Concrete mixtures are convenient for construction operations (mixing, transportation, pouring, and vibrating) under certain construction conditions. And can obtain the properties of uniform quality and dense molding - called "workability".

1. Easy to form and dense

It refers to the performance of the concrete mixture that can flow under its own weight or external force and fill the formwork surface evenly and densely.

It means that there is a certain cohesion between the materials of the concrete mixture, so that delamination and segregation will not occur during construction.

It means that the concrete mixture has a certain ability to retain moisture and prevent it from leaking out.

fluidity

cohesiveness

Water retention

The fluidity (slump) of the concrete mixture should be determined based on the structure type, component cross-section size, steel bar density, construction technology and tamping method. The basic principles are:

(1) When the cross-sectional size of the structure or components is large, select a smaller slump.

(2) When the steel bars of the structure and components are sparsely arranged, select a smaller slump.

(3) When mechanically vibrating, choose a smaller slump; when manually vibrating, choose a larger slump.

(4) The principle of selecting slump should be to use a smaller slump as much as possible to save cement and obtain higher quality concrete under the conditions of meeting the construction requirements.

①Unit water consumption W (cement slurry consumption)

③Sand content (Bs or Sp)

Adjust material composition

"C" is the concrete strength symbol, and the number after "C" is the standard value of concrete cube compressive strength.

For hydraulic mass concrete, the design age is generally not 28d, but 90d or 180d.

Strength conversion coefficient P85

The splitting tensile strength is low, generally 1/10 to 1/20 of the compressive strength.

Cement strength and water-cement ratio

⑵Quality and variety of aggregates

⑶Maintenance conditions

(4) Maintenance period

(5)Test conditions

(6)Construction technology

①Using high-strength cement and low water-cement ratio

②Incorporate concrete admixtures and admixtures ③Use moist heat treatment (steam curing, autoclave curing) ④Use mechanical stirring and vibration

The shrinkage value is small and cannot be restored. It has no destructive effect on the concrete structure, but micro cracks may occur inside the concrete and affect the load-bearing performance and durability.

Beneficial deformation of concrete

Dry shrinkage deformation is more harmful to concrete. Dry shrinkage can cause greater tensile stress on the surface of the concrete and lead to cracking, thereby reducing the durability of the concrete such as impermeability, frost resistance, and erosion resistance.

Temperature deformation refers to the thermal expansion and contraction deformation of concrete as the temperature changes.

When the temperature changes greatly, it will have an impact on the concrete; in addition, the large temperature difference between the inside and outside of the concrete structure will also lead to temperature deformation of the concrete, which can easily cause temperature cracks in the concrete.

The deformation and damage of concrete under short-term load is mainly due to the occurrence of plastic degeneration of concrete, which causes internal cracks and gradually expands.

The phenomenon that the deformation of concrete increases with time under continuous load is called creep.

Concept: Concrete impermeability - the ability of concrete to resist the penetration of pressure water. (It is not only related to the water-retaining and waterproofing effect of concrete, but also directly affects the frost resistance and erosion resistance of concrete.)

Water-binder ratio (W/B) → directly affects the impermeability of concrete.

For details, see Chapter 1: Material Durability

For C30 concrete with an impermeability grade of w8, its water-cement ratio should not be greater than (0.55). P94

Concept: Frost resistance of concrete - refers to the ability of concrete to resist freeze-thaw cycles (center temperature of specimen [-18±2]℃～[5±2]℃) without damage when in a water-saturated state. Strength There is also no significant performance degradation.

Concrete anti-freeze grade: expressed by the maximum number of freeze-thaw cycles it can withstand when the relative dynamic elastic modulus drops to 60% of the initial value or the mass loss reaches 5% under saturated water conditions. Such as: F50, F100, F150, F200, F250, F300.

Water-binder ratio (W/B) → is the main factor affecting the frost resistance of concrete.

Improve construction technology and adopt a reasonable maintenance system to improve concrete density

Control the maximum water-cement ratio and minimum cement dosage

Reasonable selection of cement and aggregates

Add air-entraining agents to improve pore characteristics and reduce open connected pores

Spillway concrete (suffered by sand-laden high-speed water flow), road pavement concrete (suffered from repeated impact loads and cyclic wear), etc., require high wear resistance.

For structures such as channels and spillways with uneven surfaces, sudden changes in section, or sharp turns, the concrete is required to have anti-cavitation properties to avoid cavitation when high-speed water flows through it, causing high-frequency, local, and Impact stress causes concrete erosion.

When concrete is corroded by environmental water, the essence is that the cement stone structure is corroded.

CO2 in the air diffuses into the interior of the concrete through the capillary pores. In the presence of moisture, it reacts with Ca(OH)2 to form CaCO3, which reduces the concentration of Ca(OH)2 in the concrete, which is called "carbonization ( or neutral)".

Main factors affecting carbonization

Beneficial effects of carbonization

Adverse effects of carbonization

Excessive alkali in the raw materials of concrete will react with the active ingredients in the aggregate. The reaction products will absorb water and expand, causing internal stress in the concrete, causing the concrete to expand, crack, and lose design properties. This phenomenon is collectively called "alkali-active aggregate reaction" .

① Excessive alkali content: The alkali content in concrete is high (For example, the alkali equivalent content of cement is >0.6% or the alkali content of concrete is >3kg/m3).

②Active aggregate: The aggregate contains active ingredients and exceeds a certain amount.

③Moisture: Humid environment, with sufficient supply of moisture or humid air.

Concrete expands, cracks, and even destroys

Precautions

Measures to improve the durability of concrete