Under-reinforced Concrete Beams Research Articles

Transient state analysis of rehabilitated RC beams using finite element modelling and prediction using an artificial neural network

The present research develops and verifies a simpler numerical approach for analyzing the thermal transient state of rehabilitated concrete beams reinforced with various types of FRP (fiber-reinforced polymer) subjected to high temperatures and specifically built as under-reinforced concrete beams. This approach offers a straightforward, efficient, and exact instrument for numerical analysis. The proposed analytical technique has been validated by load-displacement curves and cross-section temperature data, indicating its dependability and practicality. Subsequently, the validated approach was used to examine the impact of significant variables on the outcome and restoration of FRP-reinforced concrete beams at high temperatures. The methodology gives the Comparing conventional and CFRP, GFRP, AFRP reinforced beams using beam, truss, and shell elements. Thermal and UDL loads were applied, mesh at 25 mm × 25 mm. Transient analysis contrasts performance via displacement and temperature. The temperature versus displacement curve shows the FRP comparisons. Identifying the critical temperature before failure is crucial, emphasizing the curve’s significance in assessing structural performance and potential failure points. Nodal temperatures ranged 939 °C–963 °C (rehabilitated) versus 958 °C (conventional). 200 °C difference affects thermal boundary conditions for structural analysis and Conventional peaks at 320 °C, while AFRP, GFRP, and CFRP reach 358 °C, 385 °C, and 390 °C respectively. CFRP lasts 2400 min. Neural network models demonstrate effective generalizability, enabling satisfactory predictions of RC beam rehabilitation with CFRP laminates within the study’s parameter range.

The Effect of Concrete Cover Thickness on the Torsional Resistance of Fibrous, High-strength, Under-reinforced Concrete Beams

This study focuses on the role of concrete cover in enhancing the torsional resistance of peak load in under-reinforced fibrous high-strength concrete beams. In this study, the primary variable that varied from 17 mm to 53 mm was the concrete cover thickness. Four rectangular solid beams were cast and tested to determine the effect of concrete cover thickness on the behaviour of under-reinforced fibrous, high strength concrete beams subjected to pure torsional loading. The test results revealed an 80.6% improvement in the torsional resistance at peak load. In addition, at maximal loads, the twisting angle, shear strain in concrete, transverse reinforcement strain, and longitudinal reinforcement strain were reduced by 70.2%, 90%, 50.6%, and 34.5%, respectively. It is therefore possible to draw the conclusion that the concrete cover has a new function, which is an improvement in torsional resistance at ultimate load that is in addition to protecting the reinforcement from corrosion.

Flexural behavior of monolith and hybrid concrete beams produced through the partial replacement of coarse aggregate with PET waste

This study was conducted to compare the structural performance of monolith and hybrid reinforced concrete beams produced by partially replacing the coarse aggregates with polyethylene terephthalate (PET) shredded wastes. The process involved casting and testing a total of 6 beams with a dimension of 3300x150x250 mm under a four-point bending moment. The control specimens were cast using normal concrete and PET monolith specimens were fabricated using PET waste concrete. Moreover, the hybrid specimens were cast using normal concrete to a depth of 150 mm from the top and PET waste concrete at the remaining 100 mm depth at the bottom. The flexural behavior of the beams was then analyzed based on the load–deflection behavior, cracking behavior, failure mode, stiffness, and ductility. The test on the hardened concrete showed that the addition of the PET waste in the mixture reduced the compressive and tensile strength of the concrete. Moreover, the hybrid specimens were also observed to have better performance than PET monolith specimens. It was also discovered that the failure mode did not change in both PET waste monolith and hybrid specimens. Furthermore, the ultimate moment capacity of the PET waste monolith and hybrid specimens was predicted using the simple analytical model for under-reinforced concrete beams and good accuracy was obtained. The findings also showed that there is a need to consider the changes in the compressive and tensile stress distribution of concrete sections for monolith PET beams in order to extend the accuracy of the analytical model.

