# Structural Performance of Reused GFRP Bars in Reinforced Concrete Beams: A Sustainable Approach to Concrete Reinforcement

Charles K Kankam, Selase A. K. Kpo, Russell O Afrifa, Jack O Banahene, Johnson Dwomoh Boateng, Joycelyn Esi Asamoah, Maxwell Appiah-Kubi, Theophilus Kwaako, Edward C Mansal, George Oti Boateng, et al.

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## ► To cite this version:

Charles K Kankam, Selase A. K. Kpo, Russell O Afrifa, Jack O Banahene, Johnson Dwomoh Boateng, et al.. Structural Performance of Reused GFRP Bars in Reinforced Concrete Beams: A Sustainable Approach to Concrete Reinforcement. Asian Journal of Advanced Research and Reports, 2025, 19 (4), pp.331-348. 10.9734/ajarr/2025/v19i4987. hal-05046239

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# Structural Performance of Reused GFRP Bars in Reinforced Concrete Beams: A Sustainable Approach to Concrete Reinforcement

Charles K. Kankam<sup>a</sup>, Selase A. K. Kpo<sup>a*</sup>, Russell O. Afrifa<sup>a</sup>, Jack O. Banahene<sup>a</sup>, Johnson Dwomoh Boateng<sup>b</sup>, Joycelyn Esi Asamoah<sup>a</sup>, Maxwell Appiah-Kubi<sup>a</sup>, Theophilus Kwaako<sup>c</sup>, Edward C. Mansal<sup>a</sup>, George Oti Boateng<sup>d</sup>, Daniel Kofi Ati<sup>a</sup> and Enoch Tongyem<sup>a</sup>

## Authors' contributions

This work was carried out in collaboration among all authors. All authors read and approved the final manuscript.

## Article Information

DOI: https://doi.org/10.9734/ajarr/2025/v19i4987

### Open Peer Review History:

This journal follows the Advanced Open Peer Review policy. Identity of the Reviewers, Editor(s) and additional Reviewers, peer review comments, different versions of the manuscript, comments of the editors, etc are available here: https://pr.sdiarticle5.com/review-history/134629

### Original Research Article

Received: 18/02/2025

Accepted: 21/04/2025

Published: 24/04/2025

<footnote><sup>a</sup> Department of Civil Engineering, KNUST, Kumasi, Ghana.</footnote>

<footnote><sup>b</sup> Department of Urban Roads, Wa, Ghana.</footnote>

<footnote><sup>c</sup> Physical Development and Municipal Service Directorate, University of Ghana, Legon, Ghana.</footnote>

<footnote><sup>d</sup> Civil and Environmental Engineering Department, UENR, Sunyani, Ghana.</footnote>

<footnote>*Corresponding author: Email: selakay22@gmail.com;</footnote>

<footnote>**Cite as:** Kankam, Charles K., Selase A. K. Kpo, Russell O. Afrifa, Jack O. Banahene, Johnson Dwomoh Boateng, Joycelyn Esi Asamoah, Maxwell Appiah-Kubi, Theophilus Kwaako, Edward C. Mansal, George Oti Boateng, Daniel Kofi Ati, and Enoch Tongyem. 2025. "Structural Performance of Reused GFRP Bars in Reinforced Concrete Beams: A Sustainable Approach to Concrete Reinforcement". Asian Journal of Advanced Research and Reports 19 (4):331-48.
https://doi.org/10.9734/ajarr/2025/v19i4987.</footnote>
ABSTRACT

Construction demolition waste is a growing environmental concern in developing countries like Ghana due to rapid urban expansion. Glass Fiber Reinforced Polymer (GFRP) bars, known for their high tensile strength, full linear-elastic response, corrosion resistance, non-conductive and lightweight nature, offer a sustainable alternative to steel reinforcement. However, their disposal remains challenging due to their non-biodegradable nature. Reusing and recycling GFRP bars present a viable solution, minimizing waste and reducing the construction sector's carbon footprint. This research focused on reused GFRP bars recovered from previously tested beams that failed due to concrete crushing, based on concrete controlled sections, while leaving the GFRP bars intact. The mechanical properties of the reused GFRP bars were evaluated to provide holistic data for this study. Five beam specimens with varying concrete strengths (23.1 N/mm², 15.6 N/mm², 16.90 N/mm², 16.02 N/mm², and 22.86 N/mm²) were reinforced with 12 mm reused GFRP bars at tensile reinforcement ratios of 0.71%, 0.94%, 0.86%, 2.11%, and 2.11%, combined with a fixed steel shear reinforcement (8 mm bars at 200 mm stirrup spacing) aimed at hybrid reinforcement respectively. These beams were subjected to four-point monotonic loading to assess their structural performance. Key parameters examined included ultimate flexural strength, deflection, cracking mode and failure mode. Experimental results revealed that the reused GFRP bars had retained their characteristic linear-elastic stress-strain behavior and high tensile strength without notable loss in mechanical performance. All reused GFRP-reinforced concrete beams exhibited linear-elastic behavior up to failure, with average experimental failure loads exceeding theoretical predictions by a factor of 1.47. Failure modes included diagonal tension failure and concrete crushing was predicted by theoretical analysis. Increasing the concrete compressive strength and tensile reinforcement ratio of reused GFRP bars improved structural behavior under bending loads, attributed to the high tensile capacity, improved stiffness, and bonding properties of the bars, similar to the findings observed in the virgin GFRP RC beams in their maiden form. These results highlight the superior durability and reusability of GFRP bars and their potential to enhance the sustainability of reinforced concrete structures, marking a significant step toward the practical reuse of composite materials in construction.

Keywords: Reused GFRP bar; RC beams; mechanical properties; reinforcement ratio; concrete strength; monotonic loading.

