By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jun 19 2024A recent study featured in the Journal of Engineering and Applied Science explored the use of recycled aggregates from construction waste for creating new concrete. The research focused on utilizing coarse recycled aggregates as replacements for natural aggregates in concrete mixes, with substitution rates ranging from 0 % to 100 %.
Study: Producing New Concrete Using Recycled Aggregate. Image Credit: Francesco Scatena/Shutterstock.com
Background
Concrete is the most widely used construction material as it is readily available and easy to work with. However, producing concrete requires significant extraction of natural resources such as cement, sand, and gravel. The extraction and processing of these materials are environmentally harmful and cause ecological imbalance due to depletion.
Construction and demolition waste is another significant environmental issue related to the building industry. Recycled coarse aggregates (RCA) in construction can save energy, finances, and natural resources. However, previous studies have demonstrated that using coarse aggregate from older concrete instead of regular coarse aggregate reduces the characteristics of new concrete.
The concrete comprising RCA can be optimized to perform at par with regular concrete while reducing environmental degradation and natural resource depletion by the construction sector. Thus, this study comprehensively examined the effect of using construction waste as coarse aggregate instead of natural aggregate with various replacement ratios.
Methods
The researchers used conventional cement and common sand as fine aggregates. Concrete cylinders from previous laboratory experiments were crushed to generate RCA of size ten millimeters. Overall, 11 distinct concrete compositions were used in this empirical investigation by varying the quantity (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 wt.%) of reused coarse material.
The workability (slump) of the mixtures was assessed after the mixing process and before casting. The slump test is intended to ascertain the consistency of concrete classification in its initial stage.
The test samples were cast into molds shaped like cylinders and prisms with dimensions 100×200 mm2 and 100×100×500 mm3, respectively, according to the standard specifications. These specimens were subjected to curing for 7, 14, and 28 days in a water tank at a constant temperature of 27 ± 3 ºC.
Additionally, characteristics such as bulk density, flexural strength, compressive strength, and tensile splitting strength were examined for all concrete mixes. These tests were performed after 7, 14, and 28 days, according to international standards.
Results and Discussion
As the replacement ratios of coarse aggregates with RCA increased, the workability of the concrete decreased. This reduction is primarily due to the rougher surface texture of RCA compared to traditional aggregates, which also increases the surface area and, consequently, the water absorption rate. Therefore, concrete mixes incorporating RCA require more water than those made with regular aggregates.
The density of concrete with RCA also diminishes as the replacement quantity increases, largely due to mismatches between RCA and other core ingredients in the mix. Contributing to this lowered density are the permeability of the RCA and the presence of weakly bonded natural aggregates, which amplify the rate of water absorption.
A decline in the mechanical strengths—compressive, flexural, and tensile—was observed with greater proportions of RCA, irrespective of the curing period. However, these strengths improved with extended curing times, suggesting a complex interaction between RCA content and curing duration.
Notably, the largest observed reductions in compressive strength, flexural strength, tensile strength, density, and slump compared to control samples (0 % RCA) were 19.4 %, 18.3 %, 19.6 %, 19.5 %, and 25.0 %, respectively. These findings underscore the need for careful optimization of the RCA replacement ratio to maintain construction viability.
Enhanced mechanical performance in RCA-containing concrete over time was attributed to its greater absorption capacity and the beneficial effects of internal curing, which are more pronounced with longer curing periods.
Conclusion
This study demonstrated that RCA-containing concrete, while weaker than conventional concrete, still meets the strength requirements for construction use. Increasing the proportion of recycled aggregates, however, reduces the operational workability of the concrete.
The results presented in the paper affirm the feasibility of using RCA as an alternative to coarse aggregate in concrete production, albeit with some compromises in strength. The findings provide valuable insights into the viability of RCA for sustainable construction practices.
Future research should focus on the microstructural and durability properties of concrete mixes with RCA to further validate their use in practical applications. Moreover, developing innovative methods to enhance the mechanical properties—compressive, flexural, and tensile strengths—of recycled concrete is crucial for its adoption in structural applications.
Journal Reference
Ola Adel Qasim, Hilal, N., Al, M. I., Nadhim Hamah Sor, & Tawfik, T. A. (2024). Studying the usability of recycled aggregate to produce new concrete. Journal of Engineering and Applied Science, 71(1). https://doi.org/10.1186/s44147-024-00463-1, https://jeas.springeropen.com/articles/10.1186/s44147-024-00463-1
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