Category Archives: Materials and Construction

Using Debonded Strands to Reduce End Stress in Bridge Beams

A new MnDOT-funded research study has found that most agencies in states with weather similar to Minnesota’s use debonded strands in prestressed concrete bridge beams. MnDOT may begin piloting debonding as an alternative to draping, which manufacturers claim is time-consuming, challenging to worker safety and expensive.

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Low-Temperature Cracking Test Produces Repeatable, Reliable Results

Researchers ran a sophisticated low-temperature asphalt cracking performance test at multiple labs to study the test, its variability and repeatability, and its additional promise in studying reflective cracking susceptibility of overlays. Results put MnDOT closer to implementing test specifications for low-temperature cracking test for pavement mixes.

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Evaluating Iron-Enhanced Swale Ditch Checks for Phosphorus Removal

Researchers documented performance of an iron-enhanced ditch check filter to remove phosphorus from stormwater over three years. The filter was effective, but its performance decreased over time, and it will require relatively frequent maintenance. Several design changes may be considered.

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New Project: Use of Innovative Technology to temporarily Deter Bat-Bridge Use Prior to and During Construction

MnDOT has funded a study to evaluate the use of non-lethal ultrasonic acoustic devices to temporarily deter bats from bridges before and during construction projects.

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Recycling Asphalt Pavement Offers Strong Alternative to New Aggregate Base

In a newly completed study, researchers found that stabilized full-depth reclamation has produced stronger roads for commercial loads in Minnesota, and the method shows promise for uses in rural agricultural areas. How much greater the strength gained with each stabilizing agent is better understood, though not conclusively.

What Was the Need?

With stabilized full-depth reclamation (SFDR), roadway builders pulverize and mix old (hot-mix or bituminous) pavement and on-site base aggregate with asphalt to create a new, thick layer of partially bound base over the remaining aggregate base of the former roadbed. The process eliminates the cost of hauling away old pavement and hauling in new, expensive aggregate, which is in limited supply.

Cracking and other damage in older pavements usually reflect through new asphalt and concrete overlays. SFDR roads, on the other hand, tend to avoid reflective cracking while meeting the increasing load demands of an aging roadway system in reduced funding environments.

To make a road stronger and more resistant to damage from heavy loads, most rehabilitation approaches require a thicker and wider roadway. SFDR may offer a way to build stronger roads without widening the road and without transporting old material from the road site and hauling new aggregate to the location.

a tanker truck connected to a reclaimer
A train of equipment runs on an SFDR site: a tanker of new asphaltic material connected to a reclaimer that pulverizes the old pavement and mixes in part of the road base and possibly stabilizing agents.

In 2016, performance requirements of SFDR edged MnDOT and the Local Road Research Board (LRRB) closer to design standards for the technique by establishing testing, modeling and analytical methods for evaluating SFDR mixtures. Minnesota designers lack a method for giving SFDR designs structural design ratings to quantify how well the mixture will meet the needs of a new roadway. How much strength is gained by mixing  in a stabilizer and laying the reclaimed road as a thick asphalt pavement base before adding the overlay remains unquantified.

What Was Our Goal?

Most replacement roadways need to be capable of bearing heavier commercial and agricultural loads than the original roads. Researchers sought to determine the structural value of SFDR in mixtures employing various stabilizing agents to help designers better accommodate rehabilitation and increased loading needs with SFDR.

“We’re really big on recycling, and we’ve been using SFDR and FDR for quite some time. We have increased confidence in SFDR. We just don’t know how high that confidence should be,” said Guy Kohlnhofer, County Engineer, Dodge County.

What Did We Do?

Researchers visited 19 Minnesota road sites to look at 24 pavement sections and surveyed pavement conditions, cracking and potholing for each segment. The team conducted stability testing with a dynamic cone penetrometer (DCP) at each section and removed three pavement cores from each for laboratory testing.

Two researchers test the base structural strength of a rural pavement using a dynamic cone penetrometer.
Two researchers test the base structural strength of a rural pavement using a dynamic cone penetrometer.

