The definitive guide to achieving level, crack-controlled, and tested composite slabs

Composite slabs on metal deck are simple in concept – steel deck acts as formwork and tension reinforcement, concrete acts in compression. But getting it right in practice comes down to details: the mix, the mesh, the chairs, and, increasingly, the use of fibre-reinforced concrete. This guide draws from 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 (100–150 mm) for most slabs; easy to place and pump.
S4 (160–210 mm) where long hose runs or line pumps are used.
Confirm with your RMX supplier; don’t exceed target water/cement ratios.

Aggregate size

Governed by the smallest of:
- 40 % of cover above ribs
- average trough width
- one-third of minimum trough width
In practice, 10 mm aggregate is standard for pumped deck pours proven in SCI P287.

Strength and durability

C25/30 minimum for composite slabs (XC1 exposure)
C30/37 recommended for robustness and consistent finish.
Design to BS EN 206 + BS 8500 (UK NA).

Cement content / w:c ratio
Reducing aggregate size raises cement demand:
+20 kg/m³ (14 mm) or +40 kg/m³ (10 mm) as per BS 8500 guidance.
Keep w:c to the code limit for your exposure class.

2. Getting level and finish right – power-floating and laser levelling

Because metal deck and the supporting frame deflect slightly under wet concrete load, you can’t 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 laser-flat level:

Plan sequence early – deck deflection, pour order, and temperature affect finish.
Allow bleed water to dissipate before floating. Start too soon and you’ll trap moisture; too late and the surface hardens unevenly.
Use laser screeds or laser-guided floats for large bays to maintain level control across deck spans.
If very tight tolerance is required, plan for a thin levelling screed after composite curing.

3. Mesh, cover, and the right chairs

Mesh purpose

Mesh (A142 / A193 / A252) controls shrinkage and temperature cracking – it’s not for main bending strength. The deck and concrete act together to provide flexural capacity.

Cover to mesh

Per BS EN 1992-1-1 / BS 8500, nominal cover is from deck crest to underside of mesh, not from concrete surface.

Exposure	Environment	Nominal cover
XC1	Internal, dry	20 mm
XC2–XC3	Humid / sheltered	25 mm
XC4 / XF1	External / moderate	30 mm

In site terms, this means the mesh sits 25–30 mm below the finished surface for internal slabs – giving about 20 mm cover from the top of the deck crest.

Chair selection and setup

Because the deck is ribbed, the mesh will sag without proper support.

Chair height:
Chair = nominal cover + ½ bar diameter.
Example: A193 (7 mm bars) with 20 mm cover → 25 mm chair.
If the mesh must align with the crest axis (~50 mm up), use 50 mm chairs and verify cover.

Spacing:
1.0 – 1.2 m each way (0.8 m in high foot-traffic zones).

Types that work best on deck:

Type	Use
Continuous wire chairs (deckchairs / hystools)	Best for metal deck; span crests evenly
Plastic strip chairs (flat base)	Where metal contact not wanted
Single plastic bar chairs	For 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 with fibre concrete (25 kg/m³ Novocon + 0.9 kg/m³ Fibermesh 150 e3).
Results:
- Fibre slabs had 18 % higher deflection capacity but 10 % smaller crack widths.
- Load to first 0.25 mm crack: 10.6 kN/m² (fibre) vs 9.84 kN/m² (mesh).
- Cracks in fibre slabs were linear and fine; mesh slabs showed wider, multiple cracks.
SCI RT1096 (University of Bath) found the same fibre blend provided 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 offer balanced structural and service performance – proven in composite slab testing.
Both fibres comply with EN 14889 (Parts 1 & 2) and are approved for use in C25/30–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 (e.g. Metfloor 60 / 80 / 55 series).
When you want to reduce labour and mesh handling without sacrificing crack control.
Always check 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 (<100 mm) without deck composite action.
Where no test data or approval exists for the fibre/profile combination.

5. Example specification wording

Concrete: C30/37, S3 slump (S4 for long line pumps), 10 mm aggregate, w/c ≤ 0.55.
Reinforcement: A193 mesh to BS 4483, lapped 300 mm min, supported on continuous wire chairs (25 mm or 50 mm) @ 1 m centres each way to maintain 20–30 mm cover to deck crest.
Alternative (where approved): Fibre-reinforced concrete using Novocon FE 1050 (25 kg/m³) + Fibermesh 150 e3 (0.9 kg/m³) as tested with Metfloor/CF60 profiles.
Finishing: Power-float and laser-level after bleed water; expect ±15 mm tolerance. Levelling screed to follow if SR1 flatness required.
Quality control: Check cover depth before pour; confirm mix workability and finish level post-pour using laser.

6. Why this matters

By combining tested materials, the right mix, proper chairing, and smart finishing, you get a slab that:

Meets design load and durability targets
Controls cracking and shrinkage
Lays level with minimal rework
Saves time and labour on site

This is exactly what modern composite construction should be – fast, tested, and reliable.

Fibre Concrete on Metal deck: To read, click here

Getting Concrete Right on Metal Deck