How do you design and evaluate asphalt mixtures for low-temperature cracking resistance?

1 Factors affecting LTC resistance

The resistance of asphalt mixtures to LTC depends on several factors, such as the binder type and grade, the aggregate gradation and properties, the air voids and density, the mix additives and modifiers, and the environmental conditions. Generally, binders with lower stiffness and higher ductility are more resistant to LTC, while aggregates with higher angularity and fracture toughness provide better crack resistance. Air voids and density affect the thermal conductivity and expansion of the mix, as well as its resistance to moisture damage. Mix additives and modifiers, such as fibers, polymers, and anti-stripping agents, can improve the strength, flexibility, and durability of the mix. Environmental conditions, such as temperature, humidity, and freeze-thaw cycles, also influence the cracking behavior of the mix.

2 Testing methods and standards

Various testing methods and standards have been developed to evaluate the LTC resistance of asphalt mixtures. The most common ones include the Thermal Stress Restrained Specimen Test (TSRST), which measures the tensile strength and strain of a cylindrical specimen; the Semi-Circular Bend Test (SCB), which measures the fracture energy and toughness of a semi-circular specimen; the Disk-Shaped Compact Tension Test (DCT), which measures the fracture energy and toughness of a disk-shaped specimen; and the Bending Beam Rheometer Test (BBR), which measures the creep stiffness and relaxation modulus of a thin beam specimen. Each test has its own advantages and limitations, and they can be used to determine various parameters, such as cracking temperature, fracture energy, flexibility index, and m-value. Additionally, standards like AASHTO, ASTM, SHRP, and NCHRP have been established to inform selection and execution of these tests.

3 Design methods and practices

Designing asphalt mixtures for LTC resistance requires the use of several methods and practices, such as the Performance-graded binder system (PG), the Mechanistic-empirical pavement design guide (MEPDG), and the Balanced mix design (BMD). The PG system classifies binders based on their rheological properties at different temperatures, while the MEPDG uses a mechanistic-empirical approach to predict pavement performance. The BMD method balances the rutting and cracking resistance of the mix by considering both volumetric and mechanical properties. These methods can help you design and select asphalt mixtures that meet your project's performance requirements and specifications. However, other factors such as material availability, construction and maintenance practices, local experience, and cost should be considered when making your final decisions.

4 Here’s what else to consider

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