Will those cracked walls hold up? Is it safe to walk on that quay? To find the answer to these questions assistant professor of mechanics of masonry Rita Esposito studies brick masonry, often using actual pieces of damaged houses and centuries-old bridges.
Brick masonry is made up of a number of components and that makes it a much more complex material than, for instance, concrete, Esposito says. ‘Brick masonry usually consists of bricks and mortar joined together both vertically and horizontally. I am particularly interested in the process of crack formation and how a small fracture may affect the load bearing capacity of an entire building or construction. In order to understand that process I study material as well as structural mechanics.’
Esposito was one of the project leaders of research into the effects of induced earthquakes on the masonry of houses in Groningen. ‘A lot of research has been done into the effects of quakes on masonry buildings, but it had never included typically Dutch houses with their slender walls and big windows. We needed to experiment to see how they held up, so we built two houses and a couple of walls in the TU Delft lab. We also studied the masonry of actual houses that had become unfit to live in or were up for demolition.’
The research into the Groningen masonry led to a database containing the properties of different types of masonry in terms of construction years and types of bricks and mortar. Thanks to this comprehensive database engineers are able to analyse the masonry in similar Dutch houses quickly and accurately, without having to carry out potentially damaging tests of the existing masonry.
Core testing method
To achieve an accurate assessment of a building, properties such as the stiffness, strength and toughness of the masonry have to be looked at. That requires taking samples from a large part of the building’s surface and testing them until they break. ‘Of course you can’t just take a wall from a building and destroy it,’ Esposito says. Instead, she uses the core testing method, which requires only a small drilled out cylinder, or core, to provide the data she needs. The method is commonly used for testing concrete, rocks or asphalt. ‘As far as masonry is concerned we are one of the few researchers in the world to apply it.’
Thick masonry from an Amsterdam bridge
Esposito is currently collaborating with the municipality of Amsterdam and the Amsterdam Institute for Advanced Metropolitan Solutions (AMS) in a research project into the suitability of core testing in masonry that is thicker than that found in houses. The team retrieved a piece of one of the pillars of a 19th century bridge, which was undergoing renovation work at the time, measuring no less than 1.4 meters, enough for dozens of core samples of different lengths and diameters. Esposito set about testing the samples for strength, stiffness and toughness, just as she had done with the Groningen house masonry. The difference lies in the thickness of the material, Esposito explains. ‘We need to increase our understanding of the behaviour of this thicker masonry. It is important because this building technique is so typical for Dutch cities with canals.’
Another difference with house masonry is that the pillars of bridges, as well as quays, are exposed to water. ‘We don’t quite know yet what the effect of the water is on the remaining capacity of a construction. The inside and the outside of thicker masonry may also be differently affected. That is why we are testing different bits of the samples in the thickness of the pillar.’
Safety and maintenance priorities
If core testing proves to be a viable option for thicker masonry the advantages could be substantial, Esposito says, for instance when assessing the need for maintenance. It will no longer be necessary to tear down parts of quays or bridges. A simple sample will quickly provide information of the actual structure. With this information models can more accurately predict how urgent the maintenance work is. This will enable the local authorities to prioritise maintenance and renovation work accordingly. It will also cut down on unnecessary work and cost. Esposito: ‘A city like Amsterdam, with its two hundred kilometres of quay walls, would definitely profit from this approach.’
Smarter and simpler models can still make accurate predictions without the need of invasive tests.
Esposito also has a solution for historical buildings which are often too fragile to sustain even minimally invasive sampling. She is working on a new model to ascertain the capacity of the centuries old masonry without doing damage to the structure. ‘Existing models can do the job but the problem is that to calculate it you need a lot of information about the properties of the material. And that takes a lot of sampling. Smarter and simpler models should be able to use limited input from non-invasive tests and still make accurate predictions.’
Published: December 2022