Journal of Composites for Construction

The effect of varying transverse posttensioning levels and arrangements on the load response of a one-half scale 30° skewed seven box beam bridge model was investigated. The effective span of the bridge model was 9.45 m (31 ft) with a width of 3.35 m (11 ft) and depth of 355.6 mm (14 in.). The bridge model was prestressed and reinforced with carbon fiber composite cables (CFCCs). CFCCs were also used as shear reinforcement. The bridge model was provided with five transverse diaphragms equally spaced along the length of the bridge. The experimental investigation included load and strain distribution tests and a flexural ultimate load test. The load and strain distribution tests were conducted on the bridge model with and without full-depth longitudinal cracks at the shear-key locations. The investigation showed that the application of an adequate transverse posttensioning force was successful in restoring the load distribution of the bridge model with full-depth longitudinal deck cracks to that of the case without deck cracks. The ultimate load and the associated compression-controlled failure mode of the bridge model agreed well with that predicted according to ACI 440.4R-04 and numerical analysis. The behavior of the bonded pretensioned and reinforced CFCC strands was linear elastic and remained intact throughout the collapse of the bridge model. The unbonded transverse posttensioned CFCC strand also remained intact.

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 $200\times 300\mathrm{mm}$ 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.

Flexural behavior and serviceability performance of 24 full-scale concrete beams reinforced with carbon-, glass-, and aramid-fiber-reinforced-polymer (FRP) bars are investigated. The beams were 3,300 mm long with a rectangular cross section of 200 mm in width and 300 mm in depth. Sixteen beams were reinforced with carbon-FRP bars, four beams were reinforced with glass-FRP bars, two beams were reinforced with aramid-FRP bars, and two were reinforced with steel, serving as control specimens. Two types of FRP bars with different surface textures were considered: sand-coated bars and ribbed-deformed bars. The beams were tested to failure in four-point bending over a clear span of 2,750 mm. The test results are reported in terms of deflection, crack-width, strains in concrete and reinforcement, flexural capacity, and mode of failure. The experimental results were compared to the available design codes.

Steel-fiber-reinforced polymer (FRP) composite bars (SFCBs) are a novel reinforcement for concrete structures. Because of the FRP’s linear elastic characteristic and high ultimate strength, they can achieve a stable postyield stiffness even after the inner steel bar has yielded, which subsequently enables a performance-based seismic design to easily be implemented. In this study, lateral cyclic loading tests of concrete columns reinforced either by SFCBs or by ordinary steel bars were conducted with axial compression ratios of 0.12. The main variable parameters were the FRP type (basalt or carbon FRP) and the steel/FRP ratio of the SFCBs. The test results showed the following: (1) compared with ordinary RC columns, SFCB-reinforced concrete columns had a stable postyield stiffness after the SFCB’s inner steel bar yielded; (2) because of the postyield stiffness of the SFCB, the SFCB-reinforced concrete columns exhibited less column-base curvature demand than ordinary RC columns for a given column cap lateral deformation. Thus, reduced unloading residual deformation (i.e., higher postearthquake reparability) of SFCB columns could be achieved; (3) the outer FRP type of SFCB had a direct influence on the performance of SFCB-reinforced concrete columns, and concrete columns reinforced with steel-basalt FRP (BFRP) composite bars exhibited better ductility (i.e., a longer effective length of postyield stiffness) and a smaller unloading residual deformation under the same unloading displacement when compared with steel-carbon FRP (CFRP) composite bar columns; (4) the degradation of the unloading stiffness by an ordinary RC column based on the Takeda (TK) model was only suitable at a certain lateral displacement. In evaluating the reparability of important structures at the small plastic deformation stage, the TK model estimated a much smaller residual displacement, which is unsafe for important structures.

