Concrete needs stability to cure properly, and proper curing will give it the strength it needs to withstand the pressures it will face.
Engineers have designed a variety of forms as the solution to create stable curing environments.
Each project will require proper form selection, installation, and form removal for the strongest concrete structures.
Table of Contents
1. Concrete Forms
2. Which Type of Concrete Form to Use?
3. How Long Should Concrete Cure Before the Form Is Removed?
4. When to Remove Concrete Forms
5. Can You Speed Up the Concrete Drying Process?
6. How to Properly Remove Formwork
Let’s first get our bearings on concrete forms and the different types available to us.
What Is Formwork?
Concrete formwork, also known as shuttering, requires solid molds to hold liquid concrete in place until it cures.
These forms are necessary to shape and strengthen the concrete. Without a form, the concrete would just end up in one big mound of hardened liquid with no real shape or use.
Formwork can turn concrete slabs into walls, foundations, and even floors. These forms can also add surface texture to the concrete for beautification.
The 8 Types of Concrete Forms
Contractors have a choice to make when building concrete structures.
Which type of concrete form best suits the project?
Each of these eight forms has its uses, limitations, pros, and cons.
Wood forms are the most conventional type. The materials are readily available, and most contractors know the process.
To create a wooden form, wooden boards are nailed together into the desired shape. Wooden stakes are added at precise locations to help bear the weight of the concrete.
Experts recommend using boards less than six inches wide for wood forms as larger boards tend to warp.
The limitations of wood forms stem from the fact that most wooden boards aren’t strong enough to hold large amounts of concrete and cannot be used for heavy concrete construction.
Insulated Concrete Forms
ICFs are hollow blocks of insulation with steel reinforcement cages built into the form.
The concrete mix is poured and sandwiched by the insulation. These forms are permanent and never removed.
You don’t have to worry about timing or proper formwork removal with ICFs because they stay put, and the concrete will dry and strengthen with little to no adverse effects from exterior conditions.
Steel forms are the strongest type of concrete form, and you can reuse them multiple times. These forms are the most popular option for large industrial structures.
Another benefit steel forms offer is that they won’t absorb the water from the curing concrete. This hydration must evaporate from the concrete at the right speed for maximum strength.
Steel forms also present a lower risk of error after removal.
Concrete Wall Forms
Concrete wall forms are pre-manufactured forms made from wood, steel, or both, all connected with anchors. This method is often used to build large and often intricate industrial structures.
Some require the use of cranes to install, while others are designed to be fast-paced and modular for easy setup and take-down.
Decorative Concrete Forms
A decorative concrete form is used to create an artistic relief through the use of negative space as the concrete dries.
There is typically a melamine, acrylic glass, or laminated board with inserts that create the design in the mold.
Which Type of Concrete Form to Use?
The type of form you’ll need depends on several factors.
Here are some things to consider when choosing your form:
- Size of pour: The dimensions of the structure can determine which form would work best.
- Pressure and weight: The form must be strong enough to withstand the pressure it’s placed under.
- Amount of concrete: The larger the structure and amount of concrete, the stronger the form needs to be.
- Type of compaction: Certain forms handle certain types of compaction better.
Based on these factors, you can make a more informed decision. As already stated, larger projects require the use of steel forms, while wood can be used for smaller projects.
Steel is cleaner than wood, resulting in fewer bug holes in the finished product.
Wood is best for low-height structures, and steel is best for high structures where concrete can buckle under its own weight.
Of course, steel is more costly, so it’s best to reserve this form for when you need it.
How Long Should Concrete Cure Before the Form Is Removed?
The amount of time you should wait before removing your concrete forms is different for every project.
Here are some of the factors that affect curing times:
- Grade and type of concrete
- Size of concrete structure member
Let’s discuss these factors:
Frozen concrete will not dry, so if you are working in extremely cold weather, the curing process will take longer.
Rain also affects drying time, especially during the first few days when the air is still moist.
Heat aids the curing of concrete, but extreme heat can cause cracking. This can be prevented by using tarps or barriers to trap moisture.
Grade and Type of Concrete
Different types of concrete have varying drying times and reactions to outside factors.
