Technical information

3-27
bottom of the HMA goes into compression. Then, the strain direction is reversed and
the gauges go into tension. The point of maximum tension occurs when the APT
carriage has passed the gauge positions along the Y-axis by about 1 to 3 in. (25 to 76
mm). Finally, when the load is receding (APT carriage moves further along), the tensile
strains are reversed and compression is induced once more at the bottom of the HMA.
This pattern is more pronounced for the gauges aligned along the centerline (G-2 and G-
4).
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-400
-350
-300
-250
-200
-150
-100
-50
0
50
100
150
0 40 80 120 160 200 240
APT Carriage Location [in.]
Bottom of HMA Strain (in Y) [microstrains]
Pass #5,000
Pass #80,000
Loading Direction
G-2,
Centerline
G-4,
Centerline
G-5,
2ft Off-Center
G-7,
2ft Off-Center
Figure 3.5.3 Measured horizontal strains at the bottom of the HMA in the direction of
loading during pass #5,000 (solid line) and pass #80,000 (dashed line).
It can be graphically seen that the approaching branch of the strain response is
different from the receding branch, resulting in a non-symmetrical time history curve.
The two most noticeable differences are: (i) peak compressive strain is usually higher in
the approaching branch compared to the receding branch; and (ii) the spacing along the
Y-axis between the tension and compression strain peaks is larger in the receding
branch compared to the approaching curve.
Referring to the approaching branch of pass #5,000 for gauges G-2 and G-4
(both centerline gauges), peak strains were 84 and 119 microstrains in compression and