How to Calculate Concrete Pressure on Formwork

Vertical loads are exerted on formworks and may consist of dead loads such as formwork dead load, steel reinforcement embedded in the forms, molded fresh concrete and live loads such as the weight of workers, equipment, and tools.

It is recommended to compute weight of materials separately in the case of heavy reinforcement to specify accurate unit weight.

ACI 347-04: Guide to Formwork of Concrete specify that, to allow for workers and their placing tools such as screeds, vibrators, and hoses, at least 2.4Kpa live load should be used for the design of horizontal formworks and a minimum live load of 3.6 kPa should be employed in cases where motorized carts and buggies are utilized.

Furthermore, ACI 347-04 determines combined live and dead load design of not less than 4.8 kPa or 6 kPa if motorized carts are employed.

Finally, formwork self-weight is calculated using unit weight and dimensions of formwork various parts. The weight of formworks is substantially less than fresh concrete dead load and construction live load. That is why an allowance is decided as superimposed load per square meter for form work components during design.

An initial assumption is made between 0.239-0.718 kPa based on experience and checked after the member is sized. This estimation is depending on the fact that common weight of formworks is 0.239-0.718 kPa.

Lateral Pressure on Concrete Formwork

Internal pressure resulted from accumulated depth of placed concrete is imposed on vertical formworks such as walls and columns. During vibration and for short period after vibration, placed fresh concrete close to the top and to a small depth of formwork behaves like a liquid and impose lateral pressure on the formwork that is equal to the vertical liquid head. Fresh concrete is granular with internal friction but vibrations eliminate bonds in the mixture and generate liquid state.

There are different reasons such as placement rate, concrete temperature, and internal frictions that affect lateral pressure of below vibration controlled depth and make lateral pressure smaller than liquid pressure head.

 

When vertical placement is carried out at slow pace, fresh concrete could have time to start stiffening. Moreover, unless concrete temperature is low, the time to start setting is not short.

Other factors such as pore water movement, creation of friction and other parameters may lead to decline lateral pressure. Various types of cement, admixtures, cement substitutes, construction practices might influence level of lateral pressure.

Mostly, concrete lateral distribution pressure, which based on tests, is depicted as shown in Figure-1. The distribution begins close to the top as a liquid and reaches peak value at lower level. For design reasons, it is suggested that ultimate pressure is uniform at conservative value.

Calculation of Lateral Pressure on Concrete Formwork

 

ACI 347-04 specify that, concrete lateral pressure is computed as per Equation-1 if fresh concrete slump value is greater than 175 mm and does not placed with normal internal vibration to a depth of 1.2 m or less.

 

Where:

 

P: Lateral pressure of concrete, kPa

 

: Density of concrete, Kg/m3

 

g: Gravitational constant, 9.81 N/kg

 

h: Depth of fluid or plastic concrete from top of the placement to the point of consideration in the form, m

 

However, ACI 347-04 stated that, if concrete slump value is no larger than 175 mm and placed with normal vibration to a depth of 1.2 m or less, then lateral pressure of concrete is calculated as follows:

Lateral Pressure on Concrete Formworks for Columns

 

With a minimum of 30Cw kPa, but in no case greater than

 

Where:

Pmax: Maximum lateral pressure of concrete, kPa

Cw: Coefficient of unit weight which is provided in

Cc: Coefficient of chemistry that is provided in

R: Concrete placement rate, m/h

T: Concrete temperature during placing, oC

Lateral Pressure on Concrete Formworks for Walls

 

 

 

Concrete lateral pressure for walls with placement rate, smaller than 2.1 m/h and placement height is no greater than 4.2 m.

 

With a minimum of 30Cw kPa, but in no case greater than

Concrete lateral pressure for walls with placement rate of greater than 2.1 m/h and placement height exceeds 4.2 m, and for all walls with placement rate of 2.1 to 4.5 m/h.

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