By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 22 2024A recent article published in Materials investigated the restoration of strength and microstructure in high-performance concrete (HPC) through cyclic water and water-CO₂ treatments. This approach shows promise for rehabilitating thermally damaged concrete (TDC).
The XRD patterns and QXRD results of the samples exposed to 600 °C at various re-curing stages: (a,b) water re-curing; (c,d) cyclic re-curing. Image Credit: https://www.mdpi.com/1996-1944/17/14/3531
Background
Concrete is known to be inherently fire-resistant; however, its mechanical performance degrades on exposure to high temperatures. This is attributed to the decomposition of hydration products, pore structure coarsening, thermal cracking, and phase transformations at elevated temperatures.
Thus, efforts are made to develop the self-healing properties of concrete after fire damage. Various post-fire curing methods are employed to rehabilitate TDC. However, the efficacy of any such process depends on the choice of curing method, rehydration phases, and extent of thermal damage.
Water-CO2 cyclic re-curing method possesses significant potential in recovering thermally damaged HPC. However, the strength recovery efficiency at varying curing periods and the causal microstructural mechanisms are not completely understood. Thus, this study aimed to demonstrate the micro-mechanisms behind the strength development of thermally-damaged HPC using water and water-CO2 cyclic re-curing.
Methods
In this study, a mortar mixture for high-performance concrete (HPC) was prepared using Portland cement, silica fume (SF, 10 wt.% of cement), and sieved standard quartz sand. The water-to-binder ratio was set at 0.36, with a polycarboxylate superplasticizer added to achieve the desired consistency and workability. Polypropylene fibers were also incorporated to mitigate the risk of explosion at high temperatures.
The HPC samples were prepared with 20 liters of this mortar mix and molded into cubic specimens (50×50×50 mm³) in two layers. After initial curing for 24 hours, the samples were de-molded and immersed in saturated limewater at 20±3 °C for 89 days.
Following this curing period, the HPC samples were subjected to heating in a muffle furnace, reaching temperatures of 600 and 900 °C at a rate of 1 °C/minute and maintained at these temperatures for one hour to ensure uniform exposure. The samples were then allowed to cool naturally to room temperature.
For water curing, the heated samples were immersed in saturated limewater for 3, 6, 18, and 30 days. For water-CO₂ cyclic curing, the samples were submerged in saturated limewater for 3 days, followed by transfer to an air chamber for the next 3 days. This cyclic process was repeated for 3, 6, 18, and 30 days. The compressive strength of the HPC samples was evaluated after high-temperature exposure and curing.
The phase composition of the samples was analyzed before heating, after heating, and during curing using X-ray diffraction (XRD). Additionally, scanning electron microscopy (SEM) was used to observe microstructural changes throughout each process. Mercury intrusion porosimetry (MIP) was performed to record the pore structure distribution of the HPC samples.
Results and Discussion
The HPC samples’ compressive strength recovered significantly through both re-curing processes, with cyclic re-curing exhibiting higher strength recovery than water curing. This was supported by the XRD, SEM, and MIP analysis.
While the calcium-silicate-hydrate (C-S-H) gel in HPC decomposed completely after exposure to 600 °C, C3S mostly remained intact due to its limited reaction with SF. During the water re-curing process, Ca2+ and OH− ions diffused from saturated limewater into HPC samples through microcracks. Simultaneously, C3S ions leached from cement, promoting calcium-hydrate (CH) production in the microcracks and coarsened pores.
Thus, the C-S-H gel rapidly rehydrated during re-curing, filling the microcracks and coarsened pores. This resulted in enhanced mechanical characteristics of the HPC samples, which attained a compressive strength comparable to the undamaged HPC after a 3-day water curing period.
Alternatively, CO2 infiltrated the microcracks during cyclic curing and reacted with Ca2+ ions to precipitate CaCO3 within the microcracks. Consequently, the HPC samples exhibited a 10.1 % higher compressive strength after 6 days of cyclic curing than the water-cured samples. Moreover, the compressive strength recovery after 18 days of water and cyclic re-curing surpassed 95 % of the total recovery observed after 30 days.
Alternatively, the compressive strength of the HPC samples damaged at 900 °C exhibited a slow improvement after 30-day water re-curing, whereas cyclic re-curing accelerated the compressive strength recovery. Overall, 18 days were concluded as optimal for re-curing the HPC samples damaged at 600 or 900 °C.
Conclusion and Future Prospects
The researchers conducted a thorough analysis of compressive strength recovery in thermally damaged high-performance concrete (HPC) using water and water-CO₂ cyclic re-curing methods. They identified optimal re-curing regimes and durations for HPC damaged at 600 and 900 °C, providing insights into the underlying recovery mechanisms.
Both re-curing methods effectively improved the compressive strength of thermally damaged concrete (TDC) by filling microcracks and coarsened pores with hydration products. However, the cyclic re-curing method showed faster recovery rates, attributed to the formation of carbonation products within the microcracks and cement paste.
The researchers recommend increasing the carbonation depth during cyclic re-curing to further enhance strength recovery in thermally damaged HPC. They also suggest investigating the effects of temperature variations on strength recovery to optimize the practical application of these re-curing methods in engineering.
Journal Reference
Li, Y., Wang, H., & Lou, H. (2024). Strength Recovery of Thermally Damaged High-Performance Concrete during re-curing. Materials, 17(14), 3531. DOI: 10.3390/ma17143531, https://www.mdpi.com/1996-1944/17/14/3531
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