Tag Archives: pavement

Materials and Construction

Modeling Demonstrates Benefit of Geogrid-Reinforced Aggregate Base

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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.

Materials and Construction

Selecting Structural Synthetic Fibers for Use in Thin Concrete Overlays

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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.

Bridges and Structures, Maintenance Operations, Policy and Planning

New measure allows comparison between bridge and pavement conditions

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Transportation planners lack a method to directly compare bridge and road conditions. In a new MnDOT-funded study, University of Minnesota researchers have proposed a Percent Remaining Service Interval (PRSI) measure that can uniformly assess the condition of bridges and pavements, enabling planners to make the most efficient use of preservation and improvement funding.

A nighttime view of workers and heavy equipment at a road construction site
Planners would like a condition measure similar to RSL that could be used to compare and prioritize needs for highway and bridge construction.

“Both the MnDOT Bridge Office and the Materials and Road Research Office have very good management systems in place,” says Mihai Marasteanu, a professor in the Department of Civil, Environmental, and Geo- Engineering (CEGE) and the study’s principal investigator. “There is a good potential to develop a new common metric that both offices could use.”

What Did We Do?

To begin developing this new measure, researchers conducted a literature review of current methods used in asset management and life-cycle cost analysis. The review of bridge research focused on performance measures and life expectancy assessment methods, while the study of pavement literature concentrated on performance measures as well as on the use of road service life measures.

Next, the research team, which included civil engineering bridge professor Arturo Schultz, surveyed both bridge management staff and pavement management staff from state transportation agencies. Team members then analyzed the asset management practices of MnDOT’s Office of Bridges and Structures and Office of Materials and Road Research to identify methods for assessing service lives and rehabilitation needs and to highlight the similarities and differences in approaches.

Based on the findings from the survey and analysis, researchers suggested the new method of PRSI that would serve both pavement and bridge needs and offered guidelines for the next steps in developing and implementing a unified PRSI procedure.

“Ultimately, funds for guardrail repairs are drawn from the same purse that pays to fill a pothole or repair a deck joint,” Marasteanu says. “With PRSI, planners could target average values across systems to optimize life-cycle costs and pursue an even distribution of PRSI values to make planning consistent from year to year.”

What’s Next?

In the next phase of the project, researchers will work with the pavement office to identify relevant data for calculating PRSI for pavements. “In addition, we plan to identify the time and costs required to reach the evenly distributed configuration of PRSIs necessary for planning consistency, assess how preservation activities impact funding efficiency, and calculate recommended metrics for asset sustainability,” Marasteanu says.

This article originally appeared in the Center for Transportation Studies’ Catalyst Newsletter, October 2018. The full report, published July 2018, can be accessed at “Remaining Service Life Asset Measure, Phase I,” .

 

 

Materials and Construction, Policy and Planning

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

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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.

Environment, Materials and Construction

Pervious Concrete Pavement Reduces Runoff into Shoreview Lake

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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:

 

 

 

Materials and Construction

Nanotechnology Reduces Cold-Weather Cracking in Asphalt Pavements

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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.

Materials and Construction

Design Spreadsheet Offers Alternatives to Protect Pavements from Frost Damage

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Researchers have developed a simple design tool for determining the amount of frost-free materials needed for a specific site’s subgrade to prevent frost and freeze-thaw damage to pavements.

“This tool will help us optimize construction to provide the best pavement,” said Steve Henrichs, Assistant Pavement Design Engineer, MnDOT Office of Materials and Road Research

Since 1995, MnDOT has required the use of frost-free materials (FFM) in subgrade depths of 30 to 36 inches for asphalt pavements, based on traffic load requirements. It is not clear that such FFM requirements are effective. In some areas, 30 inches may be excessive and, therefore, unnecessarily expensive; in others, 36 inches of FFM may not be enough, leading to costly pavement failure and repair. MnDOT needed a research-based pavement subgrade design procedure for resisting frost damage in pavements.

“Frost protection has not been studied in depth recently. This research used inputs based on soil type, location and expected frost depth, and didn’t require advanced modeling or expensive laboratory testing,” said Matthew Oman, Principal Engineer, Braun Intertec Corp.

What Was Our Goal?

The goal of this project was to develop a procedure for optimizing subsurface materials and thicknesses based on existing subgrade soils and geographic areas in Minnesota in order to resist pavement damage from frost action.

What Did We Do?

Researchers first reviewed existing literature on frost action and frost susceptibility. They synthesized national and international research and looked at practices and standards for mitigating frost action in states and countries with climates similar to Minnesota’s. Then they reviewed MnDOT’s current and historical policies and practices.

