Lightweight Concrete for High Strength - Expanded Shale & Clay

SH Prestress losses due to shrinkage
'
f ci
’
f c
M n
M cr
I t
V u
Concrete compressive strength at release
Concrete compressive strength at time of testing
Nominal moment capacity of a member
Cracking moment capacity of a member
Transformed moment of inertia of a member
Ultimate shear force at the critical section
λ 1.0 ≤ [λ = (0.6+40ε ps )] ≤ 2.0
A.12 Elements of Bond
There are three mechanisms that allow the development of bond stress between the
prestressing strand and the concrete to include adhesion, frictional bond due to Hoyer’s effect,
and mechanical interlock. Slipping of the prestressing strand relative to the concrete is required
to develop bond stress. In general, bond resistance of seven-wire prestressing strand remains
constant or increases after initial slip.
A.12.1 Adhesion
Adhesion occurs in a very thin layer at the interface between the concrete and
prestressing strand. Acting like an “adhesive,” adhesion exhibits rigid-brittle behavior
preventing slip of the strand relative to the concrete up to a critical stress level. During the
transfer of prestress or girder testing, adhesion is lost when the bond stress exceeds the critical
stress level. Once the critical stress level is exceeded, the adhesion portion of bond is lost. In the
case of seven-wire prestressing strand, the loss of adhesion is often replaced by other
mechanisms of bond. Because failure of the adhesive bond occurs at a very small displacement,
the overall contribution of adhesion to bond is minor.
A.12.2 Frictional Bond Due to Hoyer’s Effect
Research by E. Hoyer in 1939 using small diameter smooth piano wire resulted in the
identification of a mechanism of bond now known by his name. When wire or prestressing
strand is tensioned, Poisson effects cause the diameter to become smaller. After tensioning,
concrete is cast around the strands and allowed to cure to some initial required strength. At
A-26