Houseworks: Repairing cracked concrete

Published Jul 11, 2023 • 3 minute read

Bonding agent is the key to concrete repairs that last. Brush some onto the repair area and for part of mortar mix to boost adhesion. Photo by Photo Steve Maxwell

Q How can I repair small, crumbled areas in a concrete driveway? The maximum depth of decay is about one inch.

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A Start by washing the area to remove dust and loose material. Ideally this would involve a pressure washer, though a garden hose could work. Let the surface dry completely, but don’t wait so long that fresh dirt enters the damaged areas.

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Concrete repair is best done with mortar mix, and there are two tricks for getting mortar repair patches to stick and remain solid on concrete.

Boost bonding strength: The repair area needs to have some “bonding agent” brushed on before the mortar goes down, and 1/3 to 1/2 of the liquid used to make the mortar should also be bonding agent instead of water. Bonding agent is a white liquid that gives masonry much greater holding power and a tiny amount of flexibility. Concrete bonding agent is available at all hardware stores and is made by different manufacturers. The mortar should be mixed with enough liquid to be trowel-able, but not so much that it gets soupy. Aim for the consistency of butter at slightly more than room temperature.

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2. Keep the area moist: All cement-based products rely on a chemical reaction for hardening, so if your mortar dries out before this reaction has finished, the repair will fail. And it’s not enough to just wet the area once after mortar application. Burlap bags are great for providing continuous moisture. Just saturate them, lay them on the repair areas, then keep the burlap wet.

Immediately before you trowel some mortar into the damaged area, brush a coat of bonding agent into the repair zone. Let this soak in for a few minutes, then apply enough mortar with a trowel to fill the area before smoothing it over. Lay down whatever wet fabric you’re using and keep that fabric wet for three days. After that you should have a durable repair.

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The gray heating cables of the DITRA-HEAT floor heating system keep toes toasty during cold weather. Photo by Photo Steve Maxwell

Warm floors

Q What’s the best way to insulate a raised floor on foundation piers? I’m following your online building course, Cozy Cabin, and I want a warm floor in winter.

A The original cozy cabin that I based my course on has been lived in year-round for seven or eight years, and this is what I found works well for making a warm floor.

In the course I show how to apply sheets of extruded polystyrene foam on top of the subfloor, with an additional layer of 5/8” plywood on top of the foam before finished flooring. No need for strips of wood to support the foam, just drive screws right down through everything and 1 1/2 inches into the joists. Two inches of foam does a fair job, but you can add more if you need to get the floor up to code for insulation.

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No amount of foam on the floor will deliver warm feet on its own, but it does make a big difference and it’s an essential start. If you want really warm feet, look at some kind of electric in-floor heating system. Schluter’s DITRA-HEAT product is the best I’ve seen. It’s made for use under ceramic tiles, but I’ve had great results using it under laminate flooring, too. This system is not enough to heat a room on its own, but it does make the floor warm and cozy when installed over foam insulation.

Whatever you do, don’t enclose the raised floor area with a skirt on the outside. You’ll often get people suggesting this (even some professionals), but it usually leads to long-term floor rot. I’ve seen it happen many times. Besides, skirting never turns a cold floor into a warm one. It might make a bit of difference, but definitely something I recommend against.

Steve Maxwell has come to like warm floors very much. Visit him online at BaileyLineRoad.com and join 31,000 people who get his Saturday morning hands-on how-to newsletter each week.

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