ICOLD Bulletin: The Physical Properties of Hardened Conventional Concrete in Dams
Section 2 (Strength properties)
As submitted for ICOLD review, march 2008 Section 2-39
2.5 SHEAR STRENGTH
The significance of shear strength in dam concrete is practically limited to
discontinuities in the concrete mass (joints, cracks) and to the rock-concrete contact.
Shear strength depends on the adhesion between the substrate material and the grout
(in the case of grouted block joints), and the cohesive strength of the lift joint surfaces.
Shear strength is usually assumed to follow the Mohr-Coulomb
1
relationship
T=c.A + tanφ.N
where
T= shear force,
c= cohesion of intact lift surface or contraction (block) joint surface accounting for
shear key, if constructed,
A= intact area, e.g. total area times a coefficient assessing the efficiency of joint
curing, roughness or joint grouting,
tanφ = friction coefficient,
N normal force.
The Mohr-Coulomb relationship is an appropriate presentation for shear resistance,
since shearing strength is represented by the point where the failure envelope
intersects the vertical shear axis in a σ-τ(normal stress-shear) diagram.
If doubts about effective joint treatment (e.g. thermal cracking due to lacking curing) or
if block joints are taking no or little grout, c can conservatively be put to zero.
Shear resistance of the grout material (in block joints or at the concrete-rock contact)
depends on the water-cement ratio and thus on the compressive strength f
c
. In absence
of test results a shear value of τ=0.15. f
c
can be assumed, with τ being the intercept of
the failure envelope, as mentioned above.
In addition to the Mohr-Coulomb failure envelope, a bilinear shear resistance criterion is
meaningful for assessing the shear resistance in shear-keyed block joints. The lower,
steeper branch in the σ−τ diagram is for shear resistance of the plain concrete surface
within the keys (high φ, no cohesion), whereby the higher flatter branch is for shear
resistance of the joint surface (lower φ and some c= f [total block joint surface to shear-
keyed surface]). This implies that the steeper branch being the shear resistance of
concrete, say τ
c
=0.2. f
c
and the flatter branch a function of the key geometry.
For intact concrete a curved Mohr rupture diagram can be assumed enveloping the two
uniaxial Mohr circles for compressive and tensile strength.
Quality of bonding at joint surfaces is generally checked by shear tests from cores
drilled through joints. As may be expected, such shear tests demonstrate a wide range
of joint quality values between low-shear strength (core separation) and full material
strength (invisible joints).
1
The common terminology relates a curved envelope to the Mohr rupture diagram (constructed by the
uniaxial compression and tension circles) whereby the term Coulomb relates to a linear/bilinear failure
criterion