3.6 Description features of stress-strain state of concrete methods of solid mechanics

The differences of the real structure of concrete from the model of the continuum.

The concrete structure defines the following important terms of mechanics, properties of local heterogeneity, fracture, physical nonlinearity (the nonlinearity of the relationship between stress and deformation).

The continuum model is based on the hypothesis of the continuity of the body structure: the body is continuous before and after deformation.

Design properties of concrete differ significantly from the properties of a continuous medium:

1) Violation of geometric continuity (fracture);

2) Breach of physical continuity and homogeneity in the small volume, i.e. the concrete begins to appear as the material is not infinitely small volumes and in the final volume of the macrostructure;

3) the Probabilistic nature of the factors influencing the mechanical properties of concrete.

The main elements of overcoming the differences between the ideal continuous medium and concrete

1. The identification of specific volumes of structural heterogeneity;

2. The replacement of real stress and strain within the specified volume some smooth;

3. The linking of smoothed stresses and strains (physical ratio);

4. Accounting for the effects of gradients of strain and stress;

5. The special influence of the factors of defect (fracture) on the nature of deformation and fracture of concrete;

6. The given scale factor.

The characteristic elements of the concrete structure

A characteristic element of the concrete structure is a minimum amount of material, which manifest the properties of concrete as a material, and not the properties of the individual elements of its macrostructure.

The minimum size of a characteristic element l0 appoint the following considerations: l0=5*(d3+am) , where d3 is the maximum diameter of the granule of coarse aggregate, the am – middle layer to the matrix width.

For normal weight concrete l0=50 – 200mm.

Minimal dimensions of samples, according to tests that determined the strength and deformation properties of concrete should be appointed on the basis of l0 .

The question of transfer of the mechanical characteristics of the l0-element on the elements of real structures called "problem of small elements with inhomogeneous stress state".

The given scale factor

Under scale factor understand the dependence of material properties from that in what extent these properties are manifested. For concrete the scale factor into account particularly in relation to strength.

According to the statistical theories of strength scale factor may significantly occur in the fractured bodies, which possess a large variation in the amount of defects (cracks). Among these cracks out threat. It is believed that the probability increases for the appearance of cracks in a small volume less than the great that the probability of failure of a larger sample. In accordance with this theory the strength of concrete decreases with increasing of the sample volume at an exponential rate.

Cracks in the concrete and related properties

The following types of cracks in concrete:

1) Structural cracks – defects of a macrostructure of the concrete, usually located within a single typical element of the concrete structure.

2) Crack fracture of the structural elements – cracks that cross one or more structural elements, but also pose no danger to overall structure.

3) the Main cracks – cracks formed in the merger of cracks in the structures of the elements that characterize the destruction of the whole structure or important elements.

4) Technological cracks – cracks formed in the process of manufacturing. Technological cracks are considered uniformly distributed in the volume of the material.

5) the Power of a crack – cracks, resulting from the action of the load. Power cracks are sent, causing anisotropy of the material.

The defects especially in the top of the cracking occurs stress concentration, this concentration is characterized by the stress concentration factor (CCS), which is defined as the ratio of the stress values near the hub to a medium voltage in the area of concentration.

Due to the inhibition of cracking deterioration of concrete is of a plastic nature.

The failure mechanisms of concrete

Deterioration of concrete begins with the destruction of individual elements of its structure, and then results in the destruction of larger volumes.

Table 8 – the Main mechanisms of concrete degradation

Main mechanisms of concrete destruction
Detachable		Lateral
Intraareal	Intergranular	Intraareal	Intergranular

When vnutricerepnom the mechanism of destruction of concrete main crack passes through the grains of coarse aggregate (typical for concrete with porous filler), when intergranular mechanism in the bypass of filler grains.

Pure crack separation implemented under tension, pure shear mechanisms are rare, mainly under triaxial compression, in other cases is dominated by various mixed separated-shear failure mechanisms.

Mixed mechanisms of fracture:

1.Zigzag-crack

2. Branching zigzag crack

3.Often located cracks of separation are crossed by the shear crack.

4.To the particular mechanisms of fracture of concrete relates to the loss of stability of the narrow elements of the concrete between cracks of separation.

Main crack may include in its path the various local schemes of destruction. Generally, the more complex and diverse mechanism of destruction, the more deformability and strength. The process of destruction of concrete can be represented as a process of progressive destruction of continuity.

Under the dilation (dilatancy) understand the increasing volume of material (specimen) due to the deformation under certain types of stress States that do not have to call in isotropic materials the increase. Under the dilation of concrete means the increase of the volume of the material (sample) under axial and multi-axial compression in stages close to failure. Dilatation of the concrete due to the accumulation in the concrete structure micro-cracks and damage. The dilation of concrete, usually take into account in the calculation models of concrete.