Concrete Flexural Members Research Articles

Mechanical‐performance deterioration of RC segment of shield tunnel under high corrosion degree and the transformation of eccentric‐failure mode

AbstractReinforcement corrosion in shield tunnel segments has an important impact on the structure's bearing capacity and failure mode. Generally, a shield tunnel structure is under small eccentric compression, but if the reinforcement corrosion develops sufficiently, then a transition occurs from small to large eccentric load state, thereby lowering the structural bearing capacity and endangering the safety of tunnel operation. In the study reported here, based on the compressive bending load characteristics of the shield lining structure, corrosion test columns with small eccentric loading were designed to simulate the actual force state of the tunnel lining, and the whole evolution process of their mid‐span strain, ultimate bearing capacity, and cracks under different corrosion degrees was analyzed. With increasing corrosion, the bearing capacity, concrete strain, and depth of the compressive zone of a test column decrease more and more rapidly, and at the maximum corrosion degree of 68.32%, the maximum reduction of bearing capacity is 53.23%. The tests show that the failure mode of a test column with high corrosion degree (≥46.25%) is transformed from small to large eccentric failure. Based on theoretical analysis of the normal section bearing capacity of reinforced concrete flexural members, a theoretical method is proposed for calculating the critical corrosion degree for the tunnel lining structure to transform from small to large eccentric failure. The theoretical calculation method is verified against finite‐element calculations, and the present research results offer theoretical support for the durability design and safety evaluation of shield tunnel lining structures.

Evaluation of the Flexural Behavior of One-Way Slabs by the Amount of Carbon Grid Manufactured by Adhesive Bonding

Fiber-reinforced polymers (FRPs), which are resistant to corrosion, are used as reinforcement material for concrete. However, the flexural behavior of concrete members reinforced with FRPs can vary depending on the properties of FRPs. In this study, the flexural behavior of one-way concrete slab specimens reinforced with a new grid-type carbon-fiber-reinforced polymer (CFRP) (carbon grid) manufactured by bonding pultruded CFRP strands to an adhesive was investigated. The experimental results indicated differences in the load–deflection relationships of the specimens depending on the carbon grid reinforcement amount. Specimens in which the carbon grids were over-reinforced or reinforced close to the balanced reinforcement ratio reached the maximum load due to concrete crushing and exhibited ductile failure. The specimen under-reinforced with the carbon grid exhibited brittle failure. Specimens with carbon grid reinforcement close to a balanced reinforcement ratio exhibited maximum loads ranging from 0.43 to 0.61 times the calculated flexural strength, which resulted in becoming 0.86–1.00 lower in the specimens with a wider width of the CFRP strands. This study proposes coefficients to estimate the stiffness of carbon-grid-reinforced concrete flexural members after cracking. Applying these coefficients resulted in stiffness calculations that reasonably simulated the behavior of the specimens reinforced with carbon grids after crack formation.

Experimental and analytical investigation of T-beams retrofitted through the external post-tensioning using steel and various FRP strands

This study aimed to scrutinize the potential benefits of using fiber-reinforced polymer (FRP) materials as unbonded strands in post-tensioning systems for reinforced concrete (RC) flexural members. The research meticulously evaluated the influence of these FRP materials on global and local response levels. To this end, four-point loading tests were performed on T-beam specimens, and the experimental results were compared with numerical predictions. The study also incorporated insights from previous research on external post-tensioning variables. The results of this study, with their practical implications, demonstrate a strengthening technique that significantly enhances the load-carrying capacity at ultimate and yielding points, as well as the crack initiation load of non-strengthened specimens. The strengthening technique also significantly influences deflection during yielding and maximum loads and plays a crucial role in restoring deflection after unloading. However, it is essential to note that strengthening with external post-tensioning does reduce the member’s ductility. The choice of materials also emerges as a significant factor that leads to noticeable differences in crack widths and distribution among the specimens. These practical findings hold considerable importance and offer valuable insights into the potential use of FRP materials in post-tensioning systems for RC flexural members.

THE FLEXURAL CAPACITY OF COMPOSITE BEAMS WITH EXTERNAL AND SIDE-MOUNTED PLATES

In the case of increased loading on an existing reinforced concrete structure, an effective method to strengthen the flexural members is to adhesively bond steel plates to the tension surface of the reinforced concrete flexural member, creating a composite flexural member. These plates can be bonded to various positions on the 2soffit, or side-mounted (SM) vertically to the reinforced concrete flexural members. This research study verified the mathematical model which predicts the flexural capacity of composite beams by comparing it to the results of the experimentally tested specimens. The results are within 1% for the EB and 3% for the SM composite flexural members. The experimental results indicate an increase of 63% in the flexural resistance of the EB and 53% in the SM composite members compared to non-strengthened reinforced concrete members. The experimental results also indicate that the flexural resistance of the EB member is 6% higher than that of the SM member.

