Experimental and Numerical investigations on the cyclic load behavior of beams with rectangular web openings strengthened using FRP sheets

Abstract

This study assesses experimentally and numerically the cyclic loading response of reinforced concrete beams with web openings and shear-flexure strengthened carbon fiber reinforced polymer (CFRP) sheets using an external bonded technique. The influence of the beams on the reverse cyclic effect including the parameters of the hysteretic failure modes, stiffness degradation performance, energy dissipation, pinching width ratio, and ductility, are discussed. A numerical approach is employed to evaluate an innovative rectangular web opening and external bonding strengthening with an FRP sheet. A numerical model was used to validate the results of the tested specimens. Appropriate simulation techniques, which characterize the properties of constitutive materials, and a concrete damaged plasticity model were used for modeling development. Further, they are incorporated to apply models with new parameters. The results indicated that the use of CFRP as a strengthening system around the opening can significantly improve the overall stiffness capacity and beam behavior under cyclic loading. In particular, the increase in the load-carrying capacity over the control beam reached 63.43% for the beam with an opening in the shear zone and 73% for that with an opening in the flexural zone. The load–displacement hysteresis loops of the tested specimens were compared with those simulated by numerical models. Good convergence between the predicted and measured results was observed under all cyclic loading rates.

Introduction

Recently, the requirements of web openings in modern floor beams have received considerable research attention. They are fabricated using different pre- and post-fabrication approaches to accommodate practical applications such as allowing the passage of essential utility ducts and other vital services such as water supply, heating, electricity cables, air conditioning, telephone wires, and network system [1], [2]. In practice, openings can have different shapes and sizes based on the requirements of a specific building; for example, circular openings are required to accommodate utility pipes or rectangular openings for air conditioning services, and square, diamond, triangular, and even trapezoidal openings can be used. Circular and rectangular openings are most commonly used in practice [3]. Although RC beams with web openings are of considerable interest in structural engineering practice, most design codes and guidelines provide little-to-no recommendations on the appropriate analysis, design, and classification of openings [4]. Several studies have classified openings according to their sizes as either “small” or “large.” The important classification criteria were derived by Mansur [5]. He suggested that, for circular web openings, an opening is classified as a “small” opening when the ratio of the diameter of the opening to the overall beam depth (d/D) is less than 40%; the term “large” opening refers to when the ratio d/D is greater than 50%. In addition, in the Author category, a rectangular web opening based on length opening (L₀) and the largest top or bottom chord (d_t, db) is considered “small” when L₀ < d₀, and “large” when L₀ > d₀ (Fig. 1).

The web opening can be created at different beam span areas, either in the high shear region or the high flexural zones [6]. The presence of openings in beams can change the approximate simple beam behavior to a more complex one [7]. Owing to sudden changes in the beam sectional configuration, high-stress concentration affects the opening vicinity, and this can lead to a decrease in the total stiffness, shear strength, and entire beam capacity, in addition to cutting off the normal flow of energy and stress distribution [8], [9], [10]. In addition, it leads to a considerable redistribution of the moment and internal forces, particularly in the case of continuous beams [11]. The rebar arrangement including the sufficient quantity and appropriate detailing needs to be provided to ensure beam safety and mitigate the effect of opening inclusions in the RC beam configuration [12], [13], [14].

