We get asked this question all the time. What material should be used to fill an existing expansion joint or control joint in a concrete driveway? Polyurethane caulking is the best material for this project and is available in self levelling or non sag varieties. Self levelling materials can only be used in expansion joints where there is no slope. Non sag materials require tooling in order to smooth them out. Polyurethane can be tooled with a variety of tools but a latex glove and your finger will work just fine. Also, have some dish soap and water handy. Dip your finger in the dish soap and water and run your finger along the joint to smooth it out. The water and soap will not hurt the caulking. This only applies to non sag materials. Self Levelling polyurethane finds its own level and makes this project a heck of a lot easier.
Here are some other tips to help you get this job done right!
Determine the width and depth of the expansion joint. A good rule of thumb is the depth of the caulking should be half of the width. This will give a good dimension for the caulking to remain at its optimum flexibility. If the expansion joint is really deep, consider using backer rod in the joint to ensure you get the bead thickness desired. For instance if the expansion joint is 1/2″ wide, set the backer rod to a 1/4″ depth. The backer rod should be slightly bigger than the expansion joint to allow the backer rod to compress into the joint and expand against the sides of the joint. On a 1/2″ expansion joint, choose a 5/8″ diameter backer rod. The backer rod fulfills two purposes. It sets the depth of the caulking, and the caulking will not stick to the backer rod which means the caulking will only have two sided adhesion and only move in one direction. Avoiding three sided adhesions will optimize the physical characteristics of the sealant.
What if the expansion joint is nearly level with the concrete surface? The expansion joint needs to be cut down a minimum of a 1/4″ below the concrete surface. Also, the expansion joint must be isolated as well. Filament tape can be placed over the expansion joint material prior to caulking. This will ensure that the caulking does not stick to the expansion joint preventing three sided adhesion.
The sides of the expansion joint must be clean in order for the sealant to stick. A wire wheel or wire brush can be used to accomplish this. Clean the joint after with high pressure water however, the joint must be dry when applying the sealant. Applying a polyurethane over top of a damp or wet joint will result in the sealant bubbling. If there is concern of polyurethane sticking, there are concrete primers available that will help with the adhesion of the product.
Once the sealant is applied 1/16″ below the concrete surface, you should protect the area until the polyurethane becomes tack free. This will usually take 1-2 hours at 23 degrees celcius. A good way to do this is to cover the caulking with a #32 mesh sand. Once the caulking has cured, sweep up or blow away the sand. This will give the caulking a nice grainy natural look that will blend well with the existing concrete.
Control joints or saw cuts should be addressed the same as above. The control joint can be filled with backer rod or even sand. Filling the saw cut is mandatory as control joints are 1/4 the thickness of the concrete. On a 4″ slab, that is a 1″ deep saw cut and nobody want to use that much caulking. Fill the saw cut 1/4″ to 3/8″ below the concrete surface and proceed as above.