Mix Design

The specifications and design of cold plant mix recycling of asphalt pavements are referred to in ASTM D4215.(12) Cold plant mixtures can be dense-graded or open-graded. Cold-laid asphalt mixes may be used for surface, base, or subbase courses.

Although there are no universally accepted mix design methods for cold mix recycling, the Asphalt Institute recommends and most agencies use a variation of the Marshall mix design method.(13) General procedures include a determination of the aggregate gradation and asphalt content of the processed RAP, determination of the percentage (if any) of new aggregate to be added, calculation of combined aggregate in recycled mix, selection of the type and grade of new asphalt, determination of the asphalt demand of the combined aggregate, estimation of the percent of new asphalt required in the mix, and adjustment of asphalt content by field mix trials.(14)

The percent asphalt demand of combined aggregates can be determined by means of a formula that takes into account the various sieve size fractions of the combined RAP and virgin aggregate. These size fractions include the percentage retained on the 2.36 mm (No. 8) sieve, the percentage between the 2.36 mm (No. 8) and 0.075 mm (No. 200) sieves, and the percentage passing the 0.075 mm (No. 200) sieve. The percent of new asphalt is the difference between the percent asphalt demand and the percent of asphalt contained in the RAP.(1) Using the determined asphalt content, Marshall specimens can be prepared at various emulsion percentages to determine an optimum asphalt content on the basis of applicable stability and air voids criteria.

Structural Design

The AASHTO Design Guide(15) is applicable to recycled cold mix paving mixtures. While there are no universally accepted structural layer coefficient values for asphalt cold mix, it is generally recognized that cold mix asphalt is not the structural equivalent of hot mix asphalt, but is superior to gravel or crushed stone base courses. Asphalt cold mix is generally not recommended for use as a wearing surface, but only in base course layers because of both structural and durability considerations. The structural capacity of recycled cold mix can be considered equal to that of conventional cold mix paving materials.(16)

Although most agencies have not published structural layer coefficient values for conventional or recycled cold mixes, a layer coefficient value of 0.25 to 0.35 for an asphalt stabilized base is considered within a reasonable range. Pennsylvania DOT has assigned a structural layer coefficient of 0.30 for a bituminous-aggregate stabilized base,(7) which is a conventional cold mix.