0Introduction
During the use of concrete, it is often found that water continuously appears on the surface of the concrete in a static state, and even separation of the slurry from the sand and gravel aggregate occurs. According to the characterization analysis of bleeding and segregation, concrete is a mixture made of admixtures such as cement, fly ash, ore powder, sand and gravel, and water measured in a certain proportion and mixed within a specified time. Due to the differences in the specific gravity of the materials themselves, , improper proportion and adaptability of various materials and other factors, coupled with uneven mixing and vibration, it is easy to cause quality problems such as bleeding and segregation after static vibration. In this regard, throughout the entire process from raw material control to concrete construction, necessary measures are taken for prevention and control to ensure the quality of component concrete, highlighting its importance.
1 Bleeding and segregation mechanism, classification and cause analysis
1.1 Concrete bleeding and segregation mechanism
Concrete bleeding refers to the period after concrete pouring to the initial setting During this period, due to the difference in density of the mixture of various components, the solid particles sink, the free water rises, and water continuously emerges on 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.
1.2 Classification of concrete bleeding and segregation status
(1) The workability of the mixture is seriously insufficient, and water and cementitious materials are continuously exuded, resulting in a homogeneous mixture Physical changes, particles with a density greater than the bulk density of the mixture are deposited, and the mixture is seriously segregated. The lower part is almost dry material, and the upper part is slurry. There is serious "grabbing at the bottom" and it cannot 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. However, the concrete keeps bleeding out water after it is stopped. The bleeding time is more than 2 hours, and the amount of bleeding is In severe cases, there will be streamline-like bleeding on the 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) Decreased quality of cement, admixtures, aggregates and other raw materials, poor gradation, unreasonable sand rate and other factors. 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.
“The Standard 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 below 2.5mm is too small. Too much sand will have a great impact on the workability of the concrete mixture. Often when there is too much sand above 5 mm, 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.
2 The advantages and disadvantages of bleeding and the hazards of segregation
When bleeding occurs, the mixture itself is also settling and shrinking. As a result, the water content of the surface mixture will continue to increase before initial setting, and the water-cement ratio will increase, resulting in a decrease in the strength of the concrete surface and the formation of a layer of "dead skin" that is easy to peel off. During the bleeding process, pore channels are formed, which reduces the strength, impermeability and frost resistance of the concrete. exist��. Bleeding and segregation phenomena often characterize the adaptability of admixtures and cement. Therefore, the rational use of admixtures with different water reduction rates, models, and varieties can effectively improve the adaptability problems with cement. According to many years of practical experience, ordinary high-efficiency water-reducing admixtures are used for concrete with strength grades below C40, and the water reduction rate is usually within 25%. It is more appropriate to use high-performance admixtures for concrete with C40 and above grades, which can effectively ensure the strength and workability of concrete. Avoiding quality problems such as bleeding and segregation can also reduce unit costs.
Based on the actual structure, strength and other special requirements, in the case of raw material variation, change the admixture model such as high-efficiency retarder to ordinary, high-performance to high-efficiency, etc. When the model cannot be met at the same time, change the variety such as Changing naphthalene series to polycarboxylic acids, lipids to ethers, etc. can effectively eliminate bleeding and segregation quality hazards and other performance confusion.
Of course, according to the performance defects of the admixtures, some reinforcing agents, thickeners, retarders, slump-preserving agents, etc. can be added appropriately to make slight adjustments to improve the strength and other properties of the concrete mixture.
(4) Select high-quality, continuously graded sand and gravel aggregates. The sand fineness modulus is controlled at 2.3 to 3.0. In the medium sand II zone, the mud content, mud content and other indicators meet the requirements of JGJ52-2006. Similarly, the gravel should be continuously graded from 5 to 31.5mm and 5 to 20mm and crushed. The value, mud content and mud block content all meet the above standard requirements. Selecting high-quality, continuously graded sand and gravel can effectively change the porosity of concrete, increase density, and improve bleeding and segregation of concrete.
3.3 Carry out reasonable concrete mix design, trial mix, and repeated verification, and select the optimized mix ratio
The mix design should establish the glue ratio, sand rate, and active admixture according to the requirements. Mixing materials, additive dosage, etc. The glue ratio is directly related to the strength and workability of the mixture. If the glue ratio is large, the strength is generally high and the workability is good. On the contrary, the strength is low and the workability is poor, which is detrimental to improving bleeding and segregation.
The sand rate in the mix should be reasonably designed and adjusted according to the sand and gravel gradation. In the case of 5 to 31.5mm, if the sand fineness modulus is lower than 2.4, the sand rate can be reduced; otherwise, it can be appropriately increased. sand rate. If the gravel is 5 to 20.0 mm, the sand rate can be increased and the workability can be changed based on the actual test results to effectively improve the probability of bleeding and segregation.
The amount of active admixture in the proportion should be based on "Fly Ash for Cement and Concrete" GB/T1596-2017, "Granulated Blast Furnace Slag for Cement, Mortar and Concrete" In accordance with GB/T18046-2017 and physical requirements, conduct reasonable design and trial assembly verification. Large-volume, high-strength, high-performance concrete with large amounts of fly ash, slag powder and other active admixtures can effectively improve the heat of hydration, loss over time, workability, durability, etc., and at the same time effectively improve the bleeding of the mixture. and the possibility of segregation.
The design of concrete mix proportion should first consider the workability and strength of concrete, followed by economy. Combining the three, it is most effective to reduce the single water consumption of concrete. After trial mixing, the lowest single water consumption under different water-cement ratios is selected, which can maintain better workability under the same slump, leaving the concrete with no free water to bleed, and naturally solving the bleeding problem.
3.4 Strengthen the control of concrete production, transportation and construction processes
During concrete production, the mixing time for strength grades below C40 should not be less than 35 seconds, and the mixing time for strength grades C40 and above should not be less than 1 minute. If there are special requirements, if fiber is added, the mixing time needs to be extended until the mixture is evenly mixed.
When transporting by mixer truck, observe the wear and tear of the mixing blades to ensure that the homogeneity meets the requirements. During transportation, maintain 3 to 6 r/min. Before unloading, keep the mixer truck rotating rapidly for 1 to 2 minutes. Only after mixing is even, can the material be unloaded.
During the pouring construction process, the homogeneity of the concrete in the feed hopper must be maintained and the construction must be continuous when using pump trucks to transport it; when using bucket trucks to unload, the road should not be too far to avoid long-distance vibration and slurry Sinking, sand and gravel layered. When using a chute, the height should not exceed 5m. When the structure is poured and vibrated, the thickness should not exceed 300mm. Massive concrete is poured in layers. Through production and mixing time, transportation and construction control, concrete bleeding and segregation can be effectively controlled.
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