3.5 Structure formation and concrete structure

The formation of the concrete structure is in the process of solidification (hardening) of the concrete mixture and subsequent curing of concrete.

For hydration of cement is required less than 5% of the water from its total number used in the preparation of concrete for reasons of ensuring its mobility.

Table 6 – Water in the concrete mix

The nature of communication	Terms and causes of communication	% Water of the total amount thereof in the concrete mix
		Start mixing	In setting period
Chemical	The hydration of cement and the crystallization solution	1-2	4-5
Physico-chemical adsorption	The adsorption zone of molecular force fields solids	3-5	20-25
Structural	Capturing water in the capillaries, pores and other defects	93-95	70-75

The presence of excess water largely determines the porosity structure of concrete and creep.

Table 7 – characteristics of the process of structure formation of concrete at various stages of cement hydration

Period hydration	Since time began mix	The formation of supersaturated solution of calcium hydroxide by hydrolysis techkaraoke silicate, the deposition of the first growths of calcium hydroxide
I	0-1 hour	The formation of supersaturated solution of calcium hydroxide by hydrolysis techkaraoke silicate, the deposition of the first growths of calcium hydroxide
II hidden hydration	1-6 hours	Participate in the hydration of the superficial layers of cement grains, at the beginning of this stage are vital fluconazolee power
	7-24 hours	Is absorbed by the surface shells of the cement grains water, reducing the mobility of the concrete mix. The inner layers of cement grains by reacting with water, tend to expand, resulting in easier access of water into the grain. The formation of the initial frame of the primary products of cement hydration.
III	1-28 days	1. Intensive crystallization of calcium hydroxide from solution. The formation of long fibers through the pores, crushing of the pores, which increases the adhesion between hydrate forms and grains of cement . The formation of cement stone. 2. The seal structure due to the formation of hydrate forms in the pores. The pore size decreases, the increased strength of the structure, in fully gidratirovannom cement always remain internal voids, called pores of the gel. The size of these voids is very small , so they do not possible formation of tumors. If the porosity of the gel exceeds 28%, which can form tumors that will reduce the porosity.

3.5.1 The main types of concrete structures

The structure of the concrete mixture and is maintained during curing so the concrete structure would be most appropriate to classify the content of the cement stone. However, the most important property of concrete is density or porosity, which determine its basic performance properties, so the concrete structure is classified based on density.

The main types of structural concrete.

For the following types of concrete structures made the following:

R1 – strength matrix (cement-sand-stone);

R2 – the strength of the filler;

Rb – the strength of concrete.

Dense structure R1 < Rb < R2

Dense structure of the porous filler R2 < Rb < R1

The cellular structure R6 < R1,R2 = 0

4. Granular structure R2 < R6,R1 = 0

3.5.2 Levels of the concrete structure

There are several levels of structure of concrete: micro-, meso - and macrostructure. Each level in the structure assumes the presence of two components. For microstructure, the grain of hydrated cement and pores; mesostructure grain of sand and a layer of cement stone and, finally, the macrostructure includes grain coarse aggregate and a layer of cement-sand-stone.

Various studies related to the evaluation of the deformation and fracturing of the concrete, show that the formation and propagation of cracks, eventually leading to the destruction of the concrete, mostly at the contact zone of the cement-sand-stone-grain coarse aggregate", that allows to speak about the possibility when evaluating the results of mechanical force action to limit its consideration of the macrostructure of concrete. The macrostructure it is possible to estimate the properties of concrete in General and to the characteristics of cement-sandy stone, coarse aggregate, and surface area.

The macrostructure of concrete leads to the main features of the behavior of concrete under load: a significant difference in the compressive strength and the tensile strength, the ability to large plastic deformation under compression, the dependence of the elastic-plastic properties from the loading level, the effect of loading rate on strength and deformation properties of concrete and also determines the causes of microseminoprotein in the concrete and the nature of the development of microcracks.

3.5.3 Contact with the strength of the concrete structure and its density

The strength of concrete, as well as other stone materials depends on their density, because it determines the packing density of the structural elements of the scope and nature of the defects (pores, microcracks, etc.) the density of concrete is proportional to its density.

In the first approximation, it is possible to allocate two basic factors related to the structure of the concrete affecting the strength and deformation properties of the material as a whole:

1. The properties of the contact zone around the grains of coarse aggregate that enables collaboration of core and matrix;

2. The presence of defects in the concrete structure, including:

structural cracks in the matrix and at grain boundaries of the filler;

- large pores and capillaries;

- voids under coarse aggregate grains;

- loosened pores structure of the cement stone under separate grains of coarse aggregate etc.

The unit cell of the concrete structure.

The heterogeneity of the concrete structure requires the concrete probabilistic and statistical methods (especially when assigning strength and deformation characteristics) and should be considered in the calculation of concrete structures.