Brittle concrete walls: Empa researchers find cause

"Our houses are crumbling!" reads a placard at a demonstration in Dublin. "100 % reparations," reads T-shirts worn by participants - seen in a photo in the Nov. 15 online edition of The Irish Times. For years, unstable concrete in County Donegal has caused massive damage to many homes, which a government task force has also been dealing with since April 2016. For years, there have been demonstrations, disputes in parliament - and the costs for damages and compensation are likely to amount to up to three billion euros; estimated, today's status.

The headlines on this case are now also putting Empa researchers in the spotlight. The reason: at a specialist conference on this sensitive topic in Letterkenny, Ireland, on November 15, Andreas Leemann gave a presentation on concrete technology. He described the results of analyses on unstable concrete carried out together with environmental researcher Paul Dunlop from Ulster University and other experts - and contradicted a hitherto widespread assumption: a high proportion of the mineral mica, according to the previous theory, makes the building material susceptible to frost damage when its pores are filled with water.

But no; the Swiss experts see the cause in another component that the experts in Ireland had not previously targeted: Detailed examinations of concrete samples from four affected houses showed that a mineral called pyrrhotin, made up of iron and sulfur, is present in the building material in large proportions - and apparently triggers a disastrous cascade.

When pyrrhotine in the cement paste of concrete is oxidized by oxygen present, this releases sulfur, which in turn leads to the formation of ettringite. This mineral is formed anyway during the hardening of cement - but the additional ettringite formation leads to expansion, which eventually causes cracks in the concrete.

If further sulfur is released, the mineral thaumasite is formed. This process reduces the strength of the concrete by dissolving important constituents such as calcium silicate hydrates, and may ultimately result in the disintegration of the building material.

Detective research work

In order to be able to clarify and prove the damage mechanisms inside this special concrete, the Empa team admittedly had to do some detective work - from conventional material tests to complex scanning electron microscope and X-ray examinations to thermodynamic modeling. And in addition, research into meteorological data was carried out in order to get to the bottom of the common theory of frost damage caused by an excessively high mica content.

The on-site inspection had already contradicted this assumption. Typical cracks due to the influence of cold run parallel to the walls, whereas here they only ran vertically into the walls - often from the floor up to the roof. In some cases, they had even continued downward from above, where the influence of cold is less.

As it turned out, the meteorological awards also clearly argued against the cause being frost. Weather phases of the necessary cold and duration that could have triggered such damage occurred too rarely in the Atlantic-influenced climate of the region, namely only twice: from mid-December 2009 and from November 2010. In addition, the first damage was already evident before these events, and in the affected houses, widening of the cracks can be observed even in summer months. They can also affect interior walls that are protected from the elements.

Conclusion: Frost plays no or at best a minor role in concrete damage.

Limit values exceeded in EU building regulations

But why was the new explanation due to the pyrrhotine content and its consequences overlooked before? According to Andreas Leemann, this possibility is simply not taken into account in the relevant Irish building regulation IS 465. In contrast, the corresponding EU standard EN 12620 stipulates that any pyrrhotine content in the building materials must be taken into account in the sulfur content. "We analyzed the sulfur content of the aggregate of the concrete in samples from four houses," Leemann told The Irish Times, "the values exceeded the permissible EU limit by a factor of 4 to 7."

Although the Empa experts examined samples from only four houses in detail, it stands to reason that the results can be generalized, as data from Irish engineers from nearly 100 affected houses show that they also contain considerable amounts of pyrrhotine.

Those affected sometimes in difficult situations

The insights from Switzerland have already triggered a great response in Ireland, as shown by reports on Irish television and radio and in newspapers such as the "Irish Times". And the topic will continue to receive a great deal of attention in the future, especially since the Empa team will be investigating the concrete problem in greater depth in a project financed by the Irish state together with international partners. At the symposium in mid-November, two people affected by the problem were also present and reported on their situation. Among other things, the public has criticized the fact that an application for state compensation has so far only been possible for affected primary residences. In the meantime, it has become known that the most severely affected homeowners will receive state support of up to 20,000 euros in a first step, for example to move out and find a temporary place to stay or to store their possessions.

The Empa expert was able to get an idea of the conditions for those affected during his on-site investigations and document them photographically: cracked facades, large areas of chipped plaster, completely decomposed concrete and severe mold damage. In one of the houses, they were so pronounced in the kitchen and bedrooms that the family of three has been sleeping in the living room for a long time - conditions you wouldn't expect in Europe. "These people," Leemann said, "are living in a ruin."

Source: Swiss Federal Laboratories for Materials Testing and Research