High Alumina Cement (HAC) Concrete

High Alumina Cement (HAC) was a form of cement used between the 1950s to 1970s in the UK and was predominantly used in precast construction. HAC is a cement manufactured from limestone or chalk and bauxite – clay that has extremely high alumina content. It was first produced for heat resistant lining of industrial chimneys. It gained popularity from the 1950s through to the 1970s, advertised for its ability to gain strength quickly. This allows for fast manufacture, as well as the removal and reuse of formwork.

Despite its advantages, its history is marked by controversy caused by highly publicised structural failures, in particular in the UK during the 1970s.

These were due to the HAC concrete undergoing changes in its crystalline structure – a process known as conversion. This process is triggered by changes in exposure conditions after construction, most notably warmth and moisture.

The ultimate strength of the concrete after conversion varies with its initial strength, and the severity of its effect is dependent on the way in which it is used, as well as whether it is reinforced  

One example of the failure caused by the use of HAC concrete was the collapse of two beams in the roof of the John Cass School in Stepney. Although this was initially attributed to high temperature and humidity as this was the roof of the swimming pool, serious weakening of the concrete in the gymnasium roof was also discovered, suggesting that the problem did lie with the HAC.

Following these problems, use of HAC concrete was effectively banned. However, a reassessment led to a lift in the ban, since more research was undertaken to understand the importance of the ferrite phase in improving durability and compressive strength in the material. Current advice is that suspected presence of HAC should be confirmed by chemical or laboratory testing of samples. If its presence is confirmed, it must be assessed for strength and long-term durability, although up to 50,000 buildings precast beams containing HAC are still in service in the UK now.

Refurbishment projects in the UK with precast concrete should be tested for HAC if there’s no records information for these structures. This is normally tested by taking a sample of concrete the testing should also check the level of conversion if HAC is found to be present within the concrete.

After testing there are some cases where the concrete strength has reduced so much that sections of the concrete have had to be removed completely. Alot of projects we have worked on the concrete structure can be retained both the areas where HAC were discovered require regular monitoring to ensure there is no ongoing deterioration.