Category Archives: Materials and Construction

Bridges and Structures, Materials and Construction, Research

Using Debonded Strands to Reduce End Stress in Bridge Beams

October 11, 2019 micaelaresh Leave a comment

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

What Was the Need?

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

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

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

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

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

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

What Was Our Goal?

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

What Did We Do?

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

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

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

What Did We Learn?

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

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

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

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

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

What’s Next?

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

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

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

Materials and Construction, Research

Low-Temperature Cracking Test Produces Repeatable, Reliable Results

October 9, 2019 micaelaresh Leave a comment

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

What Was the Need?

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

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

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

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

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

What Was Our Goal?

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

What Did We Do?

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

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

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

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

What Did We Learn?

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

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

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

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

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

What’s Next?

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

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

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

Environment, Materials and Construction, Research

Evaluating Iron-Enhanced Swale Ditch Checks for Phosphorus Removal

September 25, 2019 micaelaresh Leave a comment

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

What Was the Need?

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

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

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

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

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

What Was Our Goal?

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

What Did We Do?

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

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

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

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

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

What Did We Learn?

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

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

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

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

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

What’s Next?

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

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

Environment, Materials and Construction, Research

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

September 23, 2019 micaelaresh Leave a comment

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

Background

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

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

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

Project Scope

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

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

Maintenance Operations, Materials and Construction, Research

Recycling Asphalt Pavement Offers Strong Alternative to New Aggregate Base

September 16, 2019 mndotresearch Leave a comment

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

What Was the Need?

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

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

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

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

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

What Was Our Goal?

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

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

What Did We Do?

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

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

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

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

What Did We Learn?

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

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

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

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

What’s Next?

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

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

Materials and Construction, Research

Testing Methods for Crack Resistance in Asphalt Materials

September 12, 2019 mndotresearch Leave a comment

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