Optical Approach to Fracture Testing Measures Concrete Strength

A light-based approach to measuring the strength of opaque heterogeneous concrete mixes applies a light-refracting coating, designed to display stress positions, to the surface of the concrete, then takes a photograph of the concrete surface and uses photo-stress analysis tomography (PSAT) to discover where stress levels in the concrete are most extreme, before cracks or fractures occur.

By taking advantage of the optical properties of a thin birefringent coating on the surface of opaque, notched composite concrete beams, researchers at the University of Leeds were able to sense the evolution of the maximum shear stress distribution on the concrete beam samples under loading.

A birefringent coating of 300-micron average thickness was applied to the beam samples. The birefringent measurements were made using reflective PSAT. The shear stress concentration factor was evaluated from the maximum shear stress distribution profiles displayed by the concrete samples.

The location of the maximum deviator stress was tracked ahead of the crack tip on concrete samples under the ultimate load, allowing effective crack length to be characterized. Using this measurement, the fracture toughness of a number of heterogeneous composite beams was evaluated.

A picture from a photonic camera showing how using the coating can create a candle-like 'flame' highlighting shear stress distribution in a sample concrete beam. Courtesy of University of Leeds.

“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,” said professor Joseph Antony. “The photonic method we developed can directly measure shear strain, even on opaque materials. Until now, photonic and optical methods of measurement have only been associated with transparent materials.”

Results of the research compared favorably with conventional methods of stress testing.

Based on the results, a model has been proposed, correlating the optically measured shear stress concentration factor and flexural strength with the fracture toughness of concretes. The photonics-based study could be useful in evaluating the fracture toughness of even opaque and complex heterogeneous materials more effectively in the future.

The increasing use of composite concretes, made from recycled waste products as well as natural materials, is what prompted the team to look for novel ways to evaluate the strength of concrete.

“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,” said Antony. “By working with industries which recycle the waste products into micron-sized particles, we had direct insight into how they are used, meaning our study could be much more informed by industry requirements.”

He added, “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.”

In addition to researchers from the University of Leeds, the research team included researchers from a Russell Group U.K. university and the University of Qatar.

The research was published in Scientific Reports (doi:10.1038/s41598-017-04782-7).