Flexural Behavior Of Reinforced Concrete Beams Research Articles

Overloading impact on the flexural behavior of RC beams strengthened with FRP composites under fatigue: Experimental study

This study investigates the effect of periodic overloading on the fatigue life of Reinforced Concrete (RC) beams strengthened with externally bonded Fiber Reinforced Polymers (FRP). The study includes 6 RC beams with dimensions of 152.4 × 152.4 × 1500 mm strengthened with one layer of carbon fiber sheet attached to the soffits. One strengthened beam tested monotonically to serve as a control beam, and another one was tested under constant amplitude fatigue loading to serve as a reference for the fatigue testing. The rest of the beams were tested under fatigue loading with periodic overloading. The loading pattern was chosen to simulate the conditions of the traffic on bridges. Two sets of repetitive loading segments were applied with a ratio of (1:9), one overloading cycles to nine base loading cycles. When compared with the reference RC beam and the available data obtained from the literature; the results show that periodic overloading reduces the fatigue life of the strengthened beams. The Palmgren-Miner rule of linear cumulative damage overestimates the fatigue life of the strengthened beams. In addition, periodic overloading shifts the location of the failure in the strengthened beams outside the maximum moment zone. However, the tested beams had the same failure mode of the reference beam, which is the rupture of the primary reinforcing steel. In this study, a prediction model is presented of the fatigue life of strengthened RC beams subjected to overloading based on the applied stress ranges in the primary steel.

Assessing the influence of fibers on the flexural behavior of reinforced concrete beams with different longitudinal reinforcement ratios

AbstractThe use of fibers in reinforced concrete (RC) beams mainly improves both the bearing capacity and the cracking control. In this way, positive effects on the service life of RC structures can be expected. In this paper, the fiber influence on the flexural behavior of RC beams with different longitudinal reinforcement ratios (0.5% ≤ ρs ≤ 1.2%) is analyzed by testing small‐scale RC beams. Concretes incorporating 0, 25 and 50 kg/m3 of steel, 6 and 12 kg/m3 of glass macrofibers, and 5 and 10 kg/m3 of polymer macrofibers were studied. Crack and deflection control, as well as bearing capacity and crack localization were evaluated for a broad range of fiber‐reinforced concrete (FRC) toughness. It is verified that fibers, in the longitudinal reinforcement ratio considered, improve the bending behavior at serviceability limit state (SLS) and ultimate limit state (ULS) of RC beams, without limiting the structure ductility. It was also confirmed the philosophy of the fib Model Code 2010, such that FRC can be considered as a composite material where performance parameters govern its mechanical behavior. Finally, the several data available allowed to deeply analyze fib Model Code 2010 formulations (mean crack spacing and flexural bearing capacity) and to propose modifications where needed.

Flexural behavior of RC beams strengthened with prestressed and non-prestressed BFRP grids

This study was aimed to investigate the flexural behavior of reinforced concrete (RC) beams strengthened with a thin composite layer consisting of a basalt fiber-reinforced polymer (BFRP) grid and polymer cement mortar (PCM). Ten RC beams were tested in the four-point flexural experiments, and following variable parameters were examined: (1) the FRP grid type, (2) amount of FRP grid, (3) types of bonding materials, (4) prestress, and (5) number of prestress release. The experimental results demonstrated that the BFRP grid effectively improved the serviceability and ultimate behavior of RC beams. The prestressed BFRP grid further improved the cracking load, yield load, and flexural stiffness of the RC beams owing to its higher strength utilization efficiency. Moreover, the failure mode of the prestressed beams changed from “concrete cover separation” to “concrete crushing” or “BFRP grid rupture” when the number of prestress release increased. Based on the strain compatibility and classical flexural theory of RC members, the prediction of the ultimate load of the FRP grid-strengthened beams exhibited a good agreement with the experimental results.

