State Highway Engineer

The report enclosed herewith relates the construction history and interim performance of an experimental pavement in which coal tar (RT-12) was substituted for asphalt cement (PAC-5) ··all other design features were basically similar to asphaltic concrete paving projects abutting each end of the experimental section. The resulting tar mixtures were in general conformity with Mixes 4T {base) and 7T (surface) of ASTM D 1753-67. Although the Marshall stability was 3000 pounds at I00°F, the tar surface course has thinned in the wheel tracks and thickened outside the wheel paths. Tar mixes have insufficient strength to withstand testing at 140°F. The greater. sensitivity of the tar to summer temperatures-- together with very heavy wheel loads �- suffice to explain the greater rutting occurring in the tar surface than in the asphalt sections. Only the heavy wheel loads were unanticipated. Deflection measurements indicate that the tar concrete pavement is otherwise comparable to the two asphaltic concrete pavements abutting thereto. Leveling of the existing surface, together with an insulating overlay of asphaltic concrete, should restore the pavement to normal service capabilities. Whereas the report indicates that 4.5 inches would be needed to restore the pavement to the original 20-year design, I would recommend a 2.5-inch overlay (plus.5-inch allowance for leveling) at this time.. thereby deferring the remaining 2 inches until a mor