U of MD dynamic modulus results

Test Results

< U of MD dynamic modulus results

Complex (Dynamic) Modulus Evaluation

Theoretical Background (omitted*)
General Test Description

Dynamic Modulus tests were conducted on two replicate specimens for each of six mixtures evaluated in this study. For each mixture tested, a full factorial of test frequencies (0.1, 1, 10 and 25 Hz) and temperature (0, 40, 70, 100 and 130 degrees F) were use. Each specimen was tested in an increasing order of temperature, that is; 0, 40, 70, 100 and 130 F. For each temperature level, specimens were tested in a decreasing order of frequency (25, 10, 1 and 0.1 Hz). This temperature-frequency sequence was carried out to cause minimum damage to the specimen before the next sequential test. This is due to fact that at cold temperatures and high frequency levels the material behaves stronger compared to warmer temperatures and at low frequency levels. This sequence of testing resulted in a total of 240 dynamic modulus tests on twelve specimens using both types of virgin binder as control.

The dynamic modulus specimens were developed in a rather unique manner for this study. For a given mix type, a 6-inch diameter gyratory specimen was compacted at the specific design asphalt content to a target air void level of 7 plus-or-minus 1%. This compaction was achieved using the Superpave^TM-Rainhart gyratory compactor. The specimen was compacted to an approximate height of 6 inches and the precisely established relationship of N_gyr to V_a (percent maximum theoretical specific gravity) was used to determine the approximate number of gyrations necessary to achieve the target air void.

Upon extrusion of the compacted mixture, two replicate specimens were then cored from each gyratory compacted mixture. Each core had a diameter of 2.75 inches, an approximate height of 6 inches and therefore complied with the minimum height/diameter ratio of 2.0 (actual ratio was 2.2) for dynamic modulus evaluation. The specimens were weighed in air and water, air voids computed and then capped with a silicone capping compound for testing. This process was found to yield highly satisfactory results for quickly making replicate specimens at any desired air void level with a minimum amount of effort.

An MTS electro-hydraulic test system was used to load the specimens. The dynamic moduli and phase angle were measured by applying a dynamic sinusoidal stress (continuous wave) on unconfined specimens. The dynamic load was measured through the MTS load cell, whereas, the deformations were measured through two spring-loaded LVDTs (Linear Variable Differential Transformers). These LVDTs are clamped vertically on diametrically opposite specimen sides. Parallel clamps placed three inches apart and located one inch from the top and bottom of the specimen were used to secure the LVDTs in place.

All tests were conducted within an environmentally controlled chamber throughout the testing sequence (i.e., temperature was held constant within the chamber to plus-or-minus 1 degree F throughout the entire test). After testing at a given temperature was completed, the new temperature was adjusted in the chamber for the next day's test and specimens stored within the chamber to reach the new equilibrium temperature. This resulted in a time period of generally 18 to 24 hours for the specimen to reach and maintain the required test temperature. As noted from this description, all dynamic modulus tests were conducted in accordance with ASTM (D-3497) procedure.

Study Results

[Tables omitted*] summarize the measured dynamic modulus and phase angles obtained for all six types of mixtures investigated in the lab study. Appendix E also summarizes the results and contains plots of the relationships between temperature and dynamic modulus, as a function of frequency, for the six mixtures.

Analysis of Results

Phase Angle
It has been previously indicated that the phase angle is an indicator of the viscoelastic behavior of an asphaltic mixture. A value of 0 degrees is indicative of a pure elastic material, while a value of 90 degrees is one that exhibits pure viscous behavior.
[Note: detailed discussion of mathematical model and regression constants omitted*.]
. . . a review of all of the figures does point out that a reduction of the maximum f value occurs due to the addition of the Elvaloy® polymer.

By use of numerical differentiation, the maximum phase angle value, and the dynamic modulus where the value occurs, was computed for each mix and model type. The following table summarizes these results and also indicates the average values (from both model forms).

Influence of Elvaloy® Upon Peak Phase Angle

	Max Phase Angle (deg.)			E* at Max Phase Angle (psi)
Mix Type	Ln	Log	Avg.	Ln	Log	Avg.

AC 120/150	43.1	38.1	40.6	284,000	217,000	250,500
+1.5% Elvaloy®	38.6	35.1	36.8	241,000	256,000	248,500
+2.0% Elvaloy®	40.6	36.6	38.6	244,000	211,000	227,500

AC 10	38.8	36.2	37.5	469,000	499,000	484,000
+1.5% Elvaloy®	36.3	34.7	35.5	465,000	409,000	437,000
+2.0% Elvaloy®	35.8	33.6	34.7	386,000	383,000	384,500

In general, the addition of the Elvaloy® polymer reduced the maximum phase angle value by approximately 3-4 degrees. It can also be observed that the E* value, corresponding to the maximum phase angle is likewise reduced for both the AC 120/150 mixtures. For the AC 120/150 mixtures, a 10% decrease in the dynamic modulus (at peak phase angle) occurred; while a 20% reduction occurred in the AC-10 mixtures. This observation implies that the polymer tends to make the mix more elastic (less viscous) at higher temperatures (lower moduli).

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