Flexural Behavior of Preloaded Reinforced Concrete Beams Strengthened with Prestressed Carbon Textile Reinforced Concrete Plates

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

Advances in Construction Materials 2007

With increasing terrorist attacks in the global scene, civilian concrete structures have been targets of weapon threats, especially suffering from explosive and ballistic threats. As concrete structures are vulnerable to these extreme effects, the research on the behavior of concrete structures under blast and impact effects and the corresponding protective measures have gained greater and greater momentum. Fiber Reinforced Polymer (FRP), a composite material, has been used to strengthen concrete structures to improve their blast and impact resistance recently. Since the protection of concrete structures is associated with the extent of caused damage, proper prediction tools are required to provide reasonable assessments on the vulnerability of structures to specific threats. Therefore, the research focuses in this thesis are to develop practical approaches to quantitatively predict local damage induced by near-field blast and projectile impact effects. The author firstly investigates local responses on plain concrete plates and reinforced concrete (RC) plates with externally bonded FRP layers and then proposes damage prediction models for each scenario. The study reviews extensively a number of research works conducted by previous researchers from literature. A near-field blast always produces transient non-uniform pressure with very high magnitude on concrete structures and the damage on targets is localized. To investigate the behavior of concrete plates under near-field blast effects, carrying out experiments and using numerical models are two common approaches. However, there are few of available analytical methods to assess the extent of damage on concrete elements. Hence, the author proposes a new analytical model to predict the size of local damage on concrete plates under contact detonation, which represents the most extreme condition of near-field blasts. The accuracy of the analytical model is verified by test results from references. Through the proposed approach, the local responses including crater, spalling and breaching on concrete plates can be well predicted without need of high economic and computational resource. In consideration of the vulnerability of concrete material to explosion loading, adopting externally bonded FRP layers is applicable in improving the blast resistance of RC structures. To quantitively determine the blast resistance of FRP strengthened RC plates under near-field blast effects, Artificial Neural Network (ANN) techniques are introduced and applied to estimate the size of local damage on targets. ANN methods have been successfully applied in some blast incidents, e.g. prediction of blast wave characteristics and estimation of influence of barrier walls on blast wave propagation. Furthermore, some works by using ANN technique to study RC panels/walls under close-in blasts are enlightening. In this thesis, the author constructs an ANN model based on the data from high-fidelity numerical simulations and this well-trained model can provide reasonable predictions on the local damage of RC plates and FRP layers. To the author’s knowledge, due to the limited number of relevant studies, there is no model available to quantitatively predict the damage on an FRP strengthened RC plate after a near-field blast. Using the results from the ANN model, a damage prediction chart is proposed for fast assessment. Based on the literature review, researchers have used experimental, numerical and semi-analytical approaches to investigate the behavior of plain concrete under projectile impacts. Different empirical models have been proposed to evaluate local effects on concrete targets. Besides, several semi-analytical methods are available to estimate the ballistic behavior of projectiles into concrete plates. However, the emphasis of most studies is on the penetration depth but not the surface damage (quantified by the spalling crater area). Ballistic attack constitutes a potent threat to nearby personnel and equipment and proper protection against projectile and fragments is of great importance. Therefore, to investigate the total quantity of concrete debris produced in one projectile impact, not only the penetration depth but also the surface damage needs to be well considered. To fill the gap, the author conducts a series of projectile impact tests and then develops an empirical model to predict the spalling damage on the impacted face of concrete targets. The RC elements exhibit limited resistance to ballistic effects by virtue of the poor energy absorption capacity of concrete material. It is widely accepted that externally bonded FRP layers is useful to protect targets from striking projectiles. However, there are only a small number of relevant studies on FRP strengthened RC plates. In the thesis, the author focuses on the response at the impacted face and conducts tests on RC plates with front strengthening FRP layers. Based on obtained results, the author modifies the proposed empirical model for plain concrete targets to quantitatively predict the spalling damage on FRP strengthened RC plates. The damage prediction models developed in this thesis can provide proper assessments on local damage size, which possess practicability in protective design.

It is an effective way to increase the flexural performance of reinforced concrete (RC) beam by externally bonding fiber reinforcement polymer (FRP) laminate on the soffit of the beam. However, there is little investigation on flexural behavior of RC beam by side-bonding FRP laminates. To investigate the difference of flexural behavior between soffit-bonding and side-binding FRP laminates RC beams, a total of 9 RC beams were tested, including 8 strengthened beams and 1 control beam. The test results showed that: the first crack load of RC beam strengthened by side-bonding CFRP laminates is much bigger than that of RC beam strengthened by soffit-bonding CFRP laminates.The first crack load of side-bonding CFRP laminates beams improved significantly;side-bonding and soffit-bonding methods have the same effect on the flexural stiffness of RC beams with same quantity of CFRP laminates before tension steel rebar yielding. However, side-bonding can remarkably decrease the crack width and change the crack pattern.Side-bonding of CFRP laminates can extend the pre-crack stage of the strengthened beam;the first yielding load and the ultimate load of beams cannot be improved significantly by side-bonding FRP laminates.

