Journal of Composites for Construction

This study concerns assessing 20 existing models for predicting the compressive strength of concrete uniformly confined externally with composite materials. An extended database of 471 experiments on cylindrical concrete columns is utilized for the comparisons. The research classifies the experimental data in three distinguished subcategories according to the information available for the mechanical properties of the FRPs to investigate their effect on the divergences of the models. Apart from the use of the tensile strength obtained from coupon tests, the use of properties from the manufacturer data on FRP fibers can lead to minimum error of predicted strength of concrete externally wrapped or encased in FRP tubes. The study results in the proposal of an upgraded empirical model that indirectly encompasses the unique deformational characteristics of different Young’s moduli of FRP sheets and tubes. The model utilizes the strong linear dependence of the product of the varying effective strain at failure of the confining material (${\epsilon }_{je}$) times the varying confinement effectiveness coefficient ($k_1$) on the Young’s modulus of the reinforcing fibers ($E_f$). Thus, there is no need to estimate average empirical constant values for the ${\epsilon }_{je}$ or $k_1$, and a more rational approach comes up that further restricts errors in strength modeling. The proposed model provides a prediction with absolute average error of 8.6% for wrapped columns and 6.3% for FRP-tube encased columns.

During the last two decades, the use of fiber-reinforced polymer (FRP) in the retrofitting of concrete structures has been widely studied. However, there is limited research on the effect of initial concrete damage on the retrofitting effect of FRP on concrete columns, especially when the initial damage is severe. For FRP-confined concrete columns, the fracture process inside the concrete is covered by FRP and is not visible to inspectors during the service period. This paper investigates these problems through an experimental study of 28 $100\times 100\times 300\mathrm{mm}$ concrete columns using axial compression tests and acoustic emission (AE) fracture monitoring. Prior to FRP wrapping, three different damage levels were assigned to the concrete columns. The stress-strain results showed that the predamage levels had a small effect on the ultimate strength of the FRP-confined predamaged concrete columns, even when the predamage was severe, whereas the axial/lateral strains were more affected by the predamage levels. AE parameter analysis showed that at the early loading stage, AE signals generated from FRP-confined predamaged concrete columns contained less energy than those from the FRP-confined intact concrete columns. The fracture processes of unconfined and FRP-confined concrete columns were also studied through concrete failure modes, $b$-value and improved $b$-value ($Ib$-value) analyses, and fractal analysis. The following conclusions were drawn from the analyses: (1) during compression of the FRP-confined concrete columns, the concrete failure modes shifted from cracking type to cracking-crushing mixed type; (2) the fracture process of the confined concrete columns could be divided into three stages, which are different from those for unconfined concrete columns and reinforced concrete beams; (3) the fractal dimension of the crack network had a relationship with the $b$-value and decreased from a value between 3 and 2 to a final value of 2 during the fracture process for unconfined and FRP-confined concrete columns.

The majority of studies on fiber-reinforced-polymer (FRP) strengthened concrete columns deal with columns of a circular cross section. However, most concrete columns in the field have square or rectangular cross sections and resist eccentric loads. This paper presents the results of an experimental study on the performance of carbon-fiber-reinforced-polymer (CFRP) wrapped square reinforced concrete (RC) columns under eccentric loading. The influence of the number of CFRP layers, the magnitude of eccentricity, and the presence of vertical CFRP straps were investigated by testing 16 specimens. The specimens had the dimensions $200\times 200\times 800\mathrm{mm}$ and round corners with a radius of 34 mm. Twelve specimens were tested as columns and four specimens as beams. The results of this study showed that CFRP wrapping enhanced the load-carrying capacity and ductility of the columns under eccentric loading. Furthermore, the application of the vertical CFRP straps significantly improved the performance of the columns with large eccentricity.

Strengthening of RC columns through lateral confinement provided by external fiber-reinforced polymer (FRP) jackets (or wraps) has become an increasingly popular technique over the past decade. Nevertheless, relevant design provisions in existing design guidelines are only concerned with the design of FRP jackets for short columns in which the slenderness effect is negligible. Even for the safe application of the existing design provisions for short columns, there is an urgent need to define a slenderness limit for short FRP-confined columns. This paper proposes such a slenderness limit expression based on the numerical results of a comprehensive parametric study that investigates the effects of various parameters on the slenderness limit using a recently developed theoretical column model. An important feature of the proposed expression is that it separates the effect of FRP confinement on the slenderness limit from the effect of other parameters. This feature allows existing slenderness limit expressions for short RC columns of different forms to be readily upgraded for use in the design of FRP-confined RC columns by incorporating the part dealing with the effect of FRP confinement in the proposed expression.

