Concrete forms, and decorative liners for adding patterns and texture to concrete
Concrete Forms
Time: 03:49
Learn about the different sizes of forms, bracing, and how to properly square your forms.
Concrete is a unique product that begins its life as a semi-solid, can be manipulated and worked to assume most any shape, and then hardens to assume that shape. This ability to fill voids and assume shapes is what makes concrete the most-used building material on the planet. None of this would be possible without concrete forms.
In simple terms, concrete forms are nothing more than a solid barrier that holds concrete in place or forces concrete to assume a certain shape. However, many newer forming systems serve other purposes as well, such as providing insulation or imparting special decorative effects.
Shop for concrete forms
Learn about the many types of forms available, from basic wood to specialty systems for decorative concrete.
Forming Concrete With Plastic Forms
Time: 06:11
Forming concrete with plastic forms is an easy way to do curved radius patios, walkways, and other concrete slabs.
A square foot of conventional concrete weighs about 150 pounds, and a typical concrete project may require hundreds to thousands of square feet of concrete to be placed at one time. All that weight needs to be held back by concrete forms, which is why most forms are made from rigid wood or metal. In recent years, there have seen some advancements in concrete forms made of plastic, fiber glass and resins, but the cost and strength of these materials are slow to overcome the proven performance of metal and wood.
Concrete forms are often categorized by where and how they are used. The best form for a particular project is often a function of the pour size, the amount of concrete the form needs to retain, and the pressure or weight that will be pushing against the form. For example, the typical concrete forms used for flatwork (such as a patio, driveway, sidewalk or road) range in height from 3 to 12 inches. Because the majority of the weight of the concrete in flatwork applications is spread across a prepared subbase -- which relieves much of the weight pushing against the form -- these forms are most often wood, with metal being used for larger commercial or highway work. In contrast, a concrete form used to construct a bridge pier or high-rise building foundation will hold back hundreds to thousands of square feet of concrete, with the height of the form ranging from 12 inches to 20 feet. Because of the massive amount of weight being exerted against these forms, they are made of high-grade steel and can weigh thousands of pounds.
The most basic forms for concrete slabs (where the concrete will not exceed 6 inches in height) consist of wooden boards that are screwed or nailed to wood or metal stakes. The stakes are driven into the prepared subbase, and by using leveling devices (such as hand levels, laser levels or string lines), contractors set the forming boards to the proper level or slope. Additional boards are then used to secure the areas where one board butts up against another.
When turns, rounded edges or free-form designs are desired, thinner cross-section boards are used. To keep concrete from sticking to the forms, they are often coated with a low-grade oil, or form-release agent. This also keeps the forms cleaner and allows them to be used multiple times before being discarded.
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In the case of forms used to pour walls or larger structures, such as piers or foundations, pre-manufactured forming systems are often employed. These wall systems, which are typically made from engineered wood with a metal frame or entirely from metal, are designed to attach to each other through a system of pins or latches.
Symons Aluminum Wall Forms.
Tilt Up Wall Forms.
These types of form also use a system of ties to hold reinforcing bars in place inside the form and to secure one section to another when walls are being poured. These forming sections come in a wide range of sizes and shapes, and most manufactures will produce custom sizes for specific projects.
Quikrete Quik-Tube and Sonotube Concrete Forms
The trend toward energy-efficient home construction has led to the rapid growth of insulated concrete forms (ICFs). ICF systems are comprised of hollow blocks of insulating material (usually expanded Styrofoam) that fit together similar to children's building blocks. ICF systems are constructed on the foundation slab and remain in place to become the foundation and exterior wall system. A network of metal reinforcing bars is placed inside the block wall structure, and then the walls are filled with a high-slump concrete. The foam and concrete sandwich that is created is extremely energy efficient, and the foam becomes both the interior and exterior construction surface (see How Do ICFS Work?). Using ICFs eliminates the need to remove forms, since the hollow block walls remain in place once filled with concrete.
The cost to construct an ICF home is only slightly higher than for a comparable wood-frame home (about 0.5% to 4% on average), and the added upfront expense is quickly recouped through the savings in monthly heating and cooling costs. (See Do Concrete Homes Cost More?). This type of forming system and construction is more popular in high-energy-use regions where heating and cooling are required for much of the year.
Concrete Contractors: Find Concrete Form Products and Suppliers
Forms for concrete countertops, furiture and precast concrete products like chairs, tables, planters and benches.
Forming Concrete Counters
Time: 01:39
Renowned trainer Fu-Tung Cheng shows how to form concrete countertops using melamine.
View All 20 Concrete Countertop Tool Videos
The growth in popularity of concrete countertops, furniture and decorative precast products has led to new and innovative methods of forming (see these videos of concrete countertop forming materials and techniques) and mold making (see Concrete Countertop Molds). These non-traditional methods of forming often involve creating negative spaces or inverted forms, where the concrete piece is actually poured upside down to achieve a desired effect or finish. These forms are often made of materials such as melamine, laminated board or acrylic glass, screw mounted to a large board. Inserts are often attached to the inside of the forms to provide decorative edging or artistic relief.
As more states and municipalities look for ways to beautify concrete construction projects, the use of decorative form liners is growing as well. These plastic sheets with a relief of stone, tile or artistic designs are attached to the inside of the forming system. The concrete assumes the shape and relief of the liner, resulting in a decorative concrete surface. (See Form Liners for Cast-in-Place Concrete.)
Related: Step Forms
Stone Soup Concrete in Florence, MA.
Stone Passion in Salt Lake City, UT.
No matter the type of form or forming system used, the most important factor to achieving good results is how the forms are installed. Forming is often overlooked in concrete construction, but using the right forming system and having it professionally installed can have a significant impact on the concrete's strength and appearance. Proper form installation will result in a uniform and smooth surface, with clean edges and few surface defects.
With precise forms, anything is possible with concrete.
The growing availability of common forming materials at local hardware and big-box stores have led to a proliferation of do it yourselfers and weekend warriors attempting concrete forming and pouring, especially of concrete countertops. But unless you really know what you're doing, forming is best left to a professional. (See Are Concrete Countertops a DIY Project or Should I Hire a Pro?)
Here's an inside look at noteworthy forming systems:
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.
Let’s first get our bearings on concrete forms and the different types available to us.
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.
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.
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 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.
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.
Here are some things to consider when choosing your form:
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.
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:
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.
Different types of concrete have varying drying times and reactions to outside factors.
There are five basic types of 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.
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.
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:
According to the rules for Ordinary Portland Cement, these are the average drying times of various types of structures:
These drying times will differ for types 2, 3, 4, and 5.
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:
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.
Not only is timing critical in formwork removal, but the proper techniques should be used to maintain the structure’s integrity.
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:
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.
See also: How Precast and Custom Concrete Buildings Are MadeConcrete 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.
Speaking of in budget and on schedule, we want to help you with your next construction project. If you’re looking for a contractor in the Colorado area, get in touch for a job quote!