Development of FRP Shear Bolts for Punching Shear Retrofit of Reinforced Concrete Slabs

A fiber-reinforced polymer (FRP) shear bolt system has been recently developed at the University of Waterloo in Canada. The system is used to protect previously built reinforced concrete (RC) slabs against brittle punching shear failure. The system requires drilling small holes in a RC slab around the perimeter of a column, inserting bolts into the holes, and anchoring the bolts at both external surfaces of the slab. Many existing RC slabs have been built without any shear reinforcement. Also, many of these slabs are in corrosive environments, e.g., parking garages, where the use of deicing salts accelerates reinforcement corrosion and concrete deterioration. Therefore, FRPs are ideal materials to be used for such retrofit. The challenge, however, is the development of mechanical end anchorages for FRP rods that are efficient, aesthetic, cost effective, and that can be installed on site. The research presented in this paper includes development of FRP bolts with mechanical anchorages and the results of testing done using the developed systems. A new anchorage technique for the FRP rods based on crimping the rod ends with the aluminum fittings was developed. The testing was done on isolated slab-column specimens representing interior slab-column connections in a continuous flat plate system. The specimens were subjected to simulated gravity loading. The developed FRP bolts worked very well in improving the performance of the slab-column connections and showing the benefits of using FRP in punching shear retrofit of reinforced concrete slabs in corrosive environments.     

Efficient Strengthening Technique to Increase the Flexural Resistance of Existing RC Slabs

Composite materials are being used with notable effectiveness to increase and upgrade the flexural load carrying capacity of reinforced concrete (RC) members. Near-surface mounted (NSM) is one of the most promising strengthening techniques, based on the use of carbon fiber-reinforced polymer (CFRP) laminates. According to NSM, the laminates are fixed with epoxy based adhesive into slits opened into the concrete cover on the tension face of the elements to strength. Laboratory tests have shown that the NSM technique is an adequate strengthening strategy to increase the flexural resistance of RC slabs. However, in RC slabs of low concrete strength, the increase of the flexural resistance that NSM can provide is limited by the maximum allowable compressive strain in the compressed part of the slab, in order to avoid concrete crushing. This restriction reduces the effectiveness of the strengthening, thus limiting the use of the NSM technique. A new thin layer of concrete bonded to the existing concrete at the compressed region is suitable to overcome this limitation. Volumetric contraction due to shrinkage and thermal effects can induce uncontrolled cracking in the concrete of this thin layer. Adding steel fibers to concrete [steel fiber-reinforced concrete (SFRC)], the postcracking residual stress can be increased in order to prevent the formation of uncontrolled crack patterns. In the present work, the combined strengthening strategy, a SFRC overlay and NSM CFRP laminates, was applied to significantly increase the flexural resistance of existing RC slabs. Experimental results of four-point bending tests, carried out in unstrengthened and strengthened concrete slab strips, are presented and analyzed.     

Behavior of FRP-RC Slabs under Multiple Independent Air Blasts

In some terrorist attacks, it is possible that RC structures might be subjected to more than a single explosion. RC structures designed without the consideration of blast effects tend to lose their capacity after the first explosion. The use of a fiber reinforced polymer (FRP) sheet has been proven to enhance the performance and resistance of an RC member under a single explosion test. However, there appears to have been no experimental programs conducted to assess the performance of FRP-strengthened RC members subjected to multiple explosions reported in the literature. This paper, therefore, presents experimental results for the behavior of RC slabs strengthened by an FRP sheet after undergoing single, double, and triple independent explosion testing. Results from these blast tests indicate that the FRP sandwich RC slab tested was able to sustain the subsequent second explosion of greater impact. A brittle shear failure with FRP debonding was observed following the third explosion on this FRP-strengthened RC slab.     

