Porous and permeable pavement field guide for site crews

Permeable pavement is a stormwater system you walk on

Permeable pavement is a surface that lets rain pass straight through it into an open-graded stone bed underneath, where the water is stored and then soaks into the soil. It is not a paving job with a drainage feature bolted on. It is a stormwater facility that happens to carry traffic, and that reframing is the whole point. The surface, the stone, and the soil under it are one system, and any one of them done wrong takes the rest with it.

Three surfaces dominate the work: porous asphalt, pervious concrete, and permeable interlocking concrete pavers, usually shortened to PICP. All three do the same thing. Water enters at the top, drops into a clean stone reservoir, and either infiltrates the native soil or drains away slowly through a pipe. The system manages the storm where it falls instead of collecting it and shipping it to a basin downstream.

What makes it live or die is short. An open-graded surface mix with the voids left open. A clean stone reservoir with no fines in it. A subgrade that was never compacted tight, so it still infiltrates. And maintenance to keep the pores from sealing over. Miss any of those and you have built an expensive parking lot that ponds water. The base and subgrade work is its own subject, covered in the pavement base and subgrade compaction guide, and the surface QC carries over from the asphalt paving inspection guide. The warning to carry into both: a permeable section wants the opposite subgrade treatment from normal pavement.

Why use permeable pavement?

You use it to handle stormwater on the site rather than piping it off. A conventional lot sheds nearly all of its rain to inlets, pipes, and a detention pond, and that runoff carries the first flush of oil, sediment, and whatever else was on the surface. A permeable section takes the same rain in through the pavement, holds it in the stone, and lets it soak into the ground, which cuts the peak runoff rate and the total volume leaving the site.

That changes the numbers a civil engineer has to hit. Permeable pavement can earn stormwater credit toward the required runoff reduction or water-quality volume, which can shrink or delete the detention pond, the storm pipe, and the land they eat. On a tight infill site, the pavement doing double duty as the stormwater facility is sometimes the only way the project pencils. It also recharges groundwater, runs cooler than dense black asphalt because the open structure does not hold heat the same way, and keeps standing water and ice puddles off the surface because the rain drains down instead of sitting.

The driver is almost always the stormwater permit. How much credit you actually get, the design storm, and the volume you have to capture are set by the local stormwater authority and the project civil engineer, not by a rule of thumb. Treat the credit as a number the engineer calculates and the AHJ approves, and build the section that earns it.

The three surfaces, and where each one fits

The surface is a choice between porous asphalt, pervious concrete, and PICP, plus open grid systems for the lightest duty. They share the stone reservoir below. They differ in how they are placed, what they cost, and how they handle traffic and repair. The selection usually comes down to the contractor base in your area, the look the owner wants, and how the section will be patched years later.

Pick by use and by who is going to maintain it. The reservoir, subgrade, and infiltration design underneath are largely the same across all three, so the surface is the part the owner sees and the part that gets repaired.

How does permeable pavement work?

Water enters through the surface, falls into an open-graded stone reservoir, and then leaves the reservoir one of three ways: into the soil, out through an underdrain, or both. The surface is the doorway. The stone is the storage. The soil and the underdrain are the exit. Storage plus infiltration is the design, and the two are sized together.

Picture a storm hitting the lot. The rain does not run to a curb. It drops through the porous surface in seconds and collects in the voids between the clean reservoir stone, which behave like a sponge made of rock with roughly 40 percent of the volume as open space available to hold water. The void ratio is what turns a stone layer into a tank, and the engineer uses it, with the design storm, to set the reservoir depth.

Between storms, the stored water soaks down into the subgrade at whatever rate the native soil allows. If the soil is fast enough, the reservoir empties by infiltration alone and no pipe is needed. If the soil is slow, an underdrain set near the top of the reservoir carries the overflow away while the soil still takes what it can from the bottom. Either way, the goal is an empty reservoir before the next storm so the storage is there when it is needed. The numbers that set reservoir depth, void ratio, and drawdown time belong to the civil and geotechnical design.

