Flexural strengthening of over-reinforced concrete beams with highly ductile fiber-reinforced concrete layer

Highlights

• Applying an HDC layer to the compression zone of over-reinforced concrete beams effectively changed the brittle failure mode.

• The strengthening effectiveness of HDC is significantly better than normal concrete.

• The ductility of over-reinforced concrete beams is considerably improved by the HDC layer.

• Strengthening effectiveness is improved as the reinforcement ratio and the strengthening thickness increased.

Abstract

This study investigates the effectiveness of highly ductile fiber-reinforced concrete (HDC), which is characterized by its high ultimate compressive strain ability, in improving the failure mode and deformational characteristics of over-reinforced concrete beams. The flexural behavior of over-reinforced concrete beams strengthened with HDC was experimentally investigated. The variables included the tensile longitudinal steel reinforcement ratio, thickness of the layer, strengthening materials, and strengthening methods. The crack pattern, failure mode, load–deflection responses, flexural capacity, ductility, and strain were investigated. The experimental results indicated that applying HDC to strengthen the compression zone of an over-reinforced concrete beam is a highly effective method to change its brittle failure and improve the ductility. HDC-strengthened beams showed an increase in flexural capacity and deformation compared to normal concrete-strengthened beams with equal strengthening thickness. A better deformation capacity can be achieved by using a larger thickness of the strengthening layer, ensuring the coordination between the strengthening layer and the existing beam. The effectiveness of HDC in strengthening over-reinforced concrete beams was improved as the tensile longitudinal steel reinforcement ratio increased. A simplified calculation method for the flexural capacity of HDC-strengthened beams was proposed, which is in good agreement with the experimental results. The compression zone relative depth for the balanced failure could be notably improved by applying a larger thickness to strengthen the compression zone of over-reinforced concrete beams than the critical thickness of the HDC layer, which can be calculated by the proposed approach.

Introduction

The demand for repair and strengthening of existing reinforced concrete (RC) beams has increased owing to the degradation of structural materials, change of usage, increase in service load, aging, and aggressive environments. The selection of an applicable strengthening material and method is essential in strengthening operations. Researchers have extensively studied the performance of commonly employed strengthening techniques in the tension zone [1], [2], [3], [4], [5], [6], [7] and compression zone [8] of RC beams using externally bonded steel plates and fiber-reinforced polymer (FRP) laminates. Research works conducted on strengthening with steel plates have indicated some drawbacks, such as the occurrence of undesirable shear failures, difficulty in handling heavy steel plates, corrosion of steel, and the need for butt joint systems as a result of limited workable lengths [9], [10]. Meanwhile, the strengthening technique using FRP laminates has some shortcomings. For example, the FRP does not perform effectively in compression under cyclic loading, and the FRP-strengthened elements can experience brittle failure due to the mismatch of the tensile strength and stiffness of FRP and concrete [11], [12].

Fiber-reinforced cementitious composites provide a feasible method for strengthening existing RC structures. Highly ductile fiber-reinforced concrete (HDC) [13], [14], [15] is a class of fiber-reinforced cementitious composites, such as engineering cementitious composites (ECCs) [16], [17], [18], high-performance fiber-reinforced cementitious composites (HPFRCC) [19], [20], [21], and strain-hardening cementitious composites (SHCC) [22], [23], which possess high ultimate compressive strain, super-high ductility, excellent strain-hardening, and multiple cracking behavior. Research [24], [25], [26], [27], [28], [29] has demonstrated that the application of cementitious composites for strengthening RC beams significantly improves their flexural capacity and ductility. Deng et al. [30] obtained a good bond strength between an HDC or reactive powder concrete (RPC) layer and the existing beam (concrete).

The failure of over-reinforced concrete beams is sudden due to compressive concrete crushing before the tensile steel reinforcement has yielded. Various concrete structure design codes restrict the use of over-reinforced concrete beams to avoid brittle failure. However, a large amount of tensile steel reinforcement is sometimes applied to obtain a relatively small structural depth. For existing RC beams, over-reinforced concrete beams may occur in certain applications when the load increases owing to a change in the building function and the beam depth is restricted. Furthermore, an over-reinforced section may occur as the strength of concrete decreases with the increase in service time, or when the strength of the poured concrete does not meet the requirements. To improve the flexural capacity of RC beams, Al-Hassani [31] used steel plates to strengthen the tension zone. The results indicate that the improvement in the flexural capacity of the strengthened beam is limited. This is due to the crushing of the compressive concrete with the tension reinforcement still in the elastic domain. Furthermore, some researchers [32], [33], [34], [35], [29] applied FRP plates, near surface mounted (NSM) reinforcement, or ultra-high-performance fiber-reinforced concrete (UHPFRC) to strengthen the tension zone of RC beams. The results showed that the flexural capacity of the strengthened beam did not improve significantly in comparison to the control beam, owing to the compression-controlled failure of the strengthened beam. Therefore, it is necessary to strengthen both the tension and compression zones to obtain a radical increase in flexural capacity.

The main ideas of the research on over-reinforced concrete beams focus on setting an external confinement or adding a layer in the compression zone to avoid brittle compression failure. The external confinement mainly involves the use of steel plates and other materials to provide restraint on the compressive concrete. Another way is to apply high-strength concrete to the compression zone of the beam. Yulita et al. [36] found an increase of up to 300% in concrete strain for beams with confinement in the compression zone compared to those without confinement. The research on confined over-reinforced self-compacting concrete beams showed that, by using a steel helix to restrict the compressive concrete of an over-reinforced concrete beam, adequate strength, ductility, and stiffness were obtained, even if the tensile reinforcement ratio was as high as 4.79% [37]. Heba [38] showed that the ductility of over-reinforced normal and high-strength concrete (NSC and HSC) beams was enhanced by confining the compression zone with helical and rectangular ties. Many studies [39], [40], [41], [42], [43], [44], [45] have indicated that the flexural behavior of over-reinforced and pre-stressed concrete beams can be strengthened by the use of full-depth rectangular steel-wire helical reinforcement to envelop the compressive concrete. The existing research has rarely studied the bending behavior resulting from adding a compression layer, such as a cementitious composite material, to the top surface of the over-reinforced concrete beam. Deng [30] and Safdar [46] investigated the flexural behavior of RC beams repaired with cementitious composite materials in the compression zone, and this approach resulted in the improvement of the flexural capacity and ductility. Ahmed [47] investigated the effect of the connection modes between SHCC and the existing beam on flexural behavior. The results showed that the use of SHCC for strengthening an over-reinforced concrete beam can significantly improve its failure mode and ductility; the effect of shear connectors appeared only in the case of an SHCC layer with a thickness of more than 20 mm. However, the effects of other factors on flexural behavior have not been studied.

In this study, based on the high ultimate compressive strain of HDC, the application of HDC to strengthen the compression zone of over-reinforced concrete beams was proposed to change their brittle failure and improve their deformational characteristics. Ten RC beams, including three control beams, two normal concrete-strengthened beams, and five HDC-strengthened beams were prepared and tested. The effects of the tensile longitudinal steel reinforcement ratio, strengthening thickness, strengthening materials, and strengthening methods on the flexural behavior of over-reinforced concrete beams were evaluated. The properties of the beams were investigated based on crack pattern, failure mode, load–deflection responses, flexural capacity, ductility, and strain analyses. Eventually, a simplified calculation method for flexural capacity was proposed.