Examination of the asphalts used in the Zaca-Wigmore project by means of the microfilm durability test gave ratings of the relative durability to be expected of the asphalts when all are used under the same conditions. The actual order of occurrence of failures in the test road to date is in agreement with the predictions of this relatively simple and rapid laboratory test.

Viscosity measurements show that hardening during the asphalt-aggregate mixing operation was about twice as great during period 1 of this construction as during period 2 and this difference in hardening in the mix plant is still evident after several years in the road.

Progressive hardening of the asphalts in the road over a period of several years was found to parallel that found in the microfilm durability test in a few hours. In the first test road (period 1) construction, where pavement deflections are about 0.015 to 0.025 in. under a 15,000-lb axle load, failures developed when the asphalt viscosity at 77 F entered the range 10⁷ to 10⁸ poises. In the other test road (period 2) construction where deflections are about 0.010 in. under the same load, the first distress was noted in an asphalt which reached a viscosity at 77 F of about 10⁸ poises.

Recovery of asphalt from slices of pavement cores showed that hardening of the asphalt is greatest at the top and decreases with increasing depth in the pavement. Pavements with high air void content were found to harden more rapidly than those with low air voids.

Hardening by loss of volatile matter is an important fraction of the total hardening observed in these asphalts. It is shown that this can be controlled by proper selection of crude oil or by application of distillation and blending techniques so as to keep the 10 per cent distilled point of the asphalt at or above 400 C (converted to atmospheric pressure).