https://orcid.org/0009-0006-7192-7489
Figures
Abstract
The utilization of rice husk ash (RHA) as a cement substitute in concrete can mitigate the environmental issues caused by concrete production. This study is the first investigated the effect of different concentrations of multiwall carbon nanotubes (MWCNTs) on the shear behaviour of RHA-reinforced concrete (RC) beams. Three RHA RC beams incorporating different MWCNTs concentrations (0%, 0.1%, and 0.2%) were subjected to shear failure tests, with shear behaviour evaluated based on load-deflection capacity, failure load, strain distribution, and crack development morphology. The results indicate that the incorporation of well-dispersed MWCNTs (0.1%) significantly enhances the shear resistance of RHA RC beams. Specifically, with the addition of 0.1% MWCNTs, the first crack load, initial stiffness, and ultimate shear capacity increased by 34.78%, 38.4%, and 4.76%, respectively, while midspan deflection and ductility improved by 23.19% compared to the control specimen. However, excessive MWCNTs incorporation negatively impacted shear capacity, as the inclusion of 0.2% MWCNTs led to an 8.57% reduction in the ultimate shear capacity of the RHA RC beam. Additionally, the experimental results were compared with predictions from several models, including the ACI 318 model, Eurocode 2–05 model, Canadian standard, Bazant and Kim, Zsutty, and Collins and Kuchma models. The comparison revealed that the Eurocode 2–05 model, Canadian standard, and Zsutty models provide reliable predictions of the shear behaviour of RHA RC beams, with an error margin within 20%. The findings suggested that well-dispersed MWCNTs can play a role in improving the initial stiffness and ductility of RHA RC beams, enhancing the shear resistance of the beams, delaying shear failure, and preventing brittle failure of the beams. Furthermore, the improvement of the performance of RHA sustainable concrete components by MWCNTs contributes to the sustainable development of the construction industry.
Citation: Jing Y, Lee JC, Moon WC, Ng JL, Yew MK, Jin Y (2025) Shear performance evaluation of sustainable concrete beams containing rice husk ash and carbon nanotubes as cement replacement. PLoS One 20(6): e0320325. https://doi.org/10.1371/journal.pone.0320325
Editor: Parthiban Kathirvel, SASTRA Deemed University, INDIA
Received: December 17, 2024; Accepted: February 17, 2025; Published: June 4, 2025
Copyright: © 2025 Jing et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting Information files.
Funding: This work was supported by W.C.M.’s funding, which was funded by the Malaysian Ministry of Higher Education (MOHE) for supporting this work through the Fundamental Research Grant Scheme (FRGS), under project code FRGS/1/2024/TK06/UCSI/02/1.
Competing interests: Wei Chek Moon reports financial support was provided by Malaysian Ministry of Higher Education (MOHE). The other authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
1. Introduction
1.1. Background
Concrete components are widely utilised in the construction industry due to their excellent mechanical properties and durability. However, concrete production poses significant environmental challenges. The primary source of aggregates in concrete is non-renewable natural stone, while cement, its key constituent, is produced in vast quantities. Since the 2010s, the rapid expansion of the global construction industry has driven a substantial increase in cement production, with annual output averaging over 4 billion tonnes in the past decade [1]. Cement manufacturing is a major contributor to carbon dioxide emissions, accounting for approximately 7% of global emissions, with severe consequences for the global ecosystem [2]. To address the environmental impact of concrete production, researchers have explored various solutions, including geopolymer concrete, lightweight concrete, and sustainable concrete [3–6]. A fundamental approach to reducing these environmental issues is the development of viable substitutes for cement and aggregates. The utilisation of supplementary cementitious materials (SCMs) has significantly reduced greenhouse gas (GHG) emissions from cement production [7]. Common SCMs are classified as either industrial by-products (e.g., fly ash, blast-furnace slag, silica fume, and metakaolin) or agricultural by-products (e.g., rice husk ash (RHA), olive waste ash, corncob ash, and bagasse ash) [8–15]. Similarly, alternative aggregate materials have been explored, including perlite, lightweight expanded clay aggregate, waste nylon, coal waste, and waste polyethylene terephthalate [16–20]. Among these, RHA is one of the most readily available agricultural waste materials in Asia, offering distinct regional advantages for use in the construction industry. Consequently, it has attracted considerable attention from researchers.
Asia is the world’s leading region for rice production and consumption, accounting for approximately 90% of global rice output. China is the largest producer, yielding 206.6 million tonnes of rice in 2022, followed by India and Indonesia, which produced 196.25 and 54.75 million tonnes, respectively [21,22]. RHA, a byproduct of the high-temperature incineration of rice husks, poses significant environmental risks when disposed of indiscriminately, as it can harm ecosystems, pollute agricultural land, and pose health hazards [23]. However, when processed under optimal conditions—specifically, controlled combustion temperatures between 500°C and 700°C—RHA becomes an effective supplementary cementitious material due to its high amorphous silica content (over 80%) [24,25]. The amorphous silica in RHA exhibits pozzolanic activity, reacting with Ca(OH)₂ released during cement hydration to form hydrated calcium silicate (C-S-H) gel. This reaction enhances the internal structure of cement-based materials, improving their overall performance [26,27]. Extensive research has been conducted on the mechanical and durability properties of concrete containing RHA. Balapour et al. (2017) [28] examined the effects of incorporating up to 15% nano- and micro-RHA on the compressive strength and electrical resistivity of mortars. Their findings revealed that, compared to control specimens without RHA, the 90-d compressive strength increased by 36.18%, while the 28-d electrical resistivity improved by 147.45%. Similarly, Faried et al. [29] investigated the influence of RHA produced at varying combustion temperatures (300°C to 900°C) and durations (3–9 h) on the mechanical and durability properties of concrete. The results indicated that RHA produced at 700°C for 5 h yielded the most significant performance improvements. Compared with the control group, the addition of 3% RHA increased compressive strength by 21.9% and splitting tensile strength by 26.67%, while sorptivity decreased from 5*10-6 mm/sec0.5 to 0.33*10-6 mm/sec0.5. Beyond investigations into the mechanical properties, durability, and functionality of RHA-incorporated concrete, some researchers have focused on the performance of reinforced concrete (RC) beams containing RHA. Osman et al. [30] studied the shear behavior of ultra-high performance fibre-reinforced concrete (UHPFRC) beams using RHA, sugar cane bagasse ash, and fly ash as SCMs. Their results showed that a 10% RHA replacement level optimised the shear behavior of UHPFRC beams, leading to comprehensive improvements in shear strength, stiffness, and toughness. Among them, the shear strength was improved by 15%. Hassanean et al. (2024) [31] investigated the shear behaviour of RC beams incorporating varying percentages of RHA (up to 20%) and found that replacing 10% of cement with RHA significantly enhanced the shear capacity, stiffness, and toughness of the RC beams.
