Green Compression-Cast Concrete Material and Its Fiber-Reinforced Polymer (FRP)-Reinforced Concrete Structures

Yu-Fei Wu; Fang Yuan; Biao Hu

doi:10.1016/j.eng.2024.10.005

Engineering ›› 2026, Vol. 61 ›› Issue (6) :120 -138. DOI: 10.1016/j.eng.2024.10.005

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Green Compression-Cast Concrete Material and Its Fiber-Reinforced Polymer (FRP)-Reinforced Concrete Structures

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Abstract

Concrete manufacturing is a significant contributor to carbon emission. This study presents extensive research on compression-casting, which is a newly developed and highly efficient concrete manufacturing technology. Compression-casting is a mechanical process that improves the properties of all types of concrete materials without using chemical additives or mineral admixtures. It is particularly suitable for producing high-quality concrete from secondary/substandard or waste/recycled aggregate materials, thereby promoting green, low-carbon, and sustainable construction. This study introduces compression-casting and presents the experimental results of the material and relevant fiber-reinforced polymer (FRP)-reinforced structures. Various applications of the compression-casting technology on inferior concrete materials, including rubber, recycled aggregate, desert sand, and recycled powder concrete, have also been introduced. The behavior of FRP-reinforced structural members of compression-cast concrete (CCC) is evaluated and compared with that of normal concrete (NC). It is concluded that compared to NC, CCC has the following advantages: improved mechanical properties, microstructure, durability, cement saving, economy, and reduced carbon dioxide emissions. Finally, we provide recommendations for addressing CCC structure brittleness. As a result, this method can reshape the future of concrete manufacturing.

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Concrete material / Compression-cast concrete / Low carbon / Fiber-reinforced polymer / Mechanical behavior / Durability / Structural performance

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Yu-Fei Wu, Fang Yuan, Biao Hu. Green Compression-Cast Concrete Material and Its Fiber-Reinforced Polymer (FRP)-Reinforced Concrete Structures. Engineering, 2026, 61 (6) : 120-138 DOI:10.1016/j.eng.2024.10.005

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1. Introduction

The high carbon emissions from the construction industry pose severe climatic challenges worldwide. In 2019, carbon dioxide (CO₂) emissions from the construction industry peaked at approximately 7.14 billion tonnes, accounting for 20% of the total global CO₂ emissions [1]. This emission is particularly severe in China owing to the rapid development of the construction industry. In 2019, CO₂ emissions from the construction industry in China reached 2.87 billion tonnes, accounting for 29% of the total national CO₂ emissions [2]. Most CO₂ emissions from the construction industry emanate from building material production, of which approximately 39% are due to cement production. Therefore, reducing the CO₂ emissions for a greener construction industry is an urgent need.

Scholars have made considerable efforts to minimize cement use in concrete production while ensuring that its strength is uncompromised. An effective approach involves partially substituting cement with by-products such as blast silica fume, fly ash, furnace slag, or various industrial wastes [3], [4], [5], [6]. However, the substitution rate is limited by inadequate pozzolanic reactivity or the high costs of these alternatives. Another option is the complete replacement of conventional cement with innovative alternatives that require less energy and result in lower CO₂ emissions, such as alkali-activated geopolymers [7], [8] and limestone-calcined clay cement [9], [10]. This approach can considerably reduce CO₂ emissions. However, the inherent instability of raw materials and production conditions poses challenges, hindering the large-scale standardized production of these low-carbon cement alternatives [11], [12]. Moreover, studies [13], [14] have revealed that introducing nanomaterials, such as silicon oxide nanoparticles and carbon nanotubes, can enhance concrete performance, improving its strength and durability, even with lesser cement content. Incorporating high-performance fibers, such as carbon and glass fibers, can improve the toughness and crack resistance of concrete, allowing for reduced cement usage to some extent [15], [16]. However, the high costs of these two methods hinder their practical applications. Achal et al. [17], [18] proposed inducing microorganisms into concrete to enhance its mechanical properties and simultaneously reduce cement content. However, this has relatively high requirements in the construction environment and may pose potential health risks to construction workers.

Our research team recently developed a novel compression-casting technology (CCT) that can greatly improve strength and durability and reduce the cost of concrete [19], [20], [21], [22], [23], [24], [25]. Fig. 1 illustrates the production of concrete cylinders using CCT. A universal compression-casting facility for producing full-sized precast concrete members was developed by our group (Fig. 2). The facility consists of standard steel blocks that can be assembled with any number or combination of steel blocks. This facility can be used to cast standard beams, columns, and slabs. The test beams and slabs used in this study were produced at this facility. The compression-casting scheme has been investigated [22], and the general principle of compression-casting is that the casting pressure should be large enough and uniformly transmitted to all parts of the concrete unit. The conclusion section provides a summary of the novel CCT.

Compared with normal concrete (NC), the additional step required to produce compression-cast concrete (CCC) is applying high pressure (> 5 MPa) to fresh concrete and maintaining the pressure for a short time (2-5 min) before removing the formwork. The concrete mixture for CCC is the same as that for NC, and no chemical additives or mineral admixtures are required. Compression under high pressure squeezes out excess water and air from the concrete, reducing its internal porosity and condensing the concrete, thereby significantly improving its compactness and compressive strength. Microstructural observations have revealed that CCC has considerably lesser internal pores and microcracks in concrete than NC, making the microstructure of the former denser [19], [20], [21]. A denser concrete microstructure can effectively reduce the penetration of corrosive media (such as chlorine salts and sulfates) into the concrete, improving the durability of concrete structures and extending their service life. Concrete density is increased by compression-casting, and additional concrete quantity is required for the same volume of CCC as NC because of the increased density [23].