The open-graded mix: the voids are the product

A permeable surface mix is built to be full of connected holes, which is the exact opposite of a normal dense pavement. Standard dense-graded asphalt and standard concrete are designed to fill the gaps between aggregate with fines and paste so almost no air is left, because density is strength and a tight surface sheds water. A permeable mix throws that out. You leave the fines out so the spaces between the stones stay open and connected from the top down.

In porous asphalt, that means an open-graded mix on a uniform coarse aggregate with the sand and fines removed, bound with enough asphalt to coat the stone without filling the voids. The target air void content commonly lands somewhere around 16 to 22 percent, far above the 3 to 5 percent you want in a dense mat. A polymer-modified or stiffer binder, and often a fiber, is used to hold a thick film on the stone without draining down at temperature.

Pervious concrete is the same idea in concrete. Coarse aggregate, cement paste, little or no sand, and a void content commonly in the range of 15 to 25 percent, which you can see as open texture across the finished surface. The voids are the reason it drains, and they are also why it is weaker in compression than ordinary concrete and why it cannot be finished or sealed like a normal slab. The exact gradation, binder, void target, and mix design are the engineer's and the spec's call. Do not let a plant substitute a tighter mix because it places easier. A tighter mix does not drain, and at that point you have poured a regular slab on a stone bed.

The stone reservoir is the storage layer

Under the surface sits the reservoir: a thick layer of clean, open-graded, large crushed stone with no fines, and that cleanness is non-negotiable. This layer does two jobs. It stores the water in the voids between stones, and it spreads the wheel loads down to the soil. A common buildup uses a large open-graded stone for the deep reservoir or subbase, a smaller open-graded stone above it as a base, and a still-smaller choker or bedding stone right under the surface so the top layer does not fall into the gaps below. The reservoir stone is often a coarse uniform grade and the bedding a smaller uniform grade, but the gradations are set by the design.

The word that matters is clean. Open-graded means single-sized angular stone washed free of fines, because fines are what hold water by capillarity and clog the voids. Dense-graded base, the well-graded mix with fines that you compact to 95 percent Proctor under a normal road, is exactly wrong here. It would carry load fine and drain almost nothing. This is the sharpest break from the base and subgrade compaction guide: there you build a dense, tight, well-compacted structural base, and here you build a clean, open, single-sized stone tank.

Reservoir depth comes from the storage you need and the void ratio of the stone, balanced against the structural depth the loads require. Both have to be satisfied at once. Take the depth, the stone gradations, and the void ratio from the civil and geotechnical design and the spec, and confirm the delivered stone is washed and matches the gradation before any of it goes in the ground.

Do not over-compact the subgrade

On a permeable section, the subgrade has to keep infiltrating, so you do not proof-roll it and you do not compact it the way you would under a normal pavement. This is the single most important difference between this work and everything in the base and subgrade compaction guide, and it is the one crews get wrong out of habit. Roll the bottom of an infiltration system to 95 percent density and you have sealed the floor of the tank. The water now has nowhere to go and the whole system fails before it ever sees a storm.

Keep equipment off the open subgrade. The infiltration rate depends on the natural, undisturbed soil structure, and a loaded machine tracking across saturated clay smears and densifies it in a single pass. The usual practice is to excavate to grade with the lightest practical equipment working from outside the footprint or on a stone working platform, leave the subgrade in a scarified or undisturbed condition, and place the reservoir stone without a compaction effort beyond what is needed to seat it. If the bottom does get trafficked or rained-in and pugged, the soil may need to be ripped and the infiltration retested, because a smeared subgrade does not recover on its own.

Here is the tension to flag to the geotech early. Heavy loads want a compacted subgrade for support, and infiltration wants it loose. When the structure genuinely needs compaction, the answer is usually to compact and then drain the system with an underdrain rather than rely on the soil, or to use a thicker reservoir. That tradeoff is a design decision. Confirm with the geotechnical engineer how much, if any, compaction the subgrade gets, and never default to the road-building reflex of rolling it tight.

