Bricks May Not Be Perfect, But Building Will Stand
Cadets Test 18th-Century Reproduction Bricks Using Modern Engineering Standards
LEXINGTON, Va., Aug. 6, 2012 – Two cadets set out this summer to find out just how strong bricks and mortar made the old-fashioned way really are. It turns out they were using modern techniques to find out what brick masons three hundred years ago already knew.
Cadets Thomas McConnell ’13, under the mentorship of civil and environmental engineering professor Col. Gary Rogers, and Peter Kniesler ’13, under the mentorship of Col. Grigg Mullen, also a CEE professor, have been mixing mortar and smashing bricks, using the standards and techniques of 21-century engineers to find out just what the Colonial Williamsburg Foundation’s reproduction bricks and mortar can and cannot do.
A good example is the kind of brick called “salmon.” Made in the foundation’s kiln, which fires bricks just as they did in the 18th century, the salmon bricks are the ones placed farthest from the fire and therefore underbaked.
McConnell and Kniesler tested the compressive strength of the bricks using a device that supports the bottom of the brick while adding weight to the top. When the brick cracks, it has “failed,” or reached its maximum compressive strength.
“With the salmon bricks, … after the initial crack they keep taking more and more weight. You can watch the brick turn to dust and it keeps taking more weight, … even after technically they’d failed,” said McConnell.
As part of his project, McConnell traveled to Williamsburg to tend the kilns and learn about making and building with reproduction bricks and mortar. Exploring the construction of period buildings revealed just how much the 18th-century builders knew.
“At Williamsburg they found salmon bricks on the interior of the wall, brick dust,” said McConnell. “We realized that the brick masons back then knew that that’s how the bricks worked and if you placed them on the interior of the wall where there were bricks surrounding them, they would keep doing their job.
“It goes to show that back then they knew exactly what they had and how their materials reacted and they knew how to use those strengths and weaknesses to keep the structure from failing.”
Kniesler and McConnell have discovered that the reproduction bricks and mortar meet modern standards except for in compressive strength. Even there, said McConnell, “They were very close.”
Early in the project, Kniesler tried using the Colonial Williamsburg recipe to create reproduction mortar from scratch, i.e. from quicklime derived by burning oyster shells. It turned out to be surprisingly challenging to get the mixture just right for a mortar that was strangely different from modern mortar.
Modern mortar cures – a chemical reaction that bonds the lime in the mortar to the sand in it – in 28 days. Not so 18th-century mortar.
“It takes a very long time to fully hydrate. They’ve taken mortar from some of the houses and sent it off to a lab, and judging by the percentage of chemical reactions, this mortar was laid down yesterday,” said McConnell. Actually, though, “It was 200 years old.”
McConnell and Kniesler’s Summer Undergraduate Research Institute project is an outgrowth of previous collaborations between Mullen and the Colonial Williamsburg Foundation. In 2010, Mullen and VMI cadet timber framers helped the foundation raise the Charlton Coffee House, the first new building at Colonial Williamsburg in 50 years.
When the foundation approached Mullen with the request to test the bricks, he knew it would make an excellent project for his engineering students. Working with real materials makes the math more meaningful and helps develop what Mullen calls the “BS detector.”
“It’s a reality check. This is where the theory meets reality,” said Mullen. … This is where engineering judgment comes from.”
Engineers need that judgment when they’re in the field, unable to sit and calculate. They must be able to make good decisions without extensive calculations: “I don’t have perfect answers, but is this in the range that makes sense or do I have to holler no and stop it?” said Mullen.
So what about the fact that the bricks the Colonial Williamsburg Foundation is making and using don’t quite meet modern standards?
“The bricks are not perfect from a modern standpoint,” said Mullen, “but it’s awfully hard to argue with buildings that have been standing for 300 years.”