Bond Strength of Fiber Reinforced Polymer Rebars in Normal Strength Concrete

The bond behavior of reinforcing bars in concrete is a critical issue in the design of reinforced concrete structures. This study focuses on the bond strength of fiber reinforced polymer (FRP) rebars in normal strength concrete. Four different types of rebars were tested using the pullout method: aramid FRP (AFRP); carbon FRP (CFRP); glass FRP (GFRP), and steel. This involved a total of 151 specimens containing 6, 8, 10, 16, and 19?mm rebars embedded in a 203?mm concrete cube. The test embedment lengths were five, seven, and nine times the rebar diameter (db). For each rebar, the test results include the bond stress–slip response and the mode of failure. The test results showed that the bond strength of an FRP rebar is, on average, 40–100% the bond strength on a steel rebar for pullout failure mode. Based on this research, a proposal for the average bond strength of straight FRP rebars in normal strength concrete is made, which verifies an existing bond strength relationship (GFRP) and extends its application to AFRP and CFRP. It is an expression that is a function of the rebar diameter, and the concrete compressive strength.     

Tensile Capacity of GFRP Postinstalled Adhesive Anchors in Concrete

This paper presents the results of an experimental study conducted on the pullout capacity of glass fiber reinforced polymer (GFRP) postinstalled adhesive anchors embedded in concrete. A total of 90 adhesive anchors were installed using sand-coated GFRP reinforcing bars and tested under monotonic tension loading in accordance with ASTM E-488-96 in 1996. The test parameters were: (1) the GFRP bar diameter (25.4, 15.9, and 6.4?mm); (2) the embedment depth (5, 10, and 15 db where db=bar diameter); (3) the adhesive type (epoxy-based and cement-based adhesives); and (4) installation conditions (wet or partially submerged and dry holes). The tested GFRP adhesive anchors were installed in concrete slabs measuring 3,750?mm long, 1,750?mm wide, and 400?mm deep. The test specimens were kept outdoors for 7?months to be subjected to real environmental conditions including freeze-thaw cycles, wet and dry cycles, and temperature variations. The experimental results indicated the adequate performance of GFRP adhesive anchors installed in wet or partially submerged condition using epoxy-based adhesive. Similar behavior was observed for those installed with cement-based adhesive in dry conditions as well. The capacity of the GFRP bars installed with both adhesive types was achieved at an embedment depth ranging from 10 to 15 db.     

Behavior of Concrete Beams Strengthened in Shear with Carbon-Fiber Sheets

This paper presents the results of a test program for shear strengthening characteristics of continuous unidirectional flexible carbon-fiber polymer sheets bonded to reinforced concrete (RC) beams. A total of eight 150?mm×200?mm×2,600?mm concrete beams were tested. Various sheet configurations and layouts were studied to determine their effects on ultimate shear strength of the beams. From the tests, it was found that the externally adhesive bonded flexible carbon-fiber sheets are effective in strengthening RC beams in shear. Further, it was observed that the strength increases with the number of sheet layers and the depth of sheets across the beam section. Among the various schemes of wrapping studied, vertical U-wrap of sheet provided the most effective strengthening for concrete beam. Beam strengthened using this scheme showed 119% increase in shear capacity as compared to the control beam without any strengthening. Two prediction models available in literature for computing the shear contribution of carbon-fiber tow sheets to the shear capacity of fiber reinforced polymers bonded beams were compared with the experimental results.     

Flexural Strength of RC Beams with GFRP Laminates

The results of an experimental and numerical study of the flexural behavior of reinforced concrete beams strengthened with glass-fiber-reinforced-polymer (GFRP) laminates are presented in this paper. In the experimental program, ten strengthened beams and two unstrengthened beams are tested to failure under monotonic loading. A number of external GFRP laminate layers and bond length of GFRP laminates in shear span are taken as the test variables. Longitudinal GFRP strain development and interfacial shear stress distribution from the tests are examined. The experimental results generally showed that both flexural strength and stiffness of reinforced concrete beams could be increased by such a bonding technique. In the numerical study, an eight-node interface element is developed to simulate the interface behavior between the concrete and GFRP laminates. This element is implemented into the MARC software package for the finite-element analyses of GFRP laminate strengthened reinforced concrete beams. Reasonably good correlations between experimental and numerical results are achieved.     