How does the native soil decide the design?

The infiltration rate of the native soil decides whether the system fully infiltrates, partly infiltrates, or just detains and filters before draining. This is the first number the design needs, and it comes from a field infiltration test in the actual subgrade, run by or for the geotechnical engineer. The pavement surface and the stone are the same in all three cases. What changes underneath is whether you need a pipe to get the water out.

A common framework, and one you should treat as illustrative rather than universal, runs roughly like this. Where the measured soil rate is high, often cited above about 0.5 inch per hour, the reservoir can drain by infiltration alone and no underdrain is used: full infiltration. Where the rate is moderate, in the rough range of 0.1 to 0.5 inch per hour, the soil still takes some water but not fast enough, so an underdrain handles the rest: partial infiltration. Where the soil is tight, below roughly 0.1 inch per hour, or where infiltration is not allowed because of contamination, a high water table, or proximity to a foundation, the section is lined and fully underdrained so it works as detention and water-quality treatment only.

Do not lift those breakpoints onto your job. The thresholds, the safety factor applied to the tested rate, the required drawdown time, and the separation to bedrock or seasonal high water table are all set by the geotechnical report, the stormwater authority, and the design spec. Get the infiltration test done in the right spot and at the right depth before the section is designed, because that one measurement drives the entire layout.

The underdrain and the overflow

An underdrain is a perforated pipe in the stone reservoir that carries off the water the soil cannot take fast enough. You use one whenever the soil is too tight for full infiltration, or whenever the design needs a guaranteed way to empty the reservoir within the allowed drawdown time. The pipe is the safety valve that keeps the stone from staying full and the surface from backing up.

Where the underdrain sits in the reservoir controls how the system behaves. Set high in the stone, it lets the lower reservoir fill and infiltrate the soil first, only carrying off the overflow above that level, which preserves infiltration credit while bounding how full the system gets. Set at the bottom, it drains the whole reservoir to the pipe and the system works mostly as detention. The elevation, pipe size, outlet, and any internal weir or upturned elbow are design details, so build the underdrain to the drawing and the invert on it, not to a generic detail.

Every permeable section also needs an overflow path for the storm that exceeds the reservoir. That is usually an outlet structure, a raised inlet, or an edge that lets the surplus leave without flooding, and it is on the civil plan. Confirm it exists and is built to grade. A reservoir with no overflow surcharges in the storm it was never sized to hold.

Geotextile and keeping fines out of the stone

Geotextile separation is there to stop soil fines from migrating up into the clean reservoir and clogging it from below, and whether and where to use it is a design decision with real opinions on both sides. Fines are the enemy of the whole system. A reservoir that fills with silt from the surrounding soil loses its storage and its drainage just as surely as a clogged surface does.

The common practice is a non-woven geotextile on the sides of the excavation, and on the bottom and around the underdrain where the subgrade soil could pump fines up into the stone. Some designs wrap the reservoir fully. Others deliberately leave the bottom open, or use a stone choke layer instead of fabric there, on the argument that a fabric on the infiltrating surface is one more thing that can blind off and choke the very infiltration you are trying to protect. Both positions are defensible and both show up in published specs.

Follow the detail. Whether the bottom gets fabric, a graded stone separation, or nothing, and what fabric class is called for, is the engineer's and the spec's call. If the detail uses fabric, lap it correctly and keep it from getting fouled with soil during placement. If the detail leaves the bottom open, do not add fabric there because it feels safer. You may be choking the infiltration the design is counting on.

Placing porous asphalt without closing the voids

Porous asphalt is placed much like dense-graded asphalt, with two habits inverted: you mind the binder drainage at temperature, and you do not over-roll. The open mix carries a thick binder film with little to hold it, so it is sensitive to heat. Too hot and the binder drains off the stone in the truck and at the paver. Get the mix delivery temperature, the laydown window, and the binder from the producer and the spec, and pave inside that window rather than the dense-mix range you are used to.

