Journal of Composites for Construction

This study investigates the effectiveness of a new anchorage technique for fiber-reinforced polymer (FRP) shear-strengthened reinforced concrete T-beams using carbon fiber-reinforced polymer (CFRP) ropes. The rope is a bundle of flexible CFRP strands held together using a thin tissue net. In this technique, holes are drilled through the web at the web-flange intersection, where the CFRP ropes are inserted and flared onto the two free ends of the U-wrap scheme. This converts the U-wrap scheme to a full-wrap scheme. The experimental program includes three series based on the number of steel stirrups. No steel stirrups are provided in the first series, whereas the spacing between steel stirrups is 260 and 175 mm in the second and the third series, respectively. Each series includes a control specimen, with no external CFRP for shear strengthening, a specimen strengthened with CFRP sheets, a specimen strengthened with CFRP L-strips, and a specimen strengthened with CFRP L-strips and anchored with CFRP ropes. Two strengthening procedures are used in this study to investigate the effect of the FRP-width to concrete-width ratio. In the anchored specimens, the CFRP rope covers half the depth of the L-strips. The results of this study reveal that the beams anchored with CFRP ropes achieve an enhanced contribution to the shear resistance compared to the beams with CFRP sheets and CFRP L-strips with no rope. In addition, the method eliminates CFRP debonding and achieves rupture of steel stirrups.

Bolted connections are often used in pultruded glass fiber reinforced polymer (GFRP) structures. Blind bolts, which require installation from one side only, become convenient when access is limited to tighten the ordinary bolts, especially for tubular GFRP sections. This paper presents the results of a comparative study on pultruded GFRP double-lap joints using ordinary bolts and blind bolts. Static experiments were conducted on pultruded GFRP double-lap joints with two columns of bolts, whereas the number of rows varied by one, two, and three. The static behaviors of ordinary bolted and blind bolted joints were compared in terms of load-displacement response, failure modes, and failure load. Then, constant amplitude fatigue tests were carried out on the double-lap joints with two bolts in a row. The maximum fatigue load varied from 30 to 80% of the static failure load of the corresponding joint. The fatigue life of each specimen was recorded and S-N curve was obtained. It was found that the static failure loads of blind bolted joints were lower by a maximum of 21% than those of ordinary bolted joints, whereas the fatigue performance was not much affected by the type of the bolts.

The main objectives of this study are to investigate the effects of three types of predamage and postdamage on the cracking behaviors of carbon fiber-reinforced polymer (CFRP)-strengthened reinforced concrete RC beams and to develop a rational methodology for predicting the average stabilized crack spacing. The predamage is induced by either sustained loading only or by the combination of sustained loading and corrosion. The predamage involved a sustained loading with an anchor tightening system, an electrochemical process to accelerate the migration of chlorides from an external electrolyte into the tested beams, and a wetting–drying cycle process with a controlled current to accelerate the corrosion of the reinforcing steel bars in the tested beams. The postdamage was induced by wetting–drying cycles. A loading test was conducted to determine the cracking behaviors of stabilized flexural cracks in the CFRP-strengthened beams with or without damage. The crack patterns, crack spacings, and test beam widths were recorded and compared, and the related mechanism was discussed. It was found that after CFRP strengthening, the effect of predamage or postdamage on the crack spacing and width is not as distinct as in the unstrengthened cases. The sustained loading predamaged beam showed insignificant differences in crack spacing and width compared to beams without predamage. Subsequently, a model capable of evaluating the crack behaviors of CFRP-strengthened beams with or without damage was developed. The analytical approach is based on equilibrium and compatibility equations to elucidate the average stresses of concrete and the CFRP laminate of a CFRP-strengthened beam element.

Blasts, whether deliberate or accidental, are a great concern for a society’s critical infrastructure as well as expeditionary military installations. Improvement to existing construction methods that enhance blast resilience can ultimately save lives and property. Concrete-filled FRP tubes (CFFTs) are known to improve a conventional reinforced concrete member’s resistance to traditional loads by strengthening, protecting, and confining the reinforced concrete core. Glass fibre reinforced polymer (GFRP) tubes are readily available in a variety of sizes suitable for use as a stay-in-place structural formwork for midsized reinforced concrete members, which can simplify and expedite construction. These advantages point to CFFTs’ great potential in resisting blast loads. This study aimed to quantify the advantages of encasing a reinforced concrete member with a GFRP tube subjected to close-in blast loading and to investigate the effects of the presence of the tube, the internal steel reinforcement ratio, and the blast scaled distance on CFFTs’ behavior under blast loading. This was accomplished by testing four CFFT and reinforced concrete specimen pairs under blast and monotonic loading. The specimens were tested in pairs to facilitate comparisons. The CFFT specimens performed significantly better than the conventional reinforced specimens, showing greater robustness with decreased localized damage and reduced residual displacements. This indicated the need for developing analysis and design procedures for this system. Therefore, a procedure for developing Pressure-Impulse diagrams for the tested CFFT specimens was outlined and their use for the design of CFFTs under close-in blast loads was explained. A numerical procedure for developing equivalent close-in blast forcing functions was also outlined.