In this paper, the effectiveness of fiber-reinforced cementitious matrix (FRCM) materials for the strengthening of reinforced concrete (RC) beams is experimentally investigated. Bending tests on RC beams strengthened with different FRCM materials, made out of (1) carbon fiber nets; and (2) poliparafenilenbenzobisoxazole (PBO) fiber nets embedded in cement-based matrix, are performed. For case (2), different net shapes, cementitious matrices, and a number of net layers were considered. Depending on the type of fibers and matrix, different flexural debonding failure modes are identified. The fiber strain at debonding is evaluated by comparing the experimental results with those obtained with two different theoretical models. The results obtained in this study confirm the effectiveness of FRCM materials for the strengthening of RC structures and encourage further experimental and theoretical work on the topic. A better understanding of the debonding phenomenon is crucial for an optimal design of the strengthening material. The way in which the nature of fibers and matrices and the number of layers control the performance of the strengthened members is also investigated in the present paper.

A design approach, developed specifically for seismic bond strengthening of the critical splice region of reinforced concrete columns or bridge piers, is presented and discussed. The approach is based on providing adequate concrete confinement within the splice zone for allowing the spliced bars to theoretically develop enough postelastic tension strains demanded by large earthquakes before experiencing splitting bond failure. The accuracy of the approach was validated experimentally by evaluating the seismic behavior of full-scale gravity load-designed (as-built) rectangular columns that were strengthened or repaired in accordance with the proposed approach. Three types of confinement were used and compared, namely, internal steel ties, external fiber polymer reinforced jackets, and a combination of both. The repaired/strengthened columns developed sizable postyield strains of the spliced bars, considerable increases in the lateral load and drift capacities, and much less concrete damage within the splice zone when compared with the as-built columns. As a further support of the adequacy of the design strengthening approach, the backbone lateral load-drift response of the strengthened columns showed a good agreement with the envelope response generated using nonlinear flexural analysis assuming perfect bond between the column reinforcement and concrete.

The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP ($V_f$). In all, 14 tests were performed on 4,520-mm-long $T$-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1) the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2) the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3) the effect of FRP strip location with respect to internal transverse-steel location; (4) the effect of FRP strip width; and (5) the effect of internal transverse-steel reinforcement on the CFRP shear contribution.

Fiber-reinforced polymer (FRP) retrofit systems for concrete structural members such as beams, columns, slabs, and bridge decks have become increasingly popular as a result of extensive studies on short-term debonding behavior. Nevertheless, long-term performance and durability issues regarding debonding behavior in such strengthening systems still remain largely uncertain and unanswered. Because of its composite nature, the effectiveness of the strengthening system depends on the properties of the interfaces between the three constituent materials; namely, concrete, epoxy, and FRP. Certain factors, including those related to environmental exposures, can cause degradation of the interface properties during service life. This is particularly critical when predicting service life and planning maintenance of FRP-strengthened concrete structures. In this study, effect of moisture on an FRP-concrete bond system is characterized by means of the tri-layer fracture toughness, which can be obtained experimentally from peel and shear fracture tests. Fracture specimens were conditioned under various durations and numbers of wet-dry cycles at room temperature and 50°C. An irreversible weakening in bond strength was observed in fracture specimens under moisture cyclic condition. A conceptual model is developed based on the experimental results of the fracture specimens under variable cyclic moisture conditions for the bond strength prediction of the FRP-concrete bond system. A numerical study of a precracked FRP-strengthened reinforced concrete beam is then performed to show potential application of the proposed predictive model.

This paper reports the results of an experimental program to investigate the bonding behavior of two different types of fiber-reinforced polymer (FRP) systems for strengthening RC members: externally bonded carbon (EBR) plates and bars or strips externally applied with the near-surface-mounted (NSM) technique. The overall experimental program consisted of 18 bond tests on concrete specimens strengthened with EBR carbon plates and 24 bond tests on concrete specimens strengthened with NSM systems (carbon, basalt, and glass bars, and carbon strips). Single shear tests (SST) were carried out on concrete prisms with low compressive strengths to investigate the bonding behavior of existing RC structures strengthened with different types of FRP systems. The performance of each reinforcement system is presented, discussed, and compared in terms of failure mode, debonding load, load-slip relationship, and strain distribution. The findings indicate that the NSM technique could represent a sound alternative to EBR systems because it allows debonding to be delayed, and hence FRP tensile strength to be better exploited.

