Formwork is the temporary (or sometimes permanent) mold that gives concrete its shape. When you see a concrete wall, column, or slab, that structure was poured as a liquid into a carefully built shell, then left to harden before the shell was removed. That shell is formwork. It accounts for roughly 50 percent of the total cost of a concrete construction project, making it one of the single biggest line items in any concrete build.
How Formwork Works
At its simplest, formwork is a container built to the exact dimensions of the finished concrete element. Workers assemble panels, brace them to resist the outward push of wet concrete, pour the mix, then wait for it to cure. Once the concrete can support its own weight and any expected loads, the forms are stripped away to reveal the hardened structure underneath.
The supporting framework beneath the molds, called falsework, holds everything in place during the pour. For a horizontal slab, falsework includes vertical posts (shores) and horizontal beams that carry the weight of wet concrete until it sets. For vertical elements like walls and columns, the main challenge isn’t holding weight up but holding panels together against lateral pressure from the liquid concrete pushing outward.
What Affects Pressure on the Forms
Wet concrete behaves like a heavy liquid, and the pressure it exerts on formwork panels is one of the most important engineering considerations. Three factors drive that pressure more than anything else: pour rate, temperature, and the height of the pour.
A faster pour rate means more liquid concrete stacks up before the lower layers begin to stiffen, creating higher pressure. When casting rates exceed about 5 meters per hour, pressures can reach over 80 percent of full hydrostatic pressure (the theoretical maximum if the concrete behaved as a pure liquid). A slower pour gives lower layers time to begin setting, and pressure can drop to roughly half of hydrostatic levels.
Temperature plays a direct role because warmer conditions speed up the chemical reaction that hardens concrete. Higher temperatures mean faster stiffening, which reduces the duration and magnitude of lateral pressure. Colder weather does the opposite, keeping concrete fluid longer and increasing the load on forms. The height of the structure matters too. Sensors placed near the bottom of a tall wall consistently record higher pressures than those placed higher up, simply because more concrete sits above them.
Common Formwork Materials
The choice of material depends on the project’s size, budget, and how many times the forms need to be reused.
- Timber: The most traditional option. It’s lightweight, easy to cut into any shape or size, and economical for small or one-off projects. Plywood facing panels wear out relatively quickly, though, so timber forms aren’t ideal when you need dozens of identical pours.
- Steel: Much more durable, with a lifespan of over 100 reuses. Steel panels produce a smooth surface finish, resist water absorption (which reduces surface blemishes in the finished concrete), and can be assembled and dismantled efficiently. The higher upfront cost pays off on large or repetitive projects.
- Aluminum: Lighter than steel due to its lower density, which cuts handling and labor costs. Like steel, aluminum becomes more economical with repeated use because the per-use cost drops each cycle.
- Plastic: Lightweight modular systems that interlock and can also be reused over 100 times. The lower handling cost makes plastic attractive for large, repetitive layouts like residential slabs and walls.
Permanent Versus Temporary Forms
Most formwork is temporary. It goes up, concrete is poured, the concrete cures, and the forms come off. But in some applications, formwork stays in place permanently and becomes part of the finished structure. Insulating concrete forms (ICFs) are a common example: foam blocks that snap together to create a mold for walls, then remain in place as built-in insulation after the concrete sets. Permanent formwork can also serve as reinforcement or as a protective layer, eliminating the labor and time needed for stripping.
Specialized Systems for Tall Structures
Standard panel-and-shore setups work well for slabs, footings, and shorter walls, but high-rise buildings, chimneys, silos, and bridge towers call for systems that climb as the structure grows.
Slipforming uses a set of forms that rise continuously on hydraulic jacks while concrete is placed nonstop. Workers feed reinforcement into the mold as it climbs, and internal vibrators compact the concrete. Because the pour never stops, slipforming can build elevator cores, cooling towers, and storage silos very quickly. Jumpforming takes a different approach: the forms are locked in place for each pour, then “jumped” up to the next level once the concrete below has gained enough strength. Self-climbing systems combine elements of both, using integrated hydraulic devices to lift the entire platform in stages.
Fabric Formwork
A less conventional option uses sheets of fabric instead of rigid panels. Because fabric is permeable, excess water and trapped air bleed through it during the pour. The result is fewer surface blemishes and a finer finish. The cement paste at the surface picks up the texture of the fabric itself, which can range from a linen-like grain to smooth, coated sheens.
Beyond aesthetics, fabric formwork can produce structurally efficient shapes. Because the fabric flexes under the weight of wet concrete, it naturally forms curves that place material where it’s needed most. Research has shown that beams cast in fabric molds can carry over 35 percent more load than prismatic (rectangular) beams with the same amount of steel reinforcement. The optimized shape also uses less concrete overall, with studies showing a 25 to 40 percent reduction in embodied energy compared to a structurally equivalent rectangular beam.
The Formwork Process Step by Step
A typical formwork cycle follows a consistent sequence regardless of the material or system used:
- Design and planning: Engineers produce drawings that specify panel sizes, bracing locations, and load-bearing requirements. Safety codes and cost constraints shape these decisions.
- Site preparation: The ground or supporting structure is leveled, and form locations are marked according to the construction documents. Reinforcing steel is placed as needed.
- Assembly: Panels, braces, and shores are erected and connected. Workers check for gaps, leaks, and any movement outside the design tolerances.
- Concrete placement: The mix is poured and compacted. During this phase, the forms experience their highest loads.
- Curing: Concrete typically reaches its initial set within 24 to 48 hours, though full strength development takes longer. Proper moisture is maintained throughout.
- Stripping: Forms are removed once the concrete has gained sufficient strength, generally between 1 and 7 days depending on the element, ambient temperature, and rate of strength gain. Field-cured test cylinders or nondestructive testing methods help verify that the concrete is ready.
- Cleaning and storage: Stripped forms are cleaned and stored for reuse on the next pour cycle.
Design Loads and Safety Margins
Formwork must be engineered to handle several types of load at once. The American Concrete Institute’s guide (ACI 347R-14) sets minimum thresholds that are widely followed across the industry. Horizontal surfaces like slab forms must support a combined dead and live load of at least 100 pounds per square foot, rising to 125 pounds per square foot when motorized carts are used on the forms. Wind loads on exposed formwork must meet local building code requirements but never fall below about 15 pounds per square foot.
Hardware like form ties and anchors are required to carry a safety factor of at least 2.0, meaning they must be rated to twice their expected working load. Anchors that also support workers, impact forces, and live loads need a safety factor of 3.0. These margins exist because a formwork failure during a pour can be catastrophic, both structurally and for the workers on site.
Safety Risks During Construction
The most common danger during formwork operations is falls. Research examining vertical formwork activities found that the top risk factors during concrete pouring are missing or inadequate fall arrest systems, unguarded platform edges, unsuitable floor coverings, and slippery surfaces. Worker inexperience and inattention also rank among the frequent contributing causes. Because wet concrete creates inherently slippery conditions and workers operate at height on temporary platforms, fall prevention measures like guardrails, harnesses, and proper platform surfacing are critical at every stage of the formwork cycle.