Category Archives: Materials and Construction

Modeling Demonstrates Benefit of Geogrid-Reinforced Aggregate Base

Improved modeling of geogrid for use with MnDOT’s pavement design software, MnPAVE Flexible, will allow pavement designers to simulate field tests of stiffness and resiliency in pavements over bases with and without geogrid. MnDOT is using modeling results from a recent study to develop a design input that quantifies the benefit of geogrid in terms of pavement service life and aggregate thickness.

“This innovative study will be especially beneficial for designs in areas with poor subgrade. We worked closely with the geogrid manufacturer to develop codes that accurately simulate geogrid behavior in a pavement,”  said Bruce Tanquist, Pavement Computer Applications Engineer, MnDOT Office of Pavement Design.

What Was the Need?

Many highways in Minnesota are built upon soft subgrades. These weak subgrades lower the roadway pavement life. In the past, timber and cement have been used to stiffen pavement foundations with mixed success. However, for the last 20 years, geogrids have been shown to be a beneficial and cost-effective method to stiffen the existing pavement structure.

Geogrid is a stiff polymer webbing with apertures that interlock with aggregate in the base. The material is placed within the new or reclaimed aggregate base, usually two-thirds the distance from the top of the base. After the remaining aggregate is placed, the road is paved with either asphalt or concrete.

rectangular shaped geogrid
Simple, rectangular-shaped geogrid stabilizes aggregate and improves pavement resiliency.

Geogrid increases the stiffness of the aggregate base layer by locking aggregate in place for improved resilience. Though the benefit of geogrid has been observed in the past, it was not quantified for pavement design purposes, and designers were not able to include the properties in their calculations when designing a pavement. Geogrid was sometimes seen as an extra expense with no calculated benefit.

A 2016 study was also tasked to quantify the benefits of geogrid in mechanistic design, but deflection testing results were inconclusive and did not support a reliable design factor for geogrid use in aggregate base.

What Was Our Goal?

MnDOT pavement designers requested a model to show how using geogrid in the roadway base impacted pavement life. Researchers used new software to evaluate geogrid behavior in different design permutations and to quantify its benefit to pavement performance using MnDOT’s pavement design software, MnPAVE Flexible.

What Did We Do?

The updated software was used to expand the geogrid modeling capability and test modeled nonreinforced and geogrid-reinforced bases. Research began by identifying geogrid parameters useful in modeling and as inputs to MnPAVE. Investigators worked with a geogrid manufacturer to specify and code the physical characteristics and properties of triaxial geogrid (with triangular-shaped apertures) used in the field for modeling.

Researchers then worked closely with a software developer to refine modeling capabilities, expanding on previous work that focused on biaxial geogrid (with rectangular-shaped apertures) to include triaxial geogrid, and to model behavior of geogrids in variable parameters for geogrid and aggregate.

Geogrid and aggregate models were tested extensively, adjusting geogrid and aggregate characteristics and simulating dynamic cone penetrometer (DCP) and light weight deflectometer (LWD) tests. Researchers collected numerical modeling results on geogridand aggregate performance to use with MnPAVE design software and to develop design factors that quantify the impact of geogrid on pavement performance.

What Did We Learn?

Field testing from previous research was insufficiently detailed because it did not include specific pavement structure and subgrade conditions below each deflection-tested location. Additionally, lab testing, which evaluated geogrids by testing their behavior within 6-inch by 12-inch cylinders, did not correlate well with the dimensions and shapes of field geogrid installations.

Effective modeling aids in quantifying the benefits of geogrids. The modeling developed in this research effectively began to bridge the gap between field and lab examination by testing forces in 1-foot-square models with 4- to 12-inch aggregate thicknesses, which is more appropriate for estimating geogrid and aggregate behavior in the field.

“We were asked to quantify the benefit of geogrid. It is important to keep the aggregate layer thick for benefits like drainage, so it’s important to know that we were getting extra years of life with geogrid-reinforced aggregate base,”  John Siekmeier, Research Engineer, MnDOT Office of Materials and Road Research.

New modeling capabilities allow testing of various parameters, including geogrid aperture dimensions and configurations, the thickness and shape of geogrid ribs, aggregate roughness and gradation, and moisture content. Test simulations of geogrid and aggregate configurations run for hours or days, and model a wide range of behaviors to capture reliable data from DCP and LWD tests of stiffness, resilience, and strength of bases with and without geogrids.

Test results showed that depending on moisture content and the time of year, bases reinforced with geogrids offer 1.5 to 2.5 times the resiliency under loading compared to nongeogrid-reinforced bases.

What’s Next?

Investigators are working with MnDOT designers to codify a geogrid factor in MnPAVE that determines the improved service life or the aggregate thickness equivalent that geogrid provides to aggregate bases in pavements. The geogrid factor could be incorporated early in 2019.

Further research could include comparing modeling results to LWD and DCP field test results of new pavements with geogrid-reinforced aggregate bases. Such implementation and site testing could continue with new pavement installations to collect data to confirm or calibrate geogrid design factors and geogrid modeling for MnPAVE.

This post pertains to Report 2018-30, “Performance Specification for Geogrid Reinforced Aggregate Base,” published October 2018.