SFDR pavement can be difficult to properly core, and most specimens failed before laboratory testing. Researchers conducted tests of dynamic modulus in a way that simulated high and low vehicle speeds in the lab on the surviving 14 samples. The tests simulated the movement of wheels over pavement surface and examined the resiliency of the pavements in springing back from these rolling loads.

Based on these results, researchers plotted the laboratory test results in mathematical curves. They then analyzed their findings while referencing flexible pavement design procedures using the concept of granular equivalents (GEs) that is familiar to many  avement designers in Minnesota. Finally, they estimated the structural difference between stabilized and unstabilized reclaimed materials and identified how the structural value varies with selected stabilization agents.

What Did We Learn?

Field surveys found roads performing well. Few of the pavement surfaces showed noticeable distress, and more recent surface coating treatments showed almost no distress over pavements in which distresses would quickly present themselves. DCP testing suggested that asphaltic stabilizers—asphalt, asphalt plus cement and modified asphalt—offered greater stiffness than fly ash and cement stabilization.

“We confirmed that what local engineers are doing has value, even if we weren’t able to generate more optimistic numbers,” said Charles Jahren, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering.

Lab testing suggested that while SFDR mixtures offer less stiffness compared to regular hot-mix asphalt (HMA) layers, their stiffness diminishes less in comparison to HMA for slow-moving heavy loads like seasonal agricultural equipment. SFDR is worthy of additional consideration as a base layer, in such loading environments.

The most critical goal for this study was to quantify the granular equivalency of SFDR mixtures with various additives to standard aggregate bases. Foamed asphalt and engineered emulsion proved the most structurally beneficial stabilizers; SFDR mixtures with these materials offered GE values of 1.46 to 1.55, confirming the general MnDOT approach that SFDR can be used for a GE of 1.5. If road builders pulverize 4 inches of asphalt roadway with 4 inches of base aggregate and add foamed asphalt or emulsion stabilizer, the 8-inch asphalt base offers the strength of a 12-inch aggregate base. A pavement of HMA or portland cement concrete can follow to create a roadway section with greater strength than a roadway section with the same thickness of nonstabilized base.

What’s Next?

SFDR performs well in the field and shows particular promise for use on rural roadways subject to seasonal, heavy agricultural loads. Researchers confirmed current GE inputs for SFDR and documented the performance of specific stabilizer options employed in Minnesota. Continued monitoring of SFDR road performance and additional testing and analysis would add more detail to design procedures and provide designers with greater confidence.

This post pertains to LRRB-produced Report 2018-33, “Field Investigation of Stabilized Full-Depth Reclamation (SFDR),” published November 2018. For more information, visit MnDOT’s Office of Research & Innovation project page.

Testing Methods for Crack Resistance in Asphalt Materials

The Minnesota Department of Transportation is working with other state agencies in a pooled fund study to improve methods for testing crack resistance of asphalt mixtures. To expand options further, MnDOT asked researchers to evaluate alternative tests with standard lab equipment. The new tests produced repeatable results. Methods include the semicircular bend (SCB) test in a nontypical configuration, a dynamic modulus test of smaller asphalt mixture samples, a bending beam rheometer (BBR) test of mixtures, and a BBR of asphalt material for binder selection.

What Was the Need?

A number of factors lead to cracking and other damage in asphalt. Cold temperatures cause pavements to contract, triggering internal tensions that lead to low-temperature cracking. Aging asphalt binder grows brittle and under loading pressure generates bottom-up, or fatigue, cracking. A variety of causes may contribute to top-down cracking, such as mixture properties, construction practices, tire design and loading.

A road crew works at night to place a layer of asphalt pavement.

MnDOT, in partnership with the National Center for Asphalt Technology, and four other state transportation agencies are part of a pooled fund study to develop mixture performance testing focused on cracking. This group, termed the Cracking Group, installed eight different pavement cells at MnROAD in the summer of 2016 to examine pavement performance and testing approaches for low-temperature, top-down and fatigue cracking.