This study presents the latest results of our research on CCC. It comprises two parts: material and structure. In the material section, the results of various material tests are reported. The advantages of CCC over NC include superior mechanical properties, better durability, cement savings, reduced CO₂ emissions, and material recycling. The structural behavior of fiber-reinforced polymer (FRP)-reinforced CCC members is evaluated and compared with those of FRP-reinforced NC in the structure section. Finally, design recommendations for overcoming the brittleness of CCC structures are provided.

2. Research significance

Using waste/recycled materials to produce green concrete is a research focus and development trend. However, the poor performance of solid waste-based concrete is the main obstacle to its application in engineering. Currently, no method in the existing literature can make the performance of solid waste-based concrete comparable to that of NC without adding additional materials or increasing costs. Compression-casting facilitates the use of waste/recycled materials in concrete by improving the properties of concrete material and reducing costs, resulting in low-carbon and sustainable construction. Through numerous experiments, the advantages of the new technology were demonstrated in this study. This technology is useful for manufacturing any kind of concrete to improve its mechanical properties and reduce costs and CO₂ emissions. Therefore, this new technology could reshape the future of concrete manufacturing.

3. Material properties

Systematic experimental and theoretical investigations were conducted to study the performance of CCC. The carbon reduction benefits of CCC were also clarified and quantified using a life-cycle assessment (LCA) methodology. These results are described in subsequent sections to illustrate the superiority of CCC to NC.

3.1. Experiment

The experiment involved a series of material tests, including mechanical, carbonation, freeze-thaw (FT), chloride penetration, water absorption, and elevated temperature tests, as summarized in Table 1 [26], [27].

Regarding the design, a concrete specimen prepared under normal casting is represented by “NC” and is a reference sample. In contrast, a specimen prepared under compression-casting is termed “CCC-X” with “X” signifying the casting pressure. For instance, CCC-5 means a specimen that is prepared by compression-casting under a pressure of 5 MPa. This nomenclature is followed in the subsequent sections unless otherwise stated.

In addition, typical microscopic characterizations were conducted to enhance the understanding of CCC performance from the macro and micro perspectives. Some test results reported in this study have been published, whereas others have not. Specific references are provided where published results are presented and discussed.

3.2. Mechanical properties and microstructures

A previous study revealed that the compressive strength of CCC is remarkably higher than that of NC, with increments of 26%-83% [23] (Fig. 3(a)). Fig. 3(b) compares the experimentally measured elastic modulus of CCC and NC [23]. Owing to the effect of CCT, the elastic modulus of CCC was substantially increased by 26%-69% [23] compared with their NC counterparts, which is consistent with the observations on the compressive strength increment. Similar increases in strength and elastic modulus owing to CCT have been reported [19], [21], [22], [24]. The remarkable improvements in the mechanical properties of CCC are mainly attributed to the reduced air and water contents and the denser concrete microstructure [19], [21], [22].

Multiple microscopic investigations have been conducted to better understand the mechanism of enhancing the mechanical properties of CCC, including scanning electron microscopy (SEM) imaging to reveal the microstructure of the matrix, mercury intrusion porosimetry (MIP) test to quantify the mortar porosity, and backscattered electron (BSE) analysis to demonstrate the quality of the interfacial transition zone (ITZ) between the aggregate and mortar. The procedures and step-wise method for preparing samples for SEM, MIP, and BSE tests have been well documented in a previous study [21]. Figs. 4(a) and (b) reveal fewer pores and a much denser matrix for CCC-15 than for the NC, which is consistent with the porosity results illustrated in Fig. 4(c). Figs. 4(d) and (e) reveal much fewer pores in the ITZ of CCC-15 than NC. These results demonstrate that CCT can densify the matrix microstructure, reduce the mortar porosity, and improve the ITZ quality, which contributes to significantly improving the mechanical properties and durability of CCC compared with NC.

3.3. Durability

3.3.1. Carbonation

Standard procedures per Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete (GB/T 50082-2009) [26] were followed during the accelerated carbonation tests. After carbonation, uniaxial compressive strength tests were conducted following Standard for Test Methods of Concrete Physical and Mechanical Properties (GB/T 50081-2019) [27]. Fig. 5 and Table 2 present the compressive strengths of the NC and CCC specimens after 0, 3, 7, 14, and 28 d of CO₂ exposure. The compression-casting pressure (CCP) enhanced the compressive strength of the CCC specimens compared with that of the NC specimens with or without CO₂ exposure. In the absence of CO₂, the compressive strength of CCC-5 and CCC-15 increased by 81.8% and 118.2%, respectively, compared with that of the NC specimens. Similarly, the compressive strengths of the 28 d CO₂-exposed CCC specimens under casting pressures of 5 and 15 MPa increased by 36.2% and 61.7%, respectively, compared to that of the NC specimens. The denser microstructure and lower porosities of the CCC specimens compared to those of the NC specimens were the main reasons for the improved resistance to CO₂ ingress and mechanical performance of the CCC specimens after CO₂ exposure.

A strength increase is observed with up to 7 d of CO₂ exposure in Fig. 5 for NC, whereas the CCC strength generally decreases in the same period. This is because the early carbonation product formed from the reactions between CO₂ and the hydration products fills the pores of the concrete and densifies the microstructure of NC. Hence, the strength of the NC increases in the early carbonation stage before concrete cracking starts. However, the microstructure of CCC is much denser. The carbonation products have no space to expand (pores), inducing microcracks in the CCC and reducing its compressive strength.