Sixteen shear capacity tests were performed on eight decommissioned AASHTO prestressed concrete girders that had been in service for over 42 years. These bridge members presented a unique opportunity to investigate carbon fiber-reinforced polymer (CFRP) retrofit schemes to enhance the shear capacity of underreinforced girders that were nonrectangular. Four destructive tests were performed to quantify the in-service strength of the girders and the remaining 12 tests were performed on CFRP retrofitted girders. In all, five configurations of the CFRP reinforcement were evaluated. Two anchoring techniques were investigated that either involved epoxying a horizontal CFRP strip over the vertical strips or a new methodology of epoxying a CFRP laminate into a groove over the vertical strips that was cut at the web-to-flange interface. Two methodologies that predicted the shear contribution of the carbon fiber reinforcement were compared with the test results. A carbon fiber-reinforcing scheme of vertical strips and horizontal anchorage strip was found to be the most effective in resisting the applied shear.

Although previous research has demonstrated the improvement in performance of reinforced concrete structures enhanced with externally applied carbon fiber reinforced polymers (CFRP), the effect of transverse impact damage on the strength of the CFRP enhancements is unknown, and no guidelines have been provided that describe which impact events warrant CFRP repair or replacement. The impact events, such as dropped tools, collisions, and low-speed projectiles may cause critical damage to the epoxy matrix and fibers that is undetectable through visual inspection. The purpose of this research is to provide insight into the level of transverse impact needed to initiate critical damage in wet layup CFRP enhancements, which will serve as a guideline for inexpensive and immediate damage assessments. To simulate a variety of impact events, impactors (tups) of different sizes and shapes were dropped from several heights. The impacts were performed with a guided drop-weight apparatus, designed to achieve free-fall behavior. The results show that impacts that only cause indention of the FRP surface do not significantly affect the tensile strength, but impacts that cause crushing of the epoxy (seen as whitish areas) can indicate as much as a 63% residual tensile strength. Furthermore, for the test conditions considered, tests showed that impacts with a peak impact pressure greater than 21 MPa (3,000 psi) reduced the tensile strength of the CFRP.

To improve the fiber-reinforced polymer (FRP)/concrete bond capacity, this paper presents a new anchoring approach with the gluing of precast fiber-reinforced cementitious composites (FRCC) plate on top of the FRP sheets. In order to measure the improvement in ultimate load and deformation capacity and to study the failure mechanisms around the anchored area, the direct shear bond test is performed on concrete prisms with bonded FRP. Several sets of tests have been carried out with anchoring plates of different FRCC compositions and lengths. Comparison with the control sample shows that the installation of FRCC plate can significantly increase both the bond and deformation capacities (by up to 100%). On the basis of the shear bond test, two types of FRCC plate materials were found to be particularly effective and were selected for strengthening of beam members to be tested under four-point bending. Comparison with control members (without anchor) and those with conventional U-shaped FRP anchors indicates that both the ultimate load and central deflection can be improved by the new anchoring method.

