Repairing Concrete Expansion Joints in a Office/Warehouse Parking Lot

dmitschke asks,

I own a office/warehouse complex with metal expansion material instead of the redwood or ceder expansion joints. The complex is about 30 years old but it has been kept up pretty well. When it rains water seeps through the expansion joints and gets under the slab. When a heavy vehicle drives over the joint water and soil come out of the joint. After a period of time the concrete settles a little because the soil washes out underneath. I use a piece of equipment to fill the expansion joints which is a hot melt tar pit. It pours hot tar in the cracks like you see done on highways. The problem is that the tar dries out eventually and cracks allowing water to get through the tar. Any suggestions as to how to get either the tar to stick or a material I could use to seal the expansion joints? When I get too much wash out the concrete breaks up and I have to replace the concrete in that area.

Our Answer

The issue you have is not only the asphalt patch drying out but its not flexible and any movement allows for the seal between the tar and the cement to fail whenever the cement slab moves. To fix this you need to apply something that will allow some movement without loosing any adhesion from the sealant to the cement. In addition the shape of the filled area has a lot to do with the ability of the product used to maintain this adhesion.

First off you need to clean out the gap real well. There cannot be any loss debris on the cement or the sealant will fail to stick. This should also be very dry as moisture will prevent also the adhesion of almost all sealants.

Purchase what is called backer rod. This material is a round closed cell foam filler.

When pushed into the crack it should maintain its round shape but stick tightly so the size of this backer rod is critical for success. This rod should be placed down into the opening about the same depth as the crack is wide. You do not need to fill the entire opening to the bottom of the cement.

Then you need to purchase a flowable crack sealer. You simply pour this filler into the crack, then using a round dowel as this product begins to set. Curve the top to create if you were to look at a cross section of the patch to look like an hourglass on its side. Skinny in the middle and fat where it contacts the cement.

Here is why this is so critical. The sealant acts like a rubber band. It is attached to the sides of the opening. Much like a rubber band it will flex and move around if its thin. But take a fat rubber band and cut it off just about the same length as it is wide and try to hold onto it and flex it around. You simply cannot do this without losing your grip. The same holds true with the sealant. If you have a block of this between the cement slabs it will not flex and end up tearing where it comes into contact with the cement. But make it thinner in the middle as with the backer rod and curving the top with a dowel, it becomes flexible and will will not tear off of the cement but still allow for movement if the cement moves under the weight of an auto or truck.