Making mortar and concrete, and the Roman Arch

After pyramids, post-and-beam structures, and cantilevers, we learned about arches---most notably, the Roman masonry arch. We discussed the use of arches in bridges later on in the unit, but for now we concentrated on learning the principles of weight transfer in an arch, and the role of the centering and the keystone.

We had this excellent book, which I thoroughly recommend---it is FULL of fun, simple, easy-to-understand activities that demonstrate architectural principles. One activity is making your own mortar and concrete, which is much simpler than I would have imagined. You just mix water, sand, and cornstarch (which plays the role of cement, to bind the other ingredients together) to make mortar. Add in gravel and you have concrete! 

(Do you know the difference between cement and concrete? You can learn about it here.)

For this mortar, we used 1/2 cup of cornstarch mixed with 1 cup of sand, and then added in boiling water (about 1/2 c or more) until the mixture is thick, but not too thick to stir and mold.

First we used our mortar to bind stones together in an arch. Our instructions said to find stones of roughly the same size and shape (these will be the voussoirs of the arch). Ours weren't completely uniform, but I guess they were close enough. You just stack the stones with enough mortar between them to hold them together.

Just like the Romans, we constructed our arch using a centering. This distributes the weight and holds up the stones before the keystone is placed. First we tried using this cup, but it wasn't cylindrical enough. See how the side stones don't touch the centering? This arch fell down as soon as the centering was removed.

This centering worked much better.

When we removed the centering, the arch stood! The forces of compression hold the sides in against the keystone and help distribute the weight away from the center, down the sides, and into the ground. We were so surprised and pleased with ourselves when this actually worked!!

Next, we added gravel to our mortar and baked it in the oven at a low temperature to make it into concrete (275 degrees for 45 minutes or so). We formed some posts and some beams, as well as an arch, to test the capacity of this simple material.

The concrete made by the ancient Romans was really interesting. I guess they used volcanic ash as one component of their cement, which helped waterproof the concrete. People are still trying (and so far, failing) to re-create Roman concrete today because it lasts a lot longer than any concrete we've been able to come up with! Fascinating.

We tested our concrete by piling loads on top of it. We were amazed at how much it could hold! It held up this big stack of books without a problem, but when Abe and Seb tried standing on it, it collapsed. We didn't mind because we could so easily make more! :)

Later, on his own, Sebby made a Roman Arch out of blocks, using this same centering technique. I thought it was impressive because when he removed the centering, the arch actually stayed up purely from compression forces, with no mortar or other glue between the blocks.

The vertical beams above were to counteract the "hoop stress" or outward pushing forces often seen in domes and Gothic arches (Gothic architects used the flying buttress to counteract these forces).

But here, the arch held even with those buttresses removed! Good job, Seb!