1 Scope
NOTE This standard covers the design, materials, construction, and acceptance testing of permeable pavement surfaces that allow stormwater to infiltrate through or between the pavement into a subsurface reservoir aggregate base and/or the native subgrade. (1.1)
NOTE Four primary surface types are addressed, each sharing the same three-layer logic (surface course, bedding/setting layer, reservoir base) but differing in materials and placement: (1.2)
- Pervious concrete (PC) — a near-zero-slump portland-cement concrete with interconnected voids, placed and finished by specialty crews.
- Porous asphalt (PA) — an open-graded asphalt wearing course over an open-graded aggregate reservoir.
- Permeable interlocking concrete pavers (PICP) — solid concrete units set on washed stone bedding with open, aggregate-filled joints.
- Concrete grid pavers (CGP) — open-cell concrete frames filled with aggregate or turf, used mainly for fire lanes, emergency access, and overflow parking.
NOTE Three hydrologic modes are recognized and selected from subgrade soil permeability and site constraints: full infiltration (all water enters the subgrade), partial infiltration (underdrains with a raised outlet hold water for infiltration before overflow), and no infiltration (impermeable liner with full underdrain drawdown over impermeable or contaminated subgrades). (1.3)
NOTE The scope includes the surface course, the bedding or setting layer, the reservoir aggregate base, the optional geotextile separation layer, optional perforated underdrain piping, and overflow/outlet control structures. (1.4)
1.5Governing local stormwater authority criteria control where they are more stringent than this standard.
NOTE Where a Municipal Separate Storm Sewer System (MS4) NPDES permit, a state stormwater manual, or a local ordinance prescribes a specific best-management-practice (BMP) design standard, that criterion supplements or supersedes the generic requirements here. (1.6)
1.7Hydrologic sizing, contributing-area routing, and outlet connection to the site stormwater network shall be coordinated with the project civil engineer and the design shown on the Stormwater Management Systems drawings. 2 Referenced Standards
2.1Materials, design, construction, and testing shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
2.2Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
NOTE The governing local or state stormwater BMP design manual, where one applies to the project, takes precedence over the generic design criteria in the standards below. (2.3)
| Standard |
Title |
| ASCE/T&DI/ICPI 68-18 |
Permeable Interlocking Concrete Pavement |
| ASTM C936/C936M |
Solid Concrete Interlocking Paving Units |
| ASTM C1781/C1781M |
Surface Infiltration Rate of Permeable Unit Pavement Systems |
| ASTM C1701/C1701M |
Infiltration Rate of In Place Pervious Concrete |
| ASTM C1754/C1754M |
Density and Void Content of Hardened Pervious Concrete |
| ASTM C1645/C1645M |
Freeze-thaw and De-icing Salt Durability of Solid Concrete Interlocking Paving Units |
| ACI 522R-10 |
Report on Pervious Concrete |
| ACI 522.1-13 |
Specification for Pervious Concrete |
| ASTM D3385 |
Infiltration Rate of Soils in Field Using Double-Ring Infiltrometer |
| ASTM D7928 |
Particle-Size Distribution of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis |
| EPA 832-F-99-023 |
Stormwater Best Management Practice: Porous Pavement |
| NAPA IS 131 |
Porous Asphalt Pavement for Parking Lots |
| AASHTO M 43 |
Sizes of Aggregate for Road and Bridge Construction (No. 2, 3, 4, 5, 57, 8, 9 gradations) |
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following action submittals for review before commencing work:
- Product data for each surface course, paving unit, bedding aggregate, reservoir aggregate, geotextile, and underdrain pipe.
- Pervious concrete or porous asphalt mix design with target void content, aggregate gradation, and admixtures.
- Aggregate gradation reports for bedding, joint-fill, and reservoir stone with washed-stone certification.
- Shop drawings showing the pavement section, layer thicknesses, slopes, underdrain layout and invert/outlet elevations, edge restraints, and maintenance access locations.
- Manufacturer data confirming paving-unit compressive strength and, in freeze-thaw climates, freeze-thaw durability.
- Contractor qualification records (see Quality Assurance).
