Slab Formwork System Product

Overview

Slab formwork systems are the most widely used temporary structures in concrete construction, supporting horizontal floor slabs until the concrete reaches sufficient strength (typically 7 days at 28-day design strength). Unlike complex vertical formwork for walls, slab systems are primarily horizontal and distribute loads downward through adjustable props to the floor below (or ground level for the first slab). The economics of slab formwork dominate high-rise construction: a typical office building reuses the same formwork kit 20–30 times (one for each floor), with only slight modification between floors, achieving amortized costs of just USD 3–8 per m² of slab area.

The system architecture is straightforward: Adjustable Steel Props System carry vertical load, Main Support Beams distribute it horizontally, Concrete-Contact Panel Grid contact the concrete, and slab-formwork-bracing resists lateral forces. Dropheads and specialized edge support enable rapid removal of formwork from drop panels and thickened slab edges, allowing quick re-deployment to upper floors.

Adjustable Steel Props: The Vertical Load Path

The Adjustable Steel Props System are telescoping columns of remarkable engineering simplicity and reliability. Each prop consists of:

1. Inner Prop Tube: A 60 mm diameter steel tube, 2.5–3.5 mm wall thickness, forming the primary load-bearing member. Inner tubes are typically 1.5–3.0 m long in the collapsed state.

2. Outer Adjustment Tube: A 70 mm diameter tube telescoping over the inner tube, with a maximum extension of 0.3–1.5 m depending on the model. The outer tube slides smoothly and is prevented from rotation by a pin or guide.

3. Collar Screw Adjustment: A threaded collar with fine-pitch threads (typically 2 mm pitch, so one full turn raises the prop 2 mm) enables final height adjustment with precision ±5–10 mm. This is critical for achieving a level slab across bay dimensions of 10–20 m.

4. Base Plate & Load Spreader: A welded steel pad, typically 200×200 mm or 250×250 mm, distributes the concentrated load to the ground or formwork below. Base plates are designed for bearing pressures of 500–1000 kPa on firm soil, or 200–500 kPa on soft ground or lower formwork.

Props are rated in load classes: light-duty props (Class 1) handle 30–40 kN; standard props (Class 2) handle 50–70 kN; and heavy-duty props (Class 3) handle 80–100 kN. Load rating depends on both the tube wall thickness and the height of the prop—slender props at maximum extension are less rigid and may buckle under load if over-stressed.

The typical slab uses 80–120 props distributed across the floor area, arranged in a grid pattern spaced 1.2–1.5 m in both directions. A 1000 m² office floor (e.g., 30 m × 33 m), with 300 mm slab and 25 kPa concrete pressure, requires approximately 7500 kN of total support. With 100 props at 70 kN each, the system is fully subscribed with minimal safety margin—meaning the formwork designer must be meticulous about load estimates and prop distribution.

Main Beam Structure & Horizontal Load Distribution

The Main Support Beams form a two-level hierarchy:

1. Primary Beam: Main spanning beams typically 10–20 m long, running perpendicular to the building's longer dimension or aligned with the primary load path. These are often steel beams (IPE 200–IPE 330, or HEB sections) selected to span the full slab width with deflection not exceeding L/500 under full live load (i.e., 50 mm deflection maximum for a 25 m span). Primary beams rest on groups of props, typically 3–5 props per beam to distribute load evenly.

2. Secondary Beams: Perpendicular beams spaced 1.0–2.0 m apart, supporting the plywood panel grid and transmitting load back to the primary beams. Secondary beams are typically smaller (IPE 140–IPE 200) and are bolted or pinned to the primary beams or cantilevered brackets.

The two-beam system provides load redundancy and allows the plywood deck to span shorter distances, reducing plywood thickness and overall system weight. If a secondary beam were missing, plywood spanning 5+ meters would need to be extremely thick (40–50 mm) and would sag noticeably under vibration.

Concrete-Contact Decking & Surface Quality

The Concrete-Contact Panel Grid are the final contact layer between formwork and fresh concrete. They consist of:

1. Plywood Decking: Okoume or birch plywood, typically 24–30 mm thick, selected for hardness and smooth surface finish. Plywood is the industry standard because it is reusable (100+ times with care), affordable (USD 20–40 per sheet), and delivers excellent concrete finish when a release agent is applied. The plywood is typically laid with the grain running perpendicular to the secondary beams, increasing bending stiffness.

2. Panel Support Blocks: Timber or rubber bearing blocks under the plywood edges (placed every 0.6–1.0 m), preventing point-load concentrations and stress concentrations that could degrade the plywood. These blocks also allow rapid removal of plywood sheets once concrete sets, simply by pulling the block and sliding the sheet out.

