Asphalt Heating and Circulation

Figure 8-12.-Asphalt return line. samples cannot be obtained even when the sampling device is used correctly. Asphalt Heating and Circulation Provisions should be made for the circulation of the asphalt through the feeding and storage system. All storage tanks, transfer lines, and pumps should have heating coils and/or jackets to maintain the asphalt at the required temperature. Return lines discharging into the storage tanks should be submerged below the asphalt level in the tank to prevent oxidation of the asphalt. When the pump is reversed, two or three vertical slots should be cut in the return line within the tank to break the vacuum in the lines. The slots should be cut above the high level mark of the stored asphalt (fig. 8-12). To assure temperature control of the asphalt, you should place an armored thermometer or a pyrometer with a recorder in the asphalt feed line at a location near the discharge valve at the mixer unit. Also, the asphalt storage tank should be equipped with a recording thermometer, having a minimum time range of 24 hours. An approved valve or spigot should be installed in the tank or in the circulating system to provide a means for sampling the asphalt. Sufficient material must be drawn and wasted before the sample is taken to ensure the material obtained is truly representative of the storage supply. When the temperature of the asphalt is maintained by circulating heating oil, the level of the hot oil in the reservoir of the heating unit should be inspected frequently. If the hot-oil level falls, check for leakage of the hot oil into the stored asphalt. Temperature of Mixture Both asphalt and aggregate must be heated before they are combined in the pugmill. The asphalt is heated to make it fluid enough to coat the aggregate particles. The aggregate is heated to make it dry and hot enough to keep the asphalt in a fluid state while it is coating the particles. Asphalt is a thermoplastic material that decreases in viscosity with increasing temperature; however, the relationship between temperature and viscosity may not be the same for different sources or types and grades of asphalt material. The temperature of the aggregate controls the temperature of the mixture, and a mixing temperature normally is specified based on factors relating to placement and compacting conditions. Another consideration is the temperature required to dry the aggregate sufficiently to obtain a satisfactory mix. Mixing should be accomplished at the lowest temperature that provides complete coating of the aggregate particles and a mixture of satisfactory workability. Table 8-1 provides a guide for suggested asphalt temperatures ranges. Mineral Filler Mineral filler is a fine material (dust) that passes through the No. 200 sieve during a sieve analysis. Mineral filler is normally part of the asphalt mix design, used to fill in the voids of the aggregates. Mineral fillers commonly used are the following: portland cement, pulverized limestone (limestone dust), silva, and hydrated lime. High production plants often have a separate feeding system for introducing mineral filler into the asphalt mix. Part of this system is a storage silo that maintains several days supply of mineral filler. A receiving hopper, screw conveyer, and dust elevator are used to charge the storage silo, and a vane feeder meters the filler introduced into the mix. The ultimate choice of this system is usually dependent on the availability of bulk filler and their price in relation to bagged fines. In plant operations where the volume of filler required does not justify a bulk silo, a bag feeding system is used. This system consists of a ground- mounted feeder, dust-tight elevator, surge hopper, vane feeder or screw conveyer, and an overflow chute. 8-10