress–Slip Relationship between FRP Sheet and Concrete under Cyclic Load

Fifty-four bond specimens were subjected to cyclic bond tests to obtain the bond stress–slip relationships between the fiber reinforced polymer (FRP) sheets and concrete. The specimens were prepared in accordance with Japan Concrete Institute recommendations (in 1998). The tests were conducted using three experimental variables: (1) type of FRP (aramid, carbon, and polyacetal); (2) sheet layers (single layered and double layered); and (3) loading hysteresis. The bond stress–slip model was developed on a Popovics base envelope and consisted of seven empirical parameters: The maximum bond stress τmax, the corresponding slip smax, the curve characteristic constant a, the unloading stiffness K, the ultimate slip su, the friction stress τfp, and negative friction stress τfn. Numerical analyses using the model compared well overall to the experimental cyclic responses of the specimens.

Monitoring of Bond in FRP Retrofitted Concrete Structures

Debonding failure of the concrete cover in reinforced concrete beams retrofitted with fiber reinforced polymer (FRP) composites is a brittle phenomenon which in most cases occurs in an abrupt manner. A complete understanding of bond requires information about the relationship between the local bond stress and slip. Bond-slip defines the constitutive relationship of the interface, and it provides means for computation of ultimate strength and distribution of the bond stress. For FRP composites interface slip is very small even at the ultimate stage prior to failure. For this reason, the study of local bond in the earlier studies did not include slip. This article presents a fiber optic based method for measurement of local slip at the interface between concrete and FRP as well as for prediction of bond failure in reinforced concrete members. Since bond failure in FRP retrofitted concrete is a brittle phenomenon, development of effective means to predict the failure prior to occurrence plays an important role in structural health monitoring of such structures. Therefore, the scope of the study includes two types of tests, namely pull out tests and beam flexure tests. In general, the measured interface slip between concrete substrate and FRP at ultimate load stage is <60 μm. The fiber optic based system is capable of measuring the interface slip with a resolution of 1 μm. In flexure, the long gauge distributed sensors are able to predict the debonding and peeling of the FRP fabric from the concrete beam through deformation reversals. A numerical model based on finite element analysis of the beams is developed to verify the capabilities of the fiber optic sensor through computation of principal stresses at the interface.

Stress Transfer between FRP Laminates and Concrete through Deteriorated Concrete Surfaces

This study investigated the mechanism of stress transfer between glass and carbon fiber-reinforced polymer (FRP) laminates and concrete beams with deteriorated surfaces. Thirty-six beams were prepared with either a solid concrete cross section, or with a weak concrete layer at the surface, simulating the state of a deteriorated surface. The beams were reinforced with glass or carbon FRP sheets and tested in flexure. Strain development in the laminate and in the concrete layers was recorded and analyzed. The mode of failure changed from shear within the deteriorated layer of concrete to delamination at the interface between the resin and the concrete in the solid high-strength concrete. A significant amount of stress was transferred between the FRP laminates and the concrete surface probably by residual frictional stresses after shear cracks developed in the deteriorated layer, leading to a remarkable load bearing capacity of these beams.

Prediction of Interfacial Bond Failure of FRP–Concrete Surface

The use of fiber-reinforced polymer (FRP) for strengthening concrete structures has grown remarkably during the past few years. In spite of exhibiting superior properties, the safety of usage is questionable as FRP undergoes brittle debonding failure. The aim of this study is to review and compare the existing research on bond failure between FRP and concrete substrates. Among the different failure modes, there has been little research in terms of intermediate crack-induced interfacial debonding and fewer strength models are developed for predicting such failures. Conducting a simple shear test on the FRP bonded to a concrete substrate can simulate this type of failure mode. Twelve specimens were tested to study the influence of concrete strength and the amount of FRP on the ultimate load capacity of a FRP–concrete bond under direct shear. Existing experimental work was collected from the literature and consists of an extensive database of 351 concrete prisms bonded to FRP and tested in direct shear tests...