Experimental and Analytical Study of Flexural Response of RC Beams with Steel Fibers After Elevated Temperature

In the present investigation, experimental tests were performed to study the flexural response of normal- and high-strength reinforced concrete (RC) beams with and without crimped steel fibers after exposure to elevated temperature. The RC beams were subjected to 200 °C, 400 °C, 600 °C and 800 °C at an average rate of 5 °C/min for holding period of 3 h. After single heating–cooling cycle of elevated temperature, the RC beams were tested under four-point loading. The flexural behavior of RC beams at first crack load and ultimate load was studied. It has been found that the addition of fibers in RC beams yields ductile failure and delays the initiation of flexural and shear cracks, both at ambient temperature and after exposure to elevated temperatures. The ultimate load-carrying capacity of high-strength RC beams with steel fibers has been found higher than the high-strength RC beams without steel fibers after exposure to 200 °C temperature. An empirical model is proposed to incorporate the effect of temperature and steel fibers in predicting the mid-span deflection of normal- and high-strength RC beams. The model is in very good agreement with the experimental values and predicts the mid-span deflection for normal- and high-strength concrete beams.

FE modeling of concrete beams and columns reinforced with FRP composites

Compression and flexure members such as columns and beams are critical in a structure as its failure could lead to the collapse of the structure. In the present work, numerical analysis of square and circle short columns, and reinforced concrete (RC) beams reinforced with fiber reinforced polymer composites are carried out. This work is divided into two parts. In the first part, numerical study of axial behavior of square and circular concrete columns reinforced with Glass Fiber Reinforced Polymer (GFRP) and Basalt Fiber Reinforced Polymer (BFRP)bars and spiral, and Carbon Fiber Reinforced Polymer (CFRP) wraps is conducted. The results of the first part showed that the axial capacity of the circular RC columns reinforced with GFRP increases with the increase of the longitudinal reinforcement ratio. In addition, the results of the numerical analysis showed good correlation with the experimental ones. An interaction diagram for BFRP RC columns is also developed with considering various eccentricities. The results of numerical modeling of RC columns strengthened with CFRP wraps revealed that the number and the spacing between the CFRP wraps provide different levels of ductility enhancement to the column. For the cases considered in this study, column with two middle closely spaced CFRP wraps demonstrated the best performance. In the second part of this research, flexural behavior of RC beams reinforced with BFRP, GFRP and CFRP bars is investigated along with validation of the numerical model with the experimental tests. The results resembled the experimental observations that indicate significant effect of the FRP bar diameter and type ont he flexural capacity of the RC beams. It was also shown that Increasing the number of bars while keeping the same reinforcement ratio enhanced the stiffness of the RC beam.

Flexural behaviour of RC beams with steel and polypropylene fibres

The flexural behaviours of reinforced concrete (RC) beams incorporating steel and polypropylene fibres under four-point bending test is examined in this paper. A total of nine beam specimens were designed, cast and tested under flexural loadings. Three types of concrete mixture – plain, concrete with steel fibres and concrete with polypropylene fibres – were employed in this study. Fibre content is fixed at 3% by the weight of sand as sand replacement for both mixtures containing either steel or plastic fibres. Compression tests were also conducted at 7, 14 and 28 days for twenty-seven concrete cubes prepared using the three mixtures. Comparisons of results were done in term of compressive strength, ultimate flexural load and deflection capacity. The crack pattern and failure mechanism of RC beams subjected to four-point bending are discussed. Finally, the effect of steel and polypropylene fibres on the flexural behaviour of RC beams are justified.

