5000
Central Wisconsin Airport
4000
27,580 MPa
34,475 MPa
3000
2000
1000
Figure 19. Change in base course modulus
0
during spring thaw (FWD location 9, CWA).
19 Mar
24
29
3 Apr
8
13
18
23
28
found at OCA, changing the PCC modulus did not sig-
The elastic modulus of concrete (Ec), elastic sub-
nificantly affect the backcalculated subgrade modulus
grade modulus (Es) and the coefficient of subgrade re-
(Fig. 18 for location 9). However, as was also found at
action (k) can be determined from
OCA, changing the PCC modulus had a significant ef-
12(1 - 2 )
di
Ec =
Pl 2
fect on the base course modulus (Fig. 19). Trends in the
3
h
Di
data indicate that the base was in a weakened state from
Es = (1 - ν2 ) i
d
2P
the end of March through about mid-April. The values
Di
l
reported here for the base course modulus are still con-
sidered too large. By combining the change in the PCC
di 2 P
k=
modulus with changing the different layer thicknesses,
Di
l
singularly or simultaneously, we found that, in some
where =
cases, we were able to reduce the base course modulus
Poisson's ratio for concrete
ν=
to more reasonable values. However, we felt that
Poisson's ratio for subgrade
changing the layer thicknesses was introducing another
P=
applied FWD load
unknown variable into the analysis. Therefore, al-
Di =
FWD deflection measurement at sensor i
though the base course moduli are high, we have opted
di =
nondimensional deflection at sensor i
to present the results obtained for the reported
h=
PCC layer thickness.
thicknesses. It is very clear that a small coring program
Additional information on this method can be found
should be conducted with FWD testing to verify thick-
in Ioannides et al. (1989) and Barenberg and Ioannides
nesses. Irwin et al. (1989) reported that a 6-mm change
(1989). Barenberg and Ioannides have developed fig-
in layer thickness has a large impact on the backcalcu-
ures to determine l from the deflection basin area; l is
lated layer modulus. Also note that, although changing
then used to determine di. The base and subgrade were
the PCC modulus affected the base course modulus, the
modeled as a single composite structure at OCA. At
change produced very small changes in the absolute er-
CWA, the base, subgrade and bedrock were combined
ror (Table 7).
into a single layer.
ILLIBACK was also used to backcalculate the layer
Typical changes in the `subgrade' modulus during
moduli. This procedure was developed by Ioannides
spring thaw at OCA are shown in Figure 16. The `sub-
et al. (1989) as a closed form of backcalculation for
grade' modulus backcalculated from ILLIBACK was
a two-layer rigid pavement system using principles of
approximately 25% larger than that from WESDEF, as
dimensional analysis. For a given deflection sensor dis-
shown in Figure 20. The backcalculated PCC modulus
tribution, they found a unique relationship between the
for some of the FWD sites is shown in Figure 21. These
FWD deflection basin area and the radius of relative
are typical results and are also representative of other
stiffness of the pavement subgrade system (l). They
sites at OCA. They are also within the range of reported
then developed relationships between the ratios of non-
values, varying between 21,000 to 42,000 MPa. The
dimensional deflections and actual FWD deflections
data also indicate that the PCC modulus increased by
and l for a constant FWD loading plate radius (300
about 15% over the duration of the monitored period.
mm). These relationships were used with the applied
Based on the results, relationships between the sub-
load to calculate the coefficient of subgrade reaction
grade modulus and the basin area or ISM were devel-
(k), PCC modulus (Ec) and subgrade modulus (Es).
19
Previous Page