Flexural behavior and design of under-reinforced concrete beams with BFRP and steel bars

This study aims to investigate the flexural behavior of hybrid reinforced concrete beams and study the design methods. Seven concrete beams reinforced with basalt fiber-reinforced polymer (BFRP) bars and steel bars, including six under-reinforced concrete beams and one slightly over-reinforced concrete beam, were constructed and subject to four-point loading. The reinforcement ratio was the main test parameter. The experimental results showed a transition region between the tension and compression failure modes. The upper bound of the reinforcement ratio of this region was determined through a statistical analysis of available data. The under-reinforced concrete beams with BFRP and steel bars yielded large deflections and crack widths, and the deformability factors of these beams were significantly larger than those of over-reinforced concrete beams with hybrid bars reported previously. The American Concrete Institute (ACI) method and Xue’s equation were modified and employed to predict the flexural strength of under-reinforced concrete beams with hybrid bars. The experimental results were utilized to verify the accuracy of existing models to predict the pre-yielding and post-yielding deflections and assess the bond coefficients of the ribbed BFRP bars.

Experimental and analytical study of flexural performance of concrete beams reinforced with hybrid of GFRP and steel rebars

This paper studies the influence of varying the proportion and configuration of steel and glass fiber-reinforced-polymer (GFRP) rebars on the flexural behavior of reinforced concrete (RC) beams. The influence of adding steel fibers in concrete used for the hybrid reinforced beams is also studied. A total of sixteen under-reinforced concrete beams of 200 × 450 × 3000 mm were tested to failure under four-point flexure. The parameters investigated were first-crack load, initial stiffness, ultimate load, ductility, service load, and deflection at 50% and 100% of ultimate load. The increase in the area of steel rebars caused improvement in serviceability and enhancement in ductility. The incorporation of steel fibers in concrete improved the first crack-load and initial stiffness. The flexural capacity of hybrid reinforced beams was assessed based on the section analysis by incorporating strain hardening in steel rebars, which showed a reasonably good correlation with the experimental results.

Load-Deflection Behavior of Over- and Under-Reinforced Concrete Beams with Hybrid FRP-Steel Reinforcements.

The present study pertains to the load-deflection behavior and cracking moments of concrete beams with hybrid FRP-steel reinforcement. Under and over-reinforced hybrid beams were tested for failure along with reference beams with only steel or FRP reinforcement. The first-cracking moments of the beams were estimated analytically by using different uncracked moments of the inertia and modulus of rupture definitions. The uncracked moment of inertia definitions include the gross and uncracked transformed moments. The adopted modulus definitions are comprised of the experimental values from tests on prisms and the analytical values from the equations in different concrete codes. Furthermore, analytical methods were developed for estimating the deflections of concrete beams with hybrid FRP-steel or only FRP reinforcement. Two different types of elastic moduli, namely the secant modulus corresponding to the extreme compression fiber strain and the ACI 318M-19 modulus, were used in deflection calculations. Closer estimates were obtained by using the secant modulus, particularly in hybrid-reinforced beams. In the post-yielding region of the steel tension reinforcement, the deflection estimates were established to lay in closer proximity to the experimental curve when obtained by adding up the deflection increments instead of directly calculating the total deflections from the elastic curve equation. Accurate estimation of the cracking moment was found to be vital for the close prediction of deflections.

Flexural behaviour of steel fibre reinforced concrete beams made with copper slag as partial fine aggregate replacement

Lately, development of concrete using industrial byproducts has garnered major significance in the construction industry in order to overcome problems associated with depleted natural resources. As, river sand is costly and also large scale exhaustion of these sources generates environmental harms, a substitute or replacement product for concrete industry is very indispensable. In such a situation, the copper slag can be proposed as an alternative to the river sand, which is an industrial by-product obtained from the manufacturing of copper. The present investigation assesses the incorporation of copper slag in concrete (CSC). In this paper, experimental investigations are carried to understand the flexural characteristics such as such as first crack, ultimate load carrying capacity, maximum crack width and deflection at service load and at centre of steel fibred copper slag mixed under-reinforced concrete beams. Results showed that the compressive strength of concrete decreases as copper slag content increases beyond 40%. The reduction in compressive strength beyond 40% replacement of river sand due to the increase in free water content in concrete mixes due to the low absorption properties of copper slag which can cause excessive bleeding at higher copper slag content. The load-deflection behaviour of under-reinforced, normal and copper slag concrete beams is witnessed to be alike apart from the increased values of ultimate failure loads and ultimate deflections at failure in steel fibred copper slag concrete beams.

