The workability of concrete contains several indicators such as fluidity and cohesiveness. In the practice of concrete production, the problem of poor cohesiveness of concrete in low strength grade is often encountered, such as exposed aggregate, slurry stone separation, poor water retention. Sometimes concrete in the condition of normal water-reducing admixture quantity is free of segregation phenomenon. But the phenomenon of exposed aggregate is quite significant, that is, the slurry can not be well wrapped around the gravel. Relying on simple increase in sand ratio can not fundamentally solve the problem. When the water-reducing admixture is reduced by 0.1% to 0.2%, concrete mix liquidity becomes poor, and slump loss is accelerated. Concrete slump loss more than 60mm after 30min out of the machine, resulting in construction difficulties on site.
1、Sand sieve analysis
The quality and particle gradation of fine aggregate will affect the workability of concrete. In particular, the content of fine particles of 0.30mm and below 0.15mm in fine aggregate is crucial to the cohesiveness and water retention of concrete. Domestic and international standards are recommended to reach 15% and 5% or more, respectively. For low and medium strength grade concrete, sand fineness modulus is suitable for about 2.5, and particle size less than 0.30mm particle content is suitable for 20%. When concrete mix cohesiveness is poor, it can be properly compounded with fine sand to increase the content of particles below 0.30mm to improve cohesiveness. When there is no fine sand, you can increase the amount of fly ash.
The fine aggregate in concrete fills the coarse aggregate voids, and the slurry fills the aggregate voids. Slurry is composed of rubber, water, admixtures and air content. Under normal conditions, the volume of slurry for a single square of low and medium strength grade concrete is 0.28 to 0.33 m3. If the slurry content is insufficient to effectively fill and wrap the aggregate, it is easy to cause poor cohesiveness.
For the poor cohesiveness of concrete caused by insufficient slurry, you can keep the water to binder ratio unchanged and increase the water consumption appropriately to improve the quantity of slurry. Or add air-entraining component admixture to increase the air content of concrete. However, it should be noted that the air content increased by 1%, the concrete strength will be reduced by about 4%.
3, Compatibility of admixtures and cementing materials
The admixture can improve the working performance of fresh concrete. However, in production practice, it is often encountered that the admixture is poorly compatible with the raw material of concrete. The compatibility of admixtures and concrete raw materials is an important issue that should not be ignored, especially the compatibility of admixtures and cement and mineral admixtures. When the admixture is low, the concrete fluidity is poor and the slump loss is accelerated. Increase the amount of admixture is easy to lead to slurry and aggregate separation and worse adhesion, which is a typical manifestation of poor compatibility between admixture and other raw materials.
Due to the poor cohesiveness of concrete caused by poor compatibility of admixtures and concrete raw materials, it should be adjusted to the admixture adaptability.
Sand ratio has an important influence on the workability of concrete. Generally speaking, as the sand ratio increases, the cohesiveness of concrete increases first and then decreases. There is an optimal sand ratio. When the actual sand ratio is lower than the optimal sand ratio, the mixed aggregate composed of sand and gravel aggregate has a larger porosity. More slurry is needed to fill the void, and the thickness of slurry wrapped around the surface of aggregate becomes thinner, and the cohesiveness becomes worse.
When the sand ratio is greater than the best sand ratio, the amount of fine aggregate sand is too large, resulting in a porosity greater than the optimal porosity, which also causes the thickness of slurry wrapped in the surface of aggregate becomes thinner, and the cohesiveness of concrete becomes worse. In production practice, the sand ratio can be properly adjusted according to the conditions of practice.
There are many factors affecting the cohesiveness of concrete. Sometimes it is the influence of a single factor, while sometimes it is the combined influence of multiple factors. Therefore, when encountering this problem, it can not be one-sided to analyze, it should analyze from multiple perspectives to find the reasons and test arguments one by one.