1. INTRODUCTION

Structural materials play a crucial role throughout
their life cycle, supporting the development of
resilient and sustainable infrastructure in
developing countries like Ghana. As urbanization
increases, the demand for innovative materials
that enhance performance while minimizing
environmental impact has grown. In this context,
composite materials like Glass Fiber Reinforced
Polymer (GFRP) bars have emerged as a
promising alternative to traditional steel
reinforcing bars in reinforced concrete (RC)
structures. GFRP bars, known for their high
strength-to-weight ratio, corrosion resistance,
and durability, are particularly valuable in
environments prone to moisture and chemical
exposure. Additionally, their lightweight
properties reduce transportation and installation
costs, while their non-conductive nature makes
them ideal for electromagnetic-sensitive areas
(Tahmouresi Kasra & Mohseni, 2022).

GFRP composites are primarily composed of
glass fibers embedded within a matrix of
polyester, vinyl ester, or epoxy resins. The fibers
primarily provide strength and stiffness, while the
resin's role is to bind the fibers together, transfer
forces between them, and protect them (Ahmed
et al., 2020). Fibre-reinforced polymer (FRP)
composites are now being used across a broader
range of industries than ever before, including
transportation, aerospace, and construction
sectors. The use of FRP composites in civil
engineering spans a broad spectrum of
applications, from strengthening, retrofitting, and
repairing existing structures to incorporating
GFRP composites as reinforcement in new
projects, either partially or fully, similar to
concrete-GFRP structures (Attia et al., 2023;
Abbood et al., 2021). American Concrete
Institute (ACI) committee 440-06 outlines the
guide for the design and construction of concrete
reinforced with FRP bars which underscores their
alignment with global initiatives for sustainable
construction and long-term infrastructure reliability (ACI Committee 440, 2006).

The mechanical properties of GFRP bars offer
several advantages over steel bars (Feng et al.,
2022; Liu et al., 2020). Boateng et al. (2024)
evaluated the mechanical properties of GFRP
bars of various diameters. Findings revealed that
GFRP bars typically exhibit a full linear-elastic
tensile stress-strain response without a distinct
yielding point up to failure, resulting in reduced
elongation capacity and failure strain. This failure
is characterized as brittle behavior, which
happens abruptly and with little warning, due to
their lack of yielding point and plastic properties
compared to steel bars. The authors further
recorded that this unique behavior of the GFRP
bar makes it anisotropic and heterogeneous.
These findings further confirm prior research
(Chenggao & Guijun, 2021; Ifrahim et al., 2023).
Issa et al. (2011) reported that GFRP reinforcing
bars possess a comparatively low modulus of
elasticity, ductility, and stiffness compared to
conventional steel. This reduced stiffness, along
with factors such as distinct bond behavior and
lower tension stiffening, leads to greater
deflections in GFRP-reinforced members than
those observed in steel-reinforced members
under any load stage. Ohene-Coffie et al. (2024)
in another research examined the anchorage
bond strength of GFRP reinforcing bars in normal
weight and lightweight concrete. Using 48 double
pull-out prismatic specimens with varying
embedment lengths and GFRP bar diameters,
the research recorded average bond strengths of
4.684 N/mm² and 3.558 N/mm² for 12mm and
16mm bars at a 100 mm embedment length, with
a maximum strength of 6.174 N/mm² at 300 mm
embedment (Ohene-Coffie et al., 2024).

Experimental studies have validated the ability of GFRP-reinforced beams to achieve improved flexural strength (Ahmed et al., 2020; Gora, 2022; Ma et al., 2021) and shear capacity (Liang et al., 2023; Kpo et al., 2025) under varying loading conditions. Kpo et al. (2024) investigated the strength and deformation characteristics of concrete beams reinforced with virgin GFRP bars through experimental testing and theoretical validation. The results indicated that the GFRP-reinforced concrete beams exhibited linear elastic load-deflection responses with reduced stiffness after first cracking, which resulted in significant initial deflections. The authors reported that increasing the concrete compressive strength from 23.4 N/mm² to 30.4 N/mm² in GFRP-reinforced beam specimens

resulted in a 28.6% increase in ultimate failure
load, while raising the GFRP longitudinal tensile
reinforcement ratio from 0.7% to 1.13% notably
improved the beams' ultimate flexural capacity.
Kpo et al. (2024) also found that in stark contrast
to steel RC beams, GFRP RC beams
experienced sudden brittle failure by concrete
crushing at higher ultimate loads and final
deflections due to their non-yielding, relatively
higher tensile strength and reduced modulus of
elasticity. Furthermore, Johnson (2014)
examined the suitability of GFRP as
reinforcement for concrete structures. The
research findings revealed that GFRP stirrups
achieved stress levels surpassing the minimum
design requirements. This enhancement in
stirrup strength contributed to overall beam
capacity that exceeded the predicted values
based on design code provisions. Studies
conducted by Mohammed and Said (2024)
yielded encouraging results regarding the use of
GFRP bars in concrete structures. The findings
demonstrated higher failure loads and greater
deflections compared to traditional steel
reinforcement, especially in beams made of
lower-strength concrete.

GFRP-reinforced elements are typically over-
reinforced, meaning the proportion of GFRP bars
to concrete exceeds the balanced ratio due to its
high tensile strength and elastic behavior. As a
result, the design is based on concrete controlled
section and failure mode is governed by concrete
crushing in the structural member (Sasikumar &
Manju, 2024; ACI Committee, 2015). As a result,
GFRP bars in failed members often remain intact
and can be recovered for reuse. There is
therefore the need to investigate the structural
performance of beams reinforced with these
reused GFRP bars recycled from failed structural
members.