To expand the application of RHA-based sustainable concrete in the construction industry and further enhance its overall performance, researchers have introduced nanoparticles into RHA concrete. Common nanoparticles used in RHA concrete include nano-SiO2, nano-Al2O3, and nano-TiO2 [32–34]. The performance enhancement of cement-based materials by nanoparticles is primarily attributed to their nucleation, bridging, filling, and pozzolanic activity [35–37]. Anto et al. [38] reported that the combined incorporation of 3% nano-SiO2 and 10% RHA resulted in significant improvements in compressive strength, flexural strength, water sorption resistance, and acid attack resistance. Similarly, Avudaiappan et al. [33] observed that the addition of 1.0% nano-SiO2 and 10% RHA increased the 28-d compressive strength, flexural strength, and splitting tensile strength from 24.5 MPa, 3.71 MPa, and 2.56 MPa to 31.3 MPa, 4.2 MPa, and 3.28 MPa, respectively. Additionally, the synergistic effect of RHA and well-dispersed nano-SiO2 effectively reduced the porosity of the concrete. Alex et al. [34] observed that incorporating 10% micro RHA and 1% nano-Al₂O₃ increased the 28-d compressive strength and splitting tensile strength of cement mortar by 26.8% and 48.72%, respectively, while also significantly reducing water absorption. Meddah et al. [39] reported that the nucleation, pozzolanic activity, and filler effect of nano-Al₂O₃ accelerated cement hydration, generated additional C-S-H gel, filled capillary pores, and compacted the internal structure of RHA-incorporated concrete, thereby enhancing its mechanical properties. Praveenkumar et al. [32] demonstrated that the addition of 3% nano-TiO2 and 10% RHA significantly improved the overall performance of concrete. Compared with the control specimen, the 28-d compressive strength increased by 11%, while water absorption, porosity, and chloride ion penetration were notably reduced. This improvement is attributed to the synergistic effect of RHA’s pozzolanic properties and nano-TiO₂’s filler effect, which effectively refines the pore structure and densifies the cement matrix [40].
The failure of concrete structural elements poses a significant risk to building safety [41,42]. To reduce the risks associated to concrete structural failure, researchers have suggested multiple approaches to improve the structural performance of concrete. The use of concrete-filled steel tubes can diminish the brittleness of concrete components and enhance the ductility of concrete constructions [43–45]. The utilization of fiber-reinforced polymer (FRP) bars can improve bond strength, thus enhancing the mechanical performance of concrete structural elements. Furthermore, the corrosion-resistant characteristics of FRP bars can augment the durability of concrete structural elements in harsh situations [46–49]. Crack damage is one of the main reasons for the reduction of concrete structure performance, and the incorporation of fiber materials into concrete has been proven to effectively control the development of cracks [50,51]. The fibers typically utilized in concrete comprise steel fibers, polyethylene fibers, glass fibers, carbon fibers, and carbon nanotubes, etc [52–56]. Among them, CNTs, cylindrical nanomaterials formed by carbon allotropes, can effectively control the development of microcracks due to their excellent physical properties and high aspect ratio [57–60]. Demczyk et al. [61] reported that the tensile strength and elastic modulus of multiwall carbon nanotubes (MWCNTs) can reach up to 0.15 TPa and 0.9 TPa, respectively. The strengthening effect of CNTs in cement-based materials is primarily attributed to their nucleation, filling, and bridging mechanisms, which promote the formation of C-S-H gel, effectively densify the internal pore structure, and inhibit the development of microcracks [62,63]. Sarvandani et al. [64] studied the effect of COOH-MWCNTs on the mechanical and durability properties of cement mortar in sulfate solutions and saturated limewater environments. Their findings showed that incorporating 0.2% COOH-MWCNTs increased the 180-d compressive strength of cement mortar in a saturated limewater environment by 22.1%. Additionally, in a sulfate environment, the 360-d compressive strength loss and flexural strength loss decreased by 13.37% and 14.08%, respectively, while the 90-d capillary water absorption rate was reduced by 39.2%. Yao and Lu [65] investigated the mechanical properties of concrete incorporating various CNT concentrations at different temperatures (room temperature, 300°C, 600°C, and 900°C). Their results indicated that with 0.5 wt% CNTs, the 28-d compressive strength at room temperature and the residual compressive strength at 300°C reached peak values compared to control specimens without CNTs. Recent research has also explored the application of CNTs in RC beams. Irshidat [66] observed that incorporating 0.2% CNTs enhanced the compressive and splitting tensile strengths of concrete by 23% and 22%, respectively. Furthermore, CNTs improved the bond strength and stiffness between steel reinforcement and concrete. Hassan et al. [67] reported that well-dispersed CNTs significantly enhanced the bond strength of 12 mm and 16 mm diameter steel bars, with increases of 36% and 21%, respectively. Öztürk. [68] studied the engineering properties of geopolymer concrete beams containing a small amount of CNTs or carbon black (CB) under mixed failure modes (shear and flexural). The results showed that compared with the CB-based geopolymer beam, the loading-carrying capacity, initial stiffness, and energy dissipation capacity of the CNT-based geopolymer beam increased from 15.2 KN, 2.6 KN/mm, and 113.14 KN.mm to 27.8KN, 3.2 KN/mm, and 369.14 KN.mm, respectively.
In summary, the production of sustainable concrete should not only focus on minimising its environmental impact but also on enhancing its overall performance compared to conventional concrete, thereby broadening its potential applications. In the case of RHA-based sustainable concrete, researchers have explored the incorporation of various nanoparticles to develop high-performance RHA concrete. However, despite the exceptional microcrack control capabilities of CNTs, their application in RHA concrete remains largely unexplored. Notably, no studies have yet investigated the use of CNTs in RHA concrete structural components.
Given these considerations, this study is the first to evaluate the effect of different MWCNTs concentrations on the shear performance of RHA-modified concrete beams without shear reinforcement. By systematically varying the MWCNTs content (0% to 2%), a series of modified RHA RC beams were prepared, and their shear behaviour was analysed in terms of crack propagation, failure modes, shear capacity, load-deflection curves, and load-strain responses.
1.2. Research significance and novelty
To reduce the environmental impact of concrete production, RHA has been widely recognised as a low-carbon alternative to cement. While numerous studies have explored ways to enhance the overall performance of RHA concrete, the majority have focused on its mechanical properties and durability, with relatively little research on its application in structural components. At the same time, MWCNTs, due to their excellent material properties, have promoted the mechanical and durability characteristics of cement-based materials, particularly in terms of crack control [69]. To expand the application of RHA sustainable concrete, particularly in structural elements, research on MWCNTs-modified RHA concrete components is essential. This study addresses this gap by evaluating the shear behaviour of RHA RC beams containing MWCNTs, providing a foundation for the potential use of RHA concrete in structural applications and contributing to the reduction of environmental issues in the construction industry.
2. Materials and methods
2.1. Materials
Ordinary Portland cement (OPC) Type 42.5, with a specific surface area of 3430 cm2/g and a specific gravity of 3.14, was used as the binder. Crushed limestone with particle sizes ranging from 5 to 20 mm served as the coarse aggregate, while river sand with a particle size range of 0–4.75 mm and a fineness modulus of 2.9 was used as the fine aggregate. A polycarboxylate ether (PCE) superplasticiser, offering a water reduction rate of 28%, was employed. Rice husks sourced from Xuzhou, China, were incinerated in a furnace at 800°C for 60 min to produce RHA, which consisted of 87.3% silicon dioxide (SiO2), 2.8% potassium sulphate (K2SO4), and 1.3% calcium carbonate (CaCO3). The physical properties of MWCNTs are presented in Table 1.
