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.Continue reading Testing Methods for Crack Resistance in Asphalt Materials
Researchers examined mixtures of recycled asphalt pavement (RAP) and aggregate for new gravel road surface layers in the lab and in the field. Although test results did not align perfectly, and field results were somewhat uneven, findings suggest that mixtures with 70 percent RAP content can reduce dust generation. After a year of service these roadways can match all-aggregate gravel road performance in terms of strength, but with a smoother ride.Continue reading Study Suggests 70 Percent RAP for Minnesota Gravel Road Surfaces
Researchers tested sediment control logs in the lab and in the field to determine the relative filtration capabilities of these devices. They also developed design guidelines for correct selection and contributed to ongoing educational efforts.Continue reading Sediment Control Log Guidance for Field Applications
A new MnDOT research study determined that depositing concrete grinding residue (CGR) slurry at specific rates on roadside vegetation and soil may not cause lasting harm to plant growth and soil quality; however, follow-up research is recommended.Continue reading Concrete Grinding Residue Doesn’t Appear to Negatively Affect Roadside Vegetation and Soil
Several years of research have culminated in the publication of a culvert design manual that promotes the safe passage of fish and other aquatic organisms, as well as stream connectivity, throughout the state.
“Engineers designing culverts for Minnesota’s diverse ecological regions will benefit from this document, which offers sound guidance from many practicing experts about how to design culverts that allow aquatic organism passage and preserve stream integrity,” said Petra DeWall, former Bridge Waterway Engineer, Minnesota Department of Transportation (MnDOT).
What Was the Need?
Minnesota’s 140,000 miles of roads and approximately 92,000 miles of streams and rivers meet at tens of thousands of places. Culverts are a cost-effective solution to allow traffic to cross over smaller waterways. Historically, culverts have been designed with the safe passage of vehicles in mind. Recently, a state and national appeal for the safe passage of fish and other aquatic organisms, as well as for waterway integrity and connectivity, has influenced culvert design.
MnDOT has supported many research projects examining fish and aquatic organism passage (AOP) through culverts, and nationally, a number of published resources exist on appropriate design. Because of the variety of ecological regions in the state, the range of culvert geometries and many other factors, no single solution can accommodate AOP through culverts statewide. A comprehensive culvert design guide was needed to inform designers about solutions that can effectively facilitate the movement of fish and other aquatic organisms in Minnesota while maintaining healthy streams.
What Was Our Goal?
The objective of this project was to produce a comprehensive and accessible culvert design guide that could be used by Minnesota practitioners to design culverts for AOP and stream connectivity. The guide would provide the following benefits:
• More efficient culvert design and permitting process for AOP.
• A central definition of typical designs, which would improve contractors’ familiarity with designs and lower construction costs.
• Avoidance of designs that could be detrimental to the natural environment.
• Avoidance of designs likely to lead to roadway damage and need for repairs.
• Fishery improvement through increased stream connectivity.
What Did We Do?
To determine the scope of the guide, researchers worked with experts from the Minnesota Department of Natural Resources (DNR), the U.S. Forest Service and others with knowledge of civil engineering, AOP and stream geomorphology.
They then sought information for the guide from a wide range of authoritative resources. A literature search examined current and past research by the research team and others; guidance documents from federal agencies; guidance from other states; permit requirements from the DNR and other agencies; and databases of fish populations, stream attributes and culvert data. The literature search also sought to reveal gaps in knowledge where further research specific to Minnesota was needed.
Additionally, researchers surveyed a cross section of highway design engineers and managers from MnDOT, county and city agencies, resource agencies and engineering consultants to identify current design practices for AOP and stream connectivity, and the degree of their effectiveness.
What Did We Learn?
The project resulted in the Minnesota Guide for Stream Connectivity and Aquatic Organism Passage Through Culverts, a thorough guide for culvert designers, hydraulic engineers and others involved in culvert design and construction in Minnesota. Topics addressed in the guide include:
• The need for culvert designs that include AOP and stream connectivity, as well as the current regulatory context.
• An overview of culvert design, categories of design methods that incorporate AOP and waterway connectivity, and a list of best practices.
• Site characteristics, analysis and tools related to energy dissipation, hydraulic analysis for AOP and sediment transport.
• A design method selection chart, information on certain designs and references for further information.
• Further guidance about design issues such as multiple barrel and floodplain culverts, grade control, retrofits and other cost considerations.
The culvert design guide will be made available to users online. Future considerations for this project include an associated webinar and efforts to coordinate information presented in the guide with expectations and permitting requirements of MnDOT departments charged with culvert creation and implementation. Additional research is underway to assess culverts and fish passage with respect to storm vulnerability and future hydrologic scenarios.
This post pertains to the MnDOT and LRRB-produced Report 2019-02, “Minnesota Guide for Stream Connectivity and Aquatic Organism Passage Through Culverts,” published January 2019.
Designing foundations for bridges and pavements requires understanding the soil conditions and properties at the site. One of the best methods for calculating site conditions is the cone penetration test (CPT), in which a rod with a cone-shaped tip outfitted with sensors is driven into the soil. Engineers attach more rods to the first as the device is gradually driven to depths of 30 to 150 feet.
