This section discusses a range of laboratory tests used to predict the low temperature properties of HMA.
Thermal cracking is primarily an environmentally induced pavement distress. Cracks can occur as the result of one of two environmental events: one large, quick drop in temperature, or a number of moderate thermal cycles that accumulate incremental damage. Thermal cracking occurs when the tensile stress in the pavement exceeds the tensile strength of the HMA.
Cracking is initiated at the surface because this is location of the largest change in temperature. This figure shows a typical thermal gradient in an HMA layer and the corresponding stress gradient due to the contraction of the pavement.
The fracture temperature of the pavement can be estimated using both laboratory testing and predictions of thermal stress buildup in the HMA. This figure shows that the tensile strength of the mix can be determined at various temperatures (blue line). On the warmer side of the peak, thermal fatigue (cyclic) is a consideration while the colder side of the blue line would be more likely to be indicative of a one-time thermal event cracking. The thermal stress in the pavement is predicted over this same range of temperatures (red line). The temperature at which these lines cross is the fracture temperature.
A number of test methods have been used over the years in attempts to predict the temperature at which a pavement will crack. The methods listed on this slide will be discussed in this section.
The TSRST test method uses an asphalt concrete beam that has been sawn on all sides. It is epoxied to end caps, and mounted on a load frame in an environmental chamber. The distance between the two end plates is instrumented and a computer program is used to keep increasing the tensile stress on the sample so that the distance between the end caps remains constant as the temperature in the chamber is decreased at a constant rate of temperature drop. That is, as the temperature drops, the tensile load is increased to compensate for the thermal contraction of the material. The graph on this slide shows how the test results are presented. The increasing axial tensile stress is plotted against the temperature as it drops. There is a transition in the stress-temperature relationship. Above some temperature, the mix can substantially dissipate thermal stress build up due to stress relaxation. Below this critical temperature, the stress rapidly increases due to the inability of the material to flow (relax). This temperature is referred to as the transition temperature. The temperature at which the beam finally breaks is referred to as the fracture temperature. The corresponding stress is the fracture stress.
The advantage to this test method is that the thermal stress build up can be directly measured rather than have to be mathematically estimated.
This figure gives some examples of how HMA properties influence the thermal cracking potential. Note that there is a slight tendency for the higher air void contents in the crushed Watsonville granite to lower the fracture temperature. However, the asphalt source is by far the most important factor. The first group of 4 bars (AAG-1) represents an AC 20 asphalter bin; the second set an AC 10 from the same source. As expected, the softer AC 10 has a slightly lower fracture temperature. The third set of bars is for an AC 30 from a different source ( i.e., with a different chemistry). Even though the AAK-1 is graded as a stiffer asphalt binder, it has better resistance to thermal cracking. Again, within the same source of asphalt binder, the softer grade shows lower fracture temperatures.
When fracture strength is considered, lower air void contents produce higher stresses at the fracture temperature. This is possibly due to the lower fracture temperature associated with the lower air voids (previous slide). There also appears to be some dependency on the fracture strength on the type of aggregate at a given air void level and type of asphalt binder. There is no consistent trend in fracture strength with the type of asphalt binder.
This slide summarizes the observations made about the previous slides.
Test results indicate the material properties shown on this slide are desirable for resistance to thermal cracking.
These HMA properties were found to have little effect on the occurrence of thermal cracking.
There are also pavement structure characteristics that can influence the thermal cracking potential of the HMA.
HMACharacterization Thermal Cracking 3Background• Associated with volumetric contraction that occurswhen temperature decreases• Cyclic• One time• Quick, large drop• Thermal stresses develop• Crack occurs when tensile stresses exceed mixstrength• Temp at which this occurs is called fracturetemperature
HMACharacterization Thermal Cracking 8TSRST• Advantages• Direct measure of development of thermal stressbuild up• Eliminates the need to mathematically estimate• Disadvantages• Beams have to made and cut (time consuming)• Difficult to obtain good alignment in load frame
HMACharacterization Thermal Cracking 9TSRST-35-30-25-20-15-10-50AAG-1 AAG-2 AAK-1 AAK-2FractureTemp,C8%, RB4%, RB8%, RL4%, RLInfluence of Air Voids and Agg. TypeAAG-1 approx. AC 20 AAG-2 approx. AC 10AAK-1 = AC 30, AAK-2 = AC 10RB = Watsonville GraniteRL = Chert
HMACharacterization Thermal Cracking 10TSRST00.511.522.533.544.5AAG-1 AAG-2 AAK-1 AAK-2FractureStrength,MPa8%, RB4%, RB8%, RL4%, RLInfluence of Air Voids and Agg. TypeAAG-1 approx. AC 20 AAG-2 approx. AC 10AAK-1 = AC 30, AAK-2 = AC 10RB = Watsonville GraniteRL = Chert
HMACharacterization Thermal Cracking 11TSRST• Slope is greater for samples with lower voids• Specimens with lower voids fracture at higherstresses• Some influence of aggregate on fracturetemperature• Major influence of asphalt source rather thanviscosity26Influence of HMA Properties
HMACharacterization Thermal Cracking 12Desirable Material Properties• Asphalt binder• Low temperature viscosity• Temperature susceptibility• Aggregate characteristics• High abrasion resistance• Low freeze-thaw loss• Low absorption
HMACharacterization Thermal Cracking 13Material Properties ThatHave Minimal Effect• Asphalt binder content• Increasing % AC increasescoefficient of thermalcontraction• Offset by decreasingstiffness• Air voids• Minimal affect
HMACharacterization Thermal Cracking 14Pavement Structure• Narrow width pavements have closer crackspacing• Examples• 7.3 m (24 ft) width may have crackspacing of 30 m (100 ft)• 50 to 30 m (50 to 100 ft; airports) mayhave crack spacing > 45 m (150 ft)• Thicker pavements = less cracking• Friction coefficient between HMA and base• Prime coat reduces thermal cracking
HMACharacterization Thermal Cracking 15Pavement Structure (continued)• Subgrade• More cracks on sand than cohesive subgrades• Construction flaws• Steel roller compaction of HMA layers at hightemps and low mix stiffness creates transverseflaws• Cracks may be initiated at these points