Experimental analysis of reinforced concrete shear deficient beams with circular web openings strengthened by CFRP composite

Abstract

In this study, two methods of CFRP applications were utilized to strengthen the shear deficient beams with circular holes and a comprehensive experimental program consisting of 11 ½ scaled specimens was undertaken. The beams with hole diameter (D)/beam height ratio (H) of 0.30, 0.44, 0.64 ratios, symmetrically drilled in shear span were tested under vertical loading. D/H ratio of 0.30 did cause not only a decrease in load carrying capacity but also increased the ductility of the beam. However, significant decreases in load carrying capacities were observed as the hole diameters increase. The load carrying capacity and ductility were significantly improved owing to different CFRP configurations. The fact that the hole diameter and CFRP strengthening method are very important parameters for strengthening is observed. No CFRP strengthening alternative was successful in the beams with a D/H ratio of 0.64. A detailed macro and micro damage analyses are presented.

Introduction

The load carrying capacities of structural members (columns, beams and slabs etc.) in reinforced concrete structures may decrease due to manufacturing and construction defects, earthquake and climate conditions. In addition, improper applications applied to the structural members after the building construction can reduce the capacity. The studies on repair and strengthening methods that are used to enhance the deficient capacity of the structural member are available in the literature [1], [2], [3], [4], [5], [6], [7], [8]. Additional applications that are not considered in the design process are commonly observed at the structural members of existing reinforced concrete buildings. These applications are often seen as random holes drilling to members such as beams, columns and floors [9]. Holes or a hole can be opened at a member of the load carrying system of the reinforced concrete building after the construction upon the request of the building owners. Especially in the basement floor beams of reinforced concrete buildings, transverse circular holes are drilled due to the installation pipes. These holes result in a decrease in the cross-section area of the beam near the support [10]. Plumbing and similar pipes are laid without any significant loss in floor height with the aid of the holes opened in the beams. These gaps are often created in a circular form with the help of a core drilling machine or a similar machine. In some circumstances, these holes are randomly and carelessly opened. Typical holes opened in the basement beams are depicted in Fig. 1. If these holes are planned before the construction, necessary precautions can be taken by adding additional reinforcements around the hole. However, if these holes are drilled without planning, the strength and stiffness of these beams must be checked, and necessary precautions must be taken.

It is also an important issue that if these holes are drilled in the support region, this can cause a significant reduction of the beam shear capacity and this also results in an increase in its deflection under vertical loading. These holes lead to a decrease in shear capacity and also cause that the beam cannot reach its bending capacity. Moreover, these holes can cause the beam to behave brittle rather than the ductile behavior.

Shear and bending behaviors of the beams which are drilled before or after the construction were investigated by many researchers [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. In these studies, circular and rectangular holes were generally selected [23]. Mansur et al. [12] concluded that the deformations of a beam were increased due to an opening and the collapse load was decreased as the opening is moved to a more highly stressed part of the span. They found that as the opening length and depth increase, the decrease in the collapse load increases. The tests carried out by Tan et al. [24] on reinforced concrete beams with circular openings indicated that the use of diagonal reinforcement enhances crack control. Yang et al. [19] investigated the behavior and strength of reinforced concrete deep beams with openings and showed that the failure of a deep reinforced concrete beam was caused by the diagonal cracks projecting from the corners of the opening. It was stated in these studies that the behavior beams with openings changes depend on the location of holes, size of holes, geometry of holes and precaution taken before the construction. These beams failed mainly as brittle formation due to the lack of the capacity of the shear around the region where the holes had been drilled. Similarly, Abdul Hafez conducted a series of experimental studies to examine the effects of holes drilled to high strength beams on beam behavior [25]. The parameters taken into consideration in the experimental studies are beam span length, longitudinal reinforcement ratio and loading type (static and repeated static). As a result of the experimental study, it was stated that the holes decreased the capacity of the reference beam between 10% and 45%. In addition, it was stated that there was no difference in the maximum load carrying capacity in static and repeated static loading. The diagonal and vertical reinforcements placed on both sides of the holes drilled are recommended since they improved the strength of the long span beams. Shakir [26] confirmed the results of ANSYS finite element analysis program using the experimental study of Abdul Hafez [25]. Then, Shakir conducted a series of numerical studies to strengthen the beams with holes considered in the experimental study. For strengthening alternatives, increasing the lower and upper reinforcement ratios of the beam and jacketing of openings with steel plates were taken into consideration. As a result of the study, it was stated that the maximum load carrying capacity decreases due to the increase in hole dimensions. In addition, the maximum load carrying capacity was increased by 59% due to the increase in the upper and lower reinforcement ratios. It was also declared that the load carrying capacity was increased by 22% and 44% with the jacketing of openings with steel plates method. In recent years, alternative strengthening methods have been introduced rather than traditional strengthening methods [27], [28]. Among these methods, fiber reinforced polymer (FRP) has been found to be effective because of its mechanical properties. Therefore, FRPs have been increasingly used for rehabilitation and strengthening of existing reinforced concrete buildings. The effectiveness of fiber reinforced polymer (FRP) strengthening methods for shear-critical reinforced concrete members have been reported in the literature as well as worldwide field applications [29], [30], [31], [32], [33], [34], [35], [36], [37]. In the literature, there are also studies on the use of FRP material and steel plate to increase the shear capacities of the beams with openings. Elsanadedy et al. [38] used carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) in order to prevent a reduction in the stiffness and strength of the beams with rectangular holes at support region. Abdalla et al. [39] investigated the effect of rectangular holes close to the support regions on the beam behavior and enhanced load carrying capacity by strengthening the area around the holes with CFRP. Pinanmas [40] investigated the contributions of steel and FRP reinforcements which were placed during the manufacturing to load carrying capacities of the beams with rectangular and circular holes at the shear span. Nie et al. [41] conducted an experimental study in order to examine the effects of the holes opened in the support region of T-beams to bending behavior and to eliminate the loss of shear capacity with the help of FRP strengthening. The holes were selected as rectangular for most of these studies. On the other hand, for circular holes, the precautions were taken during manufacturing. Therefore, no study is available for the beams with unplanned circular holes and randomly drilled holes after the construction. The studies show that CFRP application is widely used to increase the capacity of reinforced concrete beams [42]. Hassan et al. [43] investigated the effect of holes in different positions on the behavior of self-compacting concrete beams. Ten beams with openings were tested in the study and the change of hole locations was selected as parameters. It was stated that the holes cause high displacement in the beam and a decrease in the stiffness. Furthermore, the decrease in load–displacement capacity reached up to 16% due to the change of the hole location. Hemzah et al. [44] conducted a series of experimental studies in order to strengthen the holes with CFRP application. It was reported that with the CFRP application around the hole, the crack formation was prevented and the displacement values were also decreased.

