Permeable interlocking concrete pavement (PICP) reduces stormwater runoff volumes and rates, while filtering pollutants. This material has an admirable track record in paying for itself by reducing or eliminating detention ponds. In older cities with combined sanitary and storm sewer system, PICP presents a cost-effective means to reducing flows to wastewater treatment plants and related processing costs. Meanwhile, homeowners are retrofitting driveways and hardscaping backyards with PICP. Retrofits are sometime incentivized with rebates from the local municipality.
However, when civil engineers, landscape architects, architects, and contractors recommend PICP, property owners may resist as a result of their own or someone else’s experience with PICP clogging. In other cases, the owner won’t receive education on what’s required to clean PICP, or if they are educated on cleaning needs, many do not want to spend the money for surface cleaning. These situations lead to lack of maintenance, and clogging may occur.
The notion of clogging means that water puddles on a PICP surface rather than infiltrating quickly. There may be some infiltration into the surface and base, but it is very slow. In many cases, water puddles in places on the surface, and then moves to another area that more rapidly infiltrates the stormwater.
The biggest cause of clogging is the area around the impervious pavement, or contributing drainage area (CDA), delivering sediment to the PICP. There are many projects with no CDA. These usually need little, if any, cleaning. In contrast, sediment from vehicles, and especially deteriorating asphalt surfaces, deliver small particles that enter the PICP joints and slow infiltration.
See PICP As A System
Property owners often view PICP surfaces as “just another pavement” that requires little maintenance, says Matt Otero, COO at Designs by Stonescapes in Commerce City, CO. “There is a lack of education about how PICP works and why routine maintenance is important,” he says. PICP is a drainage system that must be maintained the same way any other type of filtration system would, he points out. “There should also be some thought put into the design of surrounding landscape areas to minimize runoff debris.”
Inexpensive, routine PICP maintenance includes the use of brooms, leaf blowers, rotary brushes, and shop vacs. More expensive methods for larger areas involve using regenerative air machines that, like the other methods, remove loose dirt, leaves, and other debris from pavers and from the top of joints before it enters them via rain and tire traffic. This keeps the pavement infiltrating as designed with minimal replacement of joint aggregate. Unfortunately, most owners don’t follow a routine maintenance schedule. So, when clogging occurs, more intensive, restorative maintenance is required that involves removal and replacement of sediment and joint stone. This is a more costly, time-consuming process compared to performing regular maintenance.
Vacuums For Tough Jobs
Members and staff of the Interlocking Concrete Pavement Institute (ICPI), and other organizations, have researched and witnessed demonstrations on the effectiveness of vacuum machines withdrawing sediment and aggregate from 3″ deep (~80 mm) and ~3/16″ (5 mm) or larger width joints typical to many PICP projects. The majority of machines are effective, but a bit inefficient because they require several passes to remove these materials. Moreover, the sediment often binds the aggregate, making both difficult to withdraw. Fortunately, recent research and cleaning demonstrations have uncovered innovative approaches to restoring infiltration of clogged PICP surfaces. These require only one pass over the pavement, regardless of joint size and depth. (For an overview of PICP maintenance, ICPI Tech Spec 23: Maintenance Guide for Permeable Interlocking Pavements is available for download from ICPI.)
“We install a lot of permeable pavers and offer maintenance agreements for them,” says Otero. “Unfortunately, by the time most customers ask for maintenance, the amount of sediment in the joints requires a restorative cleaning rather than simpler routine maintenance.”
To handle larger, more complex jobs, he explored a variety of vacuums and vacuum trucks, but settled on a less expensive, more portable machine that can remove aggregate and debris from PICP joints in one pass.
Otero experimented with different sized vacuum heads that rendered different cleaning levels. “A 24″-wide (600 mm) vacuum head that pulls half of the joint material out works well for pavements that are less dirty. But a pavement that has gone five years with no cleaning requires a 6″-wide (150 mm) head that pulls most of the joint material out,” he says. The equipment can easily be operated by a two-man crew with minimum training. In fact, Otero cross-trains his employees to handle the restorative maintenance as well as installation to give him flexibility when scheduling work.
Otero focused on equipment for restorative maintenance informed by the type of permeable pavements he expected to maintain. Spending the time and money to build exactly what he needed was worthwhile, he says. “Not only does the maintenance service ensure ongoing performance of the permeable pavement, but general contractors like the fact that they don’t have to protect the pavement as they finish construction of an area,” he says. “The ability to offer maintenance services to my installation customers, general contractors, and other PICP owners is a business decision to grow my business.”
Steve Jones, inventor and then President of Pave Tech, a hardscape products provider near Minneapolis, MN, conducted a six-year effort to develop TYPHOON, a compressed air system to clean PICP and remove joint material. Jones discovered the effective simplicity of using compressed air without water. “The compressed air blows the joint material out just as a water pressure based system,” he says. “But there is no sloppy mess when using air, and no loss of surface, as one would get from water pressure washing.”
The TYPHOON system features a rotating wand with nozzles attached to a 115 HP air compressor to focus controlled, high pressure air in PICP joints to remove clogging sediment, debris, and aggregate. This process is followed by the PAVEVAC, which is attached to a high volume, high suction (27″, or 675 mm of Hg) vac truck or trailer, to complete cleaning the joints and surface by removing loosened debris and aggregate from the PICP surface.
“A three-man crew can remove silt, debris, and joint material on 1,000 square feet in about an hour,” says Jones. “It takes us a less than an hour to train someone to properly use the equipment.” Training includes how to work with compressed air, a truck-mounted vacuum exerting 27″ of Hg suction, plus safe operation of hoses. Overall, it is an effective means to restore highly clogged permeable pavements.
Smith is technical director for the Interlocking Concrete Pavement Institute (ICPI). He got an early start on permeable pavements via research in 1977, and since then has researched, consulted, and trained civil engineers and contractors in permeable pavement design, construction, and maintenance. He has written many technical papers and has contributed to state and municipal guidelines on permeable pavements. Smith is co-editor of the 2015 ASCE book, “Permeable Pavements.” His latest book is the 5th edition of “Permeable Interlocking Concrete Pavements” released in 2017. He is also vice-chair of the ASCE 68-18 national standard on permeable interlocking concrete pavements. For more information or to learn more about ICPI installer certification, visit www.icpi.org.
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