HowtoBuild.cc presents – How to Build Concrete Forms

Generally structural concrete is created by placing still “plastic” concrete into a space created by constructed form work, when the plastic concrete subsequently hardens it will harden into a shape created by the outline these “concrete forms”.

The exact dimensions of the concrete forms are established from the construction project plans and supporting documentation.

Concrete formwork must be constructed correctly to produce high quality finished concrete. Concrete forms must be both rigid and strong to be able to  safely withstand the pressures placed on them at the time of placement of the concrete. It is important that you do not attempt to create shortcuts in creating concrete forms.

When an earth face exists that is strong enough to support plastic concrete we can omit the form components at that location. For example with most footings, the footing base is cast directly against the ground with form work only used on the sides or the larger footings are called for in the specifications sometime both the bottom and the sides may be caste against the earth.

You will also sometimes see foundations that are cast with a concrete form on the inner wall and the earth surface on the outside.

Concrete Forms can be made from a wide variety of Materials

Concrete forms can be made from a variety of materials however the most common are wood, steel and earth.

Concrete Forms Built from Wood

The most commonly used concrete form used in building construction are made from wood. Wooden concrete formwork has the advantage of being cheap to construct, easy to handle, and can be adapted to almost any shape desired.

Wood used for concrete forms must be straight and strong and not fully seasoned. Kiln-dried timber will swell when exposed to the water in the concrete. If the wood is are tightly joined when this happens the swelling may cause warping and distortion. If you use green timber for concrete forms care must be taken to allow for some measure of shrinkage, alternatively green forms must be kept wetted down until the concrete is placed.

The best wood to use of concrete forms are soft woods such as spruce, pine wood and fir as these are the best quality most economical lumber are easy to work with, and cheaply available almost everywhere.

Any lumber that is going to come into direct contact with the placed  concrete should be finished at least on one side as well the edges with the surfaced side facing the concrete.

Plywood is an excellent material for creating forms provided it is of the type made with waterproof glue.  Good quality waterproof Plywood tends to be more warp resistant than softwood and can be reused many times before needing to be replaced. Plywood is manufactured in a wide variety of thicknesses, thinner pieces with require extra care with support but can be extremely versatile in creating curved form work.

Concrete Forms Built from Steel

Although more expensive and time consuming to create than wooded formwork steel concrete forms are very strong and are the preferred solution if a project is to be built more than once at different locations – eg project homes, concrete water tanks, swimming pools etc.

Prefab steel forms are easy fast to put together and very easy to strip down when the pour is finished. Steel forms provide a very smooth surface finish to the finished concrete. Steel forms are easily cleaned of excess concrete and long term maintenance is minimal.

Combination steel / wood forms are exactly like steel forms except for the form face which is constructed from a sheet of plywood.

Concrete Forms Built from Soil

Very often if the surrounding soil is able to retain the desire shape of the concrete on excavation we can use the earth itself as formwork for construction that is below the surface of the land contour.

One of the largest drawbacks to this method of creating concrete forms is the quality of the concrete surface is rough at best as such this method is  generally only used for footings and foundation formwork.

Concrete Forms Built from Cardboard

Cardboard forms are generally used to create complex shapes like round concrete columns. These forms are prefabricated ( offsite ) from cardboard that has been impregnated with some waterproofed material.

Concrete Forms Built from Nylon Fabric

Tough new nylon fabrics lend themselves very well to the formation of  a “bag” that can then be injected with structural concrete creating what one could describe as concrete “pillows.”

These “concrete pillows” have many uses including lining drainage channels and even as footing solutions for modular homes as shown in the image below