Concrete on Metal Deck: How to Get Composite Slabs Right
Quick answer
For a composite slab on metal decking, specify C30/37 concrete (C25/30 minimum), S3 slump (S4 for long-line pumps), 10 mm aggregate, w/c ratio at or below the code limit. Control shrinkage cracking with A193 mesh on continuous wire chairs at 1 m centres, holding 20 to 30 mm cover to the deck crest, or replace the crack-control mesh with an approved fibre blend (Novocon FE 1050 steel plus Fibermesh 150 e3). Power-float and laser-level after the bleed water has cleared, and expect a tolerance of roughly ±15 mm on the structural pour.
Composite slabs on metal deck are simple in concept. The metal decking acts as formwork and tension reinforcement, and the concrete works in compression. Getting it right in practice comes down to the details: the mix, the mesh, the chairs, and increasingly the use of fibre-reinforced concrete. This guide covers how to pour concrete on metal decking correctly, and it draws on BS EN 206 / BS 8500, SCI P300, Imperial College London testing, and manufacturer data from Sika and Propex.
1. The concrete mix: workability and compatibility
Workability (slump/consistence)
- Minimum S3 (100 to 150 mm) for most slabs. Easy to place and pump.
- S4 (160 to 210 mm) where long hose runs or line pumps are used.
- Confirm with your ready-mix supplier, and do not exceed the target water/cement ratio.
Aggregate size
Governed by the smallest of:
- 40% of the cover above the ribs
- the average trough width
- one-third of the minimum trough width
In practice, 10 mm aggregate is standard for pumped deck pours, as proven in SCI P287.
Strength and durability
- C25/30 minimum for composite slabs (XC1 exposure).
- C30/37 recommended for robustness and a consistent finish.
- Design to BS EN 206 plus BS 8500 (UK NA).
Cement content and w/c ratio
Reducing the aggregate size raises cement demand: add 20 kg/m³ for 14 mm or 40 kg/m³ for 10 mm, as per BS 8500 guidance. Keep the w/c ratio to the code limit for your exposure class.
2. Getting the level and finish right: power-floating and laser levelling
The metal deck and the supporting frame both deflect slightly under wet concrete, so you cannot achieve SR1 flatness on the structural pour alone. Typical tolerance is ±15 mm to datum near columns (Concrete Society guidance).
When you need a power-floated finish or a laser-flat level:
- Plan the sequence early. Deck deflection, pour order and temperature all affect the finish.
- Let the bleed water dissipate before floating. Start too soon, and you trap moisture. Start too late, and the surface hardens unevenly.
- Use laser screeds or laser-guided floats on large bays to hold level control across the deck spans.
- If a very tight tolerance is required, plan for a thin levelling screed after the composite slab has cured.
3. Mesh, cover and the right chairs
What the mesh is for. Mesh (A142 / A193 / A252) controls shrinkage and temperature cracking. It is not there for main bending strength. The deck and concrete act together to provide the flexural capacity.
Cover with the mesh. Per BS EN 1992-1-1 / BS 8500, nominal cover is measured from the deck crest to the underside of the mesh, not from the concrete surface.
| Exposure | Environment | Nominal cover |
|---|---|---|
| XC1 | Internal, dry | 20 mm |
| XC2 to XC3 | Humid / sheltered | 25 mm |
| XC4 / XF1 | External / moderate | 30 mm |
In site terms, the mesh sits 25 to 30 mm below the finished surface on an internal slab, giving about 20 mm cover from the top of the deck crest.
Chair selection and setup. Because the deck is ribbed, the mesh sags without proper support.
- Chair height = nominal cover + half the bar diameter. For example, A193 (7 mm bars) with 20 mm cover needs a 25 mm chair. If the mesh must align with the crest axis (around 50 mm up), use 50 mm chairs and verify the cover.
- Spacing: 1.0 to 1.2 m each way, or 0.8 m in high foot-traffic zones.
| Chair type | Best use |
|---|---|
| Continuous wire chairs (deckchairs / hystools) | Best for metal deck. Span the crests evenly. |
| Plastic strip chairs (flat base) | Where metal contact is not wanted |
| Single plastic bar chairs | Small infills or edges |
| Flat-bottom spacer blocks | Limited use only |
4. Fibre-reinforced concrete: what works, when and why
The best-documented fibre system for composite metal deck is the Novocon FE 1050 (steel) and Fibermesh 150 e3 (micro-synthetic) blend, proven by Imperial College London (2007) and SCI (2006) studies.