There are five basic types of cement:
- Type I: OPC (Ordinary Portland Cement)
- Type II: Moderate Sulfate Resistant Cement
- Type III: Rapid Hardening Cement
- Type IV: Low Heat Cement
- Type V: Highly Sulfate Resistant Cement
Each of these categories has subcategories with different uses and drying times.
The grade of cement used also affects drying times. We’ll discuss this more in the next section.
Size of Concrete Structure Member
Larger sections gain strength faster due to the compaction of the weight of the concrete itself.
So, the structure’s size determines when the form is ready to be removed.
When to Remove Concrete Forms
The removal of concrete forms is known as “striking-off” or “stripping off” the forms.
According to the American Concrete Institute (ACI 308), the drying times for each type of concrete are as follows:
- ASTM C 150 Type I cement: 7 days
- ASTM C 150 Type II cement: 10 days
- ASTM C 150 Type III cement: 3 days
- ASTM C 150 Type IV or V cement: 14 days
- ASTM C 595, C 845, C 1157 cements: Varies
According to the rules for Ordinary Portland Cement, these are the average drying times of various types of structures:
- Walls and columns can be removed after about 24-48 hours
- Slabs, with their props left under them, can typically be removed after 3-4 days
- Soffits, with their props left under them, can be removed after one week
- Props supporting slabs under 15 feet can be removed after one week
- Props supporting slabs over 15 feet can be removed after two weeks
- Props to supporting beams and arches under 20 feet can be removed after two weeks
- Props to supporting beams and arches over 20 feet can be removed after three weeks
These drying times will differ for types 2, 3, 4, and 5.
Can You Speed Up the Concrete Drying Process?
Time is a critical issue in the construction process. If you are pressed for time, there are a few ways to speed up the drying process.
Here are some ideas:
- Weather-resistant barriers
Let’s talk about these ideas:
Admixtures can be created by adding certain chemicals or additives to the cement mixture to speed up the drying times.
Calcium chloride and silica fume are two such additives.
When weather conditions are causing an issue, you can use weather-resistant barriers to speed up the concrete drying process.
Plastic sheeting is good for trapping moisture when the temperature is high and protecting the form from rain.
Insulation blankets help to prevent freezing when the temperature drops.
If the air is overly humid, dehumidification can also aid the drying process.
Desiccant dehumidifiers use a chemical reaction to draw moisture out of the air. This is best for use in an enclosed area where the chemicals won’t dissipate in the surrounding air.
Condensation dehumidifiers cool the air to drop the dew point, then soak up the humidity from that process.
Heating dehumidifiers heat the air, raising the dew point and drawing the moisture off the surface of the concrete.
How to Properly Remove Formwork
Not only is timing critical in formwork removal, but the proper techniques should be used to maintain the structure’s integrity.
Common Mistakes to Avoid
Learning formwork removal is easier when you understand some common but detrimental mistakes — and avoid those at all costs.
Here are a few common mistakes that you should avoid in your removal process:
- Don’t use a crowbar to remove forms. Use wooden wedges; they are more delicate and won’t damage the concrete.
- Follow the same sequence and pattern if more than one form is used. The concrete needs to be strong enough to support all loads.
- If you see defects, repair them before other structural members are added. You may have to start that form over, but it’s better than failing inspections or having structural defects.
- Use the correct type of concrete for the job at hand. Read our Concrete vs. Cement article for a refresher on the various types and their uses.
- Don’t pour concrete without proper site preparation. The site should be a level, compacted area.
- Place your reinforcements correctly to prevent warping, slumping, or uneven forms.
- Use the correct water-to-cement ratio. Too much or too little water can cause defects.
- Don’t overwork the concrete. This can bring too many fine materials to the surface and weaken the structure.
- Don’t rush the process. Don’t remove the forms too early!
What Happens If You Remove the Forms Too Early?
When you jump the gun and remove your forms too early, you will not be pleased with the results.
Your structure can slump to one side or bulge at the bottom. Multiple cracks can occur because it is not ready to bear the weight of the concrete on top.
Worse still, the structure can collapse due to multiple factors weakening the project’s structural integrity.
Concrete forms are a crucial part of the concrete construction process, and the information in this guide will ensure that proper formwork is used.
With the use of proper formwork, projects will go much more smoothly. There will be no need to repair or redo any structures with new concrete.
This will save money and reduce the amount of time it takes to complete a project, keeping you within budget and on schedule.