1. Introduction
Rehabilitation of existing structures or their individual components has been the hub for the past few decades. This can be ascribed to many reasons stretching from environmental influences, change in loading magnitudes, deterioration caused by earthquakes to upgrades in order to meet the modern design provisions. Unsatisfactory performance of any structure can lead to serious concerns from the perspective of public safety. With many older constructions either lacking to meet newly imposed loading requirements or modern code standards, rehabilitation and repair works have become fundamentals to current research activities. Various strengthening techniques have been developed over the years to achieve the aforementioned tasks.
Efficient retrofitting of existing structures has been proven by various methods. The earliest studies focused on the application of conventional concrete jackets around the deficient part of the component or a component as a whole [
1,
2,
3,
4,
5]. Extensive work has also been done on the use of steel jacketing [
6,
7,
8,
9,
10]. As much as these conventional methods work effectively for retrofits, some alarming shortcomings associated with them are still inevitable. Concrete or steel jackets are themselves heavy and impart significant weight to the structure thus increasing load demand on underlying foundations. Further, concrete or steel jackets alter the stiffness of the structure with additional concerns related to the corrosion of steel jackets [
11,
12]. The Rapid evolution of composite materials in the field of structural rehabilitation is appreciable. Their inherited high tensile strengths, corrosion resistance, lightweight, easy to handle, good fatigue, and less labor cost involved make them an excellent alternative to conventional jacketing techniques [
13,
14,
15,
16,
17].
Openings in RC members are often provided for reasons like reducing self-weights, electrical supply lines, water and sewerage lines, computer networks, etc., [
18,
19,
20,
21]. Altun et al. [
22] tested nine RC beams with dimensions 200 mm × 200 mm × 3000 mm constructed with classic steel fiber reinforced concrete (CSRC). They examined the effects of reduced dead weights by providing openings, varied wall thickness, and different ratios of the wall thickness to beam height on ultimate load carrying capacity. It was found that a reduction of approximately 44% in deadweight could reduce ultimate load capacity up to 29%. Alnuaimi et al. [
18] compared experimental findings of seven hollow beams with those of seven solid beams. Their beams measured 300 × 300 mm x mm in cross-section to 3800 mm in length. By providing an internal hollow core of 200 × 200 mm, a peripheral wall thickness of 50 mm was obtained. They concluded that the hollow core significantly affected hollow beams’ behavior and could not be overlooked under the combined load of bending, shear, and torsion. Gasham [
23] tested six moderate deep RC beams embedded with PVC pipes. Their beams measured 150 × 300 mm x mm in cross-section to 1500 mm in length. Four different PVC pipe diameters were utilized to check their effects on beams’ behavior. They concluded that pipe diameter smaller than 33% of beam’s width had a negligible effect on its performance. For larger pipes, the ultimate strength and stiffness of beams reduced as pipe diameter increased. Alshimmeri and Maliki [
24] investigated the effects of vertical reinforcement, opening size, and their orientations on the behavior of six beams each measuring 120 × 180 × 1000 mm x mm x mm. It was concluded that the presence of openings reduced ultimate capacity from 37 to 58% and increased corresponding deflections up to 76%. Murugesan and Narayanan [
25] investigated the effect of longitudinal circular opening on the flexural strength of simply supported rectangular hollow beams. They varied the diameter of circular holes as 25, 40, and 50 mm as well as their positions from the top. Flexural mode of failure was observed in all hollow beams. Moreover, a theoretical model was proposed to predict flexural strength and load corresponding to the onset of 1st crack.
It can be concluded from previous experimental findings that the presence of openings inside beams can substantially reduce their ultimate strengths with increased corresponding deflections. To preclude this deficiency, different strengthening schemes have been proposed. Dong et al. [
26] carried out flexural strengthening of seven RC beams with different cross-sections, longitudinal reinforcement ratio, and concrete cover. CFRP sheets with different layers were externally bonded to the beams to enhance their flexural strengths. A gain of 41–125% in flexural strength was reported in CFRP retrofitted beams. Khuzaie and Atea [
27] studied the behavior of hollow RC T-beams constructed with reactive powder concrete (RPC). RPC was made by adding steel fibers and silica fume to concrete in different quantities. They investigated the effect of different volumetric ratios of steel fibers and silica fume. It was found that an addition of steel fibers by 2% to concrete mix could significantly enhance cracking load and ultimate torque of RPC hollow beams. Moreover, cracking, and ultimate torques were increased up to 184 and 66%, respectively. Vijayakumar and Madhavi [
28] strengthened hollow RC beams with different fibers. They found that both compressive and flexural strengths of hollow beams were noticeably improved by the addition of micro steel fibers and nylon fibers with volumetric ratios of 0.3 and 0.1%, respectively.