Composite T-Beams Using Reduced-Scale Rectangular FRP Tubes and Concrete Slabs

A composite system consisting of rectangular glass fiber reinforced polymer (GFRP) tubes connected to concrete slabs, using GFRP dowels has been developed. Seven beam specimens have been tested, including hollow and concrete-filled GFRP tubes with and without concrete slabs. Beam–slab specimens had two different shear span-to-depth ratios and one specimen had carbon–fiber reinforced polymer (CFRP)-laminated tension flange for enhanced flexural performance. Additionally, three double-shear GFRP tube-slab assemblies have been tested to assess the shear behavior of GFRP dowels, in both hollow and concrete-filled tubes. Three compression stubs of concrete-filled tubes were also tested by loading them parallel to the cross-section plane, to study GFRP web buckling behavior. The study showed that GFRP dowels performed well in shear and that composite action is quite feasible. While hollow tubes can act compositely with concrete slabs, more slip between the tube and slab would occur, compared to a concrete-filled tube-slab system. Simplified models are proposed to predict critical web buckling load of fiber reinforced polymer (FRP) tubes. Based on the models, a critical shear span-to-depth ratio of 4 was determined, below which web buckling may occur before flexural failure.     

Bond Performance of GFRP Bars in Tension: Experimental Evaluation and Assessment of ACI 440 Guidelines

The development/splice strength and the pullout local bond stress-slip response of glass fiber-reinforced polymer (GFRP) bars in tension were experimentally investigated using beam specimens and pullout specimens, respectively. Two types of 12-mm (0.47-in.)-diameter GFRP bars were evaluated, namely, thread wrapped and ribbed. The test parameters included the concrete cover, the splice length, and the area of steel confinement for the beam specimens, and the concrete compressive strength for the pullout specimens. Companion steel reinforced beams were also tested for comparison. All beam specimens reinforced with thread-wrapped GFRP bars experienced pullout mode of bond failure, while all specimens reinforced with ribbed GFRP bars or steel bars experienced splitting mode of bond failure. It was found that the bond strength of FRP bars is largely dependent on the surface conditions of the bars. The pullout local bond stress-slip response of ribbed GFRP bars is intrinsically similar to that of steel bars reported in the literature. The bond strength of both types of GFRP bars investigated was about two to three times lower than that of steel bars. Predictions of the development/splice strength of GFRP bars in accordance with the ACI Committee 440 guidelines were unconservative in comparison with the test data. Also, in contradiction with the current ACI 440 report, the use of transverse confining reinforcement increased the bond strength by a sizable 15–30%.     

Tests on Seismically Damaged Reinforced Concrete Walls Repaired and Strengthened Using Fiber-Reinforced Polymers

The behavior of six 1:2.5-scale reinforced concrete cantilever wall specimens having an aspect ratio of 1.5, tested to failure and subsequently repaired and strengthened using fiber-reinforced polymer (FRP) sheets is investigated. Specimens were first repaired by removing heavily cracked concrete, lap splicing the fractured steel bars by welding new short bars, placing new hoops and horizontal web reinforcement, and finally casting nonshrink high-strength repair mortar. The specimens were then strengthened using FRP sheets and strips, with a view to increasing flexural as well as shear strength and ductility. In addition to different arrangements of steel and FRP reinforcement in the walls, a key parameter was the way carbon-FRP strips added for flexural strengthening were anchored; steel plates and steel angles were used to this effect. Steel plates were anchored using U-shaped glass-FRP (GFRP) strips or bonded metal anchors. Test results have shown that by using FRP reinforcement, the flexural and shear strength of the specimens can be increased. From the anchorage systems tested, metal plates combined with FRP strips appear to be quite efficient. The effectiveness of the bonded metal anchors used was generally less than that of the combination of plates and GFRP strips. In all cases, final failure of the FRP anchorage is brittle, but only occurs after the peak strength is attained and typically follows the fracture of steel reinforcement in critical areas, hence the overall behavior of the strengthened walls is moderately ductile.     

Shear Strength of Normal Strength Concrete Beams Reinforced with Deformed GFRP Bars

This paper evaluates the shear strength, Vc, of intermediate length (2.5 < a∕d < 6) simply supported concrete beams subjected to four-point monotonic loading and reinforced with deformed, glass fiber-reinforced polymer (GFRP) reinforcement bars. Six different overreinforced GFRP designs, ρ > ρb, were tested with three replicate beams per design. All samples failed as a result of diagonal-tension shear. Measured shear strengths at failure are compared with theoretical predictions calculated according to traditional steel-reinforced concrete procedures and recently published expressions intended for beams reinforced with GFRP. Recommendations are made regarding the adequacy of shear strength prediction equations for GFRP-reinforced members. The study concludes that shear capacity is significantly overestimated by the “Building Code Requirements for Structural Concrete and Commentary” (ACI 318-99) expression for, Vc, as a result of the large crack widths, small compression block, and reduced dowel action in GFRP-reinforced flexural members. Shear strength was found to be independent of the amount of longitudinal GFRP reinforcement. A simplified empirical equation for predicting the ultimate shear strength of concrete beams reinforced with GFRP is endorsed.     

