Journal of Composites for Construction

The objective of this research was to experimentally investigate the effects of the prestressing reinforcement ratio, the magnitude of the prestressing force, the loading type, and the sectional shape on the flexural behavior of beams internally prestressed with unbonded carbon-fiber-reinforced polymer (CFRP) tendons. Four rectangular and three T-beams were tested. Test results have shown that, even for the unbonded prestressing ratio less than the balanced ratio of the bonded equivalent, unbonding the CFRP tendons could relieve tendon strains along the beam span and delay the rupture of CFRP tendons. Improved ductility was observed for the rectangular beams prestressed with unbonded CFRP tendons and non-prestressed auxiliary bonded steel bars when compared with prestressed beams that have bonded CFRP tendons.

This paper presents an experimental study that investigated the physical, mechanical, and durability characteristics of newly developed vinylester- and polyester-based glass-fiber-reinforced polymer (GFRP) dowels through a collaboration research project with the Ministry of Transportation of Québec (MTQ). Durability performance was first evaluated with a preliminary set of conditionings in different solutions. The long-term performance was then assessed under harsh alkaline exposure simulating the concrete environment. The alkaline exposure was achieved by immersing the dowels in an alkaline solution at elevated temperatures to accelerate the effects. Thereafter, the properties were assessed and compared with the unconditioned reference values. The test parameters were (1) type of resin (vinylester and polyester), (2) diameter of GFRP dowels (25.4–44.8 mm), (3) temperature (23, 50, and 60°C), and (4) conditioning time (30, 60, and 180 days). The test results revealed that the vinylester-based GFRP dowels were stable and did not show significant changes in their properties after being subjected to an alkaline solution for 180 days at 60°C. The long-term predictions revealed that the transverse shear-strength retention of vinylester- and polyester-based GFRP dowels would decrease by 10 and 26%, respectively, after 100 years at 10°C. Based on the findings of this research, a new material specification for GFRP dowels was formulated by the MTQ in order to ensure product performance and longevity.

Deterioration of reinforced concrete structures caused by corrosion of steel reinforcement is currently a major concern affecting the safety and functionality of our infrastructure. Fiber-reinforced polymer (FRP) materials have shown tremendous potential as an alternative reinforcement for reinforced and prestressed concrete structures. FRPs have been used in a wide variety of structural applications, from reinforced concrete deck slabs to posttensioned parking garages. However, one promising application for FRP reinforcing bars which has yet to be investigated thoroughly is their use in reinforced concrete slab bridges. This paper presents the results of an experimental study on 10 full-scale glass FRP (GFRP)-reinforced slab bridge strips posttensioned with 0, 2, or 4 carbon FRP (CFRP) tendons with and without shear reinforcement. The tendons were either fully bonded or fully nonbonded to evaluate the effect of bond on the serviceability and ultimate performance of the slabs, particularly in terms of their deformability. Based on the test results, the addition of prestressed CFRP tendons resulted in significantly improved serviceability and ultimate load-carrying capacity with good deformability.

This paper presents the results of an experimental study for evaluating the use of external fiber-reinforced polymer (FRP) laminates for strengthening unbonded posttensioned concrete members. Twenty-four full-scale simply supported beam and slab specimens reinforced with an internal unbonded tendon system and strengthened using external FRP composites were tested. An additional 12 companion bonded prestressed concrete (PC) and reinforced concrete (RC) specimens were also tested for comparison. The test parameters included area of internal tension reinforcement, area of external FRP reinforcement, span-to-depth ratio of the member (slab, beam), and profile of the unbonded tendons. It was found that the use of FRP laminates increases the load capacity and postcracking stiffness of unbonded members. The increase in load capacity was accompanied by a reduction in the deformation capacity. Failure of the specimens occurred either by concrete crushing or by FRP debonding or FRP fracture. No distinct difference beyond expectation was observed between the flexural responses of FRP-strengthened unbonded PC and those of bonded PC or RC systems. Provided a method is available for calculating the strains or stresses in the unbonded tendons at ultimate flexural strength, the same standard guidelines for designing the FRP system for flexural strengthening of RC and bonded PC members can be applied to unbonded members.

