Stormwater | Lessons in Porous Concrete

An update on the Villanova Urban Stormwater Partnership’s demonstration site

By Robert Traver, Andrea Welker, Michael Horst, Megan Vanacore, Andrea Braga, and Leo Kob

This article provides an update on the current status of the porous concrete surface for the Villanova University Porous Concrete Best Management Practice. Originally constructed in 2002, the original surface failed and was replaced in 2003, providing valuable lessons learned as featured in Stormwater magazine (Traver et al. July/August 2004, www.stormh20.com/sw_0407_villanova.html). After a second winter, limited portions of the site failed a second time and were replaced in October 2004, again providing valuable lessons as to the use of this material. Street sweeping was performed onsite and found to be a viable means of maintaining the porosity of the concrete by removing fines and debris from the surface.

Figure 1. Villanova University porous concrete site.

Project Overview
A porous concrete infiltration best management practice (BMP) was constructed during the retrofit of an existing paved area in the center of the campus of Villanova University in August 2002 (Figure 1). The contributing watershed area is approximately 50,000 square feet and is highly impervious, consisting of pedestrian walkways, rooftops, and some grass areas. The rooftops and some adjacent paved areas are directly connected to three separate rock storage beds (4 feet deep) that are outlined by the porous concrete surface. The rock beds are linked through piping systems to distribute the runoff between beds and allow for overflow for major storm events. The porous concrete surface is edged using decorative pavers with the porous concrete outlining the site and standard concrete in the center of the site.

Installation involved demolishing the original site, extensive regrading, and construction of the infiltration BMP (Traver et al. 2004). A strength additive was used. The site was designed to capture and infiltrate the first 2 inches of runoff, thereby reducing downstream stormwater volumes, streambank erosion, and nonpoint-source pollution. The project joins the Villanova University Best Management Practice Demonstration Park as both a research and a demonstration site (Traver 2002). Funding for the project was provided through the Pennsylvania Non-Point Source Pollution Section 319 Program, and the site has been designated as an EPA National Monitoring site. Further information on this and other projects can be found through the Villanova Urban Stormwater Partnership Web site (www.villanova.edu/VUSP).

Figure 2. Demolished concrete on uppper bed.

In October 2004, during Villanova University’s fall break, construction was performed at the site to replace sections of the porous concrete that had failed or were not functioning properly, approximately 40% of the reconstructed surface (Figure 2). It was observed that the areas of porous concrete that had failed were from the later parts of each individual pour, and that the first portion of each pour was in acceptable to good condition. It had also been observed during the original reconstruction that the end of each pour was less malleable and extremely hard to work. Because only the top layer of the porous concrete had failed, it is speculated that impermeable layers were formed at the end of the pours, allowing freeze/thaw to occur.

Figure 3. Sections of removed porous concrete.

Demolition began onsite on October 12, 2004, with the uppermost bed. Twelve sections of porous concrete were removed from the site (Figure 3). The sections to be replaced had been marked prior to demolition by Villanova’s Facilities Management Office. Each section was saw-cut and subsequently jackhammered to break up the porous concrete. The large chunks that were removed appeared to be in good condition other than at the surface. The large pieces were removed by hand, and some of the gravel was shoveled out. This process did produce some fines and gravel, but the magnitude was not substantial enough to clog the pores and prevent water from infiltrating the bed. The demolition also ripped up the joints between the porous concrete and the standard concrete, which were to be replaced during pouring. Demolition was completed at the end of the first day.

Four test pads had been poured on Villanova’s campus prior to the reconstruction in 2003 and were reevaluated before this renovation. Two of these pads were formed with the same proprietary additive used in the original and the 2003 pour, and the remaining pads used the “Florida mix,” a standard porous concrete mixture, with no proprietary strength additive. Because all four pads had remained in good condition and had maintained their porosity, it was decided that the strength additive that had been used in the previous pours was not necessary.

Figure 4. Hand-troweling the porous concrete.

