Removing Forms from Poured Concrete

Knowing the optimal time to remove forms from poured concrete is crucial for a successful project. Forms contain the fluid concrete while it cures, but they must be detached before the concrete is subjected to any loads. Prematurely stripping forms can damage the concrete, but leaving them on too long wastes time and money.

Several factors determine when forms can safely be removed after pouring concrete:

Concrete Curing and Hardening Rate

Concrete curing refers to the chemical process that occurs as the concrete mix solidifies and gains strength. The rate depends on the specific concrete design, weather conditions, and other elements. In general, concrete cures slowly at first, then more rapidly, before slowing again over time. Forms can typically be removed after concrete has cured sufficiently to bear its own weight and expected loads without slumping or sagging.

Type of Concrete Mix Used

The ingredients in the concrete mix impact curing and strength gain. More cement content speeds curing, while larger aggregates slow it down. Concrete with accelerators like calcium chloride can cure very quickly. Fly ash, slag, and other supplementary materials slow the curing rate. High-early-strength concrete mixtures allow forms to be removed sooner.

Weather and Temperature

Hot, dry, and windy conditions accelerate concrete curing compared to cool, humid conditions. Concrete cures faster at higher temperatures. Forms can be removed sooner in hot weather, but curing takes longer in colder temperatures. Protective measures may be needed in extreme conditions before stripping forms.

Form Material and Shape

The construction material and shape of the concrete forms impacts form removal timing. Forms that are absorptive like wood will bond more with the concrete, requiring more time before stripping. Metal and plastic forms can be removed sooner. Intricate shapes may also need longer curing times before forms are detached.

Testing Concrete for Readiness

To confirm concrete is ready for forms to be removed, it should be tested on site. A simple method is using a hammer or rod to tap the concrete and listen for a clear, ringing sound that indicates adequate hardness. More precise strength testing involves making test cylinders from the pour to break test in a lab.

Minimum Curing Times Before Removal

As a general guideline, leave forms on vertical surfaces 6-48 hours after pouring, while horizontal surfaces need forms for 12-72 hours. High early strength concrete allows form removal in 16-24 hours. Cold weather doubles these minimum times. Being patient allows concrete to fully harden, preventing surface damage.

Preparing to Remove Forms

Proper preparation is essential before removing forms from poured concrete to avoid problems:

Gather Needed Materials

Have all necessary equipment ready for efficient form removal. This includes pry bars, hammers, wedges, slab lifters, ladders, and supports. For large projects, cranes, forklifts, or other lifting devices may be required.

Use Safety Equipment

Wear safety glasses, hard hats, back braces, gloves, and steel-toe boots when removing forms. Follow all OSHA guidelines to prevent injury from falling tools or concrete. Proper ventilation is needed if using solvents or chemicals.

Check Concrete Curing

Inspect, probe, or test the concrete to verify adequate curing before stripping forms. Look for bleed water drying on the top surface. Concrete should be hard with no crumbling or spalling when scratched.

Methods to Loosen Forms

Before attempting full removal, forms must first be loosened from the hardened concrete:

Lubricating Forms

Applying form release agents before pouring minimizes adhesion between concrete and forms. Vegetable oil, diesel fuel, or commercial releases allow easier form removal.

Tapping Forms

Use a hammer or mallet to lightly tap wood or metal forms. This helps break the concrete's grip. Avoid hitting too forcefully to prevent damaging the concrete.

Wedging Between Forms and Concrete

Insert wood or plastic wedges between the form and concrete surface. Gently tap wedges to separate the form. Place wedges gradually around the form to evenly break the bond.

Steps for Removing Forms

Follow proper techniques when stripping forms from cured concrete:

Loosen Any Attached Debris

Use a wire brush to detach any material adhered to the concrete like sawdust, chips, or debris before removing forms.

Extract Nails and Rollers

Remove any nails, screws, tie wires, anchor bolts, or rollers before attempting to detach the forms. This prevents damage to the concrete surface.

Start from Top Down

For walls and columns, begin removing forms from the top and work down incrementally. This avoids pressure damage caused by forms falling onto lower areas.

Work Slow and Careful

Stripping forms must be done with care to avoid cracking or gouging the tender concrete. Rush jobs often result in mistakes and damage.

Pry Forms Off Concrete Gently

Use wood wedges and pry bars to slowly work forms off the concrete. Apply steady, even force to avoid cracking corners or edges. Don't jerk or twist forms.

Tips and Precautions

Follow these crucial tips when removing forms from poured concrete:

Avoid Damaging Green Concrete

Concrete less than 24 hours old is extremely fragile. Premature form removal risks cracking or weak spots. Wait the recommended time before attempting removal.

Have Support Shoring if Needed

Some concrete elements like beams may require temporary posts or shoring for support when forms are removed. Prevent collapse or buckling.

Fill Holes and Defects

Fill any voids, holes, or honeycomb areas exposed after stripping forms. Use patching materials that match the color and texture.

Cure and Protect Surfaces

Apply curing compounds or moist curing methods to newly exposed concrete after forms are removed. Protect from rain, sun, wind, and freezing.

After Form Removal

Once forms are detached from cured concrete, additional steps are needed:

Clean Forms for Reuse

Thoroughly clean stripped forms of concrete residues and foreign material before reusing. Repair any damage.

Patch Small Defects

Use patching compounds to fill bug holes, gravel pockets, cracks or chips that become visible. Smooth repairs level with the surface.

Start Maintenance Procedures

Begin protective treatments on exposed concrete like sealing, painting, or coating to prevent deterioration.

Allow Concrete to Further Cure and Harden

Though strong enough for forms to be removed, concrete continues gaining strength over several weeks. Avoid heavy loads initially.

When to Call a Professional

In some situations, it's advisable to have concrete professionals handle form removal:

Large, Complex, or Dangerous Removals

For major construction projects, bridge forms, or tricky removals, a concrete contractor has the manpower, equipment, and expertise to remove forms safely.

If Concrete Won't Release From Forms

Improperly applied releases or unusual weather may prevent forms from detaching normally. A pro can determine the issue and remove forms without damage.