Principles of the health-promoting effects of steam

Is the poster from Shijiazhuang? The reason is as follows: In concrete, water and cement undergo a chemical reaction known as hydration. Whether this hydration process takes place properly has a significant impact on the quality and durability of concrete. Therefore, finding ways to retain moisture so that hydration can occur fully, or using other methods to shorten the time required for concrete to hydrate, is the key to producing high-performance concrete. Generally, for concrete using ordinary Portland cement, the curing time should be seven days; some standards even require at least fourteen days. Such a long period is necessary to maintain the humidity and appropriate temperature of the concrete over an extended period of time ； However, given the current trend in the growth curve of concrete strength – with a rapid increase in the early stages and only a slight increase later on – this approach is of little value and not economical ； Therefore, under appropriate engineering conditions, it is necessary to select the suitable curing method from moist curing, membrane curing, and steam curing in order to maximize the quality and durability of the concrete.

Steam curing is a method that accelerates the hydration reaction of concrete, allowing it to reach the desired compressive strength in a short period of time and thus significantly reducing the construction timeline. This paper will explore the comparison between on-site steam curing and laboratory steam curing methods, and discuss the associated strength development. According to the recommendations of ACI Committee 517 regarding steam curing of prestressed and precast concrete, there are many variables that interact with one another, resulting in variations in the quality and properties of the concrete after curing. Moreover, the factors that contribute to a significant increase in early strength through steam curing often conflict with those that enhance late strength; therefore, the optimal curing process can only be determined through experimentation. Under appropriate curing procedures, as long as the wet chamber can maintain a relative humidity of over 60%, the theoretical 28-day strength can be achieved within a 24-hour steaming curing period. Moreover, the longer the pre-curing time, the greater the strength loss after 28 days, which is typically around 5% to 15%; however, if curing begins without proper pre-curing directly in a high-temperature steam environment, the strength loss after 28 days can be as high as 50%. In general laboratories, steam curing is carried out in a sealed humid chamber; therefore, the actual process during steam curing tends to follow a pattern that is closer to the temperature set by the computer program, resulting in smaller errors. The higher the curing temperature, the longer the required pre-curing time; this results in higher early strength, but the 28-day strength will be relatively lower ； The cooling rate is generally considered to have little impact on the strength of concrete; however, in cases where the components are large in size, it may cause surface cracks. Therefore, as a precaution, it is still recommended to cool at a slower rate to ensure the quality and durability of the concrete. 　 In contrast, the local steam curing method offers significant cost advantages for the overall project, which is why it is also widely permitted to be used in Taiwan’s high-speed rail projects ； On-site steam curing relies mainly on boilers to supply the necessary steam; appropriate steam piping systems and curing covers are used to maintain and insulate the concrete. The curing process is similar to that in laboratories, but the main difference is that: (1) there is a lack of proper temperature control equipment on site, and temperature is controlled indirectly by regulating the steam output, which makes it difficult to maintain precise control ； (2) Since it is not possible to access the confined space on site, the temperature during the constant-temperature period is generally only in the range of 50–60℃ ； (3) During the cooling period, actual on-site operation involves shutting down the boiler to allow natural cooling, making it difficult to control the cooling rate. When the water-cement ratio is low, steam curing makes it easier to achieve the strength specified in the TRANBBS design. However, when concrete mixtures are designed with a higher water-cement ratio on site, the effectiveness of steam curing is limited. This paper will compare the data obtained in the laboratory with the results obtained on site, taking into account different water-cement ratios and curing temperatures. Under the steam curing process carried out on-site, where the preheating time is controlled at 3 hours, the heating time at 2 hours, the constant-temperature period at 12 hours, and the cooling time at 2 hours, it is possible to clearly observe that the strength curve obtained is similar to that obtained under strictly controlled laboratory conditions; however, a slightly lower strength level is observed. This is due to the greater variations in temperature and humidity during on-site testing, which is a quite reasonable outcome ； As found through this experiment, the strength values obtained from field tests are approximately 90% of those obtained in laboratory tests. In other words, when designing mix proportions and it is not possible to use field test results as a basis due to differences in time and location, laboratory test data can be used instead, with an additional reasonable safety factor of 15% applied to determine the field mix proportions.

Thanks to the person above! I’ve learned it.* :loveliness: :loveliness: :loveliness: :victory: :victory: :victory: