**1. Introduction**

The rapid growth of the construction industry has put a great demand on the natural resources that are used for construction practices. It has been suggested that the global demand for concrete will increase to approximately 18 billion tons per year by 2050 [1] and an estimated yearly consumption approaching approximately 30 billion tones [2]. This

**Citation:** Parichatprecha, R.; Rodsin, K.; Chaiyasarn, K.; Ali, N.; Suthasupradit, S.; Hussain, Q.; Khan, K. Structural Behavior of LC-GFRP Confined Waste Aggregate Concrete Square Columns with Sharp and Round Corners. *Sustainability* **2022**, *14*, 11221. https://doi.org/10.3390/ su141811221

Academic Editors: Carlos Maurício Fontes Vieira, Gustavo de Castro Xavier, Henry Alonso Colorado Lopera and Sergio Neves Monteiro

Received: 28 July 2022 Accepted: 2 September 2022 Published: 7 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

suggests that there exists an enormous usage of natural resources, mainly coarse and fine aggregates resulting in their rapid depletion [3]. From the view of sustainability, this rapid depletion of natural resources must be tackled in an effective way.

The demolition of existing buildings produces a considerable quantity of waste that demands proper disposal. Roughly 700 and 800 million tons of construction waste are generated per year in the United States and European Union [4,5]. The quantity of construction waste that is produced in China each year has been estimated at 1.8 billion tons [6]. The proper treatment of construction waste before disposal is vital. Besides occupying extensive land, untreated construction waste may produce harmful substances that pollute groundwater and air [7–10].

So far, two problems need to be addressed mainly relating to the rapid depletion of natural resources and extensive accumulation of construction waste each year. A common solution to these problems may be realized in the recycling of construction waste to produce new concrete. The present study focuses on the recycling of bricks to be used as a partial replacement for natural coarse aggregates. This is because a considerable ratio of the construction waste generated each year comprises bricks. It has been reported that the quantity of clay brick waste generated each year is increasing in a geometric manner [11]. Recycled brick aggregates are usually prepared by crushing bricks in jaw crushers having different sizes of openings. Further, the crushed bricks are then sieved into different sizes using mechanical sieves. A rough estimate indicates that 400 million tons of brick waste are generated each year in China, accounting for up to 45% of the total construction waste [12].

Early experimental investigations on the recycling of bricks as coarse aggregates date back to the late 1990s and early 20th century [13–15]. Several studies have highlighted the substandard properties of recycled brick aggregate concrete (RBAC). Desmyter [16] concluded that recycled aggregates absorb more water than natural aggregates. Therefore, the resulting concrete offers lower mechanical properties as compared to natural aggregate concrete (NAC). Further, the mortar adhering to the surface of recycled aggregates results in an increased porosity leading to a 5–10% higher water absorption. Debieb and Kenai [17] reported up to a 30% reduction of the compressive strength when 100% of the natural aggregates were replaced by recycled aggregates. Medina et al. [18] reported a 39% reduction of the compressive strength for a 40% replacement of natural aggregates. Yang et al. [19] found an 11% and 20% reduction of the compressive strength for 20% and 50% replacement of natural aggregates. Due to the low density of adhered mortar, recycled aggregate concrete exhibits 5% to 15% lower particle density [20]. Jiang et al. [21] concluded that the reduction in the mechanical properties of RBAC is minimal if the replacement ratio of natural aggregates is below 30%. The substandard mechanical properties of RBAC have so far limited its use to non-structural applications [22,23]. A prevalent solution to improve the substandard properties of concrete is external wrapping. Fiber-reinforced polymer (FRP) sheets are used for this purpose. Several studies have highlighted the improvement in the mechanical properties of concrete using external FRP wraps [24–31]. Gao et al. [32] investigated the role of carbon FRP and glass FRP (GFRP) sheets in improving the properties of RBAC. It was found that the compressive strength decreased as the replacement ratio of natural aggregates increased, whereas no effect on axial deformation was reported. The failure modes of carbon and glass FRP-confined RBAC specimens were similar to those of RAC. Tang et al. [33] confined geopolymer recycled aggregate concrete using CFRP jackets and tested it under static and cyclic compressive loads. Both the peak compressive strength and ductility were improved by the application of CFRP jackets. Han et al. [34] tested recycled aggregate concrete confined with recycled polyethylene naphthalate/terephthalate composites. The test results in terms of compressive strength and ultimate strain indicated that the confinement stiffness had a more substantial effect as compared to the replacement ratio of natural aggregates. Zeng et al. [35] strengthened recycled glass aggregate concrete using CFRP jackets. A similar behavior to CFRP-confined NAC for CFRP-confined recycled glass aggregate concrete was observed.

From the above discussion, it is recognized that synthetic FRPs are efficient in improving the substandard properties of RAC. However, the cost of synthetic FRPs has been recognized as a major hindrance in their applicability to small-scale projects [36–38]. Yoddumrong et al. [39] introduced locally available bi-directional low-cost glass-fiber-reinforced polymers (LC-GFRP) to strengthen low-strength reinforced concrete (RC) columns. A significant improvement in the hysteretic behavior of the strengthened RC column was observed. Rodsin et al. [40] strengthened extremely low-strength concrete cylinders (i.e., 5 MPa to 15 MPa) using LC-GFRP. A substantial improvement in the peak compressive stress and ductility was observed in the strengthened specimens. Rodsin [41] utilized LC-GFRP sheets to enhance the mechanical properties of circular specimens constructed with RBAC. The results revealed up to a 437% increase in the ultimate compressive stress and up to 1058% improvement in the ultimate strain of LC-GFRP-strengthened RBAC specimens. In a recent study, Rodsin et al. [42] strengthened square RBAC specimens by using LC-GFRP. A corner radius of 13 mm was provided to prevent stress concentrations near sharp corners. A considerable improvement in the compressive stress–strain curves of strengthened specimens was reported.

From the above discussion, it is clear that RBAC offers lower mechanical properties than NAC, which can be improved by providing lateral confining pressures. The present study investigates the role of low-cost glass fiber reinforced polymer (LC-GFRP) sheets in improving the substandard properties of square RBAC specimens. It has been suggested that the stress concentrations near sharp corners in rectilinear specimens can result in premature failure of external sheets [43,44]. The shape of the recycled brick aggregates used in this study is approximately round, which may cause an additional reduction in the properties of RBAC. Therefore, this study investigates the efficiency of GFRP sheets on specimens with and without the provision of a corner radius and incorporating the round shape of recycled brick aggregates.
