Wood Members Strengthened with Mechanically Fastened FRP Strips

The demand for a rapid technique to strengthen existing wood bridge structural elements is evident in military and civilian sectors. An experimental program was undertaken to evaluate the feasibility of increasing the flexural strength of Southern Pine wood members using mechanically fastened fiber-reinforced polymer (FRP) strips. Three control specimens and twelve composite members were fabricated using two FRP material types with varying fastener spacing and tested to failure. The experimental results show that the proposed strengthening technique induced a gradual failure of the composite members and increased ultimate moment, initial stiffness, and ductility over that found for the control specimens. Increased fastener spacing decreased member ultimate moment, initial stiffness, and ductility ratio. The moisture content of the wood material greatly affected the ductility ratio of the wood members.     

Finite-Element Modeling and Development of Equivalent Properties for FRP Bridge Panels

Fiber-reinforced polymer (FRP) composite materials are increasingly making their way into civil engineering applications. To reduce the self-weight and also achieve the necessary stiffness, sandwich panels are commonly used for FRP bridge decks. However, due to the geometric complexity of the FRP sandwich deck, convenient analysis and design methods for FRP bridge deck have not been developed. The present study aims at developing equivalent properties for a complicated sandwich panel configuration using finite-element modeling techniques. With equivalent properties, the hollowed sandwich panel can be transformed into an equivalent solid orthotropic plate, based on which deflection limits can be evaluated and designed. A procedure for the in-plane axial properties of the sandwich core has first been developed, followed by developing the out-of-plane panel properties for bending behavior of the panel. An application is made in the investigation of the stiffness contribution of wearing surface to the total stiffness of bridges with FRP panels. The wearing surface contribution is not usually accounted for in a typical design of bridges with traditional deck systems.     

Mechanics of Composite Sinusoidal Honeycomb Cores

Lightweight and heavy-duty fiber-reinforced polymer (FRP) composite honeycomb sandwich structures have been increasingly used in civil infrastructure. Unique cellular core configurations, such as sinusoidal core, have been applied in sandwich construction. Due to specific core geometry, the solutions for core effective stiffness properties are not readily available. This paper presents a mechanics of materials approach to evaluate the effective stiffness properties of sinusoidal cores. In particular, the internal forces of a curved wall in a unit cell are expressed in terms of resultant forces, and based on the energy method and principle of equivalence analysis, the in-plane stiffness properties of sinusoidal cores are derived. Both finite-element modeling and experimental testing are carried out to verify the accuracy of the proposed analytical formulation. To illustrate the present analytical approach as an efficient tool in optimal analysis and size selection of sinusoidal cores, several design plots are provided and discussed. The simplified analysis and formulation presented for sinusoidal cores can be used in design application of FRP honeycomb sandwich and optimization of efficient cellular core structures.     

ESPI Measurement of Bond-Slip Relationships of FRP-Concrete Interface

Fiber-reinforced polymer (FRP) composites are widely used for strengthening reinforced concrete structures because of their superior properties. Reliable performance of the bond between the external FRP and the concrete in maintaining the composite action between them is crucial for this strengthening technique to be effective. To fully understand and model this bond behavior, a rigorous bond-slip law is essential. This paper presents an experimental study in which the displacement fields in a carbon fiber-reinforced polymer (CFRP)-to-concrete double shear test were measured using the nondestructive and noncontact electronic speckle pattern interferometry (ESPI) technique. Full-field in-plane displacements were measured in 33 CFRP specimens with concrete strengths varying from 23 to 69?MPa. The measurement results were used to infer the bond-slip behavior between the FRP and the concrete. The inferred bond-slip curves include a nonlinear ascending branch and a descending branch. The strength of concrete is found to have significant effect on the peak bond stress but little effect on the slip corresponding to the peak bond stress. A logarithmic model and a simpler parabolic model are proposed to represent the experimental bond-slip constitutive curves.     

