Nabil F. Grace

Strengthening Reinforced Concrete Beams Using Fiber Reinforced Polymer (FRP) Laminates

The behavior of reinforced concrete beams strengthened with various types of fiber reinforced polymer (FRP) laminates is presented. The experimental program included strengthening and testing 24 simply supported rectangular cross section beams. Each beam was initially loaded above its cracking load. The cracked beams were strengthened with FRP laminates and then tested until complete failure. Five available strengthening systems of various types of carbon/glass fiber reinforced polymer (CFRP/GFRP) strengthening materials were used. These materials included two types of CFRP sheets, bi- and unidirectional GFRP sheets, and CFRP plates. The effects of strengthening on deflection, failure load and failure mode, strain, and beam ductility are discussed. In addition, the influence of different numbers of FRP layers, type of epoxy, and strengthening pattern on the behavior of beams was examined. The ratio of absorbed energy at failure to total energy, or energy ratio, was used as a measure of beam ductility. It is concluded that, in addition to the longitudinal layers, the fibers oriented in the vertical direction forming a U-shape around the beam cross section significantly reduce beam deflections and increase beam load carrying capacity. Furthermore, the presence of vertical and horizontal sheets, together with a proper epoxy, can lead to a doubling of the ultimate load carrying capacity of the beam. However, all the strengthened beams experienced brittle failure, mandating a higher factor of safety in design.

Durability evaluation of carbon fiber-reinforced polymer strengthened concrete beams: Experimental study and design

Carbon fiber-reinforced polymer (CFRP) materials are being used for the retrofitting and repair of deficient and old infrastructures such as bridges and buildings. Over the years, these structures tend to suffer severe strength and stiffness deterioration due to aggressive environmental conditions such as humidity, saltwater, freeze-thaw, thermal expansion, dry-heat, repeated load cycles, and alkali solutions. The authors of this study present the deflections, strains, failure loads, and failure modes of strengthened beams exposed to different independent environmental conditions and repeated load cycles. The authors propose strength reduction factors associated with these various independent environmental conditions. The authors conclude that the long-term exposure to humidity is the most detrimental factor to the bond strength between CFRP plates and fabrics and reinforced concrete (RC) beams. Beams strengthened with CFRP plates and exposed to 10,000 hours of 100% humidity experienced an average of 33% reduction in their strength. RC beams strengthened with CFRP plates are more susceptible to damage than the beams strengthened with CFRP fabrics. There is no significant effect, however, of repeated load cycles on the ultimate loads of beams strengthened with CFRP plates or fabrics for at least 2 million test cycles. Delamination was the primary mode of failure for all of the test beams, with and without exposure to environmental conditions and repeated load cycles. The authors conclude by presenting a durability-based design approach that demonstrates the evaluation of nominal and design moment strengths of strengthened beams along with failure modes.

STRENGTHENING OF CONCRETE BEAMS USING INNOVATIVE DUCTILE FIBER-REINFORCED POLYMER FABRIC

An innovative, uniaxial ductile fiber-reinforced polymer (FRP) fabric for strengthening structures is described. The fabric is a hybrid of 2 types of carbon fibers and 1 type of glass fiber, and has been designed to provide a pseudo-ductile behavior with a low yield-equivalent strain value in tension. The effectiveness and ductility of the developed fabric was studied by strengthening and testing 8 concrete beams under flexural load. Similar beams strengthened with currently available uniaxial carbon fiber sheets, fabrics, and plates were also tested to compare their behavior with those strengthened with the developed fabric. This fabric has been designed so that it has the potential to yield simultaneously with the steel reinforcement of strengthened beams and hence, a ductile plateau similar to that of nonstrengthened beams can be achieved. The beams strengthened with the newly developed fabric exhibited higher yield loads and achieved higher ductility indices than those strengthened with the currently available carbon fiber strengthening systems. The fabric shows a more effective contribution to the strengthening mechanism.