2018-06-p1-image
Researchers reviewed charts and other resources on cold temperatures and structural insulation needs, like this map from the National Oceanic and Atmospheric Administration.

The central effort in this research was to examine existing pavements in Minnesota to characterize pavement performance and winter profiles. Researchers and the Technical Advisory Panel selected 72 pavement sites for study based on soil types (such as glacial till, clay, silt, sand and peat); pavement types (including concrete, asphalt and composite); subsurface materials and thickness; and weather conditions. The team evaluated construction logs, project plans, management data and subsurface investigations, and they augmented Minnesota-specific data with performance and soil data from the Federal Highway Administration’s (FHWA’s) Long-Term Pavement Performance (LTPP) program. Researchers created winter pavement profiles of most of the sites and compared them with roughness and ride quality data collected the previous summer.

Finally, the team analyzed performance trends and design and construction details to assess the effect of frost heave on ride quality. Using the findings from this effort, the team built a design tool for determining what pavement structures require of subgrades to resist environmental effects based on project location, projected frost depth and soil type.

What Did We Learn?

The initial evaluation did not produce strong correlations between winter ride quality and factors like FFM depth, grading soil depth and region. Winter ride quality measurements were poorer than summer measurements, but the role of FFMs remained unclear. Insufficient data, outliers and other questionable information were culled from the records, which were then amplified with data from other pertinent historical sources. Results from this effort suggested that FFM depth may improve pavement performance by incrementally reducing ride deterioration, particularly at depths of 25 inches or greater.

Review of relevant LTPP data established that shallower FFM depths and greater silt content in subgrades correlate with poor pavement ride quality. Silty soils, which have low permeability and produce high capillary effects, have long been considered susceptible to frost damage.

Researchers avoided thermodynamic modeling and analysis—and kept the design tool simple—by selecting subgrade silt content as a proxy for frost susceptibility. The spreadsheet tool uses project location (latitude and longitude), predicted frost depth and subgrade soil silt content as the key factors in frost susceptibility of pavements. The tool recommends frost treatment ranges from about 30 percent of predicted frost depth for soils with zero silt to over 80 percent of predicted frost depth for soils with 100 percent silt. The spreadsheet requires limited laboratory testing of subgrade soils, is simple and inexpensive to implement, and produces results similar enough to MnDOT’s practices that they will not require dramatic change in construction needs.

What’s Next?

Researchers produced four spreadsheets, each employing a different combination of frost depth prediction and soil type characterization. Once MnDOT selects its preferred spreadsheet and determines if additional subsurface tests should be included as inputs, pilot implementation will begin. Additional study to enhance the tool could include investigating MnROAD cells further, collecting more winter ride quality data, developing uniform frost depth prediction methods and tracking more information from new construction.

This post pertains to Report 2018-06, “Designing Base and Subbase to Resist Environmental Effects on Pavements,” published February 2018. The full report can be accessed at mndot.gov/research/reports/2018/201806.pdf.

Materials and Construction

New Procedures Offer Guidance for Using Bonded Whitetopping on Asphalt Pavements

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Researchers developed procedures for selecting asphalt pavements for thin whitetopping based on site examination and lab testing. Test results do not offer definitive indications of how overlaid asphalts will perform, but procedures offer recommendations on pre-overlay pavement treatment, testing protocols and design considerations for bonded concrete overlay of asphalt.

“This research established a procedure for testing pavement cores. However, more performance data on whitetopping is needed to correlate pavement performance and asphalt properties,” said Tim Andersen, Pavement Design Engineer, MnDOT Office of Materials and Road Research.

“These procedures address collecting field data and testing pavement core samples in the lab. They also provide useful guidance for pavement repair and design considerations for overlays,” said Dale Harrington, Principal Engineer, Snyder and Associates, Inc.

A badly rutted pavement.
Rutted and otherwise damaged asphalt pavement is a candidate for a bonded concrete overlay that can mitigate damage under the right site conditions.

What Was the Need?

Many counties throughout Minnesota have used bonded concrete overlays to rehabilitate asphalt pavement. Though not widely used by MnDOT, a bonded concrete overlay, or whitetopping, normally involves milling a few inches of asphalt off the damaged surface and placing 4 to 6 inches of concrete over the asphalt pavement. A well-bonded overlay can add 20 years to a pavement’s service life.

Bonded whitetopping performance has not been care-fully tracked, and correlation of its performance with the underlying pavement condition is not well understood. Be-fore MnDOT can expand its use of bonded whitetopping, materials engineers wanted to better understand what asphalt pavement conditions are best suited to this type of overlay, how asphalt behavior influences the concrete top layer and what underlying pavement characteristics affect the expected lifetime and performance of bonded white-topping.

