Related knowledge points of mass concrete construction
Source: | Author:lichengcn | Publish time: 2017-08-09 | 31 Views | Share:
      Temperature measurement points according to the casting direction of the base plate, the structure characteristics and the temperature field is expected to decorate, in the elevator shaft bottom choose two typical temperature measuring points, foundation slab choose 10 representative temperature measuring point.For each measuring point, along the depth direction, the three sensors are arranged in the direction of the middle and the next three sensors. The sensor is 10cm from the surface, and the sensor is centered in the middle, and the distance of the base plate is 160cm, and the sensor is 10-15cm from the base.In addition to the sensor embedded in the concrete, two sensors were used to detect the surface temperature and atmospheric temperature of the concrete under the maintenance layer separately.In the second test, two sensors were used to detect the surface temperature of the concrete.
      Cracks and precautions:
      1.1 possible reasons for the crack of mass concrete
      1.1.1 the types of cracks and the formation of the causes of large volume concrete blocks or foundations are caused by a variety of factors.The main influencing factors of various cracks are as follows:
      1.1.1.1 shrinkage crack:
      Shrinkage of concrete causes shrinkage cracks.The main influencing factors of shrinkage are the amount of water and cement used in concrete. The higher the water and cement consumption in concrete, the greater the shrinkage of concrete.The amount of shrinkage and shrinkage of cement is different.The smaller cement is medium and low heat cement and fly ash cement.
      The gradual cooling and hardening process of concrete contraction, will produce a large contraction stress, if the shrinkage stress exceed the ultimate tensile strength of concrete, will produce shrinkage crack in the concrete.
      People pay a lot of attention to shrinkage, but it is not the contraction itself that is attracting attention, but because it causes cracking.There are several kinds of shrinkage of concrete, the familiar is drying shrinkage and temperature shrinkage, which is mainly introduced in its own contraction, and also mentioned plastic shrinkage problem.
      The contraction is the same as dry shrinkage due to water migration.But it is not due to water evaporation loss outward, but because of who I am when the cement hydration water consumption falling gel pore fluid level, form the meniscus, produce so-called since the drying effect, the relative humidity of concrete body is reduced, the volume decreases.The change of water cement ratio is opposite to the effect of drying shrinkage and contraction. It is that when the water ash ratio of concrete decreases and shrinkage decreases, while the shrinkage increases.If the water cement ratio is greater than 0.5, the self-drying effect is negligible compared with the shrinkage and shrinkage of the body.However, when the water ash ratio is less than 0.35, the relative humidity in the body will soon be reduced to less than 80%, and the contraction and shrinkage of the body are nearly half.
      It is born in the early age of concrete mixing, because after this, due to the self-drying effect of the body, relative humidity is reduced and the hydration is basically terminated.In other words, before the template removal, most concrete self shrinkage was produced, and even has been completed, rather than drying shrinkage, except not covered and the ground of the exposed surface is very big, many components of drying shrinkage occurs after ripping, so as long as covered surface, as drying shrinkage of concrete will not occur.
      In mass concrete, even if is not low water-cement ratio, shrinkage value itself is not big, but it has to do with the temperature contraction stack together, will make stress increases, when the hydraulic dam construction so its early shrinkage as a performance index are measured and taken into consideration.Nowadays many section size is not very big, and water cement ratio of concrete is not small, as described above, has been "to have to solve the problem of hydration heat and then cause the volume deformation, to minimize cracking effect", and therefore need to like a dam, will need to consider the influence of temperature and contraction overlay, and in these structures, the development rate of both types are much faster than in the concrete dam, which is much more intense.
      There is also plastic shrinkage, which can cause cracking under conditions of high cement activity, high concrete temperature, or low water ash.When concrete exudation significantly reduced, surface evaporation of moisture is not replenished in time, concrete is still in its plastic state at this moment, a little bit under tension, the surface of the concrete distribution of irregular cracks will occur.After the crack, the water evaporation in the concrete is accelerated, so the crack expands rapidly.Therefore, the concrete casting needs to be covered early in the above situation.
      1.1.1.2  temperature cracks of concrete cracks in the internal and external temperature difference.The main influencing factors of temperature difference fracture are the temperature difference of concrete interior and concrete surface caused by cement hydration heat.This kind of crack is more likely to occur in large volume concrete.
      One-time whole pouring mass concrete structure in general requirements, after casting, the cement hydration heat caused by hydration, due to the large volume concrete, gathered in the interior of cement hydration heat is not easy to send out, the concrete internal temperature will be increased significantly, while the concrete surface soil heat faster, formed a larger temperature difference, the concrete compressive stress is generated in surface tensile stress, at this time, the coagulation (short, tensile strength is very low.When the surface tensile stress of temperature difference exceeds the tensile strength of concrete, it will crack on the surface of concrete.
      In large volume concrete construction, due to the different internal and surface heat dissipation rate of concrete, a large temperature gradient is formed on the surface of the concrete, resulting in a larger surface tensile stress.Concrete of age at the same time, this time is very short, tensile strength is low, temperature difference of surface tensile stress, more than the ultimate tensile strength of concrete, would have a surface crack in the concrete surface.Such cracks usually occur on the third day after the concrete is poured (heating).Concrete cooling stage, the result of the cooling contraction, coupled with the concrete hardening process, due to the hydration of concrete internal mixing water and evaporation and gelation of gelatin body function, makes concrete hardening shrink.These two contractions can also produce large tensile stresses due to constraints of the base or structure, until the shrinkage cracks occur.
      1.1.1.3 stability stability cracks of cracks, mainly caused by the cement stability unqualified.
      2.1 prevention and control measures for cracks
      2.1.1 design measures
      1) elaborate design of concrete concrete mixture ratio design, in guarantee under the condition of concrete with good workability, should as far as possible to reduce the unit water use of concrete, with "three low, low sand ratio, low slump, low water-binder ratio) 2 doped (with high efficiency water reducing agent and high-performance air-entraining agent) high (high dosage of fly ash)" design criteria, produce the "high strength, high toughness, shot, low thermal and high value" the crack resistance of concrete.
      2) the addition of structural reinforcement improves the anti-cracking performance, and the reinforcement should be small diameter and small spacing.The reinforcement rate of the whole section should be between 0.3 and 0.5%.
      3) avoid structural mutation to produce stress concentration and strengthen measures in vulnerable areas prone to stress concentration.
      4) set up the dark beam on the edge of the vulnerable crack, improve the reinforcement ratio of the site, and improve the limit stretch of the concrete.
      5) in the structure design, the climate characteristics should be fully considered in the construction. After reasonable setting, the water should be watered down. Under normal construction conditions, the distance between the post-pouring seams is 20 ~ 30m, and the retention time is generally not less than 60 days.If the specific conditions of the construction can not be predicted, the design changes may be made on a temporary basis.2.1.2 construction measures
      6) strictly control the quality and technical standards of the concrete raw materials and select low hydration hot cement, and the content of coarse aggregate should be reduced as far as possible (1 ~ 1.5 percent).