Simple Performance Tests: Summary of Recommended Methods and Database (2006)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

CHAPTER 3 SUMMARY OF TEST METHODS 3.1 DYNAMIC MODULUS The NCHRP 1-37A Test Method DM-1 (available as AASHTO TP-62, Determining Dynamic Modulus of Hot- Mix Asphalt Concrete Mixtures) was followed for E* test- ing. For each mix, generally two or three replicates were prepared for testing. For each specimen, E* tests were gen- erally conducted at 14, 40, 70, 100, and 130°F and 25, 10, 5, 1, 0.5, and 0.1 Hz loading frequencies. A 60-second rest period was used between each frequency to allow some spec- imen recovery before applying the new loading at the next lower frequency. Table 1 presents the E* test conditions. The E* tests were done using a controlled stress mode, which produced strains smaller than 200 micro-strain. This ensured, to the best possible degree, that the response of the material was linear across the temperature range used in the study. The dynamic stress levels were 10 to 100 psi for colder temperatures (14°F to 70°F) and 2 to 10 psi for higher tem- peratures (100°F to 130°F). All E* tests were conducted in a temperature-controlled chamber capable of holding temper- atures from 3.2 to 140°F (−16 to 60°C). The axial deformations of the specimens were measured through two spring-loaded linear variable differential trans- ducers (LVDTs) placed vertically on diametrically opposite sides of the specimen. Parallel brass studs were used to secure the LVDTs in place. Two pairs of studs were glued on the two opposite cylindrical surfaces of a specimen with each stud in a horizontal pair being 100 mm (4 inches) apart and located approximately the same distance from the top and bottom of the specimen. Top and bottom surface friction is a very practical problem for compressive type testing. To elim- inate the possibility of having shear stresses on the specimen ends during testing, pairs of rubber membranes, with vacuum grease within the pairs, were placed on the top and bottom of each specimen during testing. Figure 8 is a schematic pre- sentation of the instrumentation of the test samples used in the dynamic modulus testing. All E* test specimens were prepared and the E* tests were carried out according to the NCHRP Project 1-37A Test Method DM-1. Before compaction, the laboratory-blended HMA mixtures were short-term aged in the oven for 4 hours at 275°F, according to the AASHTO Test Method AASHTO PP2, “Standard Practice for Short and Long Term Aging of Hot Mix Asphalt.” Any laboratory-blended or plant-obtained 6 mixture was compacted in a Servopac gyratory compactor to a 6-inch diameter by 6.7 inches high. All test specimens were sawed and cored to obtain the final 4-inch-diameter by 6- inch-high E* test specimen. Before the E* testing, AASHTO T166-93 was followed to measure the bulk specific gravity and water absorption of the specimens. All final 4-inch- diameter by 6-inch-high E* test specimens were prepared to have the target in-place air voids ± 0.5 percent. 3.2 FLOW NUMBER AND FLOW TIME Fn and Ft test methods are presented in NCHRP Report 465. Test methods adapted for the simple performance tester developed in NCHRP Project 9-29, “Simple Performance Tester for Superpave Mix Design,” are presented in Appen- dix D of NCHRP Report 513: Simple Performance Tester for Superpave Mix Design: First-Article Development and Eval- uation, which is available online at http://gulliver.trb.org/ publications/nchrp/nchrp_rpt_513.pdf. In this research at ASU, repeated load and static creep tests, confined and unconfined, were conducted using at least two replicate test specimens for each mixture. All tests were carried out on cylindrical specimens, 100 mm (4 inches) in diameter and 150 mm (6 inches) in height. For the repeated load tests, a haversine pulse load of 0.1 second and a 0.9 sec- ond dwell (rest time) was applied for a target of 300,000 cycles. This number was lower if the test specimen failed under tertiary flow before reaching this target level. For the static creep tests, a static constant load was applied until ter- tiary flow occurred. An IPC Universal Testing Machine (UTM 25 kN) electro- pneumatic system was used to load the unconfined specimens. For confined testing, an IPC UTM 100 kN machine was used. The machines were equipped to apply necessary confining pressure. The load was measured through the load cell; the deformations were measured through six spring-loaded LVDTs. Two axial LVDTs were mounted vertically on dia- metrically opposed specimen sides. Parallel studs, mounted on the test specimen, placed 100 mm (4 inches) apart, and located at the center of the specimen were used to secure the LVDTs in place. The studs were glued using a commercial 5-minute epoxy. An alignment rod with a frictionless bushing was used to keep the studs aligned at extreme failure

7conditions. Figure 9 is a photograph of an actual specimen set up for an unconfined test. For radial deformations, four exter- nally mounted LVDTs aligned on diametrical and perpendic- ular lines were located at the center of the specimen and along opposite specimen sides. The radial LVDTs set-up is also shown in Figure 9. Thin and fully lubricated membranes at the test specimen ends were used to warrant frictionless surface conditions. All tests were conducted within an environmen- tally controlled chamber throughout the testing sequence (i.e., temperature was held constant within the chamber to ±1oF throughout the entire test). Figures 10 and 11 show a typical confined test set-up. Test Temp. (°F) Freq. (Hz) Cycles Rest Period (Sec) Cycles to Compute E* 25 200 - 196 to 200 10 100 60 196 to 200 5 50 60 96 to 100 1 20 60 16 to 20 0.5 15 60 11 to 15 14, 40, 70, 100, 130 (Unless otherwise specified) 0.1 15 60 11 to 15 a. Sample Assembly b. Lateral View Figure 8. Specimen instrumentation of E* testing. Figure 9. Vertical and radial LVDTs set-up for an unconfined test. Figure 11. Test set-up within triaxial cell with mounted radial LVDTs. Figure 10. Confined test set-up. TABLE 1 Test conditions of the dynamic modulus (E*) test