The group’s approach does not embrace every potential test, including some examinations other agencies and research organizations have found potentially valuable in predicting cracking behavior of asphalt pavement materials.

What Was Our Goal?

MnDOT sought to investigate the viability of testing methods not included in Cracking Group studies. These tests would be conducted on asphalt mixtures sampled during construction of the test sections at MnROAD to help in material selection, quality control and forensic investigation of paving materials.

“This was a knowledge-building, data-gathering study that will help fill out our materials library database to correlate test results of asphalt materials to field performance.”
—David Van Deusen, Research Operations Engineer, MnDOT Office of Materials and Road Research

What Did We Do?

Preliminary testing focused on the eight MnROAD cells, pulling cores from the existing pavement before reconstructing new sections. Researchers tested these cores to refine methods for proposed tests. The team then gathered details on the binders and mixtures used in the 2016 reconstruction to use in its planned tests.

Researchers ran three tests on the eight asphalt mixtures and one test on the five asphalt binders used in the pavement mixtures at MnROAD. The asphalt mixture tests were:

  • Bending beam rheometer (BBR) test of mixtures to obtain creep stiffness and strength of asphalt mixtures. This approach uses small beam specimens useful in forensic investigations.
  • Low-temperature semicircular bend (SCB) test to measure fracture energy in mixtures. Currently there is no national standard test for fracture energy, but based on previous pooled fund work, MnDOT implemented the disk-shaped compact tension (DCT) test. The SCB results will be used to tie in the previous work and compare to the DCT.
  • Dynamic modulus test of mixture resilience that uses smaller cylindrical specimens, a benefit in forensic studies.

To obtain asphalt binder strength, researchers used a variation of the BBR test for mixtures.

A sample disk of asphalt stands vertically in testing equipment to be compressed from one edge to the other.
The SCB test applies pressure diametrically on an asphalt pavement puck along the axis of a 6-inch pavement cylinder to measure susceptibility to cracking at low temperatures.

What Did We Learn?

The four tests proved to be viable options for materials selection testing, quality control and forensic examination of samples from existing asphalt pavements. The SCB and dynamic modulus can be run with research equipment. These tests yielded repeatable results and identified differences in the eight mixtures that are expected to impact performance. In particular, the BBR test of mixture has potential for being a practical field screening test.

The BBR test of mixtures measures strength and creep of ½-inch-thick asphalt mixture specimens compared to an indirect tensile test of strength on 2-inch asphalt pucks, and the test produces similar results. The dynamic modulus test uses the same configuration as the indirect tensile test, but instead of applying vertical compression to a 6-inch asphalt core, it applies pressure on a 1.5-inch puck diametrically, yielding similar results on an asphalt mixture’s resistance to loading.

The SCB test, an alternative to the DCT test, provides similar results in measuring the fracture energy of asphalt pavement mixtures. Either of these two newer tests is viable for MnDOT use. The binder BBR strength test represents a viable alternative to the direct tension test that, due to complex sample preparation and expensive equipment, is not frequently used.

“These test methods produce repeatable, consistent results, are simple to perform and differentiate between mixtures. They could provide critical information on the evolution of pavement performance since they can be used for forensic analyses.”
—Mihai Marasteanu, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering

All tests found sample performance highly dependent on temperature. Fracture resistance does not correlate directly with other tested values; two mixtures that share similar creep stiffness, for example, may not have similar fracture resistance. Results indicate the eight mixtures tested may perform similarly, although one with high recycled asphalt content and another with a highly modified asphalt binder may be outliers. Based on the laboratory test results, mixtures with performance-graded binders do not differ markedly when one is mixed with recycled asphalt materials. As is the case with all pavement field studies, time is required for the mixes to begin to distinguish themselves from one another in terms of field performance.

What’s Next?