Compaction is where porous asphalt is won or lost. The mat needs enough rolling to lock the stone together and bond the lift, but the air voids are the product, and every extra pass crushes them closed. Standard practice is a static steel roller making a small, defined number of passes right behind the paver, with vibratory and rubber-tire rolling generally avoided because they knead the voids shut. There is no chasing density here. You roll to seat, not to compact, and then you stop.

Run a test strip and verify it drains before you commit the lot. Pave a panel with the planned mix, temperature, and roller pattern, then pour water on it and watch it disappear, or run the surface infiltration test on it. The test strip sets the roller passes that lock the mat without killing the permeability, and it tells you the mix and the temperature were right before the whole job is down. The general paving QC from the asphalt paving inspection guide still applies for joints, thickness, and segregation. The permeable twist is that density is not the acceptance criterion here. Drainage is.

Placing and curing pervious concrete

Pervious concrete is the least forgiving of the three to place, and the place it goes wrong is curing. The mix has almost no slump and very little water, so it stiffens fast, has a short haul window, and has to be placed and finished quickly before it dries out in the truck. Keep the haul short, discharge promptly, and place it fast. A mix that sat too long has used its water on hydration and will ravel no matter what you do at the surface.

Place it to a screed level, then consolidate lightly. The common method is to strike off slightly high and consolidate with a roller screed or a steel roller at low pressure, just enough to bond the paste to the stone without crushing the voids closed. Excessive pressure shuts the surface and you lose the permeability you came for. You do not trowel it, you do not float it smooth, and you do not steel-finish it. Over-finishing seals the top and turns a permeable slab into a leaky regular slab. Joints are usually rolled or cut early and shallow.

Then cure it under plastic, immediately. Because the surface is open and the water content is low, pervious concrete dries from the top fast and the raveling that follows is the classic failure. The standard move is to cover the slab with plastic sheeting within roughly the first 20 minutes of placement and keep it covered and sealed at the edges for a curing period commonly held at about 7 days. This is the step that gets shortchanged when the crew is tired and it is the step that decides whether the surface holds together. Take the cure time, method, and any minimum from ACI 522 and the project spec, and do not pull the plastic early.

Permeable pavers: the joints are the drainage

With PICP, the concrete units are solid and watertight, so the water gets in through the open joints between them, filled with small angular stone. The joints are not an afterthought to be filled with sand for lockup. They are the inlet for the entire system, and the small open-graded jointing stone, commonly a small uniform grade such as ASTM No. 8 or 9, is what lets water down while still interlocking the units.

The buildup under the pavers follows the same open-graded logic as the other two. Units sit on a thin open-graded bedding course of small clean stone, which sits on an open-graded base, which sits on the deep open-graded reservoir, all clean and free of fines. The bedding stone is a single small grade, not the screened sand you would use under standard pavers, because sand here would seal the path. Edge restraint matters more than on a normal patio because the open joints and open bedding give the units less lateral lock.

Lay the units, fill the joints with the spec stone, and seat them. The advantage of PICP is repair and verification. You can lift units to clean or rebuild a clogged area and reset them, which no monolithic surface allows, and the surface infiltration is easy to test on the finished joints. Take the unit, joint stone, bedding, and base details from the ICPI or CMHA guidance and the project spec, and keep construction sediment out of those open joints the same as any other permeable surface.

Keep sediment off it during construction

The fastest way to ruin a permeable pavement is to clog it with construction sediment before the job is even finished, and it happens constantly. Bare soil washing across a fresh porous surface, a stockpile staged on the new pavement, mud tracked in by trucks, all of it drives fines into the open voids that you cannot fully get back out. A section can be dead on infiltration on opening day, with nothing wrong but dirt.

Sequence the job to protect it. The drainage area uphill of the pavement should be stabilized and vegetated before the permeable surface is exposed to runoff, so the water reaching it is clean. Keep the reservoir and surface out of the construction traffic pattern, do not stage materials or park on it, and divert sediment-laden runoff around it, not across it. Where the permeable area must be opened before the site is stabilized, protect it and plan to clean or rebuild the top of the section at the end.