Pouring began at 6 a.m. on October 13 in the upper bed. The expansion joints had been replaced, marking each separate porous concrete pad. Initially, the pour from the first mixer appeared to be too gravelly and dry. The placement was halted, and workers spread out what had already been poured. The remaining concrete was mixed in the drum for an additional period of time and the pour was resumed. This pour was a much better consistency, and they began again and worked along in small sections. The mixture was poured 1.5 inches above the surrounding surface, as opposed to 0.5 inch higher as in the reconstruction in 2003, so that a roller with higher compaction could be used. The higher compaction allowed the workers to get the porous concrete flush to the standard concrete already in place without the elevation difference that was noted during the previous construction. A hand trowel was used after the porous concrete was raked into place to ensure the pour was even and filled all gaps (Figure 4). The concrete sections were then quickly compacted with the roller, which was hosed down each time before passing over the concrete to prevent the concrete from sticking to the roller.

Figure 5. Rolling the porous concrete.

Figure 6. Wet burlap covering.

On the first section poured, a hand tamper was used preceding the roller, presumably to compact portions of the porous concrete at the edge to ensure that it was flush with the standard concrete. However, the roller itself seemed to be accomplishing this, so this practice was not continued. In addition, on another section the contractor attempted to have water continuously flowing from the roller, rather than having it sprayed on the roller by hand, but this produced a smooth (nonporous) surface because the porous concrete became too hydrated and created an impervious section (Figure 5).

After compaction, each section poured was covered with wet burlap strips (Figure 6). Because this material is heavier than the plastic that had been used in the previous construction, it remained in place over the concrete throughout the recommended 48-hour period and allowed for better hydration.

Figure 7. Nonpourous area at end of truck.

Figure 8. Villanova University vacuum street cleaner.

The second mixer at the site had material of a similar consistency to that of the first mixer, but at the end of the second mixer the remaining material that flowed down the chute was extremely wet and gravely, leaving water ponding on the surface of the pad (Figure 7). This section was removed and repaved when construction resumed on October 16 at 6 a.m., following a two-day delay because of adverse weather conditions. Paving was completed on the second day of pouring. It was observed that the Florida mix was much easier to pour than the previous mixture, and no problems with stiffening occurred during the pours.

Once the porous concrete had cured for approximately 48 hours, the burlap strips were removed. The color of the new sections was fairly consistent and close to that of the existing sections. It is believed that over time the color will even out further. A few small sections where the different compaction techniques were attempted do appear less pervious, but overall the new sections appear to be in good shape, and their effectiveness looks promising.

It is generally recommended that porous concrete surfaces be vacuum-swept at least twice a year to keep the concrete free of pore-clogging debris. Street sweeping was performed on the Villanova site in August and December 2004 (Figure 8). This cleaning was very successful in removing the particles that could potentially clog the porous concrete. The porous concrete was visibly cleaner after the street cleaner passed over it, and the removed debris could be seen in the truck.

Villanova Urban Stormwater Partnership
The mission of the Villanova Urban Stormwater Partnership is to advance the evolving comprehensive stormwater management field and to foster the development of public and private partnerships through research on innovative stormwater management BMPs, directed studies, technology transfer, and education.

Acknowledgments
Funding for the project was through the Pennsylvania Non-Point Source Pollution Section 319 Program. This support does not imply endorsement of this project by the EPA or the Pennsylvania Department of Environmental Protection. The design and construction was overseen by Robert Morro and Leo Kob of Villanova’s Facility Management Office. The design firm for the infiltration portions of the project was Cahill Associates from West Chester, PA. The site contractor was N. Abbonizio Contractors.

References
Traver, Robert. 2002. Development of a BMP research and demonstration park. 9th International Conference on Urban Storm Drainage, Environmental and Water Research Institute, ASCE.

Traver, Robert, Andrea Welker, Clay Emerson, Michael Kwiatkowski, Tyler Ladd, and Leo Kob. 2004. Villanova Urban Stormwater Partnership: Porous concrete.

Robert Traver and Andrea Welker are professors in the Department of Civil and Environmental Engineering at Villanova University and direct the Villanova Urban Stormwater Partnership (VUSP). Michael Horst is the VUSP research associate, and Andrea Braga and Megan Vanacore are graduate assistants. Leo Kob is with Villanova’s Facility Management Office.

SW July/August 2005

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