THE FLEXURAL CAPACITY OF COMPOSITE BEAMS WITH EXTERNAL AND SIDE-MOUNTED PLATES

In the case of increased loading on an existing reinforced concrete structure, an effective method to strengthen the flexural members is to adhesively bond steel plates to the tension surface of the reinforced concrete flexural member, creating a composite flexural member. These plates can be bonded to various positions on the 2soffit, or side-mounted (SM) vertically to the reinforced concrete flexural members. This research study verified the mathematical model which predicts the flexural capacity of composite beams by comparing it to the results of the experimentally tested specimens. The results are within 1% for the EB and 3% for the SM composite flexural members. The experimental results indicate an increase of 63% in the flexural resistance of the EB and 53% in the SM composite members compared to non-strengthened reinforced concrete members. The experimental results also indicate that the flexural resistance of the EB member is 6% higher than that of the SM member.

Rehabilitation of fire-affected reinforced concrete flexural members utilizing CFRP and GFRP

Rehabilitation of fire-affected reinforced concrete flexural members utilizing CFRP and GFRP

Cyclic Behavior of Reinforced Concrete Flexural Members to Changing Design Parameters

Cyclic Behavior of Reinforced Concrete Flexural Members to Changing Design Parameters

Effect of Rebar Harsh Storage Conditions on the Flexural Behavior of Glass FRP Concrete

Nowadays, fiber-reinforced polymer (FRP) has become a widely accepted alternative reinforcement to steel bars in concrete members due to its many sustainability traits, as represented by its high strength-to-weight ratio, corrosion resistance, non-conductive properties, and electromagnet neutrality. However, FRP bar exposure for an extended period of time to harsh environmental conditions and chemicals can have an adverse effect on their mechanical properties. In this investigation, glass FRP bars were exposed to indoor controlled temperature, outdoor direct sunlight, outdoor shade, seawater, and alkaline solution for six months prior to using them as reinforcement in concrete flexural members. This research involves the fabrication and testing of five pairs of 3 m-long concrete beams with 200 mm by 300 mm cross-sections embedded in the tension zone with the exposed GFRP bars. The 10 beams were instrumented with strain gauges and tested following a four-point loading scheme using a hydraulic jack attached to a rigid steel frame. Crack width records from the tests showed the inferior serviceability of the beams that contained rebars stored in an outdoor environment relative to the control beams. GFRP bar exposure to an alkaline solution or outdoor direct sunlight slightly affected the cracking and ultimate moment capacities, reducing them by 5% and 3% in terms of the same parameters as the controlled indoor exposure, respectively. The influence of GFRP bar exposure to open-air shade or sunlight decreased the pre-cracking stiffness by 25% and flexural ductility by 10–20% when compared with the control specimens. The predicted ultimate flexural strength using the ACI 440 provisions gave comparable results to the experimentally obtained values. A simple mathematical equation that envelops the moment–deflection relationship for GFRP over-reinforced concrete beams and only requires information about initial cracking and ultimate flexural conditions is proposed.

Effectiveness of Insulation Layers for Fire Protection of FRP Reinforced Concrete Flexural Members

Effectiveness of Insulation Layers for Fire Protection of FRP Reinforced Concrete Flexural Members

A full-range fatigue life prediction model for RC beams strengthened with prestressed CFRP plates accounting for the impact of FRP debonding

To predict the fatigue life of a reinforced concrete (RC) flexural member strengthened with externally bonded prestressed Fiber Reinforced Polymer (FRP) laminate accurately, the interaction between components with different degrees of degradation must be carefully taken into account. Furthermore, as the debonding of FRP plates is a recurrent problem, this has to be incorporated into the prediction model. In this study, a nonlinear analysis method for the entire process of fatigue damage in RC beams strengthened with prestressed Carbon Fiber Reinforced Polymer (CFRP) plates was proposed. In this model, all the components of the strengthened beam had independent fatigue degradation properties and failure criteria, and debonding growth was also explicitly considered. The comparison between the analytical and experimental results were used for model validation and good agreement was found.