Strength and ductility of simple supported R/C beams retrofitted with steel plates of different width-to-thickness ratios

The concept of strengthening reinforced concrete beams using epoxy-bonded steel plates (EBSP) is a well-known solution in structural engineering, however, there is little information about the effect of the width-to-thickness ratio of steel plates on the behaviour of steel–concrete composite beams. This paper presents an experimental study of the flexural behaviour of under-reinforced concrete beams, strengthened in flexure by externally-bonded steel plates (EBSP) of varying width-to-thickness ratios. A total of 23 reinforced concrete beams were tested; 6 beams in Series 1 tests and 17 beams in Series 2 tests. One beam in Series 1 tests and two beams in Series 2 tests were regarded as control specimens, whilst the remaining beams were strengthened with steel plates of different width-to-thickness ratios. In each group, the width of the bonded steel plate varied from 75 mm to 175 mm, in increments of 25 mm. The beams were tested as simply supported, under two-point static loadings until failure. From the experimental results, it was observed that the externally bonded steel plates led to substantial increase in flexural stiffness, which resulted in an increase in the capacity and cracking load of the strengthened beams and a decrease in vertical deflections and crack-widths, compared to the control beams. It was also found that the width-to-thickness ratio of steel plates as low as 12.5 can promote flexural yielding and extensive ductility in strengthened beams.

ENHANCEMENT OF TORSIONAL RESISTANCE IN FIBROUS NORMAL STRENGTH CONCRETE BEAMS

This paper highlights the influence of thickness of concrete cover on the torsional resistance in the fibrous rectangular solid concrete beams under pure torsion. In this investigation, thickness of concrete cover vary between 18 and 53 mm. To study the influence of concrete cover in under-reinforced fibrous normal strength concrete, four fibrous concrete beams were cast and test under pure torsion. The under-reinforced concrete beams was designed based on ACI-318-14. The transverse and longitudinal reinforcement indexes are kept constant in the beams. While the aspect ratio of the beam cross section and span to depth ratio of the beams are 1.22 and 5.75, respectively. The test results were claimed that the torsional resistance provided by concrete and reinforcements plus fibre were enhanced at crack and peak loads up to 76.4% and 64.4%, respectively. In contrast, twisting angle, shear strain in concrete, strain in transverse and longitudinal reinforcements were reduced up to 57.8%, 65%, 89.3% and 95.6%, respectively. The spacing between cracks and the angle of inclination of crack at failure was increased whereas the number of cracks reduced. The dimensional analysis was used to proposed equations to predict torsional resistance at crack and peak loads including the contribution of concrete cover. In addition, the Buckingham П theorem was used for this purpose. Based on the 34 data from previous researches and the data in this research, nonlinear multiple regressions in Minitab software version 16 was used to predict the coefficients of the proposed П-groups equations. The proposed models have shown a good agreement with test results.

Recovering shear capacity of heat-damaged beams using NSM-CFRP strips

The potential of recovering shear capacity of heat-damaged beams using near surface mounted (NSM)-carbon fiber reinforced polymer (CFRP) strips was investigated in this work. Twenty-two under-reinforced concrete beams with shear reinforcement deficiency (150×250×1400) were cast and heat-treated for two hours in an electric furnace; under different temperatures (300, 500, and 600°C). Later, control beams were strengthened, whereas post-heated ones repaired with NSM-CFRP strips, directed vertically across the depth of the beams and at different spacing values ranging from 100 to 200mm. The mechanical behavior of the beams was evaluated under three-point loading test setup; with data collected and analyzed for the characteristics of the relationships between load and deflection or slippage (between concrete and reinforcing steel). Cracking and failure modes were monitored and characterized, as well. The experimental results indicated that the efficiency of repairing heat-damaged reinforced concrete beams using NSM-CFRP strips depended mainly upon damage level and spacing between NSM-CFRP strips. The result showed that using NSM-CFRP allowed restoring load capacity of the beams, damaged at 300 and 500°C; but not so for those, post-heated to 600°C. The latter achieved an utmost load capacity recovery of about 91% of original value. Lowering spacing between the CFRP strips contributed to increasing shear load capacity of repaired beams as long as peeling-off failure of the concrete cover across the depth of beam’s soffits was avoided. The flexural stiffness was not recovered for heat-damaged and repaired beams whereas toughness and ductility were noticeably improved.

