a machine for removing uncured concrete from the top surface of a cast concrete slab to expose the aggregate therein to provide a decorative surface finish . the machine utilizes a pair of counter - rotating brushes to remove concrete which has been prevented from setting by use of a surface retarder , thereby exposing the aggregate to leave an attractive exposed aggregate finish . the counter - rotating brushes deposit the uncured concrete brushed from the surface of the slab onto a conveyor which carries the concrete waste to the edge of the slab for disposal .