According to "Paver Pete" Baloglou of masonry products manufacturer Techo-Bloc, the reality that permeable paver joints range from ???(R)???AE???(R)? inch to over one inch, and that there is no clear definition which joint fill (ASTM #8, #89, or #9) "transfers energy best in what joint," are two of the inconsistencies in the available data on permeable pavers.

By Peter Baloglou, Techo-Bloc Some claim permeable pavement (PP) dates back to the early 20th century. That may be the case but the data and research trail begins closer to the half-century mark. In that data from universities, associations, product and system manufacturers, etc., there are a glaring number of contradictions and omissions. To be fair, some consensus has been reached, but that aside, to assert the contrary or delete from consideration details without a quorum is misleading. These details create almost a mythical aura. Myths pertaining to permeable pavements include the use of "the word interlock", subbases, geosynthetics, compaction equipment, maintenance procedures and intervals, site analysis, ambulation, and sustainable limitations.

The use of the word "interlocking" as it pertains to permeable pavement may or may not be a myth. In the U.S. and California, the Interlocking Concrete Pavement Institute (ICPI) categorizes the system as Permeable Interlocking Concrete Pavement (PICP), in the UK Interpave Concrete Block Permeable Pavements (CBPP) and some manufacturers, such as Techo-Bloc, use Segmental Permeable Pavements (SPP). Why Interpave chose CBPP dates back the beginnings of precast or stone block paving in Europe. The industry began there, dating back to the Greek and Roman roads approximately 2000 B.C., - they aren't changing. Some manufacturers have chosen SPP because of continuity with Segmental Retaining Walls (SRW) and the concept of unit paving.

So why then PICP? "Interlocking" is the mythical word in that categorization. How is it defined? In Interlocking Concrete Pavement (ICP) tightly fitted unit paving system it's defined as Vertical (jointing sand), Horizontal (pattern), and Rotational (thickness). Each is clearly defined and accepted. In PP interlock is less clear. Joint width for ICP is 1/16-3/16" and joint fill ASTM c33 or c144; for PP joints range from 1/4-1+" and joint fill ASTM #8, #89, or #9 but which aggregate transfers energy best in what joint (shear transfer) is undefined. Thickness of units relevant to traffic, climate, soil conditions in ICP is clear but for PP aspect ratio (Length to thickness) is less so. Patterns for both are intuitively those with the least aligning joints for strength but where's the data? For ICP, again case studies, academia, empirical data is there, but for PP our fingers are crossed. Two projects for example, the Stonedge precast plant in Chambly, Quebec, 300K plus sq. ft., and Lamtec commercial site on the Delaware River in Pennsylvania, 80K plus sq. ft. are both mechanically set (TB100si proprietary vacuum laying and traditional ride on devices). Both are preforming beyond expectation and have not exhibited "creep" horizontal movement under repetitive Equivalent Single Axel Load (ESAL) traffic. Interlocking? Maybe.

The use of subbases (additional base or bases) under the base, bedding, and wearing course has become a standard in PP, but why and when is the mythical part. The use of ASTM #2 and #3's are the most specified in PP subbases. Their use for water storage, extreme depth and thickness (2-7 feet not uncommon), and strength under load (like railroad tracks), are all good reasons. The inability to get washed ASTM #2 and #3 stone (in some areas and smaller quantities) with less than 5% passing the 200 sieve, cost of additional aggregates and deliveries on site, and difficulty of use (especially residential) are all drawbacks.

If ASTM c2940 (densely graded aggregate) is acceptable to one-foot depths in ICP, why transition from ASTM #57 at 4" depth to #2 and #3s for PP? Why not in certain climates, traffic loads and soil conditions skip the subbase and give the consumer a better value for their application?

Feeding off the more clearly defined need (or not) for subbases may have some concerned about the base's ability to perform in certain climates and under even residential vehicular traffic such as a driveway. That concern is answered with mythical, mysterious geosynthetics. Not so mysteriously, geosynthetics have revolutionized the pavement and SRW industries. The question is, "why is geo-synthetic use so infrequent in PP?" Nonwoven, woven, and woven with biaxial strength geotextiles and uni- and biaxial geogrids can bring value and performance to PP applications. These geosynthetics can be used to segregate and increase the performance between the subgrade soil and subbase or base, subbase (if applicable) and base, base and bedding, and in some designs intermittently in lifts of the sub-base or base. The use of geotextiles or geogrids (or both) can be a component of structural or hydrologic decisions but their exclusion from the conversation is in error. They add integrity, little investment compared to alternative initial and life-cycle costs, aid in filtration of petroleum from storm water via microbial generation, prevent downward migration of aggregate into preceding layers under the kinetic energy of traffic, and are easily installed.

A limitation of geogrids may be in edge retention. Pre-cast concrete curb/edging products with steel reinforced concrete backfill that encapsulate the bedding, restrain units, and are elevated to restrict sedimentation remain a superior option for restraint.

