Lately, while holed up in my home like everyone else, I’ve been spending time looking at the thousands of photographs I’ve shot over the past few years. When I travel and see something interesting about construction, homebuilding or history, I try to capture it in a photograph or short video.
Three years ago, I made a trip back to my hometown of Cincinnati. When I go there, I often drive the roads I walked along or peered at through the rear windows of my parents’ 1959 Chevy station wagon.
On this particular trip, I found myself on Central Parkway, a magnificent roadway built in the early 1900s on a gravel outwash terrace created as the last continental glacier melted some 13,000 years ago. What caught my eye was the stunning guardrail that rests atop a hidden subway tunnel that runs for miles under Central Parkway.
The architects of the project specified that modular cast concrete guardrails be installed along stretches of the subway where one side of the tunnel faces west. More than 100 years later, the guardrails are in almost perfect condition, having survived brutally cold winters, decades of salt spray from snow and ice control on the parkway, and punishingly hot summers that can cause expansion in concrete.
Other than the cement-paste coating wearing away, the concrete in this guardrail is in excellent condition. Why is this? How did this concrete survive, and yet our driveways, sidewalks or patios can crack, chip or otherwise look horrible after just five or 10 years?
The answer is simple: The architects who designed projects like the Cincinnati guardrails specified stronger concrete than what today’s contractor is most likely using. In addition, the concrete was carefully mixed, cast and cured so it would attain the maximum design strength.
Consider concrete in its most basic form. It has only four ingredients: rocks, sand, Portland cement and water. If you take out the cement, you’re basically left with what you’d find on any beach: sand and stones mixed with water that you can shovel with minimal effort. Add cement and you create artificial rock. Cement is the glue that binds the sand and stones together. The more cement you add, the stronger the concrete becomes.
I don’t have access to the original specifications for the subway guardrails. But if I had to guess, I’d say the designers called for concrete with a compressive strength of 7,000 pounds per square inch or more. That’s some strong concrete.
But there’s more to making stronger concrete than simply adding more cement to the mix. Water is both a friend and foe in the concrete-making process. You need water to activate the Portland cement, but if you add too much water, you dilute the cement within the mix and the concrete doesn’t achieve its intended design strength.
Once you pour or cast the concrete, you need to ensure the water used to mix the concrete doesn’t evaporate too quickly. The chemical reaction that transforms the plastic concrete from a mix into solid, artificial rock continues for months. The water inside the concrete needs to stay there to fuel this reaction and make the concrete stronger. You achieve this by keeping it wet or applying a spray-on membrane that stops water from leaving the mass.
Years ago, when the subway tunnel beneath the Central Parkway was created, builders understood all of this. It’s important to find someone just as knowledgeable if you need concrete work done around your home.
Tim Carter has worked as a home-improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.