Understanding hardscaping materials and tips on installation

by jason_cramp | April 12, 2018 11:02 am

By Michael Willis

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Paved hardscape surfaces are an integral component of nearly all landscaping projects. With the homeowner’s desire to extend their interior living space into their yards and the ever-popular ‘staycation,’ the era of the basic paver patio has all but disappeared. Hardscaping has expanded far beyond the poolside perimeter decking or simple rectangular patio. The myriad of paver materials available is constantly evolving as manufacturers and suppliers try to keep up with the demand of current landscaping trends. Designers and contractors now have at their disposal a palette of materials in a range of sizes, shapes, colours, and textures.

With so many options available, it can be a daunting task for a homeowner to select the ideal paving material. A designer or contractor can ease this process by educating one’s self with knowledge of the different materials. This is a valuable asset, which will not only earn confidence in the eyes of the homeowner, but also present an opportunity for builders to increase sales.

Selecting a paver material can be simplified by separating the most common types into two categories.

1. Natural stone pavers.
2. Segmental concrete pavers (i.e. interlock concrete pavers [ICPs], segmental concrete paving slabs, and permeable pavers).

Natural stone pavers

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For centuries, natural stone has been used worldwide as a building material to construct architectural structures and pave roadways. Known for its durability, strength, and permanence, these pavers are one of the few building materials that have a timeless appearance. Not only valued for its structural use, natural stone is sought for its beauty, variation in colour, texture, and its natural occurring diversity.

Deposits of stone are quarried all over the world with materials originating from locations scattered across North and South America, Europe, the Middle East, and East and South Asia. Stone deposits vary from region to region with many variations unique to the specific geographic location from which they are found. This localized uniqueness is evident in the fact no two stones are alike. The appearance and colour of a stone can even vary from quarry to quarry located in close proximity to one another. Common types of stones used as natural stone pavers include granite, limestone, marble, sandstone, and slate.

Granite

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Granite is an igneous rock that is formed when magma slowly cools beneath the earth’s crust and solidifies into a crystalized stone. Although some stone materials may be labelled as granite, they are in fact not true ‘granite.’ To be classified as such, the stone must be composed of at least 20 per cent quartz and 35 per cent feldspar. The commercial stone industry tends to classify any stone with large visible crystals and grains as ‘granite’ to simplify categorization of materials to the consumer. Granite is known for its hardness, permanence, and enduring colour. It is also not susceptible to staining or discolouration. For these reasons, granite is a popular stone for use in interior applications such as countertops and flooring. It is also widely used in exterior applications such as wall cladding material and paver for patios.

Granite has large visible crystals and grains of blended minerals. Varieties of colours are available ranging from white, pink, green, beige, and brown. Stones that are black and grey in tone are actually classified as gabbro, but for commercial labelling, they are considered granite.

Granite deposits are found throughout the world with the largest exports originating from the U.S., China, Brazil, India, South Africa, and Italy.

Limestone

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Limestone is a sedimentary rock primarily composed of calcium carbonate in the form of the mineral calcite. It is formed when layers of minerals, fine sediment, and organic matter (primarily the skeletal remains of ancient marine organisms) are deposited and accumulated over time on the ocean floor.

It is an exceptionally strong, dense rock with well-consolidated pores. This enables most limestone to have a very low water absorption rate. As a result, this stone is very durable, resistant to abrasion, and able to withstand freeze-thaw conditions, which makes it an ideal paver stone. Although it is very dense, limestone is partially soluble in water and is quite reactive to acidic solutions. This is why limestone is easily eroded by waterways and rivers, and is degraded by acidic rain. Throughout the world, this water erosion created many natural caves and rock formations.

Limestone is an essential industry material and is used in more ways than any other stone. It is mainly used in construction as a crushed aggregate for road building and as an aggregate for concrete. Ground to a fine powder and heated with other materials, it is a key ingredient in the production of Portland cement, mortar, and other cementitious products. Lime is also created from the stone, which is used in many industries in the production of metals, household products, cosmetics, medicine, livestock production, and neutralizing acidic soils in farming.

