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Non-Destructive Testing of Concrete
Rebound Hammer
Probe Penetration
Pull out TestBreak-off Test
Lecture 13
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What is NDT / NDE ?
Test methods which measure physical
properties in place or can be used to detectflaws, and cause no significant structuraldamage to concrete
Non-destructive vs. Semi-destructive tests Some test methods may cause minor damage
like drilled cores, load testing, pin penetration,
break off and pull off tests These are considered semi-destructive
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NDT / NDE
Obtain the propertiesof concrete in-situ
Primarily, all engineersneed some estimate ofstrength
Other parameters alsocan be identified
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Other advantages of NDT
Member dimensions
Location of cracking, delamination, anddebonding
Degree of consolidation, and presence of voids
and honeycomb Steel reinforcement location and size
Corrosion activity of reinforcement
Extent of damage from freezing and thawing,fire, or chemical exposure
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Why Non-Destructive Testing ?
Determine the suitability of the structure toserve its intended functions
Be able to predict the performance withoutbeing invasive
Invasive techniques are location specific andexpensive
Invasive techniques can cause further
damage to the structure
Invasive techniques do not often allow
examination of large areas
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Why Non-Destructive Testing ?
Concrete in actual structures may exhibit adifferent behavior than the lab-tested ones
how do we know if the tested material issimilar to the one in service?
Specimen preparation issues in site and theconcreting for the structure
Curing procedures between specimen and the
structure Size effects
Laboratory testing anomalies
Sample uniformity
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Application of NDT
Both new and old structures
New structures Quality control
Dispute resolution about the material /
construction quality
Old / Existing structures
Adequacy of load carrying capacity
Age related material distress
Load related overload
Construction quality related
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ypical situations when NDT is
useful - I quality control of pre-cast units or construction in
situ
removing uncertainties about the acceptability ofthe material supplied owing to apparent non-compliance with specification
confirming or negating doubt concerning theworkmanship involved in batching, mixing,placing, compacting or curing of concrete
monitoring of strength development in relation to
formwork removal, cessation of curing,prestressing, load application or similar purpose
location and determination of the extent of cracks,voids, honeycombing and similar defects within a
concrete structure
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ypical situations when NDT is
useful - II determining the concrete uniformity, possibly
preliminary to core cutting, load testing or other
more expensive or disruptive tests
determining the position, quantity or condition ofreinforcement
increasing the confidence level of a smallernumber of destructive tests
determining the extent of concrete variability inorder to help in the selection of sample locationsrepresentative of the quality to be assessed
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Typical situations when NDT is
useful - III confirming or locating suspected deterioration of
concrete resulting from such factors as
overloading, fatigue, external or internal chemicalattack or change, fire, explosion, environmentaleffects
assessing the potential durability of the concrete monitoring long term changes in concrete
properties
providing information for any proposed change ofuse of a structure for insurance or for change ofownership.
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Situations where NDT can be used
to investigate the homogeneity of concrete mixing
to determine the density and strength of concrete in astructure
to determine the location of reinforcing bars and thecover over the bars
to determine the extent of defects such as corrosion
to determine the location of in-built wiring, piping,ducting, etc.
to determine whether internal defects such as voids,
cracks, delaminations, honeycombing, lack of bondingwith reinforcing bars, etc. exist in concrete
to determine if there is a bond between epoxy bondedsteel plates and concrete members.
