Concrete Formwork Pressure: Understanding the Factors that Affect it

It is a must to be aware of and understand the factors that could affect concrete formwork pressure to guarantee a successful pour. By the time of placing the concrete, it is important to consider the different factors that have a role to play in determining the height of pour and placement rate.

 

Unit Weight

The unit weight of the concrete, which often vary geographically, is typically at 140-150 pounds per cubic foot. Concrete that weighs less would have lower concrete form pressure, while the one that weighs more will have an increased pressure.

 

Height

The height of the concrete pour means the total height of the wall where the concrete is being placed during a pour.

In case the concrete fails to achieve an initial set between lifts, then this will be the concrete formwork pressure. Typically, it is a good practice to place concrete so that every lift could reach the initial set.

 

Setting Time

Below are the factors that will affect the time for the concrete to set up. Once it has set up there will be zero concrete pressure.

 

Concrete mix chemistry

This also has an influence on the pressure of the concrete formwork. Factors are as follows: type, cement, slag or fly ash and retarders. All of these can increase the pressure on the formwork. Then, accelerators will speed up the setting time while reducing pressure.

 

Concrete temperature

Higher temperature means a shorter setting time. Ambient temperature has an effect on the concrete temperature; thus, one will use a slower rate of placement during winter time.

 

Vibration method and depth

Any merging method, whether internally or externally, need to be done by lift. Upsetting the lift would affect the setup of the previous lifts. This will result in the increase of the unset height of the concrete, thus creating greater pressure.

 

The rate of placement and method

This also affects the pressure as slow placement will eventually result in less pressure. The rate of placement is in feet per hour.

 

Concrete flowability

Concrete that has a slump of 7.0 or higher, and self-consolidating concrete (SCC), do not have initial setting time.

 

Wall thickness

In theory, the thickness of the wall does not directly affect the pressure; however, a narrow wall can produce lower pressures as a result of bridging effects.

In conclusion, the key factors that affect the pressure of the concrete formwork include the rate of placement, concrete mix, and temperature. Mostly, the rate of placement needs to be lower in the winter than in summer. Fundamentally, it does not matter how many cubic yards are being placed per hour or how large the project is. What is more important is the rate of placement per height as well as the time.