1.1 Sustainability and Recycling of GFRP Bars

The production of virgin GFRP bars poses
minimal environmental challenges due to energy-
intensive manufacturing and the disposal of
waste in comparison to steel bars. The
environmental and economic implications of
disposing of GFRP waste in landfills highlights
the increasing demand for sustainable
construction practices, including recycling and
reusing materials to reduce waste and carbon
footprints (Ghardé & Kandasubramanian, 2019;
Krauklis et al., 2021). United Nations (UN) has
established 17 Sustainable Development Goals
(SDGs), with Goal 11 focusing on creating sustainable cities and human settlements. Promoting sustainable practices within the construction industry is a key aspect of this goal. Efforts such as recycling and repurposing waste materials in building projects play a crucial role in supporting this objective and contribute toward achieving the 2030 targets outlined in the agenda (United Nations, n.d.). Recycling glass fiber reinforcement from concrete beams is crucial for promoting sustainability in the construction industry, reducing waste, conserving resources, improving the economic viability and enhancing structural material performance (Ramin et al., 2023).

While existing research has largely focused on GFRP bars as an innovative alternative to steel reinforcing bars, there remains a significant gap in understanding the structural performance, superior durability, and reusability of GFRP reinforcing bars. By promoting the use of reused GFRP in reinforced concrete structures, this research aligns with global efforts to reduce construction waste and advance eco-friendly, resilient infrastructure.

# 2. EXPERIMENTAL PROGRAM

## 2.1 Materials

The experiment utilized locally sourced materials, including fine aggregate (natural pit sand, max particle size 4.75 mm), coarse aggregate (crushed granite, max size 12.5 mm), ordinary Portland cement, 12 mm nominal size ribbed and sand-coated reused GFRP reinforcing bars, 8 mm steel stirrups aimed at hybrid reinforcement, and potable water. Three concrete mix designs 1: 1.5: 3, 1: 2: 3, and 1:2:4 (cement: fine aggregate: coarse aggregate) by weight were used in casting the beams following the guidelines of IS: 10262 (Bureau of Indian Standards, 1982).

## 2.2 Preparation of Control Specimens

Concrete cubes (150 x 150 x 150 mm) were cast in accordance with BS EN 12390-3 (British Standards Institution, 2009a), as well as concrete cylinders (150 mm x 300 mm) specified by BS EN 12390-6 (British Standards Institution, 2009b) were cast as control for compressive strength and tensile strength respectively.

## 2.3 Preparation of Reused GFRP Bars

The use of recycled GFRP bars conformed to ACI 440.1R-06 (ACI Committee 440, 2006). The

virgin GFRP bars were initially tested in beams, and the beams failed due to concrete crushing as reported by Kpo et al. (2024). However, the GFRP bars remained intact due to their high tensile strength and full elastic behavior (Boateng et al., 2024). The recycling process involved the meticulous breaking of the concrete off the tested beams, leaving the GFRP reinforcing bars intact to ensure their effective reuse. The extracted GFRP reinforcing bars were thoroughly cleaned and inspected to evaluate their condition and suitability for reuse. The recycled bars were found to be in excellent condition, making them suitable for reintegration into this experimental study. Reinforcement percentages of 0.71%, 0.94%, 0.86%, 2.11%, and 2.11% reused GFRP bars were adopted for Beams 1, 2, 3, 4, 5 respectively. Similarly, a constant shear reinforcement of 8 mm steel stirrups spaced at 200 mm was used. These reinforcement details are depicted in Fig. 1.

## 2.4 Preparation of Reinforced Concrete Beams

A total of five beams, of various dimensions (see Table 1), were fabricated using recycled GRFP longitudinal reinforcement bars for the testing procedure (Table 2). Table 3 presents the mechanical properties of the reinforcing bars. The fresh concrete was placed into wooden formworks in layers and compacted with a poker vibrator to eliminate air pockets. After pouring the concrete, the beam and control specimens were left to set for 24 hours, then demolded and cured under wet hessian sacks for 28 days. Following curing, the beams were cleaned and left to dry for approximately one hour.

## 2.5 Test Procedures

### 2.5.1 Concrete compressive strength test

After removing the cubes (150 x 150 x 150 mm) from the wet sack and wiping them clean, their actual dimensions and weight were recorded. Each test cube was positioned in the compression machine so that the load was gradually and centrally applied to opposing sides until failure occurred, at which point the maximum load was noted. The compressive strength was calculated as the ratio of the maximum load to the cross-sectional area of the cube. The average compressive strength of the cured concrete cubes was tested at 7, 14, and 28 days using a Universal Testing Machine (UTM).
<figure><img src="image_3.png" /><figcaption>Fig. 1. Reinforcement details</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Reused GFRP bar cage and wooden formwork</figcaption></figure>

## 2.5.2 Concrete splitting tensile test

The test specimens (150 mm x 300 mm) stored under wet sacks were tested immediately after removal, while still in a wet state. The actual dimensions of each cylinder were measured. Each plain concrete cylinder was tested in the UTM along its length until failure.

## 2.5.3 Tensile test of reused GFRP bars

The tensile strength test of the reused GFRP reinforcing bars was conducted in accordance with ASTM D7205 (ASTM Committee D30, 2006). This was tested in a similar procedure as was carried out by Boateng et al. (2024) on the virgin GFRP before its initial testing in RC beams by Kpo et al. (2024). The test specimens with

diameters of 10 mm, 12 mm and 16 mm were prepared by securing each end with a 25 mm steel pipe featuring an inner diameter of 22 mm. The reused GFRP bars were inserted 150 mm into the steel pipes on both ends, leaving a free length of 300 mm. To fill the gap between the steel pipe and the reused GFRP bar, an epoxy mixture (Bisphenol A) with high-strength, non-shrink, and expanding additive properties was applied. The specimens were sealed and allowed to cure for 24 hours, followed by an additional three days for hardening. The tensile testing was conducted using a 1000 kN ELE Universal Testing Machine equipped with a 50 mm gauge length extensometer (Fig. 3). A constant tension rate of 3 mm per minute was applied until the specimens failed. Tensile strength, Young's modulus, failure strain, and stress-strain curves were recorded automatically during the
test. The reinforcing steel bars also underwent tensile testing. The results for both reused and virgin GFRP bars are presented in Table 3.

