Asphalt has a different composition than concrete. Concrete, the normal base material for cement blocks, is filled with tiny pores that absorb the cement mud from the block. However, asphalt is composed of a greasy tar material that will not allow good adhesion of the cement block mud. With this in mind, you can prepare your asphalt driveway so that it bonds well with the blocks. The average do-it-yourselfer can complete this project in 1 to 8 hours, depending on the size and scope of the project.
Things You'll Need
Asphalt bonding agent
Masonry cutting saw
Apply degreaser to the asphalt where you will be attaching cement block. You can find a good degreaser at any hardware store. Spray the degreaser onto the area that will receive the cement blocks. Using a stiff bristled brush, agitate the degreaser into the asphalt and allow it to set for 15 minutes. If you are applying the degreaser on a hot, dry day, cover the degreaser with plastic to prevent it from drying out.
Spray off the work area with a pressure washer. The pressure washer will remove the degreaser and any remaining residue. Use a green or yellow tip when spraying with the pressure washer; the red tip will damage the asphalt. Allow the asphalt to fully dry before proceeding.
Cut cross-hatch marks into the asphalt using a masonry cutting saw. You can rent this saw at any tool rental store if you don't own one. Do not cut outside of the cement block placement area with the saw. This will leave exposed marks after the blocks have been laid. To be sure, measure the width of the blocks—usually 8 inches. Snap a chalk line along the length of the proposed cement block laying area. Cut cross-marks that do not extend outside of the chalk line. The marks will be perpendicular to the chalk line. Space them no more than 4 inches apart, going 1 to 2 inches into the asphalt. Spray off any cutting residue with the pressure washer.
Apply the asphalt bonding agent. This chemical may come in a spray bottle or a can. If the product is in a can, add the bonding agent to a pump-up garden sprayer and spray it onto the work area. Follow the drying time on the directions of the bottle or can.
Mix your mortar mix by following the directions on the bag. Once you have a pancake batter consistency, spread two lines of mortar inside the chalk line. Place one right on the line and the other about 7 inches over. Lay your cement blocks into this mud bed. As you tap the blocks level, the mud will press into the block pores and the asphalt cuts.