Proven performance
Imperial College tests on CF60 deck (C35/45 concrete) compared A142 mesh against fibre concrete (25 kg/m³ Novocon plus 0.9 kg/m³ Fibermesh 150 e3). The results:
- Fibre slabs had 18% higher deflection capacity but 10% smaller crack widths.
- Load to the first 0.25 mm crack: 10.6 kN/m² (fibre) against 9.84 kN/m² (mesh).
- Cracks in the fibre slabs were linear and fine. The mesh slabs showed wider, multiple cracks.
- SCI RT1096 (University of Bath) found the same fibre blend gave shear resistance exceeding A393 mesh in 130 mm slabs.
How the blend works
- Novocon FE 1050 steel fibres (25 kg/m³) provide tensile bridging and shear strength.
- Fibermesh 150 e3 (0.9 kg/m³) microfibres limit plastic shrinkage and surface cracking.
- Together, they give balanced structural and service performance, proven in composite slab testing.
- Both fibres comply with EN 14889 (Parts 1 and 2) and are approved for C25/30 to C40/50 mixes.
When to use fibre concrete
- To replace secondary crack-control mesh in internal slabs on metal deck.
- When approved with the specific deck profile (for example the MetFloor 60 / 80 / 55 series).
- When you want to cut labour and mesh handling without losing crack control.
- Always check the manufacturer and deck supplier approval for the exact profile.
When not to rely on fibres alone
- For primary structural reinforcement or load-bearing bar replacement.
- In thin toppings (under 100 mm) without deck composite action.
- Where no test data or approval exists for the fibre and profile combination.
5. Example specification wording
- Concrete: C30/37, S3 slump (S4 for long line pumps), 10 mm aggregate, w/c at or below 0.55.
- Reinforcement: A193 mesh to BS 4483, lapped 300 mm minimum, supported on continuous wire chairs (25 mm or 50 mm) at 1 m centres each way to hold 20 to 30 mm cover to the deck crest.
- Alternative (where approved): fibre-reinforced concrete using Novocon FE 1050 (25 kg/m³) plus Fibermesh 150 e3 (0.9 kg/m³), as tested with MetFloor / CF60 profiles.
- Finishing: power-float and laser-level after the bleed water clears. Expect ±15 mm tolerance, with a levelling screed to follow if SR1 flatness is required.
- Quality control: check the cover depth before the pour, and confirm mix workability and finished level after the pour using a laser.
6. Why this matters
Combine tested materials, the right mix, proper chairing and smart finishing, and you get a slab that:
- meets its design load and durability targets
- controls cracking and shrinkage
- lays level with minimal rework
- saves time and labour on site
That is what modern composite construction should be: fast, tested and reliable.
Frequently asked questions
What concrete should I use for a composite slab on metal decking?
C25/30 is the minimum for composite slabs on metal deck at XC1 exposure, but C30/37 is recommended for robustness and a consistent finish. Specify S3 slump (S4 for long line pumps), 10 mm aggregate, and design to BS EN 206 plus BS 8500.
What mesh and cover do I need?
A142, A193 or A252 mesh controls shrinkage and temperature cracking. Nominal cover is measured from the deck crest to the underside of the mesh: 20 mm for internal dry (XC1), 25 mm for humid or sheltered, and 30 mm for external or moderate exposure.
How high should the chairs be, and how far apart?
Chair height equals nominal cover plus half the bar diameter, so A193 with 20 mm cover needs a 25 mm chair. Space them 1.0 to 1.2 m each way, or 0.8 m in high foot-traffic zones. Continuous wire chairs work best on ribbed deck.
Can I use fibre concrete instead of mesh?
Yes, for secondary crack control on internal slabs, where the fibre blend is approved with your specific deck profile. The tested blend is Novocon FE 1050 steel fibres (25 kg/m³) plus Fibermesh 150 e3 (0.9 kg/m³). Do not use fibres alone for primary structural reinforcement, in thin toppings under 100 mm, or where no test data exists for the fibre and profile combination.
How level can a metal deck slab be?
You cannot achieve SR1 flatness on the structural pour alone, because the deck and frame deflect under wet concrete. Typical tolerance is around ±15 mm to datum near columns. For a flatter finish, power-float and laser-level after the water has cleared, or add a thin levelling screed after curing.
Which standards cover composite slabs on metal deck?
Design to BS EN 206 and BS 8500 for the concrete, BS EN 1992-1-1 for cover, and BS 4483 for mesh. Fibre reinforcement should comply with EN 14889 Parts 1 and 2. SCI P300 and P287 provide the design and construction guidance.
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