RETROFIT OF EXISTING BUILDINGS FOR EARTHQUAKE RESISTANCE

Buildings sited on soft soils are sometimes subjected to tremors due to earthquakes occurring some 400 to 700 kilometers away as a result of the amplifying effect of soft soils on low-frequency, long-distance waves. This study focuses on the seismic vulnerability of existing reinforced concrete (RC) buildings in Singapore that are designed primarily for gravity loads, and examines the use of externally bonded glass fiber-reinforced polymer (FRP) systems in retrofitting these buildings to resist lateral forces due to seismic action. Two case studies were considered: (1) a four-story frame building, representing typical low-rise buildings; and (2) a 25-story shear wall-frame building, representing typical high-rise buildings. Pushover tests were carried out correspondingly on 1/2-scale sub-frames and 1/5-scale shear walls. The one-and-a-half bay, two-storey frame specimens represent the critical part of the low-rise building while the I-shaped wall specimens represent the lower critical 2.6 stories of the high-rise building. Test results revealed a strong column–weak beam failure mechanism for both the un-retrofitted and retrofitted frames. The retrofitted frame was 30 percent higher in ultimate strength but 12 percent lesser in ultimate drift ratio compared to the un-retrofitted frame. For the wall specimens, sudden failure of the un-retrofitted shear wall was observed at the base of the side walls due to shear. Failure of the retrofitted wall was however more ductile with FRP debonding, followed by concrete crushing and FRP rupture at the compressive base of the side wall. The ultimate load capacity and lateral displacement of the retrofitted wall increased respectively by 45 and 66 percent.

Simulation of Crack Growth in FRP Reinforced Concrete

Experimental results show that the crack growth of fiber-reinforced polymer (FRP) reinforced concrete flexural elements experience a crack development stage followed by crack stabilization. The crack length and elastic crack mouth opening displacement (CMOD) increase during the crack development stage until reaching the crack stabilization stage. A finite-element representation was proposed to predict the initial CMOD. A debonded length was specified to account for the bond-slip between FRP bar and concrete. It was assumed that there was no tangential displacement between the reinforcement and concrete outside of the debonded length. A fatigue model was created using the Paris equation to simulate the growth of elastic CMOD. The model displayed good agreement with the test results. A size effect was also observed for the exponential parameter in the Paris equation.

Bonding and debonding behavior of FRP sheets under fatigue loading

The purpose of this study is to improve the examining and understanding of the bonding behavior of Fiber Reinforced Polymer (FRP) sheets bonded to concrete blocks and steel plates under fatigue loading. First, a series of experimental investigations is summarized in the paper. The fatigue behavior of bonding surface between FRP sheets and concrete is finally characterized by the conducted P–S–N diagram representing the relationship among the probability of FRP debonding (P), the bond stress amplitudes (S), and the number of cycles (N) at debonding on a semi-logarithmic scale. The different debonding modes for various fracturing surface are also investigated and evaluated.

Experimental Investigation and Fracture Analysis of Debonding between Concrete and FRP Sheets

The last few years have witnessed a wide use of externally bonded fiber reinforced polymer (FRP) sheets for strengthening existing reinforced and prestressed concrete structures. The success of this strengthening method relies on the effectiveness of the load-transfer between the concrete and the FRP. Understanding the stress transfer and the failure of the concrete–FRP interface is essential for assessing the structural performance of strengthened beams and for evaluating the strength gain. This paper describes an experimental investigation of the interfacial bond behavior between concrete and FRP. The strain distributions in concrete and FRP are determined using an optical technique known as digital image correlation. The results confirm that the debonding process can be described in terms of crack propagation through the interface between concrete and FRP. The data obtained from the analysis of digital images was used to determine the interfacial material behavior for the concrete–FRP interface (stress...

