Unbrushed concrete can be extremely slick if it has some dust on it, which is not uncommon outdoors at a shooting range. Sweeping thoroughly before shooting MAY help; however, the easiest fix is to go to Target/Home Depot/etc. and buy yourself a $5.00 doormat. Specifically, one that has good grippy rubber on at least one side, it really doesn't matter which. I've been using one that is carpeted on one side. I place the carpet side down with only about 1/3 sticking out from underneath my shooting mat in the front. The 2/3 that is underneath the mat with me and the rifle/spotting scope/etc. on top of it is more than enough to keep it in place when loading the bipod. I have used this setup on dirt, gravel, grass, and very smooth concrete, and it seems to work pretty well on all of them.