No abstract

The consumption of structural concrete in the construction industry is rapidly growing, and concrete will remain the main construction material for increasing urbanization all over the world in the near future. Meanwhile, construction and demolition waste from concrete structures is also leading to a significant environmental problem. Therefore, a proper sustainable solution is needed to address this environmental concern. One of the solutions can be using recycled coarse aggregates (RCA) in reinforced concrete (RC) structures. Extensive research has been conducted in this area in recent years. However, the usage of RCA concrete in the industry is still limited due to the absence of structural regulations appropriate to the RCA concrete. This study addresses a safety margin of RCA concrete beams in terms of shear capacity which is comparable to natural coarse aggregates (NCA) concrete beams. To this end, a database for reinforced concrete beams made of recycled coarse aggregates with and without shear reinforcement was established, collecting the shear specimens available from various works in the existing literature. The database was used to statistically identify the strength margin between RCA and NCA concrete beams and to calculate its safety margin based on reliability analysis. Moreover, a comparability study of RCA beams was conducted with its control specimens and with a database for conventional RC beams.

This study discusses the torsional capacity of recycled coarse aggregate (RCA) reinforced concrete beams under pure torsion based on the experimental findings available in the literature. The experimental data on RCA specimens were collected and compared with the conventional concrete specimens with key variables, such as compressive strength and longitudinal and transverse reinforcement ratios, as those variables affect the torsional capacity of reinforced concrete beams. Overall, the database consisted of experimental results from 30 RCA specimens and 256 natural coarse aggregate (NCA) specimens. The result shows that specimens with a 100 % replacement ratio have the lowest strength. In addition, as the structural mechanism of torsion is similar to the shear mechanism in reinforced concrete beams, a comparative analysis was performed with RCA specimens subjected to shear force. It was concluded that the RCA has a similar effect in strength reduction for the specimens subjected to torsion or shear with a 100% replacement ratio. However, further study and experimental evidence are required to confirm the applicability of the recycled aggregate to produce and design the structural members.

Cryogenic fracturing with liquid nitrogen (LN2) has been identified as a feasible and promising waterless fracturing method for coalbed methane extraction for its environmental safety and effectiveness. Though it performed well in certain field tests in the late twentieth century, the use of LN2 as the fracturing fluid is still largely unexplored. This research work examines the thermo-mechanical properties of coal specimens from the Karaganda basin in Kazakhstan. Coal specimens were subjected to LN2 treatment under varied lab-controlled conditions, such as the freezing time (FT) length and number of freezing-thawing cycles (FTC), both in dried and water-saturated conditions. SEM investigation for FTC and FT experiments for dried coal samples indicated that the LN2 freezing-thawing process can enhance the cryogenic fracture extent and the fracture interconnectivity. Moreover, uniaxial compressive tests indicated that compressive strength decreases beneficially with an increase in both the number of freezing-thawing cycles, while water-saturated experiments indicated substantial change after liquid nitrogen treatment compared to dried ones.