Hybrid FRP–concrete–steel tubular columns: Concept and behavior

Hybrid FRP–concrete–steel double-skin tubular columns are a new form of hybrid columns recently proposed by the first author. The column consists of an outer tube made of fiber reinforced polymer (FRP) and an inner tube made of steel, with the space between filled with concrete. In this new hybrid column, the three constituent materials are optimally combined to achieve several advantages not available with existing columns. In this paper, the rationale for the new column form together with its expected advantages is first explained. A series of axial compression tests on stub columns are then presented to demonstrate some of the expected advantages. These test results confirm that the concrete in the new column is very effectively confined by the two tubes and the local buckling of the inner steel tube is either delayed or suppressed by the surrounding concrete, leading to a very ductile response. The application of the proposed hybrid section form in beams is also examined by presenting the results of a series of tests on beams with a hybrid section in which the inner steel tube is shifted towards the tension side. The test results also show that such beams have a very ductile response and that the GFRP tube in such beams enhances the structural behavior by providing both confinement to the concrete and additional shear resistance.

Influence of Thermal Cycles on the Fracture Properties of Fiber Reinforced Polymer Concrete

In this study the fracture behaviour of fiber reinforced polymer concrete, subjected to freeze-thaw and thermal degradation, is discussed in order to compare the behavior of this material with conventional cement concrete. The specimens were subjected to freeze-thaw and higher temperature cycles in a climatic chamber and then tested according to RILEM standards. Evaluation of the stress intensity factor, Kic, and of the critical crack tip opening displacement CTODc, was conducted with the Two Parameter Method (TPM). The fracture energy, Gf, of the specimens was also evaluated. Polymer concrete was reinforced with short glass and carbon fiber with 1% and 2% in mass respectively to determine whether the reinforcement increases the performance of the material. Unlike positive thermal cycles, which affect the material properties, freeze-thaw cycles do not deteriorate the material properties significantly.

Direct Shear Behavior of Carbon Fiber-Reinforced Polymer Laminates and Concrete

Abstract This paper explores the direct shear behavior between carbon fiber-reinforced polymer (CFRP) laminates and concrete. To study the push-off strength and slip relationship of externally bonded strengthening system, a new test setup is proposed herein. In this research, a total of 27 specimens are tested. The test variables include the maximum compressive strength of concrete, from 4000 to 12000 psi. With this test setup, it has been found to be able to investigate the direct shear condition between the CFRP laminates and concrete. Based on the present test results, empirical formulas to account for the push-off strength and slip relationship of various concrete compressive strengths are developed. This relationship enables further understanding of the transfer mechanism between the CFRP laminates and concrete. Also, the effect of salt water on the ultimate direct shear strength of the CFRP strengthening system is discussed.

Sensors to Monitor CFRP/Concrete Bond in Beams Using Electrochemical Impedance Spectroscopy

The use of inexpensive electrochemical impedance spectroscopy based sensor technology for nondestructive evaluation (NDE) of bond degradation between external carbon fiber-reinforced polymer (CFRP) reinforcement and concrete is examined. Copper tape on the surface of the CFRP sheet, stainless steel wire embedded in the concrete, and reinforcing bars were used as the sensing elements. Laboratory experiments were designed to test the capability of the sensors to detect the debonding of the CFRP from the concrete and to study the effect of short-term (humidity and temperature fluctuations) and long-term (freeze-thaw and wet-dry exposure and rebar corrosion) environmental conditions on the measurements. The CFRP sheet was debonded from the concrete, and impedance measurements were taken between various pairs of electrodes at various interfacial crack lengths. The dependence of the impedance spectra, and of the parameters obtained from equivalent circuit analysis, on the interfacial crack length was studied. Capacitance parameters in the equivalent circuit correlated strongly with the interfacial crack length and can be used to assess the global state of the bond between CFRP sheets and concrete. Impedance measurements taken between embedded wire sensors can be used to detect the location of debonded regions.