Limestone is consistently found in homogenous blends of earthy tones of grey, beige, brown, black, and blue. Stone deposits may be of a singular consistent colour with little patterning. Other deposits may have a distinct veining or fleuri visible on the stone’s top surface. Impurities found in the stone such as clay, sand, organics, and iron oxides will cause the stone to exhibit various colours, as it is naturally weathered and exposed to the elements.

China is the global leader in limestone production followed by India. Limestone deposits are quite prevalent in North America with much of the production coming from the U.S. mid-west, and Canada. Northwestern Europe is also a major producer.

Marble

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Marble is a metamorphic rock that forms when limestone is subjected to extreme heat and pressure. During this metamorphosis, the primary mineral found in limestone calcite recrystallizes, transforming it to marble. Like its originating form of limestone, marble is a hard and durable stone that is valued for its architectural use, as it is the choice stone for most carved sculptures.

It is also widely used as an aggregate for construction. As a stone paver and flooring material, marble is a popular choice when a polished surface is desired.

This material is very light in colour with the most pure forms having white tones. Significant visible veining or streaks of colour can be seen throughout most marble surfaces. These streaks are the remaining mineral composition and layers from the source limestone. These minerals and other impurities present such as clay and iron will alter the stone colour with hues of blue, yellow, orange, red, pink, grey, and black.

Marble deposits are found throughout the world, but primarily in the U.S., China, Brazil, Italy, India, and Turkey.

Sandstone

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Sandstone is a sedimentary rock primarily composed of small grains of minerals, which have been weathered by the elements and transported by rain, wind, and ice. The deposited grains collect in layers and become compacted under pressure, while overlying deposits continue to accumulate. Various minerals that migrate into the voids or pores of the deposits cement the grains together.

Although sandstone is a relativity soft material, it yields high compression strength and has a low absorption rate making it ideal for construction and use in freeze-thaw conditions. It is highly resistant to weathering and tends to heighten in colour and appearance as the stone ages. Imported sandstone from Asia has become a popular paver material in North America. Variations of the material can closely blend in with many of North America’s local stone materials, but yield a more affordable price.

Deposits of sandstone can be found in a variety of colours with a single-source deposit offering numerous shades and veining. Iron oxides, manganese oxides, and other impurities can cause bright and contrasting colours in sandstones. Globally, sandstone can be found in a range of vibrant colours—from tan, brown, yellow, red, pink, white, grey, and black. In some cases, certain colours can be easily associated with their geographic source deposits, which are found globally, with primary deposits located in the U.S., India, China, Brazil, and Germany.

Slate

Slate is a metamorphic rock derived from deposits of the sedimentary rock shale. Shale is formed from layers of sediment; primarily silt and clay are deposited and compacted together. When shale is subject to heat and pressure it metamorphoses into the more dense and durable slate.

This material is most commonly used in flooring, paving stones, and as roofing tiles. A great attribute to slate is the ability to split the stone into layers, giving it a natural textured surface, yet remaining quite smooth and flat. Slate is virtually waterproof with a low water absorption rate making it impervious to freeze-thaw cycles.

This material does vary in colour; however, the most abundant is grey. Deposits of green, purple, blue, and black are also common.

Spain is the leading producer and exporter of natural slate materials followed by Brazil. Substantial deposits are also found in the eastern U.S., in a region known as the ‘slate belt,’ comprising the states of New York, Pennsylvania, Vermont, Maine, and extending into Canada in the province of Newfoundland. China and India have also become large exporters of this stone.

Manufacturing and quarrying

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The manufacturing and quarrying of natural stone pavers varies depending on the desired product and geographic source location. For the most part, operating quarries around the world function in a similar fashion. Quarries extract various stone materials, which are cut into slabs or cubes, which are then sent to a processing facility where they are cut or split into standard and custom sizes.

Standard sizes (length x width) range between 300 x 300 mm (11.8 x 11.8 in.) and 600 x 900 mm (23.6 x 35.4 in.). Most pavers are cut or split to a thickness between 25.4 mm (1 in.) and 50.8 mm (2 in.). A natural split stone may vary between 25 mm (0.98 in.) and 35 mm (1.37 in.) thick, while calibrated cut stones have a more exact tolerance of 25 mm (0.98 in.), plus or minus 1 mm (0.03 in.).