Selecting Structural Synthetic Fibers for Use in Thin Concrete Overlays

Lab testing has demonstrated that structural synthetic fibers in thin concrete overlays keep cracks tight and help transfer loads across pavement slabs. A recently released research study, co-funded by the Minnesota Department of Transportation and the Minnesota Local Road Research Board, provides recommendations for selecting fiber types and dosages in pavement design.

What Was the Need?

Concrete pavements usually measure 8 to 15 inches thick. For many of these pavements, designers recommend placing dowel bars at the joints during the pour to assist the transfer of wheel load from heavy commercial and agricultural vehicles across concrete slab joints.

MnDOT has found that dowel bars are not effective in a thin concrete overlay, a 4- to 6-inch layer of concrete over an older pavement. These slabs fracture prematurely around the dowels. Adding structural fibers to concrete offers a potential solution. Used primarily to keep cracks from widening, these fibers consist of pieces of thin synthetic material—polymers, carbon fabric, even steel—mixed into the concrete batch.

Many states do not have formal standards for fiber types or characteristics, dosage rates or other specifications for their use. MnDOT currently uses the approved products list created by Illinois Department of Transportation.

Minnesota road engineers agree that fibers work well in concrete, but how well was unknown. Research was needed to determine the optimal physical characteristics of fibers, the amount that should be mixed in to the concrete, and products currently not on the approved products list that may be effective.

What Was Our Goal?

MnDOT wanted to investigate fiber performance in thin concrete overlays, specifically to help identify fibers that are most appropriate in these overlays and recommend acceptable dosage rates for mixing and placing the thin concrete. MnDOT also needed a test procedure and design recommendations or specifications for using fibers.

“This research looked at fiber performance in terms of load transfer to see if fibers can provide an alternative to dowels in thinner concrete pavements,” Maria Masten, Concrete Engineer, MnDOT Office of Materials and Road Research.

What Did We Do?

Research began with a literature search and a survey of state transportation agencies identified by the American Concrete Pavement Association as leading users of fiber-
reinforced concrete overlays.

Laboratory testing first focused on post-crack performance, relying on ASTM C1609, the nationally recognized testing standard. Investigators tested 10 fibers of various lengths, geometries and stiffness in three dosage levels in concrete, evaluating the impact of fiber properties on post-crack performance.

 

cracked concrete beams with fiber reinforcement
Post-crack performance testing of fiber-reinforced concrete beams shows that after cracking, fibers work to keep cracks from widening.

Testing then turned to joint performance. Researchers used four fibers from the previous lab examination and added a fifth fiber, a synthetic fiber used in MnROAD test cells in 2017, to test load transfer across cracks between sections of fiber-reinforced concrete. Together, the two lab phases tested 11 fibers in 43 concrete mixtures in over 400 samples 10 beams and 10 cylinders each of 30 fiber-reinforced concrete samples for post-crack performance, one plain concrete mix and 12 additional fiber-reinforced mixtures in joint performance testing. Analysis considered post-crack performance, crack width, fiber geometry, dosage, load transfer efficiency and residual strength.

In the final step, researchers analyzed the collected data and developed recommendations for MnDOT.

What Did We Learn?

Results confirmed that fibers help keep cracks and joints tight and improve load transfer across cracks and joints in thin concrete overlays. This research indicated synthetic fibers provide equal or better performance than steel fibers, which are expensive, heavy and difficult to mix. Dosages less than 0.25 percent fiber volume fraction of concrete mixture did not improve post-crack flexural or load transfer efficiency across the joint.

In lab mixing, longer and stiffer fibers tended to ball and mat with greater frequency than shorter fibers, though researchers developed a mixing method that reduces balling and matting. Fiber dosage, stiffness and shape significantly influenced strength. Embossed, twisted and crimped fibers outperformed straight, flat synthetic fibers; longer fibers with larger diameters outperformed shorter, smaller diameter fibers that inhibit workability.

“We studied many varieties of fibers before writing a specification for using fibers in concrete overlays. This is one step forward in understanding fiber’s contribution in concrete pavements or overlays,”  Manik Barman, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering.

Fiber shape had moderate influence on load transfer and displacement in joint performance testing. Dosage levels and crack width strongly affected joint performance. Overall, it was found that fibers can increase the load transfer by 30 percent and can reduce the slab displacement by 50 percent.

Researchers suggest designers use trial batches of mixtures, submitting samples to ASTM C1609 testing and selecting fibers based on joint performance results from this study. Graphs and tables from this study correlate fiber properties with post-crack flexural strength and joint performance to help guide selection and dosage.

What’s Next?

Researchers recommend fibers with high lateral stiffness and irregular cross sections in lengths between 1.5 to 2.5 inches and at dosage levels no greater than 1 percent fiber volume fraction to avoid balling, matting and unworkability of concrete mixtures. MnDOT will issue fiber requirements so manufacturers can then submit products and test results for evaluation by MnDOT in developing a new approved products list for fibers in concrete pavements.

Future research could focus on validating design recommendations in the field; establishing fresh fiber-reinforced concrete mixture parameters by running slump, air content and other tests of fresh mixes; and analyzing life-cycle costs and benefits.