MnDOT will share test results from this study with the Cracking Group team and include them in the overall examination of the MnROAD test cells. Researchers recommend comparing results to observed distresses and core tests periodically from these pavement cells to correlate field conditions and tested mixture performance over time. MnDOT will consider some of these testing methods and findings in its continuing effort to develop a performance-based balanced mix design approach for asphalt pavement.

This post pertains to Report 2019-03, “Investigation of Cracking Resistance of Asphalt Mixtures and Binders,” published January 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Study Suggests 70 Percent RAP for Minnesota Gravel Road Surfaces

Researchers examined mixtures of recycled asphalt pavement (RAP) and aggregate for new gravel road surface layers in the lab and in the field. Although test results did not align perfectly, and field results were somewhat uneven, findings suggest that mixtures with 70 percent RAP content can reduce dust generation. After a year of service these roadways can match all-aggregate gravel road performance in terms of strength, but with a smoother ride.

What Was the Need?

Gravel roads offer a cost-effective option for road departments that wish to avoid the expense of asphalt and concrete roads in rural or low traffic areas. However, about an inch of gravel is lost from these roadways each year. Aggregate resources are diminishing, and gravel and crushed rock aggregate is growing increasingly expensive.

Gravel also generates dust that can reduce visibility, affect road performance and result in complaints from nearby homeowners. 

Recycled asphalt pavement (RAP) can be an effective component of new asphalt pavement mixtures. Many aggregate producers stockpile RAP that has been broken into the size of aggregate. But not all RAP works well mixed in asphalt, and some aggregate yards are too far from pavement projects to economically use RAP in pavement. 

Road agencies frequently use RAP in gravel roads. The asphalt content in RAP can bind with dust from crushed rock or gravel, helping manage fugitive dust. A recent study in Wyoming found that using RAP in new gravel surface applications at less than 50 percent of the aggregate resulted in good road performance and kept dust to a minimum. 

What Was Our Goal?

In light of the findings from the Wyoming study, researchers sought to determine the optimal level of RAP in an aggregate mixture for Minnesota gravel road surfaces. These new applications would offer good driving stability while also controlling fugitive dust. 

What Did We Do?

Research began with a review of the literature on RAP as an aggregate component of surface, base and subbase layers, as well as a survey of Minnesota counties on their experience with these mixtures. 

In the lab, the research team tested three RAP materials and virgin aggregate from two Minnesota locations in various RAP content levels for strength and compression. Investigators then compared the economic feasibility of 100 percent virgin aggregate use to 50 percent virgin and 50 percent RAP aggregate mixtures on a 1-mile aggregate road, including annual grading and eventual regraveling in the estimations.  

Research in the field focused primarily on six 1,000-foot gravel road test sections: four sections in Goodhue County using 15, 30, 45 and 60 percent RAP content, and two sections in Carlton County using 30 and 50 percent RAP. The studies entailed all-virgin aggregate control sections, and installations were made over roads with various subgrade soils that presented a variety of properties. Sites were tested for elasticity, bearing strength and fugitive dust generation. 

A secondary field study focused on RAP contents of 50, 70 and 80 percent in 3-inch surface courses for three test sections and one control section in Goodhue County. Sites were tested for elasticity, strength, dust generation, ride quality and surface aggregate looseness over time, and some lab tests were conducted.

“The 70 percent RAP mixture seemed to be about the best combination. We put RAP down in fall 2017, and by the next summer, it was working much like a regular gravel road.” —Charles Jahren, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering 

Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to
natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the
right proportion with gravel.
Mounds of RAP at a gravel pit in Carlton County offer road agencies an alternative to natural gravel and crushed aggregate for gravel roads. But RAP has to be used in the right proportion with gravel.

What Did We Learn?

Previous research indicated that RAP can help reduce fugitive dust, offers value as surface courses, and can reduce moisture susceptibility of gravel roads in cold or wet locations. 

Lab mixtures with 30 percent RAP consistently produced high compressive strength values, and higher RAP levels generally correlated inversely with bearing strength. Improvements in dust reduction were limited until RAP levels exceeded 50 percent. 

