Elsevier

Engineering Structures

Volume 240, 1 August 2021, 112325
Engineering Structures

Flexural strength enhancement of recycled aggregate concrete beams with steel fibre-reinforced concrete jacket

https://doi.org/10.1016/j.engstruct.2021.112325Get rights and content

Highlights

  • Influence of steel fibre jacket on the flexural behaviour of recycled aggregate concrete beams;

  • Lack of suitable model to predict the bending performance of strengthened recycled concrete beams with concrete jacket;

  • Development of a new model to forecast the flexural resistance of jacketed RC beams with steel fibres concrete jacket.

Abstract

In this study, the effect of steel fibres reinforced concrete (SFRC) jacketing on the flexural performance of coarse recycled aggregate reinforced concrete (CRARC) beams is studied. A total of 48 reinforced concrete beams, in two categories, were manufactured and tested. In the first category, 16 reinforced concrete (RC) beams were tested, then strengthened with a jacket and tested again. In another category, 16 specimens were strengthened with a jacket and then tested. All specimens were tested using a four-point flexural setup. Coarse recycled aggregate (CRA) was used at two mass replacement ratios, 0% and 100%, in both RC beams and concrete jackets (CJs). Steel fibres (SF) was also added at 0% and 2% (by volume) in both beams and CJs. In these tests, the flexural capacity, maximum displacement at mid-span and the ductility of specimens were measured. Moreover, a new modified model was proposed to predict the flexural behaviour of SF jacketed CRA and coarse natural aggregate (CNA) beams. The obtained outcomes indicate that the maximum flexural strength and displacement of 100% CRARC beams increased substantially by strengthening RC with 2% SF reinforced CJs.

Introduction

Using recycled materials is a new method to manufacture reinforced concrete (RC) elements in civil engineering. Recycled materials come from different sources such as old buildings demolition, glass waste, stone powder, among others. These materials could be partially or totally replaced cement and aggregates. In the last few decades, many studies have been made on the influence of recycled materials on the mechanical performance of concrete [1], [2], [3]. Seara-Paz et al. [4] evaluated the flexural performance of RC beams with coarse recycled aggregate (CRA). Eight RC beams were manufactured with CRA, using water-to-cement ratios of 0.50 and 0.65, and four replacement ratios: 0%, 20%, 50%, and 100%. This research demonstrated that the long-term flexural behaviour are greater for CRA concrete than for conventional concrete, regarding both strain and deflection. In another investigation, Seara-Paz et al. [5] analysed the flexural behaviour of RC beams made with CRA. RC was made with four CRA contents: 0%, 20%, 50%, and 100%, in terms of mass. As a result, bending moments, deflections, strains and curvatures were obtained at different load levels. The experiments showed that the service, yielding and ultimate state of CRA exhibits, in general, a similar trend to that of conventional concrete. In 2018, Sunayana and Barai [6] assessed the flexural performance and tension-stiffening evaluation of RC beam incorporating CRA. Parameters such as materials, mix design method and reinforcement ratios were considered for evaluation of the moment-carrying capacity, deflection, and failure pattern. The reduced tension-stiffening effect, as observed from the derived stress–strain relation of cracked concrete justifies the higher mid-span deflection in RAC. Tošić et al. [7] evaluated the long-term flexural behaviour of RC made with CNA and CRA. Six simply supported reinforced concrete beams were tested under sustained loads for 450 days. The results showed similar increases in deflections for CRA relative to initial deflections compared with those containing CNA.

Sometimes retrofitting damaged beams is an economical alternative to reconstruction. On the other hand, using additional materials such as steel fibres (SF) help to improve the flexural performance of RC beams. SF are manufactured using high-strength steel of different sizes and shapes [8], [9], [10]. Furthermore, it is noted that SF prevent brittle fracture by increasing the tensile strength and toughness of concrete [11], [12], [13].In 2008, Kim et al. [12] studied the flexural performance of steel fibres reinforced concrete (SFRC) beams with two SF contents: 0.4% and 1.2%. The specimens were tested using a four-point flexural setup. The results showed that the flexural strength and energy dissipation were significantly improved, in addition to better control of cracks’ propagation, when SF were added to the concrete mixes. Altun and Aktas [13] examined the effect of SF on the flexural performance of lightweight concrete beams. SF were added at three contents (0%, 1% and 2% per volume). Specimens were tested under a four-point bending setup. The outcomes illustrated that SF increases the toughness capacity and ductility of prismatic concrete beams. Yoo et al. [14] studied the bending capacity of low, normal and high-strength SFRC beams, and presented a new model to predict their flexural behaviour. The suggested models were confirmed through a comparison of previous flexural data. According to the conclusions, the flexural strength, ductility and deflection significantly improved using 1% SF.

