Repair of Slab–Column Connections Using Epoxy and Carbon Fiber Reinforced Polymer

Repair, strengthening, and retrofit of reinforced and prestressed concrete members have become increasingly important issues as the World’s infrastructure deteriorates with time. Buildings and bridges are often in need of repair or strengthening to accommodate larger live loads as traffic and building occupancies change. In addition, inadequate design and detailing for seismic and other severe natural events has resulted in considerable structural damage and loss of life, particularly in reinforced concrete buildings. Numerous buildings and bridges suffer damage during such events and need to be repaired. The use of carbon fiber reinforced polymer (CFRP) composite fabric bonded to the surface of concrete members is comparatively simple, quick and virtually unnoticeable after installation. The use of composites has become routine for increasing both the flexural and shear capacities of reinforced and prestressed concrete beams. Earthquake retrofit of bridge and building structures has relied increasingly on composite wrapping of columns, beams and joints to provide confinement and increase ductility. This paper presents the results of cyclic testing of three large-scale reinforced concrete slab–column connections. Each of the specimens was a half-scale model of an interior slab–column connection common to flat-slab buildings. The specimens were reinforced according to ACI-318 code requirements and included slab shear reinforcement. While supporting a slab gravity load equivalent to dead load plus 30% of the live load, the specimens were subjected to an increasing cyclic lateral loading protocol up to 5% lateral drift. The specimens were subjected to the same loading protocol after they were repaired with epoxy crack sealers and CFRP sheet on the surfaces of the slab. Repair with epoxy and CFRP on the top surface of the slab was able to restore both initial stiffness and ultimate strength of the original specimen.     

Behavior of Concrete Beams Strengthened with Fiber-Reinforced Polymer Laminates under Impact Loading

Most of the research on application of composite materials in civil engineering during the past decade has concentrated on the behavior of structural elements under static loads. In engineering practice, there are many situations in which structures undergo impact or dynamic loading. In particular, the impact response of concrete beams strengthened with composite materials is of interest. This paper presents the results of an experimental investigation conducted to study the impact effects on concrete beams strengthened with fiber-reinforced polymer laminates. Two types of composite laminates, carbon and Kevlar, were bonded to the top and bottom faces of concrete beams with epoxy. Five beams were tested: two strengthened with Kevlar laminates, two strengthened with carbon laminates, and one unretrofitted beam as the control specimen. The impact load was applied by dropping a steel cylinder from a specified height onto the top face of the beam. The test results revealed that composite laminates significantly increased the capacity of the concrete beams to resist impact load. In addition, the laminates reduced the deflection and crack width. Comparing the test results of the beams strengthened with Kevlar and carbon laminates indicated that the gain in strength depends on the type, thickness, weight, and material properties of the composite laminate.     

Fatigue and Overloading Behavior of Steel–Concrete Composite Flexural Members Strengthened with High Modulus CFRP Materials

Due to corrosion and the continuous demand to increase traffic loads, there is a need for an effective system which can be used to repair and/or strengthen steel bridges and structures. This paper describes an experimental program, recently completed, to investigate the fundamental behavior of steel–concrete composite scaled bridge beams strengthened with new high modulus carbon fiber-reinforced polymer (HM CFRP) materials. The behavior of the beams under overloading conditions and fatigue loading conditions was studied as well as the possible presence of shear lag at the interface of the steel surface and the CFRP strengthening material. The test results are compared to an analytical model based on the fundamental principles of equilibrium and compatibility, to predict the behavior of the strengthened steel–concrete composite beams. Based on the findings of this research work, combined with other work in the literature, a design guideline is proposed for the use of HM CFRP for strengthening the steel flexural members typically used for bridges and structures.     

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%.     

Natural Mallow Fiber-Reinforced Epoxy Composite for Ballistic Armor Against Class III-A Ammunition

Epoxy matrix composites reinforced with up to 30 vol pct of continuous and aligned natural mallow fibers were for the first time ballistic tested as personal armor against class III-A 9 mm FMJ ammunition. The ballistic efficiency of these composites was assessed by measuring the dissipated energy and residual velocity after the bullet perforation. The results were compared to those in similar tests of aramid fabric (Kevlar?) commonly used in vests for personal protections. Visual inspection and scanning electron microscopy analysis of impact-fractured samples revealed failure mechanisms associated with fiber pullout and rupture as well as epoxy cracking. As compared to Kevlar?, the mallow fiber composite displayed practically the same ballistic efficiency. However, there is a reduction in both weight and cost, which makes the mallow fiber composites a promising material for personal ballistic protection.     

