,

(4.

10)

n =

28 lg 28

where n – duration

R, MPa

of concrete

hardening, R –

28

concrete strength at

28 days.

28 days 1 year 2

4

6

11 years

Age

Fig. 4.6. Increasing of strength of concrete (R)

in wet (1) and dry (2) conditions

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Compressive strength, % of 28-day

moist (normal) - cured concrete

Compressive strength, %

of 28 day concrete

Temperature of curing, 0C

Fig. 4.7. Increasing of strength of fresh concrete during 28

Age, days

days at temperature (t) from +20 to –100C

Fig. 4.8. Typical relationship between strength

and duration of curing for different conditions:

1-moist (normal) curing; 2-curing in live stream at

atmospheric pressure (800C max. steam temperature);

3-curing in high-pressure-steam autoclaves

84

4.5. Kinds of strength. Tests for concrete strength

The main kind of strength concrete is compressive strength that

correlates with tensile strength, shear strength, flexural strength

and other kinds of strength.

The values of concrete strength are greatly influenced by the

features of tester machines, conditions of test, and form of

specimens.

Various nondestructive tests (rebound, penetration, pullout,

vibration and other methods) are widely used in practice for

determination of strength of hardened concrete based on

relationship between strength and indirect evaluations.

For strength evaluation of hardened concrete by nondestructive

methods calibration charts are used, which related by measured

indirect evaluation to the compressive strength of concrete.

85

Rfl, Rtn, MPa

Rcmp, MPa

Fig. 4.9. Typical relationship between flexural

strength Rfl (curve 1), tensile strength Rtn (curve 2)

and compressive strength (Rcmp) of concrete

86