We are always embarrassed at the construction site, but we still love it because we have to support our families. Therefore, we must always pay attention to the maintenance of concrete in order to ensure the quality of the project. Concrete can gradually set and harden after being cast, mainly due to the hydration of cement, which requires suitable temperature and humidity conditions. Therefore, in order to ensure that concrete has suitable hardening conditions and its strength continues to grow, concrete must be cured.
A, concrete curing matters needing attention
(1) After concrete pouring, 12h shall be covered with water for curing. Hard concrete should be cured immediately after pouring. The concrete surface should be covered with materials with strong water absorption capacity. Such as sacks and mats.
(2) The duration of sprinkler maintenance is related to cement varieties. Concrete mixed with ordinary portland cement and slag portland cement shall not be less than 7d, and concrete mixed with pozzolanic portland cement and fly ash portland cement shall not be less than14d; ; Not less than 14d concrete mixed with retarding admixture or with impermeability requirements.
(3) The number of curing and watering should be able to keep the concrete surface in a wet state. At the initial stage of hydration, the chemical reaction of cement is sufficient, which can increase the watering times. When the temperature is around 15℃, water it 2 ~ 4 times a day, and increase the watering times when the temperature rises. The water used should be consistent with the concrete mixing water, and it is not suitable for water conservation when the average temperature is lower than 5℃.
When plastic film is used for curing, all exposed parts of concrete are covered tightly with plastic sheets to ensure that there is condensed water in the plastic sheets, so as to achieve the purpose of wet curing. Spraying plastic film curing solution can be used for the maintenance of water-deficient areas and high-rise buildings that are not easy to water. That is, the curing liquid is sprayed on the concrete surface with a spray gun, and a plastic film is formed on the concrete surface after the solution volatilizes, so that the concrete is isolated from the air and the water in it is prevented from evaporating, thus ensuring normal hydration.
Second, the concrete curing in different environments
1, do a good job in general environmental maintenance.
In general, the temperature of concrete is 10℃-20℃, which is conducive to the maintenance of humidity and is the best construction stage of bridge structure. At this temperature, it is easy to keep warm and moisturize in maintenance work. Usually, we use two methods to realize it: one is to cover the molding structure that needs to be maintained with different materials. If the concrete surface requires high flatness, we should use special materials, and we can also sprinkle water on the formed structure to keep it moderate. Such as columns, collision walls, etc. , should be covered with geotextile with good water absorption. Specific protection should also have a certain system to arrange specific personnel to implement it on time. Ensure the humidity and temperature of concrete before demolition. In addition, pay attention to the method of sprinkling water. Can not be directly washed to form water flow, to prevent concrete from falling off. The correct way is to spray continuously with a sprayer, so that water can not produce water pressure on concrete. Secondly, curing agents can be used for those molded structures that are difficult to cover.
2. Concrete curing in cold season
The optimum molding temperature of concrete is 10℃-20℃. If the fresh concrete is below 5℃, the concrete will freeze, so that its hydration will stop, and the concrete surface will be brittle and lose its strength. In severe cases, cracks will even appear, and the temperature will rise, and its deterioration cannot be recovered. Therefore, curing concrete in a cold environment, we must do:
(1) heat storage method. Thermal storage method is a construction method that uses the preheating heat of raw materials and hydration heat of cement to tightly cover the concrete with thermal insulation materials to delay the cooling rate of concrete and gradually harden during the cooling process. It is a construction method for concrete to reach the critical strength of freezing at room temperature.
(2) Admixture method. When mixing concrete, this method is to add an appropriate amount of admixture to make the strength of concrete increase rapidly, reach the critical strength required before freezing, or lower the freezing point, so as to make concrete set and harden at negative temperature. (3) electrothermal method. Electric heating is a method of heating and curing concrete by converting electric energy into heat energy, which belongs to dry heat and high temperature curing. (4) Greenhouse method. The greenhouse method is to build a greenhouse around the structure or component to be maintained with thermal insulation materials, and heat it in the form of raw stove, hot air blower and steam pipe.
3. Protection in high temperature and dry environment
Water is easy to volatilize under dry and high temperature conditions. If the concrete loses too much water, the strength of the concrete on its surface is easy to decrease, and then drying shrinkage cracks are easy to appear, which are mainly plastic cracks caused by premature condensation of concrete. Especially during concrete construction in summer, if the curing methods and measures are not appropriate, premature condensation, plastic cracks, reduced concrete strength and durability will often occur, which will not only affect the construction progress, but also affect the overall quality of the formed structure. Therefore, it is very important to maintain and construct in time under dry and high temperature conditions. The specific methods are as follows: first, the construction can be carried out at night, avoiding direct sunlight, and adjusting the specific proportion according to the time. During the day, we need to cool the concrete formwork. If the formwork is too high, the water loss will be serious and the concrete will be sanded.
Third, the misunderstanding of concrete curing
1. Retention position
In Appendix D of the specification, the structural entity shall be inspected according to the strength of the specimen cured under the same conditions. D.0. 1 The retention method and sampling quantity of the specimen cured under the same conditions shall meet the following first requirement: "The structural members or structural parts corresponding to the specimen cured under the same conditions shall be selected by the supervision (construction), construction and other parties." The purpose of curing specimens under the same conditions is to verify the concrete strength of structural entity inspection, so the basis for its selection should be based on the scope of structural entity inspection. In the concrete structure sub-division project 10. 1. 1, it is clearly stipulated that "the important parts involving the safety of concrete structures should be inspected by structural entities. The structural entity inspection shall be organized and implemented by the technical director of the construction project under the witness of the supervision engineer (the professional technical director of the construction unit), and the laboratory undertaking the structural entity inspection shall have corresponding qualifications. " The scope of structural entity inspection is limited to the important parts of structural members related to structural safety, such as columns, walls and beams. Therefore, non-important parts of structural members, such as concrete cushion, do not need to retain specimens under the same conditions. ?
2. The number of liens
According to the relevant provisions of the Standard for Inspection and Evaluation of Concrete Strength (GBJ 107), the number of retained specimens with the same strength grade and conditions is not less than 10 group, which constitutes the basic condition for evaluating concrete strength by statistical method; The number of retained groups should not be less than 3 groups, so as to ensure that there is enough representativeness when evaluating concrete strength by non-statistical methods. The lien shall be carried out correctly according to the quantity and importance of concrete and the requirements of Standard for Inspection and Evaluation of Concrete Strength (GBJ 107). ?
3. Treatment of unqualified specimens cured under the same conditions
When 1 group or two groups of samples with the same conditions are lower than the design value, it is obviously wrong to give a simple treatment method that requires qualified testing units to carry out entity testing. The codeNo. 10. 1.4 stipulates that "when the strength test results of specimens cured under the same conditions meet the relevant provisions of the current national standard" Standard for Inspection and Evaluation of Concrete Strength "(GBJ 107), the concrete strength shall be judged to be qualified". Therefore, the representative value of the specimen must be kept under the same conditions, and it should be multiplied by the conversion coefficient in addition to meeting the standard of GBJ 107. The conventional conversion coefficient is 1. 10, which can also be adjusted appropriately according to the local test statistics. ?
The above are collected and sorted by Zhong Da Consulting Company.
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