L-1800 LETOURNEAU LOADER WAS SPREADING SCORIA ON ROADS DUE TO HEAVY RAINS. OPERATOR SMELLED WHAT HE THOUGHT WAS A BRAKE HANGING UP & CALLED A MECHANIC. NOTICED SPARKS & SMOKE COMING FROM BELOW WINDSHIELD, SHUT ENGINE DOWN & TRIED PUTTING OUT THE FIRE IN THE ARTICULATION AREA. 2 CO-WORKERS SHOWED UP & ALSO TRIED TO HELP. FIRE DEPT WAS CALLED. CAUSE OF FIRE UNKNOWN.