Probabilistic code-based shear capacity model for I-shaped steel reinforced concrete (SRC) beams

Effects of steel fiber addition on mechanical properties of concrete and RC beams

This paper aims to study the effect of Carbon Fiber Reinforced Polymer (CFRP) strengthening techniques on the fatigue limit of Reinforced Concrete (RC) beams. An accelerated fatigue method was utilized to find the fatigue limit based on the hypothesis of linear cumulative damage as determined by the Palmgren-Miner rule. Six RC beams with dimensions of 152.4 × 152.4 × 1,520 mm were tested under monotonic and cyclic loading using a four-point bending configuration: two non-strengthened RC beams, two RC beams strengthened with Near Surface Mounted (NSM) CFRP rods, and two RC beams strengthened with Externally Bonded (EB) CFRP sheets. Strengthened RC beams had less stiffness degradation and more energy dissipation when compared to the non-strengthened RC beam. The NSM CFRP technique demonstrated a better monotonic flexural strength than the EB CFRP technique. NSM CFRP rods and EB CFRP sheets increase the fatigue limit when compared to the non-strengthened RC beam by 19% and 33%, respectively.

Strengthening using prestressed CFRP plate is a new developed technology in recent years, which can significantly improve the flexural behavior of reinforced concrete (RC) beams. In this paper, an experimental study including four RC beams stengthened with CFRP plates using self-developed anchorage system was carried out. The nonlinear finite element models of the strengthened beams were constructed to simulate the influence of flexural behavior on RC beams strengthened with CFRP plates under different damage degrees. The experimental results indicated that initial damage degrees have greatly influence on the crack distribution and ductility, deflection and the ultimate flexural strength of the strengthened beams, as well as steel reinforcement strain and CFRP plate strain. Meanwhile, the finite element model can predict accurately performances of strengthened RC beams before CFRP debonding. In addition, the numerical analysis indicated that the sustained loading while strengthening RC beam using prestressed CFRP plates would induce adverse effect, so the live loading should be removed before starting the strengthening works.

Component damage functions for reinforced concrete frame structures

Externally bonding fiber reinforcement polymer (FRP)laminate to the soffit of reinforced concrete (RC) beam is an effective way to increase its flexural strength. However, there is little investigation on flexural behavior of RC beam by side-bonding FRP laminates.To investigate the difference of flexural behavior between soffit-bonding and side-binding FRP laminates RC beams,a total of 9RC beams were tested, including 8 strengthened beams and 1 control beam. The test results showedthat:1) As fiber reinforced concrete, side-bonding FRP laminates can also effectively increase the first crack strength of RC beams.The first crack load improved significantly; 2) side-bonding FRP laminates is not a good method to improve the flexural behavior of RC beam at the yield stage and the post yield stage. 3) Side-bonding of CFRP laminates is a good way for existing RC beams to control its crack width and deflection.

This thesis describes the tests of four full-size hollow reinforced concrete (RC) beams subjected to pure torsion. The designed width and depth of the beam specimen is 50 cm 75 cm or 71.1 cm. The length of the beam testing zone is 3.5 m; the longitudinal length of entire specimen is 5.76 m. The designed compressive strengths of concrete are 45, 70, 100 MPa, and the wall thicknesses of the hollow cross section are 12 cm, 8.5 cm, 15.5 cm, and 8.7 cm. Most previous torsion tests of RC beams emphasized large (ultimate) torque and large twist angle, and the deviation in small-twist measurement were appreciable. Since the SMMT theory can now analyze the ranges before and after cracking and the cracking torque and twist can provide linear torsional stiffness for the members, this study aimed at developing a new test setup and method that can achieve precise small-twist measurement. Most previous torsion tests of RC beams used steel members to clamp the RC specimen and exert the applied torque. Such setups, however, often had inevitable gaps and slips between the clamping steel members and the RC specimens, introducing significant errors for small twist measurement. To overcome this handicap, this experiment uses a monolithic transverse RC beam to convey the applied torque at the twisting end of the specimen, and designs a monolithic RC block at the fixed end. The RC block of a specimen is also fastened to the underneath steel seat and strong floor using prestressed steel rods. To effectively transmit the applied torque, a novel curved device is used between the upper and lower unit of the twisting end, and a new force transmission setting is added to the force delivering members linked to the actuator. The torque, twist angle, shear flow zone thickness, steel and concrete surface strains, and longitudinal elongation of the specimens are continuously measured and recorded in the tests. This thesis describes the test specimens construction, test equipments, measurement facilities, installation and test procedure, and presents the experimental results of four specimens. Keywords: reinforced concrete, hollow section, torsion, experiment, beam, torque, twist angle

