Durability of bituminous pavements helps to minimize maintenance cost. There are different factors which affect the durability of bituminous pavements are discussed.
The ability of a structure to survive and give continued required performance throughout its life is called the durability. In the construction of roads, it is necessary that they survive throughout their design life, with limited maintenance.
- Durability of Bituminous Pavements
- Ageing of Bitumen in Pavements
- Permeability as a Durability Factor of Bituminous Pavements
- Adhesion in Bituminous Pavements
Durability of Bituminous Pavements
Bituminous pavements gains a performance and life based on the material quality and behavior with which it is constructed. Here the main constituent is bitumen. There are different possibilities through which a bituminous material can undergo failure.
- The exposure to the oxygen and different temperature effects the bitumen can harden
- Bad Quality aggregates used in the road construction may break down
- Aggregate poor quality affects their binding with the bitumen, under continuous traffic load and weathering action
- Higher void content and permeability problems creating deterioration of the pavement.
Ageing of Bitumen in Pavements
The exposure of bitumen to the atmosphere will result in its aging. The hardening rate of the bitumen is dependent on the weathering conditions as well as the nature of the bitumen.
There are mainly two aging process happens with bitumen. They are:
1. Oxidation Process
The process involves the combining of the oxygen from the air with the aromatics and the resins. The reaction will give asphaltenes. This will increase the polar high molecular weight fraction in bitumen. This increase will make the viscosity of the bitumen to increase.
The increase of viscosity brings the bitumen to an unstable state. The unstable state reflects discontinuity on the component materials. The decrease of cohesion due uncertainty creates crack under stress. The oxidation rate is a detrimental factor in areas of higher temperature.
2. Loss of Volatiles
Presence of low molecular weighed substances in bitumen facilitated by higher temperature will cause the loss of volatiles. The rate of loss of volatile for the penetration grade bitumen is relatively small, once it is laid.
The aging of bitumen will cause the hardening condition. The hardening condition is determined by the low penetration value or an increase in the penetration index. This also tends to increase the softening point of the bitumen sample. Such hardening will make the material brittle and cause cracking in severe conditions.
The aging of bitumen can be represented by means of Ageing Index.
The figure-1, below shows the aging of the bitumen during mixing, storing, transportation, application and throughout it service period.
Fig.1: Ageing of Bitumen represented by Ageing Index (From the Shell Bitumen Handbook,2003)
Permeability as a Durability Factor of Bituminous Pavements
The extent to which water and the air penetration will happen to the bituminous mixture is dependent on the permeability of the bituminous pavements.
The measurement of permeability of a bitumen sample can be carried out by a simple test. Into one side of the bitumen sample, the fluid is penetrated under pressure. The water that is expelled out of the opposite side of the bitumen sample is measured.
The permeating fluid used can be either air and water. The permeability ranges for three different asphalt mixtures are given in the table-1.
Table.1: Void Classification Based on Permeability Ranges for Asphalt Mixtures
|Permeability, k (cm/s)||Permeability Condition||The Voids Type||The Mixture|
|10-4 or lower||Impervious||Impermeable||Dense|
|10-4 to 10-2||Poor drainage||Semi-effective||Stone Mastic asphalt|
|10-4 or higher||Good drainage||effective||Porous asphalt|
The factors affecting the permeability of the bituminous pavements are:
- Quantity of voids
- The void distribution size
- The void Continuity
The figure-2 shows the variation of permeability in an asphalt concrete, hot rolled asphalt, porous asphalt mixtures and stone mastic asphalt, with respect to the total voids present in the respective sample.
Fig.2: The variation of permeability with respect to percentage air void content for different asphalt mixtures
The voids are also dependent on the aggregate nature, that is used in the mix. The packing of the aggregates, along with their shape, size and texture will govern a number of voids that are present in the whole mix.
Hence the compaction has a role in the decrease or increase of the void. Here quality compaction is of greater concern.
Adhesion in Bituminous Pavements
The bitumen should possess a quality adhesion property with the aggregate, to facilitate proper binding. This factor is the main parameter questioning the durability of the bituminous structures.
The adhesion properties and the stripping resistance of the bituminous pavements are dependent on the following factors, as mentioned below:
1. The Properties of Aggregates
The main properties of aggregate that affects the bitumen adhesion property are:
- The porosity of aggregates,
- Mineralogy, shape and surface texture of the aggregates
- the aggregate surface coatings and the presence of dust
- Surface area and absorption property
- The weathering grade and moisture content
- The abrasion value of aggregates and their pH
- The additives and the exposure history of the aggregates employed
The mineralogical and the physical nature of aggregates that are employed for the bitumen design mix affect the adhesion property.
In general, the aggregates that have more siliceous content i.e. like rhyolites, quartzites, granites etc. are more prone to the failure caused due to lack of adhesion. But the lime stones and igneous rocks have worked well. These own a complex material interaction.
Other factors that govern the adhesion is the surface behavior of the aggregates. A surface energy is possessed by those aggregates that have unbalanced surface charges. This will lead to the breaking of co-ordination bonds in the whole crystal lattice and in the polar environment of the minerals.
If this surface of aggregate is coated with an opposite charge, the surface energy will help in promoting the bonding.
The total volume of the pore space that is permeable and the size of the pores are the main two factors that affect the absorption of bitumen into the aggregates.
The absorptive surface that is present in the bitumen can be increased with the presence of fine microstructure of pores, voids and the micro cracks. This, in turn, is dependent on the quality and weathering state of the aggregate.
Greater the roughness of the aggregate surface, more is the adhesion. The smooth surface of aggregates makes them wet. So, a balance must be maintained so that wettability of the aggregate and the binding property in aggregates is attained.
That is, the aggregate must not be too rough nor too smooth. The effect of mineralogy is sometimes masked by owning a rough surface.
2. The Bitumen Properties
- The composition and the source of bitumen
- The durability factors
- Weathering parameters
- The curing time and oxidation effects
- Use of additives
- Electrical Polarity
Viscosity and surface tension are the main characteristics of bitumen that affect the adhesion of bitumen with the aggregates. The extent to which the bitumen is absorbed into the pores of the aggregate is governed by these factors mentioned.
The viscosity and surface tension are parameters that vary with the temperature. Always the mixing of aggregates and the bitumen is carried out a higher temperature. It is at maximum 180 degrees of 40/60 pen bitumen. This is carried out at high temperatures to make bitumen completely coat the aggregate.
Chemical absorption of bitumen onto the surface of aggregate has been observed. Bitumen-aggregate interface has been found that is strong bond forming a layer of thickness 180 Amstron. The layer consists of Ketones, dicarboxylic anhydrides, carboxylic acids, sulphoxide and components that bear nitrogen.
The figure-3 shows the alignment of bitumen molecules in the direction of polarity of the substrate. Aggregate mainly possess a negative surface.
Fig.3: Representation of bitumen molecules attracted to negatively charged aggregates
3. Interactive Mix Proportions for Bituminous Pavements
- Method of production
- The mixed type
- Incorporation of additives
- The filler type
- The thickness of the film
- The content of binder
- Compaction and grading
- Cohesion factors
4. External Influences on Bituminous Pavements
- The pH of the water
- Temperature effects
- Design and workmanship
- The presence of salt
- The relative humidity
- Annual Precipitation, Light, Heat, and Radiation