TECHNICAL
De-bonding No More!
A new method for non-destructive integrity testing of
drilled shafts and micropiles using hollow rebars
By Horst Aschenbroich, Dipl Ing., Con-Tech Systems
Due to the possible de-bonding
problem of smooth entry tubes
with concrete for crosshole
sonic logging (CSL) in many
drilled shaft projects, test results can
be wrongly interpreted. Unnecessary
and costly remedial work is often conducted.
The Hollow Bar system, originally
introduced by Con-Tech Systems
in North America, for grout injection
bored IBO, soil nails and micropiles,
turned out to be an ideal hollow rebar
solution to solve the de-bonding
problem. As an additional bonus, tests
have shown that replacing standard
rebars with the hollow bars used for
CSL will produce a much more rigid
and stiffer rebar cage, making handling
and placing into the drilled hole
faster and safer.
This paper explains the development
of this new method for non-destructive
integrity testing (NDT) of drilled shafts.
Actual full-scale tests have proven
its validity.
Introduction
The present method used is lowering
CSL probes into drilled shafts through
non-structural PVC or steel pipes,
Figure 1. These pipes, when incorporated
within the reinforcing cage,
reduce the space between the vertical
bars, hindering a good flow of concrete
to the outside of the cage with possible
creation of anomalies. Also, smooth
pipes have a de-bonding problem in
concrete. For CSL testing, the read-out
values could be misinterpreted. For
Thermal Integrity Profiling (TIP), the
thermal wires are tight to the vertical
rebars. The thermal wires or probes
could be damaged during handling,
placing of the cage and placing of
the concrete.
CSL HOLLOW REBAR
EMBEDMENT PLATE WITH
NUT TOP AND BOTTOM
FOUNDATION OR
GRADE BEAM
HOOP STEEL
REINFORCING
CSL HOLLOW
REBAR CSL HOLLOW REBAR
0D
SECTION
A B
SECTION
NTS NTS
STEEL
EMBEDMENT
PLATE
CONVENTIONAL
SOLID STEEL BAR
REINFORCING
HOOP STEEL
REINFORCING
LONGITUDINAL REBAR
DRILLED SHAFT
B
_
A
_
SYSTEMS
Figure 1: PVC plastic tubes,
indicated by arrows
TECH CON- GRAPHICS:AND PHOTO an access tube and larger spaces INCLINOMETER
3"
PVC CSL CASING
(V1) 5#14 60–00 ✳ M✳ +
(V1) 5#14 5–00 ✳F ✳
(V2) 4#14 50–00 ✳ M✳ +
(V2) 4#14 15–00 ✳F ✳
SPIRAL 2#5 5MK3 @6" PITCH
3'–0'
Instrumented reinforcement cage
showing smooth PVC pipes
between the vertical reinforcing bars,
allowing better concrete flow and cover.
The high-strength hollow rebars,
well known for self-drilling or grout
injection bored IBO, soil nails and
The new method
Hollow rebars offer high-strength, very
ductile reinforcement – a requirement
for seismic designs – while also providing
PILING CANADA 51