Accelerated evolution and molecular surface of venom phospholipase A2 enzymes

The objective of this study was to determine the in vitro shear bond strength (in megapascals) and location of bond failure with two light-cured glass ionomer resin systems. One system was a hybrid glass ionomer cement with resin (GC Orthodontics, Aslip, Ill), and the other system a glass-filled resin system (Reliance Orthodontic Products, Inc, Itasca, Ill). These systems, Fuji Ortho LC (GC Orthodontics) and Ultra Band Lok (Reliance), respectively, were compared to a light-cured composite resin. Maxillary premolar brackets (n = 200) were bonded to the facial surface of human premolar teeth. The two glass ionomer resin systems were each evaluated by two protocols, one according to the manufacturers' direction plus a variation of their respective technique. The five distinct groups (n = 40) were stored in 37 degreesC distilled water for 30 days and subjected to thermocycling before shear bond strength testing. The findings indicated that large variations existed between the bond strengths of the materials tested. The laboratory shear bond strength of the glass-filled resin glass ionomer cement (Reliance), whether tested in a dry or moist field, was similar to the composite control with all of the previous materials being significantly (P <.01) higher than both the hybrid glass ionomer cement groups (Fuji Ortho LC). However, the hybrid glass ionomer cement with enamel conditioner demonstrated a clinically acceptable mean megapascal value. The Adhesive Remnant Index values ranged from 0.53 to 1.62. The hybrid glass ionomer cement without enamel conditioning recorded the lowest mean adhesive remnant index score and the lowest mean megapascal score. Although both products are glass ionomer resin systems, their individual chemistries vary; this affects their clinical performance. Clinically, it may be suggested that glass ionomers used in a dry field may be beneficial for orthodontic bonding, and that glass ionomer resin systems used in a moist environment need an enamel conditioner.     

Bond Strength of Tension Lap Splices in High-Strength Concrete Beams Strengthened with Glass Fiber Reinforced Polymer Wraps

This paper presents the experimental results of the first phase of a study undertaken at the American University of Beirut to examine the effectiveness of fiber reinforced polymer (FRP) wraps to confine steel reinforcement in a tension lap splice region anchored in high-strength reinforced-concrete beams. Seven beam specimens were constructed. The specimens were reinforced on the tension side with three deformed bars spliced at midspan. The splice region was devoid of any transverse reinforcement to allow a full examination of the FRP wrap contribution. Glass fiber reinforced polymer (GFRP) sheets were used. The main test variables were the GFRP configuration in the splice region (one strip, two strips, or a continuous strip), and the number of layers of the GFRP wraps placed around the splice region (one layer or two layers). All GFRP wraps were U-shaped. Except for the epoxy adhesive, no other anchorage mechanism or bonding procedure was applied for the GFRP wraps on the concrete beam. Following the application of the GFRP wraps, the beams were tested in positive bending. The test results demonstrated that GFRP wraps were effective in enhancing the bond strength and ductility of failure mode of the tension lap splices, especially when continuous strips were applied over the splice region.     

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 continuous concrete beam was also tested to compare the results. The experimental results showed that moment redistribution in FRP-reinforced continuous concrete beams is possible if the reinforcement configuration is chosen properly. Increasing the GFRP reinforcement at the midspan section compared to middle support section had positive effects on reducing midspan deflections and improving load capacity. The test results were compared to the available design models and FRP codes. It was concluded that the Canadian Standards Association Code (CSA/S806-02) could reasonably predict the failure load of the tested beams; however, it fails to predict the failure location.     

Fatigue Behavior of RC Beams Strengthened with GFRP Sheets

The objective of the presented study is to examine the effects of glass fiber reinforced polymer (GFRP) composite rehabilitation systems on the fatigue performance of reinforced concrete beams. Experiments were conducted on beams with and without GFRP composite sheets on their tensile surfaces. The specimens were 152 × 152 × 1,321 mm reinforced concrete beams with enough transverse reinforcement to avoid shear failure. The results of this study indicate that the fatigue life of reinforced concrete beams with the given geometry, subjected to the same cycling load, can be significantly extended through the use of externally bonded GFRP composite sheets. An interesting finding is that, although the fiber strengthening system increases the fatigue life of the beams, the failure mechanism, fatigue of the steel reinforcement, remains the same in both strengthened and nonstrengthened beams. Thus, it is possible to predict the fatigue life of a cyclically loaded beam using existing fatigue models.     