Protecting the subgrade is part of the same discipline. The same equipment that tracks mud onto the surface compacts the infiltrating soil if it drives on the open excavation. Fence the area off, keep it outside the limit of disturbance where you can, and treat the whole footprint as off-limits to traffic until the section is built and the upstream soil is locked down.

Why does permeable pavement stop draining?

Permeable pavement stops draining because the pores clog with sediment, and clogging is the number one failure mode of the entire category by a wide margin. The structure does not wear out the way a dense pavement ravels and cracks. It silts up. Fine sediment, dirt, organic debris, and tire and pavement dust work into the surface voids over time and choke the path the water used to take, and the infiltration rate falls year over year until the surface ponds like any sealed lot.

Where it clogs depends on what is around it. A lot ringed by bare soil, mulch beds, or heavy tree cover clogs faster because there is more fine material feeding onto it. The first sign is standing water that lingers after a storm on a surface that used to drain in seconds, and a common field rule is that water sitting on the surface for more than a day or two means the pores are clogging and the section needs cleaning.

Clogging is recoverable up to a point and permanent past it. Caught early, vacuum sweeping pulls the sediment back out of the surface voids and restores much of the infiltration. Left for years, the fines pack deep and even aggressive cleaning does not bring it all back, and the surface, or the top of the section, has to be rebuilt. The lesson is that maintenance is not optional upkeep. It is the thing that keeps the system from silting itself shut.

Does permeable pavement need maintenance?

Yes. Permeable pavement needs regular vacuum sweeping to keep the surface pores open, and skipping it is the most common reason these systems fail in service. A conventional lot can be neglected for years and still function. A permeable lot that is never cleaned silts up and stops draining, and at that point it is a worse parking lot than a normal one because it ponds. The maintenance is the deal you make when you choose the section.

The core task is removing sediment from the surface before it works in deep. Regenerative-air or vacuum sweeping is the effective method, because it lifts fines out of the voids rather than dragging them around like a mechanical broom can. Frequency is driven by the sediment load on the surface, commonly a few times a year and more often where the surroundings shed a lot of fines, with cleaning triggered whenever drainage slows or water starts to linger. The surface infiltration test gives an objective trigger so you are not guessing.

Just as important is what you keep off it. No sand, ever, because sand is exactly the fine material that clogs it. No sealcoat, which would close the surface and defeat the whole point. No resurfacing or patching with dense-graded mix, which seals the spot it covers. And keep the surrounding landscape from washing soil and mulch onto it. The maintenance is not hard, but it is specific, and a maintenance crew running on parking-lot habits will sand it, seal it, or patch it dense and kill it without knowing they did.

Winter, de-icing, and plowing

Sand is off the table in winter because it clogs the surface, which is the opposite of what every other lot gets. Spreading winter sand on permeable pavement drives fines straight into the voids and is one of the fastest ways to ruin it. If traction grit is unavoidable for a storm, plan to vacuum it back off afterward, and treat that as a clog event, not routine.

The good news is that permeable surfaces often need less salt. Because the surface drains instead of holding a film of water, it does not glaze with the same sheet of ice, and field studies have found de-icing salt use can be cut on permeable pavements compared with conventional ones. Start lighter and adjust, rather than salting on the same schedule as the dense lot next door, and confirm any restrictions with the owner and the AHJ since chlorides can be a water-quality concern on an infiltrating system.

Plow it, but plow it clean and careful. Normal plowing is fine, and the open surface tends to clear meltwater faster, but do not pile dirty, sediment-laden snow on the permeable area to melt, because that dumps a season of fines into it as it goes. Push the dirty snow to a conventional surface or a designated storage spot and keep the permeable section clear for the clean melt.

Surface infiltration testing and acceptance

Surface infiltration is tested with a ring infiltrometer poured on the finished surface, and it is both the acceptance test for a new section and the diagnostic that tells you when to clean an old one. The common methods are ASTM C1701 for in-place pervious concrete and porous asphalt, and ASTM C1781 for permeable unit pavements like PICP. Both seal a ring to the surface, pour a known mass of water, and time how fast it goes down to give an infiltration rate.