Plywood is selected based on concrete load (pressure × time): a typical 300 mm slab with normal concrete placement (2.5–3 m/hour rising rate, vibration, poking and finishing) imposes 20–30 kPa static pressure and 40–60 kPa dynamic pressure (from vibration and impact). At these pressures, 24 mm plywood can span 2.0 m; 30 mm plywood can span 2.5 m. Going thinner requires closer panel support; going thicker wastes material.

Drophead System: Rapid Re-Deployment

Drop panels (thickened concrete around columns) require special edge support that must be removed quickly to allow rapid striking and re-deployment to the next floor. The Drop Head & Slab Edge System subsystem addresses this:

1. Drop Support Column: A dedicated steel tube (60–80 mm diameter) placed directly below the drop panel, with its own base plate and adjustment collar. Unlike props that support the main slab, drop columns support only the thickened edge load (typically 1.5–3.0 m² of extra concrete, or 15–25 kN additional per column).

2. Drophead Bracket Assembly: A bolted or welded bracket at the column head, connecting to the main formwork beam grid and allowing the drop column to be unbolted and removed within minutes of concrete setting (18–24 hours).

3. Jack-Head Removal System: Specialized systems use hydraulic jack-heads that can be lowered (either manually or with a small pump) to take the load off the drop panel formwork, allowing the formwork to be struck and lowered away while the concrete continues to cure on the jack-head support. This enables the same drop formwork kit to be re-erected on the next floor while the lower floor's concrete is still curing, significantly reducing cycle time.

Lateral Bracing & Vibration Resistance

The Lateral Bracing & Stability prevents lateral sway and torsion during concrete placement and vibration:

1. Diagonal Braces: Steel tubes at 45° angles connecting prop heads or beam nodes to nearby fixed points (typically the building structure or the floor below). These diagonals are essential when concrete is being vibrated, because vibration imparts horizontal dynamic forces (0.2–0.5 g acceleration) that can cause formwork instability if props are unbraced.

2. Sway Bracing (X-Patterns): X-pattern bracing in both horizontal planes (parallel and perpendicular to the main beams) ensures the formwork behaves as a rigid structure rather than a mechanism that can rack or twist. Typical practice is to brace every other or every third bay, depending on formwork span and wind exposure.

Without adequate bracing, concrete vibrators can excite formwork resonances, causing large-amplitude oscillations that put high stress on props and plywood, and can lead to visible ripples or waves in the finished concrete surface (called "luting" or "vibration imprinting").

Assembly, Striking, and Reuse

A typical slab formwork assembly sequence proceeds as follows:

1. Prop placement and leveling (2–3 hours): Props are positioned in a grid pattern, with base plates on firm ground or the lower slab. Each prop is roughly set to height using the extension mechanism, then finely adjusted using the screw collar.

2. Beam erection (2–3 hours): Primary and secondary beams are hoisted into place and bolted or pinned to groups of prop heads. Leveling is verified with a transit level or laser level.

3. Plywood laying (1–2 hours): Plywood sheets are laid on the secondary beams, with staggered joints and bearing blocks every 0.6–1.0 m. Edges are taped to prevent concrete from seeping through joints.

4. Bracing installation (1–2 hours): Diagonal braces are welded or bolted to connect the formwork grid to the building structure or props.

5. Concrete placement and striking (varies): Concrete is placed and finished over 4–16 hours. After 18–24 hours, formwork removal can begin:

   • Plywood sheets are removed first (pulled out from below or above, depending on access).
   • Secondary beams are unbolted and removed.
   • Props are lowered (either manually using a tool on the collar, or with a ratchet-equipped base jack) and extracted.
   • Primary beams are removed last.

Entire formwork removal typically takes 4–8 hours for a 1000 m² floor. The formwork is then cleaned, inspected, and re-erected on the next floor—the entire cycle is 2–4 days for a typical office building.

Variants & Special Applications

• Single-sided formwork: Props on one side only, with the opposite side against the structural slab below (used on upper floors).
• Drop-head systems: Hydraulic jack-head technology for ultra-fast redeployment to the next floor while concrete on the current floor is still curing.
• Aluminum beam systems: Lower-weight beams for easier manual handling, though less common than steel.
• Soffit falsework: Specialized formwork supporting suspended slabs, transfer structures, or cantilevers, often requiring additional bracing and load spreading.

Slab formwork remains the most economical and widely deployed formwork type globally, with annual market value exceeding USD 5 billion because nearly every concrete building requires it multiple times.