What Was Our Goal?

This project sought to develop an integrated selection procedure for analyzing existing, distressed asphalt pavement to identify good candidates for bonded whitetopping and establish design considerations for a site-specific, effective concrete overlay. By testing pavement core samples in the lab, investigators wanted to identify asphalt pavement properties that correlate with distresses in concrete overlays that are 6 inches or less. They also sought specific recommendations for managing trans-verse cracking in asphalt to avoid reflective cracking into concrete overlays.

What Did We Do?

Researchers began with a literature review of approaches to selecting pavements for bonded whitetopping. The results of this review were used to develop testing procedures to identify the volumetric properties of existing asphalt pavements. Researchers applied these procedures to 22 pavement cores from six concrete overlay sites in Iowa, Michigan, Minnesota and Missouri. Selected projects entailed 4-inch to 6-inch overlays in fair to good condition that were built from 1994 through 2009. Data about mix design, asphalt condition, pavement thickness, overlay thickness, site conditions and other details were available for each site.

The research team compared roadway data with falling weight deflectometer measurements from pavement cores to evaluate field performance and design recommendations suggested by the selection procedure. To refine the procedures, investigators evaluated volumetric asphalt characteristics for their potential influence on premature overlay cracking due to stripping, slab migration and reflective cracking. Finally, the team developed a detailed selection process that includes steps to identify and test asphalt pavements with potential for bonded whitetopping, repair asphalt before overlays and establish design considerations for overlays based on the test results from the selected asphalt pavement.

What Did We Learn?

The selection procedure, which is based on recommended practices from the National Concrete Pavement Technology Center, has six steps:

  • Perform a desk review of available site data, including design, repair and environmental conditions.
  • Obtain pavement core samples.
  • Conduct site visits to examine existing conditions.
  • Obtain additional core samples for testing, when necessary.
  • Prepare preliminary cost and materials estimates, if practical.
  • Provide design recommendations.

Investigators tested pavement cores for air voids, density, stiffness, fatigue, aging, strip-ping potential and other distress parameters. Results were inconclusive in terms of identifying asphalt properties that lead to specific bonded concrete overlay failures or to long-term performance of bonded whitetopping projects. The pavement cores showed wide variation in material properties, but few of these distresses. Researchers framed the recommendations for testing volumetric properties in the format of MnDOT’s Pavement Design Manual, giving the agency an easily adoptable core testing protocol.

The selection procedures include information about the impact of transverse cracking, rutting, longitudinal cracking and other distresses on concrete overlays, and provide recommendations for treating various distresses before whitetopping. Design considerations for whitetopping are also provided based on site conditions and the results of core, ground penetrating radar and falling weight deflectometer testing.

What’s Next?

Tested overlay sections should be evaluated over time to determine if life expectancy is met or if asphalt stripping, slab migration or reflective cracking has decreased overlay life. Because volumetric tests failed to provide conclusive relationships between asphalt properties and overlay distress, further research is needed to identify mechanistic or field tests that could correlate asphalt properties with concrete overlay performance. Once this additional research is completed, the selection procedures identified could be refined and placed in the design guide. A life-cycle cost analysis of overlays would also be useful for decision-makers considering bonded concrete overlays of asphalt.

This Technical Summary pertains to Report 2017-24, “MnDOT Thin Whitetopping Selection Procedures,” published June 2017. 

Materials and Construction

Research Confirms Low-Binder Asphalt Pavement Mixtures Prone to Cracking

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Disk-shaped compact tension test
The disk-shaped compact tension test determines fracture energy of pavement samples, a strong predictor of cracking performance.

Research showed that lower asphalt binder mixtures are susceptible to premature cracking. The current use of coarse-graded mix designs should be adjusted to narrow the gradation difference between larger and smaller aggregates in the mixes. While the research suggests such mixes should be used sparingly in Minnesota, it did not provide definitive data suggesting the practice should be stopped altogether. The practice may continue on a limited basis.

What Was the Need?

Introduced in 1993, Superpave has successfully helped transportation agencies in northern regions design asphalt pavements that are less susceptible to thermal cracking. When tested, Superpave-compliant designs were found to resist both rutting and thermal cracking.

