Precast concrete planks - durability and potential safety concerns

A researcher is carrying out an investigation on certain types of precast units and needs information. Over recent years there have been a small number of instances where sections of the soffits of precast concrete (pcc) floor units have become detached and fallen within buildings. It is understood that no injuries have been caused, but the potential for injury to building occupants exists.

There is a need to collate information upon such instances so that an understanding of the frequency of such collapses can be established. In addition, it would be desirable to establish the circumstances leading to such collapses and to seek to assess whether the risk of this form of failure increases with the age of the units and hence is likely to become more prevalent in the future.

The soffit detachment appears to arise from corrosion of the embedded steel reinforcement which, because of the hollow core form of the pcc floor planks and the location of the reinforcing bars within the cross-section, causes cracking at the bottom of the side walls of the pcc floor planks. This appears to cause delamination of areas of the soffit of the planks affected and this can pose a falling debris hazard to building occupants. 

There may be no readily observable visual indications of the development of the cracking or of delamination of areas of the soffit of the planks prior to the collapse. The planks may be of reinforced or prestressed construction.

It is understood that the pcc units which have experienced this problem have tended to have appreciable chloride levels, with the chloride being introduced at the time of casting. Whilst the presence of chlorides may lead to some pitting corrosion of the reinforcement or other embedded steel, the development of a significant degree of corrosion product would be expected to lead to cracking of the concrete as described above. 

The risk of cracking is expected to increase with age. In addition, the corrosion of reinforcement may potentially lead to a reduction in its cross-sectional area, reducing its strength.

The presence of moisture will further increase this risk. This may result in an increasing number of pcc units experiencing cracking and delamination.

The reporter would like to hear from those who have knowledge of such failures. Details to be provided should include:

• Location (in confidence if required)

• Indication of the amount of units that may be at risk in m²

• Brief description of incident

• Manufacturer of pcc units / other details about the construction of the pcc units involved

• Age of pcc units

• Environmental conditions (dry, wet, condensing, internal, etc)

• Chloride content by mass of cement

• Concrete carbonated / not carbonated (if known)

It is also known that problems of cracking and soffit delamination have occurred in some in-situ slabs within buildings. It would be helpful to hear about these problems as well, but the form of construction should be clearly identified as being in-situ.

The reporter has knowledge of these problems occurring in a limited number of properties. The earliest are thought to date from the 1950s. Potentially this issue could extend to pcc units manufactured in the early 1970s when the use of calcium chloride as a set accelerator was stopped.

The problems experienced to date appear to relate mainly to circumstances where the pcc planks have been used to form a flat roof. In a roof structure moisture to promote corrosion would usually be more readily available than in most internal floors, perhaps due to problems with the water proofing membrane or to condensation effects. In the longer term the presence of chlorides is expected to lead to an increasing risk of corrosion-induced cracking.