Optical coating provides indication of concrete stress

Optical method will help engineers assess the suitability of composite concrete for building projects

New types of concrete, intended to use up waste polymer materials and reduce the consumption of natural resources, are becoming more widespread, but they present engineers constructing buildings or infrastucture with a problem. While the physical properties of conventional concrete — made from cement, gravel, sand and steel reinforcement — are well understood, these composite concretes, which also contain particles of waste polymers — are not so well known, and this may curtail their use as its specifiers may not be certain when it is in danger of fracturing. Research from Leeds University.

composite concrete stress
Light reflected through the coating shows stress as a “candle-flame” effect in a photonic camera image

In a project carried out in collaboration with the Centre for Advanced Materials at Qatar University and partially funded by the Qatar National Research Fund, the Leeds team, led by Joseph Anthony of Leeds’ School of Chemical and Process Engineering, tested a series of concrete beams, made using fly ash from municipal waste incineration and ground-up polyethylene, by applying a thin epoxy coating that exhibits an optical property called birefringence — light waves reflected from the surface of the concrete through the coating split in different directions in relation to the amount of stress acting on the material in that direction. Putting the concrete under stress and taking pictures using a detector known as a photonic camera reveals where the stresses are most extreme before the concrete cracks.

While the birefringent epoxy is not new, it has previously been used with transparent materials and has not been used to measure shear stress or assess concrete toughness before. “There are other methods to measure stress and strain levels in the engineering sector, but we do not believe any of them can measure shear strain directly with high precision, which is most relevant to assess the failure strength of materials,” Anthony said. The team’s work has been published in the journal Nature Scientific Reports.

Leeds University is developing a centre of excellence in concrete research, but Anthony believes this work could have relevance in other areas. “Our study was aimed at developing a method by which plastic or polymer waste materials, in this case from Qatar, could be used as valuable ingredients in developing new engineering products,” he said. “We believe this new photonic or optical approach to fracture testing could be applied not only to develop sustainable manufacturing using materials that would otherwise be discarded as waste, but also in other diverse engineering designs including mechanical, civil, materials, electronics and chemical engineering applications.”