Beam Flexural Fatigue Test

Overview

The Beam Fatigue Test, also known as the Flexural Fatigue Test, is used to determine the fatigue life of Hot Mix Asphalt (HMA) under various conditions. It is an important test for civil engineers as it can be used to predict how long the HMA layer of pavement can undergo loading due to repetitive traffic use before failure will occur. The test is performed in stress or strain controlled mode with repeated loading to simulate the traffic conditions.

How is a Beam Fatigue Test Performed?

A prismatic specimen is placed inside the testing apparatus and is clamped in place. There are four clamps across the length of the beam—the two outer clamps serve to hold the specimen in place while the load is applied at the central two clamps. The operator selects a strain value that the specimen will be tested at (i.e. 800 microstrain) and a frequency. This frequency determines how many times the beam will be loaded to the designated strain value each second. The test is allowed to run until the specimen fails, with at least 10,000 cycles occuring before failure.

Diagram of Beam Fatigue Test

What does a Beam Fatigue Test Specimen Look Like?

When performing the beam fatigue test, a total of nine specimens must be tested in order to determine the complete failure curve. These asphalt concrete beams should be construted in accordance with AASHTO Standard PP 3. Each specimen should be approximately 380 mm (14.96 in.) in length, 50 mm (1.96 in.) in height, and 63 mm (2.48 in.) in width, and are cut to size using a precise saw to ensure that the sides are all smooth and parallel.

How are Hot Mix Asphalt Characteristics Determined?

In order to conform to AASHTO standards for the beam fatigue test, the applied load, beam deflection, tensile stress, tensile strain, and flexural stiffness must all be recorded during each cycle of the test. From these calculations, the number of cycles until failure can be estimated and a plot of the Normalized Modulus times the number of cycles versus the number of cycles can be created.

Stiffness Ratio versus Number of Cycles Graph

The following equations show how these values can be calculated.

                          

In these equations,

σ=Tensile Stress a=Center to Center Spacing of Clamps P=Applied Load b=Average Specimen Width h=Average Specimen Height ε=Tensile Strain δ=Deflection at Center of Specimen

L=Length of Specimen Between End Clamps S=Beam Stiffness NM=Normalized Modulus X Cycles Si=Beam Stiffness at Cycle i Ni=Cycle i So=Initial Beam Stiffness No=Cycle Where Initial Stiffness is Calculated

When calculating the Normalized Modulus times the number of cycles, the initial beam stiffness should be calculated at approximately the 50th cycle. After performing these calculations, a plot of the Normalized Modulus times the number of cycles versus the number of cycles can be created. A sample graph is shown below.

After creating this graph, the maximum number of cycles experienced before failure can be determined. This value is given as the cycle where the Normalized Modulus times the number of cycles is maximum. For example, in the graph shown above, approximately 100,000 cycles occur before failure.

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Keywords: Asphalt Concrete — Fatigue — Flexural Bending Test — Flexural Test — Flexural Testing Machines — Testing Instruments — Beam Fatigue Test — Beam Flexural Fatigue Tester — Load — Fatigue Life — Hot Mix Asphalt

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