Structural Toppings on Precast Concrete Planks

Two different products on the same drawing

Precast plank decks get covered with one of two things: a screed or a structural topping. They look similar on a section drawing. They are priced, programmed and designed completely differently, and mixing them up at tender is one of the more expensive mistakes a bid can carry.

A screed levels the deck and receives the floor finish. It carries no structural duty. Depth comes from the deck survey and the finishes schedule.

A structural topping is a reinforced concrete layer, commonly around 75mm of C25/30 with fabric or fibre reinforcement, cast so that it acts compositely with the planks beneath. Done properly, published precast industry guidance puts the increase in capacity of a hollowcore section in the order of 20% to 60%, depending on the section, the interface condition and the detailing.

That uplift is a design output, not a site decision. The topping thickness, the reinforcement and the interface shear detail come from the structural engineer. Our job is to build exactly what the engineer has drawn, on a deck that is rarely as flat as the drawing assumes.

What the drawings must answer before you price

• Is it a screed or a topping? Where the drawing says topping and the specification says screed, resolve it before the tender goes in. The difference is measured in weeks and in reinforcement tonnage.
• What is the interface condition? Composite action depends on the plank surface. As-cast, brushed or prepared surfaces each carry different design assumptions, and the engineer's calculation only works if the surface on site matches it.
• Has the deck been surveyed? Planks arrive with camber and deflect under load. The real topping volume comes from a level survey, not from multiplying the drawn depth by the area. Volume changes the price and the load on the frame.
• How does the reinforcement land? Fabric needs laps, chairs and cover in a layer that may only be 75mm deep. Congested edge details and service penetrations need resolving on paper first.
• What finish does it need? A topping that receives a finish can be brush finished. A topping that is the finish usually wants power floating, which brings plant and late working onto an upper deck. See our note on self-compacting systems for where that matters.

Getting concrete onto an upper deck

The design is the engineer's problem. Placement is ours, and on a precast frame it is the part that decides the programme.

A topping pour on an upper deck needs concrete delivered at a steady rate to a floor plate that may have no crane availability and nowhere to stand a pump. We run two of the only three track-mounted placing booms in the country, plus our own line pumps, which means the concrete reaches the deck without craning plant up and without borrowing the tower crane from the frame contractor's cycle.

On schools, student accommodation and hospital frames, that is routinely the difference between a topping that slots into the floor cycle and one that sits on the critical path. It is the reason main contractors bring us the whole deck: screeds and toppings on precast planks are our day to day work, not an occasional job.

Programme, in one paragraph

A topping brings reinforcement fixing, a concrete rather than a screed material, curing, and usually a floated or brush finish. Allow for the fixing time deck by deck, agree the pour sequence with the frame cycle, and protect the topping while it gains strength. A screed, by contrast, is pumped, levelled and left. If your programme has assumed screed durations against a topping specification, the tender is carrying a gap. We price both, and we will tell you at tender which one the drawings actually describe.

Common questions

Pricing a precast deck now? Send the structural drawings and the finishes schedule to info@trentcs.com or call 01283 480110, and we will tell you straight whether it is a screed or a topping, and what it will take to deliver.