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U of MD load deformation results
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Repeated Load Permanent Deformation Evaluation Theoretical Background Permanent deformation behavior of paving materials is an important factor in the design and analysis of flexible pavements. The development of rutting at the pavement surface is the result of pavement deformation of all the individual pavement layers including the subgrade. However, with recent increases in truck traffic, loading and tire pressure, a significant amount of permanent deformation may occur in the upper asphaltic layer. In order to determine the permanent deformation of the asphalt layer for a given mix type, permanent deformation parameters of the mix can be developed from laboratory testing. These parameters can then be used to predict the permanent deformation of the material in the field as a function of time (traffic).Currently, several approaches have been developed to determine the permanent deformation parameters of paving materials. Most methods employ a repeated dynamic load test under several thousand repetitions and permanent deformations monitored over the given cycle period. For the purpose of this comparative mix study, a maximum of 10,000 repetitions were used (approximately 3 hour test). The repeated load permanent deformation test can be used with any type of material (subgrade soil, granular subbase/base materials and bituminous mixtures). . . . General Test Description Repeated load permanent deformation tests were conducted on two replicate samples on all three AC-120/150 mix types to define the rutting resistance. For the three AC-10 mixtures evaluated, the replicate size was increased to three replicates to improve the variability of the results. All tests were carried out on cylindrical, 2.75 inch diameter and 6 inch high specimens. Sample preparation was the same as for the dynamic modulus testing described [earlier].The repeated load permanent deformation tests were all performed at a test temperature of 100 degrees F with three levels of loading stress: 10 psi, 20 psi, and 30 psi (deviatoric stress at unconfined state). A haversine pulse load was applied (i.e., 1 load pulse per second) with a time of load of 0.1 seconds and dwell time of 0.9 seconds. Each test was conducted up to 10,000 cycles of load unless failure (flow) occurred during the test sequence. An MTS electro-hydraulic test system was used to load the specimens. The load was measured through a load cell, whereas, the deformations were measured through two spring-loaded LVDTs (Linear Variable Differential Transducers). These LVDTs are clamped vertically on diametrically opposite specimen sides. Parallel clamps placed three inches apart and located one and half inches from the top and bottom of the specimen were used to secure the LVDTs in place. This system is identical to the one used to monitor the dynamic (complex) modulus of the mixtures. 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). . . . Study Results Both the graphical and the tabular results for the repeated load permanent deformation tests are shown in Appendix G [omitted*] for all six mixes. For each mix, a set of three plots, as discussed in the previous section, are included within the appendix. From the results obtained, the permanent deformation parameters, flow point, and the sample failure are evaluated as follows.Evaluation of Permanent Deformation Parameters [Tables omitted*] summarize the repeated load permanent deformation parameters (intercept: "a" and "µ"; slope: "b" and "alpha")for the six mixtures evaluated at T=100 degrees F. In these tables, individual replicate values, as well as average values, are shown for the three stress (deviatoric) levels tested. In addition, [figures omitted*] show these intercept-slope parameters plotted as a function of the percent Elvaloy® modifier, for each of the two basic asphalts and three stress levels, used in the study.The overall impact of adding the Elvaloy® modifier upon reduction in the overall permanent strain (rutting) observed in each mix must be evaluated with the combined influence of both the intercept and slope values generated. While consistent trends are difficult to establish with the intercept parameters [figure omitted*]; it is the slope parameter that is more important and sensitive to the permanent strain. As noted in [figure omitted*], the slope parameter "b" generally decreases significantly with percent Elvaloy® at all stress levels (this implies that the slope parameter, alpha, increases with modifier percentage). In general, the slope parameter "b" is smaller for the AC-10 mixtures compared to the AC120/150 modified mix. The overall impact of the reduction in the "b" slope parameter implies that the addition of