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Flexture testing1/28/2024 The load rate is variable but is normally set at 1 to 10 Hz. During the test, the beam is held in place by four clamps and a repeated haversine (sinusoidal) load is applied to the two inner clamps with the outer clamps providing a reaction load (Figure 4). The flexural fatigue test is performed by placing a beam of HMA in repetitive four point loading at a specified strain level. Work on NCHRP Project 9-38, Endurance Limit of Hot Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements, is underway to identify the existence of a fatigue endurance limit and measure it for selected HMA mixtures. Figure 3: Typical flexural fatigue graph illustrating an endurance limit. Note that beyond a certain point the plot is essentially horizontal – indicating an infinite fatigue life. loads to failure tend to look like Figure 3. If damage due to loading falls below this “healing potential” then damage accumulation is virtually non-existent (Carpenter, 2003 ). Therefore, a material property of HMA is its ability to recover some constant amount of damage or its “healing potential”. The theory is that a continuous physical-chemical healing reaction occurs, even during continuous loading, at low strain levels (Carpenter, 2003 ). More recent studies seem to suggest that at low levels of strain (around 70 microstrain), HMA mixtures have, in effect, an infinite fatigue life. suggested that the relationship between strain at the bottom of the HMA layer and the number of cycles to failure seems to undergo a significant slope change at lower strain levels (in the vicinity of 70 microstrain). This strain level changes with HMA thickness (thicker pavements give lower strain values), stiffness and other properties. It development is directly proportional to the strain level at the bottom of the layer (Carpenter, 2003 ). HMA pavements are similar.Ī classic fatigue crack starts at the bottom of a HMA pavement layer (or structure) and grows towards the surface. A paper clip can be broken by repeatedly bending it just as a large pressure vessel can fail after being subject to many thousands of pressure cycles. The concept of a fatigue life centers around the universal idea that most materials undergo a gradual deterioration under repeated loads that are much smaller than the ultimate strength of the material. The flexural fatigue test is used to investigate fatigue as it relates to HMA construction materials. A number of factors can influence a pavement’s ability to withstand fatigue, including pavement structure (thin pavements or those that do not have strong underlying layers are more likely to show fatigue cracking than thicker pavements or those with a strong support structure), age of the pavement, and the materials used in construction. In HMA pavements, fatigue cracking occurs when repeated traffic loads ultimately cause sufficient damage in a flexible pavement to result in fatigue cracking (Figure 2). AASHTO T 321: Determining the Fatigue Life of Compacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flexural Bendingįigure 1: Flexural fatigue device loaded with a HMA beam.The standard beam fatigue procedure is found in: Because of the testing equipment complexity and long testing times, the flexural fatigue test is primarily a research test and is not a standard test in Superpave mix design or quality assurance testing. In order to relate laboratory results to normally observed field performance, a shift factor of 10 to 20 is typically needed. The basic flexural fatigue test subjects a HMA beam to repeated flexural bending in a controlled atmosphere (Figure 1). These failures ultimately result in fatigue cracking which will cause disintegration of the pavement if not maintained in time. However, when pavements are under-designed strains are sufficiently high to cause fatigue failures under repeated loads. In a well designed pavement, strains in the pavement are low enough so that fatigue is not a problem. This characterization is useful because it provides estimates of HMA pavement layer fatigue life under repeated traffic loading. The flexural fatigue test is used to characterize the fatigue life of HMA at intermediate pavement operating temperatures.
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