The variation of thickness depends on the intended application of the product and the desired appearance. Since the stone is quarried and cut to size, an almost limitless option for customization is possible. Post cutting, different finishing processes are applied to the stone to give the material its final texture and appearance. Common finishing processes include natural split, flamed, honed, sandblasted, bush-hammered, and antiqued.

Natural split

The stone is split in half leaving a natural un-cut surface. This texture is more rough and uneven, but gives the stone a natural appearance.

Flamed

A high-heat flame is passed over the stone surface, causing it to flake and spall, leaving a rough textured appearance. This process also removes any visible cutting marks remaining from manufacturing.

Honed

The stone’s surface is polished with a rotary grinder using a variety of coarse and soft sanding pads. After sanding, the stone surface remains smooth with a matte sheen.

Sandblasted

Abrasive sand blasting is used to remove any imperfections or cutting marks that remain on the stone from manufacturing. This process leaves the stone with a subtle, smooth-textured surface that is not slippery.

Bush-hammered

The stone surface is shaped (manually or mechanically) with a hammer that has many pyramidal points. The impact of the hammer leaves the surface texture of the stone with a consistent bumpy/stippled texture.

Antiqued

An abrasive rotary brush is passed over the stone’s surface, which removes the softer material, leaving behind the harder, more dense material. This finish gives the stone surface an aged appearance similar to rough leather or an orange peel.

Paving the way to modern landscaping

Paving roads and walkways with stone has been practiced since well before the Roman Empire. Most roadways were paved with natural stone until the ’50s post World War II. After the war, as Europe rebuilt, architects and engineers started to realize the benefit of using manufactured concrete pavers in place of natural stone pavers.

Concrete pavers are more easily produced, dimensionally consistent, and quite durable. Since the majority of pavers are installed on a compacted aggregate sub-base, they can be more flexible and resilient to freeze-thaw cycles. Further, thanks to decreased maintenance costs and availability, by the ’70s concrete pavers became the building material of choice. Initially, however, the selection was limited to a few shapes, sizes, and colours.

In the mid-’90s, as the landscaping industry saw rapid growth, concrete paver manufacturers started to offer far more variation in material styles, shapes, sizes, and colours. As technology progressed, manufacturers were able to replicate the appearance of natural stone pavers. These manufactured pavers not only shared the appearance of Mother Nature’s creation, but also its texture and colour. With relatively simple installation methods and affordable price points, segmental concrete pavers are by far the most popular choice for use as a paving material.

Most manufacturers offer lifetime guarantees with transferable warranties on the structural integrity of their paving stones. Pavers are also resistant to salt erosion and other staining. Modern segmental concrete pavers can be categorized into three groups: ICPs, segmental concrete paving slabs, and permeable pavers.

Interlock concrete pavers (ICPs)

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ICPs are a manufactured concrete product, which is used as a paving material for walkways, patios, and driveways. In Canada, they are classified as any manufactured stone that has a surface area less than or equal to 0.09 m² (0.96 sf) with a minimum thickness of 60 mm (2.36 in.).

These ‘interlock’ pavers are designed to fit together as a module in a variety of patterns and configurations. The combination of the concrete pavers, aggregate sub-grade, bedding sand, jointing sand, and edge restraint allows the pavers to interlock as a system.

As a single system, pavers are able to move independently while avoiding any cracking as the substrate shifts during freeze-thaw cycles. This flexibility also allows for the dispersal of weight from heavy loads such as vehicular traffic.

Manufacturing

In the past, ICP selection was limited to a few shapes and colours. Today’s manufacturers offer dozens of styles and laying patterns to choose from, which can complement any home’s architecture. Styles range from sleek, large-scale pavers for contemporary, modern-styled homes to those that are rustic in appearance and look like natural flagstone. Pavers are manufactured in thicknesses of 60 to 80 mm (2.36 to 3.14 in.) and in a multitude of sizes depending on the chosen product.

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In the ’90s, the tumbled paver revolutionized the ICP industry offering a more rustic natural appearance. This rough texture was initially a simple manufacturing process. Ordinary pavers were ‘tumbled’ through a large rotating drum, which would chip and distress the paver edges and corners giving the paver a cobblestone appearance. As manufacturing technology progressed, more elaborate pavers were being created with natural textures, special edge treatments, and finishes.

