Understanding the effects of different heating rates on green concrete structures is of primary importance to engineers and industry. This is because heating of concrete and refractory materials can result in serious problems, particularly if an explosive spalling event occurs. If an explosive spalling occurs, projectiles of reasonable mass (1-10 kg) can be thrust violently over many metres. While surface spalling is less violent the extent of damage can still be severe and in both cases repairs will be required resulting in significant costs to industry.
There are two drivers for spalling of refractory concrete – thermal strain caused by rapid heating and internal pressures due to the removal of water. Thus being able to predict the outcome of different heating rates on thermal stresses and internal pressure during water removal is particularly important to industry using refractory and other concrete structures.
The current approach to drying and heating of refractory concrete is to follow manufacturer’s procedures, which are conservative, and have not changed in the past 30 to 40 years. It is also known that even following these very conservative heating schedules, explosive spalling of refractory concrete still occurs. These dryout schedules show heating rates varying from 20°C/hr to 40°C/hr and lengthy hold periods which can result in schedules taking several days before industrial process is at full production. Such refractory dryout schedules have no scientific basis and are very costly for industry.
Our work has found:
Existing manufacturer’s heating curves are unreliable and will not prevent explosive spalling.
Increasing the heating rate can increase the maximum pore vapour pressure.
The use of thermal hold periods during drying does no aid nor will it prevent spalling if the hold period is not within the pressure peak zone.
A thermal hold period can decrease the maximum pore pressure if it is correctly placed within the pressure peak zone.
At a hotface temperature of approximately 360°C to 400°C a pressurized liquid front (build-up of water) can develop due to the increased pore pressure which is greater than the local vapour pressure.
Explosive spalling starts when the peak pore vapour pressure initiates a crack and the sudden release of the pressurized liquid water into steam that acts as the propellant.