Hosseini et al. [45] conducted a numerical study to investigate the behavior of the beam with holes that were strengthened by CFRP and GFRP sheets. Shakir and Yahya [46] drilled 150 × 150 mm holes in T-shaped deep beams which were manufactured with self compacting concrete. Afterward, nine experimental studies were carried out to investigate the effects of the hole location to the behavior of the beams by keeping the hole axes fixed in the vertical position. In experimental studies, CFRP strengthening was applied to around the hole. Then, experimental studies were verified in ANSYS finite element program and supported with parametric studies. As a result of the parametric study, it was concluded that the maximum load carrying capacity increased by 59% with the subtraction of the concrete class from 30 MPa to 50 MPa; however, the strength increased by 12% with the increase of the concrete class by 90 MPa. This situation revealed that the use of concretes with strength above 60 MPa does not have much effect on strength.

It is known that the problems in the usage of stirrups which are used to enhance the shear capacity of the beam were observed especially in the countries where inadequate building quality control is applied. Therefore, the loss of capacity can become more dramatic and sudden power depletion may occur in these beams with the holes. Nevertheless, determination and explanation of the damage modes obtained during and after the experiment should be considered both in conventional reinforcement and in FRP strengthenings. The studies in the literature on the damage modes that have occurred draw attention recently on this subject [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]. Especially, the damage analysis of the applied FRP composite materials is important for future studies. The damage development of composite materials under load varies depending on fiber configuration and stacking sequence [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71]. Gemi et al. [55] conducted strengthening studies with a series of CFRP applications to increase the shear capacity of thinned end beams commonly used in prefabricated structures. In this study, the thinned end region was tested with CFRP application in different configurations. As a result of the experimental study, a comprehensive damage analysis was included and the damage modes are presented in detail.

As can be seen from the comprehensive literature review, the following issues were generally observed to be missing in the experiments of the beams with openings, and the main construct of this study was developed based on these deficiencies.

• It is observed in the literature for the beams with openings that various precautions (diagonal stirrups, additional reinforcement and etc.) are taken around the openings. However, in real practice, such a precaution is unrealistic since the hole to be drilled is not known.

• It is extremely important for the designers to see in which hole diameter the CFRP strengthening will be effective. In particular, it may be necessary to know whether CFRP strengthening will be effective after a certain limit value and different measures may be taken accordingly. It is also important that the longitudinal reinforcement to be selected in the beams does not exceed the under balance limit value since the beams in practice are generally under balance.

• The effects of the configuration of CFRP strengthening for the beams with openings have not been investigated so far. It is also important to determine which configuration of strengthening with CFRP will be more effective depending on the hole diameter.

The purpose of this study is twofold: firstly to investigate the effects of the ratio of hole diameter to beam depth (D/H) to the load carrying capacity of the beams having inadequate stirrups for the shear formation and secondly to examine the change in the shear capacity and ductility of these beams with the use of different CFRP applications. Pursuant to these goals, an experimental program was conducted to test eleven rectangular beams having different radius circular holes under two points monotonic static loading. In order to represent the existing reinforced concrete structure for the beams, stirrup tightening was not performed and the longitudinal reinforcement ratio has been selected to form an under-balance mechanism. In this way, it was aimed to observe a reduction in the shear capacity and changes in the behavior of the beams caused by the holes more clearly. As a result of experimental studies, although CFRP applications increase the load carrying capacity, one of the important points in the study is that it is seen that CFRP works in perfect harmony with concrete. The design of the CFRP application, the behavior of the beams during the experimental study and the detailed damage analysis are explained to guide future design.