Tests on Seismically Damaged Reinforced Concrete Walls Repaired and Strengthened Using Fiber-Reinforced Polymers

The behavior of six 1:2.5-scale reinforced concrete cantilever wall specimens having an aspect ratio of 1.5, tested to failure and subsequently repaired and strengthened using fiber-reinforced polymer (FRP) sheets is investigated. Specimens were first repaired by removing heavily cracked concrete, lap splicing the fractured steel bars by welding new short bars, placing new hoops and horizontal web reinforcement, and finally casting nonshrink high-strength repair mortar. The specimens were then strengthened using FRP sheets and strips, with a view to increasing flexural as well as shear strength and ductility. In addition to different arrangements of steel and FRP reinforcement in the walls, a key parameter was the way carbon-FRP strips added for flexural strengthening were anchored; steel plates and steel angles were used to this effect. Steel plates were anchored using U-shaped glass-FRP (GFRP) strips or bonded metal anchors. Test results have shown that by using FRP reinforcement, the flexural and shear strength of the specimens can be increased. From the anchorage systems tested, metal plates combined with FRP strips appear to be quite efficient. The effectiveness of the bonded metal anchors used was generally less than that of the combination of plates and GFRP strips. In all cases, final failure of the FRP anchorage is brittle, but only occurs after the peak strength is attained and typically follows the fracture of steel reinforcement in critical areas, hence the overall behavior of the strengthened walls is moderately ductile.     

Flexural Capacity of Glass FRP Strengthened Concrete Masonry Walls

Fiber-reinforced polymers (FRP) can provide a strengthening alternative for unreinforced and underreinforced masonry. The ease with which FRP can be installed on the exterior of a masonry wall makes this form of strengthening attractive to the owner, considering both reduced installation cost and down time of the occupied structure. Six unreinforced concrete masonry walls (four at 1.8 m tall and two at 4.7 m tall) were tested in out-of-plane flexure up to capacity. The walls were strengthened with glass FRP composite composed of unidirectional E-glass fabric with an epoxy matrix. The composite was adhered to the surface of the masonry using the same epoxy with the fibers oriented perpendicular to the bed joints. General flexural strength design equations are presented and compared with the results of the testing. It was found that the equations overpredicted the actual capacity of the test specimens by no more than 20%.     

Bond Performance of Near-Surface-Mounted FRP Bars

Near-surface-mounted (NSM) reinforcement has become a well-known method for strengthening existing concrete structures. The bond between the NSM reinforcing bars and concrete is the key factor in the NSM technique. In the NSM technique, there are two bond interfaces: one between the NSM bar and the adhesive, and the other between the adhesive and the concrete. For this technique to perform efficiently, these two interfaces need to be investigated. On the other hand, concrete structures that require rehabilitation are often exposed to aggressive environments. Many of these environments are related to cold-climate conditions as can be found in Canada. Environmental factors including freeze/thaw action, exposure to deicing salts, and sustained low temperatures combine to attack the integrity of repaired structures. Consequently, repair materials for the Canadian infrastructure must be able to withstand these harsh conditions for prolonged periods of time. A total of 80 NSM-fiber-reinforced polymer (FRP) bars installed in C-shaped concrete specimens were tested in pull-out setup to failure. Sixty specimens were tested at normal room temperature, while the remaining 20 specimens were tested after conditioning in an environmentally controlled chamber for 200 freeze/thaw cycles. The dimensions of the specimens were designed, upon a preliminary phase of testing, to ensure that no transverse cracking would occur in the specimen before bond failure of the NSM bar. The results are presented in term of failure load, average bond stress, strains in FRP bar, end slip, and mode of failure. A bond-slip model was proposed for the used FRP bars.     

Out-of-Plane Strengthening of Masonry Walls with Reinforced Composites

This paper presents an investigation into the effectiveness of using fiber-reinforced composite overlays to strengthen existing unreinforced masonry walls to resist out-of-plane static loads. A total of fifteen wall panels [1,200 × 1,800 × 200 mm (4 ft × 6 ft × 8 in.)] were tested. Twelve panels were assembled with fiber-reinforcing systems attached to the tension side, and the remaining three control walls were left without any external reinforcement. Two configurations of external reinforcement were evaluated. The first reinforcement configuration consisted of two layers of fiber-reinforced plastic webbing and the second consisted of vertical and horizontal bands of undirectional fiber composites. The three wall specimens without external reinforcement were tested to evaluate the change in the system strength and behavior with application of the external reinforcing systems. In addition to the two fiber configurations, the testing program also evaluated two methods of surface preparation of the walls, sand blasting, and wire brush. All specimens were thoroughly washed by water jet, 48 hours prior to application of the fiber-reinforcing systems. Three specimens were tested for each variable. A uniformly distributed lateral load was applied to each panel using the procedures described in the ASTM Standard E-72 Test Method (airbag). Failure loads, strains in the external reinforcement (FRP), out-of-plane deformations, and failure modes were recorded. Recommendations on the usefulness of the proposed technique as a means of strengthening masonry walls for out-of-plane loads are presented. In general, flexural strength of masonry walls can be increased if the shear failure is controlled.     