Time-Dependent Deformation of Pultruded Fiber Reinforced Polymer Composite Columns

This paper describes an experimental investigation into the time-dependent deformation of pultruded glass fiber reinforced polymer (GFRP) composite columns under an axial-compressive loading at the environmental controlling room with a constant temperature and relative humidity. Tests were conducted on two types of cross-sectional columns: closed-cross section such as square tube (box) and opened-cross section such as wide flange. Both types of columns were 1,200 mm in length, and had cross-sectional dimensions of 102 mm×102 mm and with a 6.4 mm thickness. A total of eight GFRP composite columns were tested at four different stress levels; 20, 30, 40, and 50% of the average ultimate compressive strength from the short-term column tests. The experiments were conducted for approximately 2,500 h with an individual hydraulic loading jack system. The test results indicate that Findley’s power law model can be successfully used to predict time-dependent deformation of GFRP composite columns, and the time-dependent compressive elastic modulus would be decreased as much as 30% of initial value over a 50-year period.     

Percutaneous in situ pin fixation in superior femoral epiphysiolysis]

OBJECTIVES: This study determined the effect of alginate adhesive on various bond strengths of alginate to stainless steel. METHODS: Three test assemblies were designed and machined in stainless steel for tension, shear and cleavage tests. Alginate adhesive (Fix) was applied thinly and dried for 5 min. Alginate (Blueprint) was then loaded and allowed to set for 5 min before testing. The force at failure was measured by an Instron machine with a cross-head speed of 50 mm min(-1). RESULTS: Breaking stresses of alginate without adhesive were found to be 65 kPa (tension), 31 kPa (shear) and 10 kPa (cleavage). The bond strengths of Blueprint with Fix were 100 kPa (tension), 42 kPa (shear) and 37 kPa (cleavage) giving improvements of 53%, 37% and 270% respectively (p < 0.05). CONCLUSIONS: Alginate adhesive increases the bond strength of alginate, particularly cleavage, to stainless steel.     

Effect of Transverse Reinforcement on the Flexural Behavior of Continuous Concrete Beams Reinforced with FRP

Continuous concrete beams are structural elements commonly used in structures that might be exposed to extreme weather conditions and the application of deicing salts, such as bridge overpasses and parking garages. In such structures, reinforcing continuous concrete beams with the noncorrodible fiber-reinforced polymer (FRP) bars is beneficial to avoid steel corrosion. However, the linear-elastic behavior of FRP materials makes the ability of continuous beams to redistribute loads and moments questionable. A total of seven full-scale continuous concrete beams were tested to failure. Six beams were reinforced with glass fiber-reinforced polymer (GFRP) longitudinal bars, whereas one was reinforced with steel as control. The specimens have rectangular cross section of 200×300??mm and are continuous over two spans of 2,800?mm each. Both steel and GFRP stirrups were used as transverse reinforcement. The material, spacing, and amount of transverse reinforcement were the primary investigated parameters in this study. In addition, the experimental results were compared with the code equations to calculate the ultimate capacity. The experimental results showed that moment redistribution in FRP-reinforced continuous concrete beams is possible and is improved by increasing the amount of transverse reinforcement. Also, beams reinforced with GFRP stirrups illustrated similar performance compared with their steel-reinforced counterparts.     

Physical Properties of Glass Fiber Reinforced Polymer Rebars in Compression

Forty-five glass fiber reinforced polymer (GFRP) rebars were tested in compression to determine their ultimate strength and Young’s modulus. The rebars (or C-bars), produced by Marshall Industries Composites, Inc., had an outside diameter of 15 mm (#15 rebar), and unbraced lengths varying from 50 to 380 mm. A compression test method was developed to conduct the experiments. Three failure modes, that are directly related to the unbraced length of the rebar, are identified as crushing, buckling, and combined buckling and crushing. The crushing region represents the failure mode a GFRP rebar would experience when confined in concrete under compression. The experimental results showed that the ultimate compressive strength of the #15 GFRP rebar failing by crushing is approximately 50% of the ultimate tensile strength. Based on a very limited number of tests, in which strain readings were acceptable, Young’s modulus in compression was found to be approximately the same as in tension.     