On a new job, the test proves the surface actually drains at the rate the design assumed, which is the thing density and thickness numbers alone cannot tell you on a permeable mat. Run it on the test strip and on the finished work at the locations and pass rate the spec calls for. Published permeable specs often set a high minimum acceptance rate, and PICP guidance has cited a minimum on the order of 100 inches per hour for new work, but take the actual acceptance threshold, the number of tests, and the locations from the project spec and the AHJ.

Over the life of the pavement, the same test tracks clogging. A surface that tested high when new and now reads a fraction of that is telling you the pores are silting and it is time to vacuum, and a retest after cleaning confirms whether the infiltration came back. The void content of the mix, where it is verified, is a separate plant and design check, not the field surface test. Tie the acceptance numbers, the test method edition, and the frequency to the spec and the stormwater authority rather than to any figure quoted here.

What permeable pavement can carry

Permeable pavement is built for low-speed, lighter-duty traffic: parking stalls, drive aisles, walkways, plazas, overflow and fire lanes, not high-speed highway or heavy industrial loading. The open surface and the structure under it are weaker than a dense section of the same depth, and the design assumptions break down under heavy, channelized, fast traffic. Where heavy loads are unavoidable, that is a job for the engineer to size for, often with a thicker reservoir or a different surface, not a place to assume the standard section will hold.

Match the surface to the duty within that envelope. Pervious concrete and PICP handle turning and braking in parking areas well. Porous asphalt suits larger drive and parking areas where a paving crew is set up. Open grid systems are for the lightest use, like overflow parking and emergency access that sees a vehicle rarely. A common and sensible layout puts permeable pavement in the parking bays and stalls, where loads are light and slow, and keeps conventional pavement on the heavy main drives and truck routes.

The structural section, the loads, and the surface choice are an engineering call, the same as any pavement. The permeable twist is that you cannot simply compact your way to more capacity, because compacting the subgrade kills the infiltration. So added capacity tends to come from a thicker or stronger reservoir and surface, designed by the engineer, rather than from the density you would lean on under normal pavement.

Cost and lifecycle: where the money actually is

Permeable pavement usually costs more to install per square foot than a conventional section, and it can still come out cheaper for the project once the stormwater infrastructure it replaces is counted. The surface and the deep clean-stone reservoir cost more than a normal base and mat. What offsets it is the detention pond, the storm pipe, the inlets, and the land that the permeable section can shrink or eliminate, plus any value placed on the stormwater credit. Whether it pencils is a whole-site comparison, not a unit-price one.

The lifecycle cost has one line conventional pavement does not: mandatory cleaning. Budget the vacuum sweeping for the life of the pavement, because that recurring cost is the price of keeping the infiltration alive, and a section that is never cleaned fails early and gets rebuilt, which is the most expensive outcome of all. Properly maintained, the surface life is in the same general range as conventional pavement, with the reservoir below lasting much longer. Run the comparison over the full life with the maintenance in it, and let the engineer and owner weigh it against the stormwater works it offsets.

Records and the O&M plan the permit wants

Because permeable pavement is a permitted stormwater facility, the maintenance is usually not just good practice. It is a condition of the permit, and the authority can require a recorded operation-and-maintenance plan and proof that the cleaning is actually happening. Treat the documentation as part of the deliverable, not an afterthought, because an inspector can come back years later and ask for the inspection log.

Keep the design record and the maintenance record together. The design side is the infiltration test results, the as-built reservoir depth and stone gradations, the underdrain inverts, and the surface infiltration acceptance tests. The operation side is the cleaning schedule, each vacuum-sweep date, the surface infiltration readings over time, and any standing-water observations that triggered a cleaning. That history is what proves the system still works and what defends the owner if the authority asks.

