Weight optimization for flexural reinforced concrete beams with static nonlinear response

Experimental investigation on shear behavior of concrete beams with CFRP grid shear reinforcements

Circular or rectangular openings are often incorporated into Reinforced Concrete (RC) beams to accommodate the service requirements of the buildings, such as telephone lines, sewage systems, and water supply pipes. These concrete beams need to be strengthened and repaired, as the presence of openings reduces their stiffness, which results in severe deformations and a significant redistribution of load. An experimental study of the impact of vertical openings on the structural behavior of simply supported RC beams is presented in this research. Five RC beams were tested as part of the experiment. The beams were split into two groups based on the vertical opening's shape. One of the beams was kept as a reference beam (without any openings). One beam in each group was internally strengthened with reinforcing steel rebar, whereas the other was not. The experimental findings demonstrated that, for all studied specimens, independent of the opening shape, the development of openings within the RC beams resulted in an increase in the service midspan deflection and a decrease in the ultimate load carrying capacity. The maximum decrease in the ultimate load was approximately 27.7% for the un-strengthened specimens compared to the reference specimen. In contrast, compared to the beams without reinforcement, the rebar strengthening was successful in enhancing the ultimate load and reducing the service midspan deflection by approximately 19% and 29%, respectively. Additionally, the experimental results demonstrated that, when compared to a reference beam, the strain values of the top and bottom steel reinforcement at the mid-span increased at the same applied load level (at service load). However, when additional steel reinforcement was used to strengthen the beam around the vertical openings, the strain in both the top and bottom rebar decreased.

The use of fibers as mass reinforcement to delay cracking and to improve the strength and the post-cracking performance of reinforced concrete (RC) beams has been well documented. However, issues of common engineering practice about the beneficial effect of steel fibers to the seismic resistance of RC structural members in active earthquake zones have not yet been fully clarified. This study presents an experimental and a numerical approach to the aforementioned question. The hysteretic response of slender and deep steel fiber-reinforced concrete (SFRC) beams reinforced with steel reinforcement is investigated through tests of eleven beams subjected to reversal cyclic loading and numerical analysis using 3D finite element (FE) modeling. The experimental program includes flexural and shear-critical SFRC beams with different ratios of steel reinforcing bars (0.55% and 1.0%), closed stirrups (from 0 to 0.5%), and fibers with content from 0.5 to 3% per volume. The developed nonlinear FE numerical simulation considers well-established relationships for the compression and tensional behavior of SFRC that are based on test results. Specifically, a smeared crack model is proposed for the post-cracking behavior of SFRC under tension, which employs the fracture characteristics of the composite material using stress versus crack width curves with tension softening. Axial tension tests of prismatic SFRC specimens are also included in this study to support the experimental project and to verify the proposed model. Comparing the numerical results with the experimental ones it is revealed that the proposed model is efficient and accurately captures the crucial aspects of the response, such as the SFRC tension softening effect, the load versus deformation cyclic envelope and the influence of the fibers on the overall hysteretic performance. The findings of this study also reveal that SFRC beams showed enhanced cyclic behavior in terms of residual stiffness, load-bearing capacity, deformation, energy dissipation ability and cracking performance, maintaining their integrity through the imposed reversal cyclic tests.

Epoxy resin concrete, characterized by its superior mechanical properties, is frequently utilized for structural reinforcement and strengthening. However, its application in structural members remains limited. In this paper, the bond–slip behavior between steel reinforcement and epoxy resin concrete was investigated using a combination of experimental research and finite element analysis, with the objective of providing data support for substantiating the expanded use of epoxy resin concrete in structural members. The research methodology included 18 center-pullout tests and 14 finite element model calculations, focusing on the effects of variables such as epoxy resin concrete strength, steel reinforcement strength, steel reinforcement diameter and protective layer thickness on bond performance. The results reveal that the bond strength between epoxy resin concrete and steel reinforcement significantly surpasses that of ordinary concrete, being approximately 3.23 times higher given the equivalent strength level of the material; the improvement in the strength of both the epoxy resin concrete and steel reinforcement are observed to marginally increase the bond stress. Conversely, an increase in the diameter of the steel reinforcement and a reduction in the thickness of the protective layer of the concrete can lead to diminished bond stress and peak slip. Particularly, when the steel reinforcement strength is below 500 MPa, it tends to reach its yield strength and may even detach during the drawing process, indicating that the yielding of the steel reinforcement occurs before the loss of bond stress. In contrast, for a steel reinforcement strength exceeding 500 MPa, yielding does not precede bond stress loss, resulting in a distinct form of failure described as scraping plough type destruction. Compared to ordinary concrete, the peak of the epoxy resin concrete and steel reinforcement bond stress–slip curve is more pointed, indicating a rapid degradation to maximum bond stress and exhibiting a brittle nature. Overall, these peaks are sharper than those of ordinary concrete, indicating a rapid decline in bond stress post-peak, reflective of its brittle characteristics.

