Upper floor and roof decks

During the second half of the 19th century and up until about 1925, fire resisting floors of filler joist construction were popular. A system known as the 'Fox and Barrett' floor was patented in 1844 and was used widely for the construction of commercial and public buildings up to about 1880.

Older proprietary systems are sometimes encountered. The Frazzi system of flooring was introduced in the 1890s and used up until the First World War. It comprised a series of steel filler joists and hollow pots, often with clay cover pieces to protect the bottom flanges of the floor joists. The manufacturer's details illustrate a floor made up primarily of coke breeze concrete, similar to that used in filler joist construction.

While there are a few varieties of filler joist floor, the basic form of construction of each is similar, comprising small (up to 150mm deep steel or iron joists) arranged in parallel lines at 0.6 to 1m centres. The space between the joists is filled with concrete which, to reduce the overall weight, was often constructed from coke or coal breeze - the fines left over from the production of coke for blast furnaces. The joists were usually arranged such that they had a flat soffit, usually plastered with a wooden wearing layer on the top surface. The top flange could either protrude slightly or be totally encased in concrete.

Sometimes, and particularly in public buildings, the floors were finished with terrazzo or granolithic surfaces. The difficulty with the form of floor is not so much the concept (filler joist floors at the Royal Albert Hall were found to provide a floor capacity of as much as 5kNm2) but the lack of corrosion protection afforded to the steel joists. Coke breeze concrete is not a particularly strong material but this did not matter because the strength of the floor was provided by the steel joists.

Typical filler joist construction

Industrial furnaces of the period were not particularly efficient, so substantial amounts of unburnt coal or coke could sometimes be left following combustion. The resultant materials could have a high sulphur content, which on oxidisation turns to sulphur dioxide. If moisture is present, sulphur dioxide converts to sulphurous acid, which in turn combines with more oxygen to create sulphuric acid, a highly aggressive chemical, likely to attack steel.

Normally steel is protected against corrosion in concrete because it is highly alkaline. Coke breeze is different insofar as it can be acidic and it is very porous (a factor that led to its choice in the first place). Furthermore, it has low resistivity, which means that high corrosion currents can occur.

Some investigations have demonstrated that the unburnt portion of carbon in the concrete could be as much as 30%, this throws in to question whether the so-called fire-proof floor would actually be partly combustible!

Site inspection

Given the lack of corrosion protection it is not surprising that steel embedded in coke breeze can decay very significantly in the presence of moisture. While the exposed flanges may appear to be sound or only lightly corroded, it is sometimes possible for the web to have corroded away completely. For this reason it is important to make an accurate diagnosis of the condition of the steel - usually by physical opening up and non-destructive testing methods.

The form of construction can usually be identified by a series of parallel cracks along the lines of the filler joists where they have corroded. Evidence such as this should certainly be investigated in detail, particularly in cases where change of use or refurbishment is considered. To reduce the risk of corrosion it is vital that the coke breeze is kept dry - easier in internal environments but a possible vulnerability where terrazzo or tiled floors are regularly swabbed down.

Methods of repair

It would be difficult to break out the entire coke breeze surrounding a corroded joist or joists. Resin injection treatments are available to be drawn in under vacuum to encapsulate the joist and to protect it against further corrosion. However, if the joist has been seriously weakened by section loss, replacement may be the only suitable expedient. Otherwise, a strategy of regular inspection and great care to keep the floors dry would appear to be a sensible precaution.