Foundation cracks that allow water to seep into your basement should be fixed. This can be done from the inside or the outside. Working from the inside is typically easiest, so we'll start there.
Polyurethane caulk can work well on small cracks. For the best results, make sure the concrete is dry before applying it, and force the material into the crack as far as you can. If there's any loose material in the crack, remove it before you apply the caulk.
Larger cracks take a little more work, and more materials. Here's what you'll need:
Masonry chisel and heavy hammer.
Quick-dry hydraulic cement. This is a special waterproof cement that will even set up underwater. It expands as it dries.
Bucket for mixing cement.
Spray bottle of water.
Here's how to do it:
Put on your goggles.
Chisel out a groove with the masonry chisel and hammer. You want to make the inside of the crack larger than the outside. If you're familiar with furniture construction, you're basically making a dove-tail groove. This shape is best as it will lock the cement in place as it dries.
Clean out the crack with the wire brush.
Mix the cement. Be sure to follow the package directions.
Spray the patch area with water. Doing so will keep the moisture in the patch so it can set properly.
Fill the crack almost full. About 1/2 inch from the surface is good. Let this layer dry.
When the first layer is dry, completely fill the crack with a second layer. Smooth it so it matches the surface of the surrounding area.
A dove-tail shape works best for foundation crack repairs. A shape that is wider on the outside than on the inside, won't hold the patch material as well.
Cracks can also be repaired from the outside. Since these repairs prevent water from entering the foundation in the first place, they can be more effective and are typically longer lasting. However, they often require excavating the area around the crack. The repair process is pretty much the same as doing it from the inside -- you'll clean the crack, then fill it with cement. When the cement is dry, apply a coat of waterproof coating over the repaired area. Choose a product designed to be used below grade. Then refill the excavated area.
After the cracks have been patched, you can apply a waterproof sealer to your basement walls. Doing so will prevent additional moisture from seeping through porous surfaces. There are a variety of different products available for this. Make sure the walls are dry and clean for the best results, and follow the label directions when applying.