☐ Product data (surface, units, aggregates, geotextile, underdrain)
☐ Mix design (PC/PA) with target void content and gradation
☐ Aggregate gradation and washed-stone certification
☐ Shop drawings (section, slopes, underdrain, edge restraint, access)
☐ Paving-unit strength and freeze-thaw durability data
☐ Contractor qualification records
3.2.1The Contractor shall submit the following informational submittals:
- Subgrade infiltration test results (double-ring infiltrometer per ASTM D3385) confirming the design hydrologic mode.
- Mock-up acceptance record for the demonstration panel.
- Construction-phase sediment-protection plan for the pavement area.
☐ Subgrade infiltration test results (ASTM D3385)
☐ Mock-up acceptance record
☐ Construction-phase sediment-protection plan
3.3 Closeout Submittals
3.3.1The Contractor shall submit the following closeout submittals before final acceptance:
- Surface infiltration acceptance test reports (ASTM C1701 for PC, ASTM C1781 for PICP/CGP).
- Pervious concrete void-content test reports (ASTM C1754) where pervious concrete is used.
- Record drawings showing as-built layer thicknesses, underdrain inverts, and maintenance-access locations.
- Operation and maintenance manual including the required vacuum-sweeping schedule.
☐ Surface infiltration acceptance reports (C1701 / C1781)
☐ Void-content test reports (C1754, pervious concrete)
☐ Record drawings (thicknesses, inverts, access)
☐ Operation and maintenance manual
4 Quality Assurance
4.1Pervious concrete shall be placed by a crew under the direct supervision of a technician holding current NRMCA Pervious Concrete Contractor Certification (PCCP) or a documented equivalent.
NOTE Pervious concrete is placed and finished without traditional finishing tools and at near-zero slump. (4.2)
4.3Troweling, vibrating, and over-rolling the pervious concrete surface shall not be done.
4.4Porous asphalt shall be placed by a contractor with documented experience producing and placing open-graded asphalt wearing courses.
4.5PICP shall be installed by a contractor with documented experience installing permeable interlocking concrete pavement of comparable scale, or holding an ICPI PICP Specialist credential.
4.6The Contractor shall construct a demonstration mock-up of each surface type, of an area agreed with the Engineer, on the actual reservoir base before full production.
NOTE The mock-up establishes the accepted appearance, surface texture, joint fill, and infiltration performance for the work. (4.7)
4.8The mock-up shall remain in place as the standard of comparison until the work is accepted.
4.9The mock-up may be incorporated into the permanent work if it meets all acceptance criteria.
4.9.1Contractor Qualification
● NRMCA PCCP certified supervisor on site
○ Documented equivalent pervious concrete experience
○ Not applicable (no pervious concrete on project)
○ ICPI PICP Specialist credential
● Documented comparable PICP installation experience
○ Not applicable (no PICP on project)
5 Environmental and Service Conditions
5.1 Surface Type Selection
NOTE Surface type is selected from traffic loading, aesthetics, freeze-thaw exposure, and budget; all four types share the same reservoir-base design approach, so the choice is driven by the surface course rather than the storage layer. (5.2)
NOTE Concrete grid pavers are a niche variant used primarily for fire lanes, emergency access, and overflow parking, where load-bearing capacity is needed only intermittently and a vegetated or aggregate-filled surface is acceptable. (5.3)
○ Pervious concrete (PC)
○ Porous asphalt (PA)
● Permeable interlocking concrete pavers (PICP)
○ Concrete grid pavers (CGP)
○ Pedestrian / plaza only
● Passenger vehicles (parking, driveways)
○ Light trucks
○ Heavy trucks / loading areas
5.4 Hydrologic Design Mode
NOTE The hydrologic mode is the single most consequential design decision, and it is governed by the measured infiltration capacity of the native subgrade, not by assumption. (5.5)
5.6Subgrade infiltration shall be measured by a double-ring infiltrometer test in accordance with ASTM D3385 at the proposed subgrade elevation before the hydrologic mode is finalized.
5.7Subgrade soil shall additionally be characterized by particle-size distribution per ASTM D7928 where the double-ring result is marginal or the soil is fine-grained.
5.8Full infiltration mode shall be used only where the native soil saturated hydraulic conductivity is at least 0.27 in/hr; soils below this threshold, or sites within 2 ft of the seasonal high groundwater table, shall use partial or no-infiltration mode.
