EVALUATING THE ADHESIVE PROPERTIES OF ASPHALT

THE ADHESIVITY OF AN ASPHALTIC BITUMEN IS DEFINED AS ITS ABILITY TO RESIST BEING DISPLACED BY WATER FROM AN AGGREGATE. TO EVALUATE THIS PROPERTY THE FOLLOWING TESTS HAVE BEEN USED: (1) THE PARTITION TEST, WHICH MEASURES THE AMOUNT OF NON-OIL-WETTED MATERIAL AFTER SHAKING A MIXTURE OF FINE AGGREGATE, WATER, AND ASPHALTIC OIL, IS CONSIDERED TO GIVE ONLY VERY ROUGH RESULTS. (2) VARIOUS WATER-DISPLACEMENT TESTS, IN WHICH THE AGGREGATE IS COATED WITH BITUMEN AND THEN SUBJECTED TO THE ACTION OF WATER AT A GIVEN TEMPERATURE, APPEAR TO GIVE THE MOST PROMISING RESULTS. THE AUTHOR USES 20 HRS SOAKING IN DISTILLED WATER AT ROOM TEMPERATURE FOR UNCURED CUTBACKS, AND AT 140 F FOR COMPLETELY CURED CUTBACKS AND PAVING BITUMENS. A FURTHER ACCELERATED TEST IS TO BOIL IN DISTILLED WATER FOR 1-5 MIN. THE ADHESIVITY VALUE IS THE PERCENTAGE REMAINING COATED AND IS ESTIMATED VISUALLY. (3) IN THE ABRASION-DISPLACEMENT TESTS COATED MINERAL AGGREGATE IS SHAKEN UNDER WATER IN A CAN IN A SHAKING MACHINE UNTIL THE TEST IS COMPLETED, AND IS GENERALLY THE TIME REQUIRED TO REMOVE A GIVEN PERCENTAGE - E.G., 50 PERCENT. IT IS CONSIDERED THAT TEST (2) GIVES THE MOST INDICATIVE RESULTS, PREFERENCE BEING GIVEN TO THAT AT 140 F SINCE (3) IS SUBJECTED TO THE ERRORS OF (2) AS WELL AS ABRASIVE EFFECTS. TO OBTAIN RESULTS OF A MORE PRACTICAL CHARACTER CIRCULAR TRACK TESTS HAVE BEEN DEVELOPED TO A STANDARD PROCEDURE, AND DETAILS OF ITS OPERATION ARE GIVEN. THE BREAKDOWN OF THE TRACK IS TAKEN AS THE NUMBER OF REVOLUTIONS OF THE MACHINE REQUIRED TO LOOSEN 10 PERCENT OF THE TOTAL BITUMEN-AGGREGATE MIXTURE LAID. BETWEEN 78 AND 90 F. THERE IS A MEAN INCREASE OF 840 REVS./DEGREE DECREASE IN TEMPERATURE AND AT 90 F. AN INCREASE OF 450 REVS./DEGREE RISE IN SOFTENING POINT, THE TRACK BEING RUN UNDER WATER AND CONTINUOUSLY IN EACH CASE. A CORRELATION HAS BEEN INDICATED BETWEEN THE RESULTS OBTAINED ON THE CIRCULAR TRACK AND THE LABORATORY TESTS (2). BY THE USE OF SUITABLE WETTING AGENTS, PARTICULARLY HEAVY METAL SOAPS IN THE BITUMEN, THE TRACK LIFE HAS BEEN INCREASED UP TO 3.88 TIMES THAT OF THE UNTREATED BITUMEN. A CONSIDERABLE AMOUNT OF WORK REMAINS TO BE DONE TO DEVELOP A SMALL-SCALE QUANTITATIVE LABORATORY TEST WHICH CAN BE CORRELATED WITH ROAD RESULTS.