2.2. Mix proportions and specimen preparation
Table 2 presents the mixture proportions. Aggregates, cement, and RHA were sequentially added to the mixer and blended for 2 min. Subsequently, the PCE, uniformly dispersed MWCNTs, and the remaining water were introduced and mixed for an additional 5 min. To ensure effective dispersion, an ultrasonic method was employed. The MWCNTs aqueous solution, contained in a beaker, was ultrasonicated for 30 min using a 750 W ultrasonic disperser with a cycle of 3 s on and 3 s off. To maintain the solution at room temperature, the beaker was submerged in a cold-water container, which was replenished with fresh water every 10 min.
Three RC beams, each measuring 1000 mm × 125 mm × 250 mm and prepared according to the mix proportions specified in Table 2, were used in this study. All steel reinforcements were obtained from a single supplier. HRB500 bars (hot-rolled ribbed steel bars) with a 16 mm diameter were used for flexural reinforcement. Shear reinforcement was provided at the beam supports over a 150 mm span (at 50 mm intervals) using HPB300 bars (hot-rolled plain steel bars) with an 8 mm diameter. The mechanical properties of these steel bars are presented in Table 3. Additionally, 100 mm cube specimens were prepared to assess the 28-d compressive strength following the GB/T 50080–2019 test method [70]. The properties of all beams are summarised in Table 4. Both the beams and specimens were cured under standard conditions. The reinforcement arrangement of the beams is shown in Fig 1.
2.3. Test method
The shear behaviour of the beams was tested in accordance with GB/T 50152-2012 [71]. Data collection was automated using a static resistance strain gauge connected to a dynamic signal acquisition and analysis system. Testing was performed on a universal testing machine with a maximum capacity of 600 kN. Midspan deflection was measured using a linear voltage differential transducer (LVDT). A graded loading system with 12 load levels was employed, with the initial load applied at each interval using load control at a rate of 5 kN/min. To evaluate beam deformation under different loads, strain gauges were affixed to the stressed steel bars and beam surfaces during specimen preparation, while displacement meters were installed at designated locations on the specimens. Figs 2 and 3 illustrate the positions of the strain gauges on the stressed steel bars and beams, respectively. This study analysed the strains in the steel bars (SG-1 to SG-4) and in the compression zone of the midspan concrete (SG-5 to SG-7). Additionally, four groups of strain gauge rosettes (SGR1 to SGR4), each consisting of three strain gauges arranged at angles of 0°, 45°, and 90°, were positioned in a straight line from the loading point to the support to measure the principal strains in the shear span. Once the load reached 60 kN, loading was continued until specimen failure. Furthermore, shear crack widths were measured after each loading level using a crack-width meter or comparison card, with an accuracy of 0.02 mm.
3. Results and discussion
3.1. Crack behaviour
Fig 4 illustrates the crack patterns observed at the failure of all beams, while Table 5 presents the experimental results, which are discussed in the following section.
For Beam B1, no significant phenomena were observed during the initial loading phase. The first crack appeared at a load of 46 kN at the bottom midspan, measuring 4 cm in length and 0.2 mm in width. As the load increased to 52 kN, this midspan crack extended upwards, and a new crack emerged at the left support, measuring 6 cm in length and 0.2 mm in width. This crack extended diagonally towards the loading point, progressively widening. At 58 kN, both the midspan crack and the crack at the left support continued to propagate upwards, while a new crack, 8 cm in length and 0.47 mm in width, appeared at the right support. When the load reached 68 kN, a branch crack formed on the left primary crack, measuring 5 cm in length and 0.65 mm in width. Upon reaching 80% of the ultimate load, the existing cracks continued to extend along their original paths, with minimal formation of new cracks. At the ultimate load of 105 kN, the maximum crack width reached 1.6 mm, extending from the support to the load point, ultimately causing failure and significantly reducing the beam’s load-bearing capacity.
For Beam B2, the first crack appeared at a load of 62 kN at the bottom left support, measuring 4 cm in length and 0.18 mm in width. At 68 kN, another crack formed at the bottom right support, measuring 6 cm in length and 0.25 mm in width. The load was applied in 5 kN increments. As loading progressed, the existing cracks widened, but no additional cracks developed. At the failure threshold of 110 kN, the maximum crack width reached 1.8 mm.
For Beam B3, the first crack appeared at a load of 46 kN at the bottom left support, with a length of 3 cm and a width of 0.25 mm. At 53 kN, an oblique crack developed at the bottom right support, measuring 7 cm in length and 0.32 mm in width. As the load increased, cracks on both sides of the support extended diagonally from the fracture point towards the top of the loading point, with their widths progressively widening. At the failure load of 96 kN, the maximum crack width reached 1.74 mm, marking the beam’s load-bearing limit.
Fig 5 illustrates the relationship between the main diagonal crack width and nominal shear stress, calculated using the following equation:
where denotes the effective depth (mm).
As shown in Fig 5, the main diagonal crack development rates were similar for all beams. However, under the same nominal shear stress, the inclusion of MWCNTs effectively reduced crack width during crack propagation. For example, at a nominal shear stress of 1.18 MPa, the maximum main diagonal crack widths for Beams B1, B2, and B3 were 0.5 mm, 0.18 mm, and 0.45 mm, respectively. This reduction is primarily attributed to the bridging effect of MWCNTs, which effectively delayed crack propagation [72]. Similar findings were reported by Naji et al. [73], who observed that the addition of MWCNTs increased the ultimate load and reduced the number of cracks due to their bridging effect, which hindered crack propagation.
3.2. Failure mode
The cracking patterns of all beams were generally similar, exhibiting pure shear failure, prominent diagonal fissures, and signs of concrete crushing in the upper section. Upon unloading, the test beams displayed considerable rebound. As shown in Table 5, the incorporation of MWCNTs enhanced the initial stiffness of the RHA RC beams, increasing it by 38.8% for Beam B2 and 13.7% for Beam B3 compared to Beam B1. Furthermore, the inclusion of 0.1% MWCNTs led to a 34.8% increase in the first crack load and a 4.8% improvement in the ultimate shear capacity of Beam B2 relative to Beam B1. The 28-d compressive strength of Beam B2 also increased by 8.7% compared to Beam B1, confirming that the incorporation of MWCNTs improved the mechanical properties of RHA concrete. The enhancement in 28-d compressive strength, initial stiffness, first crack load, and ultimate shear capacity with the addition of 0.1% MWCNTs can be attributed to the following mechanisms:
- Nucleation effect − MWCNTs slow the early coverage of C-S-H gel on unhydrated cement and RHA particles, thereby enhancing the pozzolanic reaction between cement and RHA particles and densifying the internal structure of the concrete.
- Filler effect − Due to their nanoscale characteristics, MWCNTs effectively fill the capillaries in cement-based materials, reducing the number of macropores and capillaries while promoting the formation of high-rigidity C-S-H gel.
