Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

22 Chapter Three
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Compressive strength, psi
5000
Compressive strength, psi
Air-entrained concrete
6000
Non-air-entrained concrete
4000
5000
3000
28 day
4000
28 day
3000
2000
1000
7
3
1
2000
1000
7
3
1
0
0
0.4 0.5 0.6 0.4 0.5 0.6 0.7
Water-cement ratio, by weight
Water-cement ratio, by weight
FIGURE 3-1 Relationship between age and compressive strength of concrete for
Type I portland cement. Notes: (1) Courtesy, Portland Cement Association.
(2) Data based on compressive tests of 6- by 12-in. cylinders using Type I
portland cement and moist-curing at 70°F.
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action is the result of the setting of the cement, which may begin
within 30 min after the concrete, is mixed under favorable conditions,
namely, a warm temperature. This action may continue for several
hours, especially if the temperature is low. The latter action is the
development of internal friction between the particles of aggregate in
the concrete that restrains them from moving freely past each other.
The magnitude of the internal friction is higher in a dry concrete than
in a wet one, and it increases with the loss of water from a concrete.
Figure 3-1 gives illustrative relationships for age-compressive
strength of laboratory cured air-entrained and non–air-entrained
concrete with different water-cement ratios using Type I portland
cement, when the concrete is moist cured at 70 ° F.
Lateral Pressure of Concrete on Formwork
The pressure exerted by concrete on formwork is determined primarily
by several or all of the following factors:
1. Rate of placing concrete in forms
2. Temperature of concrete
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