The first step, as in the U. S. method (see Section 7.2), is determining the volume of the skeleton of coarse particles and the voids between them available for the remaining SMA elements. Determining the volume occupied by the coarse aggregate skeleton consists of defining its density and testing the coarse aggregate compaction (namely, the amount of air voids remaining among the coarse grains after compacting). As we know, the amount of air voids in a compacted coarse aggregate may be determined using the following methods:
• With dry aggregates, using the dry-rodded test after AASHTO T19 as in the U. S. method, or using a gyratory compactor, Marshall hammer, or on a vibrating plate
• Using a special lubricating agent[38] and chosen method of compaction
The substantial difference between the dry-rodded method and other methods is the dry compaction of aggregate used in the U. S. method and the “grease” process used in the others. Explaining this issue logically, air voids determined using the dry — rodded method must be larger because of the higher resistance to the displacement of particles relative to each other, so they will not be arranged as closely as in the grease method. The use of grease will also result in less crushing of the aggregate during compaction. After all, when compacting SMA on a site, the presence of binder in the mixture lubricates it, making the displacement of aggregate particles easier. So it seems that the method using grease, though more problematic in practice, enables us to obtain results closer to reality. Any substance with a viscosity resembling the viscosity of binder at about 150°C may be employed as a lubricating agent or as a grease in the mixture. In the Netherlands, medical oil has been used for that purpose, with 1.5% (m/m) added to the aggregate mixture. The possibility of conducting the whole operation at room temperature, without heating up the oil and aggregate, is a notable advantage of this substance.
The coarse aggregate of an analyzed mixture (a sample of 4 kg) is compacted by being placed in 150 mm diameter specimen mold and undergoing 300 rotations in a gyratory compactor[39] with the external angle of rotation set on 1°. After the density of the coarse-aggregate particles (greater than 2 mm) and the air voids in the compacted aggregate are determined, the aggregate is extracted from the oil and the gradation is measured. Consequently, apart from the result of air voids in compacted coarse aggregates, some additional information is gained on the aggregates’ resistance to crushing. A compactor, when set at 300 rotations, causes overcompaction of the mixture and, to some degree, destruction of grains corresponding with the laydown and compaction process and after several years of service.
Another important factor that should be taken into account while analyzing air voids among coarse grains is their crushing and wearing, which occurs at the production stage of a mixture, at its laydown, and during its later service. Crushing of the grains causes the displacement of particles, hence a decrease of air voids in the coarse aggregate skeleton (post-compaction). Coupling this with the knowledge of susceptibility of the aggregate to crushing and wearing that is gained by screening the aggregate after compacting it in the gyratory compactor, it is necessary to increase the air voids in the designed SMA to a certain degree (e. g., to 5% instead of 4%). This should guarantee that, even after longterm service under heavy loads, there will be no bleeding of mastic (fat spots) from among the grains of a skeleton. This method of reasoning has been adopted in the Netherlands, where the air void content in compacted laboratory samples for heavy duty traffic has been increased to 5% (v/v) (Jacobs and Voskuilen, 2004; Voskuilen, 2000).
The use of aggregates susceptible to crushing alters the volume relationships in the aggregate mix in the following ways:
• The volume of the coarse particle skeleton decreases (because some of the coarse particles becomes fine particles).
• The volume of fine particles, which are not involved in the coarse skeleton’s performance, increases.
• The content of air voids in the aggregate mixture decreases.
• Consequently the quantity of air voids in the SMA drops, so the risk of overfilling with mastic increases.
Using this method, the effect of increasing air voids in the coarse skeleton has been taken into account. We should remember that air voids have been determined using the method of dry or greased compaction of coarse aggregates. In this test, only the aggregate greater than 2 mm has been used. In a real mixture, coarse grains are coated with mastic, so naturally there are particles of filler or crushed sand among the coarse particles. These particles slightly increase the content of air voids among the coarse aggregates, particularly at the first stage of an SMA’s performance. In the Netherlands the effect of an added increase of air voids among coarse particles has been called the enlarging effect. With the passage of time, the decrease of air voids among coarse aggregates occurs as a result of post-compaction, reorientation of coarse grains, wear, and the movement of fine particles.