the Elvaloy® significantly reduces the permanent strain of the mix. This is clearly observed in [AC-120/150 and AC-10 bar charts]. In these figures, each three clusters of bar graphs refer to repeated load permanent deformation strains measured under 10, 20 and 30 psi deviatoric loads. From, these diagrams it can be observed that the addition of the Elvaloy® polymer significantly decreases the permanent deformation of the mix. . . . In general summary, it can be concluded that the addition of Elvaloy® significantly reduces the permanent deformation (rutting) of asphalt mixtures compared to mixtures using conventional (virgin) binder sources. Cyclic Flow Point Analysis Another very important characteristic of permanent deformation behavior concems the presence (and number of repetitions) of the "cyclic flow point". This property is very important in that it signifies the onset of the second stage of permanent deformation (shear deformation at DeltaV = 0). Referring to [figure omitted*], it should be appreciated by the reader, that the epsilonp models previously discussed (a,b and µ, alpha) are only applicable up to the cyclic flow point (Nfpd repetitions). In many respects, the onset and presence of the stage 2 deformation is more important and critical as a measure of the gross stability of an asphalt mixture than the stage 1 (linear log-log) portion of the permanent deformation-repetition response curve.As previously noted and shown in Appendix G [omitted*], the prediction of the cyclic flow point (Nfpd) is automatically developed from the computerized data acquisition and analysis program used for the repeated load permanent deformation tests. The flow point is varied as the point in the curve of rate of change in permanent strain versus number of load repetitions where the slope equals zero. A large flow point (or no flow point observed within the maximum number of load repetitions used in the test) is indicative of a mixture having better rutting resistance and stability compared to a mixture with a very low (small) Nfpd value. [The following table] is a summary of the Nfpd values obtained during the repeated load permanent deformation tests conducted in the study. Summary of Cyclic Flow Point (Nfpd Value)
While, the results are somewhat difficult to portray graphically due to the absence of Nfpd values within the 10,000 maximum repetitions applied during the test for some test replicates, the general trends are unmistakeable and very important in illustrating another advantage of the Elvaloy® modifier in enhancing the permanent deformation behavior of asphalt mixtures. Referring to [the above table], it can be observed that Nfpd values were obtained at all stress levels for the virgin (non-modified) AC-120/150 and AC-10 mixtures. Furthermore, it can be observed that as the stress level is increased, the Nfpd value is logically decreased. If one now observes the trends of Nfpd values as the percentage of Elvaloy® is increased, it can be clearly seen that the Nfpd value is increased. As a specific example, one can consider the influence of the Elvaloy® at a stress level of 30 psi for the AC-120/150 mix. For the virgin mix (0% Elvaloy®), the Nfpd= 250. As the percent Elvaloy® is increased to 1.5%; it can be observed that the Nfpd value is increased to 3330. Finally, for the 2% Elvaloy® mix, no Nfpd values (flow points) were found in the test. This implies that the Nfpd value, if present, must clearly exceed the 10,000 maximum repetitions used in the study. A similar conclusion can also be drawn for the AC-10 mixtures at each of the three stress levels examined. It should be observed, that for all tests generated at the 2% Elvaloy® value, no Nfpd values were found. As a consequence, another important conclusion of this study is that the addition of the Elvaloy® polymer tends to make asphaltic mixtures stable over a longer repetition level, compared to non-modified or conventional asphalt mixtures. Observation of Sample Failure During testing, it was observed that at the end of the repeated load permanent deformation test, the deformation of the samples visually (and quite graphically) demonstrated the efficiency of the Elvaloy® modifier, as well as the influence of increased modifier percentage. [A photograph] shows the three types of AC-120/150 mixes at the end of permanent deformation testing at 30 psi stress level. It can be seen that the sample made of the virgin mix is seriously damaged, the sample made of the 1.5% modified mix is also deformed, but nowhere near the extent shown for the virgin mix. On the other hand, the sample made of the 2.0% modified mix has no damage (deformation) at all. This result is consistent to the analysis of the permanent deformation parameters and flow point evaluation. . . .
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