Inspection and Evaluation of Building Structures - Retrofitting Design of Building Structures - page 16

Table 2.2 Comparison of nondestructive inspection methods

No. Method Test contents Applicability Characteristics Disadvantages Remark

Test areas
confined on
concrete surface ,
repeating test on
same place
not available

Compressive
strength ,
uniformity of
concrete

Simple , quick ,
without limits in
shape and size of
tested objects

Surface
hardness of
concrete

Rebound Often

applied

1

Ultrasonic
propagation
velocity ,
amplitude and
frequency

Without limits in
shape and size of
tested objects ,
repeating test on
same place

Great attenuation
of ultrasonic and
slightly low
accuracy for high
probe frequency

Compressive
strength ,
inner defects
of concrete

Ultrasonic Often

applied

2

Concrete
surface
hardness and
ultrasonic
propagation
velocity

Combined
rebound
and
ultrasonic

Compressive
strength
of concrete

Simple , higher
accuracy than
single method

Often
applied

3 Complicated

pre- or post-
install anchor-
ing piece and
measure
pull-out force

Causes certain
damage to
concrete
and needs repair

Compressive
strength of
Concrete

Pull-out Higher

accuracy

Often
applied

4

Compressive
strength ,
splitting strength
and inner
defects of
concrete

Cumbersome
devices , high
cost , causes
damage to
concrete and
needs repair

Causes certain
damage to
concrete and
needs repair
after test

Drill samples
with certain
size from
concrete

Drilled
core

Often
applied

5

2.2.2 Steel or Rebar

In evaluation and retrofitting for existing buildings, steel or rebar needs to be inspected
to determine its strength. Particularly when the material performance of rebar is suspected
or the rebar performance has changed after the building suffered from disasters or fire,
inspection of steel becomes indispensable.

1. Strength inspection of rebar in reinforced concrete structures

Currently mechanical performance of constructional steel or rebar, including ultimate ten-
sile strength, yield strength, extensibility and cold bending behavior, is generally determined
in a laboratory by tension test of sample rebar cut from field samples.
(1) Field sampling
Field sampling of rebar is an inspection method causing damage to building structures.
Therefore samples taken during field sampling and damage caused to building structures
should be as small as possible. Sampling should occur only at unimportant members or
subordinate parts of members if possible, for instance, inflection points of beams for tensile
rebar. As for alteration and story-adding, sample rebar can be cut directly from parts that
are to be removed anyway.
Generally speaking, three samples of each type of rebar should be secured and for suspi-
cious rebar, for instance, the same type of rebar from different batches, the number should
be increased according to specific situation.
(2) Inspection of rebar performance
For rebar in reinforced concrete there are requirements for strength, plasticity, cold bend-
ing behavior and weldability.