I. Severe bleeding occurs during concrete construction.
1. Phenomenon: After vibrating the concrete or using a vibrator to mix the materials for a period of time, more water appears on the surface of the concrete.
2. The main reasons for concrete leakage: Severe leakage of concrete is mainly related to the type of cement used, fineness, amount of cementitious materials, sand ratio, and water usage. Excessive water usage is likely to cause leakage, and prolonged vibration can also lead to leakage.
3. Preventive measures: When preparing concrete, priority should be given to using cement with better water retention properties, controlling the slump and water usage of the concrete, and avoiding excessive vibration of the concrete. Some fly ash, volcanic ash, and other admixtures can also be added to enhance the water retention of the concrete.
4. Treatment methods: Once leakage occurs, consider reducing the water usage or adjusting the proportion of the concrete, and remove the water that has seeped onto the surface. If the leakage is minor, no treatment is required as a small amount of leakage can keep the concrete surface moist and reduce the water-cement ratio in the concrete, thereby improving the actual strength of the concrete.
II. Faults caused by improper use of admixtures
1. Phenomenon:
(1) After the concrete is poured, some areas or a large area fails to fully solidify and harden for a long time;
(2) The surface of the poured concrete structure shows bulging;
(3) Before pouring, the slump of the concrete mixture is too small, making it difficult to pour.
2. Cause analysis:
(1) Excessive or uneven addition of retarders and water reducers (such as calcium lignosulfonate) will cause some or a large area of concrete to fail to solidify and harden for a long time after pouring;
(2) The admixtures added to the concrete are in the form of dry powder, containing ungrounded particles (such as sodium sulfate particles), which expand when encountering water, causing the concrete surface to "turn white" and bulge;
(3) The content of components maintaining the slump in the admixtures is insufficient, resulting in excessive slump loss, or the initial slump of the concrete mixture is low.
3. Treatment methods:
(1) The admixtures added to the concrete should comply with relevant standards and be tested before use to meet the construction requirements;
(2) The storage time of the concrete mixture should be shortened as much as possible to reduce slump loss;
(3) If the concrete fails to harden and harden for a long time due to excessive use of retarders and water reducers, the curing time can be extended, and the demolding time can be postponed (in this case, the later strength of the concrete is usually not greatly affected);
(4) When using powdered admixtures and forming lumps, they should be treated before concrete preparation. For concrete surfaces with "alkali effusion" phenomenon, the protruding parts caused by admixture particles should be removed first, and then repaired.
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