Flexural Fatigue Behavior of Reinforced Concrete Beams Strengthened with FRP Fabric and Precured Laminate Systems

Rehabilitation of existing structures with carbon fiber reinforced polymers (CFRP) has been growing in popularity because they offer resistance to corrosion and a high stiffness-to-weight ratio. This paper presents the flexural strengthening of seven reinforced concrete (RC) beams with two FRP systems. Two beams were maintained as unstrengthened control samples. Three of the RC beams were strengthened with CFRP fabrics, whereas the remaining two were strengthened using FRP precured laminates. Glass fiber anchor spikes were applied in one of the CFRP fabric strengthened beams. One of the FRP precured laminate strengthened beams was bonded with epoxy adhesive and the other one was attached by using mechanical fasteners. Five of the beams were tested under fatigue loading for two million cycles. All of the beams survived fatigue testing. The results showed that use of anchor spikes in fabric strengthening increase ultimate strength, and mechanical fasteners can be an alternative to epoxy bonded precured laminate systems.     

Shear Strengthening of Concrete Structures with the Use of Mineral-Based Composites

Rehabilitation and strengthening of concrete structures have become more common during the last 10–15?years, partly due to a large stock of old structures and partly due to concrete deterioration. Also factors such as lack of understanding and the consequences of chloride attack affect the need for rehabilitation. In addition, more traffic and heavier loads lead to the need for upgrading. Existing externally bonded strengthening systems using fiber-reinforced polymers (FRP) and epoxy as bonding agents have been proven to be a good approach to repair and strengthen concrete structures. However, the use of epoxy bonding agents has some disadvantages in the form of incompatibilities with the base concrete. It is, therefore, of interest to substitute epoxy with systems that have better compatibility properties with the base concrete, for example, cementitious bonding agents. This paper presents a study on reinforced concrete beams strengthened in shear with the use of cementitious bonding agents and carbon fiber grids, denoted as mineral-based composites (MBC). In this study it is shown that the MBC system has a strengthening effect corresponding to that of strengthening systems using epoxy bonding agents and carbon fiber sheets. Different designs and material properties of the MBC system have been tested. An extensive monitoring setup has been carried out using traditional strain gauges and photometric strain measurements to obtain strains in steel reinforcement, in FRP, and strain fields on the strengthened surface. It has been shown that the use of MBC reduces strains in the steel stirrups and surface cracks even for low load steps as compared to a nonstrengthened concrete beam.     

Corrosion of Steel Reinforcement in Carbon Fiber-Reinforced Polymer Wrapped Concrete Cylinders

The rehabilitation, repair, and strengthening of concrete structures has increased worldwide with a growing number of systems employing externally applied fiber-reinforced polymer (FRP) composites. However, the service life and effectiveness of FRP repair and strengthening techniques when applied to concrete in corrosive marine environments is still not well understood. This paper presents the results of an experimental study on the corrosion performance of embedded steel reinforcement in cylindrical reinforced concrete specimens with 13 different surface treatment options. Samples were subjected to an impressed current and a high salinity solution. Test variables included the type of epoxy, wrap fiber orientation, and the number of wrap layers. Samples were evaluated for corrosion activity by monitoring corrosion potentials and impressed current flow levels, and by examining reinforcement mass loss and concrete chloride content among samples. Test results indicated that FRP wrapped specimens had prolonged test life, decreased reinforcement mass loss, and reduced concrete chloride content. The performance of wrapped specimens was superior to that of either control samples or those coated only with epoxy. Epoxy type had a significant effect on the performance of samples regarding their resistance to corrosion. It was concluded that carbon FRP wraps were able to confine concrete, slowing deterioration from cracking and spalling and inhibiting the passage of salt water.     

Effect of Fiber Orientation and Ply Mix on Fiber Reinforced Polymer-Confined Concrete

This paper explores the effects of fiber orientation and ply mix on load–deformation behavior and failure modes of fiber reinforced polymer (FRP) confined concrete by testing under uniaxial compression a designed array of plain concrete cylinders wrapped with different fabric orientation. Depending on the jacket confinement stiffness, either a strain hardening or a strain softening behavior was observed beyond the kink point where there was a sharp reduction in slope in the load–deformation curve. Kinking was seen to have a definable graphical relationship with the critical concrete lateral strain while the kink stress was found to upshift with increasing jacket stiffness. It is concluded that while hoop fiber wrapped concrete leads to brittle failures, angular fiber wrapped concrete tends to fail in a ductile manner, attributed to a fiber reorientation mechanism. Ply mix sequence plays an important role in the overall deformation and failure behavior. Existing models are found to be adequate in describing load–deformation behavior of angular fiber wrapped concrete as long as equivalent FRP properties in the hoop direction are used.     

