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Section 2: In-place Surface Recycling

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Hot In-place Recycling (HIPR)

This rehabilitation technique is meant to address asphalt concrete surface distress and texture issues only, therefore the underlying base layers must offer adequate support.

The HIPR process involves recycling the existing asphalt surface layer by heating, scarifying, and adding a recycling agent. There are three basic HIPR processes:

  1. Heater-scarification - heating, scarifying, rejuvenating, leveling, reprofiling, and compacting.
  2. Repaving - heating, scarifying, rejuvenating, leveling, laying a new hot mix layer, reprofiling, and compacting.
  3. Remixing - heating, scarifying, rejuvenating, mixing (adding a new hot mix), leveling, reprofiling, and compacting.

A specialized train is used to heat the surface of the pavement to 300 - 350°F which allows the top 1 to 2 in. of material to be scarified, rejuvenated, remixed and replaced in a multi-step on-site process. Minor cross slope and aggregate gradation corrections can be made. Virgin hot mix can also be added to correct the recycled asphalt pavement (RAP) shortcomings as in the case of the remixing process. Coring of the existing asphaltic concrete pavement (ACP) surface is necessary to determine the material properties of the existing asphalt pavement. This will allow for evaluation of any necessary adjustments to aggregate gradation to develop the required voids in mineral aggregate (VMA) and selection of the appropriate asphalt concrete (AC) binder. It is recommended that the vendor/contractor be contacted with regard to the mix design.

Pavements with delaminations, especially saturated delaminations, in the top 2 in. should not be considered for HIPR projects. Also, the state of practice does not recommend pavements that have been rutted, heavily patched, or chip-sealed as good candidate projects. Typically, hot in-place recycling operations are conducted when the ambient air temperature is 50ºF and rising, but 2004 specifications cite a minimum surface temperature for hot mix asphalt (HMA) placement of 60ºF. An additional overlay may be placed over the recycled surface if additional structural strength is needed as in the case of the repaving process. An equivalent alternative strategy is to mill 1 in. of the existing top layer and place back 2 in. Up to 30% RAP may be allowed in the new 2-in. overlay.

Hot In-place Recycling Train. (click in image to see full-size image)

Figure 7-1. Hot In-place Recycling Train.

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Cold In-place Recycling (Bituminous Layers Only)

As with the hot in-place process, this rehabilitation technique is meant to address distress within the bituminous portion of the pavement structure, but can reach as deep as 4 to 6 in. Therefore, the base must also be sound, with repairs made to locations that have failed or show potential for failure. The process also involves a specialized train with a cold milling machine, crushing/screen unit, and mixing unit that is capable of reclaiming the old asphalt, crushing (screening and sizing) the RAP, and mixing the RAP with virgin aggregate (if necessary) and emulsion. Coring of the existing ACP surface is necessary to determine the material properties of the existing asphalt pavement to properly design proportions of virgin aggregate, emulsion and rejuvenator, if necessary. Cores are also inspected for the presence of variations in the pavement layers, delaminations and whether voids are saturated. The industry does not recommend pavements that have been rutted, heavily patched, or chip-sealed as good candidate projects. Generally, a seal coat or additional overlay will be required after adequate curing since the cold re-processed mix has higher void ratios and is more difficult to compact than hot mix.


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