Evaluation of Bending Strength of RC Beams Strengthened with FRP Sheets

This paper deals with reinforced concrete beams strengthened by means of externally bonded fiber-reinforced polymer (FRP) sheets. The scope of the work is to discuss and compare an exact and an approximate approach to the computation of the flexural load-carrying capacity of the strengthened beam. The two approaches differ from one another in the way they take into account the extent of the load already acting throughout strengthening operations. To achieve this goal a numerical model is presented and validated by comparing its output with that of 46 experimental tests taken from the literature. The numerical model is then adopted to perform a numerical parametric analysis of a wide range of practical applications, excluding all cases of FRP delamination, and useful conclusions are drawn.     

Flexural Behavior of R/C Beams Strengthened with Carbon Fiber Reinforced Polymer Sheets or Fabric

Their resistance to electro-chemical corrosion, high strength-to-weight ratio, larger creep strain, fatigue resistance, and nonmagnetic and nonmetallic properties make carbon fiber reinforced polymer (CFRP) composites a viable alternative to bonding of steel plates in repair and rehabilitation of reinforced concrete structures. The objective of this investigation is to study the effectiveness of externally bonded CFRP sheets or carbon fiber fabric in increasing the flexural strength of concrete beams. Four-point bending flexural tests were conducted up to failure on nine concrete beams strengthened with different layouts of CFRP sheets and carbon fiber fabric and on three beams with different layouts of anchored CFRP sheets. An analytical procedure, based on compatibility of deformations and equilibrium of forces, was presented to predict the flexural behavior of beams strengthened with CFRP sheets and carbon fiber fabric. Comparisons were made between the test results and the analytical calculations. The flexural strength was increased up to 58% on concrete beams strengthened with anchored CFRP sheets.     

Behavior of Reinforced Concrete Beams Strengthened with Externally Bonded Hybrid Carbon Fiber–Glass Fiber Sheets

A comparative test program including six beams was carried out. Two strengthening systems, namely hybrid carbon fiber glass fiber-reinforced polymer (H-CF/GF-RP) strengthening and CF-reinforced polymer strengthening were used. The test results showed that the H-CF/GF-RP strengthening led to a significant increase of ductility with a slight influence on stiffness of strengthened beams.     

Strengthening of Preloaded RC Beams Using Hybrid Carbon Sheets

Reinforced concrete (RC) beams subject to service loads of 40 or 60% of steel yielding were strengthened using hybrid continuous carbon fiber sheets. The hybrid systems were made of high-strength and high-modulus carbon sheets, and compared with systems using only high-strength carbon. It was found that the use of high-modulus carbon sheets in hybrid systems could increase the yielding load, the flexural stiffness, the postyielding ductility, and reduce the crack opening in concrete. The slope changes on load-deflection curves at steel yielding are not noticeable in hybrid systems. The tensile strains developed in hybrid sheets after the fracture of high-modulus carbon are higher in magnitude and distributed in a larger area, leading to an ultimate carbon fracture with concrete crushing. These unique features are attributed to the high stiffness and low ultimate tensile strain of the high-modulus carbon fibers which stiffen the structures, avoid or delay the fiber-reinforced polymer debonding, and facilitate the deformability during their subsequent breakdown.     

Comparison of Methods for Determining Specific Surface Area of Soils

The present study was undertaken to compare the capabilities of four of the known methods; more specifically, N2 adsorption, methylene blue (MB)-titration, MB-spot test, and ethylene glycol monoethyl ether (EGME) methods were evaluated to determine the specific surface area (SSA) of 16 different clayey soils. The study showed that N2 adsorption method underpredicts the SSA of soils, especially for smectitic soils. No significant differences were observed between N2 SSA, MB SSA-titration, or MB-spot test for kaolinitic soils. The SSA estimates of MB-titration and MB-spot test methods were highly correlated for all soils. The EGME method has a very different procedure from the MB methods; however, it was highly correlated with MB methods (r2 = 0.95). The N2 adsorption method had no correlation to other methods. The cation exchange capacity of tested soils was highly correlated to the SSA, as high as r2 = 0.77. No unique relationship was determined between the clay fraction and SSA.     