### 2.5.4 Testing of reinforced concrete beams

The cured reinforced concrete beams, at 28 days of maturity, were cleaned to facilitate crack detection. They were painted with white emulsion paint to enhance visibility and left to dry. The beams were mounted on a rigid steel loading frame equipped with two supports positioned 100 mm from each beam end. A 200 kN capacity hydraulic jack actuator of 2 kN intervals with an attached load cell was used to apply the load incrementally through a rigid steel spreader beam. This spreader beam distributed the load symmetrically to the specimen at two points, 400 mm apart, and located 600 mm or 700 mm from the nearest support. The testing setup included a digital dial gauge mounted beneath the beam center to measure deflection, along with permanent markers, a measuring rule, a magnifying glass, and thread. Cracks were monitored intermittently using the magnifying glass during incremental loading. Recorded parameters included deflection, initial crack load, total cracks, crack spacing, maximum crack width, crack length, and final failure load. At failure, crack propagation details—width, spacing, and types—were analyzed to evaluate beam performance. The data established the load-deflection relationship and provided insights into structural behavior. Fig. 4 shows the beam testing setup.

<table><caption>Table 1. Details of reused GFRP bar RC Beams and Virgin GFRP bar RC Beams</caption><thead><tr><th rowspan="2">Beam ID</th><th colspan="3">Beam Dimensions</th><th rowspan="2">Effective depth<br>(mm)</th></tr><tr><th>Length, L (mm)</th><th>Width, B (mm)</th><th>Overall Depth, D (mm)</th></tr></thead><tbody><tr><td colspan="5"><strong>Reused GFRP Bars RC Beams</strong></td></tr><tr><td>Beam 1</td><td>1800</td><td>150</td><td>250</td><td>211</td></tr><tr><td>Beam 2</td><td>2000</td><td>150</td><td>200</td><td>161</td></tr><tr><td>Beam 3</td><td>2000</td><td>125</td><td>250</td><td>211</td></tr><tr><td>Beam 4</td><td>1800</td><td>100</td><td>200</td><td>161</td></tr><tr><td>Beam 5</td><td>1800</td><td>100</td><td>200</td><td>161</td></tr><tr><td colspan="5"><strong>Virgin GFRP Bars RC Beams (Kpo et al., 2024)</strong></td></tr><tr><td>Beam 1</td><td>2000</td><td>120</td><td>200</td><td>172</td></tr><tr><td>Beam 2</td><td>2000</td><td>120</td><td>200</td><td>172</td></tr><tr><td>Beam 3</td><td>2000</td><td>120</td><td>200</td><td>172</td></tr><tr><td>Beam 4</td><td>2000</td><td>120</td><td>200</td><td>172</td></tr><tr><td>Beam 5</td><td>2000</td><td>120</td><td>200</td><td>172</td></tr></tbody></table>

<table><caption>Table 2. Properties of reused GFRP bar RC Beams and Virgin GFRP bar RC Beams</caption><thead><tr><th rowspan="2">Beam ID</th><th colspan="2">Concrete</th><th colspan="2">Reused GFRP Bar Reinforcement</th><th colspan="2">Steel stirrup size and spacing</th></tr><tr><th>Compressive strength (N/mm²)</th><th>Tensile strength (N/mm²)</th><th>Bar Diameter (mm)</th><th>% Main Reinforcement ratio</th><th>Bar size (mm)</th><th>Bar spacing (mm)</th></tr></thead><tbody><tr><td colspan="7"><strong>Reused GFRP Bars RC Beams</strong></td></tr><tr><td>Beam 1</td><td>23.1</td><td>2.23</td><td>12</td><td>0.71</td><td>8</td><td>200</td></tr><tr><td>Beam 2</td><td>15.6</td><td>1.89</td><td>12</td><td>0.94</td><td>8</td><td>200</td></tr><tr><td>Beam 3</td><td>16.90</td><td>1.73</td><td>12</td><td>0.86</td><td>8</td><td>200</td></tr><tr><td>Beam 4</td><td>16.02</td><td>1.84</td><td>12</td><td>2.11</td><td>8</td><td>200</td></tr><tr><td>Beam 5</td><td>22.86</td><td>2.03</td><td>12</td><td>2.11</td><td>8</td><td>200</td></tr><tr><td colspan="7"><strong>Virgin GFRP Bars RC Beams (Kpo et al., 2024)</strong></td></tr><tr><td>Beam 1</td><td>23.4</td><td>2.4</td><td>12</td><td>0.7</td><td>10</td><td>200</td></tr><tr><td>Beam 2</td><td>23.4</td><td>2.4</td><td>12</td><td>0.7</td><td>10</td><td>200</td></tr><tr><td>Beam 3</td><td>30.4</td><td>3.1</td><td>12</td><td>0.7</td><td>10</td><td>200</td></tr><tr><td>Beam 4</td><td>23.4</td><td>2.4</td><td>12</td><td>1.13</td><td>10</td><td>200</td></tr><tr><td>Beam 5</td><td>30.4</td><td>3.1</td><td>12</td><td>1.13</td><td>10</td><td>200</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Testing of tensile strength of reused GFRP bar</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Beam testing set-up</figcaption></figure>