This post pertains to the MNDOT and LRRB-produced Report 2018-29, “Comparison of
Performances of Structural Fibers and Development of a Specification for Using Them in
Thin Concrete Overlays,” published August 2018.

New Project: Development of Pavement Condition Forecasting for Web-based Asset Management for County Governments

Many counties have incomplete roadway inventories, but lack asset management programs, which are often cost-prohibitive and require advanced technical training and staff to maintain. The Upper Great Plains Transportation Institute at North Dakota State University (NDSU), has developed a low-cost asset inventory program called the Geographic Roadway Inventory Tool (GRIT). The program, which is currently available to North Dakota counties, will be offered to all Minnesota counties following further development and testing by the Minnesota Local Road Research Board.

Background

NDSU created the asset inventory program as the first step for asset management to allow local roadway managers to document and understand their existing infrastructure using the latest mobile technology and Geographic Information System technology.

The goal of the research study is to expand the program to include roadway forecasting based on the American Association of State and Highway Transportation Officials(AASHTO) 93 model with inventory, pavement condition and traffic forecasting data.

Existing input data from GRIT, such as pavement thickness, roadway structural information and construction planning information, will be spatially combined with current Pathway pavement condition and traffic data from MnDOT to automatically forecast the future condition and age of roadways using the AASHTO 93 model. This forecasting model will then allow roadway managers to use this information with comprehensive GIS web maps to prioritize roadways in construction schedule or multi-year plans.

Geographic Roadway Inventory Tool

Objective

The additional information contained in the pavement forecast system will allow county roadway managers to prioritize projects that can benefit from lower cost pavement preservation activities and understand how long roadways can last before a high cost reconstruction must take place. The online GIS output maps will also enable the public to see what projects will be conducted on a year-to-year basis.

Project scope

The research team will work with Beltrami, Pope, Faribault, Pennington, and Becker counties and the city of Moorhead in Minnesota to research, develop, test and implement an additional forecasting function of the existing asset management program. This will be done using the AASHTO 93 empirical model to calculate a future pavement serviceability rating (PSR) based on the existing pavement structure and age, forecasted traffic and the latest pavement condition. While existing pavement structure and age information will come from data entered into the GRIT program by counties, processes and procedures will be researched and developed to automatically access pavement condition and traffic data from MnDOT and geospatially combine it with inventory data.

With pavement forecast information, county roadway managers will be able to better understand which roadways will deteriorate first and which will benefit from more effective, low-cost maintenance programs rather than full-depth reconstructions. The model will not forecast suggested future projects or project costs, but rather just output the future condition of the roadways on a yearly basis. The AASHTO model can be applied for both flexible and rigid pavement sections.

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

Pervious Concrete Pavement Reduces Runoff into Shoreview Lake

The city of Shoreview, Minnesota was on the right track when it took the unusual step of paving a residential neighborhood with pervious concrete to help control stormwater and pollutant runoff into a nearby lake, according to a recently released seven-year performance study.

Typically used for parking lots and sidewalks, porous paving material allows stormwater to filter through the pavement and an aggregate base into the soil rather than run off the pavement and drain into storm sewers.

Shoreview bucked convention by using pervious concrete in a traffic application — low-volume, low-speed roads in the Woodbridge neighborhood near Lake Owasso. The city thought pervious pavement could help meet community sustainability goals and federal clean water regulations by reducing pollutants in waterways and groundwater while keeping water safely off driving surfaces.

Traditionally, pervious concrete hadn’t been used for roadways because engineers didn’t consider it strong enough for traffic (this and other projects have now demonstrated its application for low-volume roads like neighborhood streets). The impact on ride quality, tire-pavement noise and filtration was also not well understood, particularly in cold climates with freeze-thaw cycles like those in Minnesota.

Pervious concrete also presented a maintenance challenge: Organic debris, sand and other grit can clog the pavement’s pores. Periodic vacuuming is required to maintain the intended flow of water through the pavement. Concerned about how best to maintain the pavement and interested in tire-pavement noise levels and filtering performance, Shoreview, MnDOT and the Local Road Research Board monitored the Woodbridge roadways for seven years.

Installation and Evaluation

Shoreview replaced 9,000-square -feet of asphalt roads with 7 inches of pervious concrete over 18 inches of coarse aggregate base; near the lake, highly drainable sand served as the base. About twice each year for five years, researchers tested sound absorption, water infiltration and ride quality one day after the pavement had been vacuumed. In 2015, they repeated these tests without vacuuming the day before.

The pervious pavement performed well in filtering stormwater. By 2012, at least 1.3 acre-feet of water had filtered through the pavement and ground, and by 2015, nearly 2 acre-feet of water had filtered through the surface—all of which would otherwise have run directly into Lake Owasso.

Water infiltration and sound absorption rates were higher than traditional concrete, although rates declined over time because organic material continued to clog pavement pores despite vacuuming twice a year.

Conclusions

Initial construction of the pervious concrete streets and stormwater filtration system was slightly more costly than construction of comparable asphalt pavement with culverts. Life-cycle costs, including projections of maintenance costs over 15 years, however, showed somewhat lower costs for pervious pavement. While the pervious concrete pavement may require diamond grinding after 10 years, monthly vacuuming could make this unnecessary. Asphalt pavement would typically require a mill-and-overlay at year 15, and culverts would require periodic cleaning.

