Above and Below: Screeding aggregate reduces air voids and settles the aggregate, creating a flat and uniform base beneath the permeable pavers. Given that you and your crew are working in confined areas, flat pieces of lumber (shown here) can do the job because of their light weight and portability. Several other tool choices are available for as well, such as a titanium screed bar, which won?EUR??,,????'???t be susceptible to warping like wood. In addition, a rake-type screed bar will allow your crew members to screed while standing, thus eliminating any possible workers compensation claims due to constantly being hunched over. Because these tools are not perfectly flat, use a taut string to prevent pooling before and after the pavers are installed. Photos courtesy of Karl Bremer, Versa-Lok Retaining Wall Systems.

Permeable interlocking concrete pavement (PICP) occupies a major segment of the installation contractor's hardscape market. Comprised of materials such as porous asphalt, concrete, paving stones or bricks, the surface layers of pavers are separated by joints filled with small stones. During periods of precipitation, water enters the joints between the solid concrete pavers and flows through an open-graded base, i.e. crushed stone layers with no small or fine particles. The void spaces among the crushed stones store water and infiltrate it back into the soil subgrade. The stones in the joints provide surface permeability and the base filters stormwater and reduces pollutants.

A very important step when installing permeable pavers is to make sure to the drainage pipe is at the appropriate elevation to ensure proper flow. Here a standard perforated PVC pipe with a filter sock (to prevent buildup of sediment) is buried at a depth of 8 inches. If this were a commercial project the depth of the aggregate would increase due to the much heavier loads the paver would face.

Project Layout

When installing permeable pavers, site preparation must be done before excavation. First, the area to be removed should be marked with stakes located a slight distance away, so that they are not removed during excavation. The stakes should be marked to establish grades, or have string lines pulled and tied to them. Slopes should be a minimum of 1.5 percent. In the case of roads, the minimum longitudinal slope should be 1 percent with a minimum cross slope of 2 percent. Grade stakes should be checked periodically during the job to be sure that they have not been disturbed.

This 1,200 ft. residential project was completed in Afton, Minn, (in a flood plain) in the Mineapolis/St. Paul metro area. Taking around a week to complete, Aqua-Loc permeable pavers were installed by hand. Pavers can be placed in many patterns depending on the shapes. While not necessary on this lightly traveled driveway, Herringbone patterns (45 or 90 degree) are recommended in street applications, as these interlocking patterns provide the maximum load bearing support, and resist creep from starting, braking and turning tires. Chalk lines snapped on the bedding sand or string lines pulled across the surface of the pavers are used as a guide to maintain straight joint lines.

Drainage and Compaction

One worker uses a vibrating plate compactor to compact the permeable pavers, while another sweeps aggregate into the crevices between the blocks. To properly compact the pavers, he will make at least two passes over the course of an afternoon. Compaction equipment varies with the type of subgrade soil, so prior to getting to the job site, refer to the manufacturer to be sure you have the right tool for the type of soil you are dealing with.

One worker uses a vibrating plate compactor to compact the permeable pavers, while another sweeps aggregate into the crevices between the blocks. To properly compact the pavers, he will make at least two passes over the course of an afternoon. Compaction equipment varies with the type of subgrade soil, so prior to getting to the job site, refer to the manufacturer to be sure you have the right tool for the type of soil you are dealing with.

Drainage and Compaction

The stone and geotextile pipe assembly is placed along the pavement perimeter to remove excess water in the subgrade soil and base. The perforated pipe should be sloped and directed to outlets at the sides or ends of the pavement. The pipe outlets should be covered with screens to prevent animal ingress. Drainage is recommended in clay soils or other slow draining soils subject to vehicular traffic. Soil subgrade drainage extends pavement performance to the extent that the small additional investment is returned many times in additional pavement service years. Compaction of the soil subgrade is critical to the performance of interlocking concrete pavements. Adequate compaction will minimize settlement. Compaction should be at least 98% of standard Proctor density as specified in ASTM D 698. However, modified Proctor density (ASTM D 1557) is preferred, especially for areas under constant vehicular traffic. This compaction standard may not be achievable in extremely saturated or very fine soils. Stabilization of the soil subgrade may be necessary in these situations.

A worker sweeps aggregate into voids between pavers. Using and 1/4 ?EUR??,,????'??? 3/8 open graded stone aggregate (AASHTO standard: #8 and #9), he fills up the spaces roughly halfway to the top of the Aqua-Loc permeable pavers.

