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.

What Was the Need?

Bridge designers often prestress concrete beams with steel strands to improve the performance of the beams. The strands precompress the beams so that when external loads like vehicle traffic are applied, the concrete is less likely to crack under loading.

When the beams are fabricated, the strands are stretched from one end of the concrete form to the other, and then concrete is poured and hardens around the stretched cable. Once the concrete is cured, the cables are released from the precasting bed. When the cables shorten, they shorten or squeeze the concrete they are bonded to in the beam, precompressing it.

Because concrete is effective in compression and poor in tension (it cracks), precompressing the concrete leads to beams that may not crack in service conditions. It also leads to less deflection of the beams under loading. Both outcomes improve the strength, serviceability and durability of the system.

“We were assessing the current state of the practice of debonding strands in prestressed concrete, learning what other agencies have done and how much success they’ve had,” said Catherine French, CSE distinguished professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Prestressing causes high stress at beam ends, which is conventionally mitigated with some combination of two common design approaches. Cables can be debonded at the ends, typically by using a sleeve of a limited length that prevents concrete from directly bonding with the strands where covered. Draping strands can also help with end stresses by reducing the eccentricity of the strands. While regular strands run parallel to the length of the beam, draped strands are pulled in a somewhat V-shape, from the top of the beam at each end to the bottom of the beam in the middle.

MnDOT currently uses draping to relieve the end stresses, but it does not allow debonding due to its potential to provide a path for chlorides to enter the concrete along the debonding tubes, which could lead to corrosion. Manufacturers would like to rely less on draping, which requires more time, cost and care to safely fabricate. Local agencies are interested in debonding because draping requires thicker and more expensive concrete beams. National standards offer the choice of draping, debonding or a combination of both.

What Was Our Goal?

Researchers investigated the state of the practice for using debonded strands in prestressed concrete beams. MnDOT and the Local Road Research Board (LRRB) needed recommendations for using debonded strands to position the agency to adopt current and imminent national debonding standards for prestressed beams and to use debonding as an alternative to draping, where appropriate. 

What Did We Do?

Researchers studied current MnDOT prestressing specifications, prestressing and debonding guidelines established by the American Association of State Highway and Transportation Officials (AASHTO), and research on debonding and draping. They also surveyed 11 agencies in 10 states with climates similar to Minnesota’s about their use of debonding and its performance in terms of reducing beam end stresses and resisting corrosion. They followed up with some respondents to gather more detail on respondents’ practices and experience.

Debonding sleeves protect prestressing strands from bonding with concrete, reducing stress and cracking at beam ends. 

Plastic sheathing is wrapped around a portion of bonded prestressing strands.
In addition to conducting the survey, the research team met with two fabricators who produce MnDOT prestressed concrete beams to review prestressing, debonding and draping procedures, and visited the plants to observe the process.

What Did We Learn?

Debonding appears to reduce cracking at beam ends. Currently, AASHTO allows debonding of up to 25 percent of prestressing strands in concrete beams, though this may soon be revised to allow a higher debonding limit. AASHTO’s T-10 Technical Committee proposes allowing up to 45 percent debonding, while NCHRP Research Report 849 recommends allowing up to 60 percent of strands be debonded.

“Researchers did not find that there was any excessive corrosion with debonded strands. The team is recommending we start at debonding 40 percent of prestressed strands,” said Brian Homan, State Aid bridge plans engineer, MnDOT Bridge Office.

Ten of the 11 responding agencies use debonding, typically in coordination with sealing beam ends with silicone or similar material to protect sleeved cables from water and salt intrusion. Five of the 10 reported debonding as their primary method for reducing end stresses, and three indicated draping as their favored approach. Six limit debonding to 25 percent of strands, though others allow a higher percentage, including Michigan DOT, which allows up to 40 percent of the strands to be debonded. Respondents reported few problems with debonded strands.

Researchers recommend that MnDOT begin debonding up to 40 percent of its strands to refine the practice before it considers adopting the 60 percent standard. Two split sheath tubes, one over the other, should be applied to the strands to achieve debonding in the end regions.

Concrete ends should be sealed as MnDOT currently requires, and silicone sealant should be applied to exposed strand ends. The research team recommended a sequence for releasing prestressing cable to minimize cracking at the beam ends—bonded before debonded and shorter debonded lengths before longer lengths.