Gradation-based design approaches have also allowed for the use of coarse-graded, low asphalt binder mixtures. These mix designs establish a maximum aggregate size and reduce the range of usable gradations. Such coarse-graded designs meet MnDOT specifications because the maximum aggregate size falls within the acceptable gradation range. However, the reduced fine aggregate content made possible by the use of coarse aggregates leads to a mix that, while still within specifications, offers less surface area to be coated by the asphalt binder and can encourage unwelcome permeability in the field. To win low-bid competitions, contractors have embraced these low-binder, coarse-graded designs to reduce binder and aggregate costs.

Transportation engineers noticed that these pavements seemed to “gray out” or lose their dark color more quickly than previous asphalt designs. These pavements also seemed to grow somewhat more brittle and were less able to rebound from loading. Such asphalts are thought to be prone to quicker failure than mixes with finer aggregate and more binder. Road designers typically attribute thermal cracking and potholing in low-binder asphalt to the increased permeability that leads to water incursion and freeze-thaw damage.

What Was Our Goal?

The goal of this project was to determine how well low-binder asphalt pavements per-form and whether current designs make sense in terms of cost–benefit and durability. Researchers would identify any relationship between reduced bitumen use and potential for cracking, and would suggest changes to specifications for coarse-graded asphalt pavement mixtures to prevent such cracking issues.

What Did We Do?

Researchers worked with MnDOT to identify 10 pavement locations in Minnesota that used 13 coarse-graded, low-binder asphalt mix designs. Investigators extracted data on cracking, roughness and other factors for these sites from MnDOT’s pavement management system. The research team then visited the sites and inspected the pavements.

Researchers developed a coring plan, and field samples were cored for volumetric analysis to determine the binder, aggregate, air void level and other properties of each mixture. They also tested permeability and dynamic modulus, and conducted fracture energy testing to determine cracking resistance.

Investigators used performance modeling to analyze the test results of pavement proper-ties and project pavement durability. Then they compared the projected performance to actual field performance. From this assessment, they drew recommendations for modifying specifications for MnDOT low-binder, coarse-graded asphalt mixtures.

What Did We Learn?

This study suggests MnDOT should reduce its use of coarse-graded asphalt mixtures, but the findings did not provide sufficient data to justify prohibiting the use of coarse- graded, low-binder asphalt designs.

Low-binder mixtures were prone to thermal and transverse cracking. Their high permeability left them vulnerable to premature moisture and freeze-thaw damage. Field and laboratory testing and modeling demonstrated that coarser mixtures produce excessive cracking in a short period of time. Thin overlays of 3 inches or less crack more quickly than thick overlays of 4 to 6 inches. Mechanistic-empirical simulations showed that low-binder asphalt mixtures were significantly inferior to higher-binder mixtures in terms of thermal cracking.

Most of the high-cracking mixtures showed low fracture energy in testing, suggesting the value of fracture energy testing and modeling. Disk-shaped compact tension testing showed that higher permeability mixtures correlate reasonably well with lower fracture energy. Eight of the 13 mixtures were more permeable than recommended, and six significantly so. Typical volumetric properties poorly predicted cracking.

To better project pavement performance, researchers recommend that MnDOT maintain volumetric testing-based specifications, but add performance testing-based specifications and testing designs for fracture energy, fracture resistance, modulus and other parameters. For Superpave designs, investigators suggest using a narrower aggregate gradation range, reducing the gradation gap between minimum and maximum aggregates in mixes.

What’s Next?

Although the research validates MnDOT engineers’ anecdotal concerns, the pavements evaluated were mostly overlays, which are known to be susceptible to transverse cracking because of flaws in underlying pavement layers. MnDOT may weigh the results and adjust specifications, but would likely require further study of coarse-graded mixture performance before ruling out its use or identifying situations in which coarse-graded mixtures may be the best option. Additional research could consider the use of nonuniform lift designs for asphalt pavements, varying mixes for each lift in the structure rather than using a single, uniform mix for every layer in the full depth of the pavement.

This post pertains to Report 2017-27, “Impact of Low Asphalt Binder for Coarse HMA Mixes,” published June 2017. 

Materials and Construction

Using Recycled Concrete Aggregate in New Concrete Pavement Mixes

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Using recycled pavement as aggregate in new concrete mixes can save money and promote environmental sustainability. New design methods published in a new research report allow engineers to create more durable mixes from recycled aggregate than in the past, reducing the need for virgin aggregate, a diminishing and expensive resource.

“This report shows that a lot can be done with recycled aggregate,” said Matt Zeller, Executive Director, Concrete Paving Association of Minnesota. “We can get the strength up to that of concrete with virgin aggregate by bumping our mix design and lowering our water-to-cement ratio.”

“Concrete pavement made with RCA can be beneficial both economically and environmentally,” said Farhad Reza, Professor, Minnesota State University, Mankato, Department of Mechanical and Civil Engineering.

