Concrete not set for a long time
Concrete mixer truck backlog on site
According to Article 6.6.14 of GB50164-2011 Concrete Quality Control Standard, the duration time from the discharge of the concrete mixer to the completion of pouring should not exceed the provisions of Table 6.6.14. That is, when the temperature ≤ 25 ℃, the duration should not exceed 150 minutes; when the temperature > 25 ℃, the duration should not exceed 120 minutes.
Pouring time is too long, worker fatigue
In the site of pouring structural columns or secondary structure, often more than 180min, or even more than 300min. especially in the summer, more than 3h of concrete has not been easy to pour, if the admixture thinning and then line pouring, which is considered better; some sites directly add water to thinning and continue pouring, which is bound to cause the lack of concrete strength.
Concrete construction without layering
Pouring concrete to the top in one go
Article 6.6.9 of GB50164-2011 Concrete Quality Control Standard clearly states that “When pouring a structure with large vertical dimension, it should be poured in layers, and the thickness of each layer should be controlled at 300～350mm; and more vibrating rods can be arranged, and should be vibrated while pouring concrete, and should be continuously formed. “.
Pouring concrete in layers too high
Super-high pouring is likely to cause swelling of the mold, and if it is not poured with tamping, it is easy to form cavities, exposed tendons and honeycomb pitting. When pouring a wall within 3m, it can be divided into two layers, and if the pumped concrete is easy and easy to vibrate, you can increase the height of the layers to 1000-150mm, when the vibrating speed cannot keep up with the pouring speed, you should stop the pouring immediately, and start pouring again when the vibrators catch up.
Concrete site watering
Add to the ground pump
In GB/T14902-2012 “ready-mixed concrete” standard 7.5.2 clearly states: “The mixing truck should be drained of water in the mixing tank before loading, and it is strictly prohibited to add water to the concrete in the mixing tank after loading”. Some construction workers ignore this provision, in order to facilitate the pumping, reduce the resistance to paving, on their own with a water pipe to the pump bucket or tank tube to add water; what’s more, do not see the dry thin material, first put on the water pipe while adding water to the pouring.
To add to the commercial concrete
Adding water to the concrete mix will increase the water to glue ratio of the concrete, the greater the water to glue ratio, the lower the final strength of the concrete, if too much water is added, it will lead to the strength of the concrete does not reach the design requirements, cannot pass the acceptance, and finally remedied by strengthening and reinforcement, and in serious cases, even bulldozing. This will cause great losses to the construction enterprises. Therefore, the national standard clearly states that it is strictly prohibited to add water to the concrete mix, when the concrete mix slump is on the small side, you can ask the tanker driver to add the right amount of admixture to adjust the slump in accordance with the company’s technical background, so as to facilitate the pumping construction.
Watering of the concrete finish
Add water before closing.
In order to facilitate the workers at the construction site to accept the surface, sprinkling of raw water phenomenon is also common, construction site technicians will not stop, seriously affecting the quality of concrete. Sprinkling raw water on the concrete surface, increasing the water-cement ratio of the concrete surface, increasing the hardened slurry void, making the concrete surface loose, reducing the strength of the concrete surface and the ability to resist carbonation, resulting in the concrete surface of ash, sand and cracking.
Too much concrete slump.
Unauthorized treatment of quality defects in concrete
Hazards of handling
In the construction process, often due to ideological and technical shortcomings, making the construction unit to produce a variety of concrete components defects, such as honeycomb, pitting, cracks, open tendons, holes and other arbitrary repair, but without reinforcement and reinforcement, will cause lifelong defects of the concrete project and affect the service life of the building, and the concrete entity testing, the impact of the concrete performance of the real judgment.
Premature demolition of concrete formwork
Removal of wall formwork
Because of the low strength of concrete and the poor working ability of concrete and steel, under the weight of the components, construction loads and other vibrations, it is easy to cause the reinforcement to slide in the concrete, thus reducing the holding force of the reinforcement; premature demolition of the mold, leading to premature water loss on the concrete surface, accelerating the carbonation of the concrete surface to reduce the durability of concrete.
Removal of slab formwork
If the concrete strength is lower than the demolition strength and the demolition is too early, it is easy for cracks to appear in the tensile zone, and even the pressurized zone will be damaged, which will cause safety accidents; premature demolition of concrete will increase the Xu variable of concrete, leading to structural deformation; premature demolition, due to the low strength of concrete, during the demolition process, it is easy for the slumped edges and corners or local collapse phenomenon to occur, weakening the cross-section of the components, affecting the load-bearing capacity and appearance quality.
Slab surface formwork dismantling
In some sites, the concrete is poured the first night and the wall and column formwork is removed the next morning to meet the deadline. This is fine when the temperature is normal, but in cooler weather, it is too early to remove the formwork the next morning 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 hoops (horizontal bars) underneath. This is caused by the concrete sinking under its own weight. Therefore, before dismantling the formwork, you must first check whether the concrete is finally set, and it is appropriate to use fingernails to scratch the top surface of the concrete without loose mortar, otherwise, the formwork cannot be dismantled.
Concrete construction without layering
Walls, frame columns
Pouring concrete, from the top of the reinforced concrete columns, walls and other projects, if too high, concrete in the coarse aggregate separated from the concrete, rolling off the outside of the concrete pile or at the bottom, or in the dumping process, the first coarse aggregate fall, and mortar fall behind, resulting in the segregation of concrete, so that the coarse aggregate crowded in the corners of the floor slab, easy to form a “honeycomb”! “, “pockmarked”, “rotten roots” or cause uneven distribution of sand and stone. Moreover, the free fall height is too high, the concrete has great impact, it will smash bent steel, the vertical members of the stirrups dislocation, these will affect the members of the force performance.
Inadequate concrete curing
Walls, frame columns
Water is a necessary condition for the hydration reaction of cement, so the humidity of the surrounding environment has a significant impact on the normal development of concrete strength. If there is not enough moisture, the concrete will lose water and dry out, which will affect the normal hydration of the cement, or even stop hydration. The earlier the drying effect, the more serious the dry shrinkage and cracking, the looser the structure, the greater the loss of strength, the greater the carbonization of the concrete surface, and the lower the durability of the concrete.
Concrete is not poured in the order
Due to improper pouring order and fabric, it will cause uneven sand and stone material in the concrete components, which will affect the strength of the concrete components and the generation of defects such as honeycomb, pitting surface, holes, etc., causing hidden danger to the quality of the project and may also constitute a major quality accident.
Less concrete cementitious material
Low gluing material, insufficient strength, total building demolition
JGJ55-2011 “ordinary concrete ratio design regulations” 7.4.2 stipulates: “the amount of cementitious material pumped concrete should not be less than 300kg/m3”. And some of the mixing plant in order to reduce production costs, the low-grade concrete ratio of cementitious materials designed for 280 kg / m ³, what is more, the C15 concrete ratio of cementitious materials are also designed for 280 kg / m ³, resulting in a car of C15 concrete led to many times blocking the pump, and finally had to be poured, readjust the cementitious materials to 340kg / m3 before normal pumping construction.