This paper examines a technique for flexurally strengthening reinforced concrete (RC) beams using newly developed steel-wire continuous basalt fiber composite plates (SBFCPs) that comprise steel wires and continuous basalt-fiber-reinforced polymer (BFRP) composites. The test parameters consist of the volumetric ratio of steel wires in the SBFCPs, the composite plate type, and anchoring measures. The test results demonstrate that the SBFCP-strengthened specimens performed better than the unstrengthened specimen with respect to load capacity and member stiffness. Further, the flexural cracks are distributed more closely and uniformly than in the unstrengthened specimen. The SBFCP-strengthened beams have competitive advantages, particularly with respect to their ductility and performance-cost ratio, in comparison with specimens strengthened with carbon-fiber-reinforced polymer (CFRP) materials. A parametric study demonstrates that the volumetric ratio of steel wires in the SBFCPs affects the load capacity and stiffness of specimens strengthened with SBFCPs. The results also indicate that anchorage by steel plates and bolts improves the load capacity and ductility of strengthened specimens.

This paper presents a nonlinear local bond-slip model for fiber reinforced polymer (FRP) laminates externally bonded to concrete at elevated temperature for future use in the theoretical modeling of fire resistance of FRP-strengthened concrete structures. The model is an extension of an existing two-parameter bond-slip model for FRP-to-concrete interfaces at ambient temperature. The two key parameters employed in the proposed bond-slip model, the interfacial fracture energy, $G_f$, and the interfacial brittleness index, $B$, were determined using existing shear test data of FRP-to-concrete bonded joints at elevated temperature. In the interpretation of test data, the influences of temperature-induced thermal stress and temperature-induced bond degradation are properly accounted for. As may be expected, the interfacial fracture energy, $G_f$, is found to be almost constant initially and then starts to decrease when the temperature approaches the glass transition temperature of the bonding adhesive; the interfacial brittleness index, $B$, exhibits a similar trend. The proposed temperature-dependent bond-slip model is shown to closely represent the test data upon which it is based, despite the large scatter of the test data.

This study investigates three methods of strengthening existing reinforced square concrete columns under different loading conditions. Four groups of sixteen reinforced concrete square columns were made from normal-strength concrete. Reinforcement was kept at minimum ratio, simulating columns that need retrofitting. Columns of the first group were reference columns (Group N), while the corners of the second group columns (Group RF) were rounded and wrapped with three layers of carbon-fiber-reinforced polymers (CFRPs). The sides of the columns of the third group (Group CF) were bonded with four pieces of concrete with a segmental circular shape, thus changing the cross section of the column from a square to a circle before each column was wrapped with three layers of CFRP. The columns of the last group (fourth) were modified as the third group to result in a circular cross section, but were confined with steel straps. Results from the study showed that all confinement methods increased the capacity and ductility of columns. In particular, segmental circular concrete covers dramatically reduced the stress concentration at the corners and increased confinement efficacy. The interaction ($P$–$M$) diagrams of experimental results and theoretical analysis all confirmed high performance of groups RF and CF.

The paper presents an investigation into the performance of slender rectangular reinforced concrete columns strengthened with carbon fiber-reinforced polymers (CFRPs) in two manners. The first approach is a well-known form of CFRP sheet jacketing with the effect demonstrated in many studies, and a second one is a relatively new retrofit method of near surface mounted (NSM) CFRP strips. A total of eight full-scale specimens with rectangular cross sections ($210\times 150\mathrm{mm}$) were tested to failure under eccentric compressive loading. The total length of the specimens was 4,100 mm. The results of this study demonstrate a significant difference in slender and short column strengthening in accordance with the predominant stress manner. It was confirmed that the effect of CFRP wraps on the increase in column strength is proportionally greater for short RC columns subjected to predominant compression. The longitudinal fibers in CFRP strips bonded into grooves in concrete cover are more effective in enhancing the flexural load-carrying capacity of slender reinforced concrete columns subjected to eccentric loading. The most effective approach to flexural capacity enhancement was demonstrated by a synergistic effect of NSM CFRP reinforcement ensured by CFRP sheet wrapping.