In the literature, there is little information available regarding the behavior of composite beams made up of reinforced concrete (RC) and ultra-ductile fiber-reinforced concrete (UDFRC). In this study, UDFRC was examined for its effectiveness in enhancing the strength of RC beams. With a tensile strength of 4.35 MPa and a strain capacity of 2.5%, PVA-based UDFRC was prepared. The performance of 12 medium-sized reinforced concrete (RC) beams was measured under four-point flexural loading. The beams measured 1800 mm long, 150 mm wide, and 200–260 mm deep. The experimental program on beam specimens was divided into two phases. In the first, four 150 × 200 × 1800 mm RC beams with UDFRC layer thicknesses of 0, 30, 60, and 90 mm were tested. Additionally, four concrete and four concrete–UDFRC beams were investigated, measuring 150 × 230 × 1800 mm and 150 × 260 × 1800 mm, respectively. The study focused on medium-sized, slender RC beams under quasi-static loads and room temperature with additional or substituted UDFRC layers. As a result of replacing concrete with UDFRC, the load-carrying capacity at first crack and steel yield significantly increased between 18.4 and 43.1%, but the ultimate load-carrying capacity increased only in the range of 6.3–10.8%. Furthermore, beams with additional UDFRC layers could carry 30–50% more load than their concrete counterparts. An RC-UDFRC beam had a load-carrying capacity 10–15% greater than that of a comparable RC beam. Generally, there is a lower deflection response in UDFRC–concrete composite RC beams than in control concrete beams. The UDFRC layering can potentially improve the load-carrying capacity of RC beams, at least when ductility provisions are considered.

The state of Qatar is suffering from a harsh environment and coastal conditions which stand for most of the year. As a result, steel-reinforced concrete (RC) structures are subjected to rapid corrosion and deterioration. Therefore, there is a necessity to replace the conventional steel reinforcement by fiber-reinforced polymers (FRP) composites. Apart from FRP corrosion resistance property, their strength to weight ratio is higher than steel reinforcement which made them a viable alternative to steel reinforcement. This study aims to investigate the flexural behavior of basalt fiber reinforced concrete continuous beams reinforced with basalt FRP (BFRP) bars. Two RC continuous beams with a size of 200 × 300 × 4000 mm were prepared and cured for 28 days. Then, the beams were tested up to failure under five-point loading. The volume fraction of basalt macro-fibers (BMF) was the only investigated parameter in this study. Test results have shown that the addition of BMF in volume fractions of 0.75% improved the crack widths, cracking loads, failure mode, strains and displacements of the beams.

Studies on the mechanical and durability performance of textile reinforced geopolymer concrete beams

Reinforced concrete (RC) is the most preferred construction material for the civilian structural construction such as building and bridge because it is economical to build and possesses high strength. There have been several verified numerical studies on RC, but most have all been limited to the scope of a small rectangular or small square RC panel clamped at the edges. As a result, there is still a need for a complete RC structure for example RC wall with its foundation to represent as a single stand structure. With available validated complete RC structure experimentally and numerical data on blast pressure profile, detail numerical study is possible to investigate in depth. In AUTODYN commercial software, arbitrary Lagrange Euler (ALE) solvers are used to analyse the interaction between air and RC wall structure. The RC walls were built with the same moment resistance but a different hooked direction on vertical flexural steel reinforcement into the foundation. The numerical result indicated that in the hooked-in direction, the average strain at the back side of the wall peaked at 1.0625×10-3 at first 5 msec after impact, which is half of what it was in the hooked- out direction. Furthermore, the numerical simulation coincided with the experimental findings, where the proper steel arrangement for the RC wall subjected to blast was hooked-in.

This paper presents the experimental results of shear behaviour on concrete beams longitudinally reinforced with glass fiber-reinforced polymer (GFRP) reinforcement bars. Totally sixteen concrete beams were tested under static load. Half of the tested beams were longitudinally reinforced with GFRP reinforcement bars, while, the other half were reinforced with conventional steel reinforcement bars. The beams were prepared with varying test variables, such as shear span-to-effective depth ratios (a/d), amount and types of longitudinal reinforcement bars and stirrup spacing. The experimental results show that the ratios of a/d and stirrup spacing significantly influence the ultimate capacities of the beams. Moreover, more closely spaced diagonal shear cracks were resulted in GFRP reinforced concrete (RC) beams compared to steel RC beams.

This paper reports on the potential of using the impact resonance method (IRM) for detecting fatigue damage in strengthened reinforced concrete (RC) beams. In this experimental program, 1.2 m long RC beams strengthened with a carbon fibre reinforced polymer (CFRP) plate have been employed. The specimens were subjected to fatigue loading under four-point bending for up to 2 × 106 cycles at 3 Hz. The load amplitude was varying from 15% to 75% of the cycles yielding load of the beam. Throughout fatigue testing, the cycling was stopped for IRM measurements to be taken. The obtained data provided information about changes in modal properties, such as, fundamental frequencies and damping ratios. Moreover, the results have shown that the IRM technique was successfully employed in laboratory for detecting fatigue damage in concrete beams strengthened with CFRP laminates.Key words: impact resonance method, modal properties, RC beam, FRP-strengthening, fatigue test, cracking.