Flexural behavior of RC beams retrofitted by ultra-high performance fiber-reinforced concrete

This paper presents an investigation into the flexural behavior of reinforced concrete (RC) beams retrofitted by ultra-high performance fiber-reinforced concrete (UHPFRC) layers. The experimental study has been conducted in two parts. In the first part, four methods of retrofitting with UHPFRC layers in both the up and down sides of the beams have been proposed and their efficiency in the bonding of the normal concrete and ultra-high performance fiber-reinforced concrete has been discussed. The results showed that using the grooving method and the pre-casted UHPFRC layers in comparison with the sandblasting method and the cast-in-place UHPFRC layers leads to increase the load carrying capacity and the energy absorption capacity and causes high bond strength between two concretes. In the second part of the experimental study, the tests have been conducted on the beams with single UHPFRC layer in the down side and in the up side, using the effective retrofitting method chosen from the first part. The results are compared with those of non-retrofitted beam and the results of the first part of experimental study. The results showed that the retrofitted beam with two UHPFRC layers in the up and down sides has the highest energy absorption and load carrying capacity. Afinite element analysis was applied to prediction the flexural behavior of the composite beams. Agood agreement was achieved between the finite element and experimental results. Finally, a parametric study was carried out on full-scale retrofitted beams. The results indicated that in all retrofitted beams with UHPFRC in single and two sides, increasing of the UHPFRC layer thickness causes the load carrying capacity to be increased. Also, increases of the normal concrete compressive strength improved the cracking load of the beams.

Flexural behaviour of RC beams strengthened with combinations of two CFRP systems and P-SWRs

Externally bonded (EB) carbon fibre-reinforced polymer (CFRP) sheets, near-surface-mounted (NSM) CFRP bars and prestressed steel wire ropes (P-SWRs) are widely used for strengthening reinforced-concrete (RC) beams. Existing research has shown that it can be inefficient or infeasible to provide sufficient load-carrying capacity using one strengthening method because of the CFRP strain limitation at debonding or the upper bound on the amount of strengthening materials due to limited space for strengthening. Moreover, some already strengthened RC beams need to be further strengthened under heavier loading conditions. Therefore, using a combined strengthening scheme for one beam should be considered in both these situations. In this study, five concrete beams were strengthened and tested to explore the effects of different combined schemes on beam flexural behaviour. The results demonstrate that P-SWRs are of crucial importance in improving the cracking behaviour and ductility of beams originally strengthened with EB-CFRP sheets or NSM-CFRP bars. The strengthening scheme of NSM-CFRP bars and P-SWRs is found to be better than that of EB-CFRP sheets and P-SWRs. A recommendation for the appropriate selection of strengthening schemes is given based on the test results.

Finite element modeling of reinforced concrete beams externally strengthened in flexure with side-bonded FRP laminates

Reinforced concrete (RC) beams are often strengthened through externally bonded (EB) fiber-reinforced polymer (FRP) systems. In such applications, improving the bending capacity of RC beams is of main interest and which is achieved via bonding EB-FRP systems to the soffit of beams. Access to soffit of beams can be restricted by existing structural members or limited (in case a RC beam span over neighboring compartments). In those scenarios, RC beams can still potentially be strengthened in flexure through longitudinal bonding of EB-FRP systems to the sides of beam's web. This paper examines critical parameters that influence the effectiveness of side bonded EB-FRP systems through a newly developed finite element (FE) model. This model utilizes state-of-art simulation techniques and is capable to trace the flexural behavior of RC beams externally strengthened with side-boned FRP laminates throughout all loading stages till failure. The model was validated by comparing with experimental data and the predicted and measured results were in good agreement. The validated model was then utilized to study the effect of concrete compressive strength, FRP material type, FRP size, and steel reinforcement ratio on the behavior of strengthened RC beams. Outcomes of this numerical investigation show the effectiveness of side-bonding FRP systems as an alternative to conventional soffit strengthening systems to improve the flexural capacity of RC beams.