To design efficient and economical strengthening solutions, the structural performance before and after the intervention must be accurately evaluated. In the case of statically indeterminate structures or when the structure has suffered damage or deterioration, linear elastic analysis methods are not adequate to obtain the residual capacity and the structural effects of the intervention because of the nonlinear response of the structure. In such cases, refined analytical models able to capture the structural nonlinear behavior, the effects of previous damage, and those produced by any intervention are required to design safe and economical strengthening solutions. In this paper, a nonlinear and time-dependent evolutive analysis model, previously developed by the authors, is applied to the prediction of the response of fiber-reinforced polymer (FRP)-strengthened concrete structures in flexure. The model can take into account the structural effects of changes in geometry, structural scheme, material properties, and applied loads that may occur along the structure service life, including those attributable to strengthening. A criterion to predict peeling failure in FRP-strengthened beams on the basis of nonlinear fracture mechanics consisting in evaluating the maximum shear force that can be transmitted to the concrete by the FRP laminate between cracks or at the laminate end is incorporated in the model. Two previous experimental programs have been used to validate the model. First, four RC continuous beams, three of them strengthened with FRP laminates and tested to study the influence of the FRP arrangement, are analyzed. Second, two beams previously precracked owing to service loads and strengthened with FRP are analyzed under increasing load up to failure. In all cases, very good agreement between the theoretical and the experimental results is obtained in terms of deflections, strains, reactions, internal forces, and failure mode, showing the capabilities of the model to evaluate the efficiency of proposed strengthening solutions.

To address concerns regarding the seismic vulnerability of New Zealand unreinforced masonry (URM) buildings, a research program was undertaken to investigate the effectiveness of fiber-reinforced polymer (FRP) systems as a seismic retrofit intervention for in-plane loaded URM walls that are prone to fail in a shear mode during earthquakes. Seventeen URM wallettes were retrofitted with either externally bonded (EB) glass FRP fabrics, EB pultruded carbon FRP (CFRP) plates, or near-surface mounted pultruded CFRP rectangular bars. The wallettes were tested by the application of a diagonal compressive force, and data were recorded for applied force and corresponding wall drift. Results were compared with five nominally identical unretrofitted wallettes. It was determined that the FRP systems substantially increased the shear strength of the wallettes. Significant increases in the pseudoductility and the toughness were also obtained, which are all considered to be important goals of any seismic retrofit intervention.

Past experimental tests on a full-scale masonry wall with an opening evidenced the key role of the spandrel panel in the in-plane nonlinear response of the system. Recent seismic codes do not provide specific criteria to assess and to strengthen existing masonry spandrel panels with inorganic matrix-grid (IMG) composites. Numerical finite-element (FE) analyses are used to deepen the knowledge about the nonlinear response of masonry walls and the role of the IMG strengthening system. The comparison of experimental and numerical results contributes to the development of a simplified analytical model to assess the influence of the external reinforcement system on the in-plane seismic response of masonry wall systems. Some hints about the strengthening design that could change the failure mode from brittle shear to ductile flexure are given. Finally, a further enhancement of the IMG strengthening system is proposed to avoid the undesirable splitting phenomena attributable to compression forces and to exploit the full compressive strength of masonry against bending moments.

Glass-fiber-reinforced polymer (GFRP) cell-core sandwiches are composed of outer GFRP face sheets, a foam core, and a grid of GFRP webs integrated into the core to reinforce the shear load capacity. One of the critical failure modes of cell-core sandwich structures is shear wrinkling, a local buckling failure in the sandwich webs because of shear loading. The shear wrinkling behavior of GFRP laminates with different laminate sequences, stabilized by a polyurethane foam core, was experimentally and numerically investigated. Shear wrinkling was simulated by a biaxial compression–tension setup. The results show that an increasing transverse tension load significantly decreases the wrinkling load. The decreasing effect of tension is explained by the lateral contraction because of Poisson’s effect, which causes an increase in the initial imperfections and subsequent accelerated bending.

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) were initially proposed for bridge substructures in corrosive environments in the early 1990s. Systematic studies have since demonstrated the feasibility and merits of CFFTs with or without internal mild steel reinforcement. However, the experimental database in this field is still quite limited. This paper enhances the test database through a series of monotonic bending tests on one control RC specimen and five CFFT specimens previously subjected to reverse cyclic loading. Although the control RC specimen suffered shear-flexural cracks, specimens with carbon fibers experienced flexural failure by longitudinal splitting of the FRP tube in tension and its crumpling in compression. Specimens with glass or hybrid (glass/carbon) fibers, on the other hand, all failed by local buckling of FRP with either burst crushing or crumpling cracks. The specimen with hybrid fibers had higher normalized initial stiffness primarily because of its higher FRP/concrete stiffness ratio. The tests showed that the ductility of CFFT increases with FRP rupture strain. Further synthesis of flexural strength with FRP and mild steel reinforcement indexes reveals the existence of an optimized overall reinforcement index to achieve a design moment without overconfining concrete. Finally, the study confirms that shear failure is not critical for CFFT specimens at short shear span-to-depth ratios, even with internal mild steel reinforcement, as long as the FRP architecture is designed properly.

