How to Fix Crumbling Concrete
DEAR TIM: I work at a smaller municipal airport and need your help. The concrete inside some of the plane hangers is crumbling at the straight lines where one giant slab of concrete touches up against another slab. There’s a metal strip in between the slabs if that makes a difference. What’s the best way to repair this concrete? Our previous repair attempts have all failed. If you have a magic method that will not fail, will it work at my home too as I have similar issues there? You can make me look good in front of my boss if you solution will support the planes and tractors that cross over the joints. Zack K., Troutdale, OR
DEAR ZACK: I’ve been in a few airport hangers and the size of the concrete slabs is quite impressive. What’s more, the concrete is usually quite thick and formulated to be exceedingly strong. While smaller private jets may only tip the scales at just under seven-thousand pounds, a mid-sized Boeing 737 passenger jet weighs over thirteen times that! I doubt a 737 can land at your airport, but you may have larger private aircraft that weigh much more than 3.5 tons.
When you do the math about how much weight is pressing on the concrete under the small footprint of the tires at each landing gear, the numbers can be quite large. Think of a woman wearing spiked high-heel shoes and how much concentrated weight is on that tiny pad under one heel as she walks and has one foot up in the air. This is the same issue with airplanes.
The good news is you can repair the crumbling concrete in your hangers and the method will work just as well at your home or that of any other homeowner. All you need is an old circular saw or you can rent a special saw that will make the required cut into the concrete so you can proceed with the repair.
There are many reason why concrete patches fail. It’s important to use the correct patching mix. You also need to make sure the shape of the area being patched will ensure the patch won’t pop out. Most patches fail because a very important simple bonding material is not used. There are more reasons, but let’s concentrate on how I’d proceed.
The first thing to do is to prepare the area to be patched. You need to remove all bad concrete and cut into the slab away from the metal joint so that your patch is in contact with solid concrete.
The shape of the patch needs to resemble a dovetail joint that finish carpenters use to join two pieces of wood. Dentists use this same technique to install amalgams in a decayed tooth. In other words, the bottom of the area being patched needs to be slightly wider than the top of the patch at the surface. If road crews did this same thing, pothole repairs in blacktop would last so much longer.
I would make sure that the depth of the repair patch is no less than one inch. If you’ve tried to just apply a thin coat of stucco or mortar mix previously, I can understand why your patch attempts have failed. The strength of concrete relies, for the most part, on the size of the stones that are in the mix. Small grains of sand at the edges of a very thin patch are not as strong as you might think.
Use an old circular saw you might find at a yard sale equipped with a blade that will slice into concrete. Tool rental stores have special saws made to cut into concrete. There are abrasive blades made for this or you can buy blades that have tiny diamonds embedded in the blade. Wear special masks so you don’t breathe in the silica dust and take whatever precautions are necessary so this dust is not ingested into any airplane engines!
Tilt the saw blade so it makes an angled undercut into the good concrete. Once the cut is made, carefully chip out the concrete to a depth of at least one inch. Use a wet-dry vacuum to extract all chips and any dust. I’d clean the hole with water and a scrub brush to ensure all dust is removed.
Your concrete repair mix should have stones in it no larger than 1/3 the minimum thickness of the repair area. This means if your repair patch area is one inch deep by one inch wide by several feet long, no stone in the mix can be larger than an average green pea!
I’d make a high-strength patch mix using three measures of small stone, two measures of medium clean sand and 1.5 measures of Portland cement. Add just enough water so the mix resembles stiff applesauce.
The secret step to ensure the patch bonds to the existing concrete is to add a thin layer of cement paint to the existing surfaces of the concrete just before you add the new patching material. You make cement paint by mixing pure Portland cement with clean water. Mix until it’s the consistency of regular paint. Spritz the existing concrete with a small amount of water before applying the cement paint. Immediately cover the cement paint with the patching concrete mix never allowing the cement paint to air dry.
Finish the patching material to the desired smoothness and spray on a concrete curing compound or cover the patch with plastic for 72 hours. You need to keep the patch damp for at least this much time so the concrete patch cures to the desired strength.