Fig. 6 illustrates the carbonation depths of the CCC and NC specimens after CO₂ exposure for 28 d. Fig. 7 and Table 3 present the effects of the casting pressure (0, 5, and 15 MPa) on the carbonation depths of the specimens. The carbonation depths were much smaller in the CCC specimens than in the NC specimen. After 28 d of CO₂ exposure, the carbonation depths of the CCC specimens were 82.6% and 91.7% lower at 5 and 15 MPa, respectively, compared with the NC specimen. The carbonation depths of the CCC specimens were quite similar for all CO₂ exposure durations, indicating that the impact of the casting pressure on the carbonation depths is insignificant. Thus, CCC can be used to minimize CO₂ penetration into concrete to postpone or inhibit steel corrosion in reinforced concrete structures.

3.3.2. FT cycles

The FT tests were performed according to the Chinese standard GB/T 50082-2009 [26]. The appearances of the concrete specimens after various FT cycles differed (Fig. 8). As the number of FT cycles increases for NC, the cement paste on the external surface begins to fall off, followed by the gradual formation of honeycomb pits with a large volume of powdery residue [28]. The coarse aggregates were exposed after 50 FT cycles, similar to existing test observations (Fig. 8(b) ) [29]. The CCC-5 specimen suffered significantly less damage to the surface than the NC counterparts after 50 FT cycles, indicating a higher FT concrete resistance with compression-casting (Fig. 8(d)). After 125 FT cycles, slight damage was observed on the surfaces of the CCC-5 and CCC-15 specimens (Figs. 8(e) and (g)), with the CCC-5 having more damage than CCC-15, demonstrating that a higher casting pressure results in more FT resistance. A substantial longitudinal crack was observed in the CCC-15 specimen at the end of the FT cycles (Fig. 8(h)), similar to that observed in high-strength concrete [30]. The FT tests were terminated when the relative elastic modulus of the specimens was lower than the standard value [26]. The enhanced FT resistance due to compression-casting is attributed to the increased strength and improved microstructure of CCC.

Fig. 9 presents the retention values of the dynamic elastic modulus of concrete after FT cycles for different casting pressures. As expected, the dynamic elastic moduli of the NC and CCC samples deteriorated with increasing FT cycles, and the deterioration ratio was considerably affected by the casting pressure. Compared with the NC specimen, the dynamic elastic moduli of the CCC specimens decreased at a slower rate, and a higher casting pressure contributed to a lower deterioration rate. After 50 FT cycles, the retention of the dynamic elasticity moduli of specimens CCC-5 and CCC-15 were 1.62 and 1.71 times that of the NC specimen. The dynamic elastic modulus is affected by cement paste, aggregate, ITZ, and porosity [31], [32]. As the FT cycles continued, microcracks and voids appeared and propagated in the mortar and along the ITZ, reducing the dynamic modulus of the concrete’s elasticity [33]. The denser internal structure and higher strength of the cement matrix under a higher casting pressure improved the retention of the dynamic modulus of elasticity and enhanced FT resistance.

Fig. 10 presents the strength degradation of concrete under different casting pressures. As expected, the initial strength before FT cycling (Fig. 10(a)) and the residual strength during the FT cycle (Fig. 10(b)) were higher for the CCC specimens than for the NC specimens, and these findings are more profound for specimens cast under higher pressure. After 50 FT cycles, the compressive strengths of NC, CCC-5, and CCC-15 were 16.4, 68.0, and 80.2 MPa, respectively, corresponding to the residual strengths of 38.1%, 97.7%, and 99.5%, respectively. When the FT cycles reached 100, the residual rates of the compressive strengths of NC, CCC-5, and CCC-15 were 0% (completely disintegrated), 82.1%, and 99.4%, respectively. A higher casting pressure leads to higher mortar and ITZ strength of the concrete, postponing concrete damage and cracking [34].

3.3.3. Chloride penetration

The chloride diffusion coefficient, determined based on the rapid chloride migration (RCM) test, is one of the most important indices for evaluating the chloride permeability of concrete [35], [36], [37], [38]. This section describes the chloride penetration depths and chloride diffusion coefficients of NC and CCC based on standard RCM tests, per the Chinese standard GB/T 50082-2009 [26]. All the concrete samples had 100 mm diameter and 50 mm depth. The CCC and NC specimens were prepared under compression pressures of 5 and 0 MPa, respectively. The chloride penetration depths and chloride diffusion coefficients of the CCC samples were smaller than those of their NC counterpart, indicating that compression-casting can postpone chloride penetration (Fig. 11). This is because the reduced porosity and densified microstructure of concrete further inhibit chloride migration [39], [40].

3.3.4. Water absorption

Water absorption tests were performed on the CCC and NC specimens in accordance with GB/T 50081-2019 [27]. The concrete grade and geometrical dimensions of the CCC and NC samples were the same as those of the specimens used in the chloride penetration tests described previously (Section 3.3.3). Water absorption was calculated as a percentage of the mass increase according to the procedure described in a previous study [27], and Fig. 12 presents the results. As expected, the CCC samples had a significantly lower water absorption rate. Specifically, the average water absorption ratio of the CCC samples was 3.83%, which was 32.8% lower than that of the NC specimen. Concrete water absorption is closely related to its pore structure [41], [42], [43]. The compression pressure reduces porosity and improves the pore structure of concrete, resulting in less water absorption by the CCC. Thus, compression-casting decreases water absorption, making the CCC resist aggressive ion invasion better, which is consistent with the chloride penetration test results presented in the previous subsection (Section 3.3.3).

