BY SHANNON HOLLAND, MNDOT
MnDOT has performed numerous salt scatter tests throughout the years, most of which were done in the summer or fall months. Although the tests provide valuable insight into salt scatter and the performance of various sander and spinner configurations, they don’t offer realistic results.
The idea for a winter salt vs. slurry trial stemmed from previous salt scatter testing. Those results indicated a supersaturated salt slurry reduces bounce, scatter, and material loss. Testing done in the summer of 2021 illustrated that we should be able to reduce application rates by 30 percent and still achieve the same results because more material is staying on the roadway.
Since collecting salt and slurry mixtures off a snow-covered roadway would be nearly impossible, we had to find a new way to evaluate the effectiveness of the materials. Instead of attempting to collect the material applied, our new measures were going to be based on roadway friction and bare-lane regain times.
Design and winter testing
In February of 2022, a group of 11 from MnDOT rented a closed track to perform testing. Using a track instead of testing in live traffic gave several advantages: It allowed us to test high volumes of liquids without impacting traffic, experiment with smaller application rates in a controlled environment, capture time-lapse images, and compare bare-lane regain times. Also, the track—a multilane divided concrete surface—offered two identical sections of roadway.
Prior to applying material to the track, we compacted the fresh snowfall for two hours to simulate a realistic roadway scenario. After compacting both lanes, the plow truck plowed and scraped the roadway before applying material.
During application, we continuously ran 11 vehicles around the track to simulate traffic. Two of those vehicles were equipped with mobile RWIS (road/weather information systems) that allowed us to continuously collect roadway data on the track, including friction. Going one direction on the track, the truck applied 250 lbs./lane-mile of dry salt. Going the other direction, the application rate was reduced by 30 percent for a super-saturated salt slurry of 150 lbs./lane-mile of salt with 10 gallons/ lane-mile of salt brine mixed in the auger.
We ran traffic continuously until bare lane was regained, estimating the average daily traffic (ADT) to be 10,560. Our target regain time was two to five hours based off ADT.
Results
For the super-saturated slurry side of the track, bare-lane regain time was achieved in 1 hour 17 minutes. The dry salt application side regained bare lane in 1 hour 50 minutes. Although both sides regained within our target regain times, the slurry side of the track used 30 percent less material and achieved bare-lane regain faster.
Future testing
MnDOT plans to continue testing this winter on the test track. We are looking forward to testing various applications such as direct liquid applications and experimenting with lower ADT levels, different pavement surface types, and under different weather conditions.
Scatter is lower with slurry
AT 35 MPH
Dry rock salt:
40% of material stayed on target
17% of material bounced to the shoulder
Slurry:
62% of material stayed on target
3% of material bounced to the shoulder