.
.10)
f ag
.
141
From Table 7.2 the coefficient α can be found.
Table 7.2
Coefficient α (plastic consistency of concrete mixes)
Value of α at water-cement ratio
Cement content,
kg/m3
0.3
0.4
0.5
0.6
0.7
0.8
250
–
–
–
1.26
1.32
1.38
300
–
–
1.3
1.36
1.42
–
350
–
1.32
1.38
1.44
–
–
400
1.31
1.4
1.45
–
–
–
500
1.44
1.52
–
–
–
–
600
1.52
1.56
–
–
–
–
Notes: 1. Water demand of fine aggregates is equal 7%.
2. For stiff concrete mixes (C < 400 kg/m3) α = 1.05...1.15.
142
7.4. Correction of design concrete compositions
Inevitable deviations of actual indexes of properties of concrete mixes and
concretes from a calculated stipulate certain approximations of calculated
compositions of concrete. Adjustment of calculated compositions is made
experimentally in a laboratory. Depending on possibilities of testing laboratory
and terms of construction works, an amount of laboratory works at
experimental correction of composition of concrete can be different. Complete
adjustment can be at experimental correction of all parameters of concrete
mixture: water content, water-cement ratio, ratio between different aggregates,
volume of the entrained air. Sometimes, incomplete laboratory adjustment is
possible (for example, only correction of water content, providing given
consistency with subsequent correction of other parameters of mixture after
production of concrete).
It is necessary to take into account that at production conditions, sand and
crushed stone (gravel) have some humidity unlike laboratory (nominal)
compositions of concrete, which define for dry initial materials.
143
At manufacture of concrete, the quantities of fine (F
, kg/m3) and coarse
p.ag
(C
, kg/m3) aggregates increase on mass of water which is:
p.ag
F
= F ⋅ 1
( + H