Seven Common Misunderstandings Of Concrete Watering And Curing

2020-11-02

Whether it is construction engineering, bridge engineering or road engineering, as long as cement concrete is useful, it will involve the watering and maintenance of concrete. But such a seemingly common occurrence often contains details that are overlooked and misunderstood. Today, New Horizons analyzed the mechanism of watering and curing, and pointed out seven common mistakes in concrete watering and curing as a reminder.

Misunderstanding 1: watering and curing are only for the need of cement hydration

After the concrete is poured and formed, it must be covered and watered to meet the requirements of keeping the concrete surface wet for a certain period of time. At the same time, in order to prevent the rapid evaporation of the curing water, plastic film, sack pieces or straw bags and other materials are also used to cover. However, the curing of concrete is more than just watering, its functions can be summarized in two main points:

One is to keep the concrete in a sufficiently wet state for a certain period of time to meet the needs of cement hydration; the other is to ensure that the concrete can maintain a suitable maximum temperature, a suitable temperature difference between inside and outside under different ambient temperature conditions. It has a suitable temperature difference between the surface and the ambient atmosphere, as well as an appropriate cooling rate and heating rate.

Misunderstanding 2: The latest start time of watering and curing is 12h after pouring.

The first paragraph of Article 7.4.7 of the "Specification for Quality Acceptance of Concrete Structure Engineering" (GB50204-2002) stipulates that the concrete should be covered and moisturized within 12 hours after pouring. However, many construction workers misunderstood that the latest start time for watering and curing after concrete pouring is 12 hours thereafter. That is to say, as long as the concrete is poured and cured before 12h, it will meet the requirements of the specification. On the construction site, we often encounter technicians urging maintenance and watering, but some people will say: "It is only a few hours after the concrete has been poured, and it is still far from 12 hours! Don't worry!"

Many years ago, plastic concrete with high fluidity was commonly used on construction sites. Its pouring volume is not large, the concrete strength grade and cement strength grade are low, the amount of cement is small, the early hydration degree is not high, the dry shrinkage, and no autogenous shrinkage. In this case, watering and curing within 12h after pouring may be appropriate. However, with the continuous development of cement concrete technology, materials such as high-performance concrete, early-strength concrete, high-strength concrete and ready-mixed concrete have been widely used in recent years. The concrete strength grade and cement strength grade used are higher, the amount of cement is larger, the early strength is high, and the water-cement ratio is small. These reasons cause the temperature deformation, dry shrinkage deformation and auto-shrinkage deformation of concrete to be large, and concrete cracking occurs from time to time, and the late watering and curing time of concrete will further aggravate the early cracking of concrete.

Misunderstanding 3: The longer the concrete watering and curing time, the better

The second paragraph of Article 7.4.7 of "Specifications for Quality Acceptance of Concrete Structure Engineering" (GB50204-2002) stipulates that for concrete mixed with Portland cement, ordinary Portland cement or slag Portland cement, the watering and curing time It shall not be less than 7 days. For concrete mixed with retarding admixture or impermeability requirements, it shall not be less than 14 days.

It should be pointed out here that the specification only specifies the minimum time for watering and curing, but does not give the best duration and maximum time for watering and curing. The longer the watering and curing time, the higher the degree of cement hydration and the greater the irreversible shrinkage of the cement. If the cement particles are fully hydrated, the cement gel produced will not only increase the strength of the concrete, but also produce a lot The shrinkage can cause concrete cracking in severe cases.

Just like aggregates in concrete play a role in stabilizing volume, a certain amount of unhydrated cement particles or other inert materials are required to stabilize the volume in cement stone. Therefore, the watering and curing time is not as long as possible. It is obviously wrong to blindly extend the watering and maintenance time as a "strengthening maintenance" approach. The advancement and development of modern cement and concrete technology requires watering and maintenance at the right time. The length of the "just in time" curing time is related to many factors such as the selection of constituent materials, concrete mix ratio, ambient temperature and humidity, and wind speed. The lower the concrete water-binder ratio, the more it is necessary to strengthen the supply of external supplementary water in time, but the watering and curing time can be appropriately shorter; when the water-binder ratio is large, there is more free water in the concrete. If the concrete is in an area with high relative humidity, The effect of wet curing is not large, but the curing time is relatively longer to make the permeability stable.

