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Hot Mix Asphalt <br /> Facilities 300 PROCESS AND CONTROL <br /> Types of <br /> HMA 'There are two types of hot mix asphalt facilities in common use today <br /> Facilities batch and continuous mix. Continuous mix facilities are of two subtypes <br /> drum mix and continuous pugmill mix. <br /> All continuous mix facilities function alike up to the aggregate dryer. All of the <br /> approximately 60 continuous mix facilities in use in California, in 1990, are of <br /> the drum mix type, in which asphaltic cement and mineral aggregate are mixed <br /> in the last 1/3 of the dryer, with the exception of 5 facilities. These 5 facilities <br /> are of the continuous pugmill mix type where mixing takes place in a cond- <br /> nously operating pugmill. <br /> In this section only batch mix facilities and the drum mix type of continous mix <br /> facilities will be discussed. The continuous pugmill mix type of continuous mix <br /> facilities are not in common use. <br /> 301.1 BATCH MIX FACILITIES <br /> In a batch mix facility, see figure 301.1, the aggregate is hauled from the <br /> storage piles and is placed in the appropriate hoppers of the cold feed unit. <br /> The material is metered or unmetered from the hoppers onto a conveyor belt <br /> (see figure 301.2) and is transported to the rotary dryer.' <br /> Mixing <br /> Operation As it leaves the dryer, the hot aggregate drops into a bucket elevator and is <br /> transferred to a set of vibrating screens and separated into four or more <br /> different sizes (see figure 301.3). The separated aggregate is transferred to the <br /> mixing operation in specific proportions according to the desired product. The <br /> dried and separated aggregate is mixed with asphalt cement in batches.' <br /> The aggregate is dropped from the vibrating screens into three to five large bins <br /> according to size. The aggregate size distribution in the mix is controlled by the <br /> operator by opening various bins over a weigh hopper until the desired mix <br /> and weight are obtained for one batch. This material is dropped into a <br /> pugmill (mixer) and is mixed dry for about 10 to 15 seconds. The asphalt <br /> cement is pumped from a heated storage tank into a weigh bucket, propor- <br /> tioned for one batch, and injected into the mixer. The aggregate and <br /> asphalt are usually mixed for about 30 seconds and dropped into a truck and <br /> hauled to the job site.' The final mix may also be deposited into a storage silo. <br /> Page 300 - 2 August 1990 <br />