Subbase (pavement)
Encyclopedia
In highway engineering
, subbase is the layer of aggregate
material laid on the subgrade
, on which the base course
layer is located. It may be omitted when there will be only foot traffic on the pavement, but it is necessary for surfaces used by vehicles.
Subbase is often the main load-bearing layer of the pavement
. Its role is to spread the load evenly over the subgrade
. The materials used may be either unbound granular, or cement
-bound. The quality of subbase is very important for the useful life of the road.
Unbound granular materials are usually crushed stone
, crushed slag
or concrete
, or slate
.
Cement-bound materials come in multiple types. Mass concrete
is used where exceptional loads are expected, with thickness usually 100-150 mm, and optional reinforcement with steel mesh or polymer fibers. Other cement bound materials (CBM), with less strength but also lower cost, are used. They are rated by strength, from the weakest CBM 1 (also formerly known as soil cement
) through CBM 2 to CBM 3, 4, and 5, which are more similar to concrete and are called "lean mix".
The thickness of subbase can range from 75-100 mm for garden paths through 100-150 mm for driveways and public footpaths, to 150-225 mm for heavy used roads, and more for highway
s.
Highway engineering
Highway engineering is an engineering discipline branching from civil engineering which involves the design, construction and maintenance of Highway Systems. Highway Engineering become prominent towards the latter half of the 20th Century after World War 2. Standards of highway engineering are...
, subbase is the layer of aggregate
Aggregate (composite)
Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes...
material laid on the subgrade
Subgrade
In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track. It is also called formation level.The term can also refer to imported material that has been used to build an embankment....
, on which the base course
Base course
Base Course in pavements refers to the sub-layer material of an asphalt roadway and is placed directly on top of the undisturbed soil so as to provide a foundation to support the top layer of the pavement...
layer is located. It may be omitted when there will be only foot traffic on the pavement, but it is necessary for surfaces used by vehicles.
Subbase is often the main load-bearing layer of the pavement
Pavement (material)
Road surface or pavement is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past cobblestones and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete. Such...
. Its role is to spread the load evenly over the subgrade
Subgrade
In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track. It is also called formation level.The term can also refer to imported material that has been used to build an embankment....
. The materials used may be either unbound granular, or cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
-bound. The quality of subbase is very important for the useful life of the road.
Unbound granular materials are usually crushed stone
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
, crushed slag
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
or concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
, or slate
Slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
.
Cement-bound materials come in multiple types. Mass concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
is used where exceptional loads are expected, with thickness usually 100-150 mm, and optional reinforcement with steel mesh or polymer fibers. Other cement bound materials (CBM), with less strength but also lower cost, are used. They are rated by strength, from the weakest CBM 1 (also formerly known as soil cement
Soil cement
Soil cement is a construction material, a mix of pulverized natural soil with small amount of portland cement and water, usually processed in a tumble, compacted to high density...
) through CBM 2 to CBM 3, 4, and 5, which are more similar to concrete and are called "lean mix".
The thickness of subbase can range from 75-100 mm for garden paths through 100-150 mm for driveways and public footpaths, to 150-225 mm for heavy used roads, and more for highway
Highway
A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...
s.