Transverse Thermal Expansion of FRP Bars Embedded in Concrete

Fiber reinforced polymers (FRPs) have a thermal expansion in the transverse direction much higher than in the longitudinal direction and also higher than the thermal expansion of hardened concrete. The difference between the transverse coefficient of thermal expansion of FRP bars and concrete may cause splitting cracks within the concrete under temperature increase and, ultimately, failure of the concrete cover if the confining action of concrete is insufficient. This paper presents the results of an experimental investigation to analyze the effect of the ratio of concrete cover thickness to FRP bar diameter (c∕ db ) on the strain distributions in concrete and FRP bars, using concrete cylindrical specimens reinforced with a glass FRP bar and subjected to thermal loading from −30 to +80°C . The experimental results show that the transverse coefficient of thermal expansion of the glass FRP bars tested in this study is found to be equal to 33 (× 10−6 mm∕mm∕°C) , on average and the ratio between the transvers...

Accelerated aging tests for evaluations of durability performance of FRP reinforcing bars for concrete structures

This paper presents accelerated aging test results of a durability study on fiber-reinforced polymer (FRP) reinforcing bars for concrete structures. Bare FRP bars and also bars embedded in concrete, primarily for glass composites, were exposed to five different solutions: water, two types of simulated alkaline pore solutions of normal and high performance concrete, saline solution, and combined alkaline solution with chloride ions. The aging was accelerated by using elevated temperatures. Wetting and drying and freezing and thawing cycles were combined with some solutions to simulate the coupling effects as expected in field conditions. The tensile strength and interlaminar shear strength of FRP bars were determined before and after exposure, and were considered to be measures of durability performance of the specimens. In addition, pullout tests were conducted to investigate the effects of accelerated exposure on the durability of bond strength between FRP bars and concrete. The results showed that when exposed to simulated environments significant strength loss resulted from the accelerated exposure of both bare and embedded GFRP bars, including bond strength, especially for solutions at 60 °C. In contrast carbon fiber-reinforced polymer (CFRP) bars displayed excellent durability performance. For GFRP bars, continuous immersion resulted in greater degradation than exposure to wetting and drying cycling. In contrast, freezing and thawing cycling combined with solutions had little degradation effects on the FRP bars.

Mechanical characterization of fiber reinforced Polymer Concrete

A comparative study between epoxy Polymer Concrete plain, reinforced with carbon and glass fibers and commercial concrete mixes was made. The fibers are 6 mm long and the fiber content was 2% and 1%, respectively, in mass. Compressive tests were performed at room temperature and load vs. displacement curves were plotted up to failure. The carbon and glass fibers reinforcement were randomly dispersed into the matrix of polymer concrete. An increase in compressive properties was observed as function of reinforcement. The comparison also showed that Polymer Concrete, plain and reinforced, has a better performance than regular market concrete, suggesting that PC is a reliable alternative for construction industry.

Freeze–thaw and thermal degradation influence on the fracture properties of carbon and glass fiber reinforced polymer concrete

In recent years, it has been found that some concrete structures, even for some high performance concrete and ready mixed concrete under good quality control, start to deteriorate long before reaching their designed service life. Cracks are found after the structure has been completed for a few years, which results in the shortening of service life and lowering in durability. In this paper, the influence on the fracture properties of fiber reinforced polymer concrete, FRPC, subjected to freeze–thaw and thermal degradation, is discussed. The study on the damage influence was conducted using a climate chamber to perform the temperature changes on the single edge notched beams. FRPC specimens were subjected to freeze–thaw and higher temperature cycles and then tested according to RILEM standards. To determine the stress intensity factor, K Ic, critical crack tip opening displacement CTODc, the Two Parameter Method was used and the fracture energy, G f of the specimens was calculated to evaluate the fracture properties of FRPC. Three-point bending tests were carried out on notched beams with a clip gauge attached to measure the crack mouth opening displacement CMOD. Polymer concrete was reinforced with short glass and carbon fiber with 1% and 2% in mass respectively, to determine whether the reinforcement increases the performance of the material. The fracture properties of FRPC are reported in such conditions.