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Summary of NDT Methods
See attached sheets
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Schmidt Hammer / Rebound
Hammer (ASTM C 805)
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Schmidt Hammer
Based on the sameprinciple of why a
superball bounces Mainly tests the
surface hardness of
concrete, then relatedto strength
Fundamentally acomplex problem ofimpact loading andstress-wavepropagation
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Schmidt Hammer
Springs stretched
Hammer released
Hammer
ImpactsAnd
Plunger
Rebounds
Indicator
Records
rebound
Rebound number 10 to 100
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Some things to note
Can be used in horizontal, vertical or inclinedpositions
Hammer should be perpendicular to the surfaceunder testing
Should develop a correlation between the rebound
number and the concrete being tested (theaggregate type is very significant)
Higher rebound number, higher the strength
Dont rely too much on the calibration curvesupplied with the instrument
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Limitations of Rebound hammer - I
Near surface measurement will not getproperties of the core concrete
Smoothness of surface Surface has to be smooth, variations between methods
of surface finishing
Size, shape and rigidity of the surface If the concrete moves (cracking, poor surface) the
numbers are not reliable
Age of the specimen Should not do on low strength concrete at early ages
the aggregate effect dominates and will get faultyreadings
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Limitations of Rebound hammer - II
Related to stiffness than strength
Surface and internal moisture conditions
High rebound number for wet specimens than air cureddry specimens
Presaturation preferred
Aggregate dependence
Type of cement
High alumina cement shows higher strength
Supersulfated cement 50% lower than Type I
Carbonation of the surface
Carbonation increases surface density
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Rebound number-Strengthrelationship
Guidebook on NDT for Concrete
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Rebound number-strengthrelationship
Guidebook on NDT for Concrete
Different types of gravel
as fine aggregate
Different types of
Coarse aggregates
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Probe Penetration (Windsor probe)
Drives a probe into theconcrete with a known
amount of force
Another Impact test
James Instruments, Cement Assoc of Canada
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Probe penetration (ASTM C 803)
Shoots an alloy probe into concrete
6.3 mm dia, 79.5 mm length
Exposed length of the probe measured, andrelated to the compressive strength of concrete
Much higher energy than rebound hammer
Influenced by the surface smoothness of theconcrete
Influenced by the presence of aggregates Much larger damage area than the rebound
hammer
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Relating strength and penetrationdepth
HardAggregates Soft
Aggregates
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Damage zone in probe penetration
Need to identify thelocation of
reinforcing barssince the probe willbe affected
Pin penetration modified version ofthe probe requires
less energy than theprobe
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Pullout Test (ASTM C 900)
Concrete subjected tostatic loading unlike
Rebound hammerand probepenetration
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Pullout test
Measures the maximum force required to pull anembedded metal insert with an enlarged head
from a concrete specimen or structure Force is applied by a tension jack, or center-hole
ram, that reacts against the concrete surface
through a reaction ring concentric with the insert A roughly cone-shaped fragment of the concrete is
extracted as the insert is pulled out
Diameter of the conic fragment d2
is determinedby the inner diameter of the reaction ring, and thesmall diameter d1 is determined by the insert-headdiameter
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Pulled out specimens
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Advantages and Disadvantages
Produces a well-definedfracture surface in theconcrete
Measures a static strengthproperty
Empirical relationships can
be developed between thepullout strength and thecompressive strength ofthe concrete
Pullout strength governedby that portion of concretelocated next to the conicfrustum defined by theinsert head and thereaction ring
Limited to newconstruction insert hasto be embedded
The empirical relationship
is applicable to only theparticular testconfiguration and concretematerials used in thecorrelation testing
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Break-off test (ASTM C 1150)
Measures the forcerequired to break off a
cylindrical core from alarger concrete mass
The measured force anda pre-established
strength relationship areused to estimate the in-
place compressive
strength
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Break-off test
Plastic sleeve inserted into fresh concrete toform the core
Also, the test specimens can be prepared inhardened concrete by using a special core
bit to cut the core and the counter bore Can be used to evaluate concrete in both
new and existing construction
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Break-off test procedure
Sleeve is removed and a special loading jack is placed intothe counter bore
A pump supplies hydraulic fluid to the jack that applies ahorizontal force to the top of the core
The reaction to the horizontal force is provided by a ringthat bears against the counter bore
The force on the core is gradually increased by operatingthe pump until the core ruptures at its base
The hydraulic fluid pressure is monitored with a pressuregage having an indicator to register the maximum pressure
achieved during the test The maximum pressure gage reading in units of bars [1
bar = 0.1MPa (14.5 psi)] is called the break-off number ofthe concrete.
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