After the concrete is poured and tamped, it can gradually set and harden mainly because of the cement hydration, which requires appropriate temperature and humidity conditions. Therefore, in order to ensure that the concrete has appropriate hardening conditions and its strength continues to grow, the concrete must be cured.

For the poured concrete, the natural curing shall be started before the final setting of the concrete (usually within 8~12h after the completion of concrete pouring).

However, the Technical Code for Waterproofing of Underground Works also stipulates that the waterproof concrete shall be cured immediately after final setting, and the curing time shall not be less than 14 days.

Ordinary concrete is cured before final setting, while waterproof concrete is cured after final setting.

Why is this?

Because a layer of hydrophobic film will be formed after the hydration reaction of waterproof concrete. It is this hydrophobic film that plays the role of waterproof and water blocking. Therefore, it is necessary to spray water for curing after the hydration reaction of waterproof concrete is complete, so as not to damage the hydrophobic film and ensure that the waterproof concrete plays a waterproof role.

One thing must be noted for waterproof concrete: it shall be cured immediately after final setting. Because if the curing is not timely, the water inside the concrete will evaporate rapidly, making the cement hydration incomplete. The evaporation of water will cause the capillary networks to connect with each other, forming a seepage channel. At the same time, the concrete shrinkage increases, cracks appear, and the impermeability drops sharply, or even completely loses the impermeability.

Summary of concrete curing time

1 Ordinary concrete

(1) For the poured concrete, the natural curing shall be started before the final setting of the concrete (usually within 8~12h after the completion of concrete pouring).

(2) The standard curing age of concrete specimen is 28 days, and its compressive strength is tested.

(3) After concrete pouring: ① Cover and maintain moisture within 12h. ② The time for watering and curing shall not be less than 7d for concrete mixed with Portland cement, ordinary silica cement and slag cement, and 14d for concrete mixed with retarding admixture or with impermeability requirements.

(4) After the construction of the overall surface course, the curing time shall not be less than 7d, and people can walk only after the compressive strength reaches 5MPa.

2 Waterproof concrete

Curing shall be carried out immediately after final setting, and the curing time shall not be less than 14 days.

3 Post cast strip

(1) The concrete shall be covered for moisturizing and curing within 12h after the completion of concrete pouring.

(2) The post cast strip shall be kept for at least 28 days and kept for at least 15 days for wet curing.

(3) Waterproof construction of post cast strip: ① The age of concrete on both sides shall reach 42d; ② The curing time shall not be less than 28d.

4 Mass concrete

(1) For mass concrete, the duration of moisture curing shall not be less than 14d.

(2) Mass concrete: when ordinary Portland cement is used, the curing time shall not be less than 14d; When slag cement is used, the curing time of pozzolanic cement shall not be less than 21 days; The waterproof concrete shall be cured immediately after final setting, and the curing time shall not be less than 14 days.

5 Sprayed concrete

(1) The Sprayed concrete shall be cured 2h after final setting, and the curing time shall not be less than 14d.

(2) When the temperature is lower than 5 ℃, water spraying curing is not allowed.