Determination of Creep Compliance and Tensile Strength of Hot-Mix ...

n Dn PNormalization Constant = × × avg (5)bavg Davg Pnwhere:b n , D n , and P n = thickness, diameter, and creep load of specimen n, respectively.The trimmed mean of the normalized deformations (i.e. ΔX tm,t and ΔY tm,t ) is simplythe average of the remaining values (usually 4) after the maximum and minimumvalues have been discarded.Creep compliance values needed for input into the M-E PDG Thermal Crackingmodule are calculated at 1, 2, 5, 10, 20, 50, and 100 seconds of loading, at -20, -10,and 0°C. The first major step is to determine the deformations at these times duringtesting at each of the temperatures.Upon inspection of the raw acquired data, one first identifies the points in time atwhich 1) the load is first applied to the specimen and 2) the load stabilizes to ± 2% ofthe target creep load. In viscoelastic theory, the load versus time profile for creeptesting is a step function; i.e. the load is applied instantaneously, held constant forthe desired length of time, and then removed instantaneously. However,instantaneous loading in the real world is impossible. Under ideal real-worldconditions the elapsed time between the initial application of load and stabilization atthe creep load (± 2%) would be 0.1 second or less, based on the opinions ofexperts. However due to equipment limitations, elapsed load “ramp” time (i.e. theelapsed time between initial application of the load and the stabilization of the load to± 2% of the target creep load) during this study averaged 3 seconds.Per recommendations by Harold Von Quintus, MoDOT’s consultant on calibration ofthe M-E PDG, creep compliance at 1 second, for example, would be calculatedusing deformations recorded 1 second after the load stabilized to ± 2% of the targetcreep load; i.e. the point in time at which the load stabilized to ± 2% of the targetcreep load would be considered t zero . In essence, a true creep load profile was beingassumed. All creep compliance values at different times, t, are calculated relative tot zero . Designated as the “original” method throughout the remainder of this paper, themethodology described above is shown in Figure 6 using a time-abbreviateddataset. Deformations are designated as North or South (i.e. the face of thespecimen the deformations are associated with), and Vertical or Horizontal.Note that in this particular dataset, the load “drooped” to the lower limit (target creepload – 2%) immediately following the very brief overshoot, and stayed there forseveral seconds before fully stabilizing at the target creep load of 2000 pounds. Thisphenomenon occurred quite often but not all of the time, and seemed to result froma combination of the tuning of the T-O servo-hydraulic gains (i.e. Proportional,Integral, and Derivative gains or PID’s), the particular specimen and testtemperature, and inherent peculiarities of the T-O system.14