Every day you come in contact with concrete structures, whether it’s crossing Sint Sebastiaansbrug on your way to campus or sitting in your office in EWI.
Most of us never even consider the structural integrity, durability or service life of these structures. However, a lot of research is done to predict performance in order to save money and lives. Some of this research is conducted on our very own campus, by the Concrete Structures Group over in Stevin Lab II.
But how do we know when a structure will fail (preferably without actual failure)? PhD candidate, Patrick van Hemert, from the Netherlands is working with one technique to find out how. By proof loading structures and simultaneously measuring acoustic emissions van Hemert investigates whether there are indications of reaching the total load capacity. Once you stress a material, cracks begin to form internally and the propagation of these waves is measured with acoustic emission sensors on the surface. “We hope to find the relationship between the failure mechanism and the acoustic emission signals we receive,” said van Hemert. Taking his tests to the field, van Hemert applied smart proof loading to a bridge in Friesland. Since the bridge was no longer in service, they were allowed to load to the point of failure and they found that the shear capacity of the bridge was higher than expected.
In addition to extending the service life of a structure, researchers in Stevin II are also working to understand and improve durability. Andrija Blagojevic, a PhD candidate from Serbia, is working to determine the influence of cracks on the durability of reinforced concrete structures with emphasis on chloride-induced corrosion. When chloride penetrates through the concrete to the steel reinforcement, corrosion begins and the structure gradually loses its integrity. Cracks are unavoidable in practice (due to thermal effects, loading and other expansive reactions) but it is not known exactly how much cracks facilitate the ingress of chloride ions, water and oxygen- the agents necessary for the corrosion process. To better understand the influence of crack width on the durability of concrete structures, Blagojevic casted 32 reinforced concrete beams, induced predefined cracks and subsequently placed them in alternating cycles of two days submerged in a 3.5 NaCl solution and with five days of drying. “This situation simulates the most aggressive environmental conditions,” he said. The goal of these tests is to determine the relationship between surface crack width, concrete cover, crack frequency and loading conditions on the corrosion of the steel reinforcement.