Lijuan Cheng

Bond Characteristics of Various NSM FRP Reinforcements in Concrete

The rehabilitation and strengthening technique using near-surface mounted (NSM) fiber-reinforced polymer (FRP) has proved to be a reliable alternative to the existing methods for deficient concrete structures. To effectively apply this method, bond characteristics at the interface need to be fully understood. This paper presents the results of the bond performance of a wide range of embedded FRP NSM reinforcements with various surface configurations, cross-sectional shapes, material types, adhesive types, and groove sizes. A total of 109 concrete specimens (with a dimension of 350×300×150 mm) were tested under direct pull-out loading configuration. It was found that both specimen sets using NSM bars treated with spirally wound and sand coating with indentation and groove displayed the best performance. Strip shape was more effective than round and square cross-sectional shapes. Higher bond strength was obtained by using carbon FRP than glass and adhesives with higher material bond strength. Increasing the groove sizes was fairly effective in improving the bond capacity. The existing bond strength models were also assessed in comparison with the test data in this paper.

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.

Impact Response of Externally Strengthened Unreinforced Masonry Walls Using CFRP

Research reported herein investigates the out-of-plane impact resistance of unreinforced masonry (URM) walls strengthened with carbon fiber-reinforced polymer (CFRP) composites, externally applied in sheets to one face of the wall. Two analytical methods based on energy principle and wave propagation theory and a finite-element-based numerical model have been developed, assuming a perfect bond at composite–masonry interface with an equivalent stiffness of the system. Full-scale impact tests are conducted for verification purpose, where three 1.2 m tall URM concrete walls (one unstrengthened and two strengthened with continuous unidirectional and woven CFRP sheets) are vertically tested up to cracking using a pendulum drop-weight impact tester. The test results compare reasonably well with those obtained from the analyses and simulation. It is found that the energy and finite-element methods can provide reasonable estimates for peak impact force and wall deflection, whereas the wave propagation method is r...

Fatigue Bond Characteristics and Degradation of Near-Surface Mounted CFRP Rods and Strips in Concrete

AbstractThis study investigates the bond characteristics and degradation under fatigue loading of near-surface mounted (NSM) carbon-fiber-reinforced polymer (CFRP) rods and strips embedded in concrete. Thirty-six concrete block specimens (350×300×150  mm) were tested under single shear. Sinusoidal waveform fatigue loading with a range of 10–50% of the corresponding static bond strength was applied with the number of cycles varying from 0 to 10,000, which was then followed by static pullout test to determine the local bond-slip relationship between the NSM reinforcement and concrete. Two separate sets of specimens were also subjected to the same fatigue loading up to failure to obtain the corresponding fatigue life. It was found that specimens strengthened with NSM rods mostly failed due to the breakage of concrete and epoxy, while those with NSM strips all failed due to slippage of NSM reinforcements at a lower pullout strength after the fatigue cycles. For both types of reinforcement, as the number of fa...

Unreinforced Concrete Masonry Walls Strengthened with CFRP Sheets and Strips under Pendulum Impact

Impact tests using drop-weight pendulum on nine 1.2-m-high full-scale concrete masonry block walls were conducted to investigate the out-of-plane impact behavior of unreinforced masonry (URM) walls externally strengthened with carbon-fiber-reinforced polymer (CFRP) composites. Three strengthening schemes on one side of the wall were studied: continuous unidirectional and continuous woven sheets, discrete strips in a vertical pattern, and discrete strips in orthogonal and diagonal patterns. All walls were vertically positioned resting on a knife-edge support with one face leaning against two steel rollers close to the upper and lower edges of the wall. The impact load was applied at the wall center through a drop-weight pendulum impact tester with various drop heights. Test results revealed that using composite laminates or strips could significantly improve the impact performance of URM walls. The wall strengthened with continuous woven sheets performed better than the one with unidirectional sheet. With ...

