Basement Floor Slab - Horizontal Crack Repairs using Sika Crack Fix

The past three weeks we've been upgrading the basement foundation wall insulation - so I've had to move all the shelving and storage units away from the basement walls to get at the walls. Since all the heavy lifting was done to move everything away from the walls, I decided it was an excellent opportunity to give the concrete floors a coat of paint. Prior to painting, it was the perfect time to repair the horizontal cracks in the concrete slab - so I've been at this in parallel with the insulation job.

Final Crack Repair - Sika Crack Fix

Following some reasearch - I've settled on using Sika Crack Fix - a structural epoxy that is very fluid and can penetrate very narrow cracks, and is also commonly available at my local home improvement stores. It's not inexpensive, at about $20 a tube, but if I can get a permanent repair to these slab cracks I'll be satisfied.

Sika Crack Fix - Comes in a Cartridge designed for a standard small caulking gun, with two mixing tips. 

Cartridge installed in the caulking gun

The first step to a smooth repair is to open up the top of the cracks with an angle grinder and a concrete wheel. Create a groove at the top of the crack - 2 to 4 mm wide - and relatively straight - this will create a channel that will make the application of the epoxy much easier, and create a small well for the epoxy to pool before it penetrates the crack. This is the key to a neat, easy crack repair. When creating the groove with the grinder, use your shop vac with the hose end held right next to the grinder wheel, this will pick up 95% of the dust, and help keep your work area clean.

Groove the top of your cracks with an angle grinder prior to application of the epoxy

Once the cracks have been grooved, you can start applying the epoxy. Since I have no way of plugging the crack at the bottom of the slab - the epoxy is so thin it will run right down through the crack, and out the bottom of the slab. To minimize waste, I would apply one or two tubes of Sika Crack Fix to about 25 feet of prepared cracks - and allow the epoxy to penetrate the cracks. I would then wait 24 hours and repeat the process - to allow the crack to shrink in width over subsequent applications and minimize the leakage out the bottom of the crack.

Once the crack is filling up - you'll find that the epoxy will run horizontally as well - moving down any slope in the floor slab. This isn't a problem, it helps to distribute the epoxy fully through the crack, but you may find that some epoxy will pool out of the crack on the surface of the floor. The hardened epoxy is so hard that it is difficult to remove by scraping or sanding. After about 4 to 6 hours following the application of the epoxy - the epoxy will set up and start to harden - yet it will still be soft enough to scrape off any unwanted excess. This is an excellent time to level out any sections where the epoxy is convex - and extending above the surface of the surrounding floor. 

Completed crack repair - where the epoxy has been scraped flat before completely hardening. 

The Sika Crack Fix can also be used to fix broken surfaces - I have some small areas around a floor sump where the concrete surface has chipped away slighly exposing some aggregate. I've been using the Sika Crack Fix to smooth these areas prior to painting.

Wider surface repairs using Sika Crack Fix - This section is still slightly concave, and will require a subsequent application of Crack Fix to get it flat. 

I've used about 10 tubes of the Sika Crack Fix in about 40 feet of narrow cracks averaging about 1mm wide. That's a bit expensive at $20 a tube - but if it does a "permanent" long term repair - I'll be happy with that. It looks like the epoxy will have to be roughened prior to painting with sandpaper - if left to settle on its own it hardens very smooth - almost glass like, and I'll be looking for good adhesion with the floor paint. I'll post on the floor paint process later, as I get that task done.

Now - for a few other points now that I've almost finished all the crack repairs in the basement.

Sometimes, the part A and part B doesn't mix correctly, and won't harden in the crack. This may happen at the start of the tube, at the end of the tube, or if you interrupt application and restart with a new mixing tip. The instructions for the product suggest pouring out product in a waste container until all the air bubbles are gone - this ensures the product being applied has a proper mix of both components - and is very important. When you get to the end of the tube, and the mixing tube starts spouting bubbles - stop there and discard the tube. I've had a few instances where the product hasn't mixed correctly, and the epoxy stays sticky and doesn't harden. You can use a putty knife or rag to scrape off as much of this material as possible, and then finish cleanup with a solvent. The product guide recommends "Sika Equipment Cleaner" to clean up unmixed or uncured epoxy.  This product contains Xylene according to the MSDS, and so does the brake cleaner that I had in my garage, so I've cleaned some of this unmixed epoxy with a small amount of brake cleaner. (Try getting some "Sika Equipment Cleaner" on a Sunday morning....)

Unmixed epoxy cleaned up with Brake Cleaner, ready for re-application of mixed Crack Fix

Another issue that I ran into is that the Crack Fix is so watery when applied, it flows with any slight slope in the concrete slab. I have a few cracks that extend to a floor sump, with a slight grade. The crack fix kept on flowing along the crack, and leaving a trough in the crack, and puddling out of the crack at the bottom of the slope. The solution to this is to shoot some Crack Fix into a small plastic container, and let it set for 2 to 3 hours. Once the product starts setting up, and not flowing, apply it to the crack with a putty knife. It should stay in the crack without flowing down. Check and retrowel with the putty knife after 30 minutes.

Crack Fix setting up in a plastic cap, almost ready to apply.

Finally, in preparation for painting, I've found that a belt sander is really the best way to level out cracks. I had always managed to get by without a belt sander until now. I went out, purchased an inexpensive Skil 3 x 18 sander, and a few belts of 80 grit. This machine works great for levelling out the cracks and getting a nice flat surface equal with the concrete prior to painting. 

Skil 7510-01 3 x 18" belt sander, with vacuum attached. Perfect for levelling epoxy Crack Fix.

The sander accepts a 1 1/2" shop vac hose for dust collection, which also helps remove heat from the motor during operation. I highly recommend operating this tool with a vacuum attached. It's really efficient at levelling out cracks.

Epoxy filled cracks levelled with the belt sander, ready for painting. 

This has been a good job to get done - so far, I've used about a dozen tubes of Crack Fix. It's not inexpensive, but compared to bringing in a conctractor to replace your basement floor slab, this is an excellent way to prolong the life of your basement floor.

Getting near the end of the job. Perimeter of the floor - painting completed. Now filling all the cracks in the center of the floor, almost ready to complete the painting.