It is important to know and understand the factors that can affect concrete formwork pressure to ensure a successful pour. When placing concrete, we will consider the different factors that play a role in determining the placement rate and height of pour.
Unit Weight of Concrete
The unit weight of concrete, which can vary geographically, is typically 140 to 150 pounds per cubic foot. Concrete weighing less will have a lower concrete form pressure, while concrete weighing more will increase the pressure.
Height of Concrete Pour
Height of concrete pour means the total height of the wall that concrete is being placed during one pour. concretepressure=unitweightxheight
If the concrete is not achieving initial set between lifts, then this is what the concrete formwork pressure would be. Ideally, it is good practice to place concrete so that each lift reaches initial set.
The following factors will affect concrete set up time. Once concrete is set up there is zero concrete pressure.
- Concrete mix chemistry
– This affects the concrete formwork pressure. Factors include: cement type, slag or fly ash and retarders
– all of which increase formwork pressure. Subsequently, accelerators speed up the setting time and reduce pressure.
- Concrete temperature
– Higher temperature equals shorter setting time. The ambient temperature will affect the concrete temperature; therefore, one would use a slower rate of placement in the winter.
- Vibration method and depth
– Any consolidation method, internally or externally, should be done by lift. Disturbing the lift will affect the set up of previous lifts, increasing the unset height of the concrete, creating greater formwork pressure.
- Rate of placement and method – This affects the concrete pressure
– slow placement results in less pressure. The rate of placement is in feet per hour.
Example: If concrete was placed in a 10foot wall and it took two hours; the placement rate would be 5 feet per hour.
- Concrete flowability
– Concrete with a slump of 7.0 or greater, as well as self consolidating concrete (SCC), has no initial setting time.
- Wall thickness
– In theory, the wall thickness does not affect concrete formwork pressure; however, a narrow wall may produce lower pressures due to bridging effects.
In summary, the main factors that will affect the concrete formwork pressure include the rate of placement, concrete mix and temperature. Generally, the rate of placement should be lower in the winter than summer. Essentially it doesn’t matter how many cubic yards are placed per hour or how big the project is. What matters is the rate of placement per height and time (height of wall poured per hour).
This information is intended to provide knowledge that can then be used in the decision making of placing concrete by being cognizant of the different factors affecting the concrete formwork pressure.