Cyclic bond behavior in reinforced concrete flexural members exposed to elevated temperatures

After enduring fire incidents, many reinforced concrete (RC) structures require assessment of their ability to withstand future seismic loads. A critical factor contributing to seismic behavior of such structures is residual bond between steel and concrete under cyclic loading. This paper examines cyclic bond-slip behavior in RC flexural members that have experienced elevated temperatures. Twenty-six modified beam-end specimens with different diameters of longitudinal reinforcement, concrete strengths, and confinement levels were fabricated. The specimens were subjected to a variety of heating regimes and a natural cool down before loading of their longitudinal rebar in a monotonic or cyclic pattern. The decrease in bond strength after 2-hr exposure to temperatures of 400, 600, and 700 °C were 23, 58, and 65 percent for monotonic loading but 30, 60, and 65 percent for cyclic loading, respectively. Elevated temperatures also caused a decrease in slope and area of hysteretic bond-slip loops. Cyclic bond strength was 25–45% less than monotonic bond strength and better correlated with the residual compressive strength of concrete, unlike monotonic bond strength, which was better correlated with the square root of this property. A model was proposed for backbone bond-slip curves of RC flexural members that are exposed to elevated temperatures.

Evaluation of crack self-sealing in flexural concrete members with SAPs by chloride ion penetration resistance

Self-sealing concrete is a technology to repair cracks using swelling properties of superabsorbent polymers (SAPs) without external work. Many studies have been conducted on SAPs as self-sealing materials that fill cracks in concrete. However, because each study evaluated self-sealing performance using different experimental methods, each study reported different results. Therefore, a standardized test method to evaluate the self-sealing performance of SAPs is needed. This study evaluated the self-sealing performance of cementitious materials with SAPs through water absorption, chloride ion penetration, and water flow tests and analyzed the correlations between the results. Two SAPs with different particle sizes were used, and their contents were set to 0.3%, 0.5%, and 0.7% depending on the binder weight. The experimental results revealed that the self-sealing performance improved with increasing particle size and SAP content. In the water absorption test, the amount of water absorbed by the surface of the original specimen and that absorbed through cracks were measured. However, the influence of the self-sealing ability of the SAPs on the total water absorption rate could not be clarified. A chloride ion penetration test was conducted to measure the chloride ion penetration depth directly, and the corresponding results indicated that the self-healing ability of the SAPs enhanced the resistance of the cementitious materials against the penetration of harmful ions. Finally, a water flow test was performed, and the results revealed that the short-term self-sealing performance of SAPs can be evaluated relatively independently.

Unified Flexural Resistance Design Method and Evaluation Frame for the B-Regions of RC Flexural Members—Theory and Application

The load and resistance factor design (LRFD) method is normally used to design B-regions of reinforced concrete (RC) flexural members. The design includes many checks corresponding to different limit states. The LRFD method requires many loop calculation steps in the design, demonstrating its relative inefficiency. It cannot be applied to compare limit states directly and quantitatively. Different design limit states are separated and isolated. How to improve the analytical calculation efficiency of the LRFD method and to realize direct and quantitative comparisons between limit states are very important problems in structural engineering. This paper presents an innovative unified flexural resistance design (UFRD) method and a unified flexural resistance evaluation (UFRE) frame to solve these problems to some extent. The main contents include the unified flexural resistance (UFR) principles, formulas for the unified flexural resistance design (UFRD) method, the operation procedure to facilitate its usage, the UFRE framework to compare limit states, and three examples. The results show that the UFRD method can provide the same design outcomes as the LRFD one. However, UFRD calculations are simpler, requiring at most 20% of the calculation steps of the LRFD method. The UFRE frame can make different limit states compare with each other directly and quantitatively, which cannot be realized by the LRFD method. It helps expose some potential and insufficient flexural resistance hazards for some limit states, such as the only 10% relative strength reservation of one example. Thus, the UFRD method and the UFRE frame supplement and develop the LRFD method to some degree. The simplicity and practicality of the approach and the frame make them appropriate for many applications.