FLEXURAL AND SHEAR STRENGTHENING OF CONCRETE BEAMS USING NEW TRIAXIALLY BRAIDED DUCTILE FABRIC

This paper studies the effectiveness of a new ductile fabric, containing bundles of fibers triaxially braided in 3 directions, that was designed specifically to be used for strengthening concrete beams. The fabric has a yield-equivalent strain value in the 0-deg direction close to the yield strain value of steel. Thus, it has the potential to contribute significantly to the beam load before the yielding of the steel reinforcement of the strengthened beam without further sacrificing much of its ductility. 12 reinforced concrete beams were strengthened in flexure or shear using the new fabric. Similar beams were strengthened with a carbon fiber sheet and 1 with a steel plate to compare their behavior with those strengthened with the new fabric. The beams were loaded in 4-point bending until failure. Beams strengthened in flexure with the new fabric produced greater ductility than those strengthened with carbon fiber sheets. The new fabric produced yield plateaus similar to those of the unstrengthened beams and also to that experienced by the steel plate. Test results of the beams strengthened in shear demonstrated that the fabric stretched before beam failure to strain values that were very close to its yield-equivalent strain value, confirming that the fabric strength was fully exploited.

Life-Cycle Cost Analysis of Alternative Reinforcement Materials for Bridge Superstructures Considering Cost and Maintenance Uncertainties

A life-cycle cost analysis (LCCA) was conducted on prestressed concrete bridge superstructures using carbon fiber reinforced polymer (CFRP) bars and strands. Traditional reinforcement materials of uncoated steel with cathodic protection and epoxy-coated steel were also considered for comparison. A series of deterministic LCCAs were first conducted to identify a range of expected cost outcomes for different bridge spans and traffic volumes. Then, a probabilistic LCCA was conducted on selected structures that included activity timing and cost random variables. It was found that although more expensive initially, the use of CFRP reinforcement has the potential to achieve significant reductions in life-cycle cost, having a 95% probability to be the least expensive alternative beginning at year 23–77 after initial construction, depending on the bridge case considered. In terms of life-cycle cost, the most effective use of CFRP reinforcement was found to be for an AASHTO beam bridge in a high traffic volume area.

Design Approach for Carbon Fiber-Reinforced Polymer Prestressed Concrete Bridge Beams

This paper presents a design approach for carbon fiber-reinforced polymer (CFRP) concrete bridge beams prestressed using bonded pretensioning and unbonded posttensioning tendons arranged in multiple vertically distributed layers along with nonprestressing CFRP rods. Design equations to determine flexural capacity and to compute the stresses and strains in concrete and tendons are provided. Based on parabolic stress-strain relation for concrete and linear stress-strain relation for tendons, a computer program was also developed to compute the overall response of the beam. The design equations and accuracy of the nonlinear computer program were validated by comparing the analytical results with experimental results from a full-scale double-T (DT) test beam. The difference in the analytical and experimental values of the ultimate moment capacity of the DT-test beam is negligible, whereas the corresponding difference in the ultimate forces in unbonded externally draped posttensioning strands is approximately 4.1%. A detailed parametric study was conducted to examine the effect of the reinforcement ratio and the level of prestressing forces on the deflections and ultimate load-carrying capacity of the full-scale DT-beam. Other findings are given.

STRENGTHENING OF NEGATIVE MOMENT REGION OF REINFORCED CONCRETE BEAMS USING CARBON-FIBER-REINFORCED POLYMER STRIPS

The aim of this research was the experimental evaluation of the performance of carbon-fiber-reinforced polymer (CFRP) strips used for flexural strengthening (FS) in the negative moment region of a full-scale reinforced concrete beam. FS was considered for 2 categories of beams: Category I beams were designed to fail in shear tests; Category II beams were designed to fail in flexure. Five full-scale concrete beams in each category were constructed, instrumented, and tested. The deflection, strain, and failure mode responses were studied. It was noted that Category I beams failed by diagonal cracking with local debonding at the top of the beams, and Category II beams failed by delamination onset at the interface of the CFRP strips and the concrete surface, with and without concrete-cover failure. It was also noted that the CFRP strips were not stressed to their maximum capacity when the beams failed, which led to ductile failures of all the beams.