Economic analysis determined that a 50/50 percent mix of RAP and aggregate would cost 1.5 percent more than an all-virgin aggregate surface course in terms of construction and maintenance, but potential reductions in dust generation, surface aggregate loss and regraveling after three years of service may produce savings from RAP use. 

Results from field testing defied clear recommendations on optimal RAP content. Generally, higher RAP content offered greater elasticity and lower levels of loose aggregate initially, but these benefits fell to equal or below non-RAP levels after a year. Higher RAP correlated with reduced dust generation, but again fell over the first year of service. In secondary testing, initial dust generation was lower with the 50 percent mixture than the others, but after a year was lowest with the 70 percent mixture. 

Ultimately, researchers found that after a year, during which fugitive dust production was reduced, the performance of a 70 percent RAP content aggregate surface course was most like a virgin aggregate surface course and offered a smoother driving surface. 

What’s Next?

“These findings provide another tool in the toolbox. They will be most useful to engineers who haven’t used RAP in gravel roads and to county engineers who have a RAP resource.” —Joel Ulring, Pavement Engineer, MnDOT State Aid for Local Transportation

While this research did not develop a definitive recommendation for an optimal RAP content in surface courses for aggregate roads, it did produce useful data on performance. The study did encourage a general sense that 70 percent RAP content for surface courses of approximately 2 inches may be effective and warrants systematic study for a three-year period. 

A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.
A researcher scrapes a gravel road surface with a modified garden hoe to measure loose aggregate levels.

This post pertains to Report 2019-11, “Optimal RAP Content for Minnesota Gravel Roads,” published March 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Sediment Control Log Guidance for Field Applications

Researchers tested sediment control logs in the lab and in the field to determine the relative filtration capabilities of these devices. They also developed design guidelines for correct selection and contributed to ongoing educational efforts. 

What Was the Need?

Whenever MnDOT or its contractors engage in construction, maintenance or other projects that substantially disturb the soil at a project site, they are required to use practices that reduce sediment discharge from the site when it rains. Sediment control methods are used as perimeter barriers around stockpiles, for inlet protection, as check dams in small drainage ditches and also along natural waterways such as streams, ponds or wetlands. 

A commonly used method is the sediment control log (SCL)—a linear roll constructed with an outer sleeve of varying permeability that is filled with natural biodegradable infiltration materials such as straw, coconut fiber (also known as coir), compost or rocks. MnDOT’s SCLs range from 6 to 9 inches in diameter and up to 30 feet in length.

While MnDOT has used SCLs extensively for many years, these devices often fail because their performance is not well-defined or understood. SCLs are also frequently installed incorrectly or in inappropriate locations. Because SCL use represents a substantial cost to the agency, MnDOT sought to learn actual performance parameters as well as optimum locations and installation methods. 

What Was Our Goal?

The goal of this project was to improve practitioners’ ability to select the appropriate SCL for a specific purpose and location. To achieve this goal, researchers sought to:

  • Determine the hydraulic characteristics of SCLs—how SCLs constructed from different encasement fabrics and internal media allow the passage of water. 
  • Evaluate the sediment removal efficiency of these SCLs and the effect of trapped sediment on their hydraulic characteristics.
  • Develop design guidelines for selecting SCLs based on log materials and the characteristics of the watershed where they will be installed. 
  • Organize the selection guidelines into a format that can be used by field practitioners for amending or upgrading the device. 

“This study compared the sediment filtration capabilities and effective life cycles of a range of sediment control logs. This new knowledge will allow us to reduce costs in all areas of sediment control log use and more effectively protect the environment.”

—Dwayne Stenlund, Erosion Control Specialist, MnDOT Office of Erosion Control and Stormwater Management 

What Did We Do?

First, researchers conducted a literature review of studies published from 1995 to 2013 that examined a variety of sediment control methods. 