RC Slabs Strengthened with Prestressed and Gradually Anchored CFRP Strips under Monotonic and Cyclic Loading

This paper presents the test results of reinforced concrete slabs strengthened with prestressed and gradually anchored carbon fiber–reinforced polymer (CFRP) strips under monotonic and cyclic loading. To take full advantage of the externally bonded CFRP technique, it is beneficial to apply the laminates in a prestressed state, which relieves the stress in the steel reinforcement and reduces crack widths and deflection. The aim of the monotonic tests was to determine the strengthening efficiency of the new prestressing technique and to investigate serviceability and ultimate states. The cyclic tests were performed to identify the fatigue behavior of the strengthened slabs and to investigate the influence of long-term cyclic loading and elevated temperature on the bond properties of the prestressed CFRP laminates and the ductility and flexural strength of the strengthened slabs. A nonlinear analytical model of reinforced concrete members strengthened with passive and prestressed CFRP strips under static loading is proposed in the paper. A comparison of the experimental and predicted results reveals an excellent agreement in the full range of loading.     

Postrepair Fatigue Performance of FRP-Repaired Corroded RC Beams: Experimental and Analytical Investigation

This paper presents the results of an experimental and analytical study of the fatigue performance of corroded reinforced concrete (RC) beams repaired with fiber-reinforced polymer (FRP) sheets. Ten RC beam specimens (152×254×3,200?mm) were constructed. One specimen was neither strengthened nor corroded to serve as a reference; three specimens were corroded and not repaired; another three specimens were corroded and repaired with U-shaped glass FRP sheets that wrapped the cross section of the specimen; and the remaining three specimens were corroded and repaired with U-shaped glass FRP sheets for wrapping and carbon-fiber-reinforced polymer (CFRP) sheets for flexural strengthening. The FRP sheets were applied after the main reinforcing bars were corroded to an average mass loss of 5.5%. Following FRP repair, some specimens were tested immediately to failure, while the other repaired specimens were subjected to further corrosion before being tested to failure to investigate their postrepair (long-term) performance. Reinforcement steel pitting due to corrosion reduced the fatigue life significantly. The FRP wrapping had no significant effect on the fatigue performance, while using CFRP sheets for flexural strengthening enhanced the fatigue performance significantly. The fatigue results were compared to smooth specimen fatigue data to estimate an equivalent fatigue notch factor for the main reinforcing bars of the tested specimens.     

Modeling Debonding Failure in FRP Flexurally Strengthened RC Members Using a Local Deformation Model

Reinforced concrete (RC) beams and slabs can be strengthened by bonding fiber-reinforced polymer (FRP) composites to their tension face. The performance of such flexurally strengthened members can be compromised by debonding of the FRP, with debonding initiating near an intermediate crack (IC) in the member away from the end of the FRP. Despite considerable research over the last decade, reliable IC debonding strength models still do not exist. The current paper attempts to correct this situation by presenting a local deformation model that can simulate IC debonding. The progressive formation of flexural cracks, and the associated crack spacings and crack widths are modelled from initial cracking to the onset of debonding. The bond characteristics between the longitudinal steel reinforcement and concrete, and the FRP and concrete, as well as the tension stiffening effect of the reinforcement and FRP to the concrete, are considered. The FRP-to-concrete bond-slip relation is used to determine the onset of debonding. The analytical predictions compare well with experimental results of FRP-strengthened RC cantilever slabs.     

Mechanically Fastened FRP-Strengthened Two-Way Concrete Slabs with and without Cutouts

The feasibility of strengthening concrete slabs in flexure, with and without cutouts, using the mechanically fastened (MF) FRP technique is investigated. Two series of large-scale reinforced concrete slabs are tested. The first series is comprised of five slabs without a cutout, and measuring 2,600×2,600×120?mm; the second series consists of four slabs of the same dimensions with a central cutout measuring 800×800?mm. The mechanically fastened system is found to be a valid alternative to the externally bonded system resulting in a rapid, economic, and effective strengthening technique for two-way concrete slabs. The increases in ultimate capacities of the MF FRP-strengthened slabs range between 30 and 70% over those of the unstrengthened specimens. In addition, finite-element modeling of MF FRP-strengthened slabs is introduced in this study. The interfacial behavior between the MF FRPs and the concrete substrate is accounted for by using appropriate interfacial models. Very good agreement is obtained between the test results and the numerical predictions.     