Chen et al. [
29] carried out investigations on the influence of CFRP plates in restoring structural properties of rectangular hollow beams. They tested eight beams under 3-point loading out of which three beams were kept as controlled and five beams strengthened with CFRP plates with and without prestressing. They noticed that failure patterns of repaired beams were similar to those of control beams. Besides, prestressing of CFRP plates resulted in further improvement in the behavior of hollow beams.
There are limited studies on RC beams with hollow sections [
26,
27]. There is an urgent need to investigate the effect of openings on the structural behavior of large-scale hollow section RC beams. Especially openings of larger dimensions which could significantly reduce the weight and cost of the RC beams. A detailed review of existing literature indicates that so far openings of large dimensions have not yet been studied in RC beams. Therefore, the main objective of this stud was to investigate the effect of two different types of openings on the structural behavior of large-scale hollow section RC beams. Given that openings in RC beams reduce their capacities significantly, the need for their strengthening for optimal performance cannot be overlooked. To date, many studies have been published on the use of different types of FRP to alter the strength of the reinforced concrete members, such as beams and columns [
30,
31,
32,
33]. Recently, Mohammed et al. 2020 a systematic review of current practices for the repair of structures using prefabricated composite jackets. The authors conclude that the jacketing systems offer superior properties in terms of corrosion resistance, lightweight and durability compared to conventional repair systems [
34]. This paper further aims at investigating fiber reinforced polymer (FRP) application on rectangular hollow beams in enhancing their flexural strengths. Carbon FRP sheets are applied to hollow beams in different configurations to allow for the comparison and selection of the best possible configuration that could restore hollow beams’ flexural capacities up to optimum level. To achieve the research objectives, a total number of nine large-scale RC hollow section beams were constructed and tested under a four-point bending scheme. In order to further evaluate the economic and performance benefits of these beams, a cost-benefit analysis was also performed.
4. Cost-Benefit Analysis
In order to further evaluate the economic and performance benefits of these beams, a cost-benefit analysis is performed. The cost-benefit analysis is also added concerning the feasibility of the design of newly constructed beams. Cost-benefit analysis is important to scale up the design at the industrial level. The results depict some of the interesting findings. This research proposes basically three different benefits over the conventional design of beams; it reduces the weight of the beams, the construction cost is reduced, and the ultimate loading capacity of the beams is improved. As it can be seen from
Table 6, the weight of beam B02-HS50-CON and beam B03-HS100-CON is reduced with an approximate amount of 18 Kg (3.33%) and 72 Kg (13.33%), respectively with respect to the control beam (B01). The construction price for these beams is also reported in USD. It can be clearly seen from
Table 6, that there is a significant difference of 0.82 USD (3.34%) and 3.27 USD (13.33%) of cost reduction for both beams, B02 and B03, respectively. One of the important findings of this research study, which is very interesting to note that the ultimate load carrying capacity of the proposed modified beams is more than the control beam with an average increase of 1.2 kN (1.66%) and 2.74 kN (5.27%), respectively. Based on these findings, it can be suggested that the feasibility of the newly constructed beams is more as compared with the control beam (B01). Therefore, the newly proposed designs confirm the feasibility of these infrastructural elements (beams) from the sustainability (less concrete), economic (low cost), and performance (load carrying capacity) point of view. In this research study, the use and importance of high-performance beams are evaluated in order to improve the performance mechanism of structures in the face of lower expenses and higher performance as compared with the conventional/control beams. The cost-benefit analysis presented in this research study is imperative and important for the sake of checking the applicability and feasibility of the proposed technique. The proposed mechanism to enhance the performance of the beams would be useless if it is not possible to use this proposed technique on a larger scale in favor of saving the compensating costs associated with production on large scale.