Reevaluation of Deflection Prediction for Concrete Beams Reinforced with Steel and Fiber Reinforced Polymer Bars

This paper provides a critical evaluation of equations commonly used to compute short-term deflection for steel and fiber reinforced polymer (FRP) reinforced concrete beams. Numerous proposals have been made for FRP in particular, and the different approaches are linked together by comparing the tension-stiffening component of each method. Tension stiffening reflects the participation of concrete between cracks in stiffening the member response. The Branson equation used in North America and other parts of the world is based on an empirically derived effective moment of inertia to calculate deflection. The tension-stiffening component with this method is highly dependent on the applied level of loading relative to the cracking load as well as the ratio of uncracked-to-cracked transformed moment of inertia (Ig∕Icr) for the beam section. Tension stiffening is overestimated for the high Ig∕Icr ratios typical with FRP concrete, leading to a much stiffer response and underprediction of member deflection. Deflection of steel reinforced concrete with reinforcing ratios less than 1% is also likely to be underestimated because of higher Ig∕Icr ratios at these lower reinforcement levels, but not to the same extent. In both cases, service loads are less than twice the cracking load where tension stiffening is most significant. Modifications to Branson’s equation for deflection prediction of FRP concrete soften the member response by reducing the tension-stiffening component, mostly by introducing empirical factors that effectively decrease the Ig∕Icr ratio. An alternative expression for calculating beam deflection is developed with a rational approach that incorporates a tension-stiffening model adopted in Europe. The proposed equation gives an effective moment of inertia that is independent of Ig∕Icr and works equally well for either steel or FRP reinforced concrete.

Fracture and flexural characterization of natural fiber-reinforced polymer concrete

Mechanical characterization of epoxy polymer concrete reinforced with natural fibers is investigated in this work to analyze the possibility of substitution by synthetic fibers. These natural fibers studied are coconut, sugar cane bagasse, and banana fibers. All of these fibers come from their specific products after they have been used, i.e. as recycle. As the natural fibers are agriculture waste, manufacturing natural product is, therefore, an economic and interesting option. The main idea is to use the fibers like they come from nature without any kind of preparation. The comparison between epoxy polymer concrete reinforced with natural fibers, unreinforced and reinforced with synthetic fibers is made. A brief description of how the natural fibers are obtained and manufacturing process of polymer concrete is also made.

The effects of atmospheric exposure on the fracture properties of polymer concrete

In this experiment the effect of atmospheric exposure of epoxy and fiber-reinforced epoxy polymer concrete was investigated to evaluate its fracture properties, such as stress intensity factor, K Ic , and fracture energy, G f . The deterioration and structural performance of polymer concrete were investigated in a real situation of exposure during a year period and compared the same formulation in laboratory conditions. The relationship between year period, exposure time and load-bearing capacity of deteriorated polymer concrete is studied and fracture mechanics of the specimens are discussed. From the tests results and discussion it is clear that the material studied, polymer concrete, suffers a high deterioration when subjected to aggressive environments.

Mixed-mode fracture behavior of glass fiber reinforced polymer concrete

Fracture toughness of chopped strand glass fiber reinforced particle-filled polymer composite beams was investigated in Mode I and Mode III loading conditions using three-point bend tests. Effects of crack angles on fracture behavior were also studied. The specimens, which have inclined crack at an angle θ to the axis of the specimens, were used to carry out the tests. The specimens were tested with inclination angles 30°, 45°, 60° and 75°. The results are compared with the values of K IC obtained using conventional ( θ=90° ) specimens. In addition, J integrals were also determined. J IC increases continuously with increasing in crack angle from θ=30° to θ=90°. In contrast, J IIIC decreases with the crack inclination angle θ from 30° to 90°.