The Astro we use at NNRC is great. I'd say its probably around 2 or 3cm deep. It has black rubber pellets inside it instead of sand and will disperse off the racing line after a few months. The astro itself has started to pack down on the racing line.
Good grip when dry and tons of grip when the sun gets on it and warms it up! In the wet it can be very grippy too with the right tyres.
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