Upper floor and roof decks

While timber had for centuries been the standard means of constructing upper floors in all forms of building, by the 19th century it became desirable to provide floors of improved fire resistance, particularly within mill and industrial buildings. Initially, cast iron and brick vaulting was used, but this later gave way to wrought iron and concrete followed latterly by mild steel and then various forms of reinforced concrete. The widespread use of mild steel had, by the early 1900s, fuelled a demand for floors of lighter weight and fast erection.

In-situ concrete floors provide good sound and fire protection but are heavyweight structures that need some form of temporary support during construction. To reduce the overall dead load of the structure, void formers can be used. While there are a variety of void formers available today, those that gained most popularity were hollow clay pots, woodwool formers and waffle slabs.

Both hollow clay pots and woodwool formers can suffer from similar disadvantages: both can be difficult to detect in existing construction and both can lead to a need for expensive repairs.

Hollow clay pot floors were common in the post-war years but their use gradually declined and in the mid-1980s the London Brick Co ceased manufacturing them. Made from extruded clay forced around a moulded die, they formed a durable and robust method of construction when used between reinforced concrete ribs and with a structural concrete topping. Floor structures using hollow pots remain common in continental Europe.

Some hollow pot floors date back to the early 20th century. In 1906 Keine floors were first used. Later variants introduced concrete ribs between pots as tensile reinforcement in much the same way as contemporary floors. The early floors incorporated hoop iron or steel in the mortar joints between the hollow clay blocks.

During the 1950s-1970s, woodwool was often used as permanent formwork as a convenient means of providing thermal insulation properties to exposed floor slabs such as in podium car parks, ground floor slabs, etc.

The floor comprises an arrangement of narrow, closely spaced (about 750mm typically) joists, with very thin concrete slabs between them to complete the floor surface. The construction of the floor system is started by building a formwork platform on which void formers are placed in the desired pattern. Reinforcement for the joists and slab are then placed. Concrete is cast around and above the void formers to create a ribbed slab with a smooth upper surface.

Tee-beam construction comprises a series of ribs and flat sections, with the shuttering formed of either pre-made woodwool slabs or specially formed units. This form of construction suffered the same drawbacks as flat slabs, except that additional care was needed in selecting the size of the steel reinforcement bars in relation to the dimensions of the rib, such that there was adequate space for say 20mm of aggregate to pass around the bars and encase them. Furthermore, loss of cement grout through gaps in the formwork or by soaking into the woodwool could again create honeycombing.

Because the permanent shuttering conceals the steel reinforcement, it is difficult to know whether full compaction and coverage of the steel has been achieved. Until withdrawn in March 2003, BS 3809:1993 recommended the cutting of small access points in the woodwool shutter to enable quality checks to be undertaken.

Broadly speaking concrete floors constructed with hollow clay pots fall into one of two categories:

• ribbed slabs where the topping is considered to contribute to the structural strength of the floor; and
• where the topping is not considered to contribute to structural strength, as a series of concrete ribs cast in-situ between blocks which remain part of the completed structure - the tops of the ribs may be connected by a topping of concrete (not necessarily of the same strength as that used in the ribs).

While floor specifications varied, it was common to find clay spacer tiles between the larger pot components. These spacer tiles were designed to set the width of the concrete ribs that would be cast later. Incorrect placing of the steel reinforcement, the use of large aggregate or poor compaction could lead to honeycombing of the concrete rib, with a potential loss of fire resistance, durability or, in extreme circumstances, strength. The defect is very similar to that experienced with woodwool but the problem is not often reported and hollow pot floors have not suffered the same lack of confidence that befell woodwool.

Possible voiding to ribs when spacer tiles are used with hollow pots

Two typical applications for woodwool are:

• flat slab construction; and
• tee beam construction.

The former is self-explanatory and is often found as additional insulation to car parks. In these locations, woodwool cement board (WWCB) was quite often left exposed or painted and so should be readily identifiable. However, when used internally, detection may be more difficult, as a common form of construction was to plaster direct to the woodwool soffit or to apply plasterboard linings thus concealing the shuttering from view. In such circumstances a surveyor should not treat the painted, plastered soffit at face value and simply assume that it is of reinforced concrete. Careful probing with a sharp awl pushed through the plaster may reveal the true nature of the substrate.

The problem with WWCB when used in this way is that it is impossible to know whether the steel reinforcement to the slab has been placed properly, or that it is properly embedded in concrete. The steel reinforcement should have been laid on spacer blocks, but these may have been displaced during the placing of the concrete, or missed out altogether. The effect of this would be that the steel would rest directly on the top surface of the slab such that concrete would not be able to be placed around it properly. As a result, strength and fire resistance will be compromised.

Possible voiding of concrete to rib (woodwool slab construction)

Non-destructive testing techniques can be used to identify whether any problems are likely to exist, although physical sampling may be more reliable. Removal of the shuttering and repairs using hand placed mortars or sprayed concrete may be necessary if defects are found.

What appears to be solid concrete - and may sound like solid concrete when tapped lightly with a hammer - may actually be weak, brittle clay tile or woodwool slab. Care should be taken during refurbishment or alteration, to ensure that new loads, particularly point loads placed on a floor slab, can be accommodated without punching through a hollow area.

Woodwool slabs as permanent formwork - in this case the ribs are visible