Moisture Content

Monitoring soil moisture content is important to reaching the compaction levels described above. Soil moisture and density measurements should be taken to control and verify the degree of compaction. The moisture content and compacted density of the subgrade soil should be checked for compliance to specifications before installing geotextiles.

This commercial project, a 14,000 sq. ft. ?EUR??,,????'??Park & Ride transit lot in Shakopee, Minn., took about a week to complete. Using a mechanical paving machine, that can unload an entire layer of pavers from a palate of at once, the workers were able to cover a large area in a relatively short amount of time. The grippers on the machine must be configured to be able to grab the many shapes of paving stones. The voids are filled with 1/4 to 3/8 inch open graded stone to about halfway from the top of the pavers.

Spreading and Compacting the Sub-base and/or Base Aggregates

Specifications typically used by cities, states or provinces for aggregate base materials under flexible asphalt pavements are adequate for interlocking concrete pavements. Spread and compact the base in 4 to 6 in. (l00 to 150 mm.) lifts. High force compaction equipment can compact thicker lifts. Consult with compaction equipment manufacturer for guidance. Frozen base material should not be installed, nor should material be placed over a frozen soil subgrade. The thickness of the base is determined by traffic, soil type, subgrade soil drainage and moisture and climate. Sidewalks, patios and pedestrian areas should have a minimum base thickness (after compaction) of 4 in. (100 mm.) over well-drained soils. Residential driveways on well-drained soils should be at least 6 in. (150 mm.) thick. In colder climates, continually wet or weak soils will require that bases be at least 2 to 4 in. (50 to 100 mm) thicker.

Placing and Screeding the Bedding Sand

Bedding sand under concrete pavers should conform to ASTM C 33 or CSA A23.1. This material is often called concrete sand. Masonry sand for mortar should never be used for bedding, nor should limestone screenings or stone dust. The bedding sand should have symmetrical particles, generally sharp, washed, with no foreign material. Waste screenings or stone dust should not be used, as they often do not compact uniformly and can inhibit lateral drainage of moisture in the bedding sand. ICPI Tech Spec 17-Bedding Sand Selection for Interlocking Concrete Pavements in Vehicular Applications provides additional guidance on selecting bedding sand. Bedding sand should be spread and screeded to a nominal 1 in. (25 mm.) thickness. Frozen or saturated sand should not be installed. If there is an uneven base (due to inconsistent compaction or improper grading), the bedding sand should not be used to compensate for it. Over time, unevenness in the bedding sand will reflect through to the surface. Uneven areas on the base surface must be made level prior to placing the bedding sand.

Once the base is complete, screed pipes or rails are placed on it and the bedding sand spread over them. The sand is screeded or smoothed across the pipes with a straight and true strike board. Screed pipes are removed and the resulting void filled with bedding sand. After the sand is screeded it should not be disturbed. Sufficient sand is placed and screeded to stay ahead of the placed pavers. Powered screeding machines that roll on rails and asphalt spreading machines adapted for screeding sand have been successfully used on larger installations to increase productivity.

This commercial grade Bio Aquifer Storm System (BASS) was designed to handle the weight of buses on a daily basis. Because of this, the permeable pavers sit on three layers of base. The top layer of bedding consists of 2 inches of 1.4 to 3/8 inch open graded stone (AASHTO No. 8-9). Underneath that is 4 inches of 3/4 to 1 inch open graded stone with no fines (AASHTO No. 57). Finally, the sub-base is made up of 1 foot of 1.5 to 3 inch open graded stone with no fines (AASHTO No. 1). Below the sub-base is a drainage pipe leading out to a large retention pond located adjacent to the parking lot.

Placing the Concrete Pavers

Buildings, concrete collars, inlets, etc., are generally not straight and should not be used for establishing straight joint lines. Joint widths between the pavers should be consistent and be between 1/16 and 3/16 in. (2 and 5 mm.). Some pavers are made with spacer bars on their sides. These maintain a minimum joint width, allowing the sand to enter between each unit. Pavers with spacers are generally not placed snug against each other since string lines guide consistent joint spacing. Cut pavers should be used to fill gaps along the edge of the pavement. Pavers should be cut with a double bladed splitter or a masonry saw to achieve a smooth cut.

Gaps less than 3/8 in. (10 mm.) should be filled with sand or filled by shifting courses of pavers. After an area of pavers is placed, it should be compacted with a vibrating plate compactor, which should be capable of exerting a minimum of 5,000 lbs. (22 kN) of centrifugal compaction force and operate at 75-90 hertz. At least two passes should be made across the pavers to seat the pavers in the bedding sand and force it into the joints at the bottom of the pavers.

