The following paragraphs outline the different categories of pavement materials typically used on a residential site. The summary of pavement materials focuses on their appearance and design character, but does not discuss their recommended installation techniques. The reader should consult other sources for this information. Pavement or hardscape materials are categorized according to their physical characteristics as (1) loose materials, (2) unit materials, and (3) adhesive materials.
Loose materials, also called “aggregates,” are small individual stones or chips that are amassed as a pavement surface without being physically held together by an adhesive. The material is put down on the ground to a desired depth and held in place by a containing edge. The most common loose materials include gravel and various recycled materials.
Gravel Gravel is a small stone that is between 1/4" and 3/4" in size (Figure 12—1). Two common types of gravel are bank run gravel and pea gravel, so named because of its pea size. Both types of gravel are rounded, and so individual stones easily move against one another. Consequently, most gravel has a slight give to it underfoot that is often accompanied by a distinct crunching sound. A potential range of colors including black, gray, buff, golden yellow, red, and off-white augment the appealing texture of gravel. A particular mixture of gravel may consist of one color or be an assortment of many colors, giving it a more mottled appearance.
Gravel has a number of uses as a pavement surface in the landscape, including:
• to create an informal and naturalistic character like a garden path or walk through a wooded area.
• to define secondary walks or infrequently used spaces. Gravel is potentially difficult to walk on or to push wheeled equipment across and so should not be used for frequently used spaces or paths.
• to define curvilinear surfaces or irregularly shaped areas. Because gravel has no predetermined shape, it can be easily molded to any form on the ground plane (Figure 12-2).
• to establish an attractive, pliable texture on the ground plane.
• to furnish a textural contrast with smooth or precisely edged pavement materials (Figure 12-3).
• to serve as a ground cover in arid climatic zones or other locations where it is difficult for plants to grow.
• to produce a permeable pavement surface. Gravel is ideal for the sustainable landscape where the objective is to minimize surface runoff (also see Chapter 3).
The loose quality of gravel is a potential disadvantage and so it should not be used:
• for pavements where there is a need for snow removal.
• where there is need for low maintenance. Gravel is easily kicked or tracked out of its location and carried to adjoining areas of the landscape, thus requiring extra upkeep.
The principal recommendation for designing with gravel is that metal or plastic edging, wood, or another pavement material should contain it (Figures 12—4 and 12—5). Without an edge, gravel is a formless material that has indistinct edges that merge into surrounding areas.
Recycled Materials There are several recycled materials that are available in an aggregate form much like gravel. The most notable among these is crushed recycled glass, a relatively new pavement material that has become available in the past five years because of the general movement toward finding new uses for discarded materials. Glass from bottles and other products is crushed and then tumbled to slightly round off jagged edges and points. The result is smoothed angular aggregate that can vary in size from 1/8" to 1/2" across, although most glass used for pavements is in the smaller size range (Figure 12—6). The unique aspect of recycled glass in comparison to more traditional pavement materials is its availability in a range of bright, vibrant colors in addition to clear glass. The limitations and design guidelines for recycled glass and other recycled materials are similar to those for gravel.
Recycled crushed glass is a specialty material that can be used in the landscape to:
• create areas of bright color that sparkles in the sun light. Like gravel, the color may be uniform or a mixture of a range of colors.
• produce an animated, glittering look that changes with the angle of the sun and direction from which it is viewed.
• establish a decorative contrast to more earthy ground plane materials.
• produce a contemporary character and/or compatible fit with hot, arid climates where bright colors are more easily absorbed into the landscape.
• produce a permeable pavement surface like gravel.
Other recycled materials that can be used as pavement are shards from shattered clay pots and ground rubber produced from tires. Crushed clay pots create an angular, irregular aggregate that varies from about 1/2" to 2" in size depending how much it has been broken. Clay pot fragments furnish a walking surface that has a deep red color and a distinct texture (Figure 12—7). Ground rubber, or crumb rubber, generally varies in size from 1/8" to 1". The color is usually black, although it can be purchased in dark browns, a range of brick reds, or other colors depending on the manufacturer. Crumb rubber can be used as a loose material like gravel to produce a soft, flexible surface that gives somewhat underfoot or to provide a safe surface under play areas.