3.3.5. Elevated temperature

Fine cracks were observed on the surface of NC after 2 h of exposure to 400 °C, and the number and width of the cracks gradually increased with increasing temperature (Fig. 13). The initiation and propagation of temperature-induced cracks are due to the thermal strain differences between the hardened cement matrix and aggregates [44], [45]. After 2 h of exposure to 800 °C, the surface of the NC specimen was covered with cracks, with some areas having an off-white color. This appearance change was caused by the lime generated during the decomposition of calcium hydroxide and calcium carbonate [46]. In contrast, after 2 h of exposure to 400 °C, the CCC-15 specimen had almost no cracks on its surface. Fine cracks were observed when the temperature increased to 600 °C, and they increased at 800 °C. Thus, the CCC specimen damage caused by high temperatures was much less severe than that of the NC specimen in terms of crack number and width.

Fig. 14(a) reveals that the compressive strengths of the CCC and NC specimens decreased with increasing temperature. The compressive strength of the CCC specimen is higher than that of the NC specimen at 20, 400, 600, or 800 °C, and further increase can be achieved at a higher casting pressure. Compression-casting reduces excess water, densifies the pore structure, and improves the strength of the mortar and ITZ. These combined effects mitigate the temperature-induced damage and enhance the residual compressive strength of the CCC specimens compared with that for the NC sample [47], [48]. Fig. 14(b) reveals the compressive strength retention rate of the NC, CCC-5, and CCC-15 specimens, which is defined as the ratio of the strength at the current temperature to the initial strength at room temperature of 20 °C. The retention of compressive strength was higher for the CCC samples than for the NC specimens. The compressive strength retention of the NC specimen at 600 °C was 0.284; however, it was 0.359 and 0.420 for CCC-5 and CCC-15, respectively.

3.4. Cement savings

The cement industry accounts for approximately 5%-10% of the world’s total carbon emissions [49], [50], of which approximately 90% of CO₂ emissions are from ordinary Portland cement (OPC) production. Thus, reducing OPC without sacrificing the concrete properties can contribute to moving toward carbon peaking and carbon neutrality policy. Therefore, alternative binders (sulfoaluminate belite cement and calcium aluminate cement) with lower CO₂ emissions have been used to partially replace OPC, imposing various levels of side effects on the concrete performance [51], [52]. Mineral additives, such as fly ash and slag [53], at various percentages, can also replace OPC in concrete. However, the low substitution rate, high cost, and environmental impact of certain mineral additives hinder the use of this method.

CCT is also effective in reducing the cement content in concrete under the same concrete strength. Compression tests were conducted on standard concrete cylinders to determine the cement saving rates, with the main test variables being the cement content in the concrete mix, w/c ratio, and casting pressure. CCT can significantly enhance the compressive strength of concrete with a w/c ratio of 0.53 when the cement content exceeds 253 kg∙m⁻³ (Fig. 15(a)) [54]. The increased strength is the basis for reducing cement in concrete while meeting the strength requirement. Insufficient cement (e.g., 190 kg∙m⁻³) cannot provide sufficient cement paste, resulting in a porous surface alongside the low strength. The cement content of CCC should be significantly lower than that of NC to achieve the required concrete strength (Fig. 15(b)) [54]. For example, the cement saving ratio required to achieve a CCC with 40 MPa strength is approximately 30.2% less than that required for NC. When the required strength was 50 MPa, the cement saving ratio of CCC was 47.5% compared with that of NC. The results in Fig. 15 demonstrate that CCT can remarkably reduce the cement content in concrete manufacturing without strength loss while improving durability.

3.5. Comparative benefits over other concrete types

As previously stated, CCT can simultaneously reduce cement quantity in a concrete mix and enhance its mechanical properties, benefiting the environment. LCA is commonly used to quantitatively evaluate the environmental impacts of products, as defined by the European Standard Commission [55]. Tang et al. [56] used LCA to evaluate the carbon reduction benefits of adopting CCT in producing concrete components, from the extraction of raw materials to manufacturing completion, using carbon emissions per unit of compressive strength (CECSs, unit: kgCO₂eq∙MPa⁻¹) as the key indicator. The results are presented in the following sections.

3.5.1. Comparative benefits over other concrete types

Fig. 16 compares the carbon emissions and CECS of the concrete components of CCC, compression-cast waste rubber concrete (CCWRC), and compression-cast waste rubber recycled aggregate concrete (CCWRRC) with those of other types of green concrete and conventional concrete using the same compressive strength [56]. Other green concrete types include granite dust concrete (GDC) [57], e-waste concrete (EWC) [58], rice husk ash concrete (RHAC) [59], ferronickel slag and blast furnace slag powder (F-S powder) concrete (FBSC) [60], and rubber concrete (RUC) [61]. Fig. 16 reveals that all types of CCC have lower carbon emissions and CECS than uncompressed concrete.

3.5.2. Carbon reduction potential

NC was replaced by CCC of similar strength at different ratios (0%, 25%, 50%, 75%, and 100%) to evaluate the potential carbon reduction benefits of CCT, and the carbon emissions were compared. Concrete demand data for residential and commercial buildings in 26 regions worldwide were obtained from Zhong et al. [62]. Fig. 17 reveals that the carbon emissions from NC and CCC production increased annually. From 2025 to 2060, when all the concrete is NC, the carbon emission increases from 1.75 × 10¹² to 2.25 × 10¹² kgCO₂eq. When 100% of the NC is replaced by CCC, the carbon emission increases from 1.25 × 10¹² to 1.60 × 10¹² kgCO₂eq, indicating a considerable decrease in annual carbon emissions. From 2025 to 2060, the annual carbon emissions from CCC declines by 7%, 14%, 20%, and 27% at replacement ratios of 25%, 50%, 75%, and 100%, respectively, compared to that from NC. The results indicate that globally, replacing all NC with CCC could significantly reduce the carbon emissions generated in concrete production and would greatly contribute to achieving the world’s goal of carbon peaking and carbon neutrality.

