Why does concrete swell and crack?

Unfortunately, concrete does not last forever. The ravages of time are also gnawing at concrete structures in Switzerland. Not only steel-reinforced structures such as bridges are affected, but also concrete structures without reinforcement, such as dams. One of the reasons for this is the so-called alkali aggregate reaction (AAR). It can affect all concrete structures in the open air. In AAR, the ingredients of the concrete itself are the problem: cement – the “glue” of the concrete – contains alkali metals such as sodium and potassium. The moisture in the concrete thus becomes a caustic solution. The main components of concrete are sand and gravel. These in turn consist of silicates, for example quartz or feldspar. The alkaline water reacts with these silicates and leads to the formation of alkali calcium silicate hydrate. This mineral stores more and more water molecules in its structure, thereby expanding and bursting the concrete from the inside over time.

Significantly shortened durability of structures

Remarkably, the same reaction takes place in numerous gravel grains that are stuck in the concrete; the stones are blown up individually. The pressure that this microreaction can exert on an entire structure is enormous: a dam, for example, can expand by several decimetres. This can lead to damage at the lateral connection points to the rock or to deformations in the area of locks. The reaction is slow, so that the first damage to affected structures only becomes noticeable after ten to 15 years. However, continuous swelling of the concrete can greatly shorten the durability of structures.
In 2015, a team of scientists from Empa and the Paul Scherrer Institute (PSI) succeeded for the first time in identifying the structure of the water-containing crystal that triggers swelling in the concrete. Previously, the structure had been the subject of much speculation.
The discovery was the initiator for an interdisciplinary research project funded by the Swiss National Science Foundation (SNSF). In addition to Empa and PSI, two EPFL institutes are involved; research activities are coordinated by Empa researcher Andreas Leemann. “We want to investigate and understand the AAR in all dimensions, from the atomic level and the length scale in the Angström range to the effects on entire buildings in the centimetre and metre scale,” explains Leemann.

Six subprojects for all dimensions

To this end, six subprojects have been defined in the SNF-Synergia project: PSI uses synchrotron radiation to investigate the structure of the reaction products in order to explain their sources. The EPFL is investigating the conditions that determine the dissolution of the silicates and the composition of the initially formed reaction products; computer simulations are also being used to investigate the effects of swelling on buildings. At Empa, the formation of cracks in concrete is recorded spatially and time-resolved using computer tomography at the Empa X-ray Centre, while the water-containing crystals are synthesised in the laboratory. This enables researchers to obtain larger quantities of the substance that is usually found in nano- to micrometer-sized cracks in the gravel grains. However, physical properties can only be precisely determined with larger quantities of the substance in question. The knowledge gained in this way should not only serve to better understand the AAR, but also to show ways in which damage – and thus costs – can be avoided.

“We are already in the midst of deciphering this phenomenon, which has so far only been known in part,” says Leemann. The four-year research project started in May 2017. The first results are already available. The next step is to network the individual working groups more closely and build on the results of the partner groups. In the end, a complete picture of the AAR is to be created, which will make it possible to better assess the condition and hazards of concrete structures and to scientifically accompany the fate of the affected structures.