Next, they determined the physical characteristics of 12 SCLs filled with diverse biodegradable media, ranging from straw; coconut fiber; wood fiber; wood chips; light, medium and heavy compost; and rock. Then they investigated the hydraulic characteristics of the SCLs, most importantly the volumetric flow rate through logs of various media, using the flume at the University of Minnesota’s Biosystems and Agricultural Engineering Laboratory. 

A sediment flume was constructed at this laboratory that researchers used to evaluate the sediment removal efficiencies and failure rates of a subset of five logs. The subset was selected to capture the range of hydraulic response representing a variety of log materials.

Researchers also examined field installations of SCLs in locations across the state to learn how SCLs were installed and, if failing, how they had failed.

A long, black sediment control log. Dried sediment is on top of a section of the log that traverses a shallow, eroded ditch. The log is held in place with two wooden stakes on the downslope side.
Overtopping occurred at this failed SCL installation, indicated by the dried sediment on the log. 

Finally, they produced two SCL selection tools and developed training materials about SCL use. 

What Did We Learn?

From the literature review, researchers reviewed seven laboratory studies and nine field studies examining a wide range of sediment control methods. They found no studies similar to this project that compared different kinds of SCLs for their sediment removal efficiency, life cycles and appropriate siting. 

Researchers investigated the physical characteristics of 12 SCLs, including diameter, density and percent volumetric pore space. They conducted material size analysis and other tests to determine saturated moisture content, capillary moisture content, saturated conductivity and other relevant hydraulic measures. Using results from the laboratory flume, they documented the flow rates of water through the SCLs. 

The physical characteristics of the 12 SCLs varied substantially. For example, densities ranged from 2.18 pounds to 18.5 pounds per cubic foot. Hydraulic characteristics, such as the amount of water retained and the rate of fluid flow through the medium, also varied widely. 

The subset of five logs tested for sediment removal efficiency showed how much sediment each log could filter at three flow rates and how much sediment buildup would cause log failure. These results combined with earlier hydraulic data allowed researchers to extrapolate the relative comparative longevity of different SCL media and to develop two SCL selection tools: one for ditch checks and one for perimeter control. The tools will guide practitioners to select the correct SCLs using watershed area, basin and ditch slope. Researchers also adapted the results of the investigations into a set of training materials for erosion control and stormwater management.

What’s Next?

The two decision tools will guide the selection of correct SCLs for particular locations. SCL training materials have already been implemented in the erosion control and stormwater management certification workshops. 

“Sediment control log failure is a worldwide problem. This research takes a substantial step toward a better understanding of the parameters within which SCLs can be effective, clarifying with data their capabilities as well as their limitations.”

—Bruce Wilson, Professor, University of Minnesota College of Science and Engineering

This post pertains to Report 2019-23, “Sediment Control Log Performance, Design and Decision Matrix for Field Applications,” published May 2019. Visit the MnDOT research project page for more information.

Concrete Grinding Residue Doesn’t Appear to Negatively Affect Roadside Vegetation and Soil

A new MnDOT research study determined that depositing concrete grinding residue (CGR) slurry at specific rates on roadside vegetation and soil may not cause lasting harm to plant growth and soil quality; however, follow-up research is recommended.

Study results showed that CGR did not appear to hinder vegetation growth or soil quality, but did change soil chemistry. At some roadside areas, the increase in soil pH enhanced plant growth. Results cannot be generalized for all soil types, plant communities, concrete residues or water sources in Minnesota. Access to real-time slurry disposal activity is needed for a thorough investigation.

Study background

Construction crews use diamond grinders to level newly cured concrete with adjacent slabs of older pavement and to smooth new pavement surfaces for improved friction and tire traction. Diamond grinders are fitted with hoses for rinsing grinding burrs with water to keep the burrs clean and prevent overheating. Vacuum lines then collect the residual dust and rinsing fluids, generating a slurry of concrete grinding residue (CGR) that is frequently discarded on roadside slopes and vegetation. 