Strengthening of Cantilever and Continuous Beams Using New Triaxially Braided Ductile Fabric

Effectiveness of a new triaxially braided ductile fiber reinforced polymer (FRP) fabric for flexural strengthening of cantilever and continuous reinforced concrete beams is examined. The behavior of beams strengthened with the new fabric was investigated and compared with the behavior of similar beams strengthened with a commercially available carbon fiber sheet. The responses of the beams were studied in terms of deflections, strains, and failure modes. The beams strengthened with the new fabric showed greater ductility than those strengthened with the carbon fiber sheet. The new fabric provided reasonable ductility due to the formation of the plastic hinges that allowed for the redistribution of the moment between the positive and negative moment zones of the strengthened continuous beam. Redistribution of the moment enabled the full use of the strength of the beam at cross sections of maximum positive and negative bending moments.

Carbon Fiber-Reinforced Polymer Strand Application on Cable-Stayed Bridge, Penobscot Narrows, Maine

Maines first cable-stayed bridge opened to traffic on December 30, 2006. Designed as an emergency replacement for the Waldo–Hancock Bridge, the new bridge uses an innovative cradle system to carry the stays from the bridge deck through the pylon and back to the bridge deck. Each strand is anchored independently; thus, strands may be removed, inspected, and replaced while the bridge carries traffic. This advantage, coupled with Maine Department of Transportation concerns for the premature loss of the Waldo–Hancock Bridge due to corrosion, created interest in an opportunity to install and monitor representative carbon fiber-reinforced polymer (CFRP) strands in the cable stays of this bridge. CFRP strands were installed for the purposes of assessing their performance in a service condition and evaluating them for use on future bridges. Three representative stays in the bridge were designed to include two reference strands each, which may be removed and not replaced without change to the bridges structural integrity. Six epoxy-coated steel strands were removed and successfully replaced with CFRP strands in June 2007. Data are being collected from the monitoring equipment installed on all of the strands (both traditional steel and CFRP strands) in the bridge to evaluate CFRP strand performance for future bridge cable stay and post-tensioning installations. The bridge location ensures that the test strands will be evaluated under a wide range of temperatures and variety of wind loads.

Use of CFRP/CFCC Reinforcement in Prestressed Concrete Box-Beam Bridges

This paper presents the results of an experimental investigation on the flexural response of two identical box-beam bridge models reinforced and prestressed with different types of carbon fiber reinforced polymer (CFRP) tendons/strands. The first box-beam bridge model BBD-I was reinforced and prestressed using CFRP-DCI tendons, while the second bridge model BBC-I was reinforced and prestressed using carbon fiber composite cable (CFCC) strands. Each box-beam bridge model consisted of two precast prestressed box beams placed adjacent to each other and a CFRP/CFCC reinforced deck slab. The two box beams of each bridge model were prestressed using seven pretensioning tendons/ strands and consisted of four transverse diaphragms for transverse post-tensioning. Each box-beam bridge model was also prestressed using 12 longitudinal and four transverse unbonded post-tensioning tendons/strands. The changes in pretensioning forces during and after casting the box beams, ultimate loads and modes of failure, deflections, post-tensioning forces, strains, and energy ratios of both bridge models are presented in this paper. The average values of the measured transfer lengths of 9.5 mm (0.37 in.) diameter CFRP-DCI tendons and 12.5 mm (0.49 in.) diameter CFCC strands were measured as 27.4 times and 22.4 times the nominal strand diameter, respectively. As expected, both bridge models experienced similar failure modes, that is, the failure was initiated by the crushing of concrete in the compression zone followed by the immediate rupture of prestressing tendons/strands. Moreover, it was observed that the ultimate strength of Bridge Model BBD-I was higher and the energy ratio for the same was lower than that of Bridge Model BBC-I. However, the longitudinal unbonded post-tensioning tendons/strands of the two bridge models remained intact even after the complete collapse of the bridge models. In general, both Bridge Models BBD-I and BBC-I experienced identical flexural behavior, especially the cracking load, mode of failure, and variation in post-tensioning forces.