Lightweight Concrete for High Strength - Expanded Shale & Clay

V
"
pc
cw− Pr ed
= ft
−Pr
ed
1 +
"
ft
−Pr
ed
f
bd
p
(5.5)
The predicted diagonal tensile strength, f’’ t-Pred was calculated using equation 5.6:
f
"
t−
Pr ed
= 4λ
f
'
c
(5.6)
5.9.1 Initial Shear Cracking
Table 5.8 provides an overview of initial shear cracking. The predicted values were
calculated at the midpoint of the shear span. The experimental value was the applied shear at
which cracking was first recorded visually or electronically.
Examination of Table 5.8 showed that the AASHTO (1996) Standard method of
calculating concrete shear strength was conservative overall. In the case of girder tests G1C-
Center and G2C-Center, the prediction was less than 3 percent unconservative. The AASHTO
LRFD (1998) technique provided predictions of concrete strength that far underestimated their
capacity. The ACI alternate approach provided the closest prediction for V c where the cracking
shear was equated to the shear strength provided by the concrete. Overall, the predicted V c
values were within about 6 percent of experimental values for both the G1 and G2 girder tests.
Girder tests G1C-Center and G2C-Center showed the greatest difference from the ACI alternate
predicted values, about 18 percent unconservative overall. Both the tests involved the minimum
stirrup spacing of 24 inches.
5.9.2 Ultimate Shear Capacity
Table 5.9 provides an overview of predicted ultimate shear capacity calculated by the
AASHTO Standard method and the AASHTO LRFD method with the strength reduction factor,
φ s , of 1 and compares them to the experimental maximum shear values. Using the AASHTO
Standard method, the stirrup yield strength was capped at 60 ksi as required by the code, and the
results also recorded. Since the value of f y was 62 ksi, there was an insignificant difference in
predicted values.
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