Evaluation of Prestressed Concrete Girders Strengthened with Carbon Fiber Reinforced Polymer Sheets

This paper details the use of carbon fiber reinforced polymer (CFRP) sheets to repair and strengthen prestressed concrete bridge girders in flexure and shear. Three specimens that were removed from an overloaded bridge (Bridge No. 56) in Graham County, Kansas were tested. Two of the specimens were repaired and strengthened, and all three were tested to failure to determine flexural capacity. Test results showed that two layers of longitudinal CFRP sheets increased the flexural capacity of the strengthened specimens by 20% compared to an unstrengthened control specimen. Shear capacity was also evaluated on both ends of each specimen. Two different cases were evaluated in shear. One case allowed shear cracks to propagate inside the transfer length of the prestressing strand, allowing a bond failure to occur. The second case forced the shear cracks to remain outside of the transfer length, thereby preventing a bond failure. The test results show that transverse CFRP sheets increased the shear capacity of the specimens tested by as much as 28%, but did not prevent bond failures.     

Fatigue of Reinforced Concrete Beams Strengthened with Steel-Reinforced Inorganic Polymers

Steel-reinforced polymer (SRP) composite materials are very attractive due to their low weight and high strength. The ease of installation which significantly reduces repair time and expense is another major advantage. One of the main disadvantages of SRP materials is that the matrices used for their fabrication are typically organic and thus they are susceptible to fire. In this study, a newly developed retrofit system is being used. It consists of high strength steel fibers impregnated in a fireproof inorganic matrix. The objective of this study is to examine the effects of this hybrid rehabilitation system on the fatigue performance of strengthened reinforced concrete beams. Sixteen 100?mm×150?mm×1200?mm reinforced concrete beams with enough transverse reinforcement to avoid shear failure were used in this study. Nine beams were strengthened with steel fiber sheets on their tension faces. The results from the present study indicate that the fatigue life of reinforced concrete beams, subjected to the same cycling load, can be significantly extended using externally bonded sheets. A rather important finding is that although the strengthening system increases the fatigue life of the beams, the failure mechanism remains the same in both strengthened and nonstrengthened beams. Thus, it is possible to predict the fatigue life of a cyclically loaded beam using existing fatigue models. Furthermore, no delamination failures were observed due to fatigue loading.     

Fatigue Behavior of RC Beams Strengthened with NSM CFRP Rods

This paper presents experimental results of reinforced concrete beams strengthened using near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) reinforcement. A total of nine beam specimens were tested under fatigue loads. In addition, two specimens were tested for monotonic capacity. The beams were 3,500 mm long with a cross section of 254 mm deep by 152 mm wide. Different load ranges were considered in the fatigue tests to construct the fatigue life curves. The test results showed that under monotonic loading, the beam strengthened with NSM CFRP rod exhibited increases of 26 and 50% in the yield and ultimate load over the control beam, respectively. Under cyclic loading, the fatigue life for the strengthened beams was 24% higher than that of the control unstrengthened beams. An analytical model using sectional analysis and strain-life approach was developed to estimate the fatigue life of the specimens at various cyclic load ranges. A good agreement between the experimental results and analytical prediction of the fatigue life was obtained.     

Flexural Behavior of Reinforced Concrete Beams Strengthened with CFRP Sheets and Epoxy Mortar

Experiments were conducted to study the effect of using epoxy mortar patch end anchorages on the flexural behavior of reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) sheets. More specifically, the effect of the end anchorage on strength, deflection, flexural strain, and interfacial shear stress was examined. The test results show that premature debonding failure of reinforced concrete beams strengthened with CFRP sheet can be delayed or prevented by using epoxy mortar patch end anchorages. A modified analytical procedure for evaluating the flexural capacity of reinforced concrete beams strengthened with CFRP sheets and epoxy mortar end anchorage is developed and provides a good prediction of test results.     

Shear Strengthening of RC T-Beams Using Mechanically Anchored Unbonded Dry Carbon Fiber Sheets

This research studies the feasibility and effectiveness of a new method of strengthening existing RC T-beams in shear by using mechanically anchored unbonded dry carbon fiber (CF) sheets. This method eliminates the debonding of epoxy-bonded carbon-fiber-reinforced polymer (CFRP) sheets and utilizes the full capacity of dry CF sheets. In this method, dry CF sheets are wrapped around and bonded to two steel rods. Then the rods are anchored to the corners of the web-flange intersection of the T-beam with mechanical bolts. This makes a U-shaped dry CF jacket around the web which increases the shear strength of the T-beam using the privilege of higher tensile strength and modulus of elasticity of dry CF compared to composite CFRP. A total of three RC T-beams with shear span-to-depth ratio of 2.0 were tested under increasing monotonic load till failure. The pilot tests were done as a proof-of-concept of the effectiveness of the proposed method in increasing the shear capacity of the RC T-beams. The first T-beam, which was tested as the control beam, failed in shear. The second beam was strengthened by using a U-shaped CFRP sheet that was externally bonded to the web of the beam in the shear zones. The third beam was strengthened by using anchored U-shaped dry CF sheet. The test results showed that the beam strengthened by the new mechanically anchored dry CF had about 48% increase in shear capacity as compared to the control beam and 16% increase in shear capacity as compared to the beam strengthened by CFRP epoxy-bonding method.     