3.6. Upgrading solid waste-based concrete by CCT

3.6.1. Concrete with recycled concrete as coarse aggregates

Producing high-quality recycled aggregate concrete (RAC) using recycled aggregates (RAs) is an ideal approach for sustainable construction [63]. However, RA is enwrapped by a layer of old mortar with significant microcracks and high porosity, degrading RAC performance [64], [65], [66], [67]. CCT can effectively enhance RAC performance. Uniaxial compression tests on standard cylinders of compression-cast RAC and normal RAC were conducted to provide experimental evidence using w/c ratios as test variables. The bulk density, crushing index, specific gravity, and water absorption of the RA were 1254 kg∙m⁻³, 18.69%, 2.48, and 4.98%, respectively. Fig. 18 reveals that for w/c ratios of 0.71, 0.55, and 0.45, the compressive strength of normal RAC was 21.8, 28.7, and 31.6 MPa, respectively, while that of CCC was 42.2, 48.2, and 64.1 MPa, respectively. The strength increases were 93.6%, 67.9%, and 102.8%, respectively, at a casting pressure of 15 MPa. Thus, compression-casting can significantly improve RAC strength, indicating that CCT is ideal for achieving high-quality RAC without requiring further physical or mechanical modifications.

3.6.2. Concrete with recycled rubber coarse aggregate

Over the past few decades, scrap tire (rubber) disposal has become a major environmental concern globally [68]. Compared to landfilling and stockpiling, recycling and reusing waste rubber to make recycled rubber concrete (RRC) is a much better approach [68], [69]. However, incorporating waste rubber results in substantially poor concrete performance [70]. For example, concrete with all coarse aggregates replaced by waste tire chips led to 85% and 50% reductions in compressive and split tensile strengths, respectively [71]. Wu et al. [19] stated that compared to existing chemical and physical methods that modify only the waste rubber, CCT can be a much better approach. Significant improvements in the mechanical properties [20], [72], [73] and the reduced carbon footprint [74] of RRC can be achieved through CCT. Fig. 19 presents a comparison of the compressive strengths and elastic moduli of the normal-cast rubber concrete and compression-cast specimens [19]. The strength of the normal-cast series decreased from 31 MPa for natural aggregate concrete (NAC) to 24, 22, 17, 14, 5, and 4 MPa at rubber replacement ratios of 10%, 20%, 30%, 40%, 80%, and 100%, respectively (Fig. 19(a)). As the rubber replacement ratio increases, a similarly significant reduction in the elastic modulus is observed (Fig. 19(b)). CCT remarkably improves the compressive strength and elastic modulus compared with its counterparts under normal casting. Specifically, compression casting increased the strength and elastic modulus of rubber concrete by 62.5%-200.0% and 47.6%-1100.0%, respectively, with rubber replacement ratios of 10%-100%. Furthermore, the compressive strength and elastic modulus of compression-cast rubber concrete are comparable to those of normal-cast NAC (without rubber) samples at a rubber replacement ratio of up to 30%. This paves the way for the application of rubber concrete products without loss of strength and elastic modulus.

3.6.3. Concrete with recycled brick and glass powders

The use of recycled micro-powder (RMP) as a substitute for OPC in concrete is effective in reducing the use of carbon-intensive OPC in concrete products. However, owing to the low reactivity of RMP compared to that of OPC, incorporating RMP into concrete can deteriorate the mechanical properties of concrete [75]. Thus, CCT, which effectively enhances the micro and macro properties of concrete, is promising. Therefore, an investigation was conducted, and the main test variables were casting pressure, RMP type, and RMP content. In this study, the RMP included brick powder (BP) for recycled BP concrete and glass powder (GP) for recycled GP concrete. According to the Chinese standard JG/T 573-2020: Recycled Fine Powder Used in Concrete and Mortar [76], the maximum particle size of the micro-powder should be < 75 μm, without chemical modifications.

Fig. 20 reveals the average compressive strengths of the CCC and NC specimens. Incorporating RMP degrades the compressive strength of the CCC and NC specimens, and the deterioration increases with the RMP replacement ratio. For example, compared to the NC samples with 100% OPC as the binder (NC-O), the compressive strengths of the NC specimens with BP replacement ratios of 15% (NC-BP15) and 30% (NC-BP30) decreased by 29% and 46%, respectively, which changed to 13% and 36% for NC specimens with GP replacement ratios of 15% (NC-GP15) and NC specimens with GP replacement ratios of 30% (NC-GP30), respectively. The reduced compressive strength of concrete incorporating recycled BP and GP is attributed to their lower reactivity towards hydration compared to OPC. Similar results have been reported in previous studies [77], [78].

Interestingly, CCT can significantly enhance the compressive strength of concrete incorporating RMP, regardless of the type and replacement ratio (Fig. 20). The compressive strengths of specimens NC-O, NC-BP15, NC-BP30, NC-GP15, and NC-GP30 increased by 88.47% (CCC-O), 141.04% (CCC-BP15), 163.63% (CCC-BP30), 92.71% (CCC-GP15), and 110.50% (CCC-GP30), respectively, at 10 MPa CCP. This indicates that the strength enhancement owing to compression-casting increased with increasing recycled brick (or glass) powder replacement ratio. The compressive strength of the CCC specimens with 30% BP (or GP) replacement was 42% (or 34%) higher than that of the NC specimen with 100% OPC as the binder. CCT can offset the side effects of low-reactivity recycled powders on concrete strength and promote the high-quality and large-scale application of recycled powders in making stronger concrete structures.