Numerical Investigation of Fiber Reinforced Polymers Poststrengthened Concrete Slabs

The present work deals with the numerical simulation of fiber reinforced polymers (FRP) poststrengthened scaled concrete slabs in order to predict their load carrying behavior. The used strengthening materials are FRPs which are of increasing interest in civil engineering applications such as textile reinforced concrete tubes, cables of cable-stayed bridges, or even entire bridges. The numerical results are compared with experiments which were conducted at the Univ. of California, San Diego, and are described in detail by H?rmann in 1997; H?rmann et al. in 1998; and later by Seim et al. in 2001. The slabs are modeled for the numerical simulation first in a two-dimensional design space, assuming a plane stress condition for the concrete and the fiber reinforced polymer, and second in a three-dimensional design space with multilayered shell elements in order to include the varying response across the depth of the slabs. The used material model for reinforced concrete was developed by Menrath et al. in 1998 and has been enhanced for multilayered shell elements by Haufe et al. in 1999. It is based on multisurface plasticity, consisting of two Drucker–Prager yield surfaces and a spherical cap, and exhibits fracture energy based evolution laws for the softening regime. The reinforcement is taken into account as additional stress contribution in a smeared manner using a one-dimensional constitutive law based on an elastoplastic isotropic hardening model.     

Does the Use of FRP Reinforcement Change the One-Way Shear Behavior of Reinforced Concrete Slabs?

For members with no transverse reinforcement, numerous models have been proposed for determining shear capacity, most often based on a statistical curve fit to experimental beam test results. The shear provisions of the Canadian code (CSA) for steel-reinforced concrete, by contrast, are based on a theoretical model, the modified compression field theory. This paper demonstrates that the CSA shear provisions for steel-reinforced members can be safely applied to members with internal fiber-reinforced polymer (FRP) bars by adjusting the term EsAs in the method to ErAr. A database of 146 shear failures of specimens reinforced with carbon, glass, or aramid FRP or steel is presented and gives an average test to predicted ratio of 1.38 with a coefficient of variation (COV) of 17.2%. The CSA code equations were optimized for the typical strain range of steel-reinforced concrete and when an equation appropriate for the wider range of strains associated with FRP is used, then a better statistical result can be achieved. Application of this expression to the database resulted in an average test to predicted strength ratio of 1.15 with a COV of 14.9%. As both methods are based on a theoretical shear model that was derived for steel-reinforced concrete and since both methods work safely, it can be concluded that the use of internal FRP bars does not change the one-way shear behavior of reinforced concrete beams and slabs without stirrups.     

Effect of Partial Unbonding on Prestressed Near-Surface-Mounted CFRP-Strengthened Concrete T-Beams

The flexural behavior of RC T-beams strengthened with prestressed near-surface-mounted (NSM) carbon fiber-reinforced-polymer (CFRP) reinforcement was investigated. The specific objective was to study the effect of partial unbonding of the CFRP reinforcement on the beam flexural behavior to increase the deformability. A total of eight RC T-beams were tested under four-point monotonic loading. The main variables were the level of prestressing force in the CFRP bars and the unbonded length at the midspan of the beam. The test results showed that all of the prestressed strengthened beams effectively improved the ultimate load-carrying capacity and the serviceability performance compared to the unstrengthened beam. The partially bonded prestressed beams exhibited an enhancement of the deformability compared to the fully bonded beams while minimizing the reduction of the load-carrying capacity. Partial unbonding was more effective to improve the deformability at higher levels of prestressing force. The general behavior of the partially bonded beams was reasonably well predicted by an analytical model developed previously by the writers.     

