When you get up the next morning, your building will look like the first picture up there. For some people, that is good enough. I, however, am not one of those people, and I suspect you are not either. So, it's time for some extra homework to get extra kudos!
If you have the time and the paint, then it's time to go to work. First off, I like to coat the building with a black wash made from watered down black acrylic. Any black paint will do, and in fact, the cheaper the better. I got a pot for 50 cents from Wal-Mart. Add a big ol' glob and a dash of water to thin it out, then paint the wash over the entire piece. This watered-down solution should run into all the cracks and bring out the depth of the texture.
Next, using your large brush, drybrush a dark gray over the entire piece. Pay special attention to the edges, because concrete is lighter on the inside when cracked open. Drybrush, for those new to the hobby, is a painting technique where you get paint on your brush, then dab some of it off onto something you don't mind getting paint on, before lightly brushing over the area you wish to paint. This causes only the raised texture to catch the pigment on your brush.
After this coat dries, add some white paint to your grey, and mix it up good. Using the same drybrushing technique, apply this lighter grey over the entire piece again, really emphasizing the edges. You can add a third coat, this one pure white, but I usually think that's a little over the top.
After the second coat, the piece is table-top ready. You can add a clearcoat if you wish, but I find that the glue we used in the mixture is usually sufficient by itself. Later, I will share some of my tips to make the terrain piece worthy of sharing the table with your best painted miniatures!