HOT IN-PLACE RECYCLING (HIPR)
FREQUENTLY ASKED QUESTIONS.
1. WHAT IS A GOOD CANDIDATE PAVEMENT FOR REHABILITATION WITH HIPR?
Any hot mix asphalt pavement with a stable base, adequate drainage, and does not have characteristics listed in question #2.
2. WHAT IS NOT A GOOD CANDIDATE PAVEMENT FOR REHABILITATION WITH HIPR?
Roads that are NOT good HIPR candidates have the following characteristics:
1) Excessive amount of subgrade failures.
2) Mat thickness of less than 3 inches.
3) Wide cracking.
4) Low oil content – less than 4%.
5) Poor or soft aggregates.
6) Large aggregates - larger than ¾” in diameter.
7) Evidence of Stripping.
Many of the above factors can be corrected with proper engineering.
3. WHAT MUST BE DONE TO DETERMINE A CANDIDATE PAVEMENT FOR HIPR?
1) Visual inspection of the candidate pavement.
2) Cores of the roadway surface to determine:
a) Mat thickness.
b) Aggregate size, hardness and gradation.
c) Percent of Asphalt.
d) Existence of geo-tech material such a petro-mat and it’s depth.
e) Evidence of Stipping.
3) Review records from the “As-built” drawings of the roadway.
4) Interview field maintenance personnel.
After a candidate pavement has been selected, a qualified materials engineer should conduct further testing at his/her discretion.
4. WHAT IS THE EXPECTED PRODUCTION RATE OF HIPR?
Depending on weather and road conditions, 1 to 2 lane miles per day.
5. HOW DEEP DO YOU PROCESS WITH HIPR?
1-3/4 inches is the most common specified depth. The actual mat thickness after recycling will be reduced by:
a) Crack volume
b) “Rut” volume
c) Changes in gradation.
d) Correction of surface inundations and profiles both transverse and longitudinal.
This, as a “rule of thumb,” amounts to approximately 10% to 15%.
6. WHEN CAN TRAFFIC BE ALLOWED BACK ON THE ROADWAY?
Same as with asphalt overlays, or sooner, an hour or so at the outside.
7. DOES THE RECYCLED SURFACE HAVE TO BE OVERLAID?
No. However, historically over half of the projects involving HIPR are overlaid and HIPR is utilized as a surface restoration to provide a stable and uniform platform for the overlay. Other projects, in maintenance applications for example, HIPR is “capped” with a chip-seal to provide a wearing surface.
8. DOES ADMIX (CORRECTIVE MIX) HAVE TO BE UTILIZED?
No. However, addition of Virgin Mix may be utilized for 2 basic reasons:
1) As a Corrective Mix. I.e. that amount of mix required to offset the affects of cracks, gradation changes, due to an existing mat that is non-homogeneous, to correct the longitudinal grade profile (smoothness), changes in transverse profile, for example; shoulders which fall off, or modification of super-elevation, correction of horizontal geometry, rutting, etc., etc., etc¼ This usually amounts to 10%-15%.
2) To obtain additional mat thickness. Admix may be used to obtain a thickened residual mat, after the Corrective Mix Threshold has been obtained. This is recommended only for maintenance type projects where a chip-seal is being utilized as the wearing surface. Obviously this would not be a cost effective method if an overlay were to be placed over the recycled material. As Admix costs are well in excess of the cost per ton for Hot Mix in a high production overlay operation, this would not be a cost effective method.
9. CAN HIPR BE PERFORMED AT NIGHT?
Yes, at a significant increase in operational costs and decrease in production rates.
10. HOW MUCH DOES HIPR COST?
HIPR costs vary based on the differing methods and treatment depths. Normally less than $30,000.00 per lane mile.
11. WHAT PROBLEMS CAN BE CURED BY HIPR?
HIPR treats surface distress problems in the top inch or two of the existing pavements. HIPR also re-establishes the ride quality of the roadway, provides “surface restoration” and replaces the need for a leveling course in an overlay project. HIPR will not improve any base or sub-base problems.
12. WHAT IS THE WEATHER LIMITATIONS FOR HIPR?
Typical requirements are 45 degrees and rising with calm winds and no water on or in the pavement.
13. ARE THERE ANY WIDTH RESTRICTIONS IN THE HIPR MACHINERY?
HIPR is normally done in 12 or 13 foot widths.
14. HOW CAN HIPR BE UTILIZED TO IMPROVE AN EXISTING ROADWAY?
1) Corrections to the gradation can be made by the addition of Virgin aggregates or mixes.
2) Improvements to the thermal characteristics of the roadway, i.e. the addition of polymerized asphalts may be made during the recycling process.
3) Existing cracks will become “plastic” in nature through the application of heat and will be distorted during the rolling process.
4) Electronic grade control equipment will improve the rideablility characteristics of the roadway.
5) Geometrical and gradation characteristics may be made through the addition of Corrective Mix.
15. HOW ENVIRONMENTALLY FRIENDLY IS HIPR?
HIPR is the most environmentally friendly of all pavement rehabilitation methods. Because HIPR uses a minimal amount of virgin materials, the impact to the environment is minute.