1. Determining the Stripping Timeline
Side Forms (Outer Formwork): Under ambient temperatures, cure the precast elements for 12–24 hours after pouring, or wait until the concrete reaches a compressive strength of 1.2–2.5 MPa (verified when finger pressure leaves no visible indentation and corners remain firm). At this point, side forms can be safely removed.
Bottom Forms / Load-Bearing Formwork: Typically, the concrete must reach at least 75% of its design compressive strength before the bottom formwork can be completely removed or the element lifted away.
Small-Scale Plastic Molds: Demolding can be performed gently when the concrete reaches approximately 30% of its design strength, preferably during warmer periods of the day to leverage the higher thermal expansion rate of plastic relative to concrete.
2. Stripping Steel Cable Trench Formwork (Most Common)
Steel formwork generally utilizes a modular, demountable design (comprising two side panels, end plates, an inner core, and a base plate) secured with bolts or quick-release clamps:
Loosen Fasteners: First, remove the fixing bolts or clamps from the end plates. Next, loosen the connecting bolts, diagonal braces, and tie rods on both outer side forms.
Remove End Plates: Take off the front and rear end plates to release longitudinal constraints.
Strip Side Forms: Working in pairs, slide the left and right side panels outward horizontally (for split-type formwork, pull them directly apart). If necessary, tap the outer frame lightly with a rubber mallet or wooden hammer to facilitate separation. Never strike the formwork's working face directly with a steel hammer.
Extract the Inner Core / Base Plate:
Short Trenches (≤ 1m): Pull the inner core vertically upward. Leave the finished precast element on the base plate to be lifted out later.
Long Trenches (> 1m): Typically configured with a flat mold and a movable base. Release the base plate fixtures and use lifting gear to hoist the precast element vertically. The base plate remains stationary or can be rolled out along a rail track.
Cleaning and Oiling: Immediately scrape off any residual slurry using a wire brush $\rightarrow$ rinse with water and wipe dry $\rightarrow$ apply a uniform layer of form release agent (oil-based, water-based, or neat oil) to prepare the formwork for the next production cycle.
⚠️ Note: When extracting the inner core from large-scale steel formwork, account for the "clamping effect" caused by concrete shrinkage. If extraction proves difficult, use a chain hoist to pull the core out slowly and evenly. Avoid aggressive single-sided prying, which can damage the internal corners of the precast element.
3. Stripping Plastic (PP/ABS) Cable Trench Molds
Plastic molds are generally monolithic U-shaped open molds or small modular assemblies:
Apply Release Agent: Coat the interior surfaces uniformly with a high-quality release agent prior to pouring. This is a critical step for ensuring seamless demolding in plastic systems.
Inversion and Tapping Method (Most Common):
Invert the mold with the cured concrete element 180 degrees onto a clearance rack or two parallel wooden blocks.
Tap the exterior walls and bottom perimeter of the mold lightly with a rubber mallet or wooden hammer. The resulting vibration, combined with the self-weight of the concrete element, will cause it to drop out smoothly.
If necessary, activate a vibrating table at low intensity to assist the process.
Thermal Assistance: If adhesion is tight, pour warm water over the exterior surface of the mold. The localized thermal expansion will break the bond. Never use metal pry bars, as they will crack or deform the plastic mold.
Cleaning: Remove residual slurry using a soft cloth or a bamboo scraper. Wash with water, allow to air dry, and reapply release agent.
4. Stripping Cover Plate Molds
Cable trench cover plate molds (typically flat molds or shallow-tray plastic molds/steel-framed plastic bases):
Once the concrete achieves its initial set, invert the mold and tap the four corners of the back surface gently with a rubber mallet to let the cover plate slide out. Alternatively, release the steel frame latches and lift the upper mold frame directly upward, leaving the precast cover plate in place.