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Addressing the scarcity of research on the residual expansion deformation of steam-cured concrete and its underlying factors, this study introduces a testing apparatus to characterize the internal stress in fresh concrete during the thermal treatment. It explores the impact of water-to-cement ratio (w/c) and paste-to-aggregate ratio (p/a) on internal stress and elucidates the developmental patterns of internal stress in concrete throughout the thermal treatment phase. The key findings are as follows: (i) During the thermal treatment, the internal stress in concrete under constraint is predominantly compressive, exhibiting a rapid increase in the heating stage, followed by a gradual decline in the constant temperature treatment stage, and a continued decrease with a decelerating trend during the cooling stage. (ii) Both w/c and p/c significantly influence internal stress in concrete, and this stress can be enhanced by reducing the initial free water content and augmenting the initial structural strength of the concrete. This paper serves as a valuable foundation and reference for further investigations into the residual expansion deformation of steam-cured concrete.

In this study, environments of 60 % and 33 % relative humidity have been chosen to investigate the effect of internal curing of pre-absorbed superabsorbent polymers (SAPs) on the resistance of cementitious materials to the harsh environment of external drying, which will be analyzed experimentally in terms of mechanical properties, volumetric stability, degree of hydration and pore structure. The results showed that 2‰ SAP improved the hydration of cementitious materials, enhanced the ability of the cement paste to resist low humidity environments, effectively increased the compressive strength and reduced the drying shrinkage deformation of the cement mortar. The relevant tests including FTIR, XRD and TG also prove this from multiple angles. It was also discovered that a moderate amount of pre-absorbed SAP was able to reduce the drying shrinkage of the cement mortar. However, the pore structure analysis revealed that the pre-absorbed SAP increased the cement paste porosity, leading to accelerated dissipation of pore water in the cement paste. In the end, the critical pore size (Dc) micromechanical model showed that the shrinkage reduction mechanism of the pre-absorbed SAP in low humidity environment is that the maximum pore size of the SAP group forming the meniscus during the drying process generates less shrinkage stress.

Along with the construction industrialization degree, precast concrete is being utilized more and more frequently in construction to speed up the process and guarantee the quality of structural concrete. In order to make the precast concrete reach the required strength quickly and speed up the turnover of formwork, various rapid concrete curing methods emerged. Currently steam curing, direct electric curing, and microwave curing are the widely studied rapid curing methods. Rapid curing methods could promote early strength growth by changing the internal and external temperature and humidity of concrete, but may lead to late strength shrinkage, increased internal micro-cracks, coarsening of pore structure and other problems. Based on the comprehensive analysis of the curing principles of the three rapid curing methods, this paper reviews the effects of the three rapid curing methods on the hydration process, properties, and microscopic pore structure of concrete. The shortcomings of different rapid curing methods, and the topics of future researchs are also included.

In the tunnels of the Cheng-Kun Railway in China, concrete failure can usually be observed from the effect of sulfates. A field investigation was conducted to analyze the concrete damage mechanism by means of XRD, IC, AAS and EPMA. The results showed that (1) large amounts of Na2SO4 were generated on the surface layer of the concrete lining, causing concrete detachment layer by layer; (2) the concrete lining in contact with air had been neutralized to more than 50 mm in depth, and the cement hydration products had been converted into CaCO3; (3) ettringite and gypsum, the products of chemical sulfate attack, were also detected in the neutralized concrete lining. This analysis of a field case concluded that Na2SO4 crystallization caused the detachment of the surface layer of the concrete lining, and the chemical sulfate attack occurred in the inner part of the concrete lining.

Deterioration mechanisms of some premature damaged steam-cured concrete track slabs (CTS) in Chinese railway less than 4 years were investigated. Field investigation, raw materials test and suspicious products analysis were carried out. Results show that steam-cured heat damage (SCHD) of concrete takes place in steam-cured process. Expansion products are ettringite in hydrated products and alkali-silica gels between the interface of hydrated products and coarse aggregate. SCHD makes CTS surface layer loose, porous and more micro-cracks. Long-term fatigue load from high-speed train acting on CTS enlarges concrete microcracks, leading to water penetrating into concrete easily in moist and rainy environment. In the process of water ingression, alkali-silica reaction (ASR) and delayed ettringite formation (DEF) take place, hence resulting in CTS cracking and premature deterioration.

To explore the feasibility of using metakaolin and limestone in steam-cured concrete, mechanical property, sorptivity and microstructure of steam-cured concrete incorporated with the combination of metakaolin-limestone powder were studied, with steam-cured concrete incorporated with ground granulated blast furnace slag and fly ash as a reference. Due to the high reactivity of metakaolin, the presence of metakaolin reduces the heterogeneous distribution of hydration products, thereby resulting in a refined microstructure and lowered sorptivity of steam-cured concrete. The synergistic effect of metakaolin with limestone powder leads to further improvements on strength, sorptivity associated with a more uniform microstructure at late ages. Moreover, concrete incorporated with metakaolin-limestone powder and steam-cured at 40 °C can gain a comparable early strength (1 day) with concrete incorporated with the combination of ground granulated blast furnace slag - fly ash, and steam-cured at 60 °C; this indicates that the incorporation of metakaolin enables to reduce steam curing temperature without compromising early strength gain, which is of significance for mitigating steam-curing induced detrimental effects and reducing energy consumption. Keywords: Metakaolin, Limestone, Steam curing, Compressive strength, Sorptivity, Microstructure