Concrete raw materials are mixed with water to form concrete mix, which has certain fluidity, cohesiveness 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 begins to condense and harden, gradually lose fluidity and plasticity, and finally form cement stone with a certain strength. Concrete mix before solidification and hardening is usually called newly mixed concrete, and concrete after solidification and hardening is called hardened concrete.

1. Fluidity of newly mixed concrete
It refers to the work done by newly mixed concrete to overcome its own internal friction force under the action of certain external forces (gravity, vibrating force). Generally speaking, it is how far the newly mixed concrete can flow under the action of a certain external force, and whether to reduce the labor intensity of workers. There are many methods to measure the fluidity, the most commonly used are slump (expansion) test method, Wib consistency test method, density factor test method and so on. And we use the slump test method most in practice. Slump is measured by slump barrel. The size of slump barrel is 100mm in diameter at the top, 200mm in diameter at the bottom and 300mm in height. During the test, the newly mixed concrete is divided into three layers into the cylinder, and each layer is inserted 25 times with a tamper from the edge to the center. When inserted, the tamper should be inserted into the surface of the first layer of newly mixed concrete. After the three layers are installed, smooth the drum mouth and lift the cylinder vertically. The newly mixed concrete cone collapses under the action of gravity. Measure the height of the slump of the cone, that is, the slump value; measure the average diameter of the cone after the slump in two vertical directions, that is, the expansion degree. In general, the larger the slump value is, the larger the expansion value is, which means that the fluidity of newly mixed concrete is good. In general, there is a direct proportional relationship between the three. For example, the newly mixed concrete with 180mm slump is definitely better than the concrete with 140mm slump. Of course, slump has a limit value. The slump of ordinary concrete generally does not exceed 240mm.
① Factors affecting the fluidity of newly mixed concrete: The fluidity of newly mixed concrete depends on the characteristics of each component and its relative content, especially the role of water. The function of each component of concrete is interrelated.
A. Influence of water consumption on fluidity: Water consumption in concrete is one of the most important influencing factors of fluidity. The amount of water used is directly proportional to the flow. The greater the amount of water used, the greater the flow.
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 newly mixed concrete. There is an optimal sand rate of sand rate. If the sand rate is too large or too small, the fluidity of concrete is not good, and the fluidity of concrete is the best when the optimal sand rate is reached.
C, the influence of composition material characteristics on fluidity: the water requirement of cement, aggregate grading, particle shape, maximum particle size, surface state, water requirement of admixture, fineness, adaptability, water reduction rate of admixture, adaptability, quality of air entraining agent and so on. Will have an impact on liquidity
(2) Slump loss of newly mixed concrete: the fluidity of newly mixed concrete changes with time, which is the inevitable process of concrete hydration and hardening. Because slump is usually used to evaluate the fluidity of commercial concrete, slump loss is also commonly used to represent the time-dependent change of commercial concrete fluidity. Slump loss of concrete is a common problem in the use of commercial concrete.
The causes of concrete slump loss are as follows:
A. The admixture is not suitable for the cementitious material;
B. Hydration speed of cementitious materials is faster;
C, the aggregate water absorption rate is large;
D, high temperature, evaporation of water;
E. Escape of bubbles.

2. Compressive strength of hardened concrete
The main factors affecting the compressive strength of concrete are as follows:
(1) Water-binder ratio: refers to the ratio of the water consumption of single concrete to the amount of cementing material. The larger the water-binder ratio, the larger the pore size and the lower the strength.
(2) Cement strength and dosage: the higher the strength of cement, the higher the strength of concrete; The higher the amount of cement, the higher the strength of concrete.
③. Admixture: the larger the dosage of admixture is, the lower the relative strength of concrete is; The lower the activity of admixture, the lower the strength; The greater the water requirement of the admixture, the lower the strength.
(4) The maximum particle size of aggregate: under certain other conditions, the larger the maximum particle size of aggregate is, the lower the strength is. In the case of high grade concrete, the greater the influence is, the middle grade concrete is not obvious, but the lower grade concrete is the bigger the particle size is, the higher the strength is.
⑤, age: the longer the age, the higher the strength.
⑥, temperature and humidity: the higher the temperature and humidity, the higher the strength. ⑦, sand rate: the larger the sand rate, the lower the concrete strength.

3. Durability of hardened concrete
(1) Permeability resistance: refers to the permeability of concrete against water. The impermeability grade is generally divided into P6, P8, P10 and P12. "P8" means that concrete can resist the infiltration of water under 0.8MPa pressure. The old standard is represented by the symbol "S", which has the same meaning as the new standard "P".
(2) Frost resistance: it refers to the ability of concrete to resist freezing and thawing damage. In cold areas, especially where it is in contact with water, concrete is often damaged by freezing. This is due to the volume expansion of the water in concrete after freezing, which is caused by great pressure inside concrete. If it is repeatedly frozen and melted, concrete will eventually be damaged. The freeze-thaw resistance grade of concrete is generally divided into F15, F25, F50, F100, F150 and F200. "F100" means that concrete can resist freeze-thaw cycles for 100 times.
③ Chemical corrosion resistance: It refers to the ability of concrete to resist the erosion of chemical substances (sulfuric acid, hydrochloric acid).
(4) High temperature resistance: it refers to the ability of concrete to resist damage when its strength decreases at high temperature.