Influence of Reinforcement on the Behavior of Concrete Bridge Deck Slabs Reinforced with FRP Bars

This paper presents the results of an experimental study to investigate the role of each layer of reinforcement on the behavior of concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars. Four full-scale concrete deck slabs of 3,000?mm length by 2,500?mm width and 200?mm depth were constructed and tested in the laboratory. One deck slab was reinforced with top and bottom mats of glass FRP bars. Two deck slabs had only a bottom reinforcement mat with different reinforcement ratios in the longitudinal direction, while the remaining deck slab was constructed with plain concrete without any reinforcement. The deck slabs were supported on two steel girders spaced at 2,000?mm center to center and were tested to failure under a central concentrated load. The three reinforced concrete slabs had very similar behavior and failed in punching shear mode at relatively high load levels, whereas the unreinforced slab behaved differently and failed at a very low load level. The experimental punching capacities of the reinforced slabs were compared to the theoretical predictions provided by ACI 318-05, ACI 440.1R-06, and a model proposed by the writers. The tests on the four deck slabs showed that the bottom transverse reinforcement layer has the major influence on the behavior and capacity of the tested slabs. In addition, the ACI 318-05 design method slightly overestimated the punching shear strength of the tested slabs. The ACI 440.1R-06 design method yielded very conservative predictions whereas the proposed method provided reasonable yet conservative predictions.     

Assessing the Strengthening Effect of Various Near-Surface-Mounted FRP Reinforcements on Concrete Bridge Slab Overhangs

The near-surface-mounted (NSM) method has proved to be a reliable alternative to the existing externally bonded (EB) method for the repair and strengthening of concrete structures using fiber-reinforced polymer (FRP) composites. This technique is especially advantageous in bridge deck upgrades for larger barriers and slab overhang strengthening. This paper presents the results of a comparison of the flexural behavior of bridge slab overhangs strengthened in negative bending moment regions with various types of NSM reinforcement that differ in surface condition (e.g., textured and sand coated), cross-sectional shape (e.g., round and square), material type (carbon and glass), and prestressing effect. Eleven full-scale overhang specimens (1.524 m long in overhang and 0.914 m wide) were tested under a cantilever condition. Test results showed that the FRP NSM reinforcements were effective in increasing both yield and ultimate strength of predamaged slab overhangs. All surface treatments were more beneficial than the smooth condition, and the square-shaped reinforcement displayed better performance than the round shape. The prestressing unit developed in this study is simple to apply and could be further explored for field applications.     

Interfacial Behavior and Debonding Failures in FRP-Strengthened Concrete Slabs

It has been demonstrated, through laboratory investigations and various field projects, that the external bonding of fiber- reinforced polymer (FRP) laminates is an effective technique for the structural enhancement of reinforced concrete slabs. In such applications, failure is generally governed by debonding of the FRP laminate. Nevertheless, numerical simulations to date of FRP-strengthened slabs have usually been based on the assumption of full bond between the concrete and FRP. In this study, the interfacial behavior between the FRP laminates and the concrete substrate is accounted for by introducing appropriate bond-slip models for the interface in a nonlinear finite-element analysis of FRP-strengthened two-way slabs. The numerical model is capable of simulating slabs strengthened in shear or in flexure; it can be applied to arbitrary FRP configurations, and can also accommodate both passive as well as prestressed FRP strengthening schemes. Results are presented in terms of load-deflection relationships, ultimate load capacities, failure modes, and interfacial slip and stress distributions. When compared to test results reported in the literature, the analysis is shown to lead to excellent predictions in that, for the entire set of FRP-strengthened specimens considered, the average of the numerical-to-experimental load capacity ratios is 0.966, with a standard deviation of 0.066. Furthermore, in all cases when FRP debonding was observed experimentally, the analysis correctly predicted the mode of failure.     

