Understanding hardscaping materials and tips on installation

Permeable interlocking concrete pavers (PICPs)

PICPs share an almost identical resemblance to ICPs, in that they are similar in size, texture, and colour with a wide variety of styles available. These pavers are used to reduce storm water runoff and reduce pollutants from entering waterways and drinking water aquafers.

They can also be used as a paving material for roadways, parking lots, and other large surfaces. As urban areas grow and large areas are paved with impervious materials, such as concrete and asphalt, water runoff is not able to infiltrate naturally into the native soil. Instead, water is channelled on the top surface of these paved areas to storm sewers. When normal and excessive amounts of precipitation occur, storm sewers may be beyond surge capacity, which leads to flooding, sewage backup, and contamination of waterways.

Integrating permeable pavers into a design will aid in the control of water runoff. The system works by allowing water to enter into the aggregate sub-base through gaps between the pavers. The water can naturally percolate into the soil below or be channelled through a drainage system and collected in subterranean storage tanks. Water that is collected can be used for many applications, including irrigation, fire prevention systems, or sanitized and used as fill water for pools or hot tubs.

Manufacturing

Just as they are almost identical in appearance, PICPs are manufactured the same way as ICPs. The major difference, however, is permeable pavers are made with spacers along their edges that when installed allow larger gaps or joints between the paver units. These gaps typically comprise five to 15 per cent of the paver surface area. This allows storm water runoff to infiltrate into the joints, which can percolate through crushed aggregates into the soil sub-grade.

Field installation

Installation methods for natural stone pavers and segmental concrete pavers are similar. Many factors should be considered when determining the proper installation method, such as the type of material selected, soil sub-grade, drainage, site-specific considerations, and budget. Referring to guidelines and installation procedures prepared by the Interlock Concrete Pavement Institute (ICPI) can be a valuable resource.

The following steps should be practised when installing natural stone and/or segmental concrete pavers. For natural stone pavers, the installation method is known as ‘dry-lay.’ When pavers are adhered to a concrete sub-base with mortar, it is known as a ‘wet-lay’ installation. Permeable pavers follow the same ‘dry-lay’ installation method, but require modified sub-grade preparation to handle the appropriate drainage and collection of water runoff.

1. Design, planning, and layout

A proper design of the project should be prepared taking into consideration the wishes of the homeowner, the intended use and functionality of the space, budget, and proper construction techniques.

Once a design has been established, it should be laid out by transcribing the design from paper to the actual area to be constructed. This will help the contractor and homeowner visualize the design in terms of how it will appear in the selected space. It also allows for any modifications prior to excavation. Utility services should be contacted so any underground services that may be located in or within close proximity to the project are not damaged.

2. Soil conditions and engineering

Before preparing the sub-grade for the project, it is important to be familiar with the soil conditions. If the contractor is unfamiliar with the soil conditions, it is a good idea to consult with other local contractors who are familiar with the area. The services of a geotechnical engineer may be required if limiting factors such as bedrock, excessive groundwater, or unstable soil is present.

A geotechnical engineer will provide a written report specifying the soil bearing capacity, as well as the appropriate depth of excavation, what types of engineered aggregate should be used, and the amount of compaction required.

3. Excavation and sub-grade preparation

The area that will support the pavers should be excavated to a depth based on the type of sub-grade soil that is present and the intended use of the area. The area should be free of loose, undisturbed soil and, depending on the sub-grade soil condition, may require compaction with a vibratory plate or roller.

A woven or non-woven landscape fabric should be installed over the entire excavated area. This fabric allows moisture to pass through and prevents the compacted aggregates from migrating into the soil underneath.

4. Granular aggregate placement and compaction

Granular aggregates are placed on the landscape fabric in lifts of 50 to 75 mm (2 to 3 in.) and compacted with a vibratory plate or roller to a minimum of 98 per cent standard Proctor density. The Proctor density test is used to determine the optimal moisture content at which point the soil will become most dense and achieve its maximum dry density. Compacting the granular aggregates causes the material to compress whereby filling all voids and releasing water and air to create a compact surface with increased bearing capacity.

A patio and walkway should have a base depth of 150 to 200 mm (6 to 8 in.) on a silt or clay sub-grade or 100 to 150 mm (4 to 6 in.) on a sand or gravel sub-grade. For driveways, the base depth should be 200 to 350 mm (8 to 14 in.) on a silt or clay sub-grade or 150 to 250 mm (6 to 10 in.) on a sand or gravel sub-grade. These figures may not be ideal for all situations or applications.

A thicker base depth may be required for areas that are prone to excessive freeze-thaw cycles, have unstable soil, or for commercial and industrial paver installations.

Using the ‘wet-lay’ installation method, after the granular aggregates have been properly placed and compacted, forms are set-up, and a concrete sub-base is poured over top of the aggregate base. The concrete should be reinforced with steel mesh or rebar to provide the necessary tensile strength, while also allowing the concrete to flex slightly without cracking or breaking during freeze-thaw cycles.