The construction oversite concrete construction process, construction details, and the maintenance of concrete are related to whether the concrete strength occurs and develops regularly, whether the concrete surface defects can be controlled, and whether the concrete durability can meet the service life. Therefore, some unregulated ways can bring serious impact and harm to the quality of construction oversite concrete, which requires us to be vigilant.
This article will tell you 4 things to avoid for construction oversite concrete.
Arbitrary addition of water to construction oversite concrete
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 use the water pipe to add water to the pump bucket or tank in order to facilitate the pumping and reduce the resistance of leveling. What’s more, regardless of the dryness of the material, some workers directly put on the water pipe to add water while pouring.
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 use the water pipe to add water to the pump bucket or tank in order to facilitate the pumping and reduce the resistance of leveling. What’s more, regardless of the dryness of the material, some workers directly put on the water pipe to add water while pouring.
Adding water to the concrete mixture will increase the water-cement ratio of concrete. The larger the water-cement ratio, the lower the final strength of the concrete. If too much water is added, it will cause the strength of the concrete to fail to meet the design requirements and fail to pass the acceptance. In the end, it can only be remedied by reinforcing the reinforcement, and in serious cases, it is even necessary to push down and start over, which causes great losses to the construction enterprise.
Therefore, it is strictly forbidden to add water to the concrete mixture. When the slump of the concrete mixture is small, you can ask the tanker driver to add the right amount of admixture to adjust the slump in accordance with his company’s technical instructions to facilitate pumping construction.
Ultra-high pouring and delayed vibrating for construction oversite concrete
When pouring fabric with a large vertical size, it should be poured in layers. The thickness of each layer should be controlled at 300~350mm. More vibrating rods can be arranged and the concrete should be pounded while pouring.
Some site workers pour the wall and beam slab at the same time while pouring the wall to the top of the slab at one time. There are also individual construction workers who are inexperienced in pumping concrete pouring. Pouring personnel do not take into account whether the vibrating staff can catch up. As a result, the pounding bar is less and the pounding speed is slower, which leads to the distance between pouring and pounding being enlarged, coupled with the higher pouring temperature, making the concrete poured into the mold slump faster. When pounding after 20min of pouring, the concrete is not easy to vibrate, and the pounding bar will leave holes after pulling out.
Super high pouring is easy to cause expansion of the mold. If you can’t vibrate as you pour, it’s easy to form cavities, exposed bars, and honeycomb-pockmarked surfaces by unrealistic vibrating. When pouring a wall within 3 meters, it can be divided into two layers. If the pumped concrete has better consistency and is easy to vibrate, the layering height can be increased to 1000-150mm. When the vibrating speed cannot keep up with the pouring speed, the pouring should be stopped immediately, and the pouring will start again when the vibrating speed comes up.
Excessive waiting time for construction oversite concrete
The continuation time of the concrete mixture unloading from the mixer to the completion of pouring should not exceed the relevant provisions. That is, when the temperature ≤ 25 ℃, the continuation time should not exceed 150 min. When the temperature > 25 ℃, the continuation time should not exceed 120 min.
In the construction site when pouring structural columns or secondary structures, the continuation time is often more than 180 min, and even more than 300 min. Especially in the summer, more than 3h of concrete has not been easy to pour. Thinning with admixtures and then pouring, which is considered better. Some sites directly add water to adjust the thinning and continue to pour, which will inevitably cause a lack of concrete strength.
When planning the pouring work, the site materialman should determine the quantity of each truckload to be transported according to the pouring speed and the standard specified continuation time. Too much concrete should not be transported in one truck, causing overtime pouring and affecting the strength of the concrete.
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 surface of the concrete in a moist state, not less than 7 d. The concrete covered by plastic sheeting should be covered tightly with all the open surfaces, and condensation should be kept inside the plastic sheeting. Before the concrete strength reaches 1.2MPa, do not install the formwork on it or hoist the heavier reinforcement or heavy objects such as frame pipes.
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. There are also a few units of concrete cast-in-place slabs that can not be covered with plastic sheeting in a timely manner, resulting in irregular cracks in the concrete at an early stage. Most of the construction units of high-rise buildings are poorly maintained above the 20th floor. This can be seen in the depth of carbonation measured at the rebound strength of the degree of concrete maintenance.
The cementitious material in concrete is water-hard. Concrete must be in a moist environment after forming in order to hydrate. Otherwise, the faster the water loss in the concrete, the worse the hydration of the cementitious material in it, and the greater the loss of strength of the concrete.
Therefore, the newly cast concrete must be maintained in strict accordance with the specifications and protocol requirements. 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.
Premature demoulding on construction oversite concrete
Some site workers remove the wall and column formwork in the morning of the next day in order to catch up with the schedule, the concrete is poured on the first night. When the temperature is normal, it is okay, but when encountering cooling weather, the next morning demoulding is too early, because the concrete has not yet been finalized.
Removing the formwork too early can cause the formwork to stick to the skin or create horizontal cracks under the horizontal bars. 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. Using your fingernail to scratch the top surface of the concrete without loose mortar is appropriate, otherwise, the formwork cannot be removed.
Also to read:
Top 7 Causes for Slump Loss in Concrete