What’s Next?

Debonding strands costs less than draping and is favored by local agencies; the practice will reduce prestressed beam fabrication costs for MnDOT and the LRRB. Draping procedures present safety concerns that will be relieved by a reduction in draping, and debonding is expected to reduce end cracking.

MnDOT is developing two pilot projects in which up to 45 percent debonding may be used. Future research may be warranted to identify the best percentage of strands that should be debonded and evaluate debonding sleeve materials, designs and performance.

This post pertains to Report 2019-30, “Debonded Strands in Prestressed Concrete Bridge Girders,” published July 2019. For more information, visit the MnDOT project page.

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.

What Was the Need?

In very cold temperatures, asphalt pavement shrinks and fractures as it pulls from its various restraint points. Low-temperature or thermal cracking is the most widespread distress found in asphalt pavements in cold climates like Minnesota’s.

Pavement designers select an asphalt binder performance grade (PG) based on expected seven-day average maximum and minimum temperatures that the asphalt pavement is expected to experience. A PG 58-28 binder, for example, is supposed to maintain good performance at maximum temperatures of 58 degrees Celsius (136 degrees Fahrenheit) and minimum temperatures of -28 degrees Celsius (-18 degrees Fahrenheit).

PG binder ratings and tests do not characterize asphalt mixtures precisely because PG does not account for aggregate types and gradations, recycled material in the mix, and plant and field aging of asphalt mixtures. Asphalt mixtures must be tested at relevant temperatures.

“The disc-shaped compact tension fracture energy provides a good basis as a reflective cracking performance predictor, as well as for thermal cracking,” said Eshan Dave, associate professor, University of New Hampshire Department of Civil and Environmental Engineering.

For MnDOT to fully implement performance-based specifications for asphalt mixtures, it must better understand mechanistic tests of the susceptibility of asphalt mixtures to certain kinds of stresses. The disc-shaped compact tension (DCT) test has emerged from a decade of study as the best tool for measuring fracture susceptibility of asphalt mixtures at low temperatures and other conditions. Recommendations from a 2015 pilot DCT low-temperature fracture study included repeatability testing trials for DCT testing of low-temperature fracture energy.

What Was Our Goal?

The goal of this research was to evaluate the DCT test to support its use in performance-based specifications for examining fracture properties within asphalt mixtures at cold temperatures. Researchers would develop a fracture energy database for Minnesota asphalt mixtures, refine the test as necessary, characterize the variability and reproducibility of test results by testing mixtures at multiple labs, recommend test specifications or parameters, and evaluate the potential for DCT testing of the susceptibility of asphalt overlay mixtures to reflective cracking.   

What Did We Do?

Researchers identified pavement and overlay projects with asphalt mixtures that could be sampled for DCT testing. Then they compiled MnDOT data on asphalt pavement construction projects, mixture designs and DCT test results, developing a single database for this information to be updated continuously and managed by MnDOT’s Office of Materials and Road Research.   

Using this database, researchers selected 16 asphalt projects representative of pavement and overlay mixtures, and gathered samples of mixtures from each of these projects for distribution to four testing laboratories. The team analyzed test results for all samples at all labs to assess repeatability.

A disc-shaped asphalt pavement mixture sample in the test setup. The disc was instrumented with a temperature sensor prior to testing.
For the DCT test, a disc-shaped asphalt sample was instrumented with a temperature sensor and prepared for pulling tension to simulate its cracking susceptibility while contracting at low temperatures.

The research team then collected field performance data on 15 selected asphalt overlay projects also in the database and ran over 100 finite element models of the mixtures to examine how their fracture energy test results may predict reflective cracking performance in the field.

What Did We Learn?

The DCT testing database included over 6,000 test results as of the end of 2018 and can be updated with new test results as they become available to optimize the use of DCT testing in performance-based specifications.

Typically, DCT testing requires holding a specimen at low temperature during testing for up to 12 hours; investigators refined the method to reduce the holding time to approximately 2.5 hours, in part by instrumenting specimens for interior temperature monitoring. Researchers also determined that reheating plant-produced loose asphalt mixtures to compact samples for DCT testing produced lower fracture energy results.

