DRIVER WAS CHIPPING CONCRETE ON BOTTOM OF BATCH TRUCK DRUM WHEN 15 POUND PIECE OF CONCRETE BROKELOOSE STRIKING DRIVER ON BACK OF LEFT HAND. DRIVER'S HAND HAS CONTUSION, SWELLING, NUMBNESS, AND TWO WOUNDS 1/2 INCH IN LENGTH 2 INCHES APART. EMPLOYEE WAS WEARING HARD HAT, SAFETY GLASSES, GLOVES, AND SAFETY SHOES.