Dry joint sand is swept into the joints and the pavers compacted again until the joints are full. This may require two or three passes of the plate compactor. If the sand is wet, it should be spread to dry on the pavers before being swept and compacted into the joints. Joint sand may be finer than the bedding sand to facilitate filling of the joints.

Bedding sand also can be used to fill the joints, but it may require extra effort in sweeping and compacting. Compaction should be within 6 ft.(2 m.) of an unrestrained edge or laying face. All pavers within 6 ft. (2 m.) of the laying face should have the joints filled and be compacted at the end of each day. Excess bedding sand is then removed. The remaining uncompacted edge can be covered with a waterproof covering if there is a threat of rain. This will prevent saturation of the bedding sand, minimizing removal and replacement of the bedding sand and pavers.

Final surface elevations should not vary more than + 3/8 in. (+10 mm) under a 10 ft. (3 m.) straightedge, unless otherwise specified. Bond or joint lines should not vary ???????? 1/2 in. (15 mm.) over 50 ft (15 m.) from taut string lines. The top of the pavers should be 1/8 to 3 /8 in. (3 to 10 mm.) above adjacent catch basins, utility covers or drain channels, with the exception of areas required to meet ADA design guideline tolerances. The top of the installed pavers may be 1/8 to 1/4 in. (3 to 6 mm.) above the final elevations to compensate for possible minor settling. A small amount of settling is typical of all flexible pavements. Optional sealers or joint sand stabilizers may be applied.

Source: Interlocking Concrete Pavement Institute, www.icpi.org

Advice From an Expert

Chuck Taylor general manager of Advanced Pavement Technology, has over 25 years of experience in the installation of flexible pavement systems and has worked in the construction industry for over 35 years. As a proponent for permeable pavement use, he has been a design consultant to engineers, architects and manufacturers and continues to provide seminars and training for permeable pavement systems across the country. LCN spoke to Taylor to get his input on permeable paver installation.

When it comes to installing porous pavers, ?EUR??,,????'??the tools don?EUR??,,????'???t change with open graded versus sand graded aggregates,?EUR??,,????'?? said Taylor. Open graded aggregates, with their large porosity, are preferred for drainage over their sand-graded counterparts. Because they are better able to efficiently handle storm water, Taylor feels that open graded is the better method of construction.

Creating Edge Restraint
?EUR??,,????'??Contractors in southern climates use a concrete tow method for edge restraint,?EUR??,,????'?? said Taylor. ?EUR??,,????'??But, this does not change how they build their system.?EUR??,,????'??

On the other hand, contractors in northern climates have different rules. The runoff they normally experience takes place below the surface. ?EUR??,,????'??Because they are dealing with clay soils,?EUR??,,????'?? said Taylor, ?EUR??,,????'??they have use a PVC edge restraint with a spike, which will take the water to a rain garden or a cistern for water collection.?EUR??,,????'??

?EUR??,,????'??They need to look at the water runoff,?EUR??,,????'?? said Taylor.

Clay soil is a much different beast than sandy soil, and therefore requires a pipe to collect the water.

If the subgrade soils are unsuitable for infiltration a series of drainage laterals or collector pipes need to connect to either an underground water storage system or to a storm water drain. Many factors contribute to correct permeable pavement design, so utilizing an engineer may be necessary. Photos courtesy of Chuck Taylor, Advanced Pavement Technology

If the grid paving is to function as a permeable pavement, the base material should consist of a minimum of eight inches of open graded base, preferably conforming to ASTM C 33 No. 57 gradation. This base material is installed in four to eight inch lifts and compacted with a static roller.

Crushed stone is placed, leveled and roller compacted. The paver units are then placed directly on the base and can be vegetated or backfilled with aggregate.

The most important part of paver installation is establishing a starting point for paving. Paver bond lines should run perpendicular to the concrete slab at the garage front. For a simple installation with a straight sight line to the street, a string line pulled to driveway end to the street becomes the line for creating a 90????? reference line for the pattern to follow.

Pavers are fully compacted twice on the job site. First, the bond lines are adjusted for consistency and alignment, the surface swept clean and compacted without joint sand to create an initial interlock.This vibratory compactor weighs 195 lbs. and has a vibration frequency of 5,600 vibrations per minute.

After compaction is complete, spread and sweep the crushed stone into the crevices and then compact the pavers again. By sweeping away excess aggregate, you are ensuring that it won?EUR??,,????'???t hang up in the joints and reduce interlock.