# 3. RESULTS AND DISCUSSION

## 3.1 Mechanical Properties of Reinforcing Bars

Summarized in Table 3 are the mechanical properties of the reused GFRP bar used in this study as compared with the original virgin GFRP bar (Boateng et al., 2024) used as the longitudinal tension and compression reinforcement. The 10 mm, 12 mm, and 16 mm diameter reused GFRP bars developed ultimate tensile strengths of 1056.2 N/mm², 799.5 N/mm², and 713.8 N/mm² respectively by slippage failure mode (delamination of fiber) compared to the virgin GFRP bars that developed 920 N/mm²,

829 N/mm², and 573 N/mm² ultimate tensile strength respectively by slippage failure mode. The slight difference in the tensile strength was due to the heterogenous nature of GFRP bars as the tests involved the different test bar specimens in spite of being taken from the same supplied stock (Boateng et al., 2024). Additionally, the modulus of elasticity and ultimate elongation for the reused and virgin GFRP bars developed similar values as presented in Table 3. The test results revealed that the smallest bar size, 10 mm, exhibited the highest tensile strength and Young's modulus among all bar sizes. This may be attributed to its more compact molecular or atomic structure, which enhances its resistance to deformation or
failure under load. The strength and stiffness of GFRP bars appear to increase with decreasing bar size, likely due to stronger intermolecular or interatomic bonding—an observation consistent with the behavior of virgin GFRP bars in their original form (Boateng et al., 2024).

The stress-strain relationship for the 10 mm, 12 mm and 16 mm diameter reused GFRP bars in comparison with the virgin GFRP bars are depicted in Fig. 5. The graph shows that the reused GFRP bars exhibited a typical linear elastic stress-strain response (Fig. 5) without a distinct yield point up to failure, at which point they failed suddenly without prior warning, as was seen in the original GFRP bars. The results indicate that the GFRP bars retained their high tensile strength and mechanical integrity even after their initial use in the tested beams (Kpo et al., 2024), demonstrating their potential for multiple reuses. This can be attributed to the

non-yielding, perfect linear-elastic behavior of GFRP bars, which lack a defined plastic deformation stage (Boateng et al., 2024). Unlike reinforcing steel bar, which undergoes yielding and plastic strain accumulation upon exceeding its yield strength, GFRP bars remain within their elastic limit until failure. In contrast, steel reinforcing bars experience permanent plastic deformation after yielding, leading to strain hardening and eventual failure due to necking and fracture (Kankam and Adom-Asamoah, 2002). Once steel bar yields in an initial test, it cannot fully recover its mechanical properties, making it unsuitable for reuse in structural applications. On the other hand, GFRP bar, due to its inherently higher tensile strength, maintains its structural integrity after multiple usage, provided it does not reach its ultimate tensile strength and failure. This distinction highlights the superior reusability of GFRP reinforcement over conventional steel bars.

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. Stress-strain curves for reused versus virgin GFRP bars (Boateng et al., 2024)</figcaption></figure>
<table><caption>Table 3. Mechanical properties of reused and virgin GFRP bar</caption><thead><tr><th rowspan="2">Diameter (mm)</th><th colspan="3">Reused GFRP bar</th><th colspan="6">Virgin GFRP bar (Boateng et al., 2024)</th></tr><tr><th>10</th><th>12</th><th>16</th><th>10</th><th>12</th><th>16</th><th>10</th><th>12</th><th>16</th></tr></thead><tbody><tr><td>Failure mode</td><td>Slippage</td><td>Slippage</td><td>Slippage</td><td>Slippage</td><td>Rupture</td><td>Slippage</td><td>Rupture</td><td>Slippage</td><td>Rupture</td></tr><tr><td>Tensile strength (N/mm²)</td><td>1056.2</td><td>799.5</td><td>713.8</td><td>920</td><td>1193</td><td>829</td><td>1030</td><td>573</td><td>866</td></tr><tr><td>Modulus of elasticity (kN/mm²)</td><td>57.1</td><td>46.52</td><td>41.10</td><td>53.51</td><td>54.43</td><td>47.57</td><td>41.71</td><td>33.10</td><td>30.52</td></tr><tr><td>Ultimate elongation (%)</td><td>1.78</td><td>1.7</td><td>1.68</td><td>1.7</td><td>2.20</td><td>1.7</td><td>2.48</td><td>1.7</td><td>2.8</td></tr></tbody></table>

<table><caption>Table 4. Cracking and failure loads of beams</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Theoretical<br>cracking<br>load, Pcr (kN)</th><th rowspan="2">Experimental<br>cracking load,<br>P'cr (kN)</th><th colspan="3">Theoretical failure load (Pult) based on</th><th rowspan="2">Experimental<br>failure load<br>P'ult (kN)</th><th rowspan="2">P'cr/Pcr</th><th rowspan="2">P'ult/Pult</th><th rowspan="2">P'cr/P'ult</th></tr><tr><th>GFRP<br>Rupturing<br>(kN)</th><th>Concrete<br>Crushing (kN)</th><th>Shear Failure<br>(kN)</th></tr></thead><tbody><tr><td>Beam 1</td><td>7.1</td><td>8.5</td><td>71.61</td><td>*68.78</td><td>104.70</td><td>62</td><td>1.20</td><td>0.90</td><td>0.14</td></tr><tr><td>Beam 2</td><td>3.5</td><td>4.5</td><td>54.64</td><td>*27.03</td><td>81.10</td><td>28</td><td>1.29</td><td>1.04</td><td>0.16</td></tr><tr><td>Beam 3</td><td>4.6</td><td>6</td><td>71.61</td><td>*41.92</td><td>104.70</td><td>70</td><td>1.30</td><td>1.67</td><td>0.09</td></tr><tr><td>Beam 4</td><td>2.3</td><td>4</td><td>82.23</td><td>*18.51</td><td>121.65</td><td>38</td><td>1.74</td><td>2.05</td><td>0.11</td></tr><tr><td>Beam 5</td><td>2.5</td><td>3.5</td><td>82.23</td><td>*26.41</td><td>121.65</td><td>44</td><td>1.4</td><td>1.67</td><td>0.08</td></tr></tbody><tfoot><tr><td colspan="7"></td><td>Average</td><td>1.39</td><td>1.47</td><td>0.12</td></tr></tfoot></table>

Note: *Governing failure load of beam
## 3.2 Theoretical Flexural Analysis

### 3.2.1 Tensile strength of concrete

The tensile strength of plain concrete was determined using the split cylinder test results. The split cylinder strength of concrete is calculated using Eqn. 1.

$$f_t = \frac{2p}{\pi D l} \qquad (1)$$

where;

$f_t$ = Split cylinder strength (N/mm²).

