Temporary structures
  **formwork**

 
Concrete formwork is used as a temporary  structure to support the fresh (i.e., uncured) concrete until it is strong enough to support itself and the applied loads. Formwork is a very significant portion of the project, as much as 60% of the final cost of the finished concrete project.

Concrete Slab – Formwork collapsed
Formwork Requirements & Economy:

⦁ Rigid – to prevent bulging or movement from pouring wet concrete.
⦁ Tight joints – to prevent wet concrete from leaking.
⦁ Simplicity – easy erection and dismantling.  Bond-breaking agents such as grease is typically applied to the form faces to prevent concrete “sticking.” 4.  Standardized sizes – allowing reuse.
5.  Form finish – to achieve desired texture and appearance of finished product.
Design Loads on Forms :

The American Concrete Institute (ACI) publishes a document called “ACI 347 – Guide to Formwork for Concrete.”

⦁ Form Dead Loads – the actual weight of the forms, plus the weight of fresh (i.e., wet) concrete.

⦁ Form Live Loads – the weight of workers, equipment and material storage. The minimum live load is 50 psf, while a live load of 75 psf should be used if motorized buggies are used.

⦁ Primary Factors Affecting Concrete Lateral Pressure on Forms:
⦁ Concrete Density
⦁ Concrete Temperature, T at the time of placing (Deg. F)
⦁ Vertical Concrete Placement Rate, R (feet per hour)
⦁ Concrete Placement Height, h (feet)

4).   Lateral Loads on Formwork – wet concrete is like water – it exerts a lateral pressure which increases with the depth of the form.

5).  Types of Formwork – There are three main types of formwork that you need to be aware of .

⦁ Horizontal (Floor/Slab)  Formwork
⦁ Beam Formwork
⦁ Vertical (Column/Wall) Formwork


Horizontal (Floor/Slab)  Formwork

 Horizontal forms must be designed and constructed to safely support the

 vertically applied dead loads and live loads.  Horizontal Formwork will be  discussed further in Shoring/Reshoring section.

 Diagonal bracing must be designed to resist horizontal (lateral) loads  applied to the edge of the slab having a magnitude of the larger of:
⦁ 100 lbs linear ft
⦁ 2% of the total dead load of the slab

A typical site-built floor form contains the members as shown in the sketch below:

The design of the individual structural members would be based upon satisfying the requirements as dictated by the IBC.


Beam Formwork 


  Beam forms must be designed and constructed to safely support the  vertically- applied dead loads and live loads as described above.  A typical  site-built beam form contains the members as shown below: 


Vertical (Column/Wall) Formwork 


 Column Forms:
 
 Column forms are especially susceptible to large lateral forces exerted due  to high rates of placement of wet concrete.  Tighter joints and bottom lateral support is critical. 
Chamfer Strip

Vertical (Column/Wall) Formwork 


 Wall Forms:
 
 Wall forms are also susceptible to large lateral forces exerted due to high  rates of placement of wet concrete.  Tighter joints and bottom lateral support is critical.