Cold in-place recycling (CIR) is not new. If the roadway is a good candidate for the process, it allows for 100% of materials to be recycled right there on the roadway. This saves on rock and asphalt costs, trucking costs, labor and more.
The Minnesota Department of Transportation (MnDOT) Metro District recently put the CIR process to work for the first time in 30 years, saving them $800,000 on the Hwy 110 project alone.
HWY 110 Ready for CIR
Highway 110 is an east-west urban minor arterial in the southeast Twin Cities Metro Area and in the summer of 2017, it was slated for repair.
“The pavement condition was the main driver for the 2017 project,” Tim Clyne, Metro pavement and materials engineer, says. “The ride quality was in the Fair condition (2.2 to 2.5 on a 0-5 scale). Transverse cracks were prevalent and deteriorating, and longitudinal cracks in the wheelpath were present as well. MnDOT maintenance forces had been patching and filling potholes throughout the corridor.”
The roadway was originally constructed in two segments - the western portion in 1948 and the eastern portion in 1984. Both segments last received a mill and overlay in 1997, 20 years ago.
The repair was intended to restore the pavement condition and improve the ride quality for the driving public. Other aspects of the project included access management (intersection / turn lane modifications), construction of a box culvert for a local bike trail under the highway, drainage repairs (pipe replacements), ADA pedestrian upgrades and signing replacements.
“CIR was selected for Hwy 110 because a life cycle cost analysis showed CIR to be the most cost-effective rehab option,” Clyne says. “The life cycle cost analysis compared both initial construction cost and long term maintenance cost over an analysis period to a thick bituminous mill and overlay and a concrete overlay option. In addition, we've seen the success of CIR in other MnDOT districts as well as many local agencies and wanted to try it in Metro.”
MnDOT hoped that CIR would immediately reduce, delay or even eliminate reflective cracks that they would typically see in a bituminous mill and overlay project.
“CIR also recycles 100% of the material in place,” Clyne says. “This reduces costs, reduces the use of dwindling available virgin materials, reduces trucking and thereby increases safety in the workzone and is environmentally friendly.”
CIR in Action
The pavement design called for milling down 3-in. of the existing bituminous pavement followed by 4-in' of CIR. That process was followed by paving of a new 3-in. overlay. MnDOT specifications allow for both the CIR train and the single machine CIR operation. Midstate Reclamation & Trucking, the local contractor who completed the CIR work used a CIR train consisting of a 12-ft. wide Cat milling machine with a RAP crusher, sizing screen and pugmill all-in-1 machine that was hooked up to an asphalt emulsion tanker to provide the stabilizing/recycling agent.
After the CIR mixture was mixed in the pugmill, it was placed on the roadway in a windrow, picked up using a pickup machine, placed with a conventional Vogele paver and rolled with Hamm compaction equipment.
"The paver used infrared thermal profiling technology to measure the temperature uniformity (as a surrogate for mixture uniformity)," Clyne says. "Each roller was also outfitted with intelligent compaction technology to assess compaction quality and uniformity."
Existing shoulders and turn lanes received a 3-in. mill and overlay. New turn lanes were constructed with 7-in. bituminous over aggregate base over imported sand subgrade. The target emulsion rate for engineered emulsion was 2.2%, or 0.97 gallons / SY. There was about 100,000 SY of CIR on the project.
Project Presents Challenges
The weather often presents challenges during construction in Minnesota and this project was no different.
“In the eastbound direction between the CIR operation and the bituminous overlay we experienced a very significant rainfall event, which left the grade wet and soft,” Clyne says. “When McNamara Contracting, Inc., the bituminous paving contractor, came to place the overlay, their equipment sank into and left ruts/depressions in the CIR layer.”
The contractor and MnDOT inspector were able to work through the issue and get the roadway paved and when they came back to do the paving in the westbound direction, the contractor paid closer attention to their emulsion rate and compaction effort of the CIR layer, and weather was more favorable in order to pave the bituminous overlay with no issues.
Another challenge was due to the addition of turn lanes and drainage work - MnDOT had to construct the pavement to a finished profile and in doing so had to correct several issues with cross-slopes and superelevations.
“Our MnDOT surveyors solved the problem by providing elevations for the contractor to do variable depth milling to the proposed profile so that the bituminous overlay on top of the CIR could be paved uniform and meet the final profile,” Clyne says.
The original construction plan also called for CIR in turn lanes, gore areas and other miscellaneous areas. However, it is rather difficult to get the CIR train in and out of those hard-to-reach places which presented yet another challenge for the paving team.
At the pre-construction meeting, S.M. Hentges & Sons, Inc., the prime contractor on the project, and MnDOT discussed the issue and changed the project to only include CIR of the mainline pavement (two through lanes in each direction), leaving the other areas as mill and overlay.
The eastbound direction was paved over seven days in August and the westbound was done over five days in September.
"Traffic staging is a huge problem for almost any construction project in the Metro District, which is probably one reason we haven't done a CIR project in quite a while," Clyne says. "Hwy 110 sees approximately 25,000 vehicles per day (AADT). We successfully managed the traffic staging by doing a full directional closure - for example, maintaining traffic in the westbound direction in their current lane configuration while closing eastbound traffic and detouring them onto a nearby parallel route. Each directional closure lasted about six weeks - primarily due to the construction of the box culvert and deep pipe replacements. The CIR and bituminous paving operations had about a two-week duration total for each direction."
This project was over $800,000 cheaper than the alternative thick mill & overlay, representing a cost savings of over 36%. Which means MnDOT will be using this process more frequently in the future.
"MnDOT Metro District has several CIR projects on the books in future years - some on bituminous pavements similar to Hwy 110, and 1 or 2 upcoming projects where we do a CIR layer on top of an existing (old, beat up) concrete pavement prior to a new bituminous overlay," Clyne says. "Outstate MnDOT districts and local agencies continue to plan more CIR projects as well."
