Abstract
The construction industry, one of the largest contributors to worldwide infra-structural development and economic growth, plays an important role in the betterment of people’s lives through the provision of new housing, roads, bridges, and other structures. Unfortunately, it consumes billions of tons of raw material from the earth’s surface and results in the enormous production of construction and demolition (C&D) waste every year due to various construction, restoration, and demolition activities. To circumvent these issues, the industry should increase the usage of recycled concrete aggregate (RCA) in place of natural aggregates as it can help improve concrete’s sustainability, utilize C&D waste, and minimize the adverse effects on the environment. However, the use of RCA as replacement of natural aggregates results in a weak microstructure, lower mechanical performance, and weak durability of concrete. For this reason, this study aims to enhance the microstructure quality and mechanical performance of sustainable fiber-reinforced recycled aggregate concrete (FRAC) and promote its structural applications via a novel eco-friendly and effective strength-enhancing technique.In this research, a new strength enhancing technique was developed to augment the microstructure and mechanical properties of FRAC, utilizing a new vacuum-based pozzolana slurry surface treatment technique for RCA in conjunction with basalt fiber (BF). The effectiveness of this new technique was assessed for eleven different mix formulations of FRAC produced with 20% and 100% of RCA with regards to physical properties, microstructure, mechanical properties, and fracture parameters. The experimental results revealed that the coupling effect of pozzolana slurry-treated recycled concrete aggregate (TRCA) and BF reinforcing capabilities provided a marked improvement in the mechanical and fracture properties of FRAC. The vacuum-based pozzolana slurry surface treatment of TRCA resulted in the formation of additional calcium silicate hydrate (CSH) gel and constituted better interfacial transition zones (ITZs), which effectively counteracted the microstructure flaws in FRAC. Furthermore, owing to its remarkable crack bridging and stress transformation abilities, BF strengthened the microstructure of the recycled concrete, resulting in superior crack resistance and tensile strength ability, which contributed to the improved mechanical performance of FRAC. Among other notable results, FRAC containing a 1% volume fraction of BFs and 20% of TRCAs demonstrated the best overall performance.
The next stage of the research evaluated the effects of elevated temperatures (i.e., 25°C–800°C) on residual mechanical properties, physical properties, and microstructure performances of FRAC. To mimic real building fire conditions, concrete test specimens were heated to a specific target temperature under controlled heating and subsequently assessed for residual properties via a series of laboratory experiments. In addition, simplified numerical relationships based on the experimental data were established to help predict the post-fire performance of FRAC. Overall, the test results in the case of FRAC indicated the significant enhancement of residual properties and a decrease in mass loss and surface degradation, especially beyond 400°C, which was attributed to the excellent thermal stability and reinforcing effect of BF.
The scope of this thesis also encompasses the detailed experimental investigation of a novel fiber hybridization approach in which BF and disposable medical face mask (DMFM) fiber were incorporated into FRAC together with pozzolanic materials (fly ash and ground granulated blast furnace slag). The experimental results indicated a marked improvement in the overall performance of FRAC, which was attributed to the multi-scale crack bridging abilities of the hybrid fibers and an improved microstructure performance due to the inclusion of pozzolanic materials. Moreover, the concrete density and ultra-sonic pulse velocity (UPV) of FRAC indicated excellent concrete quality and fulfilled the structural concrete requirements. The water absorption rate gradually increased with an increase in the volume fractions of the hybrid fibers, yet it remained within the allowable water absorption limit for construction materials.
The last part of this thesis comprises a comprehensive multicriteria-based performance analysis of single-fiber and hybrid fiber-modified FRAC. For this purpose, a total of eleven different mix formulations were prepared using single and hybrid fibers including BF, polypropylene fiber (PPF) and glass fiber (GF). In addition to fibers, pulverized fly ash and ground granulated blast furnace slag were added to counteract FRAC’s microstructural flaws. From the mechanical properties results, the study determined that the mix formulations comprising hybrid fibers had the highest strength improvements. Moreover, a freeze–thaw assessment revealed that the use of hybrid fibers together with the two mineral admixtures was highly effective in improving FRAC durability in terms of lower length variation (0.055%), lower mass loss (0.39%), and higher UPV (4137 m/s) after 300 cycles. From the cost analysis, it was observed that single fiber-reinforced FRAC (MRC-1BF) had the highest net cost increase of all mixes at 121.7%. Lastly, the multicriteria analysis revealed that hybrid BF–GF and BF–PPF-based FRAC had the best performance scores and rankings of all the studied formulations, implying that their practical use would be highly beneficial in terms of mechanical performance as well as cost-effectiveness.
| Date of Award | 8 Aug 2023 |
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| Original language | English |
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| Supervisor | C W LIM (Supervisor) |