Concrete-filled steel tube (CFT) structures are widely used in various types of buildings and bridges. Confined concrete tube (CCFT) is a new type of structural column originally proposed in previous work. This paper reports the impact testing results of the CFT and carbon fiber reinforced polymer (CFRP) confined CFT stub columns under different impacting energy levels, conducted using a large-capacity drop-hammer machine. The time history curves of impacting force and deformation time history curves as well as failure patterns are investigated. The results indicate that the failure patterns are related to the impact energy. Increasing the thickness of the steel tube and providing additional transverse confinement by CFRP can enhance the impact-resistant behavior. Finally, the dynamic analysis software ANSYS/LS-DYNA is used to simulate the impact behaviors of the CFT and CCFT specimens, and the simulation results are reasonable compared with the test results.

Service temperature variations (thermal loadings) may significantly affect the behavior of the bond between externally bonded fiber reinforced polymer (FRP) and concrete. This paper presents an analytical solution for the full-range deformation process of FRP-to-concrete bonded joints under combined thermal and mechanical loadings. The solution is based on a bilinear bond-slip model and leads to closed-form expressions. The validity of the solution is demonstrated through comparisons with both experimental results and finite-element predictions. Numerical results from the solution are presented to illustrate the effect of thermal loading on the interfacial shear stress and slip distributions in addition to the global load-displacement response. Provided the material properties are not affected by temperature variations, a temperature rise is shown to increase the ultimate load, whereas a temperature reduction decreases the ultimate load; the latter can have serious implications for the safety of the strengthened structure. Although the solution is developed with particular reference to FRP-to-concrete bonded joints, it is also applicable to similar bonded joints made of other materials (e.g., FRP-to-steel bonded joints). A useful function of the closed-form solution lies in the interpretation of pull test results: the solution allows the effect of thermal stresses to be isolated from the effect of property changes of the bondline in obtaining bond-slip responses from pull tests.

Bond tests were conducted on 10 concrete beams strengthened with near-surface mounted (NSM) prestressed carbon fiber reinforced polymer (CFRP) rods under different fatigue load levels. In the NSM technique, grooves are cut on the tension side of the beams. The CFRP rods are then placed inside the grooves and prestressed. Then an epoxy adhesive is placed inside the groove to provide a bond between the concrete and the CFRP rod. The test variables included the type of CFRP rod (spirally wound or sand-coated) and the fatigue load level. The beams were tested in four-point bending. Unlike the bond failures for beams strengthened with nonprestressed CFRP rods, bond failure for beams strengthened with prestressed CFRP rods and tested under fatigue loading was by slipping between the CFRP rod and the epoxy that started at the support and propagated inward toward the loading point. The sand-coated rods showed a better bond fatigue performance than the spirally wound rods, where at a given load level the beams strengthened with sand-coated rods had longer fatigue lives than the beams strengthened with spirally wound rods. Also, for a given number of cycles, the beams strengthened with prestressed CFRP rods failed in bond at a lower applied load range than the beams strengthened with a nonprestressed CFRP rod. At onset of excessive slip (failure), the force distribution in the CFRP rod in the end region was the same for a given rod type. Thus, the shear stress value and distribution in the region close to the support was the same at onset of excessive slip (failure) for a given rod type regardless of the applied load level.

The effectiveness of using an externally bonded carbon-fiber-reinforced polymer (EB-CFRP) system with mechanical end anchorage to retrofit severely shear-damaged reinforced concrete (RC) beams is examined in this paper. A total of 14 tests were performed on eight RC beams with a $T$-shaped section and low compressive strength. To represent a severe shear-damage condition, five beams were tested to failure, retrofitted, then retested to failure for a second time. Test parameters included the number of EB-CFRP layers and type of end anchorage system. The results demonstrated that retrofitting of severely shear-damaged RC $T$-beams with EB-CFRP composites and proper mechanical end anchorage can fully restore the original shear capacity of the beams. The use of a sandwich composite panel in combination with a threaded anchor rod inserted through the entire web width (thru-bolt) as an end anchorage system was more effective than using the panel with side powder-actuated fasteners. Increasing the number of EB-CFRP layers did not result in additional gain in shear capacity. The accuracy and validity of four different international guidelines/standards were examined by comparing their predictions with the experimental results.