- Bridging effect − MWCNTs can effectively bridge microcracks and improve the interface transition zone (ITZ) [58,74–77].
Lee et al. [78] reached a similar conclusion in their study on the effect of CNTs on the shear behaviour of polyvinyl alcohol-engineered cementitious composite beams. Their findings indicated that the incorporation of well-dispersed CNTs increased shear capacity by up to 33% and enhanced the initial stiffness of the beams. However, when the MWCNTs concentration reached 0.2%, the 28-d compressive strength and ultimate shear capacity of Beam B3 significantly decreased by 18.14% and 8.57%, respectively, compared to Beam B1. This reduction is likely due to the re-agglomeration of MWCNTs at high concentrations. The clustering of MWCNTs around cement grains inhibits hydration reactions, increases porosity, and introduces internal defects in the cement matrix, ultimately reducing the mechanical properties of RHA concrete [69,79–81].
3.3. Load-deflection curves
The load-deflection curves for the three beams are shown in Fig 6. As illustrated in Fig 6 and Table 5, the addition of MWCNTs significantly improved the ductility of the RHA RC beams. Specifically, with 0.1% MWCNTs, the ultimate load of Beam B2 increased to 110 kN, with a corresponding midspan deflection of 7.0 mm, whereas Beam B1 exhibited a midspan deflection of 6.5 mm at its ultimate load of 105 kN. This improvement can be primarily attributed to the bridging and filling effects of well-dispersed MWCNTs, which interact synergistically with the pozzolanic reaction of RHA. This interaction promotes the formation of high-stiffness C-S-H within the cement matrix, thereby enhancing the load-bearing capacity of the RHA RC beam [75,82]. Additionally, the incorporation of MWCNTs contributes to energy absorption by inhibiting microcrack development, thereby increasing the energy required for crack propagation and enhancing the ductility of RHA RC beams [73,83]. Similar results were reported by Qissab and Abbas [84], where the addition of 0.03–0.06% long and short MWCNTs enhanced the ductility of RC beams compared to beams without MWCNTs. Kanagaraj et al. [85] observed that the incorporation of MWNCTs enhanced the ductility of geopolymer concrete beams. The ductility index of geopolymer concrete beams rose from 1.42 to 1.93 with an addition of 0.16% MWNCTs relative to the control specimen. However, excessive MWCNTs content (0.2%) led to agglomeration, reducing the load-carrying capacity of the RHA RC beams. Although the midspan deflection of Beam B3 at its ultimate load was 7.1 mm, its ultimate load was 96 kN—approximately 87.3% of that of Beam B2.
3.4. Strain in longitudinal reinforcement
Figs 7–9 illustrate the strain variations in the longitudinal reinforcements of Beams B1, B2, and B3 under applied loads. The strain patterns were similar across all three beams: as the load increased, the strain in the longitudinal steel bars at various locations also increased. Initially, strain development at each measurement point was minimal. However, with increasing load, the strain at SG-2 and SG-3 grew significantly, indicating higher tension in these steel bars due to the bending moment. The strain values at SG-1 and SG-4 rose sharply as oblique cracks formed near the supports, reflecting stress redistribution in the steel bars in the cracked regions once the concrete fractured [86]. In Fig 8, the incorporation of 0.1% MWCNTs in Beam B2 enabled the longitudinal steel bars to sustain a slower increase in strain under higher loads compared to Beam B1. This suggests that MWCNTs enhance the crack resistance of concrete, delaying crack formation [87]. A similar observation was reported by Irshidat and Al-Shannaq [88], who observed that CNT incorporation significantly increased the initial stiffness of RC beams. However, as shown in Fig 9, when the MWCNTs concentration increased to 0.2%, the strain in the steel bars rose rapidly under a relatively lower load than in Beam B2. This phenomenon is attributed to MWCNTs agglomeration at higher concentrations, which increases porosity in the RHA concrete, weakens the bond between the concrete and steel bars, and accelerates crack initiation [67,89–91].
3.5. Strain in concrete
3.5.1. Strain on midspan of concrete.
In this study, strains in the compression zone of the RHA RC beams were also analysed, with Figs 10–12 illustrating the midspan concrete strains for all beams. The strain patterns in the compression zone were generally consistent across all beams. Strain gauge SG-5, positioned at the top of the beam, recorded the highest compressive strain due to the influence of the bending moment. Meanwhile, SG-6 and SG-7 indicated that compressive strain decreased as the load increased, eventually transitioning to tensile strain. Notably, the addition of well-dispersed MWCNTs in Beam B2 reduced strain in the RHA RC beam, whereas an excess of MWCNTs in Beam B3 resulted in increased strain. This effect is primarily attributed to the ability of well-dispersed MWCNTs to enhance the stiffness of RHA RC beams. Similar results were reported by Shi et al. [92], where RC beams incorporating basalt fibre-reinforced polymer and epoxy resin modified with MWCNTs exhibited reduced midspan strain compared to those reinforced with pure epoxy resin and basalt fibre-reinforced polymer. This effect is due to the improved stiffness of MWCNTs-modified epoxy resin, which reduces deformation and strain.
3.5.1. Strain on side of concrete.
Figs 13–15 illustrate the principal strain distribution on the side of the beams. Since the reinforcement layout and measurement point arrangements on the left and right sides of the test beams are identical, the analysis below focuses on the measurement points on the left side. The principal strain values and directions were calculated using the following formula:
where is the strain in the horizontal direction,
is the strain in the vertical direction,
represents the strain at an angle of 45° to the horizontal direction, and
is the angle between the principal strain and the horizontal direction. A positive calculation result indicates principal tensile strain (PTS), while a negative value denotes principal compressive strain (PCS).
As shown in Fig 13, the principal strains at SGR-1 and SGR-2 in Beam B1 increased steadily with the applied load. When the load was below 30 kN, the strain values remained low, indicating an elastic working state. However, with continued loading, an inflection point appeared in the SGR-1 PTS curve, where the slope slightly decreased. At 60 kN, a distinct inflection point was observed at SGR-1, which occurred because a crack passed through the SGR-1 measurement point, causing a sudden change in the principal strain. The primary failure cracks in Beam B1 were concentrated in the right span of the beam, whereas the crack width on the left side was significantly smaller. The crack in the left span only extended to the edge of the SGR-2 measurement point, resulting in a considerably lower principal strain at SGR-2 compared to SGR-1.
As shown in Fig 14, Beam B2 exhibited an extended elastic working stage compared to Beam B1. In particular, the SGR-2 measurement point remained in the elastic working stage when the applied load was below 73 kN. Additionally, during the elastic phase, the principal strain at the SGR measurement points in Beam B2 was lower than in Beam B1, indicating that the addition of MWCNTs improved the initial stiffness of Beam B2. Since the primary failure crack in Beam B2 was located in the left span, a noticeable inflection point occurred in the principal strain curve when the crack passed through the SGR measurement point. Specifically, at 60 kN, the principal strain at the SGR-2 measurement point exhibited a distinct turning point. The main inflection point for the SGR-1 measurement point appeared at 73 kN, after which the strain developed rapidly. These results suggest that the incorporation of MWCNTs delayed the onset of the principal strain inflection points, primarily due to the bridging effect of MWCNTs, which effectively slowed crack propagation [93].