Fiber-Reinforced Polymer Retrofitting of Predamaged Substandard RC Prismatic Members

An experimental study was conducted to investigate the efficiency of FRP jackets in upgrading the seismic behavior of lightly reinforced concrete prismatic members previously damaged under a combination of axial compression and a reversed cyclic lateral displacement history simulating earthquake effects. The test program comprises 13 cantilever prismatic specimens, which, owing to substandard reinforcing details representative of older construction practices in southern Europe, were susceptible to various undesirable modes of damage such as web-shear cracking, longitudinal bar buckling, or lap-splice failure. After repair, the specimens were retested using the same load combination. The efficiency of the repair options considered in the study, which refer to alternative strengthening systems (with glass or carbon wraps), was investigated with reference to the design parameters of the intervention, the type of the applied lateral displacement history, and the mode of failure that had occurred previously in the initial phase of the tests. The results provided valuable insight regarding participation of the FRP jackets in the various mechanisms of resistance, their ability to reverse the effect of initial damage, and to impart deformation capacity to the structural member.     

Structural Performance of RC Beams Poststrengthened with Carbon, Aramid, and Glass FRP Systems

The use of fiber-reinforced polymers (FRPs) to poststrengthen concrete structures started to be investigated in the mid-1970s and today is widely recognized as an attractive technique to be used in civil structures, especially when aggressive environments prevent the use of materials that are susceptible to corrosion, such as steel. Different FRP poststrengthening techniques have been developed and applied in existing structures, aiming to increase their load capacity. Most FRP systems used nowadays consist of carbon fibers embedded in epoxy matrix. Regardless of the advantages and the good results achieved using carbon fiber-reinforced polymers, some new possibilities, such as the use of prestressing and lower cost fiber materials, are being analyzed in an attempt to provide viable alternatives for a more efficient, safe, and rational use of FRP systems. The main purpose of the present work was to make a comparative analysis of the behavior of reinforced concrete beams poststrengthened with carbon, aramid, and glass FRP subjected to static loading tests. Experimental results were evaluated against theoretical ones obtained through an analytical model that considers a trilinear behavior for the load versus displacement curves. The experimental results indicate that all FRP systems applied have appropriate structural performance for use in poststrengthening applications of RC. The choice of the more suitable system would, therefore, be strongly influenced by circumstances regarding cost limitations and level of reinforcement required.     

Structural Performance of Bridge Approach Slabs under Given Embankment Settlement

Soil embankment settlement causes concrete approach slabs of bridges to lose their contact and support from the soil. When soil settlement occurs, the slab will bend in a concave manner that causes a sudden change in slope grade near its ends. Meanwhile, loads on the slab will also redistribute to the ends of the slab, which may result in faulting across the roadway at the ends of the approach slab. Eventually, the rideability of the bridge approach slab will deteriorate. The current American Association of State Highway Transportation Officials code specifications do not provide clear guidelines to design approach slabs considering the embankment settlements. State Departments of Transportation are spending millions of dollars each year to deal with problems near the ends of approach slabs. To investigate the effect of embankment settlements on the performance of the approach slab, a three-dimensional finite element analysis was conducted in the present study, considering the interaction between the approach slab and the embankment soil, and consequently the separation of the slab and soil. The predicted internal moments of the approach slab provide design engineers with a scientific basis to properly design the approach slab considering different levels of embankment settlements. A proper design of the approach slab will help mitigate the rideability problems of the slab.     

Flexure of Two-Way Slabs Strengthened with Prestressed or Nonprestressed CFRP Sheets

This paper presents a study on the flexural behavior of two-way reinforced concrete slabs externally strengthened with prestressed or nonprestressed carbon fiber-reinforced polymer (CFRP) sheets. Four large-scale flat plate slabs (3,000?mm×3,000?mm×90?mm) are tested and a nonlinear three-dimensional finite-element analysis is conducted to predict the flexural behaviors of the tested slabs, including the load-deflection response, strain distribution, crack propagation, and crack mouth opening displacement. An increase in the load-carrying capacity of 25 and 72% is achieved for the slabs strengthened with nonprestressed and prestressed CFRP sheets, respectively, in comparison to the unstrengthened slab. A reduction of the deflections up to 32% in service is noted for the strengthened slabs. The unstrengthened slab shows very ductile behavior, whereas, progressive failure is observed for the strengthened slabs, exhibiting pseudoductility in postpeak behavior. Stress redistribution between the internal and external reinforcement is significant in the slab strengthened with prestressed CFRP sheets.     

