Powerful blasts of compressed air prove to be a solid maintenance method for pavers
Permeable segmental paving — using a field of precisely spaced pavers atop a bed of precisely layered aggregates — has really come into its own as a method of treating storm water on site, by directing it into the ground underneath or by catching it and redirecting it for use in irrigation. The system effectively reduces the volume of storm water going into sewage treatment plants. That enables municipalities to follow U.S. EPA regulations, and at the same time, can help save taxpayer money because municipalities can postpone, or even potentially eliminate, investing in more wastewater treatment capacity.
The system also has three added bonuses: Genuine clay permeable pavers are, arguably, more aesthetically pleasing than the alternative. They can be removed and set off to one side if you need to dig down to repair utilities. They can then be reinstalled instead of putting in an ugly patch. Finally, in cold climates, a properly operating clay permeable paver system is durable and outlasts repeated freeze-thaw cycles, and it helps limit black ice because snowmelt does not pool up and refreeze, which makes walking outdoors safer.
However, a “properly operating” system is not automatic. Permeable pavers come with a challenge, in that gaps between the pavers, which are themselves filled with chip stone, can become clogged with soil and other debris, which causes rainwater to pool on the surface instead of flowing freely underneath the pavers, as the system was designed to do. This occurs even if the area was routinely vacuumed.
Pine Hall Brick partnered with Pave Tech and its Typhoon System, to demonstrate how the spaces between permeable pavers can be effectively cleaned through a combination of compressed air and a vacuum. The demonstration was held in the fall of 2019 at an installation of Pine Hall Brick’s StormPave permeable pavers in Lancaster, Pennsylvania, which had been installed five years earlier. Architects, landscape architects, city officials, engineers and others were invited to observe.
The Typhoon System uses strong blasts of compressed air that blow the joints clean clear down to the setting bed, effectively removing both the joint stone and the very fine debris that is suspended in the joints. The debris is then vacuumed and fresh stone is swept into place. In addition to the demonstration in Lancaster, the system earned high marks in a separate demonstration, this one a scientifically conducted experiment conducted by civil and mineral engineers at the University of Toronto. Both tests showed that a clogged permeable paver installation was substantially cleaned and again able to drain storm water at approximately the same rate that it was when first installed. Both tests also provide evidence to help remove objections to permeable paver installations by pointing to the need for regular maintenance, along with a way to substantially rehabilitate clogged installations without having to take them up and reinstall them, which would be costly.
Lancaster, Pennsylvania, Project
The intersection of East Walnut and North Plum streets in Lancaster was prone to flooding during rain storms, which flowed into the combined sewer system and then into waterways during high-volume periods. The city of Lancaster, which has an older drainage system that combines storm water and septic sewer, was under increasing pressure from EPA to reduce raw sewage overflow. On a rainy day, the combination of storm water and sewage tends to overwhelm the treatment plant and dump untreated sewage first into the Conestoga River then into the Susquehanna River, and finally, the Chesapeake Bay.
A solution to build and install large tanks to store and release storm water at much lower volumes was cost prohibitive; instead, the city went with several smaller projects. Among them was a large outdoor patio and parking stalls at Lancaster Brewing Co., which is adjacent to the intersection of East Walnut and North Plum streets. The project, which used Pine Hall Brick’s StormPave pavers, was installed in 2014.
By September 2019, the parking stalls were not absorbing rainwater at all, and weeds were seen growing from the joints, even though it was routinely swept with a sweeper vac truck from the city. The patio was absorbing some rainwater, but not as much as it did when the system was first put in. The new system — which appears similar to a push lawn mower — was brought in and blew the joints clean, down to the setting bed, removing both the joint stone and the very fine debris. The joint stone and debris were then vacuumed from the surface.
More joint stone was swept into the joints, and a retest showed that surface water was again infiltrating into the patio and parking lot surfaces at the same rate as when it was installed. That test concluded that the new system was powerful enough to loosen the hardened crust of fine debris and thus reopen water flow, which ordinary vacuum cleaning was not able to do.
Testing at the University of Toronto
The University of Toronto’s Department of Civil & Mineral Engineering took a deeper dive into comparing the different methods that are used to clean segmental permeable pavements. An April 2020 study authored by Jennifer Drake, Ph.D., P.Eng., Tahmineh Sarabian, M.A. Sc. P.Eng. and Ph.D. student Jody Scott looked into how to best keep them clean, for a specific reason.
“A lack of proven cost-effective and practical approaches for permeability restoration prevents the widespread adoption of these systems in Canada and North America,” wrote Drake in the study. “Better methods are needed so the required interval between maintenance events can be lengthened and thereby reducing overall life cycle costs.”
The advantage to the research is that for the first time, a simple-to-implement and repeatable methodology has been devised to clog pavement, which means that researchers and equipment manufacturers can test, evaluate and compare maintenance equipment and experimental results. To begin, seven 10-foot-by -10-foot cells within a field of permeable pavers were prepared by taking material actually swept off Toronto streets and placing them in between the pavers, so that the outcomes would be consistent. In all, 11 kilograms of sediment was required to clog each of the sections. Five different maintenance treatments were tried. They included the same system used at the Lancaster test, which is made up of high pressurized air and vacuum; a regenerative air street sweeper; a combination of power washing and vacuuming; vacuum street sweeping; and a waterless mechanical street sweeping.
“Only the high pressurized air and vacuum system fully restored the (paver installation) to its baseline, post-construction, surface infiltration,” Drake wrote in the study.
According to the study, "the high pressurized air and vacuum system produced mean surface infiltration measurements that were 2 to 6 times higher than all other tested techniques" and restored infiltration to 109% of its original condition, or 12,900 millimeter an hour (or 507 inches.) There was a significant drop-off seen in the other methods, with the vacuum street sweeper producing the second highest amount of surface rejuvenation but also generating the most variable results.
The study also said "although surface infiltration was restored to original or higher conditions at some locations," the surface infiltration capacity on average was 7,500 millimeter per hour, or 70% of original conditions. The waterless mechanical street sweeper restored the permeable infiltration capacity to just 35% of its original capacity, or 3,540 millimeter per hour, according to the study.
The study does have some advice for property owners, such as municipalities and universities, who are maintaining permeable paver installations. It mentions to:
- Keep in mind that applied maintenance conducted early and often works better than restorative maintenance.
- Operate vacuum equipment over permeable pavers as slowly as possible.
- Use rectangular suction heads and make two passes, perpendicular to each other.
- Remember that equipment designed specifically for permeable paver maintenance outperforms generic street sweepers and is the preferred approach for restoring severely clogged or neglected pavements.