As shown in Fig 15, the principal strain inflection point at the SGR measurement points of Beam B3 appeared earlier than in Beam B2. At 50 kN, the principal strain at the SGR-1 measurement point distorted and increased rapidly, indicating that a crack had passed through this location. Additionally, during the elastic working stage, the principal strain at the SGR measurement points in Beam B3 was higher than in Beam B2. This suggests that the initial stiffness and shear capacity of Beam B3 were significantly reduced, primarily due to excessive MWCNTs incorporation. The high MWCNTs concentration led to particle agglomeration, further compromising the mechanical properties of the concrete [69,94].
3.6. Comparison of experimental and predicted shear strengths
In this study, the most widely used shear capacity prediction models were selected for comparison with the ultimate shear capacity obtained from the experiments. These models include the ACI 318 model [95], Eurocode 2–05 model [96], and Canadian standard model [97], as well as the Bazant and Kim model [98], Zsutty model [99], and Collins and Kuchma model [100]. The equations used for each prediction model are presented in Table 6.
Table 7 provides a comparison of the experimental and predicted shear capacities for each beam, while Fig 16 illustrates the experiment-to-prediction ratios across all beams. As shown in Fig 16, nearly all prediction models—except the Zsutty model—overestimated the ultimate shear capacity. While the predicted values for all models increased with MWCNTs content, an excessive concentration of MWCNTs resulted in lower predicted values. The average shear strength experiment-to-prediction ratios ( experiment/prediction) for the Bazant and Kim, ACI 318, and Collins and Kuchma models were 1.58, 1.42, and 1.42, respectively. In contrast, the predictions from the Canadian standard and Eurocode 2–05 models were more closely aligned with experimental results, with average ratios of 1.21 and 1.19, respectively. The Zsutty model was the only one to underestimate the ultimate shear capacity; however, its predictions were closer to the experimental values than those from the ACI 318, Collins and Kuchma, and Bazant and Kim models, yielding an average experiment-to-prediction ratio of 0.8. Overall, for predicting the ultimate shear capacity of RHA concrete incorporating MWCNTs, the Zsutty, Eurocode 2, and Canadian standard models proved to be the most reliable, with an approximate prediction error of 20%.
4. Conclusion
This study investigated the shear behaviour of RHA RC beams incorporating varying concentrations of MWCNTs (0–0.2%) by evaluating crack propagation, failure modes, load-deflection behaviour, and strain distribution in both longitudinal reinforcement and concrete. Additionally, the experimental results were compared with existing shear design models. Based on a comparative analysis of RHA RC beams with MWCNTs (Beams B2 and B3) and without MWCNTs (Beam B1), the following conclusions were drawn:
- i. The addition of well-dispersed MWCNTs enhanced the shear resistance of RHA RC beams. Crack propagation was effectively delayed, and crack widths were significantly reduced.
- ii. With the incorporation of 0.1% MWCNTs, the first crack load, initial stiffness, and ultimate shear capacity increased by 34.78%, 38.4%, and 4.76%, respectively, compared to Beam B1.
- iii. Midspan deflection analysis demonstrated that the inclusion of MWCNTs significantly improved beam ductility. The midspan deflection at the first crack load and ultimate load in Beam B2 increased by 9.47% and 7.69%, respectively, compared to the control specimen. Additionally, Beam B2 exhibited a 23.19% improvement in ductility relative to Beam B1.
- iv. The addition of 0.1% MWCNTs effectively delayed the rapid development of strain in the longitudinal reinforcement, significantly reduced strain in the compression zone of the RHA concrete beam, and postponed the occurrence of the principal strain inflection point on the beam’s side.
- v. Excessive incorporation of MWCNTs (0.2%) negatively impacted shear resistance, reducing the ultimate shear capacity by 8.57%. Although the deflection of Beam B3 was similar to that of Beam B2, the ultimate load required to reach this deflection was approximately 87.3% of that of Beam B2. Moreover, the strain in the compression zone of Beam B3 developed more rapidly than in Beam B2, and the principal strain inflection point appeared earlier.
- vi. Among the shear strength prediction models, the Zsutty model was the only one that did not yield conservative estimates. The Eurocode 2 and Canadian standard models exhibited the smallest prediction errors (approximately 20%) and were the most suitable for predicting the shear strength of RHA RC beams.
This study demonstrates that incorporating MWCNTs can effectively enhance the shear resistance of RHA RC beams, addressing the existing research gap regarding the shear behaviour of RHA-based sustainable concrete beams. In order to prevent the shear failure of RHA RC beams, the optimal dosage of MWCNTs is recommended to be 0.1%. The well-dispersed MWCNTs contribute to preventing the brittle failure of RHA RC beams when subjected to shear pressure. Exceeding 0.2% MWCNTs is inadvisable, as excessive inclusion can result in the reagglomeration of MWCNTs and diminish the mechanical characteristics of RHA RC beams. The findings further highlight the potential of MWCNTs in developing high-performance RHA sustainable concrete structural components, providing an experimental foundation for the broader application of RHA concrete in the construction industry.
This study has a few limitations. First, the effect of RHA produced under different combustion conditions on the shear performance of RHA concrete beams was not considered. Second, the influence of MWCNTs with varying lengths and preparation techniques on the shear performance of RHA concrete beams was not examined.
For future research, the following studies are recommended: First, investigating the effect of RHA content on the performance of concrete materials and structural members. Second. evaluating the performance of structural members containing RHA and MWCNTs under extreme conditions, such as high temperatures. Third., evaluating the impact of different types of MWCNTs on the performance of structural members incorporating RHA.
Supporting information
S1 Table. Diagonal crack width and nominal shear stress.
https://doi.org/10.1371/journal.pone.0320325.s001
(XLSX)
S2 Table. Load and deflection results of all beams.
https://doi.org/10.1371/journal.pone.0320325.s002
(XLSX)
S3 Table. Steel bars strain results of B1 beam.
https://doi.org/10.1371/journal.pone.0320325.s003
(XLSX)
S4 Table. Steel bars strain results of B2 beam.
https://doi.org/10.1371/journal.pone.0320325.s004
(XLSX)
S5 Table. Steel bars strain results of B3 beam.
https://doi.org/10.1371/journal.pone.0320325.s005
(XLSX)
S6 Table. Strain results on midspan of concrete in B1 beam.
https://doi.org/10.1371/journal.pone.0320325.s006
(XLSX)
S7 Table. Strain results on midspan of concrete in B2 beam.
https://doi.org/10.1371/journal.pone.0320325.s007
(XLSX)
S8 Table. Strain results on midspan of concrete in B3 beam.
https://doi.org/10.1371/journal.pone.0320325.s008
(XLSX)
S9 Table. Principal strain results on the side of B1 beam.
https://doi.org/10.1371/journal.pone.0320325.s009
(XLSX)
S10 Table. Principal strain results on the side of B2 beam.
https://doi.org/10.1371/journal.pone.0320325.s010
(XLSX)
S11 Table. Principal strain results on the side of B3 beam.
https://doi.org/10.1371/journal.pone.0320325.s011
(XLSX)
References
- 1. Madhumitha J. Cement production global 2023. In: Statista [Internet]. 2024 [cited 26 Sep 2024. ]. Available from: https://www.statista.com/statistics/1087115/global-cement-production-volume/
- 2. Tkachenko N, Tang K, McCarten M, Reece S, Kampmann D, Hickey C, et al. Global database of cement production assets and upstream suppliers. Sci Data. 2023;10(1):696. pmid:37833339
- 3. Luan C, Shi X, Wang Q, Utashev N, Tufail RF, Maqsoom A. Carbonation performances of steel fiber reinforced geopolymer concrete. Can J Civ Eng. 2023;50(4):294–305.
