Environmental Engineering Reference
In-Depth Information
Figure 16.13.
Ratio of splitting tensile strength versus compressive strength - of concrete without lift joints
- all USA dams data (Khabbaz and Fell, 1999).
16.4.2.3 Shear strength of concrete
The terminology used by EPRI (1992) and USBR in their testing of concrete in direct shear
is shown in Figure 16.14 . Samples are described as bonded if they are intact, and
unbonded if the sample is broken along the plane of weakness being tested.
The peak strength is typically reached at small displacements (0.25-1.25 mm). The
residual strength is reached at large displacements but for practical purposes it has been
taken as the lowest consistent strength at displacements between 2.5 mm and 12 mm. The
sliding friction strength is the peak strength on an unbonded specimen.
The shear strength can be represented in Mohr-Coulomb terms as:
S
c
tan
(16.20)
where S
shear strength; c
effective cohesion;
effective normal stress;
effective friction angle.
The strength envelope is actually curved and at low normal stresses the friction angle is
high, but approaches the residual value at high normal stresses. This non linearity needs
to be considered in design.
EPRI (1992) report the following results from testing of concrete lift joints:
Peak strength:
best fit
c
2.1 MPa,
57°
90% stronger than
c
0.95 MPa,
57°
Sliding friction strength:
best fit
c
0.5 MPa,
49°
48°
Figures 16.15 to 16.18 show test data. It can be seen, that as suggested by EPRI (1992),
it is best to use a bi-linear relationship to model the sliding friction strength.
There is little difference between sliding friction shear strength and residual strength -
only 2° to 3° in the data available. The lower bound of the sliding friction strength data is
c
90% stronger than
c
0,
34°. Many of the data near the lower bound are from Stewart Mountain
Dams, which displayed alkali-aggregate reaction, with silica-gel noted on the surfaces of
open joints. If these data are excluded the lower bound becomes c
0,
38°.
The recommended strengths from guidelines given in Table 16.4 are on the conservative
side if there has been no displacement on the lift surfaces. However only small differential
movements, e.g. from thermal expansion and contraction, would take the strength
0,
 
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