Most of today’s ICPs are manufactured in a ‘dry-cast’ process, which consists of combining fine aggregate materials with cement powder and compressing them under extreme pressure with the minimum required amount of moisture. This yields a denser, more durable product that is capable of withstanding heavy loads.

This process also provides precise control on the percentage of raw materials added during the manufacturing process. Manufacturers are able to use crushed marble, granites, and other small aggregates—in combination with natural colour pigments—to create pavers that look like natural stone. By controlling the colouring, other products such as retaining wall stone, copings, and caps can be manufactured to complement the pavers in the same colours and tones.

Segmental concrete paving slabs

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Segmental concrete paving slabs share many of the same characteristics and installation methods as ICPs. The most notable difference between the two is that concrete paving slabs are much larger. Paving slabs do not possess the same interlocking qualities as ICPs. Due to the lack of interlocking ability and the sheer size of the slabs, this material does not disperse weight as efficiently and, therefore, is more prone to cracking.

Segmental concrete paving slabs are classified as any manufactured stone that has a surface area greater than 0.09 m² (0.96 sf) with a minimum thickness of 30 mm (1.2 in.) and a maximum overall length or width of 1 m (3.3 ft).

Manufacturing

Segmental concrete paving slabs are manufactured using the same ‘dry-cast’ process used to make ICPs. Many manufacturers also use a ‘wet-cast’ process, which involves pouring a liquid mixture of cement, small aggregates, and colour pigments into a mould to cast the shape of the slab. These moulds are recreated from casting actual natural stones.

Hundreds of stones can be cast to recreate a random natural appearance with a realistic embossed surface texture. Granite, marble, and other natural ingredients can be used in the cement mixture to allow for a range in tones, veining, and textures.

Permeable interlocking concrete pavers (PICPs)

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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

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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.

5. Paver setting bed

Once the granular-aggregate base has been compacted, and has reached the desired height, a layer of fine concrete sand is spread over the entire patio area. The setting bed is then smoothened and graded to the desired thickness of approximately 25 mm (1 in.).

6. Installation of pavers

How the pavers are installed on the setting bed will vary depending on the type of paver and desired pattern. Some pavers follow a specific pattern, while others are installed at random, or in running bond, brick, or grid configurations. This applies to natural stone pavers and segmental concrete pavers.

Once the pavers are laid out in the desired pattern, areas where stones need to be trimmed to size are either split with a guillotine or cut with a powered masonry saw.

Using the ‘wet-lay’ installation method, a cement or mortar mix is applied to the desired thickness over the concrete sub-base first. Then, the stone is placed in the mortar mix and levelled into place. This method is more labour intensive and costly, but yields the most everlasting and premium finish.

7. Edging and filling joints

After the pavers have been positioned, an edging material is installed to help retain the pavers at the patio edge or border. Aluminum, plastic, and stone retention systems are the most popular choices, and are held in place with metal spikes, clips, or mortar.

Polymeric sand is then swept into the joints and the paver surface is either hand tampered or passed over with a small vibratory plate to vibrate the joints to ensure the sand penetrates all voids. Water is then sprayed on the surface of the pavers and into the joints, which allows the binding agent or glue present in the sand to harden. Polymeric sand, once cured, is resistant to weed growth and insects.

For permeable pavers, joints are filled with a small aggregate instead of polymeric sand so the joints can easily allow water to permeate through the sub-grade. Using the ‘wet-lay’ installation method, once the pavers have been set in mortar and adhered to the concrete sub-base, mortar is applied to the joints and trowel pointed or sponged smooth.

Thanks to the abundant variety of paver materials available, it is easy to satisfy every homeowner’s design wishes. With the proper guidance, research, quality materials, and correct installation techniques, any hardscaped surface can become a magazine-worthy project.

[13]Michael Willis is an aquatic designer and partner at Roger Willis Contracting Ltd., an Ottawa-based pool builder specializing in unique, custom aquatic projects. He works in various fields—from project design to completion—and builds throughout eastern Ontario and the Caribbean. Willis is a member of the Master Pools Guild, Pool & Hot Tub Council of Canada (PHTCC), and Landscape Ontario Horticulture Trades Association. He can be reached via e-mail at michael@rogerwilliscontracting.com[14].

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