Additional benefits of the pervious pavement system that were not included in cost calculations—but were clearly significant—included complying with the federal Clean Water Act, recharging groundwater and avoiding direct pollution of Lake Owasso. Shoreview’s investment in pervious concrete has paid off economically and environmentally.

For additional information about this line of research, see these resources:

 

 

 

Prioritizing Pavement Markings on Low-Volume Roads

Researchers have developed a tool to help Minnesota local agencies make cost-effective pavement marking decisions in their counties. The spreadsheet-based tool was developed as part of a recently completed research study by the Minnesota Local Road Research Board.

What Was the Need?

Minnesota has many miles of low-volume roads, most marked with yellow centerline and white edge lines. Applying and maintaining these markings is a significant financial investment for local agencies, which typically work within very constrained budgets. These agencies needed more information about the value and the initial and ongoing costs of typical 4-inch and enhanced 6-inch pavement markings on low-volume roadways. They also needed clarification and guidance for prioritizing pavement marking installation and maintenance that could work within their limited budgets.

What Was Our Goal?

The goal of this research was to develop a prioritization approach and a decision-making tool for using pavement markings on low-volume roads based on the benefits and costs of these markings. Local agencies could then use these resources to make cost-effective decisions about installing and maintaining pavement markings.

Rural road with cyclist in bike lane
This segment of Minnesota Highway 38 has yellow centerlines and white edge lines that delineate a 4-foot bicycle lane

What Did We Do?

Researchers took a multistep approach to identifying critical pavement marking information and practices:

• Conducted a literature search of existing research on typical (4-inch) and enhanced (6-inch) pavement markings, focusing on the benefits (such as crash reduction and improved lane-keeping), costs and current maintenance practices.
• Surveyed Minnesota counties to learn about their current practices and management approaches for pavement markings.
• Reviewed existing County Road Safety Plan (CRSP) methodology to learn about research and data used to rank at-risk road segments and identify CRSP improvement strategies, specifically the range of pavement markings that CRSPs recommended.

Researchers were then able to develop a prioritization approach and a decision-making tool that incorporated both past research and local state of the practice. In addition to producing a final report describing task results, they developed a brochure explaining the approach, the tool and implementation steps.

“This innovative tool will help local agencies make pavement marking decisions under tight budget constraints, where the question is always how to best allot funds for competing needs. This tool clarifies the problems and helps prioritize the possible solutions,” said David Veneziano, LTAP Safety Circuit Rider, Iowa State University Institute for Transportation.

What Did We Learn?

The literature search revealed limited research addressing traditional pavement marking use and effectiveness on local roadways. Pavement markings produce safety benefits, including reduced crash rates, but showed no real effects on vehicle speed, indicating that pavement markings may not alter driver behavior. Only limited efforts were identified in the literature aimed at investigating the prioritization and management of pavement markings.

The survey of local Minnesota agencies revealed that most counties use centerline and/or edge lines, which may be the result of MnDOT State-Aid Operation Rules. Some counties mark all their roads; most use 4-inch latex paint or epoxy markings. Repainting schedules depend upon road age, marking condition and county budgets.

A review of Minnesota counties’ CRSPs showed they included pavement marking recommendations. The CRSPs recommended, on average, 109 miles of pavement markings in every county. Applying one linear foot of centerline costs about 5 cents; 100 miles of centerline cost $26,400. Because of the extent of these recommendations, researchers directly incorporated the methods and directives from the CRSPs into their prioritization approach and tool.

The spreadsheet tool produced through this project allows users to enter road site characteristics such as pavement condition, road width, the CRSP rating and traffic volume, as well as the age of extant markings, costs, durability and the potential for crash reduction. Pavement marking options include centerline and/or edge lines, high visibility markings and enhanced durability materials. The tool uses factor weights that assign a relative importance to each criterion for any potential marking approach compared to other alternatives. The result is a performance rating score for each marking alternative. Thus, the tool assists not only in identifying the physical aspects of a road segment, it also incorporates the agency’s preferences, priorities and budget through a priority-weighting feature that generates the cost or cost range for a marking project.

What’s Next?

Recommendations for further research include conducting a follow-up survey of users
of the new spreadsheet tool to facilitate future modifications, creating databases of roadway characteristics to simplify agencies’ use of the tool, and performing additional research on the safety and other effects of pavement markings. Researchers also encouraged agencies to keep in mind a proposed national retroreflectivity rule for the Manual on Uniform Traffic Control Devices that could affect pavement marking practices on low-volume roads. This rule has not yet been finalized or implemented.

This post pertains to LRRB-produced Report 2018-21, “Investigating the Necessity and Prioritizing Pavement Markings on Low-Volume Roads,” published June 2018. The Pavement Marking Prioritization Tool can also be found on the project webpage on the LRRB website.

Nontraditional Fog Seals Offer Value, Limitations Compared to Traditional Seals

MnDOT conducted field and lab analyses of nontraditional fog seals used by local agencies around the state. Results show that agriculture-based bioseals offer value that must be balanced against temporary reductions in retroreflectivity and pavement friction. Bioseals offer greater friction and visibility than traditional fog seals.

