HMA stiffness is an important input into a number of both mechanistic and empirical models to predict pavement performance. Stiffness information is needed to use layered elastic programs to calculate critical stresses and strains in the pavement layers. Common programs are ELSM5 and WESLEA (US Corp of Engineers Waterways Experiment Station Layered Elastic Program). The use of this material property in predicting fatigue and rutting characteristics will be discussed in those modules.
Loads are applied to samples in the laboratory using either dynamic or repeated loadings. Dynamic loading means that the load is continually changing in a sinusoidal pattern. Repeated loading means that a load is briefly applied (from 0.1 to 1 seconds is common) using either a step-wave form or a triangular wave form. This load is followed by a rest period that is intended to simulate the time between tire loads. This rest period allows for some recovery of the material deformation due to the viscoelastic nature of the HMA mixtures. Frequencies for both tests are typically between 1 and 10 Hz. In the case of repeated loading, a 1 Hz frequency means that a 0.1 second load is followed by a 0.9 second rest period.
Dynamic loading provides information on both the elastic and viscous properties. If a material is elastic, then the strain response will be in-phase with the applied stress. That is, the phase angle will be 0 degrees. If a material is viscous, then the response will be 90 o out of phase (phase angle = 90 degrees).
Resilient modulus is an approximation of Young’s modulus for HMA mixtures. Each time a square wave load is applied, the material response shows a ramping up of the strain followed by an immediate drop in strain when the load is removed. The gradual decrease in the strain during the rest period is a function of the viscoelastic nature of the material. Note that the strain produced by each load cycle is not fully recoverable. This results in a gradual increase in the strain level at the beginning of each load cycle. This represents the permanent deformation of the sample as a result of the load application. While the instantaneous modulus can be calculated using the instantaneously recoverable strain, resilient modulus is usually calculated using the total recoverable strain between loading cycles.
Any laboratory test that can be used to apply a load while at the same time measuring the corresponding strain can be used to calculate modulus (stiffness). Not all tests produce similar stiffness values.
This shows how the sample is typically set up for unconfined testing. The sample is placed inside of a confining pressure chamber when lateral pressures are used.
In general, stiffness measured with axial testing decreases with increasing air voids, regardless of test temperature.
On the other hand, there is little change in stiffness with a significant change in asphalt binder content (air voids are a constant).
Diametral testing is one of the more commonly used methods because it uses conventional cylindrical (gyratory) samples. The sample is also easily loaded into the testing frame. A load is applied along the vertical axis of the sample and the corresponding horizontal deformation is measured as indicated by the arrows. The sensor collar which attaches to the sample is not shown in this picture.
Diametral stiffness measurements are slightly dependent on the asphalt binder content. Higher asphalt contents tend to result in slightly lower moduli values at intermediate to warmer temperatures.
As with the axial method of stiffness measurements, moduli decrease with increasing air voids, regardless of the test temperature.
There is little influence on stiffness between either 0.5 and 1.0 Hz.
In general, axial stiffness measurements are more sensitive to changes in air voids than diametral testing. Diametral stiffness values are significantly higher than those determined using axial testing. Testing is limited by the test temperature. With either test, there is an appreciable amount of permanent deformation at the warmer temperatures. It is best to limit stiffness measurements to intermediate or cooler temperatures.
A third method of measuring stiffness is in flexure. This is usually done as a part of flexural fatigue testing (discussed in the following module).
Typical beams used for flexural testing are cut from larger beams so that all sides have cut faces. Either dynamic or repeated loading can be used. A repeated loading with a 0.1 second load and 0.6 or 0.9 second rest period is common.
Both the axial and flexural stiffness values are similar. The diametral stiffness tends to be consistently higher than either of these two.
There is essentially no influence in asphalt binder content when determining stiffness in the axial configuration. Both the flexural and diametral configurations show a tendency for lower stiffness values with increasing asphalt binder content. Note that both of these tests subject the sample to tensile stresses.
The simple shear tester can be used to determine the shear modulus. It is standard testing procedure to use dynamic loading at a range of frequencies over several temperatures. This is referred to as frequency sweep testing. Data is used to construct master curves (discussed in the following module). The sample is restrained at a constant height by monitoring any changes with the vertical sensors (installed between the blue clamps). The sample with the top and bottom platens epoxied to the sample, is loaded in the test chamber. The ram is lowered until the desired axial load is applied. Then a dynamic shear load is applied by the horizontal movement of the bottom platen.
Stiffness, in general, tends to be sensitive to asphalt binder type (viscosity), aggregate type, air void content, and temperature.
Block 28 Sp 13
HMA Characterization Stiffness 1Stiffness of HMAMixturesSenior/GraduateHMA Course
HMA Characterization Stiffness 2Important for PredictingPavement Performance• Used to predict:– Critical stresses and strains• ELSM5• WESLEA– Fatigue cracking– Permanent deformation characteristics
HMA Characterization Stiffness 6Stiffness• Fundamental to the analysis of pavementresponse to traffic loading• Various methods– Axial resilient (ASTM D3497)– Diametral resilient (ASTM D4123)– Flexural dynamic– Shear dynamic
HMA Characterization Stiffness 8Axial Resilient Stiffness101001,00010,0000 10 20 30 40 50Temperature, CStiffness,ksiLow Air VoidsHigh Air VoidsAffect of Temperature and Air Voids(Tayebali, Tsai, and Monismith, 1994)
HMA Characterization Stiffness 9Axial Resilient Stiffness101001,00010,0000 10 20 30 40 50Temperature, CStiffness,ksi OptimumHigh AC ContentAffect of Temperature and Asphalt Content(Tayebali, Tsai, and Monismith, 1994)
HMA Characterization Stiffness 12Diametral Stiffness101001,00010,0000 10 20 30 40 50Temperature, CDiametralResilientModulus,ksi4% Air Voids8% Air Voids(Tayebali, Tsai, and Monismith, 1994)Increased air voids = decreased modulus
HMA Characterization Stiffness 13Diametral Stiffness101001,00010,0000 10 20 30 40 50Temperature, CDiametralResilientModulus,ksi0.5 Hz1.0 Hz(Tayebali, Tsai, and Monismith, 1994)No significant influence on frequency (rest period)
HMA Characterization Stiffness 14Axial vs. Diametral• Axial more sensitive to air voids than diametral• Diametral stiffness about 35 to 45% greater than axialstiffness• Elastic modulus difficult to measure even atmoderately warm temperatures (40oC (104oF))because of excessive sample deformation– Best to limit test temp to < 25oC (77oF)
HMA Characterization Stiffness 16Flexural Stiffness• Typical beams– 38 x 38 x 381 mm (1.5 x 1.5 x 15 in)• Testing parameters– 0.1 sec load, 1.67 Hz haversine– range of temperatures• 0 to 25oC (32 to 77oC)• Used to estimate stiffness at critical strainanticipated in pavement
HMA Characterization Stiffness 17Comparisons03006009001,200Flexural Axial Diametral8% Air Voids4% Air Voids20oC (68oF)Resilient Modulus, psi