Unreinforced masonry (URM) structures that are in need of repair and rehabilitation constitute a significant portion of building stock worldwide. The successful application of fiber-reinforced polymers (FRP) for repair and retrofitting of reinforced-concrete (RC) structures has opened new avenues for strengthening URM structures with FRP materials. The present study analyzes the behavior of FRP-confined masonry prisms under monotonic axial compression. Masonry comprising of burnt clay bricks and cement—sand mortar (generally adopted in the Indian subcontinent) having $E_b/E_m$ ratio less than one is employed in the study. The parameters considered in the study are, (1) masonry bonding pattern, (2) inclination of loading axis to the bed joint, (3) type of FRP (carbon FRP or glass FRP), and (4) grade of FRP fabric. The performance of FRP-confined masonry prisms is compared with unconfined masonry prisms in terms of compressive strength, modulus of elasticity and stress-strain response. The results showed an enhancement in compressive strength, modulus of elasticity, strain at peak stress, and ultimate strain for FRP-confined masonry prisms. The FRP confinement of masonry resulted in reducing the influence of the inclination of the loading axis to the bed joint on the compressive strength and failure pattern. Various analytical models available in the literature for the prediction of compressive strength of FRP-confined masonry are assessed. New coefficients are generated for the analytical model by appending experimental results of the current study with data available in the literature.

A new and general stress-strain model for concrete confined by steel or fiber reinforced polymer (FRP) is developed in this work. One additional variable and one constant are added to the well-known Popovics model to control the type and the shape of the stress-strain curve. The proposed model has one simple, continuous, and explicit expression and can exhibit either hardening or softening types of responses. This general model provides a unified platform for modeling stress-strain of concrete confined by different materials, such as steel or FRP, and help to overcome inconsistency or complexity. The parameters of the stress-strain model are determined by analytical study and data regression using a large and up-to-date test database. The proposed stress-strain model is validated with experimental results and compared with existing models; it shows good performance and superior flexibility and versatility of the model.

This study examines the effect of aggressive freeze-thaw (FT) cyclic exposure on strength and integrity of bridge decks built using glass fiber-reinforced polymer (GFRP) stay-in-place (SIP) structural forms that completely replace the bottom rebar layers. The concern has been whether entrapped moisture may cause frost-jacking of the form, negatively impacting deck integrity. Eleven scaled deck specimens were built, each using two flat GFRP plates with T-shape ribs running normal to girders. Both plates spanned the gap between girders and were spliced by overlapping, directly under the load. The study simulated the following surface treatments of the form: no treatment at all, adhesive bond to freshly cast concrete, and coarse aggregates bonded to the forms. Also, unbonded and bonded lap splices of the forms were tested. The decks were subjected to three cracking load cycles before being saturated and subjected to up to 300 FT cycles at $+5$ to $-18°\mathrm{C}$ core temperatures. Freezing was in air while thawing was by water. Some specimens were thawed without being submerged and one specimen had perforated forms for drainage. Specimens of various splice and surface treatments survived the 300 FT cycles. Subsequent testing showed no reduction in ultimate load or stiffness, relative to control specimens, despite the 23% reduction in tensile strength and 11% in modulus of GFRP coupons exposed to the 300 FT cycles. This is because failure of the decks was governed by concrete punching shear. Decks with untreated forms and unbonded splices showed 21% lower capacity than treated and bonded ones, even without any FT exposure, but this lower capacity was still 3.9 times higher than the equivalent design truck service load at this scale.

This paper presents an experimental study concerning the bond behavior at high temperatures between concrete and carbon fiber–reinforced polymer (CFRP) strips installed according to the near-surface-mounted (NSM) technique. Double-lap shear tests were performed on concrete blocks strengthened with CFRP strips installed into slits and bonded with either an epoxy adhesive (series EP) or a mixed grout composed of epoxy and cement binders (series MG). Specimens were first heated up to predefined temperatures (from 20 to 150°C, measured in the adhesive) and then were loaded up to failure. The obtained results allow drawing the following conclusions: (1) the axial strains along the bonded length became closer to linear (at high temperature) because of the softening (glass transition) of the bonding materials; (2) the effective bond length increased for elevated temperature; (3) the bond-slip curves exhibited considerable and consistent stiffness and maximum shear stress reduction with temperature; (4) the failure modes in series EP changed from cohesive (in the concrete) at ambient temperature to adhesive (at the CFRP-adhesive interface) at elevated temperatures; and (5) the bond strength was considerably reduced with temperature. The mixed grout, although providing similar bond strength at ambient temperature and in spite of containing cement binders, suffered much higher performance reduction with elevated temperature compared to epoxy. With this latter adhesive, very high residual strength was measured for temperatures well above the $T_g$ of the epoxy. The results of series EP show also that for all temperatures tested the bond strength of CFRP-concrete interfaces installed according to the NSM technique is much higher than that of similar interfaces installed according to the more conventional EBR technique; furthermore, the NSM technique also leads to lower reduction of residual strength with temperature.

