Tag Archives: Research Spotlight

The Use of Porous Concrete for Sidewalks

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In December 2017, a team of researchers at the Center for Advanced Infrastructure and Transportation (CAIT) at Rutgers University published a research study for NJDOT on “The Use of Porous Concrete for Sidewalks.” A porous concrete sidewalk typically consists of a porous concrete slab on top of an open graded stone reservoir layer. A filter fabric is placed between the underlying soil and the reservoir layer. One of the most important benefits of porous concrete is its effectiveness for stormwater management, i.e. improving water runoff quality, reducing stormwater runoff, and restoring groundwater supplies. However, there are concerns related to its construction, cost, maintenance, and durability.

Rodding and finishing of a porous slab. Photo source: Najm, 2017.

The primary objective of the research study was to evaluate the various factors that influence the performance of porous concrete in sidewalks. These include hydraulic performance to meet New Jersey Department of Environmental Protection (NJDEP) regulations and structural performance to meet typical sidewalk strength requirements as well as life-cycle cost and maintenance requirements. The researchers compared the performance of porous concrete in sidewalks with other materials, such as conventional concrete and asphalt alternatives, and tested various pervious concrete mixes to evaluate the hydrological and structural performance and energy budget versus conventional concrete mixes. The team also conducted a cost benefit analysis for use of porous concrete on sidewalks versus alternatives. The study resulted in recommendations and guidelines that NJDOT could use to inform the mitigation of stormwater runoff and development of maintenance standards.

The study found that the effectiveness of porous concrete in reducing stormwater runoff could contribute to cost savings. The research report examined a number of important considerations affecting the broader implementation of this technology. A life-cycle cost analysis of three sidewalk design alternatives—porous concrete, porous asphalt, and conventional concrete—found that porous concrete had the highest initial construction cost, with conventional concrete coming in slightly less, and porous asphalt being the cheapest. The study showed that the service life of porous concrete may be shorter than conventional concrete, driving up the life-cycle cost and potentially offsetting the savings from stormwater management best practices. For porous asphalt, which has the shortest service life, findings showed that if the service life ratio of porous asphalt compared to conventional concrete was greater than 0.60, then porous asphalt would be the most economically competitive option of the three. Another consideration in some areas is that it may be less effective to implement porous pavement if the soil has low permeability. The report notes that, once implemented, periodic maintenance is required to prevent clogging from debris and sediments, and freezing in the winter to avoid failure due to freeze and thaw cycles.

Porous concrete beam during flexural test. Photo source: Najm, 2017

The research team recommended that next steps should include construction of a porous concrete sidewalk and a porous asphalt sidewalk for long- and short-term performance testing. They noted that implementation should include geotechnical evaluation of the subsoil layers for infiltration rates, hydraulic design and storm-runoff analysis, selection of porous mix design based on NJDOT specifications and contractor recommendations, sample prisms and cylinders extracted for lab tests, scheduled maintenance based on NJDEP and NJDOT guidelines, and regular inspection. While there are environmental benefits to the implementation, such as filtration of contaminants like metals, oils and grease, to improve water quality and reduce chloride pollution, there are also concerns it can cause groundwater contamination. These concerns, along with recommended further performance testing, highlight the importance of interim steps before wider implementation.

Pervious pavement is a key component of green infrastructure methods that seek to improve stormwater management. The New Jersey Department of Environmental Protection lists the practice among others such as rain barrels, cisterns, and rain gardens/bioretention basins as strategies that can be implemented on a variety of scales in order to both treat runoff and reduce runoff volume. Yet, as the report notes, there has been little published research on performance and practical experience in the United States, highlighting the researchers’ final recommendation for further testing.

Sources

Green Infrastructure in New Jersey. (2018). Retrieved from https://www.nj.gov/dep/gi/More_Info.html

Najm, H., Wang, H., Miskewitz, R., Roda, A. M., Ali, A., He, H., Chen, X., Hencken, J. (2017). The Use of Porous Concrete for Sidewalks. Retrieved from https://www.state.nj.us/transportation/refdata/research/reports/FHWA-NJ-2018-001.pdf

Najm, H., Wang, H., Miskewitz, R., Roda, A. M., Ali, A., He, H., Chen, X., Hencken, J. (2017). Technical Brief: The Use of Porous Concrete for Sidewalks. Retrieved from https://www.state.nj.us/transportation/refdata/research/reports/FHWA-NJ-2018-001-TB.pdf

Identifying High Risk Bridges in New Jersey

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A team of researchers from New Jersey Institute of Technology have improved upon methods to identify high risk bridges in New Jersey to facilitate prioritization for repair or replacement. They have accomplished this through validating and advancing a new multi-dimensional model to analyze bridge scour and make appropriate recommendations. Bridge scour is the gradual removal of sediment around bridge abutments or piers caused by water movement, which can affect the long-term integrity of a bridge structure. By collaborating with three New Jersey consulting firms, the researchers hope to transfer their findings for statewide application.

Read a short technical brief summarizing the project background and findings (November 2017)

The researchers developed a “Scour Evaluation Model” or SEM that reflects New Jersey’s unique geological and hydrologic/hydraulic conditions while taking a more comprehensive approach than previous practices. NJDOT joins a number of other state DOTs that use a modified method for scour evaluation, as standard methods have often yielded conservative values for scour depth, or yielded disparities between predicted and observed scour.

SEM uses seven parameters to evaluate scour risk. One key parameter is the use of envelope curves, which “correlates the upper range of expected scour depth with a measurable hydraulic variable such as embankment length or pier width.” It was originally developed by USGS and original curves were based on bridge studies in 14 states. Many of the bridges were located in South Carolina’s Coastal Plain, which has a similar geology to New Jersey.

Another key parameter is determining whether a bridge has experienced a 100 year storm, and if so, how it performed. The other five include erosion resistance of streambed, bridge age, field scour observations, channel stability, and HEC-1800 scour calculations.

In their report, the team summarized the impacts of their SEM application. The bridges were rated by priority levels (1-4) based on the analysis. First, 17 bridges were evaluated using an abbreviated SEM procedure to prescreen high risk bridges. These 17 bridges were determined to be Priority 1 (high risk) or Priority 2 (medium-high risk) and in need of repair or replacement.

Secondly, the project evaluated 12 bridges fully using SEM with the participation of three consulting firms. Two of the bridges in the study were found to be Priority 1 (high risk), one bridge was found to be Priority 3 (medium to low risk) and nine bridges were found to be Priority 4 (low risk). The low risk bridges were then recommended for removal from the scour critical list.

Third, the research study was able to validate the use of “envelope curves” to evaluate scour at 15 bridges across 9 New Jersey counties with a range of characteristics and flooding histories.

The team’s goal was to accelerate the transfer of the model into statewide practice, so that it can be fully applied to New Jersey’s inventory of scour critical bridges. This was accomplished through meetings, conference calls and field visits with participating consultants.

The team’s full research and implementation process can be read in the following report:
SCOUR Evaluation Model Implementation Phase

View the team’s presentation slides from the 19th Annual NJDOT Research Showcase