$P$ = Maximum crushing load (N).

$D$ = Diameter of cylinder (mm).

$L$ = Length of cylinder (mm).

### 3.2.2 Cracking moment of reinforced concrete beam ($M_{cr}$)

The cracking moment ($M_{cr}$) for a reinforced concrete beam loaded at two points as shown in Fig. 6, is obtained using the tensile strength ($f_t$) in the expression as in Eqn. 2, assuming elastic behavior;

$$M_{cr} = f_t \frac{bd^2}{6} \qquad (2)$$

where;

$M_{cr}$ is the cracking moment (Nmm).

$f_t$ is the tensile strength (N/mm²).

$b$ = width of beam (mm).

$d$ = depth of beam (mm).

### 3.2.3 Theoretical cracking load of RC beam

From the 4-point loading system of the RC beam in Fig. 6., the theoretical cracking load of the beam can be expressed as shown in Eqn. 3;

$$P = \frac{2M}{L_1} \qquad (3)$$

where;

$P = P_{cr}$;

$P_{cr}$ is the theoretical cracking load (kN).

$M$ = $M_{cr}$ is the cracking moment (kNm).

$L_1$ is distance from the support to the nearest load point.

Results of the computations for theoretical cracking loads are captured in Table 4.

### 3.2.4 Analyses of theoretical failure load

#### 3.2.4.1 Theoretical failure load based on GFRP bar failing first

For the simply supported beam illustrated in Fig. 6, the ultimate load is expressed by Eqn. 4;

$$P_{ult} = \frac{2M_{rf}}{L_1} \qquad (4)$$

where;

$M_{rf}$ is the moment of resistance of GFRP bar in tension (kNm).

$P_{ult}$ is the ultimate failure load for GFRP bar (kN).

$L_1$ is distance from the support to the nearest load point.

Using a partial factor of safety $\gamma_m$ = 1.52 (Boateng et al., 2024), the moment of resistance of the reused GFRP in tension is given by;

$$M_{rf} = \frac{1}{\gamma_m} f_{yf} A_f \times 0.775d$$

or

$$M_{rf} = 0.66 f_{yf} A_f \times 0.775d \qquad (5)$$

where;

$f_{yf}$ is the tensile strength of GFRP bar (N/mm²).

$A_f$ is area of GFRP in tension zone (mm²).

$d$ is the effective depth of beam (mm).

#### 3.2.4.2 Theoretical failure load based on concrete crushing first

The theoretical failure load of a reinforced concrete beam, considering concrete crushing in compression (including the contribution of compression bars, if any), is given by Eqn. 6.

$$P_{ult} = \frac{2M_{rc}}{L} \qquad (6)$$

$M_{rc}$ =

$$0.156 f_{cu} b d^2 + \frac{f_{fc}}{\gamma_m} A_f (d - d^1) \qquad (7)$$

where;

$M_{rc}$ is the Moment of resistance of concrete beam (kNm).

$P_{ult}$ is the concrete crushing load (kN).

$f_{cu}$ is the concrete compressive strength (N/mm²).

$\gamma_m$ is the partial factor of safety of GFRP bar in compression.

$d^1$ is the effective depth of compression GFRP bar.

$A_f$ is the area of GFRP bars in compression (mm²).

$f_{fc}$ is the compressive strength of GFRP bar. $f_{fc} = 170$ N/mm² (McClure & Mohammadi, 1995).

This compressive strength of GFRP is relatively low coupled with a likely very high value of factor of safety $\gamma_m$, in compression and therefore contribution by GFRP bar in compression is ignored.
<figure><img src="image_9.png" /><figcaption>Fig. 6. Schematic diagram of loaded beams (a) Beams 2 and 3, (b) Beams 1, 4 and 5</figcaption></figure>

### 3.2.4.3 Theoretical failure load based on the assumption of shear failure occurring first

The shear failure load in the beams is given by:

$$V_f = 0.87 \frac{A_{sv}}{S_v} f_y * d + v_c bd \quad (8)$$

where $V_f$ is the shear failure load (kN).

$f_y$ is the yield strength of steel stirrups (N/mm²).

$v_c$ is the design concrete shear stress (N/mm²).

$A_{sv}$ is the area of shear reinforcement (mm²).

$S_v$ is the spacing of stirrups (mm).

$b$ is the width of the beam (mm).

$d$ is the effective depth of beam (mm).