When slurry dries, it leaves pale gray patches on roadside vegetation and other features, lightening the soil surface for a season or more. The effect of this slurry on vegetation, soil and drainage was unknown. Engineers and researchers presumed that the concrete dust temporarily coats roadside turf and plants, raises the soil pH, clogs soil pores and inhibits water drainage, invites invasive species to take root, and may infiltrate storm drains and waterways. 

What Was Our Goal?

MnDOT needed to study the impact of CGR on roadside vegetation and soil. Research would evaluate sites where residue has been deposited and determine its impact on vegetation and soils common to state roadsides. 

What Did We Do?

A literature review indicated that related research has been limited and that vegetation samples of only one or two species have been examined. Researchers developed two approaches for investigating the impact of CGR on plant density, plant growth and soil properties. 

First, researchers collected CGR slurry from a slurry tank at a Minnesota construction site to replicate residue application at the Kelly Farm, an Iowa State University research site near Ames, Iowa, that features prairie vegetation similar to that found along Minnesota roadsides. They applied slurry at application rates of zero, 10, 20 and 40 dry tons per acre. Plant cover, soil chemistry and soil structure properties, such as plant biomass, density, hydraulic conductivity, infiltration and pH, were measured before the slurry was applied and again at one-, six- and 12-month intervals after application. 

Second, researchers visited two roadside locations along Interstate 90 near Austin, Minnesota, where CGR had been applied. The research team evaluated vegetation content and cover, took soil samples and compared survey results to neighboring roadside environments that had not received CGR slurry.

The infiltrometer system setup at the Kelly Farm site in Ames, Iowa.
This water infiltrometer measured infiltration of water at the roadside environment test site.

What Did We Learn?

Statistical analyses established that at the Kelly Farm, CGR did not significantly impact soil physical properties and plant biomass, but did alter soil chemistry. Levels of soil pH, electrical conductivity, metals content and other properties rose significantly after CGR application. These effects increased with increases in application rate and decreased at increased soil depths. These changes did not reduce soil quality, and higher pH levels did not persist after one month. For certain warm-season grasses and legumes, increased pH improved plant growth. Some nutrients such as calcium and magnesium leached from CGR could benefit plant growth as well.

“Concrete grinding residue or slurry can, under certain conditions, be a benefit. It can act as a liming agent, changing soil pH in a positive manner.” —David Hanson, Integrated Roadside Vegetation Manager, MnDOT Roadside Vegetation Management

The two roadside environments yielded differing results. Slurries had been deposited in 2009 at the first site and in 2013 at the second. At the first site, soil bulk density and hydraulic conductivity in the slurried areas did not differ significantly from measures at the nonslurried areas; at the second site, the levels differed significantly. At both sites, electrical conductivity, calcium content and base saturation values were higher at the areas with CGR than the areas without CGR. 

Researchers concluded that at the Kelly Farm and at the roadside locations, slurry applications at a rate of up to 40 tons per acre did not reduce soil quality and vegetation growth for longer than three years. 

What’s Next?

Efforts to access grinding operations and CGR deposits in real time were not embraced by Minnesota’s concrete industry, and researchers were unable to properly assess residue composition and rates, and volumes of slurry deposition on roadway environments. A thorough investigation of residue impact will require such access and follow-up on site conditions after established periods of time. 

Researchers noted that findings cannot be easily generalized since CGR compositions may vary depending on source and water quality, influencing soil and vegetation differently, and soil and plant communities may differ in response to comparable CGR applications. Investigators recommended that MnDOT develop quick field measures of slurry pH, electrical conductivity and alkalinity to use in adjusting slurry spreading rates at grinding sites.

“This study was a great start to this topic. Follow-up research is recommended to evaluate live projects, field demonstrations and data collection.” —Halil Ceylan, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering

This technical summary pertains to Report 2019-06, “Concrete Grinding Residue: Its Effect on Roadside Vegetation and Soil Properties,” published January 2019. Visit the MnDOT research project page for more information.