Accurate modeling of the complete stress-strain relationship of confined and unconfined concrete is of vital importance in predicting the overall flexural behavior of reinforced concrete structures. The analysis-oriented models, which utilize the dilation characteristics of confined concretes for stress-strain relationship prediction, are well recognized for their versatility in such modeling applications. These models assume that at a given lateral strain, the axial compressive stress and strain of fiber-reinforced polymer (FRP)–confined concrete are the same as those of the same concrete when it is actively confined under a confining pressure equal to that supplied by the FRP jacket. However, this assumption has recently been demonstrated experimentally to be inaccurate for high-strength concrete (HSC). It was shown that at a given axial strain, lateral strains of actively confined and FRP-confined concretes of the same concrete strength correspond when they are subjected to the same lateral confining pressure. However, it was also shown that under the same condition, concrete confined by FRP exhibits a lower strength enhancement compared to that seen in companion actively confined concrete. To develop an accurate model that can describe the experimentally observed behavior, two large test databases were assembled for actively confined and FRP-confined concretes through an extensive review of the literature. Based on the analysis of the databases, a new approach is developed to establish the axial stress difference between the actively confined and FRP-confined concretes. Finally, a unified model is proposed to describe the stress-strain relationships of actively confined and FRP-confined concrete. Comparisons with experimental test results show that the predictions of the proposed model are in good agreement with the test results of both actively confined and FRP-confined concrete, and the model provides improved predictions compared to the existing models.

The poor performance and durability of traditional steel tendon and cement grout have become serious constraints to the development of ground anchor systems, especially when these systems are permanent features in large-scale projects such as ground anchors with capacities of more than 4,000 kN in Aizhai Bridge, China. This paper presents a large-scale ground anchor system with carbon fiber-reinforced polymer (CFRP) tendon and reactive powder concrete (RPC) grout as the bonding media in both above-ground and below-ground parts. Two full-scale ground anchor specimens named GA12-20 and GA12-9 were designed and installed at the construction site, and their behavior was systematically studied through monotonic and cyclic load testing. The two systems displayed acceptable displacements under both monotonic load testing with maximum tension twice the design lock-off load and in cyclic load testing under about 90% of its bearing capacity, with residual displacements all less than 5 mm. Furthermore, the bond stress distributions in the grouted portions of the rods in tendon showed both systems have reliable bonding capacities under tension. This innovative ground anchor system with CFRP tendon and RPC grout showed a good behavior and successfully served as the ground anchor of the Aizhai Bridge.

The traditional Euler’s buckling theory of slender columns indicates that column capacity depends on flexural rigidity (EI), rather than material strength. As such, the availability of ultrahigh modulus carbon fiber-reinforced polymer (CFRP) plates, which could be much stiffer than steel, can offer a unique alternative for strengthening slender steel columns, in lieu of welding or bolting steel plates. In this study, twelve 2.6 m long $\mathrm{S}75\times 8$ steel columns of 197 slenderness ratio that represents the upper limit permitted by code were tested under concentric axial loading using pin-ended conditions. The columns were allowed to buckle around their weak axes. CFRP plates were adhesively bonded to the flanges of the steel I-shape sections in nine of the columns. The main parameters studied were the level of initial out-of-straightness [$\mathrm{length}(\mathrm{L})/\mathrm{8,387}$ to $\mathrm{L}/\mathrm{1,020}$], CFRP modulus (168–430 GPa), CFRP reinforcement ratio (13–34%) and the length of CFRP plate (33–95% of L). The gain in axial strength due to CFRP retrofitting ranged from 11 to 29%, depending on the various parameters. The gain generally increased as CFRP modulus, initial out-of-straightness, or CFRP reinforcement ratio increased. Global buckling consistently governed the maximum load. In the case of the 430 GPa CFRP, buckling was followed by CFRP crushing in compression, then rupture in tension.

The primary objectives of this paper were to experimentally investigate the fatigue endurance of the bond between carbon fiber-reinforced polymer (CFRP) sheets and concrete, and to propose a valid method for determining the safe range of loading when the CFRP-concrete interface is subjected to fatigue loading. The major test variables consisted of loading amplitude, concrete strength, CFRP-to-concrete width ratio, and bond length. The failure characteristics, evolution of the strain distribution along the bond length, CFRP-concrete interface debonding during fatigue tests, and fatigue lives were analyzed. The test results indicate that the fatigue life of the CFRP-concrete interface decreased with an increase in either loading amplitude or CFRP-to-concrete width ratio, but increased with an increase in either concrete strength or bond length. A fatigue life prediction model that considered four variables [(1) CFRP sheet stress amplitude, (2) mean value of cyclic stress in the CFRP sheet, (3) concrete strength, and (4) CFRP-to-concrete width ratio] was developed. The experimental results validate the proposed model and provide important guidance for the fatigue design of CFRP-strengthened flexural concrete members.