NOTE Assuming full infiltration on a clay subgrade without testing is the most common design error in permeable pavement. (5.9)
5.10The hydrologic mode shall be based on the measured ASTM D3385 result; assuming full infiltration without testing shall not be done.
5.11No-infiltration mode with an impermeable liner and full underdrain drawdown shall be used over contaminated subgrades, over expansive or collapsible soils, or where infiltration is prohibited by the geotechnical engineer or local authority.
○ Full infiltration (no underdrain)
● Partial infiltration (underdrain with raised outlet)
○ No infiltration (impermeable liner, full underdrain)
Per drawings (deferred by default)
5.12 Setbacks and Contributing Area
NOTE Permeable pavement is sized for direct precipitation plus a limited run-on area; a maximum contributing impervious-to-permeable surface ratio of 2:1 is a common BMP rule, and routing a large impervious catchment onto a small permeable surface overwhelms the reservoir and causes failure. (5.13)
5.14The contributing impervious drainage area routed onto the permeable surface shall not exceed the ratio established by the governing stormwater authority, and in no case shall it exceed 2:1 (impervious to permeable) without specific design justification.
5.15Full-infiltration systems shall be set back a minimum of 10 ft horizontally from building foundations.
5.16Permeable pavement shall be set back from potable water wells by the distance required by the local authority, and in no case less than 50 ft.
5.17Sediment pre-treatment — a vegetated filter strip, gravel diaphragm, or inlet sump — shall be provided at upgradient edges that receive run-on, to reduce sediment loading and extend service life.
NOTE Permeable pavement located beneath concentrated tree canopy receives leaf litter and organic debris that accelerate surface clogging. (5.18)
5.19A vegetated buffer or pre-treatment filter strip should be provided to intercept leaf litter and debris where concentrated tree canopy overhangs the pavement.
0:1 (direct rainfall only)
1:1
2:1 (maximum without justification)
5.20 Freeze-Thaw Exposure
NOTE Open-graded reservoir aggregate drains freely and is generally frost-tolerant, but the surface course itself must resist freeze-thaw and de-icing-salt damage in cold climates. (5.21)
5.22In freeze-thaw climates, PICP units shall be qualified to ASTM C1645 with a maximum 1% mass loss after 50 freeze-thaw cycles in de-icing solution.
NOTE Winter sand applied for traction fills joints and surface voids and is a leading cause of premature clogging. (5.23)
5.24In freeze-thaw climates, an anti-icing or pre-wetting strategy should be specified in preference to sanding.
5.25Any sand applied for traction shall be removed by vacuum sweeping promptly after the event.
● Freeze-thaw climate (ASTM C1645 required for PICP)
○ Non-freezing climate
6 Surface Course
6.1 Pervious Concrete
NOTE Pervious concrete derives its permeability from a high interconnected void content created by a near-zero-slump, gap-graded mix with little or no fine aggregate; void content and infiltration rate, not compressive strength, are the controlling acceptance properties. (6.2)
6.3Pervious concrete void content shall be 15% to 25%, with a target of 18% to 22% for most applications, verified per ASTM C1754.
6.4Pervious concrete compressive strength shall be a minimum of 2,500 psi for vehicular applications.
6.5Pervious concrete shall achieve an as-placed surface infiltration rate of at least 100 in/hr when tested per ASTM C1701.
6.6Pervious concrete shall be cured under plastic sheeting for a minimum of 7 days.
NOTE Surface drying during curing ravels the paste and weakens the bond at the void walls. (6.7)
25004000
Default: 2500 psi
6.8 Porous Asphalt
NOTE Porous asphalt is an open-graded asphalt wearing course whose interconnected voids carry water vertically into the reservoir; its mix is gap-graded with a polymer-modified or fiber-stabilized binder to resist binder drain-down during placement. (6.9)
6.10Porous asphalt surface void content shall be 15% to 20%.
6.11The porous asphalt wearing course shall be 3 in. to 4 in. compacted thickness.
6.12Porous asphalt shall be compacted with a static steel-wheel roller; vibratory compaction crushes the aggregate skeleton and closes the surface voids and shall not be used.
6.13 Permeable Interlocking Concrete Pavers
NOTE PICP infiltrates between the units rather than through them; water passes through open, aggregate-filled joints into a thin washed-stone bedding course and on into the reservoir, so joint width, joint-fill gradation, and bedding material control the infiltration rate. (6.14)
6.15PICP units shall be solid concrete interlocking paving units conforming to ASTM C936 with a minimum compressive strength of 8,000 psi.