Reinforced Concrete T-Beams Strengthened in Shear with Fiber Reinforced Polymer Sheets

This research studies the interaction of concrete, steel stirrups, and external fiber reinforced polymer (FRP) sheets in carrying shear loads in reinforced concrete beams. A total of eight tests were conducted on four laboratory-controlled concrete T-beams. The beams were subjected to a four-point loading. Each end of each beam was tested separately. Three types of FRP, uniaxial glass fiber, uniaxial carbon fiber, and triaxial glass fiber, were applied externally to strengthen the web of the T-beams, while some ends were left without FRP. The test results show that FRP reinforcement increases the maximum shear strengths between 15.4 and 42.2% over beams with no FRP. The magnitude of the increased shear capacity is dependent not only on the type of FRP but also on the amount of internal shear reinforcement. The triaxial glass fiber reinforced beam exhibited more ductile failure than the other FRP reinforced beams. This paper also presents a test model that is based on a rational mechanism and can predict the experimental results with excellent accuracy.     

Debonding Failures of RC Beams Strengthened with Near Surface Mounted CFRP Strips

This paper presents the results of a series of tests conducted on reinforced concrete (RC) beams strengthened in flexure with near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. As the main focus of the research is on debonding failure mechanisms, the only test variable investigated was the embedment length of the NSM strip and the NSM strip was extensively strain-gauged to monitor its bond behavior. Load-deflection curves, failure modes, strain distributions in the CFRP strip, and local bond stresses at the CFRP–epoxy interface from the tests are all examined in detail and compared with the predictions of a simple analytical model where appropriate. Of the four embedment lengths investigated, all but the shortest one led to a notable increase in the load-carrying capacity and, to a lesser extent, in the postcracking stiffness of the beam. Debonding was found to be the primary failure mode in all cases except for the beam with the longest embedment length. Also reported in this paper are results from preliminary bond tests used to characterize the local bond-slip behavior of the NSM system. Apart from gaining a better understanding of debonding failures in RC beams with NSM FRP strips, the test results reported in the paper should be useful for future verification of numerical and analytical models.     

Uniaxial Tension Behavior of Reinforced Concrete Members Strengthened with Carbon Fiber Sheets

A two-dimensional (2D) nonlinear numerical analysis code by using the rigid body spring method (RBSM) was developed by the writers at Hokkaido University to simulate the behavior of reinforced concrete (RC) members strengthened with fiber-reinforced polymer (FRP) sheets. The code supports the nonlinear constitutive laws for the different materials and nonlinear bond stress-slip relationships for steel-concrete and FRP sheet-concrete interfaces. This study uses the aforementioned code to examine the uniaxial tension behavior of RC members strengthened with carbon fiber sheets (CFS). Experimental results are compared with relevant numerical outputs to validate the model and confirm its ability to simulate the experimental observations. This study also assesses the influence of the amount of CFS strengthening on the tension-stiffening behavior of strengthened members. Finally, this research also suggests new analytical expressions for the average stress-strain relationships of concrete and steel in tension in the presence of stiffening contributions from internal steel reinforcement bars and externally bonded CFS reinforcement.     

Simplified Shear Design Method for Concrete Beams Strengthened with Fiber Reinforced Polymer Sheets

This paper presents a simplified shear design method for reinforced concrete beams strengthened externally with fiber reinforced polymer (FRP) sheets. This design method combines both the strip method and the shear friction approach. The background of the strip method is presented in detail, including the interface shear strength curve, which is compared with some available bond test data found in the literature. A parametric study is performed to propose two simplified equations, which describe the FRP sheet contribution. This contribution is added to the discrete shear friction formulation and, by derivation, a continuous and simplified design equation is proposed. This method well describes the interaction between the concrete, the stirrups, and the FRP sheets. A variable concrete crack angle is used, which enhances the accuracy of the model. No iteration is required. The proposed design formulation gives conservative predictions with 35 experimental test results found in the literature.     