In 2019, Xu et al. [15] assessed the impact of SF content on the ultimate flexural strength, maximum displacement and flexural toughness of RC beams. For this purpose, 20 specimens with various SF contents were manufactured and tested. Furthermore, 10 different longitudinal reinforcement ratios (ranging from 0.2% to 2.5%) were considered. The results indicated that the post-peak stiffness of the SFRC specimens was expressively lower than that of the control specimens (without SF). In another investigation, Lim and Paramasivam [16] analysed the bending performance of SFRC beams. For this aim, 22 RC beams were manufactured and tested under a four-point bending device. The results demonstrated that SF can partially replace transverse reinforcement (TR). In 2015, Dupont and Vandewalle [17] examined the bending strength of SFRC beams. 28 RC beams were made with a different value of SF. A four-bending test was performed to evaluate the flexural behaviour of the specimens. Based on the conclusion of this research the influence of the fibres on the flexural capacity was rather small.

Kuang and Bączkowski [18] tested SFRC beams under monotonic loading. They evaluated the ultimate shear strength and post-peak loading performance of the beams. The experimental results indicated that SF increases the post-cracking tensile strength and flexural capacity of the beams. In another study, Chaboki et al. [19] investigated the flexural behaviour of RA concrete beams with SF. 27 RAC concrete beams with various RA and SF contents were manufactured to evaluate the maximum deformation, ultimate flexural strength and ductility. A four-point setup was employed to test the specimens. The results showed that SF improved the ductility of concrete and flexural performance of the RA concrete beams. So, according to previous researches, using SF is an effective method to improve the flexural performance of RC beams before failure. Nevertheless, in some cases, it is necessary to improve the behaviour of RC beams after failure. Repairing damaged concrete beams is often needed and one of the primary methods to do it is by using concrete jackets (CJs). In order to perform CJs, some equipment is necessary such as hydraulic jack and moulds. Therefore, when CJs are difficult to perform, other retrofitting techniques such as FRP sheets and steel plate are suggested that are also common nowadays [20], [21], [22], [23]. So, some studies have been performed on repairing RC beams using CJs under different loading conditions. It should be stated that retrofitting with CJs is a little challenging in some cases. So, to perform CJs on damaged RC beams, the deflection of the damaged beam should be returned to the initial shape using hydraulic jacks. Then the lateral and bottom surfaces of beams should be made rougher in order to increase the continuity between the original beam and the fresh CJs. Formerly, U shape moulds should be positioned and supported by jacks. Finally, concrete should be funneled and moulds are released after concrete hardening. In 2012, Chalioris and Pourzitidis [24] used CJs to repair damaged RC beams. Three shear-conditioned beams were initially tested using a monotonic four-bending setup. Then, the damaged beams were repaired using CJs, applied on the bottom width and both vertical sides of the beams. The results demonstrated that CJs are a useful rehabilitation method since the strength and overall performance of the jacketed beams was better than those of the initial specimens. In another investigation, Altun [25] evaluated the flexural behaviour of jacketed RC beams. In this study, SFRC was used in CJs to repair damaged beams. To measure the effect of CJs, the bending performance of the initial and strengthened RC beams was assessed. The results of this study indicated that the mechanical performance of the jacketed RC beams was slightly better than that of the original RC beams. In 2016, Behara et al. [26] studied the torsional performance of RC beams with mesh layers U-jacketing. The “U” jackets were found to provide better bearing capacity under torsion. Ruano et al. [27] used SF to manufacture CJs and tested specimens for shear. For this purpose, RC beams were designed with a high amount of longitudinal steel rebars and minimum TR content. Some of the beams were strengthened with very fluid high-strength SFRC jacketing and some others were first tested for shear until failure and then jacketed. CJs were manufactured in both plain concrete and SFRC. SF were added to the concrete mix at two contents: 30 kg/m3 and 60 kg/m3. The results showed that the strengthened beams had excellent strength and deformation capacity recovery, but the undamaged jacketed beams had a higher bearing capacity than those tested and then jacketed.