Horizontal openings in reinforced concrete (RC) beams are quite often used to accommodate service pipelines. Several research papers are available in the literature describing their effect. RC beams with vertical openings are commonly used to accommodate service lines in residential buildings in Kuwait. However, there are lack of design guidelines and best practices reported in the literature for RC beams with vertical openings, whereas the detailed guidelines are available for beams with horizontal openings. In the present paper, laboratory experiments are conducted on nine RC beams with and without vertical openings. Parametric study has been carried out using nonlinear finite element analysis (FEA) with changes in the diameter of the opening, various positions of the opening along the length and width of the beam, edge distance, etc. 50 finite element simulations were conducted. The FEA results are verified using the results from the laboratory experiments. The study showed that the load carrying capacity of the beam is reduced by 20% for the RC beam with vertical openings placed near the center of the beam compared to a solid beam without an opening. Significant reduction in load carrying capacity is observed for beams with an opening near the support (≈15%). The overall stiffness of the beam, crack pattern and failure modes were not affected due to the presence of the vertical opening. Furthermore, an artificial neural network (ANN) analysis is carried out using the FEA generated data. The results and observations from the ANN and FEA are in good agreement with experimental results.

Many researchers worldwide have extensively used fibre-reinforced polymer (FRP) strengthening materials to enhance the shear and flexural strengths of reinforced concrete (RC) beams. However, Studies on strengthening of RC beam subjected to combined torsion and bending moment using both spiral and vertical strip configuration of CFRP that explored in this study is rare. This study aims to demonstrate the behaviour of RC beams strengthened with FRP sheets (strips) with different configurations and subjected to combined actions of torsion and bending moment. Eight beams with a dimension of 15 × 25 × 200 cm were cast. One of the beams was not strengthened, but the others were strengthened with carbon FRP. The angle of twist at torque intervals, first cracking torque, ultimate torque and ultimate twist angle of the conventional and strengthened beams during the testing process were compared. Results showed a significant improvement in the torsional performance of RC beams using carbon FRP. The fully wrapped beams performed better than the beams with strip wrapping due to the influence of various wrapping configurations. Amongst the wrapping configurations of FRP fabrics, the 45° spiral strip wrapping configuration was the most effective for RC beam strengthening in terms of torsion resistance. Highlights Reinforced concrete (RC) beams strengthened with fiber reinforced polymer composite were tested under combined bending and torsional moment; The effect of composite orientation, spacing and number of plies on the torsional response; Ultimate torsional moments of RC beams; Twist angle of rotation of control and strengthened beams; Analytical prediction for CFRP material contributions to the ultimate torsional moment of strengthened RC beam.

During the service-life of Reinforced Concrete (RC) bridges, the deflections and the natural frequencies of the bridge change. It was considered that damage in a RC bridge results in changes in the deflections and in the natural frequencies of the bridge. Since these changes could be obtained by a Structural Health Monitoring system, the popularity of such system is increasing. It is, however, uncertain how the deflections and the natural frequencies of RC bridges change during their total service-life. It is also uncertain how damage in a RC bridge changes the deflections and the natural frequencies of the bridge. To obtain information about changes in the deflections of a RC beam during its total service-life, laboratory tests have been conducted. 24 RC beams have been loaded dynamically in a four-point-bending configuration. The deflections of the RC beams have been measured from the first cycle until failure. To investigate the impact of corrosion in the reinforcing bar on the fatigue failure mechanism of the RC beam, corrosion was simulated in the reinforcing bar of 12 of the 24 RC beams. The reinforcing bars of the other 12 RC beams were uncorroded. The setup of the tests and the deflections of the RC beams with and without corrosion in the reinforcing bar are presented in this paper.

This paper presented a laboratory investigation for analyzing the natural frequency response of reinforced concrete (RC) beams affected by steel corrosion. The electrochemical acceleration technique induced the corroded RC beams until the predetermined value of the steel corrosion ratio was achieved. Then, the natural frequency responses of the corroded beams were tested utilizing piezoelectric acceleration sensors. The damage states of the corroded beams were assessed through the measurement of crack parameters and the equivalent elastic modulus of the beams, which aims to clarify the fundamental characteristics of the dynamic response for the corroded RC beam with the increased steel corrosion ratio. The results revealed that steel corrosion reduces the bending stiffness of the RC beams and, thus, reduces the modal frequency. The variation of natural frequency can identify the corrosion damage even if no surface cracking of the RC beam, and the second-order frequency should be more indicative of the damage scenario. The degradations of stiffness and the natural frequency were estimated in this study by the free vibration equation of a simply supported beam, and a prediction method for the RC beam’s residual service life was established. This study supports the use of variations in natural frequency as one diagnostic indicator to evaluate the health of RC bridge structures.