Page 414 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.5

unacceptable deformations may occur. Early loading and use of high water-to-cementitious-
materials ratios both increase time-dependent deformations.

Bond Slip
For reinforced concrete to function as designed, force must be transferred between concrete
and steel. Deformed bar reinforcement transfers this force primarily through compression
on the face of lugs, as illustrated in Fig. 12.5. Methods used in Ref. 2 give a conservative
evaluation of bond strength for deformed bars. Thermal expansion properties of both con-
crete and steel are similar. Each has a coefficient of thermal expansion of about 6 millionths
per degree Fahrenheit. Consequently, thermal changes usually have no influence on bond.
Pre-tensioned pre-stressing wires or strands transfer force by a combination of friction
and compressive action. When a wire is under tension, it has a smaller diameter than when
it is at zero stress. Seven-wire strand is manufactured with its wires twisted around a
straight center wire. Under tension, the twist has a greater pitch than at zero stress. After
prestressing of a bonded tendon has been released and transferred to the concrete, the ten-
dency for wires to increase in diameter creates a compressive force at the face of the wires.
This compressive force transfers stresses from wire to concrete through the friction at the
face of the wires.
Bond failures of pretensioned strands can occur if the tendons are lubricated. Accidental
application of oil, grease, or debonding agents can cause this phenomenon. Failure of man-
ufacturers to remove lubricants used in strand manufacture can also cause bond failures.
One piece of physical evidence suggesting bond failure is the pulling in of strand at the end
3
of a pretensioned member. If the strand has pulled in more than approximately / inch,
16
investigations should be undertaken to determine if there is a potential for bond failure. The

Concrete

Reinforcement
FIGURE 12.5 Mechanism for bond development reinforcement in concrete. (Courtesy of CTLGroup.)