Influence mechanism of superplasticizer on concrete properties
Superplasticizer is essentially a kind of building materials, in the use of foam and bubbles and other sol bubbles, and they have different characteristics and formation mechanism, is caused by ionic superplasticizer. After adding superplasticizer in the concrete mixture, superplasticizer will adsorb on the surface of cement particles. Generally, about 80% of superplasticizer will be adsorbed by cement particles, so that they can maintain aggregation state for a long time without diffusion. When not adding superplasticizer, concrete will have a certain amount of gas content (about 2%), formed by the size of the bubble is uneven, and the distribution is very uneven. After adding superplasticizer to concrete, the distribution of concrete parameters can maintain good properties and improve the working performance of concrete, such as the tensile strength of concrete. Besides, adding superplasticizer in concrete mixing will help reduce water consumption in concrete mixing. The tiny sol air bubbles generated will help fill the micro-pores in concrete, significantly improve the anti-permeability of concrete, and enhance its chemical anti-corrosion and anti-aging ability. The superplasticizer can be integrated into the free water migration of internal materials, which helps to alleviate the static superplasticizer pressure and enhance the resistance of concrete to a repeated freeze-thaw cycle. Superplasticizer can slow down the hydration reaction speed 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 superplasticizer to improve the use of concrete performance.
1) Influence of superplasticizer on the slump of freshly mixed concrete
When analyzing the workability and fluidity of concrete, a very key index is a slump. The larger the value, the better the fluidity of concrete. Slump loss refers to the difference between the concrete slump measured again after a specific time and its initial slump. The influencing factors include cement, aggregate characteristics and water-cement ratio, etc. The type of superplasticizer is also a key influencing factor. In the early 30min, naphthalene series superplasticizer had the maximum concrete slump loss, accounting for 60%, followed by melamine series superplasticizer agent (30%) and polycarboxylic acid series superplasticizer (20%), which indicated that polycarboxylic acid series superplasticizer had the best concrete slump protection. From its formation mechanism, department of polycarboxylic acid superplasticizer in concrete material will be adsorbed on the surface of cement, according to the trapezoidal way lets the cement molecular polymer adsorption layer formed by the electrostatic repulsion more dispersive effect, enhance its stability, so concrete adding polycarboxylic acid system after superplasticizer, can significantly reduce the slump loss.
(2) Influence of superplasticizer on compressive strength of freshly mixed concrete.
The strength of concrete composed of 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. Besides, the type of concrete with the highest compressive strength in the early stage was the concrete mixed with polyhydroxy acid superplasticizer, and its compressive strength value was also the highest 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 the early cracking performance of freshly mixed concrete
Physical and chemical reactions will occur between concrete with admixtures, cement, and admixtures. A kind of superplasticizer is in the concrete that some kind of cement place forms relatively small slump, appear in another kind of cement however relatively big slump loss may appear, do not exist so the superplasticizer that has a good effect to any kind of cement. In the actual evaluation of concrete quality, commonly used evaluation indicators mainly include concrete slump and strength, but it is easy to ignore the volume stability of concrete, easy to induce early shrinkage cracks, may accelerate concrete cracking, affect the durability and safety of the building structure. From the factors that affect the early cracking of the concrete, in addition to wind speed and ambient temperature, the type and amount of admixture are also very important factors.
The test shows that the maximum crack width and crack area of concrete formed by melamine-based superplasticizer are the smallest, among which the crack area is only 76% of the corresponding value of polycarboxylic acid superplasticizer and 52% of the corresponding value of naphthalene superplasticizer. The maximum crack width was only about 40% of the corresponding value of polycarboxylic acid and naphthalene superplasticizer. In the early crack cracking time of concrete, the influence of admixture variety is not significant, which is related to admixture composition and other factors.
(4) Influence of superplasticizer on the durability of freshly mixed concrete
Superplasticizer on the durability of concrete has a greater impact, mainly manifested as: (1) the impact of concrete frost thaw resistance. Because of the smaller the water-cement ratio, the better the freeze-thaw resistance of concrete. By adding superplasticizer with air entrapment effect, the freeze-thaw resistance of concrete is significantly improved. The volume of air intake should be controlled at about 2%, and the maximum should not exceed 6%. Otherwise, the strength and durability of concrete will be adversely affected.
(2) the influence of concrete impermeability. Water-cement ratio, as the main factor affecting the impermeability of concrete, if its value is greater than 0.55, it will exceed the water consumption of cement hydration and form many bleeding holes in concrete, increasing the permeability of concrete. If the water-cement ratio is too small, the workability of concrete will be affected and the anti-permeability of concrete will be adversely affected.