- 4. Bao J, Zheng R, Sun Y, Zhang P, Cui Y, Xue S, et al. A state-of-the-art review on high temperature resistance of lightweight aggregate high-strength concrete. J Build Eng. 2023;69:106267.
- 5. Ahmed A, Hyndman F, Kamau J, Fitriani H. Rice husk ash as a cement replacement in high strength sustainable concrete. Mater Sci Forum. 2020;1007:90–8.
- 6. Luan C, Zhou A, Li Y, Zou D, Gao P, Liu T. CO2 avoidance cost of fly ash geopolymer concrete. Constr Build Mater. 2024;416:135193.
- 7. Habert G, Miller SA, John VM, Provis JL, Favier A, Horvath A, et al. Environmental impacts and decarbonization strategies in the cement and concrete industries. Nat Rev Earth Environ. 2020;1(11):559–73.
- 8. Hamza Hasnain M, Javed U, Ali A, Saeed Zafar M. Eco-friendly utilization of rice husk ash and bagasse ash blend as partial sand replacement in self-compacting concrete. Constr Build Mater. 2021;273:121753.
- 9. Fernando A, Selvaranjan K, Srikanth G, Gamage JCPH. Development of high strength recycled aggregate concrete-composite effects of fly ash, silica fume and rice husk ash as pozzolans. Mater Struct. 2022;55(7).
- 10. Hakeem IY, Agwa IS, Tayeh BA, Abd-Elrahman MH. Effect of using a combination of rice husk and olive waste ashes on high-strength concrete properties. Case Stud Constr Mater. 2022;17:e01486.
- 11. Safdar Raza S, Ali B, Noman M, Fahad M, Mohamed Elhadi K. Mechanical properties, flexural behavior, and chloride permeability of high-performance steel fiber-reinforced concrete (SFRC) modified with rice husk ash and micro-silica. Constr Build Mater. 2022;359:129520.
- 12. Jiang P, Zhao D, Jin C, Ye S, Luan C, Tufail RF. Compressive strength prediction and low-carbon optimization of fly ash geopolymer concrete based on big data and ensemble learning. PLoS One. 2024;19(9):e0310422. pmid:39264969
- 13. Ahmad J, Arbili MM, Alabduljabbar H, Deifalla AF. Concrete made with partially substitution corn cob ash: a review. Case Stud Constr Mater. 2023;18:e02100.
- 14. Zhan P, He Z, Ma Z, Liang C, Zhang X, Abreham AA, et al. Utilization of nano-metakaolin in concrete: a review. J Build Eng. 2020;30:101259.
- 15. Ahmad J, Kontoleon KJ, Majdi A, Naqash MT, Deifalla AF, Ben Kahla N, et al. A comprehensive review on the ground granulated blast furnace slag (GGBS) in concrete production. Sustainability. 2022;14(14):8783.
- 16. Kumar R, Srivastava A, Lakhani R. Industrial wastes-cum-strength enhancing additives incorporated lightweight aggregate concrete (LWAC) for energy efficient building: a comprehensive review. Sustainability. 2021;14(1):331.
- 17. Shahpari M, Bamonte P, Jalali Mosallam S. An experimental study on mechanical and thermal properties of structural lightweight concrete using carbon nanotubes (CNTs) and LECA aggregates after exposure to elevated temperature. Constr Build Mater. 2022;346:128376.
- 18. Arjomandi A, Mousavi R, Tayebi M, Nematzadeh M, Gholampour A, Aminian A, et al. The effect of sulfuric acid attack on mechanical properties of steel fiber-reinforced concrete containing waste nylon aggregates: experiments and RSM-based optimization. J Build Eng. 2023;64:105500.
- 19. Shariati M, Kamyab H, Habibi M, Ahmadi S, Naghipour M, Gorjinezhad F, et al. Sulfuric acid resistance of concrete containing coal waste as a partial substitute for fine and coarse aggregates. Fuel. 2023;348:128311.
- 20. Nematzadeh M, Arjomandi A, Fakoor M, Aminian A, Khorshidi-Mianaei A. Pre-and post-heating bar-concrete bond behavior of CFRP-wrapped concrete containing polymeric aggregates and steel fibers: experimental and theoretical study. Eng Struct. 2024;321:118929.
- 21. Bandumula N. Rice production in Asia: key to global food security. Proc Natl Acad Sci, India, Sect B Biol Sci. 2017;88(4):1323–8.
- 22. Food and Agriculture Organization of the United Nations. Rice production. In: Our World in Data [Internet]. 2024 [cited 14 Nov 2024. ]. Available from: https://ourworldindata.org/grapher/rice-production
- 23. Abdel-Shafy HI, Mansour MSM. Solid waste issue: sources, composition, disposal, recycling, and valorization. Egypt J Pet. 2018;27(4):1275–90.
- 24. Raheem AA, Kareem MA. Chemical composition and physical characteristics of rice husk ash blended cement. Int J Eng Res Afr. 2017;32:25–35.
- 25. Nehdi M, Duquette J, El Damatty A. Performance of rice husk ash produced using a new technology as a mineral admixture in concrete. Cem Concr Res. 2003;33(8):1203–10.
- 26. Wang J, Xiao J, Zhang Z, Han K, Hu X, Jiang F. Action mechanism of rice husk ash and the effect on main performances of cement-based materials: a review. Constr Build Mater. 2021;288:123068.
- 27. Amran M, Fediuk R, Murali G, Vatin N, Karelina M, Ozbakkaloglu T, et al. Rice husk ash-based concrete composites: a critical review of their properties and applications. Crystals. 2021;11(2):168.
- 28. Balapour M, Ramezanianpour A, Hajibandeh E. An investigation on mechanical and durability properties of mortars containing nano and micro RHA. Constr Build Mater. 2017;132:470–7.
- 29. Faried AS, Mostafa SA, Tayeh BA, Tawfik TA. The effect of using nano rice husk ash of different burning degrees on ultra-high-performance concrete properties. Constr Build Mater. 2021;290:123279.
- 30. Osman AO, Elsayed M, El-Sayed AA, Shaheen AA. Effect of recycled materials as supplementary cementitious materials on the shear strength of UHPFRC beams without shear reinforcement. Constr Build Mater. 2024;452:138935.
