The “subgrade” in general is an in situ material upon which the pavement is placed. Generally subgrade plays major role in determining the pavement performance.
Subgrade Physical Properties
Subgrade materials are typically characterized by their stiffness and strength. Two basic subgrade stiffness/strength characterizations are:
California bearing Ratio (CBR): CBR is basically a measure of strength. The california bearing ratio test is penetration test meant for the evaluation of subgrade strength of roads and pavements. The results obtained by these tests are used with the empirical curves to determine the thickness of pavement and its component layers. This is the most widely used method for the design of flexible pavement.
Resilient modulus (MR): The resilient modulus is defined as the ratio of the applied cyclic stress to the recoverable strain after many cycles of repeated loading and thus is a direct measure of stiffness for unbound materials in pavement systems. It is the single most important unbound material property input in most current pavement design procedures. It is an essential input for carrying out mechanistic pavement design.
The resilient modulus (MR) is defined simply as the ratio of the cyclic axial stress to resilient axial strain:
Resilient modulus is basically a measure of stiffness.where, the axial stress is (Δσ) induces the cyclic resilient axial strain (Δεa)
The type of pavement, choice of base and subbase materials, and the type of surfacing should involve consideration of various construction and maintenance factors as follows:
- Extent and type of drainage, stormwater and subsoil
- Use of boxed or full width construction
- Traffic characteristics including traffic volumes, percentage of heavy vehicles and turning movements
- Use of stabilisation
- Aesthetic, environmental and safety requirements including noise
- Social considerations
- Construction constraints including vibration limitations and construction under traffic
- Use of staged construction
- Ongoing and long-term maintenance costs
- Whole of life cycle costing
PAVEMENT PREPARATION BEST PRACTICE
The preparation of a granular pavement for sprayed bituminous or asphalt surfacing requires attention to the following:
(i) Shape and Level
In order to provide a smooth ride and for the efficient run-off of surface water during rain, the granular pavement must be constructed to the specified shape and level as the thin bituminous surfacing will not correct irregularities. Regular checking of line and level is essential during the spreading and compaction process. Once compaction is completed and depending upon the importance of the road, minimum departure from design surface level should be within +/- 15 mm for minor roads to +/- 5 mm for major roads (Section 304). More recently, the spreading operation is controlled by GPS programmed machines which do not require level pegs for achieving the required shape and level (pegless survey). In these situations, independent spot checks of line and level needs to be undertaken to ensure the specified requirements are being met.
(ii) Homogenous Surface and Pavement Material
In order for the required pavement performance to be achieved, it is necessary for the pavement material to be homogenous in nature throughout the layer depth. Any change to the grading of the pavement material near the surface of the layer due to segregation at time of construction, addition of fine material to the surface, or excess material being swept from the compacted layer is likely to result in a poor ride quality and/or a weakening of the inherent pavement strength. Pavement material that is durable (resists mechanical breakdown during compaction) and has good cohesion (avoid non- plastic products but not excessively plastic that will affect strength) is recommended.
(iii) Uniform in Texture
The pavement surface must be prepared to achieve uniform texture. This is an important condition in achieving a uniform and strong bond between the pavement and the bituminous prime or primerseal surfacing. A surface that is patchy in nature will respond differently to the application of bituminous material. Different penetration rates will occur depending upon whether the surface is coarse and hungry or fine and fatty. Such variations in bond are likely to lead to future loss of seal or seal flushing.
(iv) Free from Tearing and Scabbing
Tearing and scabbing can occur during the preparation phase often due to lack of cohesion in the pavement material when subjected to brooming or when a thin laminated layer exists at the surface. Both faults lead to poor ride quality and reduced service life in the surfacing.
(v) Free from Laminations
Laminations generally occur where it is discovered that the required finished surface level has not been achieved during or following the compaction process. By subsequently adding a thin layer of additional pavement material without tyning in the additional material will not achieve the necessary mechanical interlock between the two layers. Tearing is the likely consequence of this poor practice. Another common poor practice is to add a thin layer of fine material to the compacted layer in an attempt to overcome poor texture. Again, this practice often produces a laminated layer with the inevitable consequences.
(vi) Level with Adjacent Sealed Areas and Edgings
Notwithstanding the requirements to achieve the specified line and level, it is obvious that matching in with existing sealed areas and edgings at the joins is essential to ensure smooth ride, good drainage and comfortable alignment of traffic lanes. In some circumstances, this may mean a transition from design requirements to existing requirements over an agreed length or width.
(vii) Hard, Dense, Tight Surface Capable of Brooming
The prepared pavement surface needs to be hard to resist any subsequent embedment of sealing aggregate into the pavement under trafficking. It needs to be dense to provide the required strength to withstand the applied traffic loadings. It must be tight to withstand sweeping with a rotary broom or similar equipment to remove surface dust and to provide a surface with the larger stones in the pavement exposed but not loose or dislodged. Care must be exercised to ensure that the force applied by the rotary broom is sufficient to remove the dust and loose aggregate without damaging the pavement surface. If excessive force is used, the result is usually a scabbled or ravelled surface with excess fine material windrowed to the side free from Loose and Foreign Material.
Loose and foreign material if not removed from the prepared surface prior to sealing can result in poor bonding with the bituminous surfacing leading to early failure of the surfacing. Any hardened mud or embedded vegetation needs to be removed, by hand shovel if necessary.
(viii) Avoid Excessive Slurrying
The achievement of a uniform surface texture that is tight and dense can be assisted by minor wetting up of the surface followed by pneumatic multi wheeled rolling. This action will knead any loose stones remaining at the surface after the compaction process. However care must be exercised to avoid excessive slurrying up which under rolling draws too many fines to the surface. Such a practice can lead to a glassy appearance being produced, adversely affecting the ability of the prime or primerseal to adhere to the granular surface. There is also a risk that premature rutting may occur due to the loss of mechanical interlock between pavement particles weakening the pavement structure.
Care also needs to be exercised to ensure that the quality of water used in pavement construction including any slurrying up of the pavement surface during the preparation phase is clean and substantially free from detrimental impurities such as oils, salts, acids, alkalis and vegetable substances. Such impurities can lead to the primer debonding from the pavement.
Source: Technical Report Vic Roads