CFRP Composite Connector for Concrete Members

Steel plate connections are frequently used in tilt-up and precast concrete building construction to tie adjacent wall panels together for shear and overturning effects, and to provide continuous diaphragm chord connections for wind and seismic loading. These welded connectors perform poorly in regions of high seismicity and are vulnerable to corrosion. Until now, retrofit and repair strategies for in-plane shear transfer strengthening were limited to attaching steel sections across panel edges. In the present paper, an experimental program is described that utilizes carbon fiber reinforced plastic (CFRP) composites to develop a viable retrofit scheme for precast concrete shear walls and diaphragms. Nine full-scale precast wall panel assemblies with CFRP composite connectors have been tested. The results show that the CFRP composite connection is an effective solution for the seismic retrofit and repair of precast concrete wall assemblies and other precast concrete elements, such as horizontal diaphragms, that require in-plane shear transfer strengthening.     

Effect of Localized Bending at Through‐Flaws in Pressurized Composite Cylinders

An analytical and experimental investigation was conducted to determine the effect of localized bending at through‐flaws in pressurized composite cylinders. A finite‐difference solution was formulated to determine the stress, strain, and displacement fields in the vicinity of a slit. Tests were conducted on 305‐mm‐diameter cylinders made from graphite/epoxy fabric in a (0,?45)s configuration, with axial slits. Surface‐strain‐field measurements made in the vicinity of the slit showed a significant bending, which verified the finite‐difference solution. This significant bending near the slit tip results in a large magnification of the stresses there. An average stress criterion was employed to predict the failure response of these cylinders based on data obtained from coupon specimens. The finite‐difference solution provided correction factors to account for the localized bending. The prediction utilizing this methodology was excellent in all cases. A generalized methodology to assess damage tolerance of structural configurations with notches is proposed.     

Structural Upgrade Using Basalt Fibers for Concrete Confinement

This paper aims to appraise the opportunities provided by a new class of composites based on using basalt fibers bonded with a cement-based matrix as an innovative strengthening material for confinement of reinforced concrete members. The effectiveness of the proposed technique is assessed by comparing different confinement schemes on concrete cylinders: (1) uniaxial glass-fiber-reinforced polymer (FRP) laminates; (2) alkali-resistant fiberglass grids bonded with a cement-based mortar; (3) bidirectional basalt laminates preimpregnated with epoxy resin or latex and then bonded with a cement-based mortar; and (4) a cement-based mortar jacket. The study showed that confinement based on basalt fibers bonded with a cement-based mortar could be a promising solution to overcome some limitations of epoxy-based FRP laminates.     

Effect of Severe Environmental Exposures on CFRP Wrapped Concrete Columns

Deterioration of concrete structures caused by corrosion of reinforcing steel, aging, and weathering is a major problem in harsh environments such as coastal areas and cold regions. In addition, a hot environment, such as in the Arabian Gulf, is recognized as one of the most severe and aggressive environments that affects concrete durability. The purpose of this study is to investigate the effectiveness of strengthening plain concrete cylinders, subjected to extreme temperature variations, by wrapping with two layers of unidirectional carbon fiber-reinforced polymer (CFRP) sheets. Thirty-six plain concrete cylinders (150×300?mm) were tested. Nine specimens served as unstrengthened controls and the remaining cylinders were strengthened with two layers of CFRP sheets. Cylinders were subjected to high temperatures (45°C), to heating and cooling cycles (23 to 45°C), and to prolonged heat exposure (45°C). Some of the cylinders that were subjected to heating and cooling, were later subjected to freezing and thawing cycles, while others were submerged in fresh water or salt water. The specimens were loaded to failure under uniaxial compressive load and the axial and lateral deformations were monitored. High temperature exposure was not found to decrease the strength of the wrapped concrete cylinders.     

Composite Reinforcement to Strengthen Existing Concrete Structures against Air Blast

The Wright Laboratory Air Base Survivability Section has been studying the development, application, and effects of externally applied composite reinforcing materials. The strengthened facilities would be capable of surviving an air-blast load at relatively short stand-off distances (11–15 m). An agreement was reached with the Israeli officials to conduct full-scale explosive tests in Israel using 860 kg of TNT on structures that had been reinforced externally with composite reinforcing materials. The strengthening procedure employed in this study involved two types of material: (1) an autoclave-cured, three-ply, carbon fiber-epoxy laminate; and (2) a knitted biaxial E-glass fabric. The Air Base Survivability Section applied the composite materials in Israel after the facilities had been constructed using an epoxy adhesive to bond the composite materials to the concrete substrate. This provided a simple, effective, and quick method of retrofitting an existing structure. The free-field and reflected pressures and accelerations on the walls were measured. The results of these tests were considered successful, considering the fact that the externally reinforced walls suffered high displacements, yet did not fail. The pressure and impulse data indicate that both structures would have failed catastrophically without the externally applied composite reinforcing materials.     