Fatigue Life Evaluation of Concrete Bridge Deck Slabs Reinforced with Glass FRP Composite Bars

Since bridge deck slabs directly sustain repeated moving wheel loads, they are one of the most bridge elements susceptible to fatigue failure. Recently, glass fiber-reinforced polymer (FRP) composites have been widely used as internal reinforcement for concrete bridge deck slabs as they are less expensive compared to the other kinds of FRPs (carbon and aramid). However, there is still a lack of information on the performance of FRP–reinforced concrete elements subjected to cyclic fatigue loading. This research is designed to investigate the fatigue behavior and fatigue life of concrete bridge deck slabs reinforced with glass FRP bars. A total of five full-scale deck slabs were constructed and tested under concentrated cyclic loading until failure. Different reinforcement types (steel and glass FRP), ratios, and configurations were used. Different schemes of cyclic loading (accelerated variable amplitude fatigue loading) were applied. Results are presented in terms of deflections, strains in concrete and FRP bars, and crack widths at different levels of cyclic loading. The results showed the superior fatigue performance and longer fatigue life of concrete bridge deck slabs reinforced with glass FRP composite bars.     

Fatigue Performance of RC Beams Strengthened in Shear with CFRP Fabrics

In recent years, a tremendous effort has been directed toward understanding and promoting the use of externally bonded fiber-reinforced polymer (FRP) composites to strengthen concrete structures. Despite this research effort, studies on the behavior of beams strengthened with FRP under fatigue loading are relatively few, especially with regard to its shear-strengthening aspect. This study aims to examine the fatigue performance of RC beams strengthened in shear using carbon FRP (CFRP) sheets. It involves six laboratory tests performed on full-size T-beams, where the following parameters are investigated: (1) the FRP ratio and (2) the internal transverse-steel reinforcement ratio. The major finding of this study is that specimens strengthened with one layer of CFRP survived 5 million cycles, some of them with no apparent signs of damage, demonstrating thereby the effectiveness of FRP strengthening systems on extending the fatigue life of structures. Specimens strengthened with two layers of CFRP failed in fatigue well below 5 million cycles. The failure mode observed for these specimens was a combination of crushing of the concrete struts, local debonding of CFRP, and yielding of steel stirrups. This failure may be attributed to the higher load amplitude and also to the greater stiffness of the FRP which may have changed the stress distribution among the different components coming into play. Finally, comparison between the performance of specimens with transverse steel and without seems to indicate that the addition of transverse steel extends the fatigue life of RC beams.     

Full Torsional Behavior of RC Beams Wrapped with FRP: Analytical Model

Torsion failure is an undesirable brittle form of failure. Although previous experimental studies have shown that using fiber-reinforced polymer (FRP) sheets for torsion strengthening of reinforced concrete (RC) beams is an effective solution in many situations, very few analytical models are available for predicting the section capacity. None of these models predicted the full behavior of RC beams wrapped with FRP, account for the fact that the FRP is not bonded to all beam faces, or predicted the ultimate FRP strain using equations developed based on testing FRP strengthened beams in torsion. In this paper, an analytical model was developed for the case of the RC beams strengthened in torsion. The model is based on the basics of the modified compression field theory, the hollow tube analogy, and the compatibility at the corner of the cross section. Several modifications were implemented to be able to take into account the effect of various parameters including various strengthening schemes where the FRP is not bonded to all beam faces, FRP contribution, and different failure modes. The model showed good agreement with the experimental results. The model predicted the strength more accurately than a previous model, which will be discussed later. The model predicted the FRP strain and the failure mode.