Lateral Pressure of Concrete Forms for Wall
Formwork should be designed to resist the lateral pressure loads exerted by the newly placed concrete in the forms. If concrete is placed rapidly in wall or column forms, the pressure can be equivalent to the full liquid head pressure. This requires that rate of placement exceed the initial set time of the concrete mix. Excessive deep vibration can liquefy the initial set of concrete within the effective coverage of the vibrations. Retarder additives or cool weather can also delay the initial set and result in higher than anticipated lateral pressure. The formula for wall pressure established by the American Concrete Institute (ACI-347) considers the mix temperature and the rate of placement of concrete. The rate of placement is expressed in terms of feet per hour of concrete rise in the forms. Table 7.1 shows pressure values for concrete walls of different temperature and rate of filling.
1. For columns and walls with rate of placement less than 7 ft/h (2.1 m/h);
with a maximum of 3000 psf (1.47 kgf/cm2) for columns, 2000 psf (0.98 kgf/cm2) for walls, a minimum of 600 psf (0.29 kgf/cm2), but no greater than 150h (0.24hst).
where p = lateral pressure (lb/ft2)
R = rate of placement, ft/h T = temperature of concrete in the form °F h = height of the form, or the distance between construction joints, ft
pM = 0.073 + – r (metric equivalents)
Tq + 17.8
+ 2.49^s, Tc + 17.8
with a maximum of 2000 psf [0.98 kgf/cm2], a minimum of 600 psf [0.29 kgf/cm2], but no greater than 150h (0.24h*).
3. For rate of placement > 10 ft/h: p = 150h or
pM = 0.24hst (metric equivalents)
The above three formulas can only be applied if concrete satisfies the following conditions:
• Weighs 150 pcf (2403 kg/m3)
• Contains no admixtures
• Has a slump of 4 in. (100 mm) or less.
• Uses normal internal vibrator to a depth of 4 ft (1.22 m) or less
If concrete is pumped from the base of the form, the form should be designed to resist the lateral hydrostatic pressure of fresh concrete plus minimum allowance of 25 percent to account for pump surge. Caution must be taken when using external vibration or concrete made with shrinkage-compensating or expansive cements as pressure higher than the hydrostatic pressure is expected to occur. It is a good practice to reduce the allowable stresses to half its original value when using external vibrators.