6.16PICP units shall be 80 mm thick for vehicular applications.
6.17PICP units 60 mm thick may be used in pedestrian-only areas not subject to vehicular loading.
6.18Joint openings shall be 3/16 in. to 1/2 in. wide and filled with clean, washed ASTM No. 8 or No. 9 aggregate.
6.19Sand shall not be used as bedding or joint fill in any permeable pavement layer.
NOTE Sand migrates into the reservoir voids and causes rapid, irreversible clogging and loss of infiltration capacity. (6.20)
6.21The PICP bedding course shall be 2 in. of washed ASTM No. 8 stone placed over the reservoir base; no sand setting bed is permitted.
● 80 mm (vehicular)
○ 60 mm (pedestrian only)
800011000
Default: 8000 psi
0.18750.5
Default: 0.25 in
● ASTM No. 8 washed stone
○ ASTM No. 9 washed stone
6.22 Concrete Grid Pavers
NOTE Concrete grid pavers are open-cell concrete frames whose cells are filled with washed aggregate or topsoil and turf; they spread intermittent vehicle loads while leaving most of the plan area open to infiltration, and are intended for fire lanes, emergency access, and overflow parking rather than continuous traffic. (6.23)
6.24Concrete grid paver cells shall be filled with clean washed aggregate or with a topsoil/turf medium as shown for the application.
6.25Grid pavers shall be set on a washed-stone bedding course over the reservoir base, with no sand setting bed.
● Washed aggregate fill
○ Topsoil and turf fill
7 Reservoir Base and Bedding
NOTE The reservoir base performs two jobs at once — it supports the traffic load structurally and it stores stormwater in its open voids — and its depth is set by the thicker of the two demands. (7.1)
7.2The reservoir base shall be clean, washed, open-graded crushed stone with a minimum void ratio of 40%.
7.3Reservoir aggregate shall be the AASHTO gradation appropriate to the surface type: AASHTO No. 57 for pervious concrete, AASHTO No. 2 or No. 3 for porous asphalt, and AASHTO No. 2, No. 3, or No. 4 for PICP.
7.4The reservoir base depth shall be sized to store the design storm runoff volume required by the governing stormwater authority (commonly the 1-year or 2-year, 24-hour storm) and to satisfy the structural thickness for the traffic class, whichever is greater.
7.5Reservoir aggregate shall not be choked with fines or stone dust.
NOTE Only clean washed stone maintains the void ratio and the structural angularity that carry load and store water. (7.6)
7.7Open-graded reservoir aggregate shall be compacted with a vibratory plate compactor in lifts.
7.8Conventional Proctor-based density specifications shall not be used as the acceptance basis for single-size open-graded stone.
NOTE Conventional Proctor-based density specifications do not apply to single-size open-graded stone, because the test method assumes a continuous particle-size distribution that is absent in open-graded aggregate. (7.9)
7.10The number of compaction passes per lift shall be established on the demonstration mock-up and held for production.
○ AASHTO No. 57 (pervious concrete)
○ AASHTO No. 2 (porous asphalt / PICP)
● AASHTO No. 3 (porous asphalt / PICP)
○ AASHTO No. 4 (PICP)
8 Geotextile Separation
NOTE A non-woven geotextile at the subgrade interface prevents fine subgrade soil from pumping up into the reservoir voids and clogging the storage; it is specified where the subgrade carries appreciable fines and omitted where the subgrade is clean and granular, since an unnecessary fabric can itself become a clogging plane. (8.1)
8.2A non-woven geotextile separation layer shall be provided at the reservoir-base-to-subgrade interface where the subgrade soil contains appreciable fines, as determined from the ASTM D7928 gradation.
8.3Where a geotextile is provided, it shall be lapped a minimum of 12 in. at all seams and turned up at the perimeter against the edge restraint.
8.4Geotextile shall not be placed between the surface course and the reservoir base, where it would obstruct vertical infiltration.