In another study, Monir et al. [28] tested the bending behaviour of RC beams strengthened using CJs. The analysis of jacketed RC beams considering the interfacial slip effect is a complicated problem. They neglected the slip between the CJs and the original beams in the analysis. Therefore, a simplified model was developed to evaluate jacketed RC beams taking into account the interfacial slip distribution and the actual nonlinear behaviour of both concrete and steel rebars. In 2018, Monir et al. [29] assessed the bending performance of RC beams strengthened with CJs. In this research, an iterative calculation algorithm was developed to forecast the moment–curvature relationship of a jacketed RC beams by considering the influence of cross-section dimension of the original beam. The presented scheme allowed the assessment of the interfacial slip and shear stress distributions in ductile RC beams, and it was also used to conduct an extensive parametric study, which led to modification factors that can be used to evaluate the flexural strength and deformation of a strengthened beam considering interfacial slip. In 2019, Anvari et al. [30] assessed the flexural behaviour of jacketed CRARC beams. They tested eight specimens under four-point bending setup. For this aim, specimens were tested, then strengthened with CJ, and then tested again. RA were going to use at two volume percentage of 0% and 100%. Results of this research illustrated that CJs is an efficient scheme to improve the ductility of CRARC beams, and the DR was raised when steel fibres, recycled aggregate and both of them were used by 160%, 24% and 146% respectively.

The ductility ratio (DR) of RC beams is a suitable parameter to assess the bending performance of specimens. Cohn and Bartlett [31], [32] proposed a relatively more appropriate definition of DR. According to it, DR can be estimated as the ratio between the displacement at 85% of the maximum load in the post-peak portion of the curve and the displacement at first yield of the beam (Eq. (1) and Fig. 1).i=Δ0.85Δy

Standards suggested a different model to predict the DR of RC beams [33], [34]. The ductility ratio may be defined as the ratio between deflection at failure and deflection at yield or the first crack, as shown in Fig. 2 and Eq. (2). The yield point is defined as the intersection of the initial tangent to the curve with the tangent to the curve from the maximum bearing capacity.i=ΔmaxΔy

Section snippets

Research significance

A review of the literature showed that much research work has been made on the flexural behaviour of CRARC beams and repaired beams using CJs. Moreover, the improvement of the flexural behaviour of RC beams using SF was evaluated. However, the effect of SF CJs on the flexural behaviour of CRA RC beams has not been assessed. Thus, in our study, the effect of SF CJs on the flexural behaviour of CRA RC beams was studied, focusing on the effect of TR’s spacing. The main reason for incorporating SF

Geometry and properties of the specimens

A total of 48 RC beams, with a cross-section 150 mm wide and 200 mm high, and a length of 1500 mm, were manufactured and tested. They were tested under four-point bending until failure in two categories. In the first one, they were loaded until failure, and were then reverted approximately to the initial shape and repaired with a concrete jacket 50 mm thick. In the second category, specimens were strengthened using CJs and tested. Since the evaluation of the behaviour of jacketed recycled

Test setup and loading condition

The specimens were tested under a flexural strength setup at 28 days. The 1500 mm span beams were supported by round bars, and two concentrated linear loads were applied as seen in Fig. 5. These loads were 300 mm apart. The test was performed under displacement control conditions, and the stopping condition was the failure of the specimen. The deflection of the beam was recorded at each load step using LVDTs. Also, strain gauges are used to measure the strain of concrete. For this aim, two

Flexural behaviour and bearing capacity

In this section, the load–displacement relationship at mid-span, maximum bearing resistance, flexural cracks propagation, maximum deformation, and DR of the specimens and the effect of SFRC jackets were evaluated. Increasing the maximum bearing capacity indicates a higher resistance against loads. Additionally, raising the maximum displacement and preventing a sudden drop in resistance is also positive. For this purpose, first, the influence of SF and CRA on the bending performance of specimens

Theoretical model to predict the bending resistance of jacketed RC beams

The diameter of the longitudinal tensile and compressive rebars and stirrups of the 48 RC beams was 20 mm, 10 mm and 8 mm, respectively. There was no reinforcement in the CJs. ACI318-19 [34] presents some formulas to predict the flexural behaviour of reinforced concrete beams with and without CJs. [30], [34]. The previous models are appropriate only for normal concrete beams and strengthened with normal CJs (without CRA and SF). So, according to this study, the results are compared with

Conclusions

In this research, the flexural behaviour and ductility of CRA concrete beams before and after strengthening using a CJ were investigated. The research included tests of 48 RC beam specimens. SF contents varied between 0% and 2%. Furthermore, CRA were used at mass replacement ratios of 0% and 100% CNA and two TR alternatives were used: no TR and 100 mm spacing. In the first group, the specimens were tested until failure and then repaired and strengthened using NWC and SFRC jacket and tested

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (38)

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