Studies on flexural behavior of reinforced concrete beams with copper slag and fly ash

This paper presents the fresh and hardened concrete properties with fly ash (FA) and copper slag (CS). M30 grade of concrete has been designed with a constant water–cement ratio as 0.4. Twenty four concrete mixtures are arrived by cement is partially replaced by FA from 0 to 30% with 10% increment and natural sand is replaced by CS from 0 to 100% at 20% increment. The compressive strength of the concrete was verified at 3, 7, 14, 28, 56, and 90 days curing periods. Five reinforced concrete (RC) beams of size 150 mm × 250 mm × 3200 mm were cast based on the optimum mix proportion and flexural behavior of RC beams was monitored by a four‐point bending test. From the experimental results, compressive strength, and flexural strength were increased for concrete with 30% FA and 80% of CS because the smaller surface area of CS per unit volume is exposed with a large quantity of concrete matrix and workability of concrete also increased.

Effect of steel fibers on the flexural behavior of RC beams with very low reinforcement ratios

This study aims to investigate the implications of hooked-end steel fibers on the flexural performance of reinforced concrete (RC) beams with very low reinforcement ratios. For this, four different fiber volume fractions, vf, of 0.25%, 0.50%, 0.75%, and 1.00%, were incorporated into the concrete mixture and plain concrete without fibers was considered as a control specimen. Four reinforcement ratios of 0.178%, 0.267%, 0.317%, and 0.406%, which are 44%, 66%, 78%, and 100% of the minimum reinforcement ratio, ρmin, were also adopted to evaluate the steel fiber effect on the flexural behavior of RC beams with various very low reinforcement ratios. The test results indicated that the overall flexural performance of RC beams, in terms of flexural strength, deflection capacity, post-cracking flexural stiffness, and cracking behavior, was improved by increasing the reinforcement ratio up to ρmin. Higher initial cracking and yield loads, post-cracking stiffness, and better cracking performance of RC beams were also obtained by including steel fibers. However, the enhancement of ultimate load carrying capacity by steel fibers was relatively minor, and the ductility index and flexural strength margin, used to guarantee a ductile failure mode, deteriorated with the inclusion of steel fibers. The lower reinforcement ratios and higher fiber volume fractions clearly led to lower ductility indices. Therefore, it was concluded that longitudinal steel rebar could not be replaced with discontinuous steel fibers at moderate volume fractions, vf ≤ 1.0%, in terms of ultimate load carrying capacity, ductility, and flexural strength margin. Lastly, analytical results considering material models for steel fiber-reinforced concrete (SFRC), given by the RILEM recommendation, generally overestimated the flexural capacities of reinforced SFRC beams, and the inaccuracy increased with increasing fiber contents.

Flexural behavior of RC beams retrofitted with ultra-high strength concrete

The performance of popular techniques of rehabilitation and strengthening of concrete elements using externally bonded steel plates and fiber-reinforced plastic (FRP) laminates has been extensively investigated and several de-lamination failures have been reported primarily due to the mismatch of their tensile strength and stiffness with that of the concrete structure. To overcome some of these drawbacks, this paper critically examines the performance of Reinforced Concrete (RC) beams retrofitted with a thin ultra-high strength concrete (UHSC) strip. All the reinforced concrete beams were retrofitted with UHSC strips of single thickness after they had been subjected to several levels of damage. It is found that the damaged reinforced concrete beams can be successfully strengthened and rehabilitated by using a thin precast UHSC strip adhesively bonded to the prepared tensile surface of damaged beams. A finite element model was developed to predict the failure load and load–deflection behavior of the retrofitted RC beams. The model accounts for the (i) degree of pre-damage, (ii) fracture behavior of concrete and UHSC through their respective specific fracture energy and stress-crack opening relation, and (iii) elasto-plastic behavior of the reinforcing steel. The model predictions are in good agreement with the test results. It is concluded that UHSC is an excellent prospective candidate for the repair and rehabilitation of damaged RC flexural elements.