Misunderstanding 4: The concrete has only finally set, and the surface is still wet, so do not worry about watering and curing

Many people mistakenly believe that the watering and curing of concrete can be started at any time within 12 hours after the concrete is poured. That is to say, the watering and curing can be done sooner or later within the 12h time frame. This kind of understanding and practice is obviously wrong. If the early strength of concrete is regarded as the internal cause of its early cracking, then its watering and curing lags behind the external water replenishment and interruption of water replenishment after the rapid evaporation of surface water, which is the external cause of the early cracking of concrete. Therefore, it is necessary to greatly advance the time when the concrete starts to be watered and cured, so that the evaporated water on the surface of the concrete can be recharged in time, so as to achieve "early and timely" watering and curing.

Specifically, after the concrete has been poured, and at the beginning of its initial setting, watering and curing is limited to prevent the surface of the concrete from being artificially washed out, and watering and curing "as soon as possible". The word "as soon as possible" is particularly emphasized here to ensure that the concrete has sufficient water replenishment conditions in the early stage to avoid the combined effects of plastic shrinkage, auto-shrinkage and dry shrinkage of the concrete.

Misunderstanding No. 5: The best way to water and maintain concrete should be heavy water, so that the water can be fully and thoroughly

In order to save covering materials, some people do not cover the concrete and use high-pressure water to pour it violently. This not only wastes water, but also easily damages the concrete surface. More importantly, the pressure water flows across the surface of the concrete and quickly removes its heat, causing the temperature of the concrete surface to drop sharply.

If the concrete hydration heat peak is encountered, and the temperature difference between the curing water and the concrete surface is large, it is likely that the temperature of the concrete drops sharply, causing the temperature difference between the concrete surface and the environment to be too large, resulting in "thermal shock" and causing the concrete surface to crack. At the same time, remember that curing and watering cannot be intermittent, and repeated "thermal shocks" may aggravate concrete cracking. Suitable watering and maintenance should be small flooding.

Misunderstanding No. 6: In order to accelerate the hardening of concrete, only heat preservation during the curing stage without cooling down

The initial pouring temperature of concrete is an important part of the maximum temperature of concrete. Cooling down the temperature of concrete in a plastic state will reduce the maximum temperature while also reducing the cracking temperature of the concrete. Therefore, cooling the concrete in the plastic state is one of the effective methods to prevent the concrete from cracking.

But it should be noted that it is not advisable to pour cooling water that is too much below the ambient temperature. As we mentioned earlier, a sudden drop in temperature may cause concrete "thermal shock" and cause concrete cracking. Therefore, the cooling treatment and watering maintenance at this stage must be careful. Before tensile stress is generated inside the concrete, it should be cooled in time.

Misunderstanding No. 7: Thermal insulation coverage starts when watering and covering

Before the concrete reaches the maximum temperature of hydration, it should be in the heat dissipation stage in order to obtain a lower cracking temperature. If the heat preservation is advanced to the time of watering and covering, it will increase the maximum temperature and cracking temperature of concrete. Therefore, the correct heat preservation time should start from the concrete cooling, not in advance.

One of the purposes of thermal insulation during the cooling stage of concrete is to reduce the internal heat loss of the concrete to reduce the temperature gradient of the section. The second purpose is to delay the heat dissipation time of concrete, so that it can give full play to the potential of strength growth, and make the relaxation of concrete fully manifest, and finally the internal tensile stress of concrete can be reduced accordingly. At the same time, due to the growth of concrete age, the tensile performance of concrete improves faster than its compressive performance, which can also reduce concrete cracking.

Avoiding the above seven misunderstandings of concrete watering and curing, you would have a good quality concrete floors. Contractors need suitable diamond tools to grind and polishing concrete to get smooth floor surface. DTS diamonds (DTS diamond tools) manufacturer produces and supplies premium quality diamond tooling for current grinders, such as HTC, Husqvarna, Werkmaster, Scanmaskin, STI Prep/Master, XPS, Klindex, Terrco, Blastrac, Diamatic, Stonkor grinding machines etc, you will find a wide range of floor grinding tools, hybrid pads, floor polishing pads, PCD tools etc for floor restoration business.

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