Technical information

4-10

)min(

ERR obtained for pass #5,000 were also used for the backcalculation of pass

#80,000.

4.2.3 Interim Results and Discussion

Table 4.2.1 presents the backcalculated layer moduli for pass #5,000 and pass #80,000.

The global error term (equation 4.2.7) was 4.89% for pass #5,000 and 6.27% for pass

#80,000. In both cases it can be seen that the stiffness of the pavement structure is

decreasing from top to bottom. During pass #5,000 the HMA is 14.6 times stiffer than

the underlying aggregate base. The aggregate base is seen to be twice as stiff as the

subgrade. By comparing these results with pass #80,000, it is clear that during the APT

experiment the individual layer moduli increased: (i) the HMA experienced a slight

stiffness increase of about 8.5%; (ii) the stiffness of the base increased significantly by

about 54%; and (iii) the subgrade increased in stiffness by about 16.5%. Subsequently,

the relative stiffness within the structure also changed, with the HMA ending up 10.3

times stiffer than the underlying base, and the base becoming 2.6 times stiffer than the

subgrade. In lieu of direct test data, these changes are believed to be the result of further

densification under the APT carriage passes, especially of the unbound materials.

Table 4.2.1: Backcalculated layer moduli for pass #5,000 and pass #80,000.

Layer

Thickness,

in. (mm)

Poisson’s

Ratio

Pass #5,000 Pass #80,000

Backcalculated Moduli, psi (MPa)

1 HMA 5 (127) 0.30 350,000 (2,412) 380,000 (2,618)

2 Base 6 (152) 0.35 24,000 (165) 37,000 (255)

3 Subgrade 61 (1,549) 0.40 12,000 (83) 14,000 (96)

4 Concrete

Semi-

infinite

0.20 4,000,000 (27,580)

Figures 4.2.2 and 4.2.3 show both the measured and calibrated model responses

for pass #5,000 and #80,000 (respectively) vs. offset distance from the gauge. Each

figure contains six charts. The two topmost charts show horizontal strains in X (left)