12.1.2. Stress Relaxation and Creep

• Stress Relaxation: A constant strain results in decreasing stress over time.

Stress relaxation describes a scenario where a material is held at a fixed deformation or strain. While the deformation is constant, the internal stress within the material gradually decreases over time. This phenomenon happens because the material can 'relax' some of the internal forces that were created when the deformation was first applied. In practical terms, if you imagine stretching a rubber band and holding it in that stretched position, over time, the tension in the rubber band may reduce even though it remains stretched.

Consider the analogy of an elastic band being stretched. When you stretch the elastic band and hold it in place, if you check the tension in it after a few minutes, you'll notice that it feels looser than when you first stretched it. This is because some of the energy has dissipated, similar to the way materials in pavements may behave under constant strain.

• Creep: A constant stress causes increasing strain over time.

Creep is the phenomenon where a material experiences continuous deformation or strain when subjected to a constant load or stress over an extended period. Unlike stress relaxation, where stress decreases while strain remains constant, creep involves a gradual increase in strain as the material yields under the sustained load. This is particularly evident in bituminous mixes used in pavements, where the weight of vehicles can lead to progressive deformation over time.

Think of a sponge being pressed down under a heavy object. Initially, the sponge compresses under the weight, but if the weight stays there for a long time, the sponge continues to flatten slightly more and more. This is similar to what happens in pavement structures where they may slowly deform under constant traffic loads, leading to issues like rutting.

• These behaviors are analyzed using creep compliance and relaxation modulus functions.

To understand and quantify the concepts of stress relaxation and creep, engineers use two important functions: creep compliance and relaxation modulus. Creep compliance indicates how much a material will deform over time under a constant stress, while relaxation modulus measures how much stress decreases over time under constant strain. These functions provide critical insights into the material’s performance, enabling engineers to predict how bituminous mixes will respond to loads and environmental factors throughout the lifespan of a pavement.

Imagine a sponge being pressed under a weight. If you tap the weight at regular intervals, creep compliance is like measuring how much the sponge squishes down further each time. On the other hand, if you stretched a rubber band and held it there, the relaxation modulus is like observing how much the tension decreases in the band even though you keep it stretched. These measurements help engineers design better pavements that can withstand constant loads without failing.

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