It's Fun to Choose the Right Paving Materials
Paving materials can be concrete, asphalt, unit pavers, or loose aggregates. For public projects, paving material needs to be smooth enough for people with walking disabilities and easy for maneuvering in wheelchairs. To be smooth, there can be no uneven tree roots, freezing and thawing cracks, slippery gravel, or clumpy bits of mulch. Choose paving materials that can provide long-term stability for a walking or driving surface.
The sub base for paving must be solid and unshifting. For areas without ground that freezes, landscape sub base material can be compacted soil or a mixture of soil and cement. For colder climates, a sub base of deep gravel and even reinforced concrete is required to keep the paved surface safe and navigable.
The installation process for concrete can determine the usability of the surface. Concrete must be textured to prevent slipping. Concrete uses reinforced rods for strength, and these characteristics are very important for vehicular areas. It can be colored with dry shake powders (which fade) or color can be incorporated into the concrete mixture of Portland cement and aggregates prior to pouring. Concrete has to be finished and floated. Special forms are required to hold newly poured concrete in place before it cures. Other special forms can be used to imprint artistic textures into the surface. All of this fuss makes it expensive, but durable. As a designer, you shouldn’t propose concrete without having learned the installation process from start to finish.
Asphalt is a bit easier to install, but has some drawbacks. The mixture of aggregates and oil must be heated, poured, and then pressed flat and even, much like a ribbon of taffy is formed into candy. It’s a sticky mess at first, but creates an attractive, dark, semifluid surface. If asphalt doesn’t receive regular traffic, it can fall apart at the edges. Smaller equipment is available for asphalt, so it is a good option for the installation of extensive woodland and bicycle paths.
Unit pavers, individually have tremendous density and strength. Unit pavers need some kind of permanent edging to hold them in place. In warmer climates, they can be placed on a soil-cement or sand base, without the need for a gravel or concrete sub base. Mortar can provide a bed and grout for unit pavers if they must remain solidly in place. There are standards for concrete, asphalt, and pavers. Be sure if you use pavers for vehicular traffic, they are dense enough to handle heavy truck loads.
Loose materials need some kind of containment or your paths could wash away after a few rain events. Small, angular pieces of stone or brick dust do a good job of locking together to provide a good surface for wheel chairs, but loose sand or pebbles do not. There’s a wonderful, nostalgic sound created by gravel driveways and loose stone walkways, but they need frequent maintenance and fresh layers as they shift and move with time. Loose materials are pervious, an asset that is appreciated as more and more stormwater regulations are added. If loose materials like pebbles or brick dust are chosen for the paving surface, then the path will need to be edged with a strong, stiff edging material.
Gravel, small stones with fines, crushed brick, or hardwood mulch make nice loose paving. The edging material can be innovative, but it must be strong. Buried rocks can be used to hold them in place. Staked plastic edging is not strong enough to hold gravel unless it is at least one-eighth of an inch thick. Aluminum or steel edging secured below grade with long stakes is ideal for loose paving. It should extend no more than one-quarter of an inch above grade to be unobtrusive and to avoid a tripping hazard. Logs or strong, stiff edging metal can be used to edge loose paving, too. Railroad ties are not as well suited to woodland paths as they are to woodland parking lots. Their scale is too large for a path. The beauty of using loose materials is the permeability of the final product.
Stone paving is very beautiful, but takes skilled crews for installation. The uneven character of natural stone requires a custom subsurface for each individual stone. Often, stone pieces are hammered into deep beds of sand or soil cement to create a level surface. Thick mortar beds can be used, too. With the right installation crew, stone paving can be every bit as stable, level, and permanent as concrete or asphalt.
Choosing the right materials for paved surfaces is one of the fun things landscape architects get to do. They can suggest innovative solutions other engineering professionals may not have considered. The materials chosen for paved surfaces can become the focal point of a project.