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 … Continue reading Getting Concrete Right on Metal Deck