The temperature difference between the concrete pad and the night air doesn't have to be great to cause shimmering - trust me on this one.
You are actually using very high magnifications, especially for an inexpensive telescope with less-than-perfect optics. To find the power you are using, simply divide the focal length of the eyepiece into the focal length of your telescope. If your telescope has a 2400mm focal length, just divide by 4mm and you'll see that you are viewing at 600X. That is far too much magnification for commercial telescopes of modest size. If you have a 4" diameter objective (mirror or lens), probably the highest practical magnification you can ever use will be about 200X (thats a 12mm eyepiece with a 2400mm objective). Keep at it - astronomy is fun.