Flexural Load Testing of Concrete-Filled FRP Tubes with Longitudinal Steel and FRP Rebar

The flexural performance of reinforced concrete-filled glass-fiber reinforced polymer (GFRP) tubes (CFFTs) has been investigated using seven specimens, 220?mm in diameter and 2.43?m long. Specimens were reinforced with either steel, GFRP, or carbon–fiber reinforced polymer (FRP) rebar of various sizes. Prefabricated GFRP tubes with most of the fibers oriented in the hoop direction were used in five specimens. One control specimen included conventional steel spirals of stiffness comparable to the GFRP tube and the other had no transverse reinforcement. Test results have shown that CFFT beams performed substantially better than beams with a steel spiral. Unlike CFFTs with FRP rebar, CFFTs with steel rebar failed in a sequential progressive manner, leading to considerable ductility. An analytical model capable of predicting the full response of reinforced CFFT beams, including the sequential progressive failure, has been developed, verified, and used in a parametric study. It is shown that laminate structure of the tube affects the behavior, only after yielding of the steel rebar. Steel reinforcement ratio significantly affects stiffness and strength, whereas concrete strength has an insignificant effect on the overall performance.     

Large Shear Studs for Composite Action in Steel Bridge Girders

Shear studs used in composite steel bridge construction are typically 19.1 mm (? in.) or 22.2 mm (? in.) in diameter. This paper presents the development and implementation of the 31.8 mm (1??in.) stud diameter. Because the 31.8 mm (1??in.) stud has about twice the strength and a higher fatigue capacity than the 22.2 mm (? in.) stud, fewer studs are required along the length of the steel girder. This would increase bridge construction speed and future deck replacement, and reduce the possibility of damage to the studs and girder top flange during deck removal. Studs also can be placed in one row only, over the web centerline, freeing up most of the top flange width and improving safety conditions for field workers. This paper provides information on the development, welding, quality control, and testing of the 31.8 mm (1??in.) stud. Information on the first bridge built in the state of Nebraska with the 31.8 mm (1??in.) studs is provided.     

Analytical Method for Pullout of Anchor from Anchor–Mortar–Concrete Anchorage System due to Shear Failure of Mortar

Depending on the relevant material properties, failure of grouted anchors can take forms of pullout of concrete cones, debonding at either anchor–grout or grout–concrete interface, fracture of anchor and combination of some of these failure modes. Further, if the thickness of the grout layer is thin enough, the shear strength of the grout is relatively low or the anchor is in the form of a steel bar with ribs or spirals, the grout would be sheared off so that the anchor is pulled out. The present study presents an analytical method for the last scenario, i.e., anchor pullout from an anchor–mortar–concrete anchorage due to shear failure of mortar. Two different boundary conditions are considered: fixed bottom surface of concrete as Boundary 1, and top surface of concrete with uniform distributed force as Boundary 2. A shear-lag model was introduced to analyze the behaviors of the mortar and the interfacial properties of both the anchor–mortar and the mortar–concrete interfaces were also considered. Based on the deformation compatibilities of the interfaces and the mortar layer, the distributions of the tensile stresses in the anchor and shear stresses in the mortar along the embedment length were obtained analytically during different loading stages for both Boundaries 1 and 2. Moreover, the probabilities and sequences of shear cracks induced by the mortar failure were determined according to the boundary conditions and the comparison between the shear stresses at the loading and nonloading ends. Double shear crack propagation from both ends with different crack lengths was then investigated. Besides, the pullout load was expressed as a function of the shear crack lengths. Then the maximum load and the corresponding critical crack lengths were obtained by using the theories of extremum. Finally, a series of material, structural, and interfacial parameters were adopted to study their influences on the calculated results using the proposed method, including the critical crack lengths, initial cracking load and maximum pullout load. It was found that the initial cracking and maximum loads in Boundary 1 are larger than those in Boundary 2. However, as the longitudinal rigidity of the concrete increases, the values of the maximum pullout loads in both of the boundary conditions approach each other. It was also found that there exists an effective bonding length, beyond which the critical crack length and maximum pullout load are no longer increased.     

Experimental Analysis of Crack Growth in GFRP Reinforced Concrete

For decades, bridge slabs have been troubled by the corrosion of steel reinforcement. The unique corrosion resistance of glass fiber-reinforced polymer (GFRP) bars makes them a promising alternative to steel bars. Experiments have been conducted to investigate the bond performance of GFRP reinforced concrete under constant amplitude cyclic fatigue loading. Each specimen was an identical length beam with a single GFRP bar at the bottom, intended to simulate a transverse strip of a typical bridge deck slab. The crack growth was monitored for specimens of different widths, simulating different transverse reinforcement spacings. Up to 2?million?cycles of cyclic loads were applied at 100% typical service load levels. No fatigue failure was encountered in the testing. The effects of moderate overloads were also investigated.