I received bad chemical burns and permanent scaring to my knees after kneeling in wet concrete while pouring a small slab. Someone told me that iron oxide in the color additive that was in the concrete can quickly cause severe chemical burns. I have asked around professional concreters but none of them know of a case where this has happened - I assume because they don't kneel in it. Does anybody know of a case where color additive has caused a problem? I know that straight wet concrete can burn, as has happened to me before, but that was after kneeling in it for a long time. These burns occurred after kneeling in it for only about 20 minutes.