3.4 The essential elements of mechanics and concrete technology

Concrete is considered as an artificial conglomerate, in which individual grains of fine and coarse aggregate bonded into a monolith of cement-sand-stone complex of the crystal structure. Due to the peculiarities of hardening of cement-sandy gel under conditions of excess of free water required to ensure sufficient mobility of the concrete mix, the concrete is formed as a material that includes all three phases of matter: solid, liquid and gaseous. The presence of numerous concrete then lets talk about him as a capillary-porous material.

Based on this definition, we emphasize that concrete is an artificial stone material, and this means that by applying the right technologies and high culture of manufacture it is possible to obtain concrete with the required performance properties, the most important of which are durability, resistance to external influences, water resistant and some others. Objectives improve the properties of concrete, creating a concrete new kinds of occupations such as science building chemistry and building physics, materials science (in which there is a separate section on "concrete technology"), etc. But an important role plays and the mechanics of concrete, which is seen in the experimental determination of strength and deformation characteristics of the new and improved concrete forming fundamental approaches to the problem of strength, microseminoprotein and deformability of concrete, the development of methods of VAT assessment of concrete structures etc.

The close connection of mechanics of concrete with other Sciences about the concrete makes it necessary in the framework of introductory lectures on mechanics of concrete consider some of the questions concrete technology.

Figure 29 – Components of concrete

The most common binder is Portland cement.

Portland cement is a hydraulic binder, which is a gray powder obtained by fine grinding of clinker with the inclusion of additives.

Clinker is produced by a uniform firing to sintering carefully controlled raw mix that contains approximately 75-78% CaCO3 and 22-25% CaO2+Al2O3+Fe2O3.

The filler has the following impact on the structure formation of concrete:

1) Creates a rigid frame, reinforced concrete structure in the first stage of hardening;

2) Affects the hardening of the cement stone (the hardening occurs in the thin films between the grains of filler);

3) Increases the water retention capacity of cement paste;

4) Constrains the shrinkage deformation of concrete;

5) Promotes the formation of a crystalline frame of the cement stone.

On the properties of concrete is influenced by:

- The composition and quality of raw materials;

Technology of production of concrete;

- The impact of the environment;

- Force action.

For a long time in the concrete changes of the porous structure in two ways: have a place as a structure-forming and destructive processes.

Concretes are classified according to the average density, type of binder, as directed.

Table 3 – Classification of concrete density

№	Name	Density (кг/м3)	Note	Application area
1	Especially heavy concrete	>2500	Filler – metal sawdust, shavings, ore.	At influence of radiation, x-ray radiation, in military construction.
2	Heavy concrete	2100-2500	Concrete on filler from dense rocks (granite, limestone).	Has the broadest application
3	The facilitated concrete	1800-2100	Filler – rocks with the lowered density of 1500-1600 kg/m3, concrete without sand.	Has broad application.
4	Light concrete	500-1800	On porous fillers (expanded clay, slag, pumice).	In low construction, in the protecting designs. + Low weight - Rather high water penetration, Rather low durability, rather low stickiness of concrete with fittings
5	Especially light concrete	до 500	Cellular filler (gas concrete, foam concrete

The density of concrete depends on the density of cement, type and density of aggregate, structural concrete.

Table 4 – Classification of concrete in appearance of the binder

№	Type of binder	Name	Application area
1	Various types of cement, including Portland cement	Cement concrete	Up to 70% of total concrete volume production
2	Lime	Silicate concrete	Only precast concrete because for the production necessary to use autoclave curing method
3	Polymeric binders (resins), curing in concrete with additives	Polymer concrete	Used to work in hostile environments

Table 5 – Classification of concrete for the purpose

№	Title, application.	Special requirements for this type of concrete
1	Normal concrete for concrete and reinforced concrete structures	Strength (compression), frost
2	Hydraulic concrete (for the dams, sluices, water channels)	The high strength, water resistance, frost resistance, low shrinkage, resistance to leaching during infiltration, low heat hardening
3	Concretes for enclosing constructions (light)	Low weight, relatively low thermal conductivity
4	Concrete floor, road and airport paving	High impact strength, flexural strength, resistance to aggressive environments
5	Special-purpose concretes (heat-resistant, acid-resistant)	Requirements defined purpose

In addition to the properties listed in table 5, all concrete should have the following properties: should have a degree of hardening rate, specific consumption of binder and the cost of concrete should be minimal.

Concrete artificial stone belong to the conglomerates, which are a kind of composite materials, therefore, for different concrete characteristic as your own (private) and the General properties and regularities.

In the process of manufacture and hardening of concrete can be divided into two stages, when a material which has significantly different properties and the state:

Stage 1 – prior to the cement setting and transformation of concrete into solid concrete mixture;

Stage 2 – the period of hardening and exploitation material possessing all the properties of solid concrete.

Formation properties of concrete begins with the preparation, laying and curing of concrete mix, so it is important to know the order of structure formation and properties of concrete to understand the peculiarities of the structure and properties of concrete.

Concrete mix is a complex multicomponent system polydispersity obtained when mixed with water cement mixture with a filler.

In its properties the concrete mix occupies an intermediate position between viscous liquids and solids. Unlike a viscous fluid: the presence of structural viscosity, i.e. the presence of a certain strength to its structure. Unlike solid: the lack of sufficient ability to maintain its shape and capacity for significant plastic deformation even at low loads.

The essential features of a concrete mix:

- The ability of a compound to increase the mobility under the influence of mechanical action;

- Change of properties (primarily loss of motion) under the influence of physico-chemical processes of interaction of cement and water until the setting of the mixture and turning it into a solid.

Capillary forces – the forces of surface tension in the water menisci at the contact of solids in the absence of excess amount of water.

The strength of flocculation (flocculonodular power) is force of surface interaction between solid and liquid phase, has an electrical nature (the attraction of water molecules to the oppositely charges of the crystals of cement).

Colloidal forces are interaction forces between the particles occur on the surface of the shell consisting of water, adsorption is associated on the surface of the solid phase.

Forming the concrete mix it is necessary to simultaneously solve three tasks: to provide sufficient connectivity, the mobility of the concrete mix with minimum water consumption.

To reduce flocculonodular forces using:

1) Increasing water-cement ratio (VAC) – increases the distance between particles.

2) Vibration (temporary destruction of flocal under the action of external forces);

3) Introduction of chemical additives: plasticizers and super-plasticizers (large molecules are adsorbed by the cement particles and prevent the formation of flocal);

4) Introduction of air-entraining additives (air bubbles play the role of sliding bearings).