Assessment of Design Formulas for In-Plane FRP Strengthening of Masonry Walls

Masonry structures have demonstrated their seismic vulnerability during recent world seismic events. This paper investigates in-plane seismic performance of unreinforced masonry (URM) walls before and after they are retrofit using fiber-reinforced polymer (FRP) materials. An assessment of available design formulas for evaluating both the in-plane performance of URM walls and the contribution of FRP strengthening systems was performed. Walls with two configurations of the FRP reinforcement have been analyzed: one based on FRP strips installed parallel to the mortar joints, the other characterized by FRP strips arranged along the diagonals of the wall. Based on shear–compression tests carried out on FRP-strengthened masonry walls available in the literature, a comparison between theoretical and experimental data is performed. A discussion about the FRP strains at failure of the walls is provided and values of effective FRP strains to be used for design purposes are proposed.     

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.     

Seismic Behavior of Concrete Columns Reinforced by Steel-FRP Composite Bars

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.     

Hysteretic Behavior of Bridge Columns with FRP-Jacketed Lap Splices Designed for Moderate Ductility Enhancement

This paper presents test results of six specimens representing older bridge columns with inadequate reinforcement detailing consisting of short lap splices at the base and widely spaced transverse reinforcement. Four of these specimens were rehabilitated using fiber-reinforced polymer (FRP) jackets of two different composite materials (carbon and aramid) to avoid premature failure of the lapped bars after a limited number of postyield cycles. The test results indicate that thin FRP jackets can be used to avoid failure of short lap splices at moderate displacement ductilities. Displacement capacities consistent with expected demands in regions of moderate or low seismicity were achieved after jacket retrofitting. The hysteretic behavior of rehabilitated columns was assessed with emphasizing issues related to variation of stiffness and damping ratio as a function of ductility demand for this class of columns. Equations that account for the effect of axial load level on estimates of effective stiffness and damping as a function of displacement ductility are proposed for this class of columns.     

Strengthening of Interior Slab–Column Connections Using a Combination of FRP Sheets and Steel Bolts

The results of an experimental investigation undertaken to evaluate a new technique for strengthening interior slab–column connection in combined flexural and shear modes are presented. The technique consists of using a combination of shear bolts inserted into holes and prestressed against the concrete surface for improving the punching shear capacity, and external [fiber-reinforced polymer (FRP)] reinforcement bonded to the tension face of the slabs in two perpendicular directions for increasing the flexural strength of the slabs. Square slab specimens of 670×670?mm dimensions were tested and the main test variables included the ratio of steel reinforcement (1.0 and 1.5%), span–depth ratio or thickness (55 and 75?mm) of the slabs, area, and configuration of steel bolts, and area of FRP reinforcement. It was found that the use of shear bolts alone improves the punching shear strength and increases the ductility of failure by changing the failure mode from punching to flexural. However, the use of a combination of shear bolts and a moderate amount of FRP reinforcement increased the flexural strength and resulted in a substantial improvement of the punching shear capacity of the slabs. The corresponding increases attained levels between 34 and 77%. A design approach is presented for evaluating the ultimate capacity of the slab–column connections when strengthened using the proposed strengthening technique. Strength results predicted by the proposed approach were in good agreement with the experimental results.     

Case Study of Application of FRP Composites in Strengthening the Reinforced Concrete Headstock of a Bridge Structure

Worldwide interest is being generated in the use of fiber-reinforced polymer composites (FRP) in the rehabilitation of aged or damaged reinforced concrete structures. As a replacement for the traditional steel plates or external posttensioning in strengthening applications, various types of FRP plates, with their high strength-to-weight ratio and good resistance to corrosion, represent a class of ideal material in externally retrofitting. This paper describes a solution proposed to strengthen the damaged reinforced concrete headstock of the Tenthill Creeks Bridge, Queensland, Australia, using FRP composites. A decision was made to consider strengthening the headstock using bonded carbon FRP laminates to increase the load carrying capacity of the headstock in shear and bending. The relevant guidelines and design recommendations were compared and adopted in accordance with AS 3600 and Austroads bridge design code to estimate the shear and flexural capacity of a rectangular cracked FRP reinforced concrete section.     