Reza served as the project’s principal investigator.

What Was the Need?

When pavements are due for reconstruction, the old pavement is frequently crushed to aggregate-sized particles and used as the base course for new pavement. In the 1980s, MnDOT and other state transportation agencies began using such recycled aggregate in the concrete course as well. But this latter practice was discontinued by the early 1990s due to mid-slab cracking observed in pavements constructed with such concrete. Using recycled concrete aggregate (RCA) in the base course has continued, however.

Newer mechanistic-empirical design methods and performance engineered mixtures have led to improved RCA mixtures. For example, concrete mixtures now have lower water-to-cement ratios. These advances present an opportunity to re-evaluate the use of recycled aggregates in concrete mixes, which aligns with two important trends: the diminishing availability of virgin, high-quality aggregate, and the growing federal emphasis on sustainable design. Using recycled concrete as aggregate fulfills the three basic principles of sustainability: performance, environmental stewardship and cost-effectiveness.

What Was Our Goal?

Researchers sought to evaluate the performance of selected sections of concrete pavement in Minnesota that had been constructed with RCA; examine field samples and lab mixes; and develop guidelines for successful use of recycled aggregate in new concrete pavements.

What Did We Do?

Researcher vibrates RCA mix samples in a box.
Investigators vibrated RCA mixes in sample boxes to prepare the mixes for mechanical analysis.

After a literature search on the use of RCA in new concrete pavements, investigators examined the following issues:

  • Historical Performance. The research team gathered and compared data on performance, ride quality and durability for 212 miles of RCA pavement and for 212 miles of regular concrete pavement in the state. Both pavement samples had been built in the same time period and had had similar traffic levels.
  • Materials and Constructability. Investigators analyzed the ride quality of two-lift (or two-layer) concrete pavement test sections built in 2010 at the MnROAD test facility, using modeling to project long-term performance based on the historical evaluation. They conducted tests on nine cores pulled from the RCA pavements and tested new mixes made with recycled aggregate from Olmsted County, Minnesota. For comparison, they tested virgin aggregates from a Mankato, Minnesota, plant and fines from a Henderson, Minnesota, site.
  • Life-Cycle Cost Analysis. The research team conducted a life-cycle cost analysis of new RCA mixes and traditional concrete mixes, comparing their performance and cost-effectiveness.
  • RCA Guidelines. Based on the historical analysis, laboratory testing and modeling, and life-cycle cost analysis, the researchers developed new guidelines for the design and construction of pavements containing RCA in concrete mixes.

What Did We Learn?

Results showed that using RCA in concrete pavements can save money and is a sustain-able practice that produces durable concrete pavement.

  • Historical Performance. Most of the existing pavement studied had not reached the terminal ride quality index of 2.5—the level that generally indicates a major pavement rehabilitation must be performed. Analysis showed that rehabilitation is required, on average, at about 27 years of service for RCA pavements and at 32 years for standard concrete pavements.
  • Materials and Constructability. Mix design can be adjusted to achieve traditional strength levels that older RCA mixes did not reach. Elimination of fines and stricter adherence to gradation specifications for concrete aggregate can achieve workable and durable mixes that are less likely to suffer excess drying shrinkage. Pavements designed in this way meet the standards of the Federal Highway Administration’s INVEST program for sustain-ability in highway construction.
  • Life-Cycle Cost Analysis. Long-life RCA pavements are more economical in cost-benefit terms than are thinner, shorter-life RCA pavements.
  • RCA Guidelines. Researchers developed specification recommendations and design guidelines for the use of RCA in new pavement construction. Trial mixes are critical, and absorption and compressive strength must be examined before use. Recycled fines are not recommended, but otherwise RCA can be used in the full range of aggregate sizes between minimum and maximum. Recycled concrete pavement may not produce enough aggregate for both pavement and base course, but acquiring extra RCA to make the base course 70 percent recycled and 30 percent virgin makes the new pavement economical and sustainable.

What’s Next?

Keeping detailed records on mix designs used and tracking mix performance over time will help MnDOT to further refine its use of recycled aggregate in concrete mixes and will provide robust data on the performance of more sophisticated RCA mixes. A research team may want to consider using lower-quality recycled concrete as a bottom lift and higher-quality recycled concrete with virgin aggregate in the top lift. Methods for managing water input with recycled aggregate to achieve proper water-to-cement ratios warrant further study.

This Technical Summary pertains to Report 2017-06, “Evaluation of Recycled Aggregates Test Section Performance,” published February 2017.