Tackifiers Put to the Test: Erosion and Sediment Control Comes to a Natural Conclusion
by Brett Carlson, freelance writer
Two years after completion of the Highway 10 project, stable, lush and aesthetically pleasing shoulder beds are the norm.
In addition to vegetation establishment, engineering specification and NPDES permit requirements, the poor in-slope soil conditions and gusty winds posed a challenge to minimizing the effects of erosion. The soil composition after construction lends little or no vegetative growth potential to roadside shoulders. This soil condition makes it even more difficult for seeding, mulching and tackifier products to properly adhere to the ground.
To properly maintain the project area, MnDOT and contractors, which included Cory Bernu of Chemstar, Vicky Dosdall of Lawn and Driveway Service and David Sherbrooke of Sherbrooke Turf, Inc. had to establish a three to five foot wide canopy of vegetation along the entire east and westbound shoulder areas. The vegetation would limit water and wind erosion, thus meeting the project goal: to establish a stable, aesthetically pleasing shoulder with 70 - 100 percent vegetation cover within 30 days of project completion. But, getting vegetation to grow and stay in place is very difficult in conditions such as those experienced at the Highway 10 project.
"It was very important for us to find a viable substitute for emulsified asphalt," said Stenlund, MnDOT. "One of our goals during the project was to utilize a variety of different tackifier agents both guar and starch based. You have to keep in mind that emulsified asphalt is an excellent adhesive. However, there are a number of environmental concerns with putting that type of tackifier on the ground and into our environment. So, we made sure to test as many tackifier agents as we could in order to determine an appropriate alternative to guar and emulsified asphalt."
In all, the project utilized approximately 3,147 pounds of tackifier, including: four types of guar-based tackifiers (roughly 1,112 pounds in all) and the starch-based product (about 2,035 pounds) as a test to see how natural tackifiers held up against the proven petroleum-based tackifiers in the field. To properly judge the results, MnDOT monitored important factors, including: overall performance, chemical composition, mixing ratio, cost and ease of application.
"The performance of guar products is well-documented in our state," said Stenlund. "However, we never had the opportunity to test a starch-based tackifier in the field. So, we were extremely interested to see how this type of product would stand up against the traditional guar products in the conditions present in the Highway 10 project."
Like guar-based products, a starch-based product is extremely versatile and can be used as a mulch tackifier, straw/hay overspray binder or as a soil stabilizer/dust abatement agent. However, the erosion control and turf establishment values of starch-based tackifier agents have additional benefits to those of its guar-based competitors. For example, starch tackifiers are a 100 percent active polymer that contains 7 - 8 percent natural nitrogen, which acts as a slow-release fertilizer to help increase the seed germination and plant establishment process. Such natural fertilization characteristics are often absent or at levels of less than 1.0 - 0.5 percent protein in guar, plantago and other synthetic tackifier products.
Another important factor regarding starch-based tackifiers is their high moisture retention characteristics. Starch-based products help improve seed germination rates and reduce irrigation requirements through the hydrophilic nature of their organic polymers. These polymers allow the tackifier products to readily absorb and retain water at the same time as they are helping to stabilize the mulch and hydroseeding layer from wind and/or water erosion possibilities.
As MnDOT geared up for the project, it had to properly map out the distribution of its five tackifier products. So, it split the 19.85 acres of in-slope shoulders into 25 sections of a little less than one acre each, with three replicates randomly assigned. This would provide a very good testing ground regarding the performance of the various tackifier agents. Now, the next challenge mixing ratio.
"Our specs for the state of Minnesota were well-documented with guar-based products," said Stenlund. "However, you need to understand that all guar products are not the same. The consistency from manufacturer to manufacturer is considerable. So, we had a rough idea of what guar in general could do at 100 - 200 pounds per acre, but no real indication of how a starch-based product would perform. Basically, we needed to see for ourselves if a starch-based product was worse than, comparable to, or better than traditional guar products."
Bernu knew that prior applications of his company's starch-based tackifier demonstrated that the product remained extremely pumpable up to 12 bags of product per 3,000 gallons of water. This far outweighed the maximum of three bags per 3,000 gallons of water typically associated with guar products. Number of bags per truckload, or mixing ratio, was an important point because the more bags of tackifier a crew could mix per truck can save a lot of valuable project time and money. For example, the higher the mixing ratio, the less travel time from water source to spraying area. In addition, a higher mixing ratio allows the crew to apply more product in a shorter amount of time.