- 31. Hassanean Mahmoud AM, Hussein Sara AM, Elsayed M. Shear behavior of reinforced concrete beams comprising a combination of crumb rubber and rice husk ash. Eng Struct. 2024;319:118862.
- 32. Praveenkumar TR, Vijayalakshmi MM, Manigandan S. Thermal conductivity of concrete reinforced using TiO2 nanoparticles and rice husk ash. Int J Ambient Energy. 2019;43(1):1127–33.
- 33. Avudaiappan S, Prakatanoju S, Amran M, Aepuru R, Saavedra Flores EI, Das R, et al. Experimental investigation and image processing to predict the properties of concrete with the addition of nano silica and rice husk ash. Crystals. 2021;11(10):1230.
- 34. Alex AG, Kemal Z, Gebrehiwet T, Getahun S. Effect of α: phase nano Al2O3 and rice husk ash in cement mortar. Adv Civ Eng. 2022;2022(1).
- 35. Zheng Y, Zhuo J, Zhang P. A review on durability of nano-SiO2 and basalt fiber modified recycled aggregate concrete. Constr Build Mater. 2021;304:124659.
- 36. Huseien GF. A review on concrete composites modified with nanoparticles. J Compos Sci. 2023;7(2):67.
- 37. Li S, Hu M, Chen X, Sui S, Jin L, Geng Y, et al. The performance and functionalization of modified cementitious materials via nano titanium-dioxide: a review. Case Stud Constr Mater. 2023;19:e02414.
- 38. Anto G, Athira K, Nair NA, Sai TY, Yadav AL, Sairam V. Mechanical properties and durability of ternary blended cement paste containing rice husk ash and nano silica. Constr Build Mater. 2022;342:127732.
- 39. Meddah MS, Praveenkumar TR, Vijayalakshmi MM, Manigandan S, Arunachalam R. Mechanical and microstructural characterization of rice husk ash and Al2O3 nanoparticles modified cement concrete. Constr Build Mater. 2020;255:119358.
- 40. Praveenkumar TR, Vijayalakshmi MM, Meddah MS. Strengths and durability performances of blended cement concrete with TiO2 nanoparticles and rice husk ash. Constr Build Mater. 2019;217:343–51.
- 41. Krentowski JR. Assessment of destructive impact of different factors on concrete structures durability. Materials (Basel). 2021;15(1):225. pmid:35009371
- 42. Kiakojouri F, De Biagi V, Chiaia B, Sheidaii MR. Progressive collapse of framed building structures: current knowledge and future prospects. Eng Struct. 2020;206:110061.
- 43. Shariati M, Pourteymuri M, Naghipour M, Toghroli A, Afrazi M, Shariati M, et al. Evolution of confinement stress in axially loaded concrete-filled steel tube stub columns: study on enhancing urban building efficiency. Sustainability. 2024;16(17):7544.
- 44. Alatshan F, Osman SA, Hamid R, Mashiri F. Stiffened concrete-filled steel tubes: a systematic review. Thin-Walled Struct. 2020;148:106590.
- 45. Ayough P, Sulong NHR, Ibrahim Z. Analysis and review of concrete-filled double skin steel tubes under compression. Thin-Walled Struct. 2020;148:106495.
- 46. Gravina RJ, Li J, Smith ST, Visintin P. Environmental durability of FRP bar-to-concrete bond: critical review. J Compos Constr. 2020;24(4):03120001.
- 47. Nepomuceno E, Sena-Cruz J, Correia L, D’Antino T. Review on the bond behavior and durability of FRP bars to concrete. Constr Build Mater. 2021;287:123042.
- 48. Rahimi M, Davoodi M-R, Nematzadeh M, Yousefpour H, Azarbera M, Fattahi Z. A comparative study between beam and pull-out tests on bond behavior of ribbed GFRP bar in concrete conforming to RILEM standards. Arch Civ Mech Eng. 2024;25(1).
- 49. Su C, Wang X, Ding L, Wu Z, Ma X. Durability of seawater sea sand concrete beams reinforced with carbon nanotube-modified BFRP bars in a marine environment. Compos Struct. 2022;292:115642.
- 50. Paul SC, van Zijl GPAG, Šavija B. Effect of fibers on durability of concrete: a practical review. Materials (Basel). 2020;13(20):4562. pmid:33066618
- 51. Golewski GL. The phenomenon of cracking in cement concretes and reinforced concrete structures: the mechanism of cracks formation, causes of their initiation, types and places of occurrence, and methods of detection–a review. Buildings. 2023;13(3):765.
- 52. Bogas JA, Ahmed HH, Diniz T. Influence of cracking on the durability of reinforced concrete with carbon nanotubes. Appl Sci. 2021;11(4):1672.
- 53. Zheng Y, Lv X, Hu S, Zhuo J, Wan C, Liu J. Mechanical properties and durability of steel fiber reinforced concrete: a review. J Build Eng. 2024;82:108025.
- 54. Zhou S, Xie L, Jia Y, Wang C. Review of cementitious composites containing polyethylene fibers as repairing materials. Polymers (Basel). 2020;12(11):2624. pmid:33171827
- 55. Devi Ch, Vijayan DS, Nagalingam R, Arvindan S. A review of the implementations of glass fiber in concrete technology. Mater Today: Proc. 2022;62:2010–5.
- 56. Muthukumarana TV, Arachchi MAVHM, Somarathna HMCC, Raman SN. A review on the variation of mechanical properties of carbon fibre-reinforced concrete. Constr Build Mater. 2023;366:130173.
- 57. O’Rear EA, Onthong S, Pongprayoon T. Mechanical strength and conductivity of cementitious composites with multiwalled carbon nanotubes: to functionalize or not? Nanomaterials (Basel). 2023;14(1):80. pmid:38202536
- 58. Ahmad J, Zhou Z. Properties of concrete with addition carbon nanotubes: a review. Constr Build Mater. 2023;393:132066.
- 59. Adhikary SK, Rudžionis Ž, Tučkutė S, Ashish DK. Effects of carbon nanotubes on expanded glass and silica aerogel based lightweight concrete. Sci Rep. 2021;11(1):2104. pmid:33483572
- 60. Lu H, Yao Y. Spalling mechanism of carbon nanotube concrete at elevated temperature. Constr Build Mater. 2022;314:125594.
- 61. Demczyk BG, Wang YM, Cumings J, Hetman M, Han W, Zettl A, et al. Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes. Mater Sci Eng A. 2002;334(1–2):173–8.
- 62. Yesudhas Jayakumari B, Nattanmai Swaminathan E, Partheeban P. A review on characteristics studies on carbon nanotubes-based cement concrete. Constr Build Mater. 2023;367:130344.
- 63. Zhang J, Ke Y, Zhang J, Han Q, Dong B. Cement paste with well-dispersed multi-walled carbon nanotubes: mechanism and performance. Constr Build Mater. 2020;262:120746.
- 64. Mansouri Sarvandani M, Mahdikhani M, Aghabarati H, Haghparast Fatmehsari M. Effect of functionalized multi-walled carbon nanotubes on mechanical properties and durability of cement mortars. J Build Eng. 2021;41:102407.