Experimental Performance of RC Hollow Columns Confined with CFRP

Column jacketing with fiber-reinforced polymer (FRP) composite materials has been extensively investigated in the last decade to address the issue of seismic upgrade and retrofit of existing reinforced concrete (RC) columns. Researchers have mainly focused their attention on solid columns, while very little research has been done on hollow columns strengthened with FRP. To study the behavior of noncircular hollow cross sections subjected to combined axial load and bending and to contribute to the comprehension of the resistant mechanisms present in FRP confinement, a total of seven specimens have been tested. The present work is the first step in a broader endeavor aimed at evaluating the benefits generated by a FRP wrapping, computing (P-M) interaction diagrams for hollow columns confined with FRP, and defining design criteria for the strengthening of these elements using composite jackets. The theoretical analyses will also assess under which conditions the standard approaches for columns with solid cross sections could be extended to the case of hollow columns.     

Stress Transfer and Fracture Propagation in Different Kinds of Adhesive Joints

To effectively and efficiently utilize fiber-reinforced plastic (FRP) laminates (plates or sheets) in strengthening civil infrastructures, a design strategy integrating the properties of FRP reinforcement and composite structural behavior needs to be adopted. The interfacial stress transfer behavior including debonding should be considered to be one of the most important effects on the composite structural behavior. In this paper, two kinds of nonlinear interfacial constitutive laws describing the pre- and postinterfacial microdebonding behavior are introduced to solve the nonlinear interfacial stress transfer and fracture propagation problems for different kinds of adhesive joints in FRP/steel-strengthened concrete or steel structures. Expressions for the maximum transferable load, interfacial shear stress distribution, and initiation and propagation of interfacial cracks (debonding) are derived analytically. In addition, numerical simulations are performed to discuss the factors influencing the interfacial behavior and the theoretical derivations are validated by finite-element analysis.     

Experimental Evaluation of Axial Behavior of Strengthened Circular Reinforced-Concrete Columns

Insufficient or deteriorating reinforced-concrete piers in many existing bridges are required to be strengthened using economical, fast, and efficient methods. Currently, only a few methods can be used to strengthen circular columns. Steel jackets and fiber-reinforced polymer (FRP) composites are the two commonly used methods. In this study, along with these two strengthening methods, concrete jackets reinforced with spiral rebar, welded wire fabric (WWF), and a new steel reinforcement called PCS are investigated under different axial-load applications. Fifteen identical specimens were constructed, strengthened, and tested: one column with no strengthening; three columns strengthened with FRP; two with steel jacketing; and nine with concrete jacketing (two with WWF, three with spiral rebar, and four with the new reinforcement). The bare or unretrofitted specimens had a 152?mm (6?in.) diameter, while the outside diameter of concrete-jacketed specimens was 254?mm (10?in.). Effectiveness of each strengthening method in increasing the stiffness, axial capacity, and displacement ductility was investigated using the experimental data.     

Bond Efficiency of EBR and NSM FRP Systems for Strengthening Concrete Members

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.     

Strengthening of Infill Masonry Walls with FRP Materials

This paper evaluates the effectiveness of different externally bonded glass fiber–reinforced polymer (GFRP) systems for increasing the out-of-plane resistance of infill masonry walls to loading. The research included a comprehensive experimental program comprising 14 full-scale specimens, including four unstrengthened (control) specimens and 10 strengthened specimens. To simulate the boundary conditions of infill walls, all specimens consisted of a reinforced concrete (RC) frame, simulating the supporting RC elements of a building superstructure, which was infilled with solid concrete brick masonry. The specimens were loaded out-of-plane using uniformly distributed pressure to simulate the differential (suction) pressure induced by a tornado. Parameters investigated in the experimental program included aspect ratio, FRP coverage ratio, number of masonry wythes, and type of FRP anchorage. Test results indicated that the type of FRP anchorage had a significant effect on the failure mode. Research findings concluded that GFRP strengthening of infill masonry walls is effective in increasing the out-of-plane load-carrying capacity when proper anchorage of the FRP laminate is provided.     

Experimental Behavior of Carbon Fiber-Reinforced Polymer (CFRP) Sheets Attached to Concrete Surfaces Using CFRP Anchors

Fiber-reinforced polymer (FRP) composite sheets have gained popularity as a viable strengthening technique for existing reinforced concrete structures. The efficiency of the strengthening system largely depends on adequate bond between FRP sheets and the concrete substrate. In recent years, techniques to anchor FRP sheets have been proposed in applications that have limited distance to develop FRP sheet strength. One promising technique consists of fabricating and bonding FRP anchors during the FRP sheet saturation and embedding them into predrilled holes in the concrete substrate. This paper presents experimental results highlighting the complex behavior between FRP sheets and anchors. The primary failure modes that the sheet-anchor system can experience are identified. The experiments identify the main variables that influence the FRP anchor-sheet system behavior. This research contributes to the needed experimental database that will aid in future development of design recommendations of this anchorage system.