To ensure that the proper product specifications where followed during the project, Bernu was on hand to offer technical support, advise all parties of what to expect from the starch-based product and answer any and all questions regarding the mixing and application process.
The mixing ratios for the tackifier products were held at a constant. The starch-based tackifier was mixed at 8 fifty-pound bags per truckload and the guar averaged 3 fifty-pound bags per truckload. And, each product was tested at 100 and 200 pounds per acre. This would give MnDOT a good picture regarding how many pounds of tackifier, or concentration rate, were needed per acre for maximum mulch and soil adhesion. Thus, by monitoring and comparing the mixing ratios and concentration rates with the cost of product per pound, MnDOT could directly correlate how much time and money the state and its contractors could save by applying the least amount of product in order to achieve the greatest amount of adhesive impact.
However, in order for MnDOT's tackifier field test to show consistent results, it had to monitor more than just mixing ratios and concentration rates. For example, vehicle and application speed, nozzle velocity, water temperature and purity were all variables that had to be closely monitored and regulated to the best of the crew's ability.
On-site preparation began on the long stretch of east and westbound shoulders with Lawn and Driveway Service thoroughly loosening the soil to a depth of three inches with a conventional agricultural field disk cultivator. By properly disking the seedbed area, the soil now stood a better chance at establishing vegetation growth. The area was then drop seeded with a common mix including part Kentucky bluegrass and smooth brome, followed by harrowing.
Next, Lawn and Driveway Service applied straw mulch at a rate of approximately 2 tons per acre. The combination of 10 - 15 mile per hour wind gusts on day one, 20 - 25 mile per hour wind gusts on day two, and constant vehicle traffic displaced a significant amount of the mulch from the shoulders and onto the road and surrounding landscape. Thus, it was necessary for Lawn and Driveway Service to add 25 percent more straw mulch per acre than called for in the plans to properly cover the project area. Normally, mulching would not be allowed under these wind conditions, but with attention paid to tight sequencing of operations and overrunning the quantity, the 90 percent soil coverage as per specifications was obtained.
After the straw mulch was applied to the shoulder areas, Lawn and Driveway Service then quickly crimped the straw mulch into place in order to minimize mulch movement and loss before the tackifier application. Once again, the job of crimping the mulch was made difficult by the all-too-familiar windy conditions and traffic speeds. These conditions forced the crew to accelerate production sequence timing so that the mulch would stand a better chance of staying in place before the wind whisked it away.
The final step in the Highway 10 project involved the tackifier application.To do this, Sherbrooke Turf, Inc. utilized a 2,500 gallon Finn Hydroseeder® to mix each tackifier product with water. In a perfect world, the water utilized for the Hydroseeder would have been of constant temperature and composition. However, such a scenario was not available to the Highway 10 crew.
The project crew used a single cold water source with a temperature around 55 degrees from a centrally located stream throughout the two-day project. The water could have different amounts of bacteria, algae and other impurities than might be expected on other projects and different parts of the state. The crew was aware that all of these variables could affect the mixing rates and mixing times for each batch of tackifier. The crew also knew the results would not be too far off from those obtained by using other natural water resources.
As each tank was filled with water, careful records of the number of tackifier bags placed into each tank was made. Once the tackifier was added, it was allowed to mix for at least 15 minutes, which was roughly the travel time from water source to spray site. This allowed the tackifier ample time to thoroughly disperse and hydrate in the water solution leading to a more consistent spray and adhesion rate.
Staying true to form, the tackifier application process also was altered due to windy project conditions. Normally the contractor would use a back mounted chair and hose to directly overspray the mulch under low pressure. Due to the requirement of tacking both sides of the road shoulder, and moving with the flow of traffic, the top turret nozzle was used. The pump pressure was increased to minimize over spray onto the bituminous surface and outside of the shoulder. With increased nozzle velocity, the straw mulch tended to be displaced and spread away from the shoulder areas. In addition, winds made it challenging for the hydroseeding crew to hold and establish a five foot shoulder spray pattern. The rolling terrain made it difficult to maintain constant travel speed.