Ductility evaluation and flexural failure mode recognition of reinforced Ultra-High performance concrete flexural members

Although ultra-high performance concrete (UHPC) exhibits material tensile ductility far in excess of that expected from conventional or high-strength concretes, structural UHPC members typically fail in flexure due to reinforcement rupture after crack localization, which may result in a low structural ductility. In the flexural design of reinforced UHPC (R-UHPC) members, attention should be paid to addressing the issue related to the low structural ductility due to crack localization. This study firstly develops a two-dimensional numerical model to simulate the flexural response of R-UHPC beams, and the numerical results agree well with the experimental results of the load–deflection curves as well as the failure modes. Parametric analyses are then carried out to examine the effect of the flexural reinforcement ratio, tensile strength of steel reinforcement, tensile behavior of UHPC and compressive strength of UHPC on the flexural failure mode and ductility behavior of R-UHPC members. The results indicate that when the failure is controlled by fracture of tensile steel bars after crack localization, higher reinforcing ratio, higher steel post-yield hardening strength, and lower steel fiber volume lead to more ductile behavior. When the failure is controlled by crushing of UHPC, however, higher reinforcing ratio and lower UHPC compressive strength lead to a lower ductility. In addition, a flexural failure mode recognition method is proposed for R-UHPC flexural members, whose accuracy is verified based on the available experimental results of 105 R-UHPC flexural members. Finally, a minimum flexural reinforcement ratio is proposed to avoid low structural ductility in R-UHPC flexural members.

Effect of concrete strength and tensile steel reinforcement on RC beams externally bonded with fiber reinforced polymer composites: A finite element study

Due to the continual deterioration of concrete infrastructures, the construction industry faces challenges in terms of their serviceability, structural strength, and durability. Therefore, efficient rehabilitation strategies are required to enhance their performance. Externally bonding fiber reinforced polymers (FRP) have evolved as a simple and convenient retrofit technique when compared to conventional techniques of strengthening. FRPs can be powerful toolsfor strengthening concrete structural members in shear and flexure because of their exceptional mechanical characteristics. Flexural strengthening is generally achieved by externally bonding FRP strips or laminates in the soffit side of beams, resulting in improved ultimate strength and bending capacity. This study aims to conduct a numerical parametric investigation on reinforced concrete (RC) beam specimens flexurally strengthened by externally bonding FRP. Three-dimensional finite element models of four RC beams strengthened with FRP and with different concrete compressive strength and flexural steel reinforcement were developed to predict their load capacity. Models were developed and simulated using finite element software called ABAQUS. The beam behavior was evaluated with the help of different material models available in the ABAQUS library. Study considerations included concrete damage plasticity, Hashin's damage in CFRP, and perfect bonding between CFRP and concrete interfaces. The parametric study included two groups of beams: one with varying concrete compressive strength (i.e., 25 MPa and 50 MPa) and the other with varying tensile steel reinforcement (5#14 mm diameter bars and 2# 22 mm diameter bars). The ABAQUS results were verified using experimental data from the previousliteratures.The computed numerical resultsand experimental results demonstrated reasonable accuracy for load–displacement curves and ultimate load capacity. It was found that the ultimate load increased significantly in high-strength (M50) beams with a higher number and smaller diameter (5#14 mm) tensile bars. In conclusion, RC beams with more compressive strength and tensile steel reinforcement boost the effectiveness of the external bonding technique.

Numerical modelling of reinforced concrete flexural members strengthened using textile reinforced mortars

Numerical modelling of reinforced concrete flexural members strengthened using textile reinforced mortars

Shear and flexure of FRP-reinforced concrete beams and slabs – A review

The article reviews structural response of flexural members reinforced with fiber-reinforced polymer (FRP) as a sustainable alternative to conventional carbon steel. It overviews experimental studies and code-based models that characterize shear and flexural response of concrete beams and slabs reinforced with FRP bars. Discussion on lower resistance to fire being key limitation of FRP bars is also included. The most commonly used fibers to manufacture FRP bars include carbon, glass, basalt, aramid, or combinations of these materials. Consequently, each material plays its role in contributing to shear and flexure differently. Basalt FRP are characterized by higher tensile capacity and resistance to corrosion, carbon FRP bars can withstand exposure to harsh environments; glass FRP bars possess lower modulus of elasticity that may cause large deflection and wider cracks. FRP bars are generally preferred over carbon steel for use in concrete elements where exposure to chlorides or chemicals is expected, or places where non-magnetic properties are essential. Beams and slabs reinforced with FRP bars are likely to undergo larger deflections due to lower stiffness of FRP bars. The dominant failure mode of FRP-reinforced concrete flexural members is discussed in comparison to conventional steel-reinforced members. The article identifies increasing interest and research in using green concrete reinforced with FRP bars that helps to make construction serviceable and sustainable. However, premature de-bonding of bars remains a concern with more research needed to avoid deterioration of mechanical properties by underutilization of materials. Although concrete is practically fire-resistant, FRP is likely to undergo damage of resin due to the presence of combustible organic polymers. The bulk of research implies that the thermal instability of the resin in FRP reinforcing bars is not a disadvantage that outweighs the possible benefits of its usage in the construction industry.

