Asphalt Pavement Analyzer for rutting susceptibility, and Cantabro for durability, a deviation up or down on production AC content versus design AC content can have dramatic impacts. Minimizing Aggregate Moisture The first step in minimizing moisture in aggregate stockpiles is good stockpile practices. If possible, consider covering (preferred) and/or paving underneath aggregate stockpiles (Figures 1 and 2). This is especially critical for fine aggregate and recycled products (RAP and RAS). Fine aggregate (e.g., screenings and manufactured sand) will likely be more prone to moisture retention than clean stone. Water retention will be maximized with well-graded fine aggregate with high minus No. 200 contents. The stockpile area should be sloped to ensure moisture drains away from the stockpile. While paving under stockpiles is a good practice, covering the aggregate is ideal (i.e., keeping the water out instead of draining water away). When considering a covered or paved stockpile area, evaluate the payback period by calculating the decreased moisture content’s impact on plant production rate, drying energy, and overall mix variability. Aggregate Moisture Measurement Methods Most commonly, moisture content is measured with direct or gravimetric methods in which the sample dry mass and water are measured using conventional oven (ASTM D2216) or microwave (ASTM D4643) methods. However, some locations have had success measuring moisture with indirect methods, such as a nuclear moisture content gauge. APAC Central, A CRH Co. (Tulsa, Okla.), has used a nuclear gauge to provide results that align with conventional oven measurements for a variety of materials (clean stone to fine aggregate) and moisture contents (0.5 to >7%). It should be noted that difficulties were experienced when using the nuclear gauge for recycled materials’ moisture contents due to the hydrogen atoms in the asphalt binder. APAC Central’s method of using the nuclear gauge for moisture testing is briefly described below. Such a protocol should be strongly considered as a way to obtain accurate moisture contents in a more time-efficient manner. 1. Build a miniature stockpile. 2. Back drag a smooth surface. 3. Place the nuclear gauge on the surface. A 6-inch probe depth and 1-minute count time has been used successfully by APAC Central. 4. Ideally, the moisture content will be checked at three or more locations, and the average will be taken as the stockpile moisture. Impacts Aggregate moisture significantly impacts the production, cost, and GWP of asphalt mixes. With lower aggregate moisture, plants can run at lower burner temperatures while still maintaining production rates. Just as has been seen with the GWP reductions using warm mix asphalt technologies through lower plant temperatures, the same exists with lower aggregate and RAP moisture levels—up to 6% GWP reduction at 50 degrees Fahrenheit. Therefore. reduce and control aggregate moisture; reduce and control RAP moisture; monitor moisture content frequently, especially for fine aggregates and fine RAP materials. References Oldcastle Quality Bulletin #PRD-012015. Aggregate Moisture Importance and Measurement. Oldcastle Materials. Asphalt National Performance Committee. GWP Levers for EPDs. SAPA EPD Workshop 18 May 2023. National Asphalt Pavement Association. Joseph Shacet (formerly NAPA Director of Sustainable Pavements). BACK TO BASICS: CONTROLLING STOCKPILE MOISTURE Figure 2: Paved Aggregate Stockpiles VAASPHALT.ORG 25
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