## 3.3 Theoretical and Experimental Analysis

### 3.3.1 Load-deflection response

A comparison between the load-deflection curves for the reused GFRP RC beams and the virgin

GFRP RC beams (Kpo et al., 2024) is presented in Fig. 7. Each of the beams was subjected to a monotonic loading, wherein load increments of 2 kN were applied, and deflection measurements were recorded at each increment. The load at which cracking initiated was carefully recorded, and the beam underwent incremental loading until the failure point was achieved. The load applied to the beam was increased until failure, at which point the failure load was recorded. The load-deflection behavior of the beams (Fig. 7) initially exhibited elastic responses by their steep and linear curves before cracking occurred. This linear phase of the curve depicts the beam's capacity to withstand loads without permanent deformation. However, as the load increased, flexural cracks began to develop, leading to a noticeable change in the slope of the curves. Despite this shift, the curves remained relatively linear for a period, reflecting the beam's continued resistance under load. Eventually,
after significant deflection and a reduction in the flexural stiffness, the beams predominantly failed due to the crushing of concrete, with cracks extending deep into the compression zone. Furthermore, the load-deflection (Fig. 7) illustrates the similarity in ultimate load values between the reused GFRP RC beams (28 kN – 70 kN) and the original GFRP RC beams (28 kN – 80 kN), with comparable deflection ranges of 16.63 mm – 25.21 mm for the reused beams and 15.4 mm – 20.38 mm for the original beams. This indicates that reused GFRP RC beams displayed comparable load-deflection behavior to their original (virgin) form, as previously observed by Kpo et al. (2025, 2024).

## 3.4 Cracking Loads

The theoretical and experimental cracking loads for the monotonically loaded beams are presented in Table 4. The average ratio of experimental cracking loads (P'cr) to theoretical cracking loads (Pcr) for beams reinforced with reused GFRP bars was 1.39. Beam 5 recorded the lowest experimental cracking load of 3.5 kN, followed by Beam 4 of 4 kN cracking load with Beam 1 recording the highest experimental cracking load. The average experimental cracking load for the five beams was of 5.3 kN. Also presented in Table 4, the ratio of experimental cracking load to the experimental failure load (P'cr/P'ult) ranged from 0.08 to 0.16 with an average of 0.12. This demonstrates that reused GFRP RC beams can carry an average of 12% of its ultimate load before the initial crack develops. Due to the lower modulus of elasticity of GFRP bars compared to steel, GFRP-RC beams exhibited larger deflections and wider cracks after first cracking (Kpo et al., 2025, 2024). However, their high tensile strength allowed them to sustain higher loads beyond cracking, further increasing the gap between cracking and

ultimate load, resulting in a low cracking-to-failure load ratio.

## 3.5 Failure Loads

Table 4 summarizes the theoretical and experimental failure loads of the beams subjected to monotonic loading. The average experimental failure loads (P'ult) for the reused GFRP RC beams exceeded the theoretical failure load (Pult) by a factor of 1.47. The experimental failure loads (P'ult) for beams 2, 3, 4 and 5 exceeded the theoretical failure loads (Pult) by 4%, 67%, 105% and 67%. Furthermore, Table 4 shows that the experimental failure loads (P'ult) of the reused GFRP RC beams ranged from 28 kN to 70 kN. Beams 1, 2, 3, 4, and 5, with respective average compressive strengths of 23.1 N/mm², 15.6 N/mm², 16.9 N/mm², 16.02 N/mm², and 22.86 N/mm², recorded ultimate failure loads of 62 kN, 28 kN, 70 kN, 38 kN, and 44 kN, respectively. It is worth highlighting that the ultimate failure loads were similar to those recorded for the original GFRP RC beams tested by Kpo et al. (2024), which ranged from 28 kN to 80 kN (Fig. 7). A noticeable trend, is that an increase in the concrete compressive strength of the beam generally corresponded to an increase in its load-carrying capacity when reinforced with reused GFRP bars, and vice versa as confirmed by earlier literature (Attia et al., 2023; Al-Sunna et al., 2012). Similarly, reused GFRP-reinforced beams with higher longitudinal reinforcement ratios exhibited higher failure loads compared to the lower failure loads observed in the beam with a lower tensile reinforcement ratio of GFRP bars due to the enhanced stiffness and bonding properties. This underscores the preservation of structural integrity, indicating that the GFRP RC beams maintained their performance even in the reuse application, consistent with the findings from their original form as reported in previous studies (Kpo et al., 2025, 2024).

<table><caption>Table 5. Deflections and crack modes</caption><thead><tr><th rowspan="2">Beam ID</th><th rowspan="2">Final deflection at failure (mm)</th><th rowspan="2">Average crack width (mm)</th><th rowspan="2">Average crack spacing (mm)</th><th colspan="3">Types and No. of cracks</th></tr><tr><th>No. of pure flexural cracks</th><th>No. of flexural shear cracks</th><th>No. of diagonal shear cracks</th></tr></thead><tbody><tr><td>Beam 1</td><td>25.21</td><td>1</td><td>86.51</td><td>11</td><td>8</td><td>8</td></tr><tr><td>Beam 2</td><td>23.59</td><td>2</td><td>100.0</td><td>6</td><td>6</td><td>7</td></tr><tr><td>Beam 3</td><td>22.63</td><td>1</td><td>71.62</td><td>8</td><td>4</td><td>3</td></tr><tr><td>Beam 4</td><td>16.63</td><td>1</td><td>98.56</td><td>4</td><td>7</td><td>6</td></tr><tr><td>Beam 5</td><td>17.99</td><td>1</td><td>68.22</td><td>8</td><td>8</td><td>12</td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>Fig. 7. Load - deflection curves (a) Reused GFRP RC beams (b) Virgin GFRP RC beams (Kpo et al., 2024)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Beam 1 failure mode</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 9. Beam 2 failure mode</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. Beam 3 failure mode</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 11. Beam 4 failure mode</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Beam 5 at failure</figcaption></figure>
## 3.6 Cracking Mode

