Concrete isn't a material that you associate with absorbing smells, but it can happen occasionally. If it has been damp the concrete may have a musty mildewy smell. If gasoline or oil spill, they can leave an odor as well. Since concrete isn't really an absorbent material, removing the smell is generally as easy as cleaning its cause.
Sprinkle the concrete with powdered laundry detergent, concentrating on the cause of the odor. One cup of detergent should be plenty for a room, home parking area or any concrete up to the size of a basketball court. If you need more, add it as you go.
Add water to the detergent lightly; you don't want to wash it away, just activate it. This should leave the concrete damp and the detergent pasty.
Use the push broom or scrub brush to scrub the concrete, spreading the soapy water all over and giving the detergent time to work in all the nooks and crannies of the concrete. Add water as you go to help spread dissolve the detergent completely.
Rinse the concrete with either the water hose if you are outside or with a mop and bucket inside. Allow the concrete to dry for 2 to 3 hours. At this point you should not see or smell anything but clean.
To ensure that the odor is truly gone, sprinkle baking soda on the area that the odor was emanating from and allow it to sit overnight. Baking soda absorbs odors and will not leave a cloying cover-up scent. Brush the baking soda away in the morning with a dry broom.