BASE & SUB-BASE LAYERS
Hydraulically bound materials are obtained by mixing aggregates or granular soils with cement (or a hydraulic road binder) and a reduced amount of water, enabling compaction with rollers.
Once hardened, the mix forms a durable paving material with a high elasticity modulus. Consequently, pavements with this type of layer and a wearing course of asphalt are often referred as semi-rigid.
When naturallly occuring soils are mixed with cement, the resulting material is usually called soilcement.
If granular materials from quarries or pits are used, a range of mixes can be obtained depending on the amount of binder: from cement-treated gravels (with cement contents usually between 80 and 120 kg per cu m), also called cement - bound granular materials, to compacted concretes (cement contents similar to those of vibrated concretes, i.e. 300 kg per cu m). Intermediate mixes are the so-called high-performance cement-treated gravels or (dry) lean concretes. Both mechanical strength and modulus of elasticity vary in accordance with cement content.
With the exception of compacted concretes, cement-bound layers cannot withstand continuous passage of vehicles and need to be protected with one or more layers or asphalt.
It is not unusual that a semi-rigid pavement includes two layers of cement-bound materials, eg a cement treated gravel base 25 to 20 cm thick and a soil-cement subbase 25 to 20 cm thick.
Cement bound subbases are also used under concrete pavements, sometimes interposing an intermediate layer of asphalt.
Proper compaction is a key factor in obtaining a durable cement - treated material, since there is a strong relationship between RCC density and mechanical strength. Thus, cements or other types of hydraulic binders with a high content of active additions are preferred, since they allow more time to compact the mix.
Although not strictly necessary, in some cases creation of transverse joints at short distances (2 - 3 m) in the fresh material (also known as precracking) is very beneficial and strongly recommended.
Cement treated layers offer a number of technical, economic, ecological and environmental advantages.
Through treatment with a hydraulic binder, layers with a high elasticity modulus can be achieved. Thus, stresses in both the subgrade and the upper bituminous layers are greatly reduced. This results in long-life pavements when properly designed and constructed.
Hydraulically bound layers can be constructed with equipment that is also used for other materials (unbound granular layers, bituminous mixes) and therefore widely available.
As well as being technically and economically beneficial, they are especially suited to regions with a shortage of high quality mineral aggregates for unbound bases. The incorporation of a hydraulic binder provides the opportunity to use lower quality aggregates and alternative materials (sands, slag, fly ash, etc). Thickness of upper asphalt layers can also be lower than those needed for the same design traffic when using unbound granular layers.
Ecological and environmental advantages
Cold treatment reduces pollution and discharge of fumes into the atmosphere. Moreover this technique allows significant global energy savings by reducing the volume of high quality aggregates required, which are non-renewable natural resources. This results in less transport of materials (thus limiting indirect effects, such as nuisance to users and residents and fatigue of the road network adjacent to the site) and contributes to preserving the environment.
Cement treated bases and subbases are widely used in highways, roads, streets, parking areas, airports, materials handling and storage areas and other types of infrastructures.
Hydraulically bound materials are covered by several European standards:
EN 14227-1: Unbound and hydraulically bound mixtures
Specifications: Cement bound granular mixtures.
EN 14227-5: Hydraulically bound mixtures
Specifications.:Hydraulic road binder bound mixtures.
EN 14227-10: Hydraulically bound mixtures
Specifications: Soil treated by cement.
EN 14227-13: Hydraulically bound mixtures
Specifications: Soil treated by hydraulic road binder.
Conclusion - benefits
The growing use of hydraulically bound bases and subbases and the focus on sustainability issues bring increasing recognition of the benefits of these techniques:
speed of project completion;
reduced use of imported aggregates;
less local construction traffic;
reduced project costs.