Site analysis and its complexity is another mythical barrier in PP use and understanding. The first step, where to begin, "geotechnical, hydrologic, and civil engineers"?(R)?are you kidding?" are all barriers to entering the PP world. The first steps always seem to be the hardest. Start with a common percolation test of onsite soils, $100-$1,000 depending on excavator/backhoe expenses and local engineer/dept. of health fees. Step two: soil analysis for sieve size, bearing ability, and classification an additional few hundred dollars with a geotechnical firm or university annex. These may be one (or two or three) time expenses due to geology of surrounding area. Next step consult manufacturer of PP's website and spec guide for recommendations on wearing course and cross section of installation options. You may also purchase or web search municipal, industrial, or academic publications on PP relevant to your application. In critical applications with adjacent subterranean drainage structures, bio-swales, vegetated filter strips, rain-harvesting components, etc. Consult an engineer armed with permeable design software. Residentially, light commercial and municipal sites work with your town officials and product producers and design a PP good for the property owner yet also shows your company's commitment to sustainability. Ten-ton static roller minimum four passes is a common recommendation for the compaction of aggregate layers in a PP installation. Really? Even the two inch ASTM #8 bedding layer? Four inch base of ASTM# 57 stone layer? ASTM #2 and #3's sub-base stone? And in what lift height and level of moisture? More questions than answers are created by the above recommendation of compaction! In order to get answers let's start with compaction's definition, mechanically increase weight per unit volume or density. To increase the density you must know what you're compacting, moisture percentage, when to stop (maximum density), and equipment needed. The aggregate layers are all open, poorly graded materials and similar to SRW construction (compaction/filtration/drainage zone) -they don't require the lubricating effect of water and are self-consolidating. How do we know they are self-consolidating? How do we measure open graded materials level of density for consistent replicable data? Measuring density on open graded stone will remain a mystery but for now using "compaction" equipment and moisture to lubricate its passes across the aggregate layers will give us only a smooth surface for which to continue construction.

If you feel consolidation of aggregate layers is possible, consider your equipment is simply pushing the stone into the sub-grade or surrounding excavated side soils. That's displacement not compaction.

Maintenance, maintenance, maintenance three things to remember about PP. Maintenance has been stressed since the systems inception. Agitation of joint fill will be needed due to sedimentation from Total Suspended Solids (TSS), heavy metals such as brake dust, petroleum distillates, etc. Determining the need for maintenance will depend on one thing, visual observation of ponding or pooling water (or run-off on steep slopes) on your total site. Things that may affect the need for maintenance:

1) Environment: like shedding trees
2) Wind and snow (piled) born debris
3) Pedestrian and vehicular pollutants
4) Quality of Erosion and Sediment (E&S) measures
5) Designed for real world use i.e. sustainable site
6) Joint width and jointing material
7) Etc.

So, an annual, bi-annual, quarterly, monthly maintenance schedule will be needed. Myth! Depending on your total site's rate of infiltration maintenance may never be needed or intervals maybe every 3-5 to 10 plus years. What about joint aggregate on the surface of the units, weeds, ants, etc.? Jointing aggregate on the surface is an exaggerated issue and only a concern in rapid water flow situations like flooding or power washing pavement. Aggregate remains below the chamfer of the units and virtually glued together with friction and contaminants from the environment. Weeds, ants, etc., are prevalent in any pavement outdoor and addressed with common cures.

The last myth is that of safe ambulation. Manufacturers of PP systems, such as Techo-Bloc, test their products for Americans with Disabilities Act (ADA) compliance. Regardless of whether the ADA is applicable (public site) testing to a standard has everyday residential relevance for pool sides, children and elderly use, steep slopes, etc. The act requires that the pavement is firm, stable and skid resistant. "Firm" applies to the rigid units in PP. The block units must pass ASTM c936 or CSA a23.1 (more stringent Canadian standard, tested in saline solution) in order to be relevant in the marketplace. "Stability" is subject to the pavement cross section and design. If the aggregate layers, geosynthetics, pre-cast curb edging are installed correctly pavement will be stable under foot, tire, and static or dead loads. Finally "skid resistance" is subject to a test method, the Brungraber Mark 2 test using a pendulum and simulated human skin. Is it the most relevant to a shoed foot, wheel chair and high heel? Maybe not, but it gives us the worst case scenario. The skid resistance testing gives us reliable results that can be part of submittals.

In colder climates PP are best for snow melt due to geothermal heat and with no refreeze "black ice" after the thaw makes it the safest option.

SPP use in sustainable sites is only limited by the openness of our design, engineering and municipal communities. With an eye out for misinformation and myths the system of SPP is poised to overcome the next obstacle. Future myths are already starting to rear their ugly heads. Myths like:

Future myth #1) "Can't build patios with SPP because of foundation walls." Answer) Why build any patio against a foundation wall when you can build out in the yard?

Future myth #2) "Steep slope driveways will let water out the bottom." Answer) Not with a bench cut excavation following the angle of repose of the soil and protective dikes for each landing/bench.

Future myth #3) "SPP are good for the environment but ugly." Answer) With large and small scale pavers and slabs, multi-piece configurations, tri-color blends with heavy pigment loading, and cobbled and random textures, visit your local hardscapes dealer to see displays and be surprised.

Future myth #4) "SPP cost too much more than ICP." Answer) 10-15% less in an apples to apples comparison!

If by airing these myths a debate has begun, mission accomplished. Talk to some soil, pavement (all types) and water experts and research their assertions to formulate your own theories, share those theories with others. SPPs represent the largest growth segment of the hardscape industry, so get involved. Now go out there and get "#Boot Deep In" some Segmental Permeable Pavements!