Now, to make things even more challenging, MnDOT had to make sure that the straw mulch, mulch crimper, hydroseed and tackifier applications were all done in perfect succession. This would ensure that the mulch and soil would remain in the shoulder areas and not become dislodged by variations in wind and/or traffic speed. With greater adhesion and coverage comes more robust germination rates and thicker vegetation cover. In addition, MnDOT has a rule that no contractor or subcontractor shall be paid for their services if mulch and seed are not spread uniformly and/or remain properly adhered to the seedbed areas within 48 hours of application. So, everybody involved in the project had added incentive to make sure that their job was done and done well.
"Making sure all of the applications were done in concert was quite a production of timing," admits Stenlund. "In the end, timing was everything. We had to make sure that all the applications were done at the same time and slow enough so that I could accurately measure the speed of the vehicles and rates of application in order to have sound test results."
Two weeks after the conclusion of the project, Stenlund returned to the Highway 10 area to survey the test results. He closely monitored the mulch cover and corresponding vegetation establishment at various points throughout the 11.7 miles of westbound and 15.1 miles of eastbound shoulder areas (both sides of the road). During his survey of the shoulder areas, he relied on a rating system he created in order to judge the percentage of mulch cover remaining on the shoulder bed. The rating system (see Table 1) was based on a number of factors, including:
By utilizing his "mulch cover" rating system, Stenlund had the opportunity to thoroughly document the bonding properties of each type of tackifier and see how those properties helped promote stable and green in-slope shoulders via accelerated germination rates and vegetation cover. Stenlund's rating system concluded that the starch-based tackifier well outperformed its four guar-based competitors. Overall, the starch based tackifier performed in the B+ to C+ range while the four guar-based tackifiers performed in the C+ to D categories.
Additional conclusions from the study revealed other benefits of starch-based tackifiers. For example, the starch-based tackifier has a stable price point, compared to the fluctuating price points of gaur products. Stenlund pointed out that road and maintenance crews are more likely to select a product that is more appealing to their project budget. In addition, Stenlund found that the starch-based tackifier was less rate dependent than guar. Thus, a steady and constant application rate could be followed at all times when using the starch product. This provides construction crews with a better understanding of how much product they need to use on any given project and what price point they need to budget for that project.
It was noted after the project was completed that application rate was a key factor to the success of the guar products. Minnesota state regulations recommend 200 pounds per acre for all guar products. However, Stenlund notes that not all guar products are identical, so MnDOT needs to re-examine the viscosity index for guar products in general because the application becomes messy as you cross over product lines.
Stenlund also concluded a number of variable factors such as vehicle speed, shoulder location, prevailing wind, mulch anchoring and mulch type, soil type, etc. all play important roles in how successful tackifier products are in the final outcome of stable, green shoulders. However, with all of the facts and figures gathered at the conclusion of the project test, Stenlund was quite confident that starch-based tackifiers were at least as good if not better than guar-based products in regards to cost, application rate and adhesiveness. In addition, Stenlund noted that the starch-based tackifier product would gain additional favor within the state because contractors and subcontractors are interested in supporting products that are harvested and manufactured in Minnesota.
"We were very pleased with the consistent results provided by the starch-based tackifier during our Highway 10 project," admitted Stenlund. "In fact, it was the field test results for Chemstar's tackifier product that led MnDOT to revise its (Type I Natural Tackifier Specification) regulations for its Standard Specifications for Construction manual to include starch-based products."
So, what does the future of starch-based tackifier products hold? Both Stenlund and Bernu are convinced that a variety of road construction and erosion control projects will someday utilize starch-based tackifiers to hydro staple erosion control blankets, provide dust control for dirt and gravel roads, help control migrating dust from road shoulders from drifting to other environments during times of drought, and combining the product with seed and mulch to create an all-in-one product application system.
More information regarding this project is available from the Minnesota Department of Transportation at: 395 John Ireland Blvd., St. Paul, MN 55155-1899; 651-284-3787 or Dwayne.firstname.lastname@example.org. For more information regarding StarTak 100 or Chemstar's extensive line of product offerings, please contact 1-800-328-5037 or visit www.chemstar.com.
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