Concrete-filled steel tubes are widely used as columns in many structural systems, and a common failure mode of such tubular columns is inelastic outward local buckling near a column end. The use of fiber reinforced polymer (FRP) jackets/wraps for the suppression of such local buckling has recently been proposed and has been proven by limited test results to possess great potential in both retrofit/strengthening and new construction. Against this background, this paper presents the results of an experimental study into the behavior of FRP-confined circular concrete-filled steel tubes (CCFTs) under axial compression, as part of a larger study undertaken at the Hong Kong Polytechnic University aimed at the development of a good understanding of the structural behavior of and reliable theoretical models for CCFTs. The experimental program included three series of tests in which the main parameters examined were the thickness of the steel tube and the stiffness of the FRP wrap. The test results showed that the FRP wrap can substantially delay and even completely suppress the development of local buckling deformation in the steel tube. As a result, both the load-carrying capacity and ductility of concrete-filled thin steel tubes can be substantially enhanced with FRP confinement. The behavior of the concrete was also found to be significantly enhanced by the additional confinement from the FRP wrap.

Glued-laminated wood beams (glulams) have low allowable shear stresses relative to competitive engineering wood products such as parallel strand lumber and laminated veneer lumber. For heavily loaded applications such as garage door headers, the lower shear allowable stresses typically necessitate the use of larger glulam members relative to other engineered lumber. This paper reports on experimental research aimed at increasing the shear capacity of glulams. To increase the shear strength, a series of fiber-reinforced polymeric (FRP) pins are inserted into holes drilled transversely across the plies of the glulam. These pins are epoxy-bonded into place after the glulam has been produced. The test results show that the shear strength scatter in the pinned set of glulams is significantly reduced as compared to the unpinned specimens. Two- and three-parameter Weibull estimates of the service-level allowables increases between 40% and 100% for specimen sets reinforced by the FRP pins. The transverse reinforcing scheme also shows promise for aiding in engagement of composite flexural reinforcement plies into the laminate stack. Previous research has shown that the bonding between wood and FRP plies has potential long-term durability problems. Therefore, the pinning technology reported on in this work shows promise for providing a means for direct engagement (through bearing of the pins) of the FRP plies to the wood plies. This behavior has been demonstrated though the testing of larger-scale specimens with both transverse shear reinforcements and longitudinal FRP plies. The reinforcing strategies show potential structural and economic advantages, especially when the FRP is coupled with low-grade, low-value finger-jointed lumber.

An analytical model is presented to simulate the behavior of prestressed concrete girders strengthened with various carbon fiber-reinforced polymer systems and subjected to static and cyclic loading. The initial concrete strains owing to prestressing and girder self weight load at the moment of the application of the strengthening system and the concrete cyclic creep as a result of the cyclic loading are considered in the model. Experimental results are used to validate the analytical model. Additionally, deflection and concrete strain increases on account of the cyclic loading are compared to the values provided by Comité Euro-International du Béton and Fédération Internationale de la Précontrainte (CEB-FIP) and Assoçião Brasileira de Normas Técnicas (NBR) 6118 codes. Deflection and concrete strain obtained from the analytical model were above those observed in the tested girders, especially after 100,000 cycles, owing to the logarithmic function used to express the fatigue behavior of concrete. In general, deflection provided by CEB-FIP was above experimental and analytical deflection, but otherwise concrete strain values provided by NBR 6118 were close to experimental results.