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4.1.1 Marshall stability and flow are asphalt mixture characteristics determined from tests of compacted specimens of a specified geometry. The Marshall Test can be conducted with two different types of equipment: (1) Method A—using a loading frame with a load ring and a dial gauge for deformation or flow meter (Traditional Method); or (2) Method B—using a load-deformation recorder in conjunction with a load cell and linear variable differential transducer (LVDT) or other automatic recording device (Automated Method).

4.1.2 Typically, Marshall stability is the peak resistance load obtained during a constant rate of deformation loading sequence. However, depending on the composition and behavior of the mixture, a less defined type of failure has been observed, as illustrated in Fig. 1. As an alternative method, Marshall stability can also be defined as the load obtained when the rate of loading increase begins to decrease such that the curve starts to become horizontal, as shown in the bottom graph of Fig. 1. The magnitude of Marshall stability varies with aggregate type and grading and bitumen type, grade, and amount. Various agencies have criteria for Marshall stability.

FIG. 1 Flow Determination for Two Types of Specimen Failure

4.1.3 Marshall flow is a measure of deformation (elastic plus plastic) of the asphalt mixture determined during the stability test. In both types of failure, the Marshall flow is the total sample deformation from the point where the projected tangent of the linear part of the curve intersects the x-axis (deformation) to the point where the curve starts to become horizontal. As shown in Fig. 1, this latter point usually corresponds to the peak stability; however, as an alternative when the failure condition is not clearly defined, it can be selected as the point on the curve which is six flow points or 0.01 in. (1.5 mm) to the right of the tangent line. There is no ideal value but there are acceptable limits. If flow at the selected optimum binder content is above the upper limit, the mix is considered too plastic or unstable and if below the lower limit, it is considered too brittle.

4.1.4 The Marshall stability and flow test results are applicable to dense-graded asphalt mixtures with maximum size aggregate up to 1 in. (25 mm) in size. For the purpose of mix design, Marshall stability and flow test results should consist of the average of a minimum of three specimens at each increment of binder content where the binder content varies in one half percent increments over a range of binder content. The binder content range is generally selected on the basis of experience and historical testing data of the component materials, but may involve trial and error to include the desirable range of mix properties. Dense-graded mixtures will generally show a peak in stability within the range of binder contents tested. Stability, flow, density, air voids, and voids filled with asphalt binder may be plotted against binder content to allow selection of an optimum binder content for the mixture. The above test properties may also be weighted differently to reflect a particular mix design philosophy. In addition, a mixture design may be required to meet minimum voids in the mineral aggregate based on nominal maximum aggregate size in the mixture.

4.1.5 Field laboratory Marshall stability and flow tests on specimens made with plant mix laboratory compacted (PMLC) asphalt mixture mix may vary significantly from laboratory design values because of differences in plant mixing versus laboratory mixing. This includes mixing efficiency and aging.

4.1.6 Significant differences in Marshall stability and flow from one set of tests to another or from an average value of several sets of data or specimens prepared from plant-produced mix may indicate poor sampling, incorrect testing technique, change of grading, change of binder content, or a malfunction in the plant process. The source of the variation should be resolved and the problem corrected.

4.1.7 Specimens will most often be prepared using Practice D6926, but may be prepared using other types of compaction procedures as long as specimens satisfy geometry requirements. Other types of compaction may cause specimens to have different stress strain characteristics than specimens prepared by Marshall impact compaction. Marshall stability and flow may also be determined using field cores from in situ pavement for information or evaluation. However, these results may not compare with results from lab mix lab compacted (LMLC) asphalt mixture, plant mix laboratory compacted (PMLC) asphalt mixture, or reheated plant mix lab compacted (RPMLC) asphalt mixture specimens and shall not be used for specification or acceptance purposes. One source of error in testing field cores arises when the side of the core is not smooth or perpendicular to the core faces. Such conditions can create stress concentrations in loading and low Marshall stability.