RAP Technologies
Plant technology
Separate cold feed bins for fine and coarse RAP fractions volumetrically meter design blends onto incline conveyers that delivers them to the heating drum. Due to differences in ratio of thermal mass and surface area, the fine RAP fractions require less time to reach mix temperature than coarse aggregates. Therefore, coarse RAP is introduced in the drum at the beginning of it, while the fine RAP is introduced at dryer midpoint via a conventional “center entry” RAP collar. The mix discharge temperature is around 150°C. Recycling agent is sprayed on the hot RAP at the dryer discharge chute as demonstrated in Fig. 2. It mechanically mixes with the RAP binder during transportation by drag slat conveyor. The diffusion continues during storage, transportation, and laying of the asphalt.
A critical element of the RAPTech process is pollution control. Since most of the fine dust is encapsulated by RAP binder there is little need for dust collections. Instead, blue smoke generated by the direct contact of RAP with flame has to be removed prior to releasing combustion gases to atmosphere. RAPTech employs a multiple stage filtration system (Figure on left) to comply with local air quality rules as follows: - Inertial separator drops out small quantity of coarse fines that are then manually removed a few times per year; - Disposable fiberglass pocket filters remove micron size particles with up to 99% control efficiency; - Recirculated water spray cools air stream and condenses hydrocarbons stripped from RAP during drying forming aerosol mist; - Fiberbed filters remove aerosol mist by Brownian capture and release zero opacity gases to atmosphere; - Exhaust gases comply with 0.04 g per SCF (Standard Cubic Foot) and 10% opacity limits for conventionoal asphalt plants established by federal New Source Performance Standards located at 40 CFR 60; - Air flow is approximately 30,000 ACFM (Actual Cubic Feet per Minute) at 30% moisture.
- The dryer is maintained at slight negative pressure to vent combustion gases and fugitive emissions to the air pollution control device.
Current operation, RAP Processing and Mix Design
The RAP is run through a screening plant and separated into fractions using 6.4mm, 12.5mm and 19.0mm sieves. A combination of these fractions is used to produce 4.75, 12.5 or 19mm NMAS Superpave mixes. The material is not crushed to avoid generation of excess fines. Oversized material is collected for offsite disposal. The fines are processed shortly before mixture production to avoid high moisture content due to weathering.
Optimum binder content is ensured by varying the proportions of aggregates and rejuvenator type and dose. The fine RAP fractions have higher binder content and therefore changing the dose of 6.4mm fraction will affect the total binder content. The recycling agent dose is chosen based on extracted binder penetration test results. Switching between different recycling agents allows changing the binder content, since the optimum dose can vary by a factor of two to provide the same effect.
100% RAP is produced and placed at temporary city street pavements, parking lots, industrial areas etc. Recently a demonstration project of 100% RAP along with conventional asphalt has been paved by the city DOT in Jewel Avenue & 147th Street in Kew Garden Hills, Queens (89). Long term performance can be evaluated by visiting 78th street that was paved in 2001.