Culvert Design Manual Provides Guidance for Accommodating Fish Passage

Several years of research have culminated in the publication of a culvert design manual that promotes the safe passage of fish and other aquatic organisms, as well as stream connectivity, throughout the state.

“Engineers designing culverts for Minnesota’s diverse ecological regions will benefit from this document, which offers sound guidance from many practicing experts about how to design culverts that allow aquatic organism passage and preserve stream integrity,” said Petra DeWall, former Bridge Waterway Engineer, Minnesota Department of Transportation (MnDOT).

What Was the Need?

Minnesota’s 140,000 miles of roads and approximately 92,000 miles of streams and rivers meet at tens of thousands of places. Culverts are a cost-effective solution to allow traffic to cross over smaller waterways. Historically, culverts have been designed with the safe passage of vehicles in mind. Recently, a state and national appeal for the safe passage of fish and other aquatic organisms, as well as for waterway integrity and connectivity, has influenced culvert design.

A pair of Topeka shiner fish
The Topeka shiner, once found throughout the state, is one species of federally endangered fish in Minnesota that must traverse culverts to survive.

MnDOT has supported many research projects examining fish and aquatic organism passage (AOP) through culverts, and nationally, a number of published resources exist on appropriate design. Because of the variety of ecological regions in the state, the range of culvert geometries and many other factors, no single solution can accommodate AOP through culverts statewide. A comprehensive culvert design guide was needed to inform designers about solutions that can effectively facilitate the movement of fish and other aquatic organisms in Minnesota while maintaining healthy streams.

What Was Our Goal?

The objective of this project was to produce a comprehensive and accessible culvert design guide that could be used by Minnesota practitioners to design culverts for AOP and stream connectivity. The guide would provide the following benefits:

• More efficient culvert design and permitting process for AOP.
• A central definition of typical designs, which would improve contractors’ familiarity with designs and lower construction costs.
• Avoidance of designs that could be detrimental to the natural environment.
• Avoidance of designs likely to lead to roadway damage and need for repairs.
• Fishery improvement through increased stream connectivity.

What Did We Do?

To determine the scope of the guide, researchers worked with experts from the Minnesota Department of Natural Resources (DNR), the U.S. Forest Service and others with knowledge of civil engineering, AOP and stream geomorphology.

They then sought information for the guide from a wide range of authoritative resources. A literature search examined current and past research by the research team and others; guidance documents from federal agencies; guidance from other states; permit requirements from the DNR and other agencies; and databases of fish populations, stream attributes and culvert data. The literature search also sought to reveal gaps in knowledge where further research specific to Minnesota was needed.

Additionally, researchers surveyed a cross section of highway design engineers and managers from MnDOT, county and city agencies, resource agencies and engineering consultants to identify current design practices for AOP and stream connectivity, and the degree of their effectiveness.

What Did We Learn?

The project resulted in the Minnesota Guide for Stream Connectivity and Aquatic Organism Passage Through Culverts, a thorough guide for culvert designers, hydraulic engineers and others involved in culvert design and construction in Minnesota. Topics addressed in the guide include:

• The need for culvert designs that include AOP and stream connectivity, as well as the current regulatory context.
• An overview of culvert design, categories of design methods that incorporate AOP and waterway connectivity, and a list of best practices.
• Site characteristics, analysis and tools related to energy dissipation, hydraulic analysis for AOP and sediment transport.
• A design method selection chart, information on certain designs and references for further information.
• Further guidance about design issues such as multiple barrel and floodplain culverts, grade control, retrofits and other cost considerations.

What’s Next?

The culvert design guide will be made available to users online. Future considerations for this project include an associated webinar and efforts to coordinate information presented in the guide with expectations and permitting requirements of MnDOT departments charged with culvert creation and implementation. Additional research is underway to assess culverts and fish passage with respect to storm vulnerability and future hydrologic scenarios.

This post pertains to the MnDOT and LRRB-produced Report 2019-02, “Minnesota Guide for Stream Connectivity and Aquatic Organism Passage Through Culverts,” published January 2019.