PICP Receives High Marks
The University of New Hampshire Stormwater Center (UNHSC) released a two-year study on the performance of a permeable interlocking concrete pavement (PICP) site built on the university campus. The 13,500 sf (1,350 m2) Hood House Drive and parking lot in Durham, NH, reduced runoff volume and pollutant mass removals some 95 percent. Water infiltrating from about half of the PICP area was monitored for pollutants such as sediments, zinc, petroleum hydrocarbons, and nutrients (i.e., phosphorous and nitrogen forms). Built over a mix of moderate and low infiltration soil, the pavement saw significant volume reductions such that no single rain event generated more than 5 gal. (20 L) of discharge to underdrains in the base. Additionally, the study confirmed that open-graded bases and soil subgrades do not heave from winter freezing and thawing.
Surface infiltration testing was conducted using a test method similar to ASTM C1781 Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems. Testing showed a decline in surface infiltration for PICP areas subject to run-on from adjacent impervious or grassed areas. Surface infiltration rates declined 69 percent over 21 months, yet retained greater than 1,000 in./hr (2,540 cm/hr). Surface maintenance included vacuuming twice annually with regenerative air equipment typically used on the UNH campus. Figure 1 illustrates infiltration rates measured from September 2010 to May 2012.
Summertime thermal analyses compared four pavement surface types at three different times of the day. Surfaces included pervious concrete and porous asphalt sites on the campus. PICP surface temperatures were observed to be lower than that for porous asphalt, pervious concrete, and standard asphalt. Figure 2 provides data for measurements in June 2012.
SURFACE INFILTRATION RATE
Thermal Performance comparison of Picp
Hood House Drive and an adjoining parking lot were retrofitted from a standard asphalt surface to a PICP system in the summer of 2010. The existing condition included no stormwater control measures and conveyed surface runoff directly into the municipal storm sewer. The PICP was designed by Appledore Engineering, Inc. with input from UNHSC and the Interlocking Concrete Pavement Institute (ICPI). The lower end of the drive receives rainfall and run-on from three pedestrian walkways and an adjoining road. Pavers and the surrounding grass landscaping are separated by new granite curbing. Rainfall enters the PICP surface and passes into the reservoir consisting of an open-graded, crushed stone base and subbase. Excess stormwater not infiltrated into the soil subgrade drains through internal perforated pipes along the bottom that discharge into the municipal storm sewer system.
Pollutant loads from conventional pavement were estimated by monitoring runoff from an adjacent road and parking lot similar in size, usage and location. The following two years consisted of monitoring the PICP, which received 26 storms with 18 water-sampling events. The PICP performance for volume reduction and pollutant load reduction was exceptional for an installation on a sandy clay soil. Some parts of the site are very rocky, with fine, sandy loam. Soil subgrade infiltration tests done prior to construction were about 3 in./hr (7.5 cm/hr) in one area, which appears to be the major reason for such little water leaving the base/subbase via perforated pipes.
PARTNERS AND PARTICULARS
The PICP cross section consists of 3 1/8 in. (80 mm) thick concrete pavers with ASTM No. 8 stone, bedded on about 2 in. (5 cm) of the same material. The 4 in. (10 cm) thick base was compacted over a crushed stone subbase of No. 2 stone with a thickness of 24 in. (6 cm) in the parking area and 16 in. (40 cm) in the drive area. Since the soil subgrade in the drive area slopes three percent, geotextile wrapped berms about 4 in. (10 cm) high were used to delay water and allow it to infiltrate into the soil subgrade rather than continue into perforated pipes to drain into a nearby storm sewer.
UNHSC analyses and procedures for this study comply with the Technology Acceptance and Reciprocity Partnership (TARP), and the Technology Acceptance Protocol – Ecology (TAPE) guidelines. The research project was funded by ICPI and the ICPI Foundation for Education & Research. The removal of the existing asphalt pavement and old granite curbs, new curbs, pipes and base installation were provided by the UNH Facilities Department. Pavers, bedding materials and installation were co-funded by eight ICPI members, the New England Concrete Masonry Association and the Northeastern Cement Shippers Association. Machine-assisted installation by an ICPI contractor member holding an ICPI PICP certification enabled timely installation of the bedding layer and pavers for about $4/sf ($43/m2).
Since the early 1980s, there have been at least 18 studies lasting one year or longer on in-situ permeable interlocking and concrete grid pavements performed by stormwater agencies, universities and consultants world-wide. Of these, only three have been conducted in cold latitudes, including the UNH study. The takeaways from this study underscore past findings while presenting new ones that further demonstrate PICPs performance in cold climates:
- PICP eliminated practically all of the stormwater runoff from the storms at UNH;
- PICP effectively removes sediment, nutrients and metals through infiltration even during winter months;
- The surface was vacuumed twice annually over the two-year monitoring period with regenerative air equipment;
- Surface infiltration rates decline over time due to sediment and other debris, but rates can be increased with vacuum maintenance;
- The PICP surface is cooler compared to porous asphalt and pervious concrete;
- Winter snow plowing was done with no problems and there was no deicer damage;
- PICP does not heave from winter freezing and thawing; and
- The PICP surface provides opportunities for brining of deicing materials to prevent ice buildup, a feature not offered by other permeable pavement surfaces.
Project cost: approximately $9.90/sf including removal of the existing asphalt pavement, excavation, drainage, aggregates, new granite curbs and machine-installed concrete pavers. Costs do not include engineering and site soil testing or monitoring.