Experimental analysis on flexural behaviour of RC beams strengthened with CFRP laminates and under fire conditions

Carbon fibre reinforced polymer (CFRP) laminates have been used on strengthening reinforced concrete, steel and timber members due to their excellent flexural performance, lightness, ease of application and corrosion resistance. However, in a fire, other detrimental factors may arise due to the high thermal exposure. This paper presents the results of an experimental investigation on the flexural behaviour of reinforced concrete (RC) beams strengthened with CFRP laminates in fire conditions. The main objective was to assess the behaviour in fire of the strengthening system on the beams thermally insulated with different fire protection systems. Fire resistance tests on beams protected with sprayed vermiculite-perlite (VP), expanded clay aggregates (EC) or ordinary Portland cement (OP) based mortars, were carried out. The tests carried out under the present research were innovative because unlike the ones conducted by other authors the beam in testing and its supports were completely inside the furnace. Furthermore, the testing beam supported a concrete slab that tried to simulate its interaction with the surrounding building structure. The results showed above all that the beams with passive fire protection materials provided an integrity of their resistance for long periods of fire exposure, especially the ones protected with VP mortar.

Experimental study of flexural behaviour of RC beams strengthened by longitudinal and U-shaped basalt FRP sheet

Abstract Fiber Reinforced Polymer (FRP) composite products such as Carbon FRP (CFRP) or Glass FRP (GFRP) have been intensively studied for strengthening reinforced concrete (RC) and masonry structures. It has been reported that FRP strengthening is effective to enhance the structural load-carrying capacity. Basalt FRP (BFRP) is a promising material for the application to structure strengthening with its advantages of low cost, corrosion resistant and sound mechanical property, but only limited studies of using Basalt FRP to externally strengthen RC beam are available in the literature. This study is to experimentally explore the effectiveness of application of Basalt FRP to strengthen RC beam under three-point bending test. The damage modes and structural response of unstrengthened and BFRP strengthened RC beams were recorded and identified. The effects of various BFRP wrapping schemes, U-jacket anchorage and epoxy adhesives on the flexural capacity of RC beams were analysed and discussed. In addition, the formulae used to predict the flexural behaviour of RC beam strengthened by other FRP composites (e.g. CFRP/GFRP) were evaluated for their applicability to Basalt FRP strengthening.

Erratum for “Fatigue and Flexural Behavior of Reinforced-Concrete Beams Strengthened with Fiber-Reinforced Cementitious Matrix” by Zena R. Aljazaeri and John J. Myers

Erratum for “Fatigue and Flexural Behavior of Reinforced-Concrete Beams Strengthened with Fiber-Reinforced Cementitious Matrix” by Zena R. Aljazaeri and John J. Myers

Flexural behavior of RC beams strengthened with steel-FRCM composite

This paper presents the results of an experimental investigation of the flexural response of reinforced concrete (RC) beams strengthened using externally bonded steel fiber reinforced cementitious matrix (steel-FRCM) composite. Steel-FRCM composite strips were bonded to the tension face of four RC beams, which were tested in four-point bending. Parameters varied were the presence/absence of the external (coating) layer of the matrix, presence/absence of U-wrap anchorages, and loading rate. Results are compared with those from single-lap direct-shear tests conducted on the same composite. The direct-shear tests showed that debonding of steel-FRCM joints is characterized by fiber slippage and fracture of the matrix layer at the internal matrix layer-fiber interface. In the beam tests, the strengthening system increased the yield load by 15–21% relative to the unstrengthened beam. The ratio of the load at which debonding occurred to the load at yielding ranged from 1.11 to 1.19 for each strengthened beam. The load rates employed and the presence of the external matrix layer did not appear to significantly affect the failure mode or the load and midspan displacement at debonding. The presence of the U-wraps helped restrain the peel-off of the composite observed in strengthened beams without the U-wrap, however, they did not restrain the fiber slippage at the ends of the composite, which inhibited composite action. Average values of the maximum fiber strain at composite debonding determined using strain profiles from strain gages, an approximate method, and moment-curvature analysis were 0.54%, 0.73%, and 0.83%, respectively.

