6 Factors Affecting Lateral Pressure of Fresh Concrete on Forms

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There are
several factors that influence the magnitude of lateral pressure of fresh
concrete on formwork sides. For instance, concrete weight, rate of placement, vibration,
temperature at the time of pouring, chemistry, and slump. Formwork design need
to take these factors into considerations during design process. The freshly placed
concrete behaves temporarily like a fluid, producing a liquid pressure that acts
laterally on vertical formed surfaces.

The designer
may need to set certain concrete placement conditions in order to limit
detrimental influence of some factors such as setting certain rate of concrete placement.
Rapid vertical placement can result in a full liquid pressure for the entire
depth. However, when rate of placement is slow, concrete at the bottom of the
form begins to stiffen. Consequently, the lateral pressure is reduced to less than
full liquid pressure by the time concrete placement is completed in the upper
parts of the form

1. Weight of Concrete

The unit weight of concrete directly influences the lateral pressure of fresh concrete imposed vertically on the formwork surface. The lateral pressure is equal to the concrete unit weight times depth or fluid or plastic concrete.

However, if the concrete slump is 175mm or less, then maximum lateral pressure is equal to the temperature of concrete during placement times rate of placement times unit weight coefficient which can be obtained from ACI 347-04.

2. Rate of Placement

Initially, the rate
of concrete placement affects lateral pressure, but this influence decreases as
concrete stiffens and/or consolidated. This because stiffening and
consolidation makes concrete to support itself. Maximum lateral pressure is
proportional to the rate of placement up to a limit equal to the full liquid
pressure. The rate of placement is the average rate of rise of the concrete in
the form.

Pumping permits rapid placement of concrete but can increase lateral pressure when forms are filled to full height before any stiffening of the concrete takes place
Fig. 1: Pumping Permits Rapid Placement of Concrete But Increase Lateral Pressure When Forms Filled to Full Height Before Stiffening of Concrete Occur

3. Vibration

Internal vibration leads to increase lateral pressure temporarily by 10 to 20%. This is because it makes fresh concrete to behave like a liquid for the full depth of vibration.

Therefore, forms should be designed to withstand the greater pressure and depth of vibration must be controlled during concrete placement. Re-vibration and external vibration generate greater loads on forms compare with internal vibration.

Internal Vibration of Concrete
Fig. 2: Internal Vibration of Concrete

4. Concrete Temperature

Temperature of concrete during its placement affect the lateral pressure greatly because it is one of the factors that govern concrete setting time. When temperature is low, greater amount of concrete can be poured before fresh concrete at lower portion of the form stiffens and support itself.

This lead to increase liquid head depth which results in higher lateral pressure. That is why cold weather need to be taken into consideration while forms are designed.

5. Concrete Slump

Slump is a
measure of concrete workability. The slump of ordinary concrete does not create
much influence on lateral pressure. However, when self-consolidating is used in
construction, full liquid head should be used to evaluate lateral pressure of
fresh concrete on forms.

6. Concrete Chemistry

Cement type, use of retarding admixtures, and use of fly ash or slag cement as a cement replacement can all have a significant effect on lateral pressure. ACI 347-04 provides chemistry coefficient to account for the influences of these components. For instance, the chemistry coefficient for type I, II, and III Cement without retarders is 1 whereas for type I, II, and III Cement with retarders is 1.2.



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