1.What is superplasticizer?
Superplasticizer ,is a kind of concrete admixture which can reduce the amount of water used for mixing under the condition of keeping the slump of concreteunchanged.Most of them are anionic surfactants, such as lignin sulfonate, naphthalene sulfonate formaldehyde polymer, etc.After adding concrete mixture, it can disperse cement particles, improve its workability, reduce unit water consumption and improve the fluidity of concrete mixture.Or reduce the amount of unit cement, but concrete superplasticizer too much, not only can not improve the mobility, but will lead to sand slurry segregation and bleeding water.
2.How does superplasticizer work?
Superplasticizer is essentially a kind of building materials, in use will produce foam and bubble and other sol bubbles, and they have different characteristics and formation mechanism, is caused by ionic superplasticizer.After adding the superplasticizer to the concrete, the superplasticizer will be adsorbed on the surface of cement particles. Generally, about 80% of the superplasticizer will be adsorbed by cement particles, which can keep the aggregation state without diffusion for a long time.Without superplasticizer in concrete , concrete will have a certain amount of air (2% or so), the size of the bubbles formed is uneven, and the distribution is very uneven.When superplasticizer is added to concrete, the distribution of concrete parameters can maintain good properties and improve the working performance of concrete, such as tensile strength of concrete.
In addition, adding superplasticizer in concrete mixing can help reduce the water consumption in concrete mixing, and the tiny sol air bubbles generated can help fill in the micro pores inside concrete, significantly improve the impermeability of concrete, and enhance its chemical corrosion resistance and aging resistance.The water-reducing agent can be integrated into the free water migration of internal materials, which is helpful to ease hydrostatic pressure and enhance the ability of concrete to resist repeated freezing-thawing cycle.Water-reducing agent can slow down the hydration reaction rate of cement in the middle and late stage, improve the strength of cement and enhance its anti-expansion performance.Therefore, when mixing concrete, in addition to considering sand, stone, cement and other conventional materials, appropriate addition of a certain amount of water reducer to improve the performance of concrete.
1. Influence of superplasticizer dosage in concrete
(1) Influence of water-reducing agent on slump of fresh mixed concrete
In the analysis of concrete workability and fluidity, a very key index is slump, the higher the value, the better the fluidity of concrete.Slump loss refers to the difference between slump and initial slump measured by concrete after a specific time. The influencing factors include cement, aggregate characteristics and water-cement ratio, etc. The type of water-reducing agent is also the key influencing factor.
2) Influence of water reducer on compressive strength of fresh mixed concrete
The concrete strength formed by different types of superplasticizer has different effects.By applying superplasticizer can significantly reduce the mixing of concrete mixing water consumption, improve the hydration degree of cement, improve concrete each age (3 d, 7 d and 28 d) intensity, can be from three consists of high efficiency superplasticizer of concrete compressive strength were significantly higher than that of blank concrete compressive strength, 3 d, 7 d and 28 d compressive strength increase rate is as high as 15%, 20% and 30% respectively.In addition, the type of concrete with the highest compressive strength in the early stage is the concrete mixed with polyhydroxyacid superplasticizer, which has the highest compressive strength after 28 days.However, the concrete composed of naphthalene series superplasticizer has relatively low compressive strength in the early stage, which indicates that its early strength performance is relatively poor.
3) Influence of superplasticizer on early cracking performance of fresh mixed concrete
Physical and chemical reactions may occur between cement and concrete with admixtures.One kind of superplasticizer has a relatively small slump in concrete made up of one kind of cement, while in another kind of cement, a relatively large slump loss may occur, so there is no superplasticizer that has a good effect on any kind of cement.In the actual concrete quality evaluation, the commonly used evaluation index mainly includes concrete slump and strength, but it is easy to ignore the concrete volume stability, easy to induce early shrinkage crack, may accelerate the concrete crack, affect the durability and safety of the building structure.From the point of view of the factors affecting the early cracking of concrete, apart from wind speed and ambient temperature, the types and amount of additives are also the key factors.
4) Influence of superplasticizer on the durability of fresh mixed concrete
Superplasticizer has a greater impact on the durability of concrete, mainly for: (1) the impact of concrete frost resistance.Considering that the smaller the water-cement ratio is, the better the freezing-thawing resistance of concrete will be. By adding water reducing agent with air-entraining effect, the freezing-thawing resistance can be significantly improved.The induced air volume should be controlled at about 2%, and the maximum should not exceed 6%, otherwise it will adversely affect the strength and durability of concrete.Effect of concrete impermeability.Water-cement ratio is the main factor affecting the impermeability of concrete. If its value is greater than 0.55, it will exceed the hydration water consumption of cement to form many bleeding holes in concrete and increase the permeability of concrete.If the water-cement ratio is too small, it will affect the workability of concrete, and is not conducive to the permeability of concrete.(3) concrete resistance to steel corrosion and the impact of carbonization performance through the selection of superplasticizer, can significantly slow down the carbonization rate of concrete, help to improve the carbonization performance of concrete, reduce the damage of steel corrosion.
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