One of many problems facing the hot-mix asphalt designer, specifier, and producer is how to properly predict the behavior of the asphalt concrete when exposed to moisture. At present, a broad spectrum of accelerated test procedures are available to the engineer that to some degree “predict” the moisture susceptibility of the mix. These procedures include the Lottman, the modified Lottman, and the boiling water stripping test. With the number of procedures available, and with different procedures being specified in different parts of the country, the engineer must answer the question of which procedure best suits the prediction of future performance.
Once a procedure is chosen, then the method by which the data are interpreted becomes important. For the Lottman procedures, splitting tension specimens are often prepared at 7 ± 1% air voids. Grossly different and misleading results will be obtained if conditioned specimens having 8% air voids (7 + 1%) are tested and compared to control specimens having 6% air voids (7 - 1%). While the preceding combination would suggest less than actual performance, the opposite is possible if the extremes are reversed. Either case is undesirable and can prove costly if a local aggregate is excluded for a false low value or if a poor material is accepted for a false passing value.
The authors suggest that a better procedure is to prepare specimens representing the low (4 to 6%), midpoint (6 to 8%), and high (8 to 10%) end of the air void range allowed for the test. This should be done for both the conditioned and the control specimens. After determination of the splitting tensile strength for each specimen, the results should be plotted as tensile strength (St) versus percent air voids for each condition. The strength at 7% voids for each test condition can be determined by using graphical interpolation and a valid comparison can be made for all conditions.
Another suggestion of the authors is that a minimum conditioned tensile strength at 7% air voids be specified in conjunction with a minimum retained tensile strength ratio (TSR). The TSR requirement may be waived for treated asphalt mixes in which the conditioned tensile strength exceeds the minimum specified tensile strength and is also higher than the tensile strength of the unconditioned untreated mix.