Mixing, Placing and Compacting

There are three basic HIPR construction processes in use: heater scarification, repaving, and remixing. All involve a specialized plant in a continuous train operation.

Heater scarification involves a plant that heats the pavement surface (typically using propane radiant heaters), scarifies the pavement surface using a bank of nonrotating teeth, adds a liquid rejuvenating additive, then mixes and levels the recycled mix using a standard auger system. The recycled asphalt pavement is then compacted using conventional compaction equipment. The process is limited in its ability to repair severely rutted pavements, which are often overlaid with conventional hot mix asphalt.

Repaving is a more sophisticated process that includes removing (by heating and scarification and/or grinding) the top 25 to 50 mm (1 to 2 in) of the old asphalt pavement, adding and mixing in a rejuvenating agent to improve asphalt viscosity, placing the recycled material as a leveling course using a primary screed, and simultaneously placing a thin (usually less than 25 mm (1 in) but up to 50 mm (2 in) in some systems) hot mix asphalt overlay. Conventional equipment and procedures are used immediately behind the train to compact both layers of material to ensure a monolithic bond between the new and recycled layer.(30)

The remixing process is used when additional aggregates are required to improve the strength or stability of the recycled asphalt concrete. Scarified or milled RAP is blended with rejuvenator and new virgin aggregate or new hot mix asphalt, then placed by a compacting screed. Conventional equipment and procedures are used to place and compact the remixed material.

Quality Control

The initial step in the quality control of hot in-place recycled mixes is in the selection of the pavement to be recycled. Not all pavements are good candidates for this type of recycling. Cores of the pavement being considered for HIPR must be taken during the early planning for the project. The cores should first be visually examined for pavement problems such as delaminations, stripping, or stripping potential, or water in the voids or delaminations. Pavements with delaminations, especially saturated delaminations, in the top 5 cm (2 in) should not be considered for HIPR projects. Also, pavements that have been rutted, heavily patched, or chip-sealed are not good candidates for HIPR projects.

Next, as noted in the Mix Design section, field core specimens should be analyzed in the laboratory to determine (based on the asphalt content, viscosity, and penetration of the recovered binder) the required amount of rejuvenating agent to be added to the mix in order to attain the desired viscosity of the recycled mix. If too much rejuvenating agent (1.0 percent or more by weight of mix) must be added in order to attain this viscosity, the mix should probably not be recycled in place. As a guideline, pavements being considered for HIPR should not be too severely aged. It is recommended that such pavements have an absolute viscosity lower that 200,000 poises (and preferably below 100,000 poises) in order to be considered for HIPR projects.(31)

Field core specimens should also be evaluated for air voids content during the pavement selection process. An existing pavement being considered for HIPR should have an air voids content in excess of 6 percent, in order to accommodate the addition of a rejuvenating agent without the loss of stability in the recycled mix. If material properties are not completely satisfactory for 100 percent recycling, the addition of 20 to 30 percent by weight of virgin hot mix during recycling should be considered.(31)

Field quality control measures during HIPR operations include monitoring the depth of scarification, the temperature of the recycled mix, the visual appearance and homogeneity of the scarified or milled RAP, the compaction procedure, and the visual appearance of the recycled pavement surface after compaction. Loose samples of the recycled mix should be obtained and extraction tests performed to monitor RAP gradation, asphalt cement and air voids contents, and penetration and viscosity of the recovered asphalt binder for comparison with the job mix formula.(32) The recycled mix should be monitored for in-place density in accordance with ASTM D2950.(29)