What is a concrete admixture?
Concrete additive is an admixture added during concrete mixing. It is mainly used to improve the performance of concrete. From the point of addition ratio of admixture, it is generally not more than 5%. Although it contains a small amount, it has played an active role in improving the performance of concrete and has become an indispensable part of the current development of concrete. The additives used in concrete mainly refer to different types of water reducing agents. The extensive promotion of concrete admixtures has led to significant improvements in the strength of the concrete, its brittleness and its own weight, and has improved the applicability of concrete construction. For example, adding antifreeze to concrete can make concrete construction work in the cold winter. The use of superplasticizers enables the renewal of concrete placement and flow regimes. Regardless of China's environmental protection or economic benefits, the emergence of concrete additives has greatly improved the level of technological development in the construction industry.
Superplasticizers can reduce the amount of water used for concrete mixing by at least 10%, or significantly increase the flow rate of concrete. For concrete aged 3 days, the strength of 砼C30 can be increased by 69 mpa, and the concrete strength at 28 days of age is increased to at least 87 mpa. Commonly used superplasticizers are mainly polyalkyl aryl sulfonates and melamine water reducing agents. The effects of superplasticizers on concrete performance are mainly as follows: (1) In terms of the properties of freshly mixed concrete. For the water reducing effect of the superplasticizer, the molecular size and specific structure type of the water reducing agent used should also be considered. The bleed air effect of the water reducing agent is affected by the surface tension of the aqueous solution. The more the surface tension capacity is reduced, the more obvious the bleed air effect. In terms of concrete setting time, naphthalene and melamine can advance the concrete coagulation time, and the sulfamate superplasticizer can slow the setting time. Although the superplasticizer is not adaptable to different cements, the use of superplasticizers can reduce the segregation and bleeding phenomena. The slump of the concrete can be improved by the addition of the superplasticizer. The specific slumping time and extent are specifically determined by parameters such as the type and amount of water reducing agent used. (2) The effect on the hardening properties of concrete. Cement incorporating a superplasticizer can improve the degree of hydration. The concrete compression and bending strength are improved. Superplasticizers also reduce the amount of cement by changing the shrinkage value of the concrete. However, the change in the telescopic value generally does not exceed the standard value of 1X10-4. (3) The impact on the durability of concrete. The high-efficiency water reducing agent effectively improves the anti-freeze and anti-thaw properties of concrete due to the high water reduction rate and trace amount of bleed air. And high-efficiency water reducer can effectively improve the ability of concrete to resist sulfuric acid corrosion. Experiments have shown that the resistance to sulfuric acid corrosion of the superplasticizer is not worse than that of blank concrete. (4) The anti-corrosion protection effect of steel bars. Concrete with high-efficiency water reducer can bond well with steel bars, and the adhesion of straight-slide steel to concrete 7D can be improved from 1.2MPA to 8.5MPA. The adhesion of the bent steel to the concrete 7D can be increased from 15MPA to 27.5MPA. The superplasticizer can also effectively protect the steel in the concrete.
The adaptability of concrete admixtures. For all kinds of concrete admixtures, the role of precipitants is mainly to reduce water consumption, increase the density and elasticity of concrete, and make concrete durable. The air entraining agent can improve the solubility of the concrete, and reduce the internal crack of the concrete by improving the internal structure of the concrete, so that the concrete can improve its antifreeze and impermeability. For concrete with higher frost resistance requirements, it is possible to properly perform bleed air or a mixture of precipitation additives. For concrete with higher impermeability requirements, the density of concrete must be increased by high-performance precipitants. At the same time, the air entraining additive is used to reduce the occurrence of water seepage and improve the impermeability of the building structure. In addition, considering the corrosive phenomenon of steel bars in concrete, the use of precipitation agents can effectively reduce the carbonization rate of concrete. Once the pH in the concrete drops, the rust of the steel will also decrease. For the use of various types of concrete admixtures, it is necessary to make a correct choice in combination with different concrete performance requirements, and to achieve the greatest improvement in concrete performance through scientific proportional control.