“There is some value to the bioseals. They seal the pavement, and they’re clear so they have a minimal effect on striping. These applications are appropriate in certain areas,” said Bruce Hasbargen, County Engineer, Beltrami County.

What Was the Need?

Maintenance crews often spray pavement surfaces with a “fog” of liquid sealant after pavement has been in service for a year or more. These fog seals extend the water resistance of asphalt and protect pavements from oxidation.

Fog seals wear off after a few years, but can be inexpensively reapplied. The seals lengthen maintenance cycles, protecting asphalt between activities such as crack repair and surface treatment. Traditional fog seals, however, are dark, asphaltic mixtures that obscure pavement striping and reduce the reflectivity of materials. Fog seals also reduce friction, and so typically suit pavements with low-speed service conditions.

In recent years, city and county road agencies in Minnesota turned to bioseals—agriculture-based, clear liquids that manufacturers claim seal pavement against oxidation and water damage without concealing pavement markings. Bioseals are currently not less expensive than petroleum industry products, and little independent work had been performed to identify performance benefits.

What Was Our Goal?

To provide local agencies with more information about bioseal performance, the MnDOT Office of Materials and Road Research studied selected bioseal products in the lab and in the field (MnROAD test site pictured above), comparing them to traditional seals to determine product performance, durability and impact on friction and pavement marking visibility.

What Did We Do?

Following a literature review of fog seal treatments, investigators selected four seals for analysis: a traditional asphalt-emulsion sealer; a nontraditional, polymerized maltene emulsion longitudinal joint sealant (Jointbond); and two soy-based bioseals (RePlay and Biorestor). These seals were applied in 2014 to 8-foot shoulder sections built in 2013 on County Highway 75 in Wright County, north of Monticello. Seals were sprayed on shoulders outside painted markings, in shoulder space where investigators applied geotextile patches and strips of highly reflective striping tape commonly used on some roads. Untreated shoulder areas of 500 feet and 1,320 feet served as control sections.

Reflective marking tape on road shoulder
Researchers placed a swatch of geotextile and reflective pavement marking tape on shoulders before the shoulders were sprayed with nontraditional fog seals. Investigators then moved the textile and tape to MnROAD to study application rates and stripe performance.

After spraying, investigators removed the geotextiles to evaluate the quality of application work by bioseal distributors. They also removed some striping tape and reapplied it as shoulder striping to Cell 33 at the MnROAD test facility, where they could reliably monitor traffic passes over the biosealed markings and evaluate retroreflectivity over time. At the Wright County site, researchers examined pavement distress, friction properties and permeability on the shoulders for three years.

Lab studies included testing seal residue and stiffness in field-aged cores taken from the sealed test sections in year three. Finally, in year three researchers surveyed local agencies in Minnesota about their use of nontraditional fog seals.

What Did We Learn?

Geotextile coating levels showed that vendor application of bioseals is consistent and well-executed. Nontraditional seals do not obscure striping, but bioseals leave residue that temporarily reduces the retroreflectivity of sealed markings to below MnDOT-required levels. Acceptable levels of retroreflectivity returned to the Jointbond samples after 800 truck passes at MnROAD, and to Biorestor and RePlay samples after 1,600 truck passes.

Every tested seal reduced pavement friction. Recovery of friction for the three nontraditional products, which reduced friction by 11 to 17 percent, took about 200 days with no traffic. The traditional, asphaltic fog seal reduced friction by 67 percent and took longer to recover, remaining slippery for turning in wet conditions for over two years.

“Bioseals affect pavement friction, so agencies need to use some caution when using them. City streets are probably going to be very good for nontraditional seals,” said Eddie Johnson, Research Project Engineer, MnDOT Office of Materials and Road Research.

Each seal reduced pavement permeability for about two years; after two years, only the traditional seal continued to provide water protection. The permeability benefit of fog seals lasts significantly longer than the retroreflectivity reduction; when reflectivity recovers, the seals still provide water resistance. Field surveys also found that Biorestor and RePlay may help resist cracks.

Laboratory studies showed that high-temperature stiffness for every treatment was greater than control samples in the top layer than in the middle of cores, suggesting that seals may improve rut resistance of treated pavements in hot weather. Low-temperature stiffness was higher in the top sections for every treatment except the traditional fog seal.

Of the 57 agencies that responded to the survey, 32 have used nontraditional fog seals, preferring Biorestor and RePlay to others. Over half of these users recommend the use of such seals; responses suggest that bioseals offer sealing benefit for two years and, in some cases, up to six years.

What’s Next?

Nontraditional fog seals protect pavements from water and may help prevent cracking. Traditional seals offer longer-lasting water resistance, but also longer-lasting and greater friction reduction. Agencies must consider temporary reductions in retroreflectivity and friction for any seal, and may wish to continue using fog seals only in lower-speed environments.

Continued monitoring of applications would be helpful in determining long-term performance. The study observed that overlaying biosealed asphalt with a traditional fog seal should be effective in extending permeability.

This post pertains to the LRRB-produced Report 2018-18, “Nontraditional Fog Seals for Asphalt Pavement: Performance on Shoulder Sections in Minnesota,” published May 2018.  

