Mix design methods and design requirements are an essential part of all asphalt concrete mixes. The agency or authority responsible for paving construction (the Department of Transportation) usually establishes the mix design method and design requirements.
Once these are installed, it becomes the responsibility of the contractor/manufacturer and his technician to develop the mix within the framework of the specification requirements.
An asphalt concrete mix must be designed, produced and placed to achieve the following desirable mixing properties:
- fatigue resistance
- slip resistance
The stability of asphalt pavement is its ability to resist shaking and cramping under load (traffic). A stable pavement retains its shape and smoothness under repeated loads; An unstable pavement develops ruts (channels), ripples (wash boarding or corrugation) and other signs of mixture transfer.
Because stability specifications for a pavement depend on the traffic expected to use the pavement, requirements can only be established after a thorough traffic analysis. The stability specification must be high enough to adequately handle the traffic, but not exceed what the traffic conditions require. Too high a stability value produces a pavement that is too hard and therefore less durable than desired.
The consistency of the mixture depends on the internal friction and cohesion. The internal friction (inter-particle friction) between aggregate particles is related to aggregate characteristics such as size and surface texture. Cohesion occurs as a result of the bonding ability of the binder.
A reasonable degree of both internal friction and cohesion in a mixture prevent the aggregate particles from moving past each other by the forces exerted by the traffic. In general, the more angular the size of the aggregate particles and the rougher their surface texture, the higher the consistency of the mixture. Where aggregates with high internal friction characteristics are not available, a more economical mixture can be used using aggregates with lower friction values where light traffic is expected.
The binding strength of the cohesion increases with increasing loading (traffic) rate. Cohesion also increases as the viscosity of the binder increases or with a decrease in the temperature of the pavement. Additionally, the cohesion will increase with the increase in binder content up to a certain point. Before that point, increasing binder content forms a very thick film on the aggregate particles, resulting in losses on inter-particle friction. Inadequate stability in pavement has many causes and effects.
The durability of asphalt pavement is its ability to resist factors such as changes in the binder (polymerization and oxidation), dissolution of the aggregate, and separation of the binder films from the aggregate. These factors can be the result of weather, traffic, or a combination of the two. Generally, the durability of a mixture can be increased in three ways. They:
- Using maximum binder material,
- using a dense gradation of stripping-resistant aggregates, and
- Designing and condensing the mixture for maximum impermeability
Maximum binder material increases durability because thicker binder films do not age and harden as fast as thin ones. As a result, the binder retains its original characteristics for a long time. In addition, the maximum binder material effectively closes a greater percentage of the interconnected air spaces in the pavement, making it difficult for water and air to enter. Of course, a certain percentage of air voids in the pavement must be left open to allow for expansion of the binder in hot weather.
A dense sequence of sound, tough, stripping-resistant aggregates contribute to pavement durability in three ways. The dense grading provides close contact between the aggregate particles. This increases the impermeability of the mixture. A sound, tough aggregate resists disintegration under traffic loading; And the stripping-resistant aggregate resists the action of water and traffic, separating the binder film from aggregate particles and causing the pavement to crumble. Under certain conditions, the resistance of the mixture to stripping can be increased by the use of anti-stripping additives, or mineral fillers such as hydrated lime. Design and compact the mix to give maximum impermeability to the pavement, minimizing air and water infiltration into the pavement. The lack of adequate durability in pavement can have many causes and effects.
Impermeability is the resistance of an asphalt pavement to the passage of air and water in or through it. This characteristic is related to the void content of the compacted mix, and much of the discussion on voids in the mix design sections relates to impermeability.
Even though void content is an indication of the ability of air and water to pass through a pavement, the character of these voids is more important than the number of voids. The size of the voids, whether or not the voids are interconnected, and the accessibility of the voids to the surface of the pavement all determine the degree of impermeability.
Although impermeability is important to the durability of concrete paving mixes, almost all asphalt mixes used in highway construction are permeable to some degree. It is acceptable as long as it is within the specified limits.
Practicality describes the ease with which a paving mix can be placed and deposited. Mixtures with good pliability are easy to store and compact; People with poor working capacity are difficult to place and compact. Performance can be improved by changing mix design parameters, aggregate source and/or gradation.
Harsh mixtures (mixtures that contain a high percentage of coarse aggregates) have a tendency to separate during handling and may also be difficult to compact. Through the use of test mixtures in the laboratory, additional fine aggregates, and perhaps binders, can be added to the hardened mixture to make it more practical. Care must be taken to ensure that the converted mix meets all other design criteria, such as void content and consistency. Too much filler material can also affect workability. This can make the mixture sticky, making it difficult to compact. Practicality is especially important where considerable hand-holding and raking (looting) is required around manhole covers, sharp curves, and other obstacles. It is important that the mixtures used in such areas are highly workable.
Mixtures that can be worked or stirred very easily are called tender mixes. Tender mixes are too unstable to place and compact properly. They are often caused by a lack of mineral filler, too much medium-sized sand, and smooth, rounded aggregate particles and/or too much moisture in the mixture. Although not in general a major contributor to practicality problems, asphalt tying does have some effect on efficiency. Because the temperature of the mixture affects the viscosity of the binder, too low a temperature will render the mixture unusable, too high a temperature to make it pliable. Binder grade can also affect workability, as can the percentage of binder in the mixture.
Flexibility is the ability of an asphalt pavement to accommodate gradual settlements and movements in sub-grades without cracking. Since almost all subgrades either settle (under loading) or rise (by expansion of the soil), ductility is a desirable characteristic for all asphalt pavements. An open-grade mix with a higher binder content is generally more flexible than a dense-graded, low binder content mix. Sometimes the need for flexibility conflicts with the stability requirements, so that trade-offs have to be made.
6. fatigue resistance
Fatigue resistance is the resistance of the pavement to bending repeatedly under wheel load (traffic). Research shows that air void (related to binder material) and binder viscosity have a significant effect on fatigue resistance. As the percentage of air spaces in a pavement increases, either by design or lack of compaction, pavement fatigue life (the length of time during which an in-service pavement is sufficiently fatigue resistant) becomes significantly shorter. Similarly, binder-containing pavement that has aged and hardened has reduced resistance to fatigue.
The thickness and strength characteristics of the pavement and the supporting strength of the subgrade also have a lot to do with determining the life of the pavement and preventing cracking associated with the load. Thick, well supported pavements do not bend under load as much as thin or poorly supported pavements do. Therefore, they have prolonged fatigue.
7. slip resistance
Skid resistance is the ability of an asphalt surface to reduce slipping or slipping of vehicle tires, especially when wet. For good skid resistance, the tire tread must be able to maintain contact with the composite particles, rather than riding over a film of water on the surface of the pavement (hydroplaning). Skid resistance with standard tread tires is typically measured at 40 mph in the field under controlled wetting of the pavement surface. A rough pavement surface with many small peaks and valleys will have greater skid resistance than a smooth surface. The best skid resistance is achieved with a rough-textured aggregate in relatively open-grade mixes with a maximum size of approximately 3/8 inch–1/2 inch (10–13 mm). In addition to having a rough surface, the aggregate must resist polishing (smoothing) under traffic. Calcareous aggregates polish more easily than siliceous aggregates. Volatile mixtures that rut or bleed (asphalt flush to the surface) present serious skid resistance problems.