By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Nov 5 2024A recent article published in Engineering introduced green compression-casting for fabricating reinforced concrete structures. The article examined the effects of various inferior concrete materials, including rubber, desert sand, recycled aggregate, and recycled powder concrete, on compression-cast concrete (CCC). Additionally, fiber-reinforced polymer (FRP)-based structural members were compared with those of normal concrete (NC).
Study: Green Compression-Cast Concrete Material and Its Fiber-Reinforced Polymer (FRP)-Reinforced Concrete Structures. Image Credit: Another77/Shutterstock.com
Background
High carbon emissions from the global construction industry pose severe climatic issues. Consequently, efforts are being made to reduce cement usage in concrete production while maintaining its strength. Partially substituting cement with industrial by-products, such as fly ash, blast silica fume, etc., is an effective approach.
Another approach is substituting traditional cement with innovative alternatives that need less energy and emit lesser CO2 emissions, such as alkali-activated geopolymers and limestone-calcined clay cement. Additionally, high-performance fibers, such as glass and carbon fibers, can improve the concrete’s toughness and crack resistance while reducing cement usage.
However, all these methods have certain limitations. To address these, the researchers recently developed a novel compression-casting technology (CCT) that can considerably enhance durability and strength and lower the cost of concrete.
CCT utilizes high casting pressure while ensuring its uniform transmission to all parts of the concrete unit. The resulting CCC has the same composition as NC with no chemical or mineral additives.
Methods
The performance of CCC was evaluated through several systematic theoretical and experimental investigations. The carbon reduction advantages of CCC were quantified via a life-cycle assessment (LCA).
The experiments involved several material tests, including mechanical, chloride penetration, carbonation, water absorption, freeze-thaw, and elevated temperature tests. A concrete specimen prepared through normal casting was considered as the reference sample. Alternatively, the samples prepared using CCT were termed “CCC-X,” with “X” representing the casting pressure.
Multiple microscopic investigations, including scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and backscattered electron (BSE) analysis, were performed to comprehend the role of CCT in enhancing the mechanical properties of CCC.
Carbonation and uniaxial compressive strength tests were conducted according to standard procedures. Notably, the chloride diffusion coefficient of CCC specimens was determined through the rapid chloride migration (RCM) test.
NC was replaced by CCC of similar strength at 0%, 25%, 50%, 75%, and 100% ratios to evaluate the carbon reduction potential of CCT. Furthermore, the structural behavior of CCC specimens was examined in different configurations, including flexural tests of CCC beams, shear tests of CCC slabs, axial compression tests of FRP-confined CCC columns, and axial compression tests of steel fiber-reinforced CCC columns.
Results and Discussion
CCC samples’ compressive strength and elastic modulus were remarkably higher than the NC samples. The significant improvements in the mechanical characteristics of CCC were primarily attributed to the reduced water and air contents and the denser microstructure.
SEM images revealed fewer pores and a denser matrix for CCC-15 than for NC, consistent with the porosity results. Therefore, CCT could densify the matrix microstructure, reduce mortar porosity, and improve the interfacial transition zone (ITZ) quality, significantly enhancing the CCC’s mechanical properties and durability.
The compressive strength of the CCC specimens with or without CO2 exposure was higher than that of the NC specimens. Without CO2 exposure, the compressive strength of CCC-5 and CCC-15 increased by 81.8% and 118.2%, respectively, relative to that of the NC specimens. Moreover, the compressive strengths of the 28 d CO2-exposed CCC-5 and CCC-15 specimens increased by 36.2% and 61.7%, respectively.
The CCC samples’ chloride penetration depths and diffusion coefficients were smaller than those of their NC counterparts, indicating that CCT could postpone chloride penetration. This was attributed to the inhibited chloride migration due to the reduced porosity and densified microstructure of concrete.
Considering the period from 2025 to 2060, carbon emissions from NC and CCC production increased annually. However, CCC production had lower annual carbon emissions. Notably, the annual carbon emissions from CCC decreased by 7%, 14%, 20%, and 27% at replacement ratios of 25%, 50%, 75%, and 100%, respectively, compared to NC.
CCC brittleness could be overcome by FRP/steel confinement, adding steel fibers to concrete, and increasing the compression reinforcement in the flexural design. Moreover, the molding cost in the CCT method was estimated to be lower than NC casting.
Conclusion
Overall, the researchers comprehensively examined CCC materials and FRP-reinforced CCC structures to conclude that CCC had a higher strength and better mechanical performance than NC while substantially reducing greenhouse gas emissions. Moreover, CCC samples exhibited better resistance to water, fire, and chloride penetration.
CCT is generally regarded as more complex than regular concrete casting. However, it is much faster and more cost-effective than NC casting. The preliminary results on FRP-reinforced CCC structural members demonstrated this. The researchers suggest extensive characterization of CCC materials for practical applications.
Journal Reference
Wu, Y.-F., Yuan, F., & Hu, B. (2024). Green Compression-Cast Concrete Material and Its Fiber-Reinforced Polymer (FRP)-Reinforced Concrete Structures. Engineering. DOI: 10.1016/j.eng.2024.10.005, https://www.sciencedirect.com/science/article/pii/S2095809924006283
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