stay Gb50164-2011 standard for quality control of concrete 6.6.9 The article clearly stipulates that “when pouring structures with large vertical dimensions, they shall be poured in layers, and the pouring thickness of each layer should be controlled at 300 ~ 350mm; more vibrating rods can be arranged, and they shall be vibrated while pouring concrete, which should be formed continuously”.
When pouring structural columns or secondary structures on the construction site, it often takes more than 180 minutes, even more than 300 minutes.
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Therefore, before removing the formwork, it is necessary to check whether the concrete is finally set, and it is appropriate to scratch the top surface of the concrete with your fingernails without loose mortar, otherwise the formwork cannot be removed.
If the humidity is not enough, the concrete will lose water and dry, which will affect the normal progress of cement hydration, or even stop hydration.
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In order to facilitate surface finishing, it is common for workers on the construction site to sprinkle raw water, and the technicians on the construction site do not stop it, which seriously affects the quality of concrete.
It is better to pour concrete after thinning with admixture; Some construction sites directly add water to dilute and then continue pouring, which is bound to cause insufficient concrete strength.
If the formwork is removed too early, there will be formwork peeling or horizontal cracks along the stirrup (horizontal reinforcement).
In order to catch up with the construction period, some construction sites remove the wall column formwork the next morning after pouring the concrete the night before.
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If too much water is added, the strength of the concrete will not meet the design requirements and cannot pass the acceptance.
When pouring concrete, if the reinforced concrete column, wall and other works poured from the top are too high, the coarse aggregate in the concrete will be separated from the concrete and roll to the outside or bottom of the concrete pile, or in the process of dumping, the coarse aggregate will fall first and the mortar will fall behind, resulting in the segregation of the concrete, causing the coarse aggregate to squeeze into the corners of the floor slab, which is easy to form “honeycomb” and “pitted surface” “Rotten roots” or cause uneven distribution of sand and stone.
When the vibration speed cannot keep up with the pouring speed, the pouring shall be stopped immediately and the pouring shall be started when the vibration personnel keep up.
It will smash and bend the reinforcement and dislocation the stirrups of vertical members, which will affect the mechanical performance of members.
Especially in summer, it is not easy to pour concrete for more than 3H.
This is caused by the sinking of the concrete due to its own weight.
If it cannot be vibrated with pouring, it is easy to form cavities, exposed reinforcement and honeycomb and pitted surface.
According to article 6.6.14 of gb50164-2011 standard for quality control of concrete, the duration from the discharge of concrete mixture from the mixer to the completion of pouring shall not exceed the provisions in table 6.6.14.
Article 7.5.2 of ready mixed concrete (GB / t14902-2012) clearly stipulates: “the mixer truck shall drain the ponding in the mixing tank before loading, and it is strictly prohibited to add water to the concrete in the mixing tank after loading”.
When the temperature is normal, it is OK, but in case of cooling weather, it is too early to remove the formwork the next morning because the concrete has not yet set.
Moreover, the free fall height is too high, and the concrete has great impact force.
If the workability of pumped concrete is good and it is easy to vibrate and compact, the layer height can be increased to 1000 ~ 150mm.
The greater the water binder ratio, the lower the final strength of the concrete.
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In serious cases, it will even be pushed over and over again.
Article 7.4.2 of jgj55-2011 code for mix proportion design of ordinary concrete stipulates that “the amount of cementitious material for pumped concrete shall not be less than 300kg / m3”.
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Super high pouring is easy to cause formwork expansion.
Water is a necessary condition for cement hydration reaction, so the ambient humidity has a significant impact on the normal development of concrete strength.
Adding water to the concrete mixture will increase the water binder ratio of the concrete.
In order to reduce the production cost, some mixing plants design the cementitious material in the mix proportion of low-grade concrete as 280 kg / m ³， Moreover, the cementitious material of C15 fine aggregate concrete mix proportion is also designed as 280 kg / m ³， As a result, a truck of C15 fine aggregate concrete blocked the pump for many times, and finally had to be poured out.
The earlier the drying action, the more serious the dry shrinkage cracking, the looser the structure, the greater the loss of strength, the greater the carbonation of concrete surface and the lower the durability.
This has caused great losses to construction enterprises.
When pouring the wall within 3M, it can be divided into two layers.
When the slump of the concrete mixture is too small, the tank truck driver can add an appropriate amount of admixture to adjust the slump according to the technical disclosure of his company, so as to facilitate pumping construction.
That is, when the temperature ≤ 25 ℃, the duration shall not exceed 150 minutes; When the temperature ＞ At 25 ℃, the duration should not exceed 120 minutes.
Some construction workers ignore this provision and use water pipes to add water to the pump bucket or tank barrel without permission in order to facilitate pumping and reduce the resistance of leveling; What’s more, regardless of the dryness and thinness of the incoming materials, put on the water pipe and pour while adding water.
Finally, it will be remedied by reinforcement.
In the construction process, due to ideological and technical defects, the construction unit often produces various defects on concrete components, such as honeycomb, pitted surface, cracks, exposed reinforcement, holes and so on, but without reinforcement, which will cause lifelong defects of concrete engineering and affect the service life of buildings, and affect the real judgment of concrete performance when testing concrete entities.
Improper pouring sequence and distribution will cause uneven sand and stone in concrete components, affect the concrete strength of components and the occurrence of defects such as honeycomb, pitted surface and holes, cause hidden dangers to project quality and may also constitute a major quality accident.
Because of the low strength of concrete and the poor working capacity of concrete and reinforcement, it is easy to cause the reinforcement to slide in the concrete under the action of component self weight, construction load and other vibration, so as to reduce the gripping force of reinforcement; Premature formwork removal will lead to premature water loss on the concrete surface, accelerate the carbonization of the concrete surface and reduce the durability of the concrete If the concrete strength is lower than the formwork removal strength and the formwork is removed too early, it is easy to crack in the tensile area and even damage the compression area, resulting in safety accidents; Premature formwork removal of concrete will increase the creep of concrete and lead to structural deformation; Early formwork removal: due to the low concrete strength, it is easy to collapse edges and corners or local collapse during formwork removal, weakening the member section and affecting the bearing capacity and appearance quality;.
Therefore, the national standard clearly stipulates that it is strictly prohibited to add water to the concrete mixture.
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The normal pumping construction can be carried out only after the cementitious material is adjusted to 340kg / m3 again.
Sprinkling raw water on the concrete surface increases the water cement ratio of the concrete surface, increases the gap of the hardened slurry, loosens the concrete surface, reduces the strength and anti carbonization ability of the concrete surface, and causes ash rising, sand returning and cracking on the concrete surface.
