The application of concrete greatly improves the quality of construction projects, and promotes the development of ready-mixed concrete industry, but ready-mixed concrete due to untimely maintenance of concrete structure caused by early cracking is difficult to eradicate, has become a hot spot of the quality of construction project complaints, but also the focus of quality supervision. Concrete maintenance concrete casting molding should be timely watering cover moisture conservation to meet the concrete surface to maintain a certain wet state. At the same time, in order to prevent rapid and rapid evaporation of water, the surface should also be covered with plastic film, sacks or grass bags and other covering materials, but only to do these points is not enough.
Concrete maintenance, one of the purposes is to ensure that the concrete in a certain period of time moisturizing, resulting in concrete cracks is complex, multi-faceted, but, the lack of experience in the construction of ready-mixed concrete and maintenance of untimely is one of the main causes of cracks in commercial concrete. After pouring concrete, the reason why it can gradually set and harden, mainly because of the result of cement hydration, and hydration requires appropriate temperature and humidity conditions, so in order to ensure that the concrete has suitable hardening conditions, so that its strength continues to grow, must be maintained on the concrete. Maintenance of the essence – watering maintenance, maintain appropriate humidity, to ensure that the hydration reaction of cement. The secret of curing – time control.
First, the choice of concrete curing program and the development of curing time determination
(1) Determination of the starting time of curing
Acceptance specification for the construction quality of concrete structures should be within 12h after the completion of pouring, the concrete should be covered and moisturized curing, most of the current construction is also in accordance with the provisions of this, that is, about 12h after the pouring of the beginning of moisturizing curing.
Concrete shrinkage from its initial set before the beginning of the early development of rapid, 1d can be completed within most, especially mixed with water-reducing agent concrete starting curing time on the early contraction of the impact is very significant, after the initial set of 8h, is the contraction of the sharp increase in the period, even if the initial set of 8h after the beginning of the curing, on the control of the early contraction cracks will also lose any effect, the time has obviously lagged behind the cracking of concrete The time has obviously lagged behind the dangerous period of concrete cracking, the starting time of maintenance from when in the end, due to the influence of many factors and the lack of sufficient experimental data and theoretical basis, it is difficult to have a specific conclusion. However, in principle, the concrete should be made to reach sufficient initial strength to withstand the operation of curing and moisturizing without damaging the early structure of the concrete.
Specifically, that is, after the concrete is poured, before and after the initial set should be watered to maintain, but not to the extent that the surface of the concrete to wash out artificial.
(2) Determination of the duration of maintenance
Curing duration is not enough is a common problem in the construction, often in the first few days of pouring someone to maintain, and finally on the form, to cope with the matter, or even no one maintenance. Concrete structure engineering construction specification stipulates that the concrete prepared with silicate cement, ordinary silicate cement or slag silicate cement should not be less than 7d, and the curing time of the concrete prepared with retarder-type additives, large admixture of mineral admixtures, and impermeable concrete, concrete with strength class C60 and above, and concrete with backing zone should not be less than 14d.
The time specified in the specification is the minimum duration of curing. The duration of curing is specifically related to the composition and proportion of concrete, ambient temperature and humidity, curing methods, wind, wind speed and many other factors, the concrete quality control standards provide that when steam curing is used for concrete components or products plant, the temperature measurement should be carried out before leaving the pool or removing the curing measures, and the curing measures can be withdrawn or components can be removed from the pool only when the difference in temperature between the surface and the outside world is not more than 20 ℃; for the Winter construction of concrete, concrete strength to reach 50% of the design strength level, before removing the conservation measures; the use of wet natural conservation, according to the construction specifications for specific implementation.
Second, the choice of maintenance methods
Concrete curing program should be based on site conditions, environmental temperature and humidity, structural parts, components or products, raw materials and concrete performance requirements and other factors to develop a curing program, on the selection of curing methods are generally used in sprinkler curing, water curing, covering the moisturizing curing, painting curing agent curing, winter heat storage curing and other methods. In construction, when developing the conservation program, it often appears that in the whole construction process, only a single method of conservation is used, and for convenience, most of them are based on water spraying curing. In order to ensure the realization of concrete performance, should be based on different structural parts of a reasonable choice of curing methods.
(1) The exposed surface of the foundation mass concrete should be covered with curing methods. The exposed surface of the foundation mass concrete is more convenient to use sprinkler curing method, but it is difficult to meet the number of sprinkler curing in the actual construction, which is easy to cause the interruption of nighttime curing, for mass concrete, to control the internal and surface of the concrete and the surface of the concrete and the temperature difference with the outside world that is, to keep the concrete inside and outside of the appropriate temperature gradient, the uninterrupted 24h curing is crucial. According to past construction experience, the use of forced or uneven cooling cooling measures in the process of mass concrete curing is not only relatively high cost, poor management is easy to produce penetrating cracks in mass concrete, and sprinkling curing is easy to lead to waterlogging of the pit, which is not conducive to backfilling. Therefore, it is appropriate to cover the exposed surface of the foundation mass concrete.
(2) concrete structure engineering construction specification: with mold curing can be used after the end of sprinkling curing to continue curing, if necessary, can also be used to cover the curing or spray curing agent curing to continue curing; according to the test, different curing methods of seepage resistance in the following order: water curing > cover the grass bag watering > coating curing agent > natural curing.
Third, the determination of maintenance temperature
During concrete curing, insulation measures should be taken to prevent the surface temperature of the concrete from undergoing drastic changes due to environmental factors (such as exposure to the sun, sudden temperature drops, etc.). The temperature difference between the core and surface of the concrete and between the surface and the environment during the curing period should not exceed 20℃. A strict curing plan should be formulated before the construction of large-volume concrete to control the temperature difference between the inside and outside of the concrete to meet the design requirements.
After the concrete is demolded in winter and the hot season, if there is a sudden change in the weather, appropriate insulation (cold season) and heat insulation (summer) measures should be taken to prevent excessive temperature stress in the concrete.