Concrete Design Software Easier-to-Use, Capabilities Expand

MnDOT has upgraded its concrete pavement design software, MnPAVE-Rigid, to make it easier to use and allow more design inputs.

“In the original software, we only allowed one aggregate base thickness and one aggregate type. MnPAVE-Rigid 2.0 allows two base thicknesses and three base types,” said Tim Andersen, Pavement Design Engineer, MnDOT Office of Materials and Road Research.

MnDOT hired American Engineering Testing to update the design software as part of a research project advised by Andersen and funded by the state research program.

Background

MnDOT developed its own pavement design software, MnPAVE-Rigid, in 2014 that incorporated the methodology of the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic–Empirical Pavement Design Guide (MEPDG). Minnesota’s pavement designers use MnPAVE to apply AASHTO’s most sophisticated design principles for both rigid and flexible pavement, focusing on mechanical properties of the pavement and prevention of early cracking and other distress.

2018-17-p2-image

AASHTO’s mechanistic–empirical (M–E) design methods entail hundreds of inputs, each a mechanical parameter, a measure of site-specific characteristics or a design goal. To simplify the input selection process, AASHTO’s M–E design software offers various input levels to reduce the data gathering and input burden. The most basic level uses default values for most of the inputs based on national averages, but still requires dozens of inputs for the number of pavement layers, traffic expectations, climate and other features.

MnPAVE-Rigid for concrete pavement design reduced that number of inputs to nine, operating like a module of AASHTO’s M–E software. MnPAVE-Rigid inputs work with a set of default values for jointed plain concrete selected by the MnDOT Office of Materials and Road Research in 2014, as described in the MnPAVE-Rigid 1.0 report.

“Many states ignored the challenge of adopting AASHTO M–E or they bought an AASHTO
software license. MnDOT used its accumulated knowledge of AASHTO M–E and Minnesota conditions to build MnPAVE-Rigid, and so can account for its M–E design results firsthand,” said Derek Tompkins, Principal Civil Engineer, American Engineering Testing, Inc.

Since implementing MnPAVE-Rigid 1.0, MnDOT has gathered feedback from users about their experience with the software. In the current project, MnDOT wanted to address this feedback, and expand and improve the original software by exploring additional options with some of the default parameters for concrete pavements.

What Was Our Goal?

The goal of this project was to update MnPAVE-Rigid 1.0 by expanding the range of inputs for traffic, subgrade type, base type and thickness, and to make the user interface more accessible.

What Did We Implement?

MnPAVE-Rigid 2.0 allows users to enter 11 inputs, including inputs related to specific traffic levels and aggregate base types; calculate the new design thickness; and print a project report that summarizes the inputs and the recommended thickness. The upgraded software is more user-friendly, and MnDOT can maintain or make future upgrades to the source code.

How Did We Do It?

Researchers met with the Technical Advisory Panel and reviewed the list of software improvements requested by pavement designers and the MnDOT Office of Materials and Road Research.

Because every change to an input affects a large number of default input variables, investigators ran over 21,000 simulations to analyze the impact of changes made to inputs for base type, base thickness, subgrade type and traffic level. The research team also modified the traffic input calculator to allow designers to enter traffic values from MnDOT’s weigh-in-motion and traffic counting data. The calculator runs input traffic data in software simulations and assigns the input an appropriate axle value for design.

MnPAVE-Rigid 1.0 ran designs based on Class 5 aggregate base over a subgrade like clay loam. Other aggregate types were added to simulations to determine how the software responds to these changes. Investigation also explored the addition of subgrade material options in design simulations.

The code developer modified elements of the advanced inputs tab and PDF report generation features to improve performance for software users, and rebuilt the software in JavaScript 2.0 code, including an installer for use with Windows software.

What Was the Impact?

MnPAVE-Rigid 2.0 is more user-friendly. Its tabs better match designer needs, and the software offers a design report PDF file for export. Instead of selecting from limited options for traffic volumes (default, normal and heavy), users can now input traffic data that the software will categorize. Designers can input Class 5 aggregate, Class 5Q (a higher quality aggregate with fewer fines) and open graded aggregate (no fines). Users can also choose 4-inch or 12-inch aggregate base thicknesses. An additional subgrade option was not included, as simulations indicated a sand subgrade input did not discernibly impact structural thickness outputs.

The AASHTO M–E software is expensive, and agencies that use it have to work closely with consultants to receive training and to explore or modify the code. MnDOT owns and manages the source code for MnPAVE-Rigid 2.0, can keep it secure, and can continue to change and upgrade it internally for Windows and Linux platforms.

What’s Next?

The updated MnPAVE-Rigid is now available online. Presentations about the software upgrades will be made at meetings for materials and soils engineers through the fall of 2018.

Still underway is an effort to further incorporate recycled material properties into MnPAVE Flexible, the design software for asphalt pavement.

This Implementation Summary pertains to Report 2018-17, “MnPAVE-Rigid 2.0,” published May 2018.

New research to explore innovative solutions to aging pavement infrastructure

New solutions are urgently needed to address Minnesota’s aging pavement infrastructure since current materials and technologies can’t keep up with the rate of deterioration and limited funding.  MnDOT recently entered into a contract with the University of Minnesota to further explore new materials and technologies – including taconite and Graphite Nanoplatelets (GNP) – that could offer cost-effective solutions for longer-lasting pavement.