Researchers identified a fracture energy limit of 90 joules per square meter over which low-temperature fracture energy test results reproduce poorly and found that testing of 12 replicated specimens lowers variability and minimum and maximum fracture energy range values. Increasing binder content and the high- and low-temperature range of PG binders improves fracture resistance in mixtures, as does lowering recycled asphalt content.

“We need to continue to evaluate DCT for mix design. The researchers made good recommendations for overlay projects to combine thickness and fracture energy,” said Shongtao Dai, research operations engineer, MnDOT Office of Materials and Road Research.

Overlay fracture resistance, determined by combining overlay thickness and mixture fracture energy, correlates with improved reflective cracking performance in asphalt overlays. A fracture-resistance value of 50 joules per square meter found in this study may be used to minimize reflective cracking potential in overlays.

What’s Next?

MnDOT is closer to implementing DCT test specifications for asphalt pavement. Turnaround time in DCT testing limits its use in quality assurance and control of asphalt mixtures during production, and MnDOT is researching alternative tests for this purpose. The DCT database may be valuable in calculating and evaluating other performance parameters for their ability in predicting field cracking performance.

Use of the DCT for overlay performance testing and specifications proved promising. Validation and field implementation, as well as further analysis of fracture energy, mixture composition and overlay thickness relationships to reflective cracking performance all warrant more research. 

This post pertains to Report 2019-24, “Disc Shaped Compact Tension (DCT) Specifications Development for Asphalt Pavement,” published June 2019. For more information, visit the MnDOT project page.

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.

What Was the Need?

The Minnesota Pollution Control Agency (MPCA) requires that developments adding more than an acre of impervious land must try to include methods to infiltrate the first inch of stormwater runoff. This “first flush” of stormwater from impervious road pavement contains pollutants that could contaminate watersheds.

To meet its MPCA permit requirements, MnDOT might consider constructing strategically designed ditch blocks and swales: wide, shallow ditches with mounds of engineered soil and sand mixtures and vegetation along roadways that manage stormwater flow.

“Our success in removing phosphorus from stormwater runoff using iron particles in filtering ditch checks showed strong proof of concept. Modification of the filters’ orientation in the ditches could result in a device that is both more effective and low maintenance for MnDOT,” said John Gulliver, department head, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Previous research has shown that MnDOT’s linear swales are effective filters of contaminated highway runoff. That study also conducted laboratory investigations into enhanced ditch checks—low permeable mounds placed in a swale to reduce water velocity. Researchers tested the ability of various media in an experimental ditch check filter to remove more pollutants. A subsequent MPCA study examined the capability of an iron-enhanced sand trench to remove phosphorus.

In September 2014, MnDOT constructed two iron-enhanced ditch checks in a swale along CR15 (TH5) in Washington County to test two design versions. One failed to function effectively; the other was monitored for four months in 2015, showing effective phosphorus and metals retention.

What Was Our Goal?

The goals of this project were to investigate the long-term effectiveness of iron-enhanced ditch checks in retaining pollutants and to develop recommendations for maintenance actions needed to support effective filter performance.

What Did We Do?

Researchers monitored the performance of the CR15 (TH5) iron-enhanced ditch check from 2016 through 2018 while temperatures were above freezing. A tipping-bucket rain gauge was connected to a data logger to record rainfall at the site. Water samples were collected automatically through tubing at four points: the inflow and outflow of the check dam and filter insert. (Two monitoring wells had been built into the filter.) An automated sampling instrument was triggered by flow through the filter insert. The data logger recorded water levels, flow rate, cumulative flow volume and rainfall depth information continuously at five-minute intervals.

Near the end of the study, workers used a boring machine to take core samples of the ditch check and the filter insert.

Three workers in safety vests and hard hats stand near a tall boring machine taking a core sample of the ditch check filter.
Near the end of the study, workers used a boring machine to take core samples of the ditch check and the filter insert.

Pressure transducers installed inside the monitor wells measured upstream and downstream water levels at the filter insert section. Flow rate through the filter was calculated. Researchers retrieved inflow and outflow samples within 24 hours of the end of a rain event. The University of Minnesota St. Anthony Falls Laboratory tested these samples for phosphorus and metals.