● Non-woven geotextile provided (fines present)
○ Omitted (clean granular subgrade)
9 Underdrains and Outlet Control
NOTE In partial- and no-infiltration modes, perforated underdrain pipe relieves the reservoir; its invert and outlet elevations set how much water is held for infiltration and how quickly the reservoir draws down, which in turn governs stormwater compliance. (9.1)
9.2Perforated underdrain pipe shall be provided in partial-infiltration and no-infiltration modes; full-infiltration systems do not require an underdrain.
9.3In partial-infiltration mode, the underdrain outlet shall be raised to hold 6 in. to 12 in. of water above the subgrade, so that water infiltrates the subgrade before it overflows through the underdrain.
9.4The reservoir shall draw down within 24 to 72 hours; 48 hours is the most common state stormwater BMP requirement and shall be the design basis unless the governing authority specifies otherwise.
NOTE Holding water indefinitely defeats both the structural and the water-quality function — a saturated reservoir cannot store the next storm and a permanently submerged subgrade loses bearing — so the drawdown time is a firm design constraint, not a target. (9.5)
9.6The underdrain pipe size, invert elevation, raised-outlet elevation, and the connection to the site stormwater collection or detention system shall be coordinated with the project civil engineer and shown on the Stormwater Management Systems drawings. ● Perforated (partial / no infiltration)
○ Solid outlet pipe only
○ None (full infiltration)
9.8 Impermeable Liner
NOTE In no-infiltration mode an impermeable liner isolates the reservoir from the subgrade so that all captured water leaves through the underdrain rather than infiltrating; the liner is used over contaminated, expansive, or otherwise unsuitable subgrades. (9.9)
9.10In no-infiltration mode, an impermeable liner shall be installed at the base and up the sides of the reservoir excavation, with all seams sealed continuous and watertight.
9.11The liner shall be protected from puncture by a cushion geotextile above and below where it bears on or is covered by angular reservoir stone.
○ HDPE geomembrane
○ Reinforced flexible membrane
● Not applicable (infiltration mode)
10 Geometry and Slopes
NOTE A minimum surface slope keeps an alternate drainage path available if the surface partially clogs, while a maximum slope keeps water from running off the surface and bypassing the reservoir before it can infiltrate. (10.1)
10.2The finished surface shall slope a minimum of 1% to maintain a positive overland drainage path if surface infiltration is reduced by clogging.
10.3The finished surface slope shall not exceed 5% for PICP.
10.4The finished surface slope shall not exceed 3% to 5% for pervious concrete and porous asphalt, as shown for the project.
10.5On sloped sites, the reservoir base shall be terraced or check-dammed so that stored water is retained across the slope rather than draining laterally to the low end.
10.6Edge restraints shall be provided at all PICP and grid-paver perimeters to resist lateral creep of the units and joint-fill loss under traffic.
10.7Surface grades, slope directions, and the limits of the permeable pavement shall be shown on the drawings. paving plan and grades 11 Testing
NOTE Acceptance testing confirms that the as-built surface actually infiltrates at the design rate; surface infiltration is the property that degrades in service and the one the owner will re-test over the system's life, so it is the primary acceptance metric. (11.1)
11.2Pervious concrete surface infiltration shall be tested in place per ASTM C1701 and shall meet or exceed the specified minimum (commonly 100 in/hr).
11.3PICP and concrete grid paver surface infiltration shall be tested per ASTM C1781 and shall meet or exceed the specified minimum (commonly 100 in/hr).
11.4Pervious concrete void content shall be tested on extracted cores or molded specimens per ASTM C1754 and shall fall within the specified range.
11.5Infiltration acceptance tests shall be performed at the frequency set for the project, distributed across the pavement area rather than clustered.
NOTE A distributed sampling plan is required because tests clustered in one clean location can mask a clogged zone elsewhere. (11.6)
11.7Any test location that fails the minimum infiltration rate shall be remediated by vacuum cleaning and re-tested, and shall be removed and replaced if it still fails.
○ ASTM C1701 (pervious concrete)
● ASTM C1781 (PICP / grid pavers)
1 per 5,000 ft2
1 per 10,000 ft2
1 per 15,000 ft2
12 Installation
12.1 Construction Sequencing
NOTE The reservoir base is uncovered storage for the entire construction period, and any sediment that washes into it clogs the voids permanently; protecting the surface and the open base until upgradient disturbance is stabilized is the difference between a working system and a failed one. (12.2)
12.3Permeable pavement shall not be placed until all upgradient soil disturbance contributing run-on to the pavement is permanently stabilized.
