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Concrete is the most commonly used man-made material on earth. Given its particular properties, it is also among the most widely employed options for construction. This mixture of cement, sand, gravel and water begins as a wet substance unable to support its own weight, thereby requiring the use of a mold to achieve the desired shape and structure. In construction applications involving concrete, that mold is referred to as the formwork.
Because formwork not only controls the concrete’s position and alignment, but also bears the weight of materials, equipment and workers, it must be expertly planned and applied. Here, we’re breaking down the specific elements of superior concrete formwork to help you develop an approach that meets the needs and specifications of your construction project.
1. Appropriate Material Selection
Concrete formwork, also sometimes referred to as shuttering, is the configuration of forms (or molds) that are utilized to create an overall shape for the concrete. Just as the applications for formwork are varied, so are the types of materials used. Following are some of the different categories of form materials available for concrete projects.
- Timber: Timber, specifically plywood, is perhaps the most commonly used type of formwork material. Derived from trees, it is widely accessible, highly economical and available in a number of different species, sizes and grades. Timber is easy to cut and shape on the job site, and it is a reusable entity if managed and stored properly.
- Steel: This is a stronger and more durable material, and it has the advantage of upholding longer stretches of reusability when appropriately maintained. While the material demands a higher upfront cost, it retains value for a lengthier span of time. Other advantages to steel forms include their ability to deflect moisture, warping and shrinkage, their exposed surface quality and their capacity to be erected and removed quickly.
- Aluminum: This material is both strong and lightweight, making it a valuable source for certain formwork projects. Aluminum formwork is often prefabricated and then assembled on site. This material requires a higher initial expense, but its minimal weight brings down overall handling costs. Because pure aluminum is susceptible to corrosion, alloys are often applied to resist this effect.
- Plastics: Two different types of plastics—glass-reinforced and vacuum-formed—are sometimes employed for formwork applications. Although lighter than metal variations of formwork, plastic formwork is not as durable. Because it is a very flexible material, it’s often leveraged to form more nuanced shapes. It also produces a high-quality surface finish. While plastic formwork can be an economical choice that enables reuse, it’s important to note its vulnerability to sun and heat exposure.
- Other: Newer (though less commonly employed) iterations of form material include options like fiberglass and fabric. It’s important to consider a number of factors pertaining to the needs of your concrete project, such as cost, strength, weight, flexibility, durability, sustainability and reusability, before making an informed decision about which type of material (or combination thereof) is best suited to meet your requirements.
2. Considerations for Strength & Capability
Formwork’s fundamental role is to bear all the loads present while casting the concrete, as well as to retain its shape throughout the process. That means superior concrete formwork requires great consideration for:
- Accurate load estimations
- The properties and behaviors of the chosen form materials
- The pros and cons of different forming systems
The design and application of your formwork should be based on its ability to produce the required shape and surface finish, withstand the necessary loads, prevent cement leakage throughout and enable the removal of formwork materials without incurring damage to the concrete. These are critical considerations to be made prior to the installation of concrete formwork.
3. Proper Rate of Pour Based on Height
Conventional concrete, when wet, can weigh from 140 to 150 pounds per cubic foot. That’s a lot of weight to account for when calculating the load capacity of concrete formwork. Because of this weight, there’s a need to factor pressure into the equation, and pressure can be impacted by both the height and weight of the actual pour.
The greater the height of the poured concrete, the more lateral pressure is exerted on the interior of the formwork. Therefore, it’s important to integrate appropriate drying intervals into the rate at which the concrete is poured. This allows a portion of the concrete to cure and strengthen before adding more of it to the mold.
Otherwise, if too much concrete is poured all at once, immense pressure can cause the forms’ connecting devices to give way. When that happens, concrete can burst through the connections and out of the formwork—a messy, expensive and potentially hazardous outcome that could be avoided by ensuring a more sustainable pour rate.
4. Focus on Concrete Characteristics
The characteristics of the concrete itself can have an impact on the formwork and overall project. That’s why it’s important to understand, test and account for the properties of fresh concrete based on attributes such as:
- Type: The choice of concrete mix will depend on factors like required strength and surface finish, performance needs, weather conditions, local codes and cost.
- Weight: The unit weight of fresh concrete is determined by density, air content, water content and cement content.
- Slump: This is the consistency of fresh concrete, or its ability to flow. The project’s slump tolerances will determine how the consistency of the concrete must be managed.
- Temperature: Temperature has a vital influence on the properties of fresh and hardened concrete, which is why minimum and maximum limits are specified. Variations in temperature can affect the rate of hydration, thereby impacting slump loss, rate of strength gain and setting and finishing times.
- Vibration: When concrete is poured, it contains air bubbles that can substantially weaken the structure. Concrete vibrators are used to eliminate these bubbles by shaking the freshly poured concrete.
5. Safety Planning & Supervision
Because there are real hazards associated with applying concrete formwork, organizations like the Occupational Safety and Health Administration issue guidelines around it. It’s important to heed these guidelines and enforce adequate safety planning.
Without proper safety precautions, there’s risk of experiencing the kinds of accidents and liability that stem from improperly designed formwork, hurried removal of forms, bends and breaks during the pouring process and so on.
The best way to mitigate dangerous risks is to partner with a construction expert with the knowledge and experience to plan ahead, leverage a qualified staff and ensure proper supervision.