In July 2018 researchers collected three core samples from the ditch check sides and the filter to test for retained phosphorus and signs of diminished performance. 

What Did We Learn?

The iron-enhanced ditch check filter successfully removed phosphorus during the majority of the 40 rain events, reducing the phosphate mass loads between 22 percent and 54 percent. However, the cumulative phosphorus retention in the filter decreased from 42 percent in 2015 to 30 percent in 2016, 25 percent in 2017 and 23 percent in 2018. The core tests confirmed that the bottom 3.9 inches of the filter media filtered most of the inflow volumes of the runoff. This heavy runoff load reduced its sorption capacity over three years while the upper part of the filter was active only infrequently.  

“Enhancing ditch checks with iron filings will aid in the removal of phosphorus in stormwater. However, since the bottom of the filter receives the most flow, more frequent mixing of the iron filings is required than originally anticipated,” said Beth Neuendorf, water resources engineer, MnDOT Metro District.

The ditch check itself showed a somewhat lower phosphorus retention performance than the filter insert, though performance varied some years. Researchers considered that the soil and sod covering may have leached phosphates into the ditch check and filter insert, affecting its overall performance.

Neither the ditch check nor the filter insert were very effective in copper and zinc retention, although the metal concentrations in the inflow and treated runoff were generally lower than in typical highway runoff.

Regarding maintenance, researchers recommended the filter insert medium be mixed up every other year to redistribute the filter media at the bottom. They also recommended that the entire filter insert be replaced after six years.

What’s Next?

Researchers presented possible design changes that could improve performance, such as using peat instead of soil and sod to cover the ditch check. They also suggested installing ditch checks in series and re-engineering the filter to address the findings revealing the heavy runoff load taken by the filter’s bottom 3.9 inches. Reducing the depth of the filter berm could avoid the excessive inundation of the bottom media while the upper portion remains unused. Maintenance frequency could then be reduced.

This post pertains to Report 2019-27, “Iron-Enhanced Swale Ditch Checks for Phosphorus Retention,” published July 2019. For more information, visit the MnDOT project page.

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.

Background

Brown bat on bridge

When protected bats roost or form colonies on bridges, bridge repair and replacement projects have to follow regulatory requirements to minimize impacts on species protected by state and federally regulations – including the Endangered Species Act. These regulations protect bat populations that have already declined due to white-nose syndrome, which is estimated to have killed more the 5.7 million bats in eastern North America since 2006. The northern long-eared bat (Myotis septentrionalis) is listed as a threatened species as it is one of the species most impacted by white-nose syndrome. Other species are anticipated to be protected as the disease spreads.

To minimize disturbance to protected bats, which may use bridges both for day roosting habitat and sites for maternity colonies where bats give birth and raise their young, MnDOT is evaluating the feasibility and efficacy of ultrasonic acoustic devices to temporarily deter bats from work areas on bridges. These devices emit a sound, inaudible to humans, that disrupts the bat’s ability to echolocate and therefore discourages bats from approaching. This new technology has been utilized for wind turbines with positive results.

Ultrasonic device to deter bats

Temporarily deterring bats from a work site saves taxpayers money and increases bat safety. Regulations for protected species can limit activity that is potentially harmful, including bridge work during times when bats may use bridges for roosts or maternal colonies. Without the use of deterrents, work may be delayed until bats vacate the bridge, which may not occur until bats retreat to hibernacula (such as caves and mines) for the winter. Having control over when bats are present will provide more predictable timelines to projects and reduce engineering and administrative costs associated with delays and changes to work plans. Without a control measure, projects must adhere to timing restrictions that increase construction costs and may even reduce bridge life expectancy. And if bats are kept away from construction sites, they will not be directly harmed or disturbed by the activity.

Project Scope

This one-year study will investigate the efficiency and feasibility of ultrasonic bat deterrent technology for temporary exclusion of bat species on bridges by monitoring bat presence before the ultrasonic devices are installed, during a trial period, and after devices are removed. This technology will be tested on two bridges in Minnesota. Findings (expected in May 2020) will determine if ultrasonic bat deterrent technology can be utilized to exclude bats from construction and maintenance work zones, thereby reducing costs and ensuring the safety of protected species.

Watch for new developments on this project.  Other Minnesota research can be found at MnDOT.gov/research.

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.