Millions of tons of concrete are poured every year. As one of the most common construction materials available, it’s not rare to find contractors and DIYers that use concrete differently.
Perhaps you, or a contractor, are planning on using a wood form to shape your concrete project. If you are wondering if leaving wood forms in concrete is acceptable, this article will answer your questions.
Can You Leave Wood Forms in Concrete?
The general advice is to not leave wood forms in concrete. This is for a number of reasons that have been outlined in the next section.
Undoubtedly, you will find projects that have left wood forms in place after a concrete pour. The main reason people try to leave these forms in place is because of spatial confinement and time restrictions. However, in general, you shouldn’t leave wood forms in concrete.
Depending on what you are creating, a form will ideally be removed after different intervals.
Walls, columns, and vertical supports need between 24 – 48 hours. Propped-up slabs take 3 – 4 days. Propped-up soffits need seven days. Props for slabs that are 15’ or less need seven days. Props for slabs over 15’ need 14 days. Props for arches and beams over 20’ require 21 days. Keep in mind that all of these times are averages and that other factors can impact the drying time.
6 Reasons Not to Leave Wood Forms in Concrete
There are six main reasons you shouldn’t leave wood forms in concrete: obstructing workers, mold, termites, fire risk, material cost, and appearance. We’ll dive into more detail about each of these points in this section.
1. Obstructing Other Workers
If a concrete slab is planned to be one size, the formwork on the outside increases the final dimensions. This can cause issues for other trades and projects. If a particular size is specified, then the slab should be made to those dimensions. Alongside this, the formwork can get in the way of other workers and cause potential hazards.
2. Mold
Wood is a natural material that is subject to natural processes such as decomposition. A confined, moist, space without ventilation creates the perfect environment for mold to develop. Mold is a concern because it can be a health hazard. The mold spores can travel into habitable rooms, covering furnishings and affecting people’s health.
3. Termites
Along with mold, termites are another big concern here. If wood is left on the form it provides termites with a food source directly beneath the structure you are building. This can cause a major issue if the termites decide to travel up into your building and eat more structural material. In this situation, you’d be forced to hire a professional to deal with the infestation.
4. Combustion/Fire Risk
Without adequately planning the wood forming, it can become a fire risk. If a fire lights on the wooden formwork, there’s a danger of it spreading up to other parts of the building.
5. Cost of Material
Wood sheet materials are more expensive than other alternatives that can be used for formwork. In fact, wood has increased in price significantly over the last few years. Even in the past, contractors favored cheaper materials instead of sacrificial wood. If they choose to use wood, the preference is to be able to remove it and reuse it in some way.
6. Appearance
If a form will be visible, for instance on outside foundation walls or on the edge of slabs, leaving the formwork on display will create a look that’s unfinished and ugly. On top of this, the appearance will worsen when the wood begins to rot.
2 Alternatives to Wood If You Want to Leave Forms in Concrete
If you want to leave formwork in place, this section discusses two alternatives to wood that you should consider.
1. Blockwork
Consider using blockwork to build the form if you want a solid wall that won’t rot or invite pests into the building. This option could take some more time and require another trade to complete the job, but well-laid blockwork can also provide a nice visual element. Blocks can be laid in different patterns and with a range of bricks or blocks that come in different colors.
2. Ground/Earth
If you are creating foundations, one of the most common ways to pour concrete is to dig the foundations into the earth. If the conditions are right, the concrete will solidify within the trenches. The benefits of this include not having to buy formwork material and not having to remove it.
Summary
Leaving wood forms in concrete is widely considered to be unacceptable. If you have hired a contractor to create the concrete form, ensure they remove it from the concrete and take the wood away as well.
Not removing the forms can cause a range of issues, from inviting termites and pests into the building, creating an ugly finish, creating a health hazard from mold, as well as potential fire risks.