What are the quality defects of concrete/house extension

After pouring concrete, it is immediately subjected to a long period of negative temperature, especially -5 to -10 ℃. A large amount of free water collects on the surface of the structure and aggregate, forming lens shaped ice crystals. In the subsequent aging period, the temperature slowly rises, the ice crystals melt and the water evaporates, leaving internal defects. The strength is almost irreparable, and the impermeability and frost resistance are almost completely lost.

1.2 Suffering from negative temperature freezing before final setting

Concrete subjected to low temperatures of -5 to -10 ℃ before final setting will form ice crystals at a certain depth below the surface of the structure, manifested as local bulges, bulges, and short and wide cracks. Even if the temperature rises, the space occupied by ice crystals cannot be filled with uniform slurry, and the durability will be severely reduced.

1.3 Exposure to negative temperature during hydration temperature peak

The temperature peak of large volume structures generally occurs 24-72 hours after pouring. If they are subjected to low negative temperatures and unnecessary insulation measures at this time, they will crack. The basic characteristics of cracks in structures with low reinforcement ratio are sparse, long, wide, and deep; The basic characteristics of cracks with high reinforcement ratio are dense, short, narrow, and deep.

1.4 Rapid instantaneous cooling in winter

For structures with small volume, the internal temperature is not high. Even if the steam curing temperature is not high (around 20 degrees), in winter, if the timing of removing the steam curing shed is not paid attention to, surface temperature cracks may occur due to the rapid cooling of the surface concrete. The length and width of the cracks are irregular, and the depth is generally shallow.

1.5 Damaged concrete under anchor

The anchor pad at the end of the beam often has multiple holes, irregular shapes, and multiple bolts in the formwork, and the vibration is often insufficient; Due to location limitations, continuous insulation and maintenance are often difficult to achieve, and the joints of the covering material are prone to opening up, resulting in a continuous decrease or significant change in temperature; The concrete stress under the anchor pad is most concentrated, and during the tensioning process, it often cracks and sinks due to insufficient concrete strength.

1.6 Improper plastering causing cracking

Affected by low temperature, the effect of chemical additives decreases, the workability of concrete deteriorates, the construction time is prolonged, and the connection effect of various processes is poor, which makes it difficult to form the pouring surface. The phenomenon of watering (spraying water) during the plastering process is common, and manual repeated rubbing and plastering leads to the development of crack width and depth.

1.7 Cracking caused by loss of water due to strong winds

Affected by strong winds, the moisture on the surface of the concrete dissipates quickly. If it is not covered and cured or plastered in a timely manner, the concrete will crack due to drying and dehydration. These cracks mostly occur within a few hours after pouring, with varying lengths and widths, but mostly penetrate the steel bars.

1.8 Early age exposure to chloride pollution

After 3D exposure to seawater, the chloride ion concentration at 11mm had reached 0.4% by 180d, while after 14d exposure to seawater, the depth at which the concentration reached 0.4% by 180d was only 6mm. Early pollution of concrete should be avoided.