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. Concrete segregation and its hazards
3.2 Impact of sand and gravel aggregates
Sand and gravel materials are the most commonly used materials in concrete. The quality fluctuation of sand and gravel materials directly affects the quality of concrete and easily causes the segregation of concrete.
The quality fluctuations of sand and gravel materials are mainly manifested in the following aspects:
3.2.1 The increase in the particle size of the gravel and the deterioration of the gradation affect the workability of concrete, leading to concrete segregation. Therefore, quality control must be strictly controlled during the construction process, and unqualified materials must be prevented from entering the construction site to ensure the stability of aggregate quality. 3.2.2 Excessive stone content in sand will seriously affect the workability of concrete and lead to concrete segregation. In this case, the sand rate can be appropriately increased to adjust the mix ratio of the concrete to solve the problem of segregation. If the problem of segregation cannot be solved well by adjusting the sand rate alone, the cementing materials of the concrete (especially the admixtures) must be increased. ) dosage.
3.2.3 The water content of sand and gravel is too high (especially sand). If the water content in the sand is in a saturated state (≥10%), the water in the capillary tubes on the surface of the sand cannot be released in time during the mixing of the mixture in the mixer, which will easily lead to excessive mixing water consumption, and the aggregate capillary tubes will not be used during transportation. The water in the concrete is continuously released, destroying the bond between the aggregate and the cement slurry, causing the concrete to separate and bleed. In this case, measures should be taken to extend the mixing time to solve or increase the amount of admixtures such as fly ash to control the segregation problem of concrete.
3.2.4 Excessive mud content in sand and gravel will cause the bonding force between the cement slurry and the aggregate to decrease, and the wrapping ability to decrease, resulting in the separation of the aggregate and the segregation of concrete.
In this case, the supplier must be required to strengthen quality control or flush it to reduce the mud content.
3.3 The influence of admixtures
Admixtures are substances added during the mixing process of concrete to improve the properties of concrete. It can improve the strength of concrete, increase the rheological properties of concrete, and save cement. Dosage, etc. However, if used improperly, it can also cause the problem of concrete segregation
. Its influence is mainly reflected in the following two aspects:
3.3.1 The compatibility of admixtures and cementitious materials. Admixtures are mainly surfactants, which have surface adsorption, dispersion, wetting, foaming and defoaming effects. Different surfactants have different effects on cement hydration due to their different molecular structures. Various components in cement Different amounts and amounts have different adaptability to admixtures. If the compatibility between admixtures and cement is not good, its effect cannot be fully exerted, and the concrete mixture will appear hardening, bleeding, bubbles and other phenomena. This situation occurred in Jiashao 6 standard. The water-reducing agent used at the beginning was Shanxi "Huanghe" brand. The concrete showed dispersion and segregation. After adjustment, the water-reducing agent of Shanghai "Sanrui" brand was used and the improvement was improved. improve the construction performance of concrete. Therefore, during the concrete testing process, the compatibility test between the admixture and the cementitious material must first be done, and it must be done frequently, because the quality of cement is not always stable, and its composition changes, so the composition of the admixture must be adjusted accordingly. Adjust, otherwise they will not get used to each other.
3.3.2 The dosage of admixtures. If the amount of admixture in the concrete is too large, the water-reducing rate is too high, and the water consumption per concrete is reduced, it is possible that the water-reducing agent will not fully play its role in the mixer, and will continue to work during the transportation process, causing the concrete to be damaged during construction. The slump at the site is greater than the slump when it comes out of the machine, causing segregation of concrete. Therefore, when we make the concrete mix ratio, we must make several sets of mix ratios with different dosages to find the optimal dosage. Be economical and easy to construct.
3.4 Influence of external factors
There are many external factors that cause the segregation of concrete mixtures, mainly in the following aspects:
3.4.1 The measurement of the mixing equipment in the mixing station is inaccurate. The mix ratios of various materials and the laboratory are inconsistent, resulting in instability of the concrete and segregation. In this case, the metering system of the mixing station should be checked frequently, and if the problem is found to be solved in time, the manufacturer should be notified as soon as possible to solve it.
3.4.2 The mixing station operators have a weak sense of responsibility. They use the wrong materials or the water used to flush the mixer and tanker is not drained cleanly, causing the segregation of concrete. For such situations, supervisory personnel should strengthen their responsibilities to the mixing station operators. Careful education is required to check whether there is such a phenomenon before the concrete is opened.
3.4.3 The mixing time is too short and the admixture does not fully exert its effect, so that more water is added during mixing. During the transportation process, the admixture continues to play its role, causing excessive slump and segregation. In this case, the mixing station is required to meet the mixing requirements of the specifications.Allow time to mix, and do not shorten the mixing time arbitrarily.
3.4.4 If the on-site construction personnel do not carry out the construction according to the specifications, for example, if the chute or channeling barrel is not installed, or if it is installed but the length is not enough, the concrete will segregate during the construction process. In this case, on-site construction is required. The personnel shall strictly follow the construction methods required by the specifications and shall not cut corners.
.Conclusion
The reasons for segregation problems in concrete construction are not just the above. During the construction process, we should strictly control the quality of raw materials, strictly control the construction process, and start from the root cause. It can truly solve the common quality problem of concrete segregation, thereby ensuring construction quality and saving construction costs.
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