Adding water arbitrarily during the construction process
Article 7.5.2 of GB/T14902-2012 “Ready Mixed Concrete” clearly stipulates that “the mixing truck should drain the water in the mixing tank before loading, and it is strictly forbidden to add water to the concrete in the mixing tank after loading”. Some constructors ignore this provision, in order to facilitate pumping and reduce the resistance of leveling, they use the water pipe to add water to the pumping bucket or tank; worse still, they do not seem to be dry or thin, and put on the water pipe first while adding water to pour. Adding water to the concrete mix will increase the water-cement ratio of concrete, the greater the water-cement ratio, the lower the final strength of concrete, if too much water is added, it will lead to the strength of concrete can not reach the design requirements, can not pass the acceptance, and finally through the reinforcement to remedy the situation, and in serious cases, even push back. This causes great losses to the construction company. Therefore, the national standard clearly stipulates that it is strictly prohibited to add water to the concrete mix, when the slump of the concrete mix is small, you can ask the tanker driver to add the right amount of admixture to adjust the slump in accordance with its company’s technical instructions to facilitate pumping construction.
Ultra-high pouring and delayed vibrating
In GB50164-2011 “Concrete Quality Control Standard”, Article 6.6.9 clearly states that “when pouring vertical size of the structure, should be poured in layers, each layer should be controlled at 300 ~ 350mm; and can be arranged more vibrating bar, should be pounded while pouring concrete, should be continuous molding “. Some sites according to 1.5 meters for a layer, and even to the wall and beam plate at the same time when pouring, once the wall to the top of the plate. There are also individual construction personnel on the pumping concrete pouring inexperienced, in the front of the pouring personnel do not take into account whether the back of the vibrating personnel can catch up, the result is that the vibrating rod is on the less, the vibrating speed is slower, resulting in the distance between the pouring and vibrating stretched, coupled with the pouring temperature is higher, making the concrete poured into the mold slump loss faster, when the pouring 20min after the vibrating again, the concrete is not easy to vibrate solid, the vibrating rod pulled out There will be holes left after the vibrator is pulled out. This is the time to know to stop the front pouring work. After demoulding, the section of wall had many cavities, exposed bars or honeycomb pockmarks. The over-height pouring easily caused the expansion of the mold, and if it could not be poured and pounded, it was easy to form hollows, exposed reinforcement and honeycomb pockmarked surface with poor pounding. When pouring a wall within 3 meters, it can be divided into two layers, if the pumped concrete has better compatibility, easy to vibrate and compact, you can increase the height of the layers to 1000 ~ 150 mm. when the pounding speed can not keep up with the pouring speed, should immediately call a halt to the pouring, and then start pouring when the pounding staff catch up.
Concrete waiting time at the site is too long
In GB50164-2011 “Concrete Quality Control Standards” in 6.6.14 stipulates: concrete mix from the mixer after unloading to the completion of pouring the continuation time should not exceed the provisions of Table 6.6.14. That is, when the temperature ≤ 25 ℃, the continuation time should not exceed 150 minutes; when the temperature > 25 ℃, the continuation time should not exceed 120 min. and in the construction site when pouring structural columns or secondary structure, often more than 180 min, and even more than 300 min. especially in the summer, more than 3 h of concrete has not been easy to pour, if the admixture thinning and then pouring, which is considered Better; some sites will directly add water to adjust the thinning and continue to pour, which is bound to cause the lack of concrete strength. The solution is, the site material clerk in the plan to pour sporadic projects, should be based on the speed of pouring and the standard provisions of the continuation of time to determine the amount of each car transport, rather than a car transport too much, resulting in overtime pouring.
Too poor maintenance after concrete pouring
GB50204 “Concrete Structure Construction Quality Acceptance Specification” in 7.4.7 states: “The concrete should be covered and moisturized within 12 hours after the concrete is poured; when using the watering method, the number of watering should be able to keep the concrete surface in a moist state, not less than 7d; the concrete covered with plastic sheeting for maintenance, its open The entire surface should be covered tightly, and should be kept inside the plastic sheet condensation water; concrete strength of 1.2MPa before the installation of its formwork or lifting heavy reinforcement or racking pipe and other heavy objects. Some construction units are not set up to maintain the concrete, and even individual units of concrete columns and shear walls are not maintained at all. The author once participated in a C25 structural column rebound, rebound age of 29d, the date of pouring in April. There are a few units of concrete cast-in-place slab can not be covered with plastic sheeting in time to maintain, resulting in irregular cracks in the early concrete. Most of the construction units are poorly maintained for the part of high-rise buildings above 20 floors, which can be seen in the depth of carbonation measured at the rebound strength of the concrete maintenance degree. The cementitious material in concrete is water-hardened, and concrete must be hydrated in a humid environment after forming; otherwise, the faster the water loss in concrete, the worse the hydration of the cementitious material in it, and the greater the loss of strength of concrete. Therefore, the newly cast concrete must be maintained in strict accordance with the specifications and regulations. Before the strength of the concrete cast-in-place slab reaches 1.2MPa, no bundles of reinforcement, racking pipes or heavier large steel formwork should be lifted. Otherwise, it is easy to cause cracks in the cast-in-place slab due to the impact load of lifting.
Demoulding too early
Some sites, in order to catch up with the schedule, the concrete poured on the first night, the next morning to remove the wall column formwork. When the temperature is normal, but encounter cooling weather, the next morning demoulding is too early, because the concrete has not yet finalized. Removing the formwork too early can cause the formwork to stick to the skin or create horizontal cracks along the hoop reinforcement (horizontal reinforcement) under. This is caused by the concrete sinking under its own weight. Therefore, before removing the formwork, make sure to check whether the concrete is finalized, and use finger nails to scratch the top surface of the concrete without loose mortar as appropriate, otherwise the formwork cannot be removed.