Background

The university has investigated the use of taconite aggregates for more than a decade (see ongoing and completed research), and started investigating GNP-reinforced asphalt materials more than three years ago (see recently completed research). Both materials present very unique properties that can be used to better build and maintain asphalt pavements.

This research project will focus on two applications with significant potential in the pavement area:

  • Early detection and repair of cracking by developing a novel asphalt material in which GNP materials, taconite concentrate, and conventional asphalt binders are combined for damage sensing and healing. The material damage will be assessed by measuring the electrical resistance, while the damage healing will be achieved by applying microwave to the material.
  • Thermal enhancement of tack coat bonding between asphalt overlay lifts, using GNP and taconite concentrate and microwave heating. Poor bonding can result in many different pavement distresses that decrease the pavement structural strength and life, ranging from top-down cracking, potholes and fatigue failure.

Improving pavement durability

The latest data shows that 15 percent of roads in Minnesota are in poor condition, at a cost to each motorist $480 per year. Low-temperature cracking is one of the main causes of pavement failure in Minnesota. Studies have shown that early detection of damage and cracking and timely repair is essential for extending the lifespan of the pavements.

Each dollar spent in the early-stage of pavement life could eliminate or delay $6 to $10 in future rehabilitation or reconstruction costs.

A series of recent studies funded by the National Cooperative Highway Research Program (NCHRP) and MnDOT showed that the GNP-modified asphalt binders and mixtures exhibit a significant improvement in both mechanical and compaction properties. The combination of the previous research and the proposed research will fully explore the properties of GNP-taconite modified asphalt binders and mixtures as a multi-functional pavement material, which will address various needs of MNDOT, including high fracture resistance, efficient compaction process, and cost-effective pavement preservation operations. By addressing these needs, the result of this research will lead to an innovative and efficient means to improve the long-term durability and resilience of asphalt pavements in Minnesota.

 Project scope

The two-year research project aims to explore the damage sensing and healing capability of asphalt binders and mixtures modified by GNP and taconite concentrates. The essential idea is to combine GNP and taconite concentrates with asphalt binders to make the final asphalt products electrical conductive. By measuring the change of electrical resistance, researchers will be able to determine the damage extent. When the damage extent reaches a certain level, the University will apply microwave to the pavement to generate heat, which will heal the cracks through viscous flow of warm asphalt binder. In addition, the thermal bonding capabilities of a novel tack coat material also modified with GNP and taconite concentrate will be investigated. The research will consist of four parts:

  1. Electrical conductivity tests on GNP-taconite modified asphalt binders and mixtures
  2. Modeling of relationship between electrical resistance and damage extent
  3. Investigation of self-healing capability through microwave
  4. Investigation of a microwave-based tack coat system to enhance thermal bonding in asphalt paving

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

 

Affordable Bridge Girder End Repair Method Restores Concrete Beams

By load testing part of a bridge that was removed over Nine Mile Creek, researchers have proven that an innovative and cost-effective method for repairing damaged bridge girders restores them to their original strength.

The findings will help MnDOT and other transportation agencies avoid lengthy traffic closures and more costly techniques when repairing other bridges.

“This innovative method works remarkably well. The amount of damage the crew repaired was pretty extensive. In the end, the strength of the repaired damaged girders was slightly more than the strength of the undamaged girders,” said Carol Shield, Professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Background

The salting of bridge and roadways during Minnesota winters can create highly corrosive conditions that damage bridges. Such was the case with the Highway 169 Nine Mile Creek Bridge near Edina and Minnetonka, where leaking expansion joints caused corrosion to elements responsible for the strength of bridge girders: shear reinforcement, prestressing strands, and the surrounding concrete.

During a 2013 repair, crews encountered two locations of severe beam deterioration. To repair these areas, MnDOT used a novel method developed in Michigan that involved removing deteriorated concrete and cleaning the area, placing steel reinforcement cages around the damaged beam ends and then encasing the beam ends with concrete. The repair concrete was a specific form of concrete placement called “shotcrete”—a mix of sand, aggregate and cement that is applied with a hose that is wetted at the nozzle before the mixture is sprayed at high velocity onto the repair surface. When the desired thickness of the concrete placement is reached, the placement is troweled and shaped to finish to the desired cross section. The beam end repairs were made in October 2013 and allowed the bridge to continue its function to the public.

MnDOT was able to make the repairs without traffic interruption.

Several years later, the bridge was scheduled for replacement. The repaired girder ends appeared to be in good condition, but the repair technique had not been studied for strength. The bridge replacement presented MnDOT with an excellent opportunity to evaluate the repair method for use on other damaged girder ends.

What Was Our Goal?

When the southbound lanes of the bridge were taken out of service in spring 2017, four prestressed girders were removed from the structure and brought to the University of Minnesota’s Theodore V. Galambos Structural Engineering Laboratory for testing.

Researchers examined and tested the beams to evaluate the effectiveness of the reinforced shotcrete repair method.