Researchers have developed a new manual to show geotechnical engineers how to conduct the CPT and use the data it gathers. The guide walks engineers through the process of CPT-based foundation design for sand and clay soils in deep and shallow foundations, helping engineers put the best technology to use.
A supplement to the Minnesota Department of Transportation’s Geotechnical Engineering Manual, this resource will provide improved methods for using CPT data in geotechnical design.
What Was the Need?
Designs for new bridges and structures require geotechnical investigation of a site’s soil conditions to evaluate the strength, settlement and drainage of a proposed foundation. Common design procedures rely on boring samples from the site and on standard penetration tests (SPT), which entail driving a weighted steel rod into the soil and recording the number of blows it takes to drive the rod a specified distance. Using lab analysis of samples and on-site tests, engineers determine foundation properties for the new design.
The cone penetration test (CPT) has become an attractive alternative to the SPT. CPT employs a probe with a cone-shaped tip outfitted internally with various sensors. Equipment in a CPT truck pushes the probe into the soil at the site; engineers attach rod sections behind the probe to continue pushing it in the soil to the desired investigation depth, which is usually 30 to 150 feet for transportation projects. Standard sensors allow the CPT to directly measure tip stress, pore water pressure and soil resistance; other parameters can be measured with additional sensors.
“One of the biggest impediments to deploying cone penetration testing more widely has been the lack of a practical document that integrates the latest findings and best approaches, and puts that information to use,” said Derrick Dasenbrock,
Geomechanics/LRFD Engineer, MnDOT Office of Materials and Road Research.
The CPT safely and efficiently produces accurate data and repeatable results, yet relatively few engineers in the United States know how to employ these tests and use the data for geotechnical design inputs. Users can search geotechnical engineering resources to learn how CPT results can be applied, but no standard procedure or manual is widely available for transportation projects.
What Was Our Goal?
Investigators sought to develop a new CPT design guide based on the most current CPT in situ testing research and development. The guide is intended for use in evaluating the performance of proposed bridges and structures, embankments and roadway features.
What Did We Implement?
The research team produced the 2018 CPT Design Guide for State Geotechnical Engineers, with step-by-step instructions for using the CPT to evaluate soil properties at sites and to design shallow footings and deep foundations. The document provides an overview of the CPT, its use in analyzing and characterizing soils, background on computing engineering parameters derived from CPT measurements, and detailed procedures for using those parameters to design and analyze shallow and deep foundations. Also included are derivation background, case studies and examples to help guide the user through the design process.
How Did We Do It?
Investigators began by reviewing guidelines for geotechnical engineering design based on CPT methods. The research team identified the key soil properties measurable by the CPT that are required for designing shallow and deep foundations. Then team members evaluated numerous CPT-based methods used for shallow foundations and over 40 use for deep foundations. Using the results of this evaluation, investigators identified methods with sufficiently robust and reliable performance that could be easily implemented by design engineers.
The team used CPT data from MnDOT geotechnical site investigations and developed short design case studies applying the recommended CPT design methods. After reviewing the CPT procedures with the Technical Advisory Panel, investigators organized design modules for soil characterization, shallow foundations and deep foundations, and documented the process in the design guide.
What Was the Impact?
The new guide is based on the current best practices for the CPT and was developed to establish MnDOT’s geotechnical design process while accommodating ongoing research. The guide presents recommended design methods and offers step-by-step instruction on how to calculate engineering parameters from CPT measurements and apply those design inputs to efficiently design foundation systems. Examples of problems and solutions are provided in the context of Minnesota cases, although the techniques are broadly applicable.
“Engineers can start using this design guide immediately in Minnesota—and elsewhere. The format is adaptable; California could add another module about earthquakes, for instance,” said David Saftner, Associate Professor, University of Minnesota Duluth Department of Civil Engineering.
The guide begins with a focus on characterizing soil properties from CPT measurements, providing an example for both sand and clay soils. The shallow foundation design module describes how to determine strength and soil settlement characteristics from CPT sensor readings using a method based on 166 full-scale field load tests. The deep foundation design module explains how to use the CPT to determine the required axial compression capacity of piling from a method based on 330 pile load tests.
The guide is a much-needed resource for geotechnical engineers both within MnDOT and outside of the agency. The improved methods for using CPT data will encourage more frequent and widespread use of the method, improving the quality and reducing the time and cost of site investigations.
Available on MnDOT’s Geotechnical Engineering website as a supplement to the 2019 revision to the Geotechnical Engineering Manual, the guide will also be shared with a Federal Highway Administration CPT users group. Future considerations for the guide include a module on characterizing peat in organic soils and on seismic soil analysis.
This post pertains to Report 2018-32, “Cone Penetration Test Design Guide for State Geotechnical Engineers,” published November 2018.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
- Seven-year study final report
- Technical summary of preliminary study of pervious concrete
- Related ongoing study: Permeable Pavement for Road Salt Reduction