Here is a concrete example showing how props and formwork work together, step by step, for a common construction task: pouring a reinforced concrete suspended slab (e.g., a second-floor ceiling/floor) in a multi-story building.

In this example, we assume the ground floor slab has already been poured and cured.

Scenario: Pouring a Suspended Concrete Slab (5m × 5m) at 3m Height

Goal

Create a horizontal concrete slab 3 meters above the ground.

• Formwork creates the flat, smooth underside of the slab.

• Props hold that formwork up against gravity while the wet concrete (which is extremely heavy) cures.

Step 1: Layout and Setup

Props (Vertical Support)

• Acrow Props (adjustable steel props) are selected for this medium-height application.

• They are arranged in a grid pattern (e.g., every 1.2 meters in both directions) to evenly support the load.

• Each prop is adjusted to the correct height (approximately 2.9 m to allow for the thickness of the formwork and beam).

• Base plates are placed under each prop to distribute the load onto the ground floor slab.

• Horizontal bracing (scaffolding tubes or frame braces) is clamped between props to prevent lateral buckling under load.

Formwork Deck

• Plywood sheets (18–21 mm thick phenolic film-faced plywood) are laid across the tops of the props.

• Joists (timber or aluminum beams) are placed between the props and the plywood to distribute the load evenly and prevent the plywood from sagging.

• The plywood sheets are butted tightly together and sealed with tape or caulk to prevent cement slurry leakage.

Step 2: Integration at Beams and Edges

Edge Formwork

• Along the perimeter of the slab, vertical timber formwork (called “edge forms” or “stop ends”) is installed to contain the concrete.

• These edge forms are propped horizontally using raking props or nailed to the supporting structure to resist the outward pressure of the wet concrete.

Beam Formwork

• If the slab includes a dropped beam (a deeper structural element), separate beam formwork is created.

• Beam sides (vertical plywood panels) are clamped together and supported by props directly underneath the beam.

• The slab formwork sits on top of or butts into the beam formwork.

Step 3: Load Transfer – How Props Support Formwork

When the concrete is poured:

1. Wet concrete weighs roughly 24 kN/m³ (approx. 150 lbs/ft³). A 200 mm thick slab exerts approximately 4.8 kN/m² (100 lbs/ft²) just from the concrete weight, plus formwork self-weight and live loads (workers, equipment).

2. This load is transferred:

   • From concrete → plywood (formwork)

   • From plywood → joists

   • From joists → prop heads

   • From props → ground floor slab (or ground)

3. The props remain in place until the concrete reaches sufficient compressive strength (usually 70–75% of design strength, typically after 7–14 days depending on mix and temperature).

Step 4: Stripping (Removal)

Props are not all removed at once.

1. Initial strip: After the concrete cures, the formwork deck (plywood and joists) may be removed while the props remain in place. This is called “propping back.”

2. Prop removal: Props are systematically removed starting from the center of the slab and working outward, following a sequence that ensures the slab is not overstressed before it reaches full strength.

3. Reuse: The props and formwork panels are cleaned, repaired if necessary, and moved to the next floor for reuse.

Visual Summary

Why This Matters

Without props:

• The formwork deck would collapse under the weight of wet concrete (often 50–80 tons for a typical residential slab bay).

Without formwork:

• The props alone cannot shape the concrete or provide a finished surface.

Together, the formwork defines the geometry, and the props provide the structural stability to resist gravity until the concrete becomes self-supporting.

If you would like, I can also illustrate a different scenario—such as a column and beam system or wall formwork with raking props—to show how the relationship changes depending on the structural element being poured.