Flexural Behavior of RC Beams Strengthened with Carbon Fiber Reinforced Polymer (CFRP) Fabrics

This paper explores the flexural behavior of carbon fiber reinforced polymer (CFRP) strengthened reinforced concrete (RC) beams. For flexural strengthening of RC beams, total ten beams were cast and tested over an effective span of 3000 mm up to failure under monotonic and cyclic loads. The beams were designed as under-reinforced concrete beams. Eight beams were strengthened with bonded CFRP fabric in single layer and two layers which are parallel to beam axis at the bottom under virgin condition and tested until failure; the remaining two beams were used as control specimens. Static and cyclic responses of all the beams were evaluated in terms of strength, stiffness, ductility ratio, energy absorption capacity factor, compositeness between CFRP fabric and concrete, and the associated failure modes. The theoretical moment-curvature relationship and the load-displacement response of the strengthened beams and control beams were predicted by using FEA software ANSYS. Comparison has been made between the numerical (ANSYS) and the experimental results. The results show that the strengthened beams exhibit increased flexural strength, enhanced flexural stiffness, and composite action until failure.

Finite Element and Experimental Serviceability Analysis of HSC Beams Strengthened with FRP Sheets

The use of externally bonded composite sheets or laminates is by now a diffuse technique to strengthen existing RC structures. However, some aspects of flexural condition still need experimental and numerical analysis; furthermore, especially for serviceability checks, there is a lack of code provisions. Six reinforced high strength concrete (HSC) beams strengthened with FRP sheets were fabricated and tested, the finite element (FE) models adopted by ANSYS was performed to examine the structural behavior of tested beams was performed. A comparison between the finite element analysis results and the experimental data available on the specimens was made and by using trial and error method, the finite element model was calibrated. Six under-reinforced concrete beams were fabricated and tested to failure. With the exception of the control beam, one or four layers of CFRP were applied to the specimens. The structural response throughout the loading regime was primarily captured in terms of the load deflection behavior. The load deflection plots obtained from numerical study show good agreement with the experimental results. The serviceability characteristics of the test beams were evaluated in terms of the crack width, deflection and stress in steel and concrete. The crack patterns in the beams are also presented.

Repair of heat-damaged RC shallow beams using advanced composites

Several repair techniques for restoring the structural capacity of heat-damaged high-strength reinforced concrete shallow beams using advanced composites are proposed. A series of 16 under-reinforced concrete hidden beams were cast, heated at 600°C for 3 h, repaired, and then tested under four point-loading. Tests were conducted to study the effectiveness of externally applied composite materials on increasing the flexural capacity of beams. The composites used include high strength fiber reinforced concrete jackets; ferrocement laminates; and high-strength fiber glass sheets. The beams repaired with steel and high performance polypropylene fiber reinforced concrete jackets regained up to 108 and 99% of the control beams’ ultimate load capacity, with a corresponding increase in stiffness of up to 104 and 98%, respectively. The beams repaired with fiber glass sheets and ferrocement meshes regained up to 126 and 99% of the control beams’ ultimate load capacity, with a corresponding increase in stiffness of up to 160 and 156%, respectively. Most of the beams repaired showed a typical flexural failure with very fine and well-distributed hairline cracks in the constant moment region.

Flexural behavior of strengthened concrete beams with corroding reinforcement

This paper presents the results of an experimental investigation on a total of 30 under-reinforced concrete beams which were subjected to corrosion after the cover zone was replaced with different high-performance fiber reinforced cementitious composites. A power supply was used to induce different degrees of corrosion into reinforcement. The beams were then tested under four-point bending for their load–deflection relations. The results show significant reductions in flexural strength due to reinforcement corrosion. The beams cast with a cover containing 50/50 blend of brass coated and hooked steel fibers have the best flexural performance. The beams cast with a cover containing glass fiber showed the smallest amount of strength reduction and a reasonable ductile failure after corrosion.

Flexural layered deformational model of reinforced concrete members

Based on a method recently proposed by the author and his co-workers, new average-stress/average-strain relations of cracked tensile concrete have been derived from experimental moment–curvature diagrams of under-reinforced concrete beams reported in the literature. Using these and previously obtained relations, a new constitutive relationship of cracked tensile concrete has been proposed for short-term deformational analysis of flexural reinforced concrete members. The stress–strain relationship, called the Flexural, accumulates tension-softening and tension-stiffening effects. A quantitative dependence has been established between the length of the descending branch and the reinforcement ratio. The Flexural constitutive relationship has been applied in a simple engineering technique based on a layered approach and the principles of strength of materials. To assess the accuracy of the technique, deflections/curvatures have been calculated for a large number of experimental reinforced concrete beams reported in the literature. Comparison has been carried out with well-known constitutive relationships and design code methods such as the ACI, EC2 and SNiP (Russian code).