3.6.4. Concrete with desert sand (DS)

River sand, a key concrete ingredient, has been overexploited, resulting in supply shortages and environmental challenges [79], [80]. In contrast, DS reserves are large and can serve as fine aggregates to promote the sustainable development of concrete material [79], [80], [81]. However, existing studies indicate that the compressive strength of DS concrete (DSC) decreases as the DS content exceeds 10%-20% [82], [83], [84]. This was attributed to the fact that excessive DS forms a sliding band, wherein a high specific surface area of DS reduces the paste surrounding the aggregate, reducing the cohesive force between the aggregates. Moreover, excess DS disorders the particle gradation of the fine aggregate [79]. Thus, large-scale replacement of river sand with DS is impossible if the strength reduction due to excess DS cannot be resolved.

CCT was used to enhance the concrete properties to overcome the limitation of strength reduction at a larger DS replacement ratio (up to 100%). Compression and splitting tests were conducted on standard concrete specimens. The gradation curves for river sand and DS were compared according to ASTM C33: Standard Specification for Concrete Aggregates [85], as shown in Fig. 21. Fig. 22 reveals that the compressive and split strengths of concrete specimens with 100% DS increased by 111.8% and 66.6%, respectively, under 15 MPa casting pressure compared with the normal-cast specimens. The remarkable enhancement in the compressive and split strengths of CCC demonstrates that CCT is effective for facilitating large-scale DS use.

4. Structural behavior

4.1. Aims of this section

CCT can improve the compressive strength and durability of concrete considerably while making the concrete more brittle—the descending portion of the stress-strain curve becomes steeper or missing. This CCC characteristic negatively impacts the mechanical behavior of reinforced CCC structures. Consequently, existing designs for NC structures may not be applicable to CCC structures. CCC structures would exhibit insufficient ductility owing to the brittleness of the CCC material if existing design guidelines are followed. Therefore, studying CCC structures through structural testing and developing means of overcoming their brittleness are essential.

Therefore, we conducted tests on the structural behavior of CCC members and proposed approaches for overcoming this brittleness. Measures for increasing ductility are required only in critical areas of the structures, such as beam-column joints and plastic-hinge regions of beams and columns, to reduce the material cost and ensure the ductility of CCC structures simultaneously. Table 4 presents an overview of the completed tests, including the flexural tests of the CCC beams, shear tests of the CCC slabs, axial compression tests of the FRP-confined CCC columns, and axial compression tests of the steel fiber-reinforced CCC columns. The test results are described in the following sections.

4.2. Effect of CCC brittleness on structural behavior

4.2.1. Flexural behavior of FRP-reinforced CCC beams

FRP reinforcement was used instead of steel reinforcement to determine the worst possible effect of material brittleness on the behavior of the CCC structural components. As recommended by ACI 440.1R-15 [86], all beams were designed to be over-reinforced to avoid the abrupt rupture of the FRP reinforcement. The configurations of the longitudinal and transverse reinforcements and their material properties are listed in Table 4. This study focused on flexural performance, and the stirrups did not affect the behavior of the members in flexure; thus, steel bars were used as the transverse reinforcement/stirrups to reduce material costs and facilitate the easy construction of the members. The CCP for the structural members of CCC was 10 MPa.

The compressive strength of concrete increased from 29.6 MPa for NC to 46.9 MPa owing to compression-casting. Fig. 23 presents the ultimate failure modes and crack distributions of all the specimens. “NC” and “CCC” indicate NC and CCC beams, respectively, and the following numerical values denote the longitudinal reinforcement ratio (ρ_f) of carbon FRP (CFRP) bars in percentage. Fig. 23 reveals that all the FRP-reinforced NC beams failed in flexural compression. Additionally, the small deflection of some beams in Fig. 23 does not reflect their displacement at failure because the FRP is elastic, and the beams rebounded after unloading. In contrast, the CCC beam of B-CCC-0.75 split in the tension zone, followed by beam breakdown owing to a large vertical crack in the flexural-shear zone. This is attributed to the more pronounced brittleness of CCC compared to NC. Concrete crushing and splitting occurred simultaneously in B-CCC-1.13 in the compression and tension zones, respectively. Specimen B-CCC-1.78 exhibited localized concrete crushing in the pure bending zone, typical of flexural compression failure. A higher reinforcement ratio led to a lower tensile stress in the FRP bars and a smaller bond stress between the CCC and the FRP bars. No splitting failure was observed in B-CCC-1.78.

Fig. 24 illustrates the influence of compression-casting on the moment-midspan deflection curves of the FRP-reinforced beams. In Fig. 24(a), with a ρ_f of 0.75%, the ultimate bending moment of specimen B-CCC-0.75 (66.2 kN·m) was lower than that of specimen B-NC-0.75 (89.6 kN·m) despite the CCC specimen having a higher compressive strength than the NC specimen. This was unexpected and attributed to the different failure modes. B-NC-0.75 experienced normal flexural failure with concrete crushing in the compression zone, allowing the beam to fully use its flexural load-carrying capacity. In contrast, B-CCC-0.75 suffered a splitting failure, resulting in an abnormal failure mode with a significantly lower flexural capacity. This indicates that a low reinforcement ratio should be avoided in FRP-reinforced CCC members. In Figs. 24 (b) and (c) with ρ_f values of 1.13% and 1.78%, despite the CCC specimen having a higher compressive strength than the NC specimen, the difference in the ultimate bending moment of both designs of FRP-reinforced beams was negligible because of the brittleness of CCC materials. The steeper post-peak portion of the stress-strain curve of the CCC specimen resulted in a lesser ultimate concrete strain and a smaller area of compression stress block at the onset of flexural strength [87]. Therefore, despite the higher strength of the CCC specimen, the beam reached its ultimate strain and flexural strength earlier, in which case the flexural strength was negatively affected.