Flexural Strengthening of Fire Damaged Reinforced Concrete Structural Member

Fire causes extreme damage leading to huge loss of life and property. An extensive damage also occurs to structural members when exposed to fire. Therefore, after fire structural members need to be strengthened to restore their load carrying capacity to withstand the future service loads. The selection of strengthening technique depends on the residual capacity of fire exposed structures and required ultimate load carrying capacity. Further if strengthen structure is exposed to fire in future then strengthening techniques should also have good fire performance and remain intact with the base concrete. In this paper, efforts have been made to study and understand the effectiveness of fire damaged reinforced concrete jacked flexural member. Limited information is available on the flexural performance of fire damaged strengthen members with reinforced concrete jacket and concrete jacket. This work presents a simple and efficient technique to strengthen reinforced concrete beam in flexure using traditional materials. This paper discusses in detail the strengthening strategy program of two-hour fire exposed beam to restore its original load carrying capacity. The reinforced concrete beam is simulated in finite element-based software to compute the thermal profile, which used to calculate residual load capacity. The obtained result of thermal distribution and residual moment capacity are compared with proposed literature models and design code equations. The performance of fire damaged and strengthen beams are studied through moment-curvature relationship load-deflection behaviour. This paper provides a simplified analytical approach to determine the residual capacity of fire damaged beams along with that expected improvements in their capacity after strengthen by reinforced concrete and concrete jacket.

Finite element modelling of reinforced concrete one-way slabs strengthened using basalt textile reinforced mortars

PurposeThis paper employs a textile reinforcement strain comparison to study the response of Textile Reinforced Mortars (TRM) strengthened reinforced concrete one-way slab members in flexure using the finite element method. Basalt TRM (BTRM) is a relatively new composite in structural strengthening applications. Experimental data on BTRMs are limited in the literature and numerical analyses can help further the understanding of this composite. With this notion, Abaqus finite element software is utilised to create a numerical method to capture the mechanical response of strengthened slab members instead of time-consuming laboratory experiments.Design/methodology/approachA numerical method is developed and validated using existing experimental data set on one-way slabs strengthened using Basalt TRMs from the literature. An explicit solver is utilised to analyse the finite element model created using calibrated Concrete Damage Plasticity (CDP) parameters according to the experimental requirements. The generated model is applied to extract load, deflection and rebar strains sustained by strengthened reinforced concrete slabs as observed from the experimental reference chosen. The applicability of the developed model was studied beyond parametric studies by comparing the generated finite element tensile strain by the textile fibre with available formulae.FindingsCDP calibration done has shown its adaptability. The predicted results in the form of load versus deflection, tensile and compressive damage patterns from the numerical analysis showed good agreement with the experimental data. A parametric study on various concrete strength, textile spacing and TRM bond length obtained shows TRM’s advantages and its favourability for external strengthening applications. A set of five formulae considered to predict the experimental strain showed varied accuracy.Originality/valueThe developed numerical model considers strain sustained by the textile fibre to make results more robust and reliable. The obtained strain from the numerical study showed good agreement with the experiment results.

Bending Performance of Precast Ceramsite-Concrete-Insulated Sandwich Panel with Stainless Steel Shear Connectors

With the continuous improvement of building energy-conservation requirements, both traditional concrete external insulation and internal insulation have been unable to meet energy-saving needs. In order to meet the demands of building energy-saving in the new era, new precast concrete external-wall-insulation technology should be developed. In this study, a bending static test and numerical simulation were carried out to evaluate the influence of the thickness of inner concrete wythe and insulation and the length of plate-type shear connectors on the cracking condition, bearing capacity and composite degree of a precast ceramsite-concrete-insulated sandwich panel (PCCISP) under the outside-plane load. The results show that the failure modes of four precast ceramsite-concrete-insulated sandwich panels were all ductile failure of the concrete flexural members. The ultimate bearing capacity of the PCCISP decreased with the decrease in the thickness of the inner concrete wythe. Reducing the thickness of insulation had no significant influence on the ultimate bearing capacity. When the thickness of insulation was reduced by 30%, the composite degree of rigidity and bearing capacity of the PCCISP were increased by 8.85% and 2.67%, respectively. Increasing the length of the plate-type shear connector slightly increased the ultimate bearing capacity, but it had no obvious influence on the rigidity and bearing capacity composite degree.