Behavior of Cyclically Loaded Squat Reinforced Concrete Bridge Columns Upgraded with Advanced Composite-Material Jackets

This paper summarizes comprehensive experimental studies on scaled models of squat bridge columns repaired and retrofitted with advanced composite-material jackets. In the experimental program, a total of 14 half-scale squat circular and rectangular reinforced concrete columns were tested under fully reversed cyclic shear in a double bending configuration. In order to provide a basis for comparison, a total of three as-built columns were tested. Another 10 column samples were tested after being retrofitted with different composite jacket systems. One circular as-built column was repaired after failure. The repair process involved both crack injection as well as addition of carbon/epoxy composite jacket. The repaired column was then retested and evaluated. Experimental results showed that all as-built columns developed an unstable behavior and failed in brittle shear mode. The common failure mode for all retrofitted samples was due to flexure with significant improvement in the column ductility. The repaired column demonstrated ductility enhancement over the as-built sample.     

Mineral-Based Bonding of Carbon FRP to Strengthen Concrete Structures

The advantages of fiber-reinforced polymer (FRP)-strengthening have been shown time and again during the last decade. Several thousand structures retrofitted with FRPs exist worldwide. There are various reasons why the retrofit is needed, but it is not uncommon for the demands on the structure to change with time, as buildings and civil structures usually have a very long life. The structures may have to eventually carry larger loads or fulfill new standards. In extreme cases, a structure may need repair due to an accident or to errors made during the design or construction phases, and must therefore be strengthened before it can be used. Different methods to retrofit with FRPs also exist, such as bonding of plates or sheets, with their use of epoxy as the bonding agent being the commonality. Epoxy provides very good bond to concrete and is durable and resistant to most environments in the building industry. However, epoxy may also create problems in the working environment, needs a minimum application temperature, and creates diffusion-closed surfaces. These drawbacks can be overcome if the epoxy can be replaced with a cementitious bonding agent. In this paper tests are presented where the epoxy has been replaced with a cement based bonding agent for retrofitting. Pilot tests show that very good composite action can be achieved and that only minor changes in the design procedure need to be taken.     

Long-Term Deflections of Reinforced Concrete Beams Externally Bonded with FRP System

External bonding of fiber-reinforced polymer (FRP) composite laminates to the tension soffit of reinforced concrete beams has become a popular method for flexural strengthening. However, the long-term performance of FRP-bonded beams under service loads is still a concern. This study was therefore aimed at investigating, both analytically and experimentally, the long-term deflection characteristics of FRP-bonded beams under sustained loads. Nine reinforced concrete beams, six of which were externally bonded with glass FRP composite laminates, were subjected to sustained loads for 2 years. The test parameters were the FRP ratio and sustained load level. The long-term deflections of the beams were reduced 23 and 33% with a FRP ratio of 0.64 and 1.92%, respectively. The total beam deflections were accurately predicted by the adjusted effective modulus method, and overestimated by about 20% by the effective modulus method.     

Damage Containment and Residual Strength of Composite Laminates

Damage in composite laminates caused by low‐velocity impact may produce significant reductions in compressive strength. Lockheed conducted an experimental program to investigate the damage‐containment capability of simple composite laminates of two graphite‐epoxy systems: T300/5208 and AS/3501‐6. Four different lay‐ups, including two hybrid laminates, were investigated for each material system. Results are presented for comparison. Laminates with Kevlar layers, especially where the layer is on the surface of the laminate, demonstrate better impact resistance.     

Effect of cement modulus on the shear properties of the bone-cement interface

Shear tests of the bone-cement interface were performed in vitro using two types of bone cement, standard poly(methyl methacrylate) (PMMA) and a reduced-modulus formulation with poly(butyl methacrylate) beads in a methyl methacrylate matrix (PBMMA). Tests of shear properties were also calculated on cancellous bone and on each cement alone. The tests were done using the Iosipescu shear test method which generates a pure shear force in a zero-moment section of the specimen. With this method, shear properties can be determined at specified locations throughout the specimen. Tests were performed across the entire interface region, specifically in the middle of the region of cement bone interdigitation and at both the bone and cement ends of that region. Ultimate shear strengths and shear moduli were calculated. The shear modulus of the PBMMA is less than 3% that of PMMA. The strength and modulus of cancellous bone had a direct relation to the apparent density of the bone, as did the strength and modulus of the bone-composite interface and the composite region. Strength and modulus were dictated by the bone at the bone-composite interface, and by the cement at the cement composite interface. Through the composite region, the stiffer of the two materials in the composite determined the shear properties. Reduced-modulus bone cement substantially decreases the interfacial shear stresses at the bone-cement interface which should decrease the rate of resorptive bone remodelling at this interface.