Flexural behaviour of RC beams reinforced with compressive steel bars and two-piece enclosed stirrups

Compressive reinforcement buckling accounts for the flexural behaviour of reinforced concrete (RC) beams with two-piece enclosed stirrups featured with relatively weak transverse restraining. Specimens of six beams and ten hooks were tested to gain experimental evidence on the flexural behaviour of beams and tie stiffnesses of stirrups, respectively. The reinforcement buckling possibility was investigated using an analytical approach in literature for an isolated elastic bar axially loaded under compression and laterally supported by springs. A finite element method based model was developed, verified and used for parametric studies to clarify the mechanics mechanism of flexural behaviour of RC beams. Results revealed that, compared to the case of traditional one-piece stirrups, premature reinforcement buckling was prevented due to sufficient restraining effect provided by two-piece enclosed stirrups. Reinforcement buckling could occur prior to peak load which differed from the assumption in the classical analytical method. The load carrying capacities of the RC beams were insignificantly affected by the reinforcement buckling. Consequently, identical flexural behaviour was confirmed for RC beams reinforced with one- and two-piece stirrups.

Fatigue and Flexural Behavior of Reinforced-Concrete Beams Strengthened with Fiber-Reinforced Cementitious Matrix

AbstractThe need for repair and rehabilitation is a crucial issue today in order to maintain a safe and efficient transportation network. One of the most recent innovative composite materials that has been proposed to the market is a fabric reinforced cementitious matrix (FRCM) strengthening system. This system consists of two components: a structural reinforcement mesh and a cementitious matrix. The structural reinforcement mesh consists of a polyparaphenylene benzobisoxazole (or PBO) fiber composite, while the cementitious matrix is a nontoxic grout system based on portland cement with a low dosage of dry polymers. The first aim of this study was to evaluate the fatigue resistance of reinforced concrete (RC) beams strengthened with FRCM under fatigue loading and postfatigue flexural strength to failure. The second aim was to investigate the durability performance of the FRCM after exposure to environmental conditioning including temperature, moisture, and sustained stress. All of the beam specimens exam...

Flexural behavior of RC beams strengthened by NSM GFRP Bars having different end conditions

Near surface mounted (NSM) fiber reinforced polymer (FRP) bars became more effective in strengthening reinforced concrete (RC) beams. This is because it increases the bond capacity and makes a protection against external damage. Most of previous related researches stated that the failure of the tested RC strengthened beams with NSM FRP is due to debonding or concrete cover separation. In this research the ends of the NSM glass fiber reinforced polymer (GFRP) bars were bent to delay or prevent NSM FRP debonding and concrete cover separation and thus increasing the load carrying capacity of the strengthened beams. The inclination angles of GFRP bars with bent ends were 90° and 45°. Straight GFRP bars with variable lengths were also used for comparison. The test results demonstrated that the GFRP bars with bent ends prevented the concrete cover separation and increased the load carrying capacity of the strengthened beams. The load carrying capacity of the strengthened beams by straight NSM bars and those having 45° and 90° inclined ends were 177%, 201%, and 185% of that of their control beam, respectively.

Cracking localization and reduced ductility in fiber-reinforced concrete beams with low reinforcement ratios

The effect of adding steel fibers to concrete mixes on the flexural behavior of reinforced concrete (RC) beams was studied experimentally. Tests were conducted on conventionally reinforced beams with different reinforcement ratios, with and without fibers. Results show that the application of steel fibers leads to a pronounced reduction in flexural ductility of beams with low conventional reinforcement ratios (ρ<∼0.5%). Ductility ratios of fibrous RC beam specimens were reduced by 50–80% compared with corresponding ratios measured on control specimens with the same reinforcement ratios but without fibers. The effects of added fibers on the flexural behavior of RC beams were examined and three aspects were reported: ductility ratio (based on deflection), distribution of curvature, and load-carrying capacity.