The spot test is used to determine whether or not an asphalt cement has been damaged during processing due to overheating. This damage, called “cracking”, occurs because the actual molecules are thermally broken apart. Cracked asphalt cements tend to be less ductile and more susceptible to aging effects. Since modern refining practices rarely cause cracking, the spot test is not often specified.
Basically, the spot test is a form of paper chromatography (a method for analyzing complex mixtures by separating them into the chemicals from which they are made). A small drop of prepared asphalt cement is dropped onto a filter paper. If the spot formed is uniformly brown then the test is negative. If the spot formed is brown with a black center then the test is positive. Today, the spot test is rarely used.
Standard Test Methods
- AASHTO T 102: Spot Test of Asphaltic Materials