3Properties of Asphalt Important properties of asphalt include: AdhesionConsistencySpecific GravityDurabilityRate of curingDuctilityAging and hardeningResistance to reaction with waterTemperature SusceptibilityPenetration is the consistency of a bituminous material expressed as the distance in tenths of a millimeter that a standard needle penetrates a sample of material vertically under standard conditions of loading, time and temperature (above). The standard loads are 100 and 200 grams, and the time is 5 seconds. A penetration depth of 2.2 cm. is 220.
4Temperature Susceptibility is a measure of the change in viscosity with change in temperature. For best durability, asphalts used in climates with wide variations in temperature should have low temperature susceptibility
5Viscosity is a measure of a material’s resistance to flow when subjected to a load. It is expressed as the ratio of shear stress (t) to shear ratetg1 cm.gUnits for viscosity are Pascal seconds (Pa-s, N/m2*s) or poise (P). 10P = 10 Pa-sViscosity varies widely with changes in temperature. At room temperature, water = 106 Pa-s, light motor oil = x 106 Pa-s, and honey = 1011 Pa-s. Glass is considered solid at a viscosity of 1012 Pa-s.
6Specific Gravity is the density of asphalt at 77oF divided by the density of water at the same temperatureDurability is defined as the property that permits a pavement to withstand the detrimental effects of moisture, air and temperature.Oxidation is a chemical reaction that occurs when a material is exposed to air. Hydrogen in the asphalt combines with oxygen to make water, leaving behind a material rich in carbon, leading to hardening and loss of ductility and adhesion.Volatilization occurs when lighter hydrocarbons evaporate from asphalt.When asphalt is heated and allowed to cool, its molecules will rearrange to form a gel-like structure, which hardens with time. This is called age hardening
7Asphalt Grading Systems Asphalt is graded according to its viscosity (at various temperatures) or its penetration. Note that the AC-XX number corresponds to the viscosity divided by 100.
10Performance Grading requires that tests be performed at the critical pavement temperature – which is different for different grades depending on the temperature during service – with the criteria fixed or the same for all grades.The first number represents the maximum service temperature, and the second the minimum. For example, PG is suitable for application when the maximum temperature is between 39 and 64 oC, and the minimum is not less than 034oCThree reasons pavement fails:Deformation or rutting at high temperatures as asphalt softensFatigue resulting in cracks due to high loads or agingLow-temperature cracks, as asphalt becomes brittle and shrinks in cold weather
11Asphalt cement is held together by aggregate interlock or internal friction. Gravel has very little internal friction and interlocking, while crushed stone has high interlocking friction.Particles should always be at the surface, to provide traction when the surface is wet.Too much asphalt results in segregation of the asphalt and aggregate, called bleeding or flushing.Air pockets account for 2-6% of the volume.Aggregates constitute 70-75% by volume, or 90-95% by weight.Excessive amounts of binder tend to lubricate the particles and lower the stability of the pavement
12Aggregate GradingOpen graded aggregate contains little or no fine aggregate. It has relatively large void space, and is good for roads requiring high permeability.Intermediate-graded aggregate contains more sand than coarse aggregate. Dense-graded aggregate has high fill fraction, and slow curing rate.Both Coarse and Intermediate-graded require a seal coat to make them impermeable to water, while Dense-graded aggregate does not require a seal coat.Coarse aggregate is graded aggregate made up of particles that are retained on a No. 4 Sieve. Fine aggregate almost entirely passes through a No. 4 sieve.
14There are two types of asphalt concretes: Hot mixed, hot-laid mixtures (HMA) andCold-mixed, cold laid bituminous mixturesHot-mixed asphalt is:DurableResistant to ruttingCan sustain high loads and wider temperaturesHot-mixed asphalt cement is aggregate mixed with asphalt cement, tar or emulsified asphalt. It must be heated to ~300oF prior to mixing.Cold-mixed asphalt is:Made for lighter useCheaper to applyGood for road repairs, resurfacingCold-mixed asphalt is aggregate mixed with emulsified asphalt, cutback asphalt, or tar, and applied at ambient temperature.
15Asphalt pavement is flexible, requires less preparation than concrete pavement, and it can be repaired quickly.It also requires a higher amount of maintenance, periodic surface treatments, and becomes hard and brittle with age and under load.Pavement is made up of four elements:SubgradeSubbase courseBase courseSurface courseThe subgrade acts as the foundation, and may be stabilized.The subbase is made of aggregates, sometimes mixed with lime.The base course supports the wearing surface, and may be made of asphalt or untreated aggregate, such as crushed stone, gravel, sand, or cement.The surface coarse is the finished asphalt concrete, sometimes topped with a sealant
16The base course may be designed to provide good drainage. Various spray applications to pavement include:Seal Coats – sprayed asphalt followed by application of stone/gravel cover. The largest aggregate is never more than twice the size of the smallest.Fog seal is a light application of slow-setting emulsified asphalt, with or without aggregatesA prime coat is liquid asphalt applied to an untreated foundation layer or subgrade of stabilized soil, gravel, or water-bound macadam.A tack coat is a thin coat of bituminous material applied to an existing surface to provide bond between the new construction and the existing surfaceA slurry seal is a mixture of slow-setting emulsified asphalt, fine aggregate, mineral filler and water applied to the pavement without heat