Cadbury Castle and South Cadbury Village
Cadbury Castle in Somerset is a tremendous Iron Age camp covering some 18 acres (73,000 m²), one of the most impressive sites in Britain. It is the focal point of many ancient trackways and is guarded by four huge banks with a height in places of over 40 feet (12 m) from the bottom of the ditch.
Unit 17 Pavement (material)
This article is about the American English usage of pavement as the durable surfacing of roads and walkways. In British English, pavement is usually taken to mean a footpath next to a road, the same as sidewalk in American English.
Pavement in American English refers to the durable surface for an area intended to sustain traffic, which can be either vehicular traffic or foot traffic. The most common modern paving methods are asphalt and concrete. In the past, brick was extensively used, as was metalling. Today, permeable paving methods are beginning to be used more for low-impact roadways and walkways.
Metalling
Metal or metalling has had two distinct usages in road paving. Metalling originally referred to the process of creating a carefully engineered gravel roadway. The route of the roadway first would be dug down several feet. Depending on local conditions, drains may or may not have been added. Next, large stone was placed and compacted, followed by successive layers of smaller stone, until the road surface was a small stone compacted into a hard, durable surface.
Road metal later became the name of stone chippings mixed with tar to form the road surfacing material tarmac. A road of such material was called a “metalled road” in British usage, although this would be very rare in modern usage. It would be more common to refer to a macadam road. The word metal is derived from the Latin metallum, which means both “mine” and “quarry”, hence the road building terminology.
Asphalt paving
A road in the process of being resurfaced, showing both old and new asphalt surfaces.
Asphalt (specifically, asphalt concrete) has been widely used since 1920-1930, though in ancient times asphalt was already used for road-building. The viscous nature of the bitumen binder allows asphalt concrete to sustain significant plastic deformation, although fatigue from repeated loading over time is the most common failure mechanism. Most asphalt pavements are built on an imported gravel base which is generally at least as thick as the asphalt layer, although some “full depth” pavements are built directly on the native subgrade. In areas with very soft or expansive subgrades such as clay or peat, thick gravel bases or stabilization of the subgrade with Portland cement or lime can be required. In some countries with soft soils, a foundation of polystyrene blocks is used instead. The actual material used in paving is termed HMA (Hot Mix Asphalt).
Advantages of asphalt roadways include relatively low noise, relatively low cost compared with other paving methods, and ease of repair. Disadvantages include less durability than other paving methods, less tensile strength than concrete, the tendency to become slick and soft in hot weather and a certain amount of hydrocarbon pollution to soil and groundwater or waterways.
Concrete Paving
Concrete pavements (specifically, Portland cement concrete) are created using a concrete mix of Portland cement, gravel, and sand. The material is applied in freshly-mixed slurry, and worked mechanically to compact the interior and force some of the thinner cement slurry to the surface to produce a smoother, denser surface free from honeycombing. Cement concrete can be either reinforced or non-reinforced. Non-reinforced pavements will typically have joints at a 5 meter interval. Reinforced concrete pavements can have a much longer joint spacing, or no built-in joints at all. Typical reinforcement used includes “rebar” (reinforcing steel) or wire mesh or both.
Vertical misalignment of the joints, known as joint faulting, can be caused by differential settlement of the substrate, and are a source of driver annoyance. A common failure mode of concrete pavements is loss of support of the slab edges or corners due to erosion of the foundation material. If this condition is caught before it leads to breakup of the slab, support can be restored by filling the void with grout or foam in a process known as “mud jacking” or “slab jacking”.
Advantages of cement concrete roadways include that they are typically stronger and last longer than asphalt concrete pavements. They also can easily be grooved to provide a durable skid-resistant surface. Disadvantages are that they have a higher initial cost, are more difficult to repair, and are also somewhat noisy if jointed, but unjointed concrete pavement is actually a method of road noise mitigation.
The record for first mile of concrete pavement to be laid in the United States is claimed by Michigan.