Table 5 provides details on the types of cracks, their propagation, and crack widths. For the reused GFRP reinforced beams, the average crack spacing ranged from 68.22 mm to 100.0 mm. Additionally, Table 5 presents the types and quantity of cracks formed. The types of cracks observed on the beam at failure were pure flexural cracks, flexural-shear, and diagonal-shear cracks. Beam 1 recorded 11 pure flexural cracks within the constant moment area, 8 flexural-shear cracks outside the constant moment area, and 8 diagonal-shear cracks. Beam 2 exhibited 6 pure flexural cracks, 6 flexural-shear cracks and 7 diagonal-shear cracks. Beam 3 recorded 8 pure flexural cracks, 4 flexural-shear cracks and 3 diagonal-shear cracks. A total of 4 pure flexural cracks, 7 flexural-shear cracks and 6 diagonal-shear cracks were observed for Beam 4. Furthermore, Beam 5 exhibited 8 pure flexural cracks, 8 flexural-shear cracks and 12 diagonal-shear cracks. The discernible trend observed is that, higher concrete strength reduced crack width by enhancing the tensile resistance of the concrete prior to cracking. Additionally, it increased crack spacing, as higher strength concrete delayed crack initiation and facilitated stress redistribution over a larger area. A higher GFRP reinforcement ratio reduced crack width by providing greater resistance to tensile forces and improving stress distribution by bonding along the reinforcement. The cracking mode of the concrete beams reinforced with reused GFRP bars were similar to those with original virgin GFRP bars as studied by Kpo et al. (2024).

## 3.7 Failure Mode

The mode of failure for beams subjected to monotonic loading is influenced by various factors, including the bond between the reused GFRP bar and the concrete, the strength of the concrete, amount of the reinforcing bar in the compression and tension zones, as well as the size and spacing of the stirrups. The simply supported beam analyzed in this study, under monotonic loading, exhibited a maximum constant bending moment and zero shear force across the central span. Conversely, the outer spans experienced maximum shear forces and varying bending moment magnitudes depending on the applied loads. The first crack consistently appeared in the central span of each beam specimen, highlighting it as the region of highest strain. Figs. 8 to 12 illustrate the failure modes of

the beams at failure. Beam 1 developed its first crack at a load of 8.5 kN and experienced sudden brittle failure at 62 kN, with a final deflection of 25.21 mm due to concrete crushing. Similarly, Beam 2 developed its first crack at 4.5 kN and failed abruptly at 28 kN with a final deflection of 23.59 mm. Beam 3 showed its first crack at 6 kN and reached failure at 70 kN, recording a final deflection of 22.63 mm. For Beam 4, the first crack occurred at 4 kN, and failure ensued at 38 kN with a final deflection of 16.63 mm. Lastly, Beam 5 developed its first crack at 3.5 kN and underwent sudden brittle failure at 44 kN, with a final deflection of 17.99 mm. This was attributed to diagonal tension failure within the concrete, and flexural failure caused by concrete crushing, as was seen in the GFRP RC in its original form (Kpo et al., 2024). The findings reveal that the structural performance of reused GFRP-reinforced concrete beams is primarily influenced by the combined effect of the reused GFRP bars' high tensile strength and the concrete's compressive strength, which together improve post-cracking stiffness, consistent with observations from the earlier study on virgin GFRP-RC beams by Kpo et al. (2024).

## 4. CONCLUSIONS

This research explored the structural behavior of concrete beams reinforced with reused GFRP bars, offering a sustainable alternative to traditional steel reinforcing bars. The study focused on the impact of varying concrete compressive strengths and tensile reinforcement ratios with constant hybrid steel shear reinforcement on the structural performance of beams, which were subjected to a four-point monotonic loading system. Through a detailed evaluation of both theoretical and experimental findings, the following conclusions were reached:

I. Reused GFRP bars retained their mechanical properties comparable to their original form, highlighting their potential for multiple reuse in structural applications, as demonstrated in this study. This presents a clear advantage over steel reinforcement, which yields and undergoes permanent deformation at relatively lower tensile strength, limiting its suitability for repeated structural use.

II. The comparable stress-strain response between the reused and the virgin GFRP bars clearly demonstrates that the reused bars retain their linear-elastic behavior and
high tensile strength, underscoring their
mechanical integrity and potential for reuse
in structural applications without significant
degradation in performance.

III. The experimental failure loads (P'ult) of the recycled GFRP RC beams averaged 1.47 of the theoretical failure loads (Pult), highlighting the improved flexural performance of reused GFRP bars in the beams.

IV. Reused GFRP-reinforced beams developed initial cracks at approximately 12% of their ultimate load due to the bars' lower modulus of elasticity, which led to greater initial deformations. However, their high tensile strength enabled them to carry significantly higher loads beyond the cracking point, thereby widening the gap between the cracking and ultimate loads and resulting in a low cracking-to-failure load ratio.

V. The reused GFRP-reinforced concrete beams displayed a comparable linear-elastic load-deflection response to those in their original form from the previous study and ultimately failed in a sudden brittle manner—either through diagonal tension or concrete crushing—as confirmed by theoretical analysis. This behavior is attributed to the inherently non-yielding nature of GFRP bars.

VI. The beam specimen exhibited remarkable deformability prior to failure, showcasing superior flexibility. However, there was minimal residual deformation after failure of beam, evidenced by the beams apparently returning to their original position after failure by concrete crushing.

VII. Increasing concrete strength and tensile reinforcement ratio significantly improved the flexural performance of the reused GFRP RC beams, consistent with findings from the previous study on the original (virgin) GFRP bars. Higher concrete strength enhanced stiffness and crack spacing, while a greater reinforcement ratio reduced crack width, spacing, and deflections.

DISCLAIMER (ARTIFICIAL INTELLIGENCE)

Author(s) hereby declare that NO generative AI
technologies such as Large Language Models
(ChatGPT, COPILOT, etc.) and text-to-image
generators have been used during the writing or
editing of this manuscript.

COMPETING INTERESTS

Authors have declared that no competing
interests exist.

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