Concrete raw materials are mixed with water to form concrete mixture, which has a certain fluidity, cohesion and plasticity. With the passage of time, the hydration reaction of the cementitious material continues, and the hydration products continue to increase, forming a condensed structure. At this time, concrete began to set and harden, and gradually lost fluidity and plasticity, and eventually formed cement stone with a certain strength. Concrete mixture before condensation hardening is usually called new mix concrete, concrete after condensation hardening is called hardening concrete.

1. The fluidity of newly mixed concrete
It is to show new mix concrete is in certain external force (gravity, vibrating force) the inside friction force that overcomes oneself below action and the work that do. Popular saying is under the action of a certain external force new mix concrete can flow far, whether to reduce the labor intensity of workers. There are many methods to measure fluidity, the most commonly used are slump (extension) test, Webb consistency test, density factor test and so on. And the slump test that we use the most in practice. Slump is measured with slump cylinder, slump cylinder size is 100mm in diameter, 200mm in diameter, high 300mm. When the test will be fresh mixed concrete into three layers of the tube, each layer with ramming bar from the edge to the center of the spiral insert ramming 25 times, insert ramming bar to insert the surface of a new layer of mixed concrete. After the three layers are installed, wipe the barrel, lift the cylinder vertically, and the new mixed concrete cone collapses under the action of gravity. Measure the height of the cone slump, which is the slump value, and measure the average diameter of the two vertical directions of the cone slump, which is the expansion degree. Under normal circumstances, the greater the slump value, the greater the expansion value, which means that the fluidity of the new mixed concrete is good, the relationship between the three is directly proportional under normal circumstances. For example 180mm slump of the new mix concrete is certainly better than 140mm slump of concrete fluidity, of course, slump has a limit value, the slump of ordinary concrete generally does not exceed 240mm, when more than 240mm May be concrete water consumption too much has been segregated.

(1) factors affecting the fluidity of freshly mixed concrete: the fluidity of freshly mixed concrete depends on the characteristics of each component and its relative content, especially the effect of water. The action of each component of concrete is interrelated.
A, the impact of water consumption on fluidity: water consumption in concrete is one of the most important factors affecting fluidity. The water consumption is directly proportional to the fluidity, and the greater the water consumption, the greater the fluidity.
B. Influence of sand rate on fluidity: Sand rate refers to the percentage of fine aggregate content in total aggregate. Sand rate has a great influence on the fluidity of freshly mixed concrete, and there is an optimal sand rate. If the sand rate is too large or too small, the fluidity of concrete is not good. When the optimal sand rate is reached, the fluidity of concrete is the best.
C. Influence of composition material characteristics on fluidity: water requirement of cement, aggregate gradation, particle shape, maximum particle size, surface state, water requirement of admixture, fineness, adaptability, water reduction rate of admixture, adaptability, quality of air entraining agent, etc. Will have an impact on liquidity

②, the slump loss of new mixed concrete: the fluidity of new mixed concrete changes with time, which is the inevitable process of concrete hydration hardening. As slump is usually used to evaluate the fluidity of commercial concrete, slump loss is also commonly used to represent the fluidity of commercial concrete according to the time change. The slump loss of concrete is a problem often encountered in the process of using commercial concrete. Causes of concrete slump loss are as follows:
A, admixtures and cementitious materials do not adapt;
B. The hydration speed of cementitious materials is faster;
C, the aggregate water absorption rate is large;
D. High temperature and evaporation of water;
E. Escape of bubbles.

2, the compressive strength of hardened concrete
The main factors affecting the compressive strength of concrete are:
(1) Water-binder ratio: refers to the ratio of water consumption and cementitious material consumption of unilateral concrete. The larger the water-binder ratio is, the larger the pores are and the lower the strength is.
(2) cement strength and dosage: the higher the strength of cement, the higher the strength of concrete; The higher the cement content, the higher the concrete strength.
(3) admixture: the greater the admixture amount, the lower the relative strength of concrete; The lower the activity of admixture is, the lower the strength is. The greater the water requirement of the admixture, the lower the strength.
(4), the maximum particle size of aggregate: under certain other conditions, the greater the maximum particle size of aggregate, the lower the strength, in high grade concrete this effect is greater, medium grade concrete is not so obvious, low grade concrete is the larger particle size, the higher the strength.
Age: the longer the age, the higher the strength.
⑥, temperature and humidity: the higher the temperature and humidity, the higher the intensity. ⑦, sand rate: the greater the sand rate, the lower the strength of concrete.

3. Durability of hardened concrete
(1) Permeability resistance: refers to the permeability ability of concrete to resist water. Impermeability grade is generally divided into P6, P8, P10, P12, "P8" refers to concrete can resist 0.8mpa pressure water infiltration. The old standard was represented by the symbol "S", which has the same meaning as the "P" of the new standard.
(2) Frost resistance: refers to the ability of concrete to resist freezing and thawing damage. In the cold area, especially in contact with water, concrete is often damaged by freezing. This is because of the volume expansion of water in concrete after freezing, is the internal concrete by a great deal of pressure, if repeated freezing and melting, concrete will eventually be destroyed. Concrete freeze-thaw resistance grade is generally divided into: F15, F25, F50, F100, F150, F200, "F100" refers to concrete can resist 100 times of freeze-thaw cycle effect.
(3) Chemical corrosion resistance: refers to concrete resistance to chemical substances (sulfuric acid, hydrochloric acid) erosion ability.
(4) high temperature resistance: refers to the ability of concrete to resist damage when strength is reduced at high temperature.