Experimental Investigation on Torsional Behavior of Solid and Box-Section RC Beams Strengthened with CFRP Using Photogrammetry

The construction boom over the last century has resulted in a mature infrastructure network in developed countries. Lately, the issue of maintenance and repair/upgrading of existing structures has become a major issue, particularly in the area of bridges. Fiber- reinforced polymer (FRP) has shown great promise as a state-of-the-art material in flexural and shear strengthening as external reinforcement, but information on its applicability in torsional strengthening is limited. The need for torsional strengthening in bridge box girders is highlighted by the Westgate Bridge in Melbourne, Australia, one of the largest strengthening projects in the world for externally bonded carbon FRP (CFRP) laminates. This paper reports the experimental work in an overall investigation of torsional strengthening of solid and box-section reinforced concrete beams with externally bonded carbon FRP. This was found to be a viable method of torsional strengthening. Photogrammetry was a noncontact measuring technique used in the investigation. The deformation mechanisms were found to be unchanged in the strengthened specimens. Furthermore, it was found that the crack widths were reduced and aggregate interlocking action improved with the strengthened beams.     

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.     

Debonding in RC Beams Shear Strengthened with Complete FRP Wraps

Substantial research has been conducted on the shear strengthening of reinforced concrete (RC) beams with bonded fiber reinforced polymer (FRP) strips. The beams may be strengthened in various ways: complete FRP wraps covering the whole cross section (i.e., complete wrapping), FRP U jackets covering the two sides and the tension face (i.e., U jacketing), and FRP strips bonded to the sides only (i.e., side bonding). Shear failure of such strengthened beams is generally in one of two modes: FRP rupture and debonding. The former mode governs in almost all beams with complete FRP wraps and some beams with U jackets, while the latter mode governs in all beams with side strips and U jackets. In RC beams strengthened with complete wraps, referred to as FRP wrapped beams, the shear failure process usually starts with the debonding of FRP from the sides of the beam near the critical shear crack, but ultimate failure is by rupture of the FRP. Most previous research has been concerned with the ultimate failure of FRP wrapped beams when FRP ruptures. However, debonding of FRP from the sides is at least a serviceability limit state and may also be taken as the ultimate limit state. This paper presents an experimental study on this debonding failure state in which a total of 18 beams were tested. The paper focuses on the distribution of strains in the FRP strips intersected by the critical shear crack, and the shear capacity at debonding. A simple model is proposed to predict the contribution of FRP to the shear capacity of the beam at the complete debonding of the critical FRP strip.     

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Using Nonmetallic Anchor Systems

This paper presents the flexural behavior of reinforced concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets using nonmetallic anchor systems. The developed nonmetallic anchor systems replace the permanent steel anchorage. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The flexural behavior of the tested beams, including detailed failure modes of each nonmetallic anchor system, is investigated. The study shows that the developed nonmetallic anchors are more effective in resisting peeling-off cracks compared to the permanent steel anchors and the beams strengthened with the nonmetallic anchors provide comparable load-carrying capacity with respect to the steel anchored control beam.     

Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio

The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP (Vf). In all, 14 tests were performed on 4,520-mm-long T-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1)?the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2)?the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3)?the effect of FRP strip location with respect to internal transverse-steel location; (4)?the effect of FRP strip width; and (5)?the effect of internal transverse-steel reinforcement on the CFRP shear contribution.     

Ductility and Cracking Behavior of Prestressed Concrete Beams Strengthened with Prestressed CFRP Sheets

This paper investigates the flexure of prestressed concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets, focusing on ductility and cracking behavior. Structural ductility of a beam strengthened with CFRP sheets is critical, considering the abrupt and brittle failure of CFRP sheets themselves. Cracking may also affect serviceability of a strengthened beam, and may be especially important for durability. Midscale prestressed concrete beams (L = 3.6?m) are constructed and a significant loss of prestress is simulated by reducing the reinforcement ratio to observe the strengthening effects. A nonlinear iterative analytical model, including tension of concrete, is developed and a nonlinear finite-element analysis is conducted to predict the flexural behavior of tested beams. The prestressed CFRP sheets result in less localized damage in the strengthened beam and the level of the prestress in the sheets significantly contributes to the ductility and cracking behavior of the strengthened beams. Consequently, the recommended level of prestress to the CFRP sheets is 20% of the ultimate design strain with adequate anchorages.