The authors address the structural behavior of masonry arches strengthened through carbon-fiber-reinforced polymer (CFRP) strips at intrados or extrados surfaces and subjected to horizontal forces. The paper reports and compares results from experimental tests on 1:2-scale arches and from an analytical model that considers no-tensile strength, infinite rigid, and infinite compressive strength material. Under the assumption that the structure fails because of the formation of a four-hinge collapse mechanism, the model allows the calculation of the collapse load and the evaluation of the load-displacement diagram. The model is based on the equilibrium conditions and considers finite displacements. Results provide an evaluation of the dependence of arch structural behavior on the length of CFRP strengthening. In particular, the postpeak behavior is evaluated in the perspective of performance-based seismic design criteria, which regard the ultimate displacement as a key parameter for seismic capacity. The evaluation shows that, for the considered case study, an increase of intrados reinforcement length produces an increase of maximum load but a decrease of the ultimate displacement.

The majority of research studies on the behavior of reinforced concrete members with externally bonded fiber reinforced polymer (FRP) sheets have been focused on beams, columns, and beam-column joints. However, limited experimental studies have been conducted to investigate the performance of structural walls retrofitted by wrapping FRP strips or sheets, especially on structural walls with openings. The validated retrofitting schemes for strengthening damaged walls without openings may not be suitable for walls with openings. Therefore, a series of experimental studies were carried out at Nanyang Technological University, Singapore, to study the effectiveness of the proposed repair and strengthening schemes in recovering the seismic performance of the damaged walls with irregular or regularly distributed openings. The strut-and-tie approach was utilized to design the repair schemes. The repaired walls managed to recover their strength, dissipated energy, and stiffness reasonably, indicating that the strut-and-tie approach can be a good design tool for FRP-strengthening of structural walls with openings. Moreover, the shear and sliding capacities of repaired walls were enhanced by using fiber anchors. The repaired walls failed primarily because of debonding of the fiber reinforced polymer at the base of the walls.

Hybrid fiber-reinforced polymer (FRP) concrete steel, double skin tubular columns (DSTCs) are a new form of hybrid columns. The most common sectional form of hybrid DSTCs consists of a layer of concrete sandwiched between a circular inner steel tube and a circular outer FRP tube whose fiber directions are close to the hoop detection to provide effective confinement to the concrete. Much recent research has been conducted on circular hybrid DSTCs, which has demonstrated that the combination of the three constituent materials leads to several advantages not available with existing forms of columns. In practical applications, for aesthetic and other reasons, square hybrid DSTCs may be needed. This paper thus extends the existing work on circular hybrid DSTCs to square hybrid DSTCs in which the outer FRP tube is square while the inner steel tube is still circular. Results from a series of axial compression tests are presented and interpreted to examine the compressive behavior of square hybrid DSTCs. In these tests, FRP tubes formed through a wet-layup process were used instead of filament-wound FRP tubes because the latter were not readily available to the authors at the time of the study. The test results show that the concrete in these square hybrid DSTCs is effectively confined by the two tubes, and the behavior of the confined concrete is similar to that of concrete in FRP-confined solid columns. A stress-strain model for concrete in square hybrid DSTCs is also proposed and is shown to provide reasonably accurate predictions of the test results.

This paper reports on the development and testing of three new concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) systems. These CFFT systems were designed to enhance the effectiveness of square and rectangular FRP tubes in confining concrete. In the design of the rectangular CFFTs two different enhancement techniques were considered; namely, corner strengthening and provision of an internal FRP panel. The technique used in the development of the square CFFT system involved the incorporation of four internal concrete-filled FRP cylinders as an integral part of the CFFT. The performance of these systems was investigated experimentally through axial compression tests of 10 unique CFFTs. The results of the experimental study indicate that the new CFFT systems presented in this paper offer significantly improved performance relative to conventional CFFTs with similar material and geometric properties. Examination of the test results have led to a number of significant conclusions with respect to the confinement effectiveness of each new CFFT system. These results are presented and a discussion is provided on the parameters that influenced the compressive behavior of these CFFT systems.