A field tool earns its place here. Logging each inspection and cleaning, attaching the dated photo of the surface and the infiltration reading, and tying it to the specific lot is exactly the kind of recurring, provable record FieldOS is built to capture, so the O&M proof exists when the permit calls for it instead of being reconstructed from memory.

Common mistakes

• Compacting or proof-rolling the infiltrating subgrade like a normal pavement, which seals the floor of the system.
• Letting fines into the reservoir, from dirty stone, dense-graded base, or soil pumping up from below without the called-for separation.
• Allowing construction sediment to wash across or stage on the surface, clogging it before the job is even open.
• Skipping the vacuum-sweep maintenance, so the surface silts up and stops draining.
• Sanding or sealcoating the surface, both of which close the pores and defeat the whole system.
• Patching or resurfacing with dense-graded mix, which seals the spot it covers.
• Over-rolling porous asphalt or over-finishing pervious concrete, which crushes or closes the voids.
• Pulling the cure plastic off pervious concrete early, which causes raveling of the surface.

What to document

The record splits into what you built and how you keep it working, and a permitted stormwater facility needs both. Capture the as-built section and the acceptance tests at construction, then log every cleaning and infiltration reading over the life of the pavement so the permit obligation is provable.

Field checklist

Standards and references

Permeable pavement sits at the intersection of paving practice and stormwater design, so several bodies own different pieces, and the controlling document is almost always the project civil and geotechnical design with the local stormwater permit on top. The civil engineer sizes the reservoir and the storage, the geotechnical engineer provides the soil infiltration rate and the compaction stance for the subgrade, and the stormwater authority sets the design storm, the required capture or runoff reduction, and the credit. Those govern. Everything below is supporting guidance.

For the surfaces, the National Asphalt Pavement Association publishes design and construction guidance for porous asphalt and its open-graded mix. The American Concrete Institute covers pervious concrete in its Committee 522 report, ACI 522, including placement, consolidation, finishing, and curing. The Interlocking Concrete Pavement Institute, now under the Concrete Masonry and Hardscapes Association, publishes the construction guidance for permeable interlocking concrete pavement. Use each for its own surface.

Surface infiltration is tested under ASTM C1701 for in-place pervious concrete and porous asphalt and ASTM C1781 for permeable unit pavements. Aggregate gradations for the reservoir and bedding stone reference ASTM and the state DOT specs. The exact void targets, infiltration thresholds, reservoir sizing, acceptance rates, and any section or article numbers shift between editions and jurisdictions, so confirm them against the edition the engineer used and the stormwater authority adopted before citing them. Two rules carry across all of it: keep the surface and reservoir open-graded and clean, protect the infiltrating subgrade, and maintain the surface or it clogs. The design owns the numbers.

Units and terms

Permeable pavement carries a few interchangeable names and a handful of rates and ratios, and the same idea reads differently across a stormwater report, a paving spec, and a geotechnical memo.

Permeable and porous are often used interchangeably for the category, though porous tends to describe the asphalt and concrete where water passes through the body of the material, and permeable describes the pavers where water passes through the joints. Infiltration rates appear in inches per hour in US stormwater work and millimeters per hour in metric. Void content is a percentage of the material volume, and the reservoir void ratio, commonly near 40 percent, is the share of the stone layer available to store water.

Permeable / porous pavement

A surface that passes water through it into a stone reservoir; porous usually means through the body (asphalt, concrete), permeable through the joints (pavers)

Open-graded

Single-sized aggregate or mix with the fines left out so the voids stay open and connected

Stone reservoir

The clean open-graded stone layer under the surface that stores water in its voids and spreads load

Void content / void ratio

The share of a material or stone layer that is open space; the storage capacity of the reservoir

Infiltration rate

How fast water moves into the soil or through the surface, in inches or millimeters per hour

Underdrain

A perforated pipe in the reservoir that carries off water the soil cannot infiltrate fast enough

Drawdown time

How long the reservoir takes to empty after a storm, set by the design to be ready for the next one

PICP

Permeable interlocking concrete pavement, solid units with open stone-filled joints

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