12.4Sediment-laden construction runoff shall be prevented from flowing across the completed permeable surface or into the open reservoir base by temporary protection such as straw wattles, silt fence, or geotextile covers.
12.5Final surface course installation shall be deferred until the surrounding site is stabilized, where the construction sequence allows.
12.6Equipment shall not track soil onto the reservoir base or the finished surface; any tracked sediment shall be removed by vacuum sweeping before it is worked into the voids.
12.7 Subgrade Preparation
NOTE The subgrade is prepared to receive and pass water, not to be sealed; over-compaction or rubber-tired rolling of the subgrade smears the soil surface and destroys the very infiltration the system depends on. (12.8)
12.9The subgrade shall be excavated to grade with equipment that does not over-compact the infiltrating surface, and shall not be proof-rolled with rubber-tired equipment in infiltration modes.
12.10The exposed subgrade shall be protected from sedimentation and from compaction by construction traffic until the geotextile or reservoir stone is placed.
12.11Where the subgrade has been smeared or compacted, it shall be scarified to restore infiltration before placing the overlying layers, except where an impermeable liner is specified.
12.12 Maintenance Access
NOTE Post-construction inspection and the cleaning that keeps the surface infiltrating both depend on physical access to the reservoir and the outlet; many MS4 permits require this access to be built in, and a system with no observation point cannot be documented as functioning. (12.13)
12.14Cleanout risers shall be provided on underdrain runs and brought to the surface for rodding and inspection.
12.15At least one observation well shall be provided per pavement cell to monitor the reservoir water level and confirm drawdown.
12.16The outlet control structure shall be accessible for inspection and maintenance.
12.17Maintenance-access locations — cleanouts, observation wells, and the outlet control structure — shall be shown on the drawings. maintenance access locations ☑ Underdrain cleanout risers
☑ Observation well per cell
☑ Accessible outlet control structure
13 Maintenance
NOTE Permeable pavement is a stormwater BMP with a service obligation, not a one-time installation; the dominant failure mode in service is gradual surface clogging from sediment and debris, and scheduled vacuum sweeping is what preserves the design infiltration rate over the system's life. (13.1)
13.2The surface shall be vacuum swept a minimum of twice per year — after the spring sanding season and in the fall — and more frequently where sediment loading is high.
13.3Sanding for winter traction shall be followed by vacuum sweeping promptly after each application event to recover joint and surface infiltration.
13.4The operation and maintenance manual shall record the sweeping schedule, the observation-well drawdown check procedure, and the surface infiltration re-test method so the owner can verify continued performance.
2 times per year
3 times per year
4 times per year
14 Delivery, Storage, and Handling
NOTE Aggregate cleanliness is the property most easily lost between the quarry and the reservoir; stockpiling washed stone on bare ground or letting it sit under runoff reintroduces the fines that the washing removed. (14.1)
14.2Washed bedding, joint-fill, and reservoir aggregate shall be delivered clean and shall be stockpiled on a clean, hard, well-drained surface, separated from soil and from other aggregates.
14.3Paving units shall be delivered banded on pallets and stored to prevent chipping and contamination of the units before placement.
14.4Geotextile and liner materials shall be stored protected from prolonged ultraviolet exposure and from puncture before installation.
15 Warranty
NOTE Early clogging traceable to construction sediment or defective materials is the Contractor's responsibility rather than the owner's maintenance burden. (15.1)
15.2The warranty shall cover defects in materials and workmanship and shall include the as-placed surface infiltration performance.
15.3The Contractor shall warrant the permeable pavement against defects in materials and workmanship for the period specified for the project, and in no case less than 1 year.
15.4The Contractor shall warrant that the surface infiltration rate measured at substantial completion is maintained, excluding reductions caused by owner failure to perform the specified maintenance.
16 Spare Parts
NOTE Replacement of individual clogged or damaged units is part of normal PICP and grid-paver maintenance, so a small attic stock of matching units lets the owner repair locally without re-tendering and without a color or texture mismatch. (16.1)
16.2For PICP and concrete grid paver projects, the Contractor shall deliver attic stock of each paver unit type, color, and thickness used, in the quantity specified for the project.
1% of installed units
2% of installed units
3% of installed units