Researchers at the Massachusetts Institute of Technology have been ageing concrete by up to what in Nature would take 300 years, in order to establish how suitable the material is for the disposal of nuclear waste.
The team has also subjected the small cylinders of cement to all sorts of pressures, in an effort to simulate the forces exerted when the containers are buried underground.
The tests have led to new insights on what happens to concrete when it is weakened and put under stress. They ‘show the importance of thinking 3D when monitoring the durability performance of concrete in nuclear waste containment’, writes Professor Ulm and colleagues at the Institute in a paper to be presented to the American Society of Civil Engineers.
The aim of the team is to be able to merge the experimental results with a theoretical model being developed by Dr Mainguy, a CEE postdoctoral associate. Thus if a parameter changes, such as if groundwater begins to seep in around the structure, the team will be able to predict its eventual effect, and intervene in time to slow down and reverse the ageing.
The ageing of concrete has much in common with that occurring in human bones, where calcium is the important strength-giving component. Concrete weakens when water leaches the calcium from it, in a process akin to osteoporosis. To accelerate this process, the MIT researchers replaced the water with a highly concentrated solution of ammonium nitrate, which makes the leaching occur at a much higher rate.
To expose the weathered material to stress, the team placed samples into a triaxial high-pressure cell that applies pressure from all sides. When they applied a shear, or slightly larger stress from one side, shivers of the material slipped apart, marking the first time a team has studied the behaviour of weathered concrete under triaxial stress.
The current lab test can comfortably predict ageing up to about 300 years and Professor Ulm is confident that the work can be extrapolated to over 1000 years, the time that spent nuclear fuel containers buried at Yucca Mountain is supposed to maintain integrity.