The secret to ancient concrete steps
A few days ago, I dedicated one of my new livestreaming videos to one of the first building products used by man. Discovered thousands of years ago, hydrated lime is perhaps one of the most amazing multipurpose building products ever put into widespread use.
A modern analogue to this revolutionary building material might be steel. While steel was first invented in India thousands of years ago, the widespread manufacturing of what we know as steel didn’t start until the middle of the 19th century. Steel has countless uses and can be found in tens of thousands of items. Hydrated lime can be found all over the world in countless buildings, used both as a mortar and as plaster.
You might wonder what lime has to do with ancient concrete steps. Well, let’s begin by talking about modern concrete that you might have at your home or all around you in your city or town. The internet is littered with hundreds and thousands of woeful stories from homeowners about how their new concrete is crumbling. I’ve got no fewer than 10 older columns in my archive at Askthebuilder.com that explain why this happens. I suggest you read those.
I wish you and I could take a road trip to Cincinnati. I’d take you to a few neighborhoods, one being Pleasant Ridge on the east side of town. There I’d be able to show you at least 20 or 30 outside sets of concrete steps that are just like the one in the photo appearing with this column.
These ancient concrete steps are original in almost all cases and installed when the quaint houses were built in the early 1900s. How is it that these concrete steps – all of them coated with cement stucco – have survived nearly 100 years and look almost brand new?
That’s a great question, and one you should ask each time you see some new product fail and wonder about how an older product that was pushed aside is still in service.
The reason why these old concrete steps lasted so long is really pretty simple. Back then concrete masons knew that if you added more Portland cement to the concrete mix, it made it stronger. This magic gray powder is very similar to hydrated lime. When you mix both with water, you start a chemical reaction where trillions of microscopic crystals start to grow.
These crystals act like modern Velcro. They’re the glue that holds together the sand and stones in the concrete mix. There’s a very good chance the concrete masons who installed the concrete steps in Pleasant Ridge also added hydrated lime to their mix, although it would take a chemical analysis to confirm.
I get excited about hydrated lime for two reasons, one being my college degree in geology. I grew up in Cincinnati, and as a youngster had no idea that geologists from all across the world visited my city. It turns out Cincinnati is the world-type section for Upper Ordovician sedimentary rocks. If you want to see the best plant and animal fossils of that time period, you come and split apart the limestone rocks interbedded in the shale at all the road cuts in greater Cincinnati.
I knew limestone was hard and durable as a geology student, but I wasn’t aware that dense, fine-grained limestone can be heated to create the lime that’s then used as brick mortar, plaster and a super-adhesive additive in regular concrete. When you heat up limestone, its chemical composition is CACO3, you drive off the CO2 and are left with CAO.
Once you grind up this CAO into a powder and then add water, you end up re-creating the limestone. This is why plaster is so hard. This is why true whitewash made from lime and then painted on brick buildings, fences or other walls lasts for decades. You’re basically painting on or troweling on a thin layer of limestone, for goodness sake!
You can now see why it’s a good idea to add hydrated lime to concrete, can’t you? The Portland cement is strong, but why not give it a little boost. Keep in mind the normal minimum standard for exterior concrete exposed to cold temperatures is a six-bag mix. This means six 94-pound bags of Portland cement are in each cubic yard of concrete. It’s probably a better idea to add seven or eight bags to your exterior concrete.
You can bet the old masons building the steps added extra cement, and they most likely put in a shovelful or two of hydrated lime. The railroad bridge engineers did the same. This is why you see concrete railroad bridge abutments in your city or town in such great shape after 100 or more years.
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