Classification and cause analysis of bleeding and segregation mechanisms

Concrete bleeding and segregation mechanism
Concrete bleeding refers to the sinking of solid particles due to the difference in density of the various components of the mixture between the concrete pouring and the initial setting period. The phenomenon in which free water rises and continuously bubbles to the surface. Concrete segregation means that the water consumption of the mixture seriously exceeds the limit or the additives are over-mixed. Factors such as the different molecular sizes and specific gravity of various raw materials, and even the incompatibility of cement and additives lead to the complete separation of the slurry from the sand and gravel aggregates. Loss, bottom grabbing, aggregate exposure and other phenomena.
Classification of concrete bleeding and segregation states
       (1) The workability of the mixture is seriously insufficient, water and cementitious materials are continuously exuded, resulting in changes in the homogeneity of the mixture, and the density is greater than that of the mixture Particles with bulk density settled down, and the mixture was seriously segregated. The lower part was almost dry material, and the upper part was slurry. There was severe “grabbing at the bottom” and it could not be stirred with a shovel, highlighting the illusion of being very sticky. For example, if it is slight, the concrete will start to “catch the bottom” after it stops, but if it is stirred evenly again, its segregation and bleeding will disappear, and its workability will become very good.
(2) The workability of the mixture is very good, and the homogeneity is also very good, but the concrete will continue to bleed out clear water after it is stopped. The bleeding time is more than 2 hours. When the amount of bleeding is serious, There will be streamline-like bleeding on its surface, but there will be no “grabbing at the bottom”.
1.3 Analysis of causes of bleeding and segregation
(1) The adaptability of admixtures and cement is incompatible. The poor adaptability between admixtures and cement is mainly reflected in the rapid slump loss of the mixture or severe bleeding or segregation. When the amount of admixture is increased, the fluidity, cohesion, and water retention cannot be effectively improved. This may be related to the poor water retention of cement, changes in variety composition, unreasonable cement placement time, etc. The water reduction rate of surface similar admixtures is low. The subsequent strength is obviously low and cannot meet the requirements.
 (2) Factors such as reduced quality of raw materials such as cement, admixtures, and aggregates, poor gradation, and unreasonable sand rate. At present, the cement gradation and water retention in the market are poor, the quality of admixtures such as fly ash and slag powder is poor, and there is a lack of sand and gravel aggregates for comprehensive environmental management. The quality is uneven, which seriously affects the amount of admixtures and the workability of concrete. Frequent occurrences Poor fluidity, water retention, and low sand rate result in poor concrete workability, leading to bleeding and segregation.
  “Standards for the Quality and Inspection Methods of Sand and Stone for Ordinary Concrete” JGJ52-2006 stipulates that the amount below 0.315mm should not be less than 15%. If the gradation is unreasonable, the amount below 0.315mm is too small, and the amount above 2.5mm is too much. Too much sand has a great impact on the workability of the concrete mixture. Often when the amount of sand above 5mm is too much, the sand rate of the concrete will be low and segregation and bleeding will occur.
(3) The concrete mix proportion design failed to meet the requirements. If the water-cement ratio is too large in the concrete mix design or production project, the cementitious material will be reduced, which can easily lead to bleeding in the concrete mixture. When designing the mix ratio, the appropriate gray-water ratio and sand rate should be determined after trial mixing and adjustment. The production process should strictly follow the determined mix ratio, regularly monitor the moisture content of the sand and gravel, and control changes in single-party water consumption to prevent serious water consumption. Exceeding the design dosage and changing the gray-water ratio and sand rate may lead to bleeding or segregation.
(4) Concrete production, transportation and construction control are unfavorable. The mixing time in concrete production was insufficient and the homogeneity did not meet the requirements. When the mixer truck was used for transportation from the factory, the mixing blades were severely worn, causing the concrete to stratify and segregate in the tank. During the pouring construction, it is difficult to control the quality of the mixing station. Due to the uneven quality and operation level of on-site workers, water is added indiscriminately at the construction site and admixtures are added at will. The formwork is often too dry or there is serious water accumulation, and the vibrating operations are not performed according to the specifications. , over-vibration and vibration leakage often occur, resulting in uneven mixture and bleeding and segregation during the construction process.
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