The largest use of asphalt/bitumen is for making asphalt concrete for road surfaces and accounts for approximately 85% of the asphalt consumed in the United States. Asphalt concrete pavement material is commonly composed of 5% asphalt/bitumen cement and 95% aggregates (stone, sand, and gravel). Due to its highly viscous nature, asphalt/bitumen cement must be heated so it can be mixed with the aggregates at the asphalt mixing plant. There are about 4,000 asphalt concrete mixing plants in the U.S., and a similar number in Europe.
Asphalt concrete road surface is the most widely recycled material in the U.S., both by gross tonnage and by percentage. According to an industry survey conducted by the Federal Highway Administration and the National Asphalt Pavement Association and released in 2011, more than 99% of the asphalt removed each year from road surfaces during widening and resurfacing projects is reused as part of new pavements, roadbeds, shoulders and embankments and driveways.
Asphalt (specifically, asphalt concrete) has been widely used since the 1920s. 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 surfaces are laid on a gravel base, which is generally at least as thick as the asphalt layer; although some 'full depth' asphalt surfaces are laid 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 may be required.
Asphalt deterioration can include alligator cracks, potholes, upheaval, raveling, oxidation and rutting. In cold climates, freezing of the groundwater underneath can crack asphalt even in one winter. Filling the cracks and maintaining the surface can temporarily fix the surface although only proper construction (allowing water to drain away from under the surface can slow this process.
Heavy, strong and durable. So its guarantee should be cast in stone, right?
Actually; just the opposite and that can be a point of frustration for homeowners and contractors alike. I would like to share with you many of the reasons for this, since most all homeowners will at one time or another have concrete installed either as a driveway, patio or sidewalks.
The Greater Cleveland Chapter of the National Association of the Remodeling Industry held a seminar on the installation of concrete and why it is usually not warranted by most contractors. Mark Steadman of Collinwood Shale Brick & Supply Co. explained the reasons why and also offered additional information about concrete.
Let's start with the basics. Concrete is made from mixing cement, limestone, sand, water and admixtures at prescribed rates. There are also standards that manufacturers of concrete should abide by from the American Concrete Institute. Independent testing laboratories inspect all concrete plants yearly to insure certain standards are met. These recipes are modified for hot-weather pours and cold-weather pours.
Just like painting, surface preparation is crucial to a good concrete job. For the exterior, say a driveway, you have to start with an acceptable base material that can be compacted. The base must be saturated before the concrete can be poured. Interior preparation is much the same, except a moisture barrier is usually placed beneath the base material.
Concrete is usually delivered to the work site in a workable state. Once the concrete has been poured, it is usually screened. That is the excess concrete is removed by dragging a long straight board across the tops of the forms. Once the concrete has taken its initial set, the finishing process can begin. Two main types of finishing are called broom finishing and float finishing. According to Steadman, broom finishing is the best type of finish because float finishing is generally overworked and can lead to a weakening of the surface.
Steadman says that all concrete will crack. It is a natural function of concrete. Controlling where the cracks occur is the key. Control joints are cut or troweled into the surface during the finishing process, typically every 10 feet or less. Just like the perforation on a ticket stub, these joints give the concrete a place to crack during expansion and contraction, which won't harm the appearance of the job. However, Steadman adds that there is some abnormal cracking, such as shrinkage cracks, that can generally appear within the first week after placement. A concrete expert can tell the difference.
Aside from cracking, two common problems with concrete are spalling and scaling. According to Steadman, spalling is the term for isolated “pop outs” appearing on the concrete surface. These “pop outs” are caused by either certain materials such as many of the salts or ice melters that we put on the driveway or sidewalk or impurities in the base material working up to the surface. Scaling is the delamination of the finished surface that can be either a small size or can affect an entire block of concrete. Some of the most important questions that we receive from consumers at the NARI office is why don't contractors warrant concrete work and why isn't the product guaranteed? Steadman’s answers that by stating that workmanship and concrete do carry an implied warranty. “As a consumer you should expect the concrete to perform its intended function and to have aesthetic quality for at least two years.”