The reason to mix a cutback yourself is cost. Another reason is that in some localities cutbacks are no longer available from the former manufacturers because of environmental and liability concerns, yet certain owners and/or agencies continue to insist on their use. I started manufacturing cutbacks about 20 years ago because to purchase the product cost more than double the cost of the ingredients, and all the supplier did was meter both asphalt and fuel into the customer's distributor truck. The company has produced their own specification compliant cutbacks using the proportions I determined ever since.
It is more complicated than just getting the correct viscosity because you must also get the correct cure time for the kind of product you trying to produce, either rapid, medium, or slow set. This requires use of a solvent, or mixture of different solvents, that provide the evaporation rate needed to provide the correct cure properties for the grade of cutback produced. Typical solvents include kerosene (jet fuel), diesel, naptha and gasoline.
When I first determined the required proportioning, I didn't have a lab readily available that could do the required testing so I devised a "down and dirty" method intended to get me close. My intent was to then send in a sample of my product for testing, adjust, retest, etc. However, the "down and dirty" method worked so well we successfully produced spec MC-70, and SC-3000 the first time we sent the products in for testing. The method was simply to obtain a sample of product in the desired grade that is used the same neat asphalt as I intended to use in my manufacture and that was known to comply with the specifications. Then, I heated that sample until it was in a liquid, but still somewhat viscous state and recorded the temperature. Then I poured the material into a small metal funnel and adjusted the flow rate by squeezing the bottom of funnel with pliers until the filled funnel would drain at a rate so that the steady stream of flow would break into a broken flow (or drip) in about 30 to 45 seconds, and recorded the required time. Then, I heated my various lab mixed combinations of asphalt and differing types and amounts of solvents to the previously established temperature and performed my funnel test on each one, adjusting until the flow time matched the flow time of my compliant sample. I determined the proportion of asphalt/solvent in this manner, using the solvents I thought most likely to have provide the correct cure time for the application, then mixed larger batches and applied them in the intended use and verified by visual inspection that the cure time was typical for the product. Then, I finally sent in samples for independent lab testing and certification. All passed, and always have since then.