4.2.2. Shear behavior of FRP-reinforced CCC slabs

Fig. 25 reveals the failure modes and crack distributions of FRP-reinforced slabs. In the specimen identification, the numerical values following “NC” or “CCC” represent the concrete w/c. The reinforcements for all slabs were identical and are listed in Table 4, and no stirrups were used. All the slabs failed under compression shear, as manifested by the appearance of a critical diagonal shear crack. The cracking load of the FRP-reinforced CCC slabs was greater than that of the FRP-reinforced NC slabs, owing to the higher strength of CCC. In addition, concrete crushing at the onset of shear strength was less pronounced in the FRP-reinforced CCC slabs. This is attributed to the higher brittleness of the CCC. Once the CCC exhibited signs of crushing, the load-carrying capacity of the concrete compression zone collapsed drastically, rapidly reducing the shear capacity of the slabs [87]. In contrast, the resultant compression force of NC decreased at a slower pace; hence, the crushing of the concrete developed further before the shear failure of the slabs.

Fig. 26 presents the shear force versus midspan deflection curves of the test slabs. The load-deflection curves of the slabs are in three stages. The load first increased linearly with deflection. Subsequently, the slopes of the curves gradually decreased owing to cracks. Finally, the load-carrying capacity decreased abruptly owing to a critical diagonal crack. Compared with the FRP-reinforced NC slabs at each w/c ratio, the FRP-reinforced CCC slabs experienced greater cracking loads and higher stiffness throughout loading. This is due to the larger elastic modulus, higher compressive strength, and stronger aggregate interlocking effect of CCC compared to that in NC. Nevertheless, the ultimate deflection of the FRP-reinforced CCC slabs was smaller than that of the FRP-reinforced NC, owing to the lower compression ductility of the CCC. Consequently, no evident improvement was observed in shear strength for FRP-reinforced CCC slabs, even though the concrete compressive strength improved by 39.8%-68.6% after compression-casting.

4.3. Overcoming the brittleness of CCC structures

4.3.1. Methodology

More attention must be paid to improving the ductility of the reinforced CCC members because CCT can increase the brittleness of the material. The flexural ductility of a concrete member depends on the rotational capacity of its plastic hinge, which is closely related to the ultimate curvature of the section [87]. Fig. 27 illustrates the strain profile of the reinforced concrete section at the onset of the flexural strength. The ultimate curvature (Φu) in the figure can be expressed as

(1)$\Phi_{\mathrm{u}}=\frac{\varepsilon_{\mathrm{cu}}}{c}$

where ε_cu is the ultimate strain of the concrete at the onset of flexural strength and c is the height of the compression zone. Therefore, the ultimate curvature of the section can be increased by increasing the ultimate strain or reducing the height of the concrete compression zone.

Based on the flexural theorems stated in previous studies [87], [88], ε_cu is only determined by the shape of the compressive stress-strain curve of concrete for under-reinforced reinforced concrete sections. Additionally, a more brittle concrete will have a smaller ε_cu value. Therefore, the ultimate curvature was smaller when the concrete was more brittle (Eq. (1)). To offset this effect, the following three measures were taken:

(1) Compression bars were added to the compression zone of the reinforced concrete section (A_sc in Fig. 27). This reduced the compression zone height c, thereby increasing the curvature based on Eq. (1).

(2) Adding fibers to concrete. This can increase ductility, which is the ultimate strain on the concrete ε_cu, thereby increasing the ultimate curvature of the section.

(3) Confining the concrete. The deformability and ductility of concrete can be significantly improved by confining it with steel or FRP tubes/stirrups, thereby increasing ε_cu and its ultimate curvature.

In the following sections, the feasibilities of the three approaches are evaluated in detail.

4.3.2. FRP confinement

Fig. 28 illustrates the failure modes of the CFRP-confined columns. In the specimen identification, “C-NC-F” and “C-CCC-F” represent the FRP-confined NC and FRP-confined CCC specimens, respectively. The nominal thickness of the CFRP was 0.167 mm, and the ultimate tensile strength and elastic modulus of FRP were 4525 MPa and 265 GPa, respectively. For the unconfined columns, compression-casting resulted in fewer cracks but significantly larger crack widths. With external FRP confinement, the middle portion of the specimen ruptured in the FRP-confined NC columns and FRP-confined CCC columns. The difference between both types of columns is that the explosion sound of the CCC specimens was louder, and the corresponding concrete spalling was more severe.

Fig. 29 illustrates the effect of compression-casting on the axial stress-strain curves of FRP-confined specimens. Bilinear strain hardening occurred in the stress-strain curves of the FRP-confined NC and CCC specimens, regardless of the FRP confinement layers. Compression-casting enhanced the axial stresses in the transition and ultimate states. This is primarily because compression casting significantly increases the unconfined compressive strength of concrete. However, the ultimate axial strain of the FRP-confined CCC specimens decreased owing to the compression-casting.