Recently, the use of fiber-reinforced polymer (FRP) in reinforced concrete (RC) structures has increased significantly. This is mostly because of its excellent mechanical properties such as high tensile strength and stiffness, low weight, good corrosion resistance, superior resistance to fatigue load, and ease of installation [15], [16], [17], [50]. There are many application techniques for strengthening using FRPs; for example, attaching it externally to the structural element, discontinuous mixing of FRPs with concrete, or using near-surface mounted techniques for FRP bars or sheets [42]. A new method for strengthening the grooving method (GM) can be used as an alternative to the conventional externally bonded reinforcement (EBR) of FRP sheets. This method uses the groove of the structural elements and is called EBR on grooves (EBROG) or it uses GM in direct contact with the internal surfaces of the tension face of the concrete beam and is called EBR in grooves (EBRIG) [51]. Based on some previous experimental investigations, both EBROG and EBRIG can considerably postpone the debonding phenomenon, and in some cases, they may eliminate it or postpone it up to a limited stage. This can lead to a significant improvement in the load-carrying behavior of the strengthened elements [43], [44], [45], [46]. Mostofinejad et al. [47] introduced an innovative method named warp and woof strap (WWS) that anchors the FRP sheets in the concrete beam to prevent the debonding phenomenon completely and increase the efficiency of using FRP composites. A total of 20 RC beams were used in their study to evaluate the efficiency of the new anchoring system in preventing undesired debonding. The results indicated that the WWS method was more efficient in preventing debonding failure mode compared to that of a conventional U-shaped anchor, and the load-carrying capacity and ductility were enhanced by up to 32% and 80%, respectively.

Thus far, there have been limited studies on RC beams with different types of FRP-strengthened web openings to explore the effect of different parameters and variables on its efficacy. Pimanmas [18] studied the use of FRP rods in RC beams with rectangular and circular web openings. A total of 13 beams and 2 FRP rod schemes were used. They concluded that the FRP rode placed close to the opening had no clear effect on beam behavior; however, a fundamental increase in load capacity and ductility was observed when the FRP rod was placed along with the beam depth Osman et al. [19] investigated the repairing technique of a RC deep beam with a circular opening that is strengthened using an AFRP sheet under sustained load experimentally. They reported that both the FRP scheme and pre-existing damage level exhibited considerable effects on the strengthening and failure modes. The enhancement of the strengthened beam was obtained in the range of 21.8–66.4%. Elmaaddawy [20] tested the potential of the EB of the CFRP laminate for strengthening deep beams with square web openings. His study confirmed the effectiveness of the EB of CFRP in improving the shear strength of the RC deep beams in the range of 35–73%. Further, Elsanadedly et al. [21] studied the behavior of a large rectangular opening in the RC beam strengthened with CFRP and a hybrid system of GFRP sheets under monotonic loading. Their study confirmed the effectiveness of the hybrid system and indicated a new classification of rectangular openings in the shear zone based on size (small, large, and very large). Nie et al. [22] utilized the FE model to simulate the post-created rectangular opening in the RC beam and strengthened it using the EB of the FRP. Two model approaches—concrete damage plasticity and brittle cracking model—that are available in the explicit dynamic analysis were used. The results of the analysis indicated the efficiency of the numerical model in the actual simulation of this type of structural element, in addition to the reliable recommendation for practical use of the FRP‐strengthening system.

Against this background, no experimental investigations were reported in recently published works that studied beams with transverse web openings and those strengthened with the external bond of FRP composite materials/steel plates under cyclic loading besides the research conducted by Salil et al. [23]. Further, most studies investigated the behavior of the opening beam with the conventional reinforcement with an EBR system using FRP composites or steel plates under monotonic loads, as explained earlier.

The limited test data and lack of availability of numerical studies motivated us to study the behavior of a strengthened beam web opening subjected to a slow cyclic load and to provide an essential understanding of behavior under this type of load. Further, the experimental and numerical studies on the RC beam with a web opening and that strengthened with FRP under dynamic or cyclic action continue to remain limited. Currently, FE modelling is being considered a powerful method and economical alternative to study the effect of the new important parameters. Salih et al. [23] investigated the behavior of circular web openings in RC beams strengthened externally by CFRP sheets under cyclic loads experimentally and numerically. A total of 7 RC beams were tested as the first group and 35 FE models were created; this was followed by model validation and a parametric study.

This study is a continuation of the existing experimental investigations for the second group of RC beams with rectangular openings strengthened using CFRP composites. The present study aims to focus on a new location of the opening in FRP‐strengthened RC beams through numerical investigation. The studied parameters included the opening location and strengthening orientation. The results of the test of six full-scale specimens are used to validate the finite element model.