It is stipulated that the winter construction conditions are satisfied when the average daily outdoor temperature is below 5℃ for 5 consecutive days. In this condition, concrete in the production and construction process if not take appropriate measures, it is very easy to swell and crack, strength decline and other quality problems. So what do you need to pay attention to in winter concrete production and construction? Is it necessary to prepare for winter construction in all winter construction projects?
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Problems of winter construction
Winter construction often rushes the deadline, and winter construction also has the characteristics of high input, low return and high risk. Most of the quality defects can only be found after the removal of the formwork, which is often two or three months later, making it extremely difficult to deal with, and also facing the risk of repair and removal. Many key details if you can pay attention to, the risk can be reduced to a minimum, the quality of maximum protection.
Several problems to pay attention to in the winter construction of concrete
Winter dry project is winter construction?
A key project, pier construction, coinciding with the cold weather into the winter, very serious winter preparation. The ratio was adjusted, increasing the amount of cement, adding early strength agent and antifreeze, while building a warming shed, 24-hour heating to protect the temperature at 15 ~ 25 ℃, can be said to consider all the consideration. As a result, when the mold was demolished in spring, we were shocked to see that it was cracked and had to be processed again. The reason lies in the wrong understanding of winter construction technical measures. Because of the low temperature of the winter environment, it is not good to take insulation measures, and therefore need to do a lot of work on the ratio, but since it can create 15 ~ 25 ℃, it does not belong to the winter environment, no need to adjust the ratio! Add more cement, add early strength agent antifreeze agent, hydration speed up, shrinkage increased, cracking is inevitable!
The concrete with antifreeze can not be maintained?
Antifreeze is commonly used in the winter in the north, but the antifreeze itself is not to make the concrete is not afraid of freezing or that freezing is not bad, but to make the concrete reach the critical strength and then freeze is not a big deal! Otherwise, the same freezing! It is important to know this point! Antifreeze is generally inorganic salts or organic products, can reduce the freezing point of the aqueous solution, so as to ensure that even a little negative temperature, there is a liquid phase, in order to make the cement hydration continue, producing strength growth, consuming part of the water, will not freeze and swell.
The use of antifreeze, another role is to make the ice crystal lattice distortion, will not produce excessive expansion stress, the temperature is too low will also be frozen, which has a critical strength requirements, above this strength, expansion stress will not make the concrete freeze, as soon as possible to reach the critical strength is the key to the problem, insulation is necessary.
Concrete solid strength according to 28d appraisal
Into the winter, the concrete strength growth slowly, but many technical personnel still identified 28d strength, including the same condition test block, including rebound strength or so. This understanding is wrong, many supervisors, quality control personnel are often guilty of this low-level error, strength assessment to be equivalent to the standard maintenance 28d as standard.
When the raw materials of a certain concrete, composition ratio is known, the growth of its strength is mainly determined by temperature and time. The product of the two is called maturity (degree of time product). A certain ratio of concrete, regardless of temperature and time composition, as long as the maturity is equal, its strength is approximately the same, therefore, for a known concrete, can be based on its maintenance history – maintenance temperature and hardening time to estimate the strength of concrete has reached. The code of construction quality acceptance of concrete structures states that the equivalent curing age can be taken as the age corresponding to when the average daily temperature reaches 600℃-d day by day, and the age at 0℃ and below is not taken into account; the equivalent curing age should not be less than 14d, nor more than 60d, which means that the concept of maturity is recognized by the national standard.
Why the mixing time should be extended
Winter construction, concrete mixing time requirements than the normal season to extend 50%, this is because of the following factors: One is, with the lower temperature, the dispersal efficiency of the admixture in reducing, because the molecular thermal movement slowed down, must be forced to mix the admixture dispersion, in order to play the efficiency of the admixture. In addition, the winter sand and gravel inevitably have frozen blocks, not after extended mixing is difficult to melt, often out of the machine concrete state is very good, but to the scene there is segregation and water layer, the concrete state is very poor. Another point is that the gravel temperature is very low, easy to form ice shells on the surface of the stone, reducing the strength and durability of concrete, must also be extended mixing time to complete the heat exchange between the gravel and mortar, the negative temperature of the gravel heated to a positive temperature to avoid the emergence of ice shells.
Raw material heating and discharging
Winter construction, in order to protect the temperature of concrete out of the machine, often have to be heated raw materials, because the heat capacity of water is four times greater than gravel, generally do not preheat gravel, but heated water, water temperature up to 80 ℃ do not exceed. Feeding order is generally water, admixture, gravel; preheat beforehand, and avoid direct contact with cement. Then put in cement, fly ash and other powders.
Once a mixing station, summer construction down no problem, winter but always slumped a lot, out of the machine on the loss, and later to find the problem, found that the root cause in not fully mixing water and gravel, water temperature has not yet completely down, cast cement and admixtures, high temperature cement quickly promote condensation, so that the concrete lost fluidity.
Cold weather, steel should also be insulated and heated
This is similar to the previous point, rebar in the ambient temperature is very low, but also to heat insulation heating treatment, otherwise in the surface of the rebar will form an ice shell, not only affect the grip of the rebar and concrete, while in the underground works will also cause water seepage channel, which is necessary to pay attention to, as for the way to take the rebar insulation heating, but also according to the actual site to choose.
Winter construction is a time of high quality problems, to systematically control winter construction, from materials, proportioning, production to late, every link should be strictly managed, especially to understand some of the technical points, only fully aware of these key points, in order to consciously avoid the prevention of quality problems!