Whenever you are working with cinder block, you can strengthen them significantly by filling them with concrete. Cinder blocks are less expensive to work with than poured concrete in home construction projects, but they are not as strong. You can make up the difference by spending a little more to reinforce the blocks, a project that is still less expensive than using just poured concrete. This is something that is not difficult to do and will strengthen your cinder blocks quite a bit. You may also find that if you have cinder blocks that are cracked, concrete can help. However, you should use caution whenever you are working with cracked cinder blocks, as the procedure for these can be tricky.
Do the Math
Figure out how much concrete you will need for your project before you begin. While it does not take very much to completely fill them, the amount of concrete that you need will obviously have a lot to do with how many blocks that you have. A single bag should go a long way in most projects, but for a closer estimate, measure the length, width, and depth of the holes in your block. This will give you an approximate calculation for how many cubic feet of concrete each block can be filled with.
It is a good idea to begin your project by drilling the holes into the cinder blocks if there aren't holes already. You will need to make sure that you use a hammer drill for this task, as any other type of drill that you use will not be able to do the job properly. Attempting to use a regular drill could damage it or even the cinder block. Make sure that you proceed with a 12-inch drill bit; this will be the best size considering what you are doing. After these holes have been drilled, you can measure the approximate volume using a procedure similar to the first step, adapted for cylindrical holes.
It is a good idea to wait to prepare the concrete until you are ready to use it. Make sure to read the directions on the packaging for your product so you know you are doing it correctly. Each type or brand can have slightly different directions.
Pour the necessary amount of water to go along with the amount of mix that you are using. Typically, a wheelbarrow, or even a five-gallon bucket or will suffice for a mixing container. Use your concrete mixer until the material has been properly stirred. Then, you are ready to begin.
You will pour the concrete into the holes, either the pre-existing ones or the ones you drilled. However, if you are using the concrete to attempt to fix cracks, you will need to add rebar to the other side of the crack in order to tie it all together. Once the concrete has been successfully inserted into the cinder block, you will need to allow it time to dry completely.