“Two of the girders have ends that were repaired by MnDOT, and two girders have ends that never needed to be repaired,” Shield said. “We [tested] the four girders and [compared] their strengths to determine if the repair actually returned the girders to the strength they had prior to the corrosion-related damage.”

The fact that researchers tested good girders alongside repaired girders gave MnDOT a high level of confidence, said Paul Pilarski, Metro North Regional Bridge Construction Engineer, MnDOT Bridge Office.

Bridge girder ends can be repaired for only $5,000 to $10,000, using this new method.

2018-07-p2-image
Repaired and unrepaired girders were tested to failure in a laboratory. This repaired beam end remains firmly connected to the beam, even after the girder was crushed.

What Did We Learn?

All repairs had been done in field conditions that have the potential to adversely affect the results. But when the beams broke in the lab, the shotcrete repair did not separate from the bonding surface. The repaired reinforced concrete beam ends were found to be at least as strong as similar beams that were in good condition and had not needed repair. The initial repair methods and subsequent testing of the prestressed beam ends are demonstrated in a video created by the research team (testing starts at 3:30 min).

Using this method, severely deteriorated beam ends can be repaired with reinforcement cages and shotcrete for $5,000 to $10,000. The alternative to this type of repair involves constructing a new beam, closing traffic, removing the bridge deck over the damaged beam as well as the beam itself, and recasting the bridge deck and barrier—an intrusive replacement that costs hundreds of thousands of dollars and more than a month of bridge lane closures.

What’s Next?

Results have been presented internally at MnDOT, at state and Midwest conferences in late 2017, and at the National Bridge Preservation Partnership Conference in April 2018. Presentations have impressed transportation engineers from around the country and have increased confidence in dealing with aging infrastructure. MnDOT will continue to refine repair methods with the shotcrete treatment based on best industry practices, and will continue to use the beam end repair method if similar conditions are encountered in the state.

This post pertains to Report 2018-07, “BR27568—Experimental Shear Capacity Comparison Between Repaired and Unrepaired Girder Ends,” published February 2018. More information can be found on the project page. (Part of this article was adapted from an October 2017 article by the Center for Transportation Studies.) 

 

Nanotechnology Reduces Cold-Weather Cracking in Asphalt Pavements

Adding graphite nanoplatelets (GNP) to asphalt binders and applying the methodology developed in a new MnDOT study could provide a cost-effective approach to reducing cold-weather cracking and increasing the durability of Minnesota pavements.

“This project gives MnDOT a low-cost way to incorporate the latest nanotechnologies into our asphalt mixtures, reducing cold-weather cracking and increasing the durability of Minnesota pavements,” said Shongtao Dai, Research Operations Engineer, MnDOT Office of Materials and Road Research.

What Was Our Goal?

The objective of this project was to develop a cost-effective method to determine the optimum mix design of GNP-reinforced asphalt binders and mixtures. This method would predict the fracture behavior of these materials using a combination of simple laboratory testing and computer modeling.

What Did We Do?

Researchers developed a method for determining the quantity of GNP to add to an asphalt binder to achieve optimal asphalt mixture performance. The method used a computer model to predict the low-temperature fracture behavior of mixtures based on bending beam rheometer (BBR) tests on fine aggregate mixtures. This test applies a load to the center of a thin, rectangular specimen that has been cooled to a low temperature while its edges rest on two elevated supports, and then measures how the specimen bends over time. The results of this test determine the stiffness of materials and their ability to relax the stresses of contraction.

The BBR test is simpler, less expensive and less labor-intensive than the more accurate semicircular bend (SCB) test, which measures fracture resistance—the way cracks in a material form—by loading a semicircular sample from its apex. However, the SCB test can determine the properties of all the particles within a mixture; the BBR test can only evaluate the mechanical properties of coarse aggregates. To obtain the accuracy of the SCB test without the labor and expense, the computer model developed by researchers in this study uses BBR results as inputs to simulate SCB tests and infer the properties of fine aggregates.

2018-02-p1-image
Although simpler and less expensive than a SCB test, a BBR test only evaluates the properties of a mixture’s coarse aggregates.

What Did We Learn?

Researchers validated their computer model by comparing its results with those of  actual SCB tests. They found that the model was able to predict the results of SCB tests for both conventional and GNP-modified mixtures. By performing only a BBR test on the fine aggregates mixture and inputting the results into the computer model, researchers obtained a reasonable prediction of the fracture response of the final asphalt mixtures.

In turn, the model showed that using GNP in asphalt binders can significantly improve the strength and fracture resistance of a mixture compared to mixtures with unmodified asphalt binders. The model can be used as a design tool to determine what percentage of GNP is needed to achieve the necessary tensile strength for a target value of fracture energy.

What’s Next?

Using GNP in asphalt binders, in combination with the methodology developed in this project, could potentially provide MnDOT with a cost-effective approach to improving the cold-weather performance of Minnesota pavements, preventing cracking and increasing pavement durability. MnDOT will continue to evaluate the use of GNP in its asphalt mixes.

This post pertains to Report 2018-02, “A Mechanistic Design Approach for Graphite Nanoplatelet (GNP) Reinforced Asphalt Mixtures for Low-Temperature Applications.” Further GNP research is underway. Find related projects at MnDOT.gov/research.