【Haichuan Chemical Construction Technology】Don’t you know the standard requirements for concrete curing time?

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Understand construction techniques to build chemical engineering projects well. 【HaiChuan Chemical Construction Techniques】 – continuously updated summary posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719396 -------------------------------------------------------------------------------- Let’s answer one question first: Is the curing time the same for C30 slabs and C60 columns? The curing time is not determined by mood or based on the schedule; rather, it depends on what type of concrete is used, what admixtures are added, and whether the concrete has waterproof properties. The regulations are clearly stated, but unfortunately too many people choose to turn a blind eye to them. 01 For ordinary concrete, 7 days is the minimum requirement; there should be no further reduction in this time. For ordinary concrete that does not contain any admixtures and for which no impermeability requirements apply, the curing time must be at least 7 days – this is a strict requirement stipulated in the \"Code for Acceptance of Construction Quality of Concrete Structures\" GB50204. What exactly is the concept of 7 days? At this point, the cement undergoes hydration, and its strength reaches around 70% of the designed value; the density of its surface is initially established, and the first peak in drying shrinkage has also passed. If less than 7 days go by, the hydration process will stop, and any resulting lack of strength cannot be compensated for. Do not use the project timeline as an excuse – the reduced strength resulting from those shortened days will still be recorded. 02: Retarder admixtures are used, and water resistance is required; curing is carried out for 14 days. Retarder admixtures slow down the rate of hydration, resulting in a slower increase in early strength. If curing is stopped prematurely, the hydration reaction comes to an immediate halt. The situation is even more serious for waterproof concrete. Its resistance to water penetration relies on a tight internal structure, and this tightness depends on continuous hydration to fill the capillary pores. If curing is stopped before those pores are filled, dehydration occurs and the pores become connected, allowing water to find its way in. That’s why the standards specify that curing must take place for at least 14 days – there is no room for negotiation. For large-volume, high-strength C60 and above grades, 21 days is required. The heat of hydration in large-volume concrete represents a significant challenge; once the peak internal temperature is reached, cooling-induced shrinkage stresses remain. The curing period serves to give time for these stresses to be gradually released, thereby preventing the formation of temperature cracks. For high-strength concrete of grade C60 and above, a larger amount of cement is used, and the hydration proceeds to greater depths; therefore, water supply is required for a longer period during the latter stage of strength development. 21 days of curing is the minimum requirement to ensure sufficient hydration. If curing is stopped prematurely, the curve showing strength growth will level off, and the C60 concrete produced may actually have the strength of C50 only. The three time periods specified in the standards are clear: 7 days for ordinary applications, 14 days for applications requiring delayed setting and water resistance, and 21 days for large-scale, high-strength applications. These numbers are not suggestions but rather minimum requirements. The curing period is not something that can be handled casually; it is determined by the physical laws governing the hydration and hardening process of concrete. If this period is not respected, not only might there be problems, but defects will definitely occur. In the project’s curing records, someone is required to sign for each day. Before the specified time has passed, the supports under the slab cannot be removed, nor can any load be applied to the top slab. The curing time should be managed as an important aspect of the project, rather than being treated as a trivial matter. The last thing to ask is whether it’s possible to reduce the breeding period by a few days; what should be asked instead is in which direction the cracks will appear during those reduced days.