Investigations on fracture in reinforced concrete beams in 3-point bending using continuous micro-CT scanning

Corrosion of steel reinforcement has been identified as a key factor of deterioration and structural deficiency (Masoudi et al., 2011) in reinforced concrete (RC) structural members. The corrosion state of current RC bridges and high-rise buildings has been a source of concern to designers and engineers. In addition, such structures have been invulnerable to harsh environmental exposures, with little or no maintenance. Furthermore, such structures are experiencing larger amount of loads than their original capacities due to the increase number of users over the years (Bisby, 2003). Several different solutions were proposed to retrofit deteriorated structural members (Masoudi et al., 2011; Hawelih et al., 2011; AlTamimi et al., 2011) by replacing cracked concrete, using epoxy injected supplements, and FRP externally bonded systems. The use of embedded FRP bar reinforcement seems to be a promising solution (Masoudi et al., 2011; Bisby, 2003; Abbasi & Hogg, 2005; Abbasi & Hogg, 2006; Qu et al., 2009; Aiello & Ombres, 2002) to strengthen structural RC members in flexure and shear. Compared to the conventional reinforcing steel bars, the FRP bars seem to have a high strength to weight ratio, moderate modulus of elasticity and resistance to chemical and electrical corrosion. Although FRP materials were shown to have a brittle failure, due to their natural composition, still if designed properly they can show considerable amount of ductility (Rasheed et al, 2010; De Lorenzis & Teng, 2007). One of the draw backs of using FRP embedded bars is their low glass temperature and tendency to change state; from solid to liquid at elevated temperatures. Hence, the performance of FRP reinforced structural members under elevated temperatures draws many doubts and concerns and warrants further investigation. Few experimental tests have been conducted in the previous years on the fire performance of RC beams reinforced with FRP bars due to the high costs of such tests, tremendous amount of preparation, and shortage of specialized facilities (Franssen et al., 2009). Sadek et al. (Sadek et al., 2006) conducted a full scale experimental program on the fire resistance of RC beams reinforced with steel and Glass Fibre Reinforced Polymer (GFRP) bars. The test matrix composed of different reinforcing rebars used along with different concrete compressive strengths. The testing took place in a special testing facility and the beams were loaded statically at 60% of their ultimate load capacity during the course of the fire test. The tests followed the ASTM E119 (ASTM E119, 2002) standard and fire curve.

The bond strength of reinforced concrete with confinement of recycled steel strap

Alternatives to steel reinforcement in concrete are being actively investigated for environmental, economic, and durability concerns. Several studies suggest that bamboo is a potential substitute for steel reinforcement. In this study, the shear behavior of five reinforced concrete beams incorporating bamboo strips as shear reinforcement at different spaces and configurations were assessed. Structural concrete having a compressive strength of 25 MPa was used for this purpose. The experimental program involved applying four point bending test to the beams to determine their load deflection curves, crack pattern, and strain distribution. In addition, a numerical analysis was conducted for validation and prediction purposes. It was observed that including bamboo strips as shear reinforcement influenced a more brittle behavior with marginal differences when changing their spacing. On the contrary, the spacing was decisive for the load carrying capacity, as smaller spacing caused higher capacity. Strain distribution results followed a similar pattern to that of the deflection. All the curves exhibited a brittle shear failure evidenced by the crack propagation process. Further, the numerical study performed produced accurate results in comparison with the results obtained experimentally, in terms of both the load deflection curves and the crack pattern.

The objectives of this research are to compare the effect of corrosion on shear behavior in particular, and the overall structural response in both NC and SCC beams in general. Twenty reinforced concrete beams were used, with ten specimens cast using normal concrete (NC), and the other ten were cast using self-consolidating concrete (SCC). The dimensions for each beam were 150mm x 220mm x 1400mm. Using accelerated corrosion through the application of a constant current of one ampere, four stages of corrosion were established at 5%, 10%, and 20% of mass loss. Simply supported beams were loaded with two concentrated loads, and a four-point loading test was applied to the reinforced concrete (RC) beams. If (a) is the distance from the concentrated load to the reaction, and (d) is the distance from the center of the tensile flexural rebars to the top of the concrete beam, then a/d=2.5 was applied to assure the highest probability of shear failure mode. The data collected from load cell, LVDTs, corrosion crack patterns and loading cracks patterns were used to study the effects of multiple stages of corrosion on the shear behaviour of reinforced NC and SCC concrete beams. The corroded rebars were then retrieved and cleaned to compare the calculated mass loss with real mass loss. The results showed high correlation between the calculated mass loss (according to Faraday law) and real mass loss. The accelerated corrosion resulted in a corrosion crack pattern, which was documented and analyzed. In this research, the use of NC and SCC showed minor influences on failure mode, while the different states of corrosion showed a higher degree of influence on failure mode and the structural capacity of beams made from both types of concrete. The apparent changes in failure mode were associated with the increased corrosion stage.