A concrete form is a temporary structure or mold used to support concrete until the hardening process gives it sufficient strength to be self-supporting.
Carpenters generally assemble the job-built concrete forms that are used to construct foundations, walls, columns, and other concrete structures.
A concrete form is subjected to tremendous pressure from wet concrete.
The pressure increases as the height of the wall increases. The rate at which the concrete is poured also affects the pressure on the form. The faster the rate of pour, the greater the pressure.
If the forms are not properly constructed and supported, or if the concrete is poured too quickly, the forms will not be able to withstand the pressure of the wet concrete. They will fracture, releasing the concrete.
Footings provide the base of a foundation system for walls, columns, and chimneys.
A footing bears directly on the soil and is made wider than the object it supports to distribute the weight over a greater area.
Footings must be located accurately and must be built to the specified dimensions.
There are four basic types of footings. Click on each type to know more:
Also called spread footings, they are commonly used to support poured concrete or concrete block foundation walls.
When a foundation is to be built on hilly or steeply sloped land, it is necessary to step the footing at intervals to achieve grade levels.
Piers are isolated footings set in soil to directly support posts, columns, or grade beams.
Grade beams are foundation walls that receive their main support from concrete piers that extend deep into the ground.
A wall form consists of two sets of panels separated by the thickness of the concrete wall. The following are different types of wall forms:
• Forms that are constructed primarily from wood panels, lumber, and nails, with some specialized hardware.
• Forms that are constructed using wood panels and lumber support members secured with patented attaching hardware.
• Patented form systems that use metal panels and actor-built attaching hardware.
Components of a Wall Form
Click on each component of a wall form panel to know more:
• Top and Bottom Plates
They are horizontal members of a wall frame panel.
These upright members support the sheathing. They are nailed to the top and bottom plates.
It gives the wall surface its shape and texture, and keeps the concrete in place.
Also known as wales, these horizontal members are used to support the form and the studs.
These upright members are used to stiffen or reinforce the form.
• Form ties or spreaders
Also known as spreaders, they prevent the form from being spread apart by the weight of the wet concrete. They also prevent the walls from shifting while the concrete is being poured.
Fastened to one side of the form and nailed to stakes driven into the ground, braces are usually placed 8’ to 10’ apart.
Sheathing and Edge Forms
Sheathing can be made of exterior plywood sheets. Just about any type of exterior plywood may be used. However, a special product called plyform is popular because it is specifically made for use in concrete forms.
Plyform is an exterior plywood that is made only from certain grades and types of wood. Plyform panels are sanded on both sides and are usually oiled at the mill. The oil reduces the moisture penetration and aids in releasing the form from the concrete.
Sheathing and Edge Forms
Edge forms are low-height perimeter forms constructed to contain concrete poured for flat surfaces (flatwork) such as on-grade building slabs with or without a foundation, or for outdoor slabs such as parking lots, driveways, streets, sidewalks, and approaches.
A slab is defined as a section of concrete that is larger in its horizontal dimensions than in its thickness. A slab-on-grade (slab-at-grade) is a concrete slab supported by the ground.
Depending on strength and deflection considerations, forms for floor and similar kinds of slabs may have to be left in place for several days before removal. The time of removal should be determined by a structural engineer.
On large construction jobs, the recommended times for removing various types of forms are often spelled out in the specifications.