This can be explained as follows: On the one hand, compression-casting improved the strength and compactness of concrete, and brittleness and strain localization became more pronounced, decreasing the deformation capacity of unconfined and FRP-confined concrete. On the other hand, compression-casting increased the compressive strength of the concrete, implying that the confinement ratio, defined as the ratio of the confining pressure to the unconfined concrete strength (σ_l/f_c), was reduced under the same FRP confinement. Consequently, the ultimate axial strain decreased. The confinement efficiency of the FRP-confined CCC was lower than that of the FRP-confined NC; however, the brittleness of the CCC could be overcome by external FRP confinement. For instance, the ultimate axial strain of FRP-confined CCC exceeds 0.015 under two-ply FRP confinement, which is much larger than that of normal unconfined concrete and satisfies the ductility requirement of reinforced CCC structures.

4.3.3. Steel fiber addition

Fig. 30 illustrates the effect of the steel fiber volume fraction on the axial stress-strain curves of the CCC specimens. The numerical values following “CCC” indicate the volume fractions of steel fiber in percentage. Table 5 lists the physical and mechanical properties of the straight steel fibers. The descending portion of the stress-strain curve of CCC columns without steel fibers is almost nonexistent (crushed shortly after the peak load) owing to the high brittleness of CCC. As the number of steel fibers increases, the descending slope gradually decreases. The greater the steel fiber volume fraction, the gentler the descending portion of the stress-strain curve, indicating better ductility.

Fig. 31 compares the peak strain and 0.85f_u strain of the CCC columns with various steel fiber volume fractions, where f_u is the peak stress. The 0.85f_u strain is defined here as the strain at the point where the stress drops to 85% of the peak stress on the descending branch. Fig. 31(a) reveals that the peak strain increased with the fiber volume fraction. As shown in Fig. 30, adding steel fibers had almost no effect on the elastic modulus of the CCC. Therefore, the peak strain increase was primarily due to the increase in the compressive strength of the CCC. The improvement in the 0.85f_u strain of CCC was more significant owing to the addition of steel fibers (Fig. 31(b)). The 0.85f_u strain of CCC increased by 52.8% when a 2% volume fraction of steel fiber was added. The 0.85f_u strain of CCC with 2% steel fibers (0.00376) was even higher than that of NC at 0.0030-0.0035. This significant improvement overcomes the brittleness of CCC and increases the ductility of CCC structures.

4.3.4. Increasing the compression reinforcement

Another effective method to overcome the brittleness of CCC structures is increasing the compression reinforcement. Thus, more compression force is shared by the compression reinforcement; hence the compression force by the concrete decreases to balance the tension force. Consequently, the height of the concrete compression zone decreased. Fig. 27 and Eq. (1) reveal that a smaller compression zone height, c, results in greater ultimate curvature, Φu, and higher flexural deformation capacity. A design with detailed procedure has been developed for the flexural design of reinforced concrete beams constructed with highly brittle concrete materials, including those without a descending branch of stress-strain curve (crushed immediately after the peak concrete stress). The reinforced concrete beams are designed in two steps.

(1) Tensile reinforcement is determined based on the design moment using the flexural theorem-based approach [87], [88], [89], [90];

(2) If the design resulting from the above step satisfies the minimum ductility requirement, the calculation is complete. Otherwise, suitable compression reinforcement is added, and the relevant tension reinforcement is adjusted to satisfy the ductility requirements.

Details of the design procedure are reported in a seperate study.

5. Conclusions

From the extensive studies on CCC materials and FRP-reinforced concrete structures, the following conclusions can be drawn.

(1) CCC has a higher strength and better mechanical performance compared to normal-cast concrete.

(2) Compared with NC, CCC has better durability in terms of reduced carbonization depth, less damage under FT cycles, smaller chloride penetration depth, better resistance to water penetration, and greater fire resistance.

(3) The compression-casting method can remarkably improve the mechanical properties and durability of concrete containing solid waste-based aggregates (e.g., RAs, waste rubber particles, and DS) or micro-powders (e.g., recycled brick and GPs). It can facilitate the large-scale recycling of solid waste and the use of substandard aggregates in concrete construction without compromising concrete quality.

(4) CCT can reduce cement usage in concrete and its costs considerably without compromising the quality and mechanical properties. Consequently, CCC can substantially reduce greenhouse gas emissions in the construction industry.

(5) CCC is more brittle than NC. Design guidelines for NC structures might not apply to CCC structures.

(6) CCC brittleness can be overcome by FRP/steel confinement, adding steel fibers to concrete, and increasing the compression reinforcement in the flexural design.

Compression casting requires a sturdy mold to withstand the high compressive pressure applied to fresh concrete, making it more suitable for mass production of standard precast concrete elements/products. In principle, each CCC product requires a customized mold. Moreover, concrete can be demolded shortly after casting (2-5 min). Consequently, molds can be used much more efficiently in CCC than in NC casting; hence, only a few molds are required for the mass production of standard products. Therefore, the molding cost can be lower than that of NC casting. A universal casting facility (Fig. 2) can be designed to cast different types of reinforced concrete elements. Generally, compression casting is more complex than regular concrete casting; nonetheless, it is much faster and may not increase the costs of concrete casting.

This study only introduces the performance of CCC materials and a prelimimary study on it FRP-reinforced structural members. For structural application of CCC, more extensive research is required on characterization of CCC materials, behavior of CCC structures, relevant design theories, and the response of CCC structures under extreme loads. Due to the significant advantages of CCC, the compression-casting technology could reshape the future of concrete manufacturing.

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