- 65. Yao Y, Lu H. Mechanical properties and failure mechanism of carbon nanotube concrete at high temperatures. Constr Build Mater. 2021;297:123782.
- 66. Irshidat MR. Bond strength evaluation between steel rebars and carbon nanotubes modified concrete. Case Stud Constr Mater. 2021;14:e00477.
- 67. Hassan A, Elkady H, Shaaban IG. Effect of adding carbon nanotubes on corrosion rates and steel-concrete bond. Sci Rep. 2019;9(1):6285. pmid:31000753
- 68. Öztürk O. Multifunctional behavior of CNT- and CB-based composite beams. Constr Build Mater. 2021;296:123453.
- 69. Ramezani M, Dehghani A, Sherif MM. Carbon nanotube reinforced cementitious composites: a comprehensive review. Constr Build Mater. 2022;315:125100.
- 70.
GB/T 50080–2019. Standard for test methods of concrete physical and mechanical properties. China Archit Build Press; Beijing China: 2019.
- 71.
GB/T 50152-2012. Standard for test method of concrete structures. Minist Hous Urban-Rural Dev PRC; Beijing China: 2012.
- 72. Hong S-H, Choi J-S, Yoo S-J, Yoon Y-S. Structural benefits of using carbon nanotube reinforced high-strength lightweight concrete beams. Dev Built Environ. 2023;16:100234.
- 73. Naji HF, Khalid NN, Alsaraj WK, Habouh MI, Marchetty S. Experimental investigation of flexural enhancement of RC beams with multi-walled carbon nanotubes. Case Stud Constr Mater. 2021;14:e00480.
- 74. Choudhary M, Sharma A, Aravind Raj S, Sultan MTH, Hui D, Shah AUM. Contemporary review on carbon nanotube (CNT) composites and their impact on multifarious applications. Nanotechnology Rev. 2022;11(1):2632–60.
- 75. Konsta-Gdoutos MS, Metaxa ZS, Shah SP. Multi-scale mechanical and fracture characteristics and early-age strain capacity of high performance carbon nanotube/cement nanocomposites. Cem Concr Compos. 2010;32(2):110–5.
- 76. Silvestro L, Jean Paul Gleize P. Effect of carbon nanotubes on compressive, flexural and tensile strengths of Portland cement-based materials: a systematic literature review. Constr Build Mater. 2020;264:120237.
- 77. Makar JM, Chan GW. Growth of cement hydration products on single‐walled carbon nanotubes. J Am Ceram Soc. 2009;92(6):1303–10.
- 78. Lee D, Park S-H, Sim Y, Kwon O-S, Lee S-C. Effect of carbon nanotubes on shear behavior of PVA-ECC beams. Structures. 2025;72:108243.
- 79. Sikora P, Abd Elrahman M, Chung S-Y, Cendrowski K, Mijowska E, Stephan D. Mechanical and microstructural properties of cement pastes containing carbon nanotubes and carbon nanotube-silica core-shell structures, exposed to elevated temperature. Cem Concr Compos. 2019;95:193–204.
- 80. Morsy MS, Alsayed SH, Aqel M. Hybrid effect of carbon nanotube and nano-clay on physico-mechanical properties of cement mortar. Constr Build Mater. 2011;25(1):145–9.
- 81. Torabian Isfahani F, Li W, Redaelli E. Dispersion of multi-walled carbon nanotubes and its effects on the properties of cement composites. Cem Concr Compos. 2016;74:154–63.
- 82. Liew KM, Kai MF, Zhang LW. Carbon nanotube reinforced cementitious composites: an overview. Compos Part A: Appl Sci Manuf. 2016;91:301–23.
- 83. Hunashyal AM, Lohitha SJ, Quadri SS, Banapurmath NR. Experimental investigation of the effect of carbon nanotubes and carbon fibres on the behaviour of plain cement composite beams. The IES J Part A: Civ Struct Eng. 2011;4(1):29–36.
- 84. Qissab MA, Abbas ST. Behaviour of reinforced concrete beams with multiwall carbon nanotubes under monotonic loading. Eur J Environ Civ Eng. 2016;22(9):1111–30.
- 85. Kanagaraj B, Anand N, Lubloy E, Andrushia A D. Influence of Multi-walled Carbon Nanotube (MWCNT) on flexural behavior and microstructure characteristics of geopolymer concrete beams. Case Stud Constr Mater. 2024;20:e03317.
- 86. Odero BJ, Mutuku RN, Nyomboi T, Gariy ZA. Shear performance of concrete beams with a maximum size of recycled concrete aggregate. Adv Mater Sci Eng. 2022;2022:1–17.
- 87. Lan Y, Zheng B, Shi T, Ma C, Liu Y, Zhao Z. Crack resistance properties of carbon nanotube-modified concrete. Mag Concr Res. 2022;74(22):1165–75.
- 88. Irshidat MR, Al-Shannaq A. Using textile reinforced mortar modified with carbon nano tubes to improve flexural performance of RC beams. Composite Struct. 2018;200:127–34.
- 89.
Olalere A, Ige J. Effect of coarse aggregate size on the compressive strength and the flexural strength of concrete beam. 2015:67–75.
- 90. Hawreen A, Bogas JA. Influence of carbon nanotubes on steel–concrete bond strength. Mater Struct. 2018;51(6).
- 91. Alsubaie A, Alfaqih I, Al-Osta M, Tounsi A, Chikh A, Mudhaffar I. Porosity-dependent vibration investigation of functionally graded carbon nanotube-reinforced composite beam. Comput Concr. 2023;32:75–85.
- 92. Shi C, Jin S, Wang C, Yang Y. Enhancing flexural behavior of reinforced concrete beams strengthened with basalt fiber-reinforced polymer sheets using carbon nanotube-modified epoxy. Materials (Basel). 2024;17(13):3250. pmid:38998332
- 93. Qin R, Zhou A, Yu Z, Wang Q, Lau D. Role of carbon nanotube in reinforcing cementitious materials: an experimental and coarse-grained molecular dynamics study. Cem Concr Res. 2021;147:106517.
- 94. Chen J, Akono A-T. Influence of multi-walled carbon nanotubes on the hydration products of ordinary Portland cement paste. Cem Concr Res. 2020;137:106197.
- 95.
ACI 318. Building Code Requirements for Structural Concrete (ACI 318M-14) and Commentary (ACI 318RM-14). Farmington Hills; 2014.
- 96. Code P. Eurocode 2: Design of concrete structures-part 1–1: general rules and rules for buildings. Br Stand Inst Lond. 2005;668: 659–68.
- 97. Bentz EC, Collins MP. Development of the 2004 Canadian Standards Association (CSA) A23.3 shear provisions for reinforced concrete. Can J Civ Eng. 2006;33(5):521–34.
- 98. Bazant ZP, Kim J-K. Size effect in shear failure of longitudinally reinforced beams. J Proc. 1984;81:456–68.
- 99. Zsutty TC. Beam Shear Strength Prediction by Analysis of Existing Data. J Proc. 1968;65:943–951.
- 100. Collins M, Kuchma D. How safe are our large, lightly reinforced concrete beams, slabs, and footings? Struct J. 1999;96:482–90.