The use of fiber-reinforced polymer (FRP) composites for strengthening, repairing, or rehabilitating concrete structures has become more and more popular in the last 10 years. Irrespective of the type of strengthening used, design is conditioned, among others, by concrete-composite bond failure, normally attributed to stress at the interface between these two materials. Single shear, double shear, and notched beam tests are the bond tests most commonly used by the scientific community to estimate bond strength, effective length, and the bond stress-slip relationship. The present paper discusses the effect of concrete strength and adhesive thickness on the results of beam tests, which reproduce debonding conditions around bending cracks much more accurately. The bond stress-slip relationship was analyzed in a cross section near the inner edge, where stress was observed to concentrate. The ultimate load and the bond stress-slip relationship were visibly affected by concrete strength. Adhesive thickness, in turn, was found to have no significant impact on low-strength concrete but a somewhat greater effect on higher strength materials.

Unreinforced masonry (URM) structures have shown their vulnerability to major events like as earthquakes, severe wind, blast, and impact. The present work started from experimental programs available in scientific literature related to masonry walls made of clay or natural stone units. The finite-element method (FEM) was used to describe the global behavior of tested specimens in terms of shear/displacement curves, shear capacity, and cracking pattern. FEM, and particularly detailed micromodeling, was adopted as a numerical simulation tool for masonry walls. International design codes underline that some fiber reinforced polymers (FRP) dimensions (e.g., width, thickness, and spacing of FRP strips applied as external strengthening of URM walls made of different brickworks) may influence the global behavior of strengthened masonry. The present work, starting from experimental programs by other research groups, related to walls made of solid and hollow clay units as well as natural tuff units subjected to compression/shear loading aims at identifying the actual influence of those dimensions. Diagonal and horizontal strips were investigated. Different brickwork panels having the same FRP strengthening (quantity and geometry) showed different behaviors. Present outcomes highlighted that the influence of some FRP strengthening parameters (e.g., strip spacing) is not so meaningful if compared to FRP total amount even if the code formulations predict significant differences.

An experimental study on the flexural and shear behavior of the full-scale glue-laminated composite sandwich beams in three different layouts was conducted to evaluate the suitability of this construction system for railway turnout sleepers. The building block of this innovative beam is a novel composite sandwich structure made up of glass fiber composite skins and modified phenolic core material that has been specifically developed for civil engineering applications. The sandwich beam is produced by gluing layers of fiber composite sandwich structure together in flatwise (horizontal) and in edgewise (vertical) orientations. The glued sandwich beams with edgewise laminations presented appropriate strength and stiffness for replacement turnout timber sleeper. The mechanical properties of these glue-laminated sandwich beams are comparable with the existing timber turnout sleepers, demonstrating that the innovative composite sandwich beam is a viable alternative sleeper material for railway turnouts. From this study, it is concluded that the glue-laminated composite sandwich structures have the potential to be used for replacement railway turnout sleepers. An enhanced understanding of the behavior of fiber composite sandwich structures for potential civil engineering applications is also an outcome of this investigation.

Fiber-reinforced polymer composites (FRP) are an attractive repair option for reinforced concrete structures, but whereas laboratory durability tests have indicated that FRP generally performs well in many environments, their long-term performance in field environments is not well understood. This case study describes an effort to collect in situ data about the FRP used to repair a concrete arch bridge 8 years after the FRP was originally placed. On-site assessment efforts included inspection for voids between the concrete and FRP using acoustic sounding and thermographic imaging and pull-off tests to check the bond strength. Large debonded regions of FRP were also cut from the structure and tested in tension in the laboratory. Results generally pointed to some level deterioration. Many new voids were found, existing voids had grown, pull-off testing showed weaker bond strengths, and the tensile strengths were quite low compared to design values. However, a lack of initial data make it difficult to distinguish between deterioration over time and the possibility of lower strengths because of field manufacture techniques.

This paper presents a new, efficient, and economic confinement technique for square reinforced concrete (RC) columns with unbonded glass fiber-reinforced polymer (GFRP) wrapping and lightweight concrete elements at the side faces. Three confined columns with different reinforcement ratios were tested under concentric axial force. The force-deformation responses show a distinctly increased load-carrying capacity at plastic deformations up to 4% average axial strain. This confinement technique is a considerable improvement for square columns with a large side-to-corner radius. Only a slight change in column geometry has to be accepted because of the use of unbonded lightweight concrete panels.