It is mainly composed of Portland cement clinker, including: 1 Portland cement: both early strength and late strength are high, the strength increases faster at low temperatures than other types of cement, and has good frost resistance and wear resistance, but High heat of hydration and poor corrosion resistance; 2 Ordinary Portland cement: Except for early strength, which is slightly lower than Portland cement, other properties are close to Portland cement; 3 Slag Portland cement: Low early strength, strength increases in low temperature environment Slow, rapid strength growth in the later period, low heat of hydration; good resistance to sulfate erosion and heat resistance, but large dry shrinkage deformation, large water separation, and poor wear resistance; 4. Pozzolana Portland cement: early strength Low, low temperature environment strength growth is slow, high temperature and humidity environment (steam curing) strength growth is fast, low hydration heat, good resistance to sulfuric acid erosion, but large dry shrinkage deformation, large water separation, poor wear resistance; 5 fly ash silicon Salt salt cement: low early strength, lower heat of hydration than pozzolana cement, good workability, good corrosion resistance, small dry shrinkage, but poor frost resistance and wear resistance; 6 Composite Portland cement: Between 2 and 4, 3 and 5, when there is a small amount of mixed materials (less than 20%), its performance is similar to that of ordinary cement. As the amount of mixed materials increases, its performance also tends to be that of the mixed materials. Material cement.

Performance

Mix water

Aggregate moisture content: When designing the mix ratio, consider the influence of aggregate moisture content

Fine aggregate: The sand for concrete is divided into three grading areas according to the 0.63mm sieve hole: Area I is used for concrete with a strength level greater than C60; Area II is used for concrete with C30~C60 and frost resistance, impermeability or other requirements (preparation Concrete should be used first); Zone III is used for concrete and building mortar less than C30. When pumping concrete, medium sand should be used.

Coarse aggregate: gravel, pebbles, continuous particle size is suitable; larger particle size can save cement, but construction is difficult. Regulations: The maximum particle size of stones shall not exceed ¼ of the smallest structural section, the maximum particle size of solid slabs shall not exceed ½, and the particle size shall not exceed 50mm, and the maximum particle size shall not exceed ¾ of the minimum spacing of steel bars.

Concept: A comprehensive performance of the mixture that satisfies the convenience of pouring and obtains uniform compactness under certain conditions, and is evaluated by slump. It includes three meanings: 1. Fluidity: the ability of the mixture to flow when subjected to gravity or vibration; 2. Cohesiveness: the ability of the mixture to resist segregation. 3. Water retention: the ability of the water in the mixture to prevent precipitation (bleeding).

Influencing factors: 1. Cement type; 2. Water-cement ratio: too high will reduce the cohesion of the slurry and make it easy to bleed; 3. The amount of cement and water: the amount of water has an important impact on the slump and is the basis for determining its fluidity. Factors; 4. Sand rate: sand: the total mass of the aggregate. If it is too small, the slump will be small and the stones will easily segregate; if it is too high, the mixture will be dry and the slump will also become smaller. The mix ratio adopts a reasonable sand rate and has a maximum slump.

The strength level is determined by the compressive strength value of a 15cm square under standard curing (20±2°C, humidity >95%) for 28 days (indicated by C).

Influencing factors: 1. Basic composition of concrete; 2. Degree of mixing and vibration; 3. Maintenance: covering and watering within 12 hours, watering and curing for not less than 7 days, pozzolana cement, fly ash cement, impermeable concrete, retarder concrete Both are 14d

Measures to improve strength: 1. Increase cement strength grade and reduce water-cement ratio. If the water-cement ratio is lowered and the workability requirements are not met, a water-reducing agent can be added; 2. Use well-graded and clean coarse sand and high-strength stones; 3. Strengthen mixing and vibration molding; 4. Strengthen maintenance to ensure appropriate temperature and higher humidity (moist heat treatment-steam curing can also be used to improve early strength); 5. Add reinforcing materials: silicon powder, steel fiber, etc.

Volume deformation includes shrinkage deformation, dry shrinkage deformation, and temperature deformation. The key to construction is to control the deformation cracks of large-volume concrete and improve the durability of the structure. Mass concrete cracks are divided into two types: 1 surface cracks and 2 penetration cracks. 1 is caused by different heat dissipation conditions on the surface and inside, mostly in the early stage; 2 is shrinkage cracks (vertical cracks) caused by the temperature drop and evaporation of excess water after a few days of pouring, and occur in the middle or middle of the structural section close to the bedrock. throughout the structure. Controlling the hydration temperature rise of concrete, delaying the cooling rate, reducing concrete shrinkage, increasing the ultimate tensile strength of concrete, and improving restraint conditions are the starting points for preventing temperature cracks in large-volume concrete.

Measures to control cracks in mass concrete: 1. Reasonably select cement varieties and concrete mix ratios. Try to use cement with low hydration heat such as slag and volcanic ash, and use admixtures or admixtures to reduce the amount of cement; 2. Adopt layered or block pouring, and rationally set horizontal or vertical construction joints; set up post-pouring belts at appropriate locations. to relax the degree of restraint; 3. Extend the curing time and formwork removal time to slow down the cooling rate; 4. Embed cooling water pipes inside the concrete, and use circulating cooling water to forcibly reduce the hydration heat temperature of the concrete.

Including impermeability p, frost resistance F, erosion resistance, carbonization resistance, alkali aggregate reaction characteristics, etc.

Density and quality of material are key factors. Measures: 1 According to the concrete work requirements and environmental characteristics, rationally select cement, and select appropriate mixing materials and fillers; 2 Control the cement dosage and use a smaller water-cement ratio, limit the maximum water-cement ratio and the minimum cement dosage; 3 Use grade Prepare well-prepared and clean sand and gravel aggregates, and choose sand and gravel aggregates with larger or moderate particle sizes; 4. Select appropriate admixtures (water-reducing agents, air-entraining agents); 5. Increase the pouring density, including full mixing and vibration , strengthen maintenance, etc.; 6. Add a certain amount of mineral admixtures to the mix ratio.