Flexural layered deformational model of reinforced concrete members

Based on a method recently proposed by the author and his co-workers, new average-stress/average-strain relations of cracked tensile concrete have been derived from experimental moment–curvature diagrams of under-reinforced concrete beams reported in the literature. Using these and previously obtained relations, a new constitutive relationship of cracked tensile concrete has been proposed for short-term deformational analysis of flexural reinforced concrete members. The stress–strain relationship, called the Flexural, accumulates tension-softening and tension-stiffening effects. A quantitative dependence has been established between the length of the descending branch and the reinforcement ratio. The Flexural constitutive relationship has been applied in a simple engineering technique based on a layered approach and the principles of strength of materials. To assess the accuracy of the technique, deflections/curvatures have been calculated for a large number of experimental reinforced concrete beams reported in the literature. Comparison has been carried out with well-known constitutive relationships and design code methods such as the ACI, EC2 and SNiP (Russian code).

Nonlinear analysis of reinforced concrete structures using a new constitutive model

An analytical model, which can simulate the biaxial description of the nonlinear behavior of reinforced concrete structures, is introduced. The behavior of concrete is assumed orthotropic inside the ultimate failure surface and a compressive softening law of concrete is presented. The behavior of cracked concrete is simulated using the smeared crack model, which the tension stiffening effect based on a cracking criterion derived from the fracture mechanics principles is considered. A computer program is developed for analyzing the over and under-reinforced concrete beams. Several parameters such as the non-linearity proprieties, the cut off and tension stiffening models and shear retention factor are studied. The correlation between analytical and experimental results shows the validity of the proposed models and the significance of various effects. The global responses are evaluated to verify simultaneously the reliability of the proposed model and the performance of the numerical program.

Flexural strength of reinforced concrete T-beams with steel fibers

The ultimate strength of reinforced concrete T-beams reinforced with conventional steel bars and short discontinuous steel fibers are studied. It is found that the presence of steel fibers reduced effectively the deflection, width of cracks and also improved the ductility and flexural rigidity of the concrete beams. Hence, an appreciable increase to the ultimate compressive strain is observed as well as the increase in the ultimate compressive strength. These are reflected by an increase in the value of the compressive block parameters. In addition, an increase in tensile strength is achieved and a rectangular tensile stress distribution is proposed. It was found that a negligible difference in moment capacity between overreinforced and underreinforced concrete beams. Therefore, it may be economical to use more amount of tension reinforcement than that allowed by the codes. Theoretical equations are developed to calculate the ultimate strength of reinforced concrete T-beams taking into account the effect of amount of compression reinforcement and amount of steel fibers. Theoretical equations show good agreement when compared with experimental results.

Strength and serviceability of repaired reinforced concrete beams undergoing reinforcement corrosion

The paper presents the results of an experimental study on 99 under-reinforced concrete beams which were subjected to corrosion before and after the cover zone was repaired with three different generic materials. The repairs represented a wide range of tensile strengths, elastic modulii, ductility and permeabilities. The beams were reinforced with two longitudinal bars, and shear reinforcement was provided externally by steel collars. Different degrees of reinforcement corrosion were induced before and after repair, ranging incrementally from 0% to 5% at a corrosion rate of 3 mA/cm2. The results show that low ultimate flexural strengths and high deflections under service loading result in corroding beams which incorporate repairs made with materials having the following combination of properties: low strain capacity and low tensile strength; high stiffness, high flexural strength and very low permeability. A relatively ductile repair material with a high coarse aggregate content provides significantly superior structural performance. Time-performance functions for the corroding beams have also been derived by non-linear regression analysis of the experimental data. These can be used to assess (a) long-term ultimate flexural strength of repaired beams undergoing known rates of reinforcement corrosion, and (b) the limiting flexural load capacity required of corroding beams in order to prevent excessive deflections. Empirical coefficients representing the different repair materials have been derived for use in these time-performance relationships in order to assess the long-term strength and serviceability (deflection criteria) of repaired beams.

Strengthening of under-reinforced concrete beams with mechanically attached steel plates

An experimental study of the strengthening of under-reinforced concrete beams, by bolting thin steel plates to the tension faces of the concrete, is described. The results indicate that a significant increase in both the stiffness and ultimate moment of resistance can be achieved by such a technique and that the improved performance is quantifiable using conventional calculations.