Do not for any reason go to silicone for hard surface racing. You will have the stuff coming apart all over the place. You have two ways to do this. Get some asphalt sealer which rc cars cannot run on. Then put a bunch of sand in the mix and spread it over the track. Spray the track the day before the race (very important) and you may get a season out of it before recoating but it still may not give the traction you want. Do a small test first. Slicone will work if you are racing on smoot dirt like a clay oval or, a basketball court. It works great there. The best option for concrete would be to run cars that have rubber tires. Touring, F1, rally, Mini's etc. Or find a church with a nice asphalt lot and have races on saturday.