Fatigue Behavior and Prediction of NSM CFRP-Strengthened Reinforced Concrete Beams

AbstractIn this study, a total of 16 reinforced concrete beams (dimensions of 203×305×3,861  mm) strengthened with near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods and strips were tested under four-point bending fatigue loading. Monotonic loading was applied to their corresponding control specimens to obtain the static monotonic flexural strength. Fatigue loading with a range of 10% of the corresponding static strength to various upper limits was applied to determine the empirical relationship for each type of specimen. It was found that the dominating failure mode under fatigue was rebar rupture. Mixed failure modes of rebar rupture and unstable cracks in concrete were also observed in specimens with longer fatigue life. As the specimens underwent more fatigue cycles, an increase was observed in their tensile strains of CFRP and deflections due to accumulated damages and the losses of stiffness. A fatigue bond model was developed, which simulated the process of fatigue loading as fa...

Cohesive Model-Based Approach for Fatigue Life Prediction of Reinforced-Concrete Structures Strengthened with NSM FRP

This paper presents a fatigue-life prediction model of reinforced concrete beams strengthened with near-surface mounted (NSM) fiber-reinforced polymer (FRP) composite rods under flexure. This model is based on the fracture-mechanics approach using a cohesive model, and accounts for different bridging properties in steel and FRP reinforcement in strengthened concrete structures. It predicts the number of fatigue cycles to ultimate failure in the strengthened members, in which the failure is defined by unstable propagation of an effective crack in concrete. Several sets of experimental results in the literature are selected to verify the efficiency and precision of this model, in which the writers compare typical stress versus fatigue-life curves. This model is capable of predicting the fatigue life of NSM-strengthened concrete beams to a good precision and relates the fatigue life to the structural stress in an explicit manner.

Steel-Free Hybrid FRP Stiffened Panel-Concrete Deck System

Synopsis: This paper presents a design, analysis, and characterization of a hybrid deck system incorporating a thin Fiber Reinforced Polymer (FRP) stiffened hat-panel configured stay-in-place formwork that serves as flexural reinforcement with steel-free concrete poured on top. Quasi-static tests were conducted to first investigate the flexural behavior of the system. To understand the deck performance under traffic loads that induce repetitive stress cycles during the service life, a two-span continuous deck specimen (1.22 m wide) was tested by subjecting it to a total of 2.36 million cycles of load that simulates an AASHTO design truck with inclusion of the impact factor at both low and high magnitudes. The concrete-panel interfacial response due to the presence of sand and interlocking ribs was characterized by performing a series of 610 mm wide deck section tests, the results of which were used to calibrate a finiteelement (FE) based analytical model. The effect of the shear span-to-depth ratio, carbon fiber reinforcement ratio, and rib spacing were then evaluated by performing a parametric study using the calibrated nonlinear FE model. A simplified design approach is also proposed.

Novel Type of Temperature Self-Compensating Acceleration Transducer Based on OFBG

Recently, optical fiber bragg grating (OFBG) sensors with different functions have been developed and applied in Structure Health Monitoring. This letter introduces a noval acceleration transducer with a characteristic of temperature self-compensating. It is a cantilever structure model with an equal strength beam; two OFBGs are bonded on both the upper and lower surface of the beam. According to the temperature self-compensating principle and acceleration measurement principle shown in this letter, the temperature self-compensating function can be achieved by simple calculations. This type of acceleration transducer has high sensitivity and its measurement range can be easily changed according to the practical requirements.

Steel-Free Bridge Decks Reinforced with FRP Composites

Successful utilization of advanced polymer composite materials in civil infrastructures have long been recognized in bridge deck applications. The concept of ‘steel-free’ in concrete bridge decks can be achieved by replacing the conventional steel reinforcement with fiber reinforced polymer (FRP) based reinforcing materials. Reviewed in this chapter are steel-free bridge decks reinforced with these composite reinforcement in forms of round or square rods, 2-D or 3-D grids or gratings, flat or curved FRP plate, sandwich panel with foam/balsa/steel inserts, stiffened or corrugated stay-in-place formwork, and polymer decks.