Minimizing Bounce and Scatter

Shaughn Kern, Technical writer and Alexander Slepak, Technical writing intern, Center for Technology & Training, Michigan Technological University

This article is full version of an abridged article found in the winter 2015 Nuggets & Nibbles newsletter.

Thousands of years ago, salt was prized for its ability to preserve food; it was also sown into the soil of enemy lands by invading armies to make the soil unsuitable for agriculture. Whether our ancestors understood the science of soil salinity is debatable, but they did have one thing in common with today’s winter maintenance professionals: they knew the value of salt as a resource, and they appreciated the environmental damage salt could cause if misused.

Salt truck spreading salt

According to a study conducted by the Michigan Department of Transportation (MDOT) in the early 1970s, 30 percent of dry salt used on roads is lost immediately to bounce and scatter. The study concluded that pre-wetting the salt before spreading it reduced bounce and scatter by improving the application pattern and accelerating the melt-rate. Today pre-wetting has become commonplace and is recognized by state and local transportation agencies as a significant cost-saving measure. However, further research was necessary to determine the influence of other distribution variables on the effectiveness of salt.

Building on Past Experience

In the summer of 2012, MDOT Operations Field Services Division built on the research from the 1970s, with the goal of determining an optimum vehicle speed and distribution method for applying salt. MDOT’s Operations Field Services Division provides training and support for maintenance garages that conduct summer and winter maintenance on all state trunk lines in Michigan.

Diagram illustrating the grid pattern used to measure salt bounce and scatter
Figure 1: Grid layout for testing salt bounce and scatter

The new study reexamined the effectiveness of salt treated with a liquid chloride solution, and correlated it to truck speed and salt distribution systems. The comparison of two salt types (untreated and treated), three truck speeds (25, 35, and 45 mph) and two distribution systems (Y-chute and cross-conveyor) made for a total of twelve tests. To conduct the tests, MDOT staff laid out a grid on a 100-foot stretch of unused portion of freeway in Southwest Michigan. This location made for an ideal test site where traffic would not disturb the salt or create a dangerous situation for the staff conducting the tests.

The test grid was made up of 12 four-foot lanes, which simulated a two-lane road with 12-foot paved shoulders (see figure 1). Trucks driving in the left travel lane dropped salt into the “target area,” which spanned four feet on each side of the centerline. The amount of salt recovered from the target area and each four-foot grid lane was tabulated as a percentage of the total amount of salt that was dropped. The results were presented in a graphic form, as shown in figure 2.

Diagram illustrating the results of the salt bounce and scatter study
Figure 2: Graphical representation of test results

Special attention was paid to salt recovered in the target zone and the rest of the travel lane, since only salt in the travel lane is considered effective. Over the course of the entire study, salt recovered in the travel lane ranged from 95.3 percent to 35.7 percent, depending on the speed of the truck, the distribution system used, and whether the salt was treated or untreated. As expected, the results of treated vs. untreated salt verified those found 40 years ago: treated salt performed significantly better at all speeds and through all distribution systems. The comparison between cross-conveyor and y-chute systems resulted in slightly better performance for the conveyor type. For untreated salt, nine percent more stayed in the travel lane when distributed through a conveyor; with untreated salt, 13 percent more stayed in the travel lane.

Speed = Bounce and Scatter

Regardless of salt type or delivery system, truck speed had the most profound effect on how much salt was lost to bounce and scatter. The most effective method of spreading salt on roads, a truck driving at 25 mph spreading treated salt with a conveyor, lost only nine percent to bounce and scatter. The same test at 35 mph resulted in 32 percent loss, with 45 mph showing a 45 percent loss. Table 1 shows the projected cost associated with the salt loss at each speed, based on the seasonal cost of salt in MDOT’s Southwest region of Michigan.

The main recommendation from this study, which is available in a project summary report published by MDOT in November 2012, is crystal clear. “The most effective scenario… occurs when a treated salt product is applied with a cross conveyor from a truck traveling at 25 mph,” according to summary. “Conversely, salt bounce and scatter is at its highest when applied from a Y-chute delivery system in a truck traveling at 45 mph.”

However…

Although the study proves that driving at 45 mph can waste over four times as much salt as driving at 25 mph, the waste calculated by the study only applies to salt. The study methodology does not account for staff time, fuel, or other costs associated with slow truck speeds. “Mobility/user delay costs, route length (time) and staffing costs need to be considered when choosing a salt application speed,” the report reads.

Table 1: Projected cost of salt loss based on
$3.96 million total seasonal salt expense

Speed

Percent Wasted

Projected Cost of Wasted Salt

25 mph

9%

$355,080

35 mph

32%

$1,247,400

45 mph

45%

$1,762,200

In an apparent attempt to reconcile the results of the bounce and scatter study and the perceived costs associated with slow truck speeds, the report reiterates MDOT’s policy of a 35 mph speed limit for applying salt, and also adds that “many operators [believe] an application speed of 45 mph is more efficient and perceive it to be safer.”

Less Salt is Better

Reduction of salt waste has benefits beyond cost savings. Salt causes deterioration of the road, corrosion of the vehicles travelling on it, and it can negatively affect roadside vegetation. Further, effective salt use can limit the need for abrasives such as cinders and sand, for which cleanup costs can be significant.

The report called for further testing using other delivery systems such as zero velocity spreaders (which eject salt in a way that compensates for truck speed), salt slurry generators, and a variety of y-chute heights. This past summer, MDOT ran a second phase of testing to cover these additional variables.

Phase 2: Dialing it in

In the second phase of testing, which was conducted during the summer of 2013 at the same site as the first phase, the MDOT team established a simplified method of quantifying results. Instead of reporting results in graphical form organized by grid lane, the team combined all grid lane values into a single point value for each test (see figure 3).


Graphical representation of test results
Figure 2: Graphical representation of test results

Results indicated that the most effective methodology was to spread salt from a zero-velocity system at 25 mph, with an effectiveness score of 0.93 on a scale of 0.00 to 1.00. Even at 35 mph, the zero-velocity system had an effectiveness score of 0.82, which was better than all other systems running at 25 mph. Notably, when accelerated to 45 mph, the effectiveness of the zero-velocity system dropped to two-thirds of what it was at 25 mph. Complete results are shown in figure 4.

Effectiveness of salt spreading methodologies using a single point value rating
Figure 4: Effectiveness of salt spreading methodologies
using a single point value rating

In Conclusion

Based on results from the two phases of the study, MDOT released a Maintenance Advisory this past August which states that “all salt applied to a roadway should be pre-wet with a liquid chloride product.” The advisory recommends 7 to 10 gallons of liquid per ton of dry salt. MDOT also updated their application rate chart for solids to reflect the use of pre-wet salt at slower truck speeds. The advisory and the application rate chart are both available for download from MDOT’s web site.

Tim Croze, region support engineer of the MDOT Operations Field Services Division, is pleased with what his team learned from the study. “It’s nice to assign actual effectiveness numbers to the many different options we have for spreading salt,” he said. “The right combination of salt type, distribution system, and truck speed can help a great deal adjusting our practices. This will enable us minimize salt waste by keeping more of it in the travel lane where it to keep more salt on the travel lane.”

Links

MDOT Salt Bounce and Scatter Study – Summary Report (pdf)

MDOT Maintenance Advisory - Best Practices for Applying Deicing Materials (pdf)

Reprinted with permission from the Michigan LTAP Center newsletter
(The Bridge) article 27.3

Winter 2015

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This work by the Cornell Local Roads Program (CLRP) is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.