Indexes of concrete density

The concrete types according to the compactness	Water impermeability grade	Water absorption, % by weight	W/C not more
Ordinary dense	W4	5.7...4.8	0.6
High dense	W6	4.7...4.3	0.55
Especially dense	W8	4.2	0.45

The crack-resistance of concrete in a great extent is bound with it deformation properties. Deformations of concrete arise up both as a result of hardening (shrinkage and swelling) and under the action of external forces.

Concrete shrinkage takes a place as a result of deriving of water remains at the drying. It ranges, as a rule, from 0.2 to 0.4 mm/m in annual age and grows with the increasing of W/C, cement content, at introduction of active mineral additives. The concrete shrinkage changes approximately proportionally to the time logarithm and especially quickly increases in the first 28 days (Fig. 10.13).

The concrete deformations at hardening essentially decrease at introduction of steel reinforcement. The shrinking deformations in concrete conduce to the crack formation and violation of integrity nature of buildings. Especially it affects on the thin-walled structures from the prestressed reinforced concrete.

One of criteria of crack-resistance of concrete is its conditional extensibility:

, (10.10)

де - ultimate tensile strength; - dynamic modulus of elasticity.

T he irreversible deformations of creep are developed under operating of the constant load on a concrete during the long time. The creep of concrete is increased at loading of structures in early age, increasing of water-cement ratio, cement content, humidity of concrete and duration of imposed loading. It depends on the type of cement and it mineralogical composition, and also from mechanical properties and content of aggregates. Concrete on the high-strength low- aluminate Portland cement is characterized by a lower creep.

The creep of concrete causes enhanceable bends and losses of previous tension in the reinforced-concrete structures. At the same time, creep deformation of can compensate the tension, which grows as a result of exothermic processes and shrinkage of the cement stone in the massive constructions.

The greater dimensional instability is characteristic for light-weight concrete, than for heavy. The module of elasticity, for example, of claydite concrete is at 20...50 % smaller than module of resilience of heavy-weight concrete of the same class. The module of elasticity diminishes the more deformative is the aggregate and the higher its content is. Maximum extensibility of light-weight concrete in 4...5 times is higher, than heavy, and arrives at 0,03...0,4 mm/m. The enhanceable extensibility and low module of elasticity of light-weight concrete promotes its crack-resisting and allows effectively using him in different buildings.

General shrinkage of light-weight concrete is in15...30 % higher than shrinkage of heavy-weight concrete. The shrinking deformations go down at diminishing of water and cement contents.