ACTEL
S. Bhaskar Principal Scientist
Advanced Concrete Testing & Evaluation Laboratory (ACTEL)
CSIR-Structural Engineering Research Centre (CSIR-SERC)
Taramani, CHENNAI-600113 www.serc.res.in
E-mail: [email protected]
Non-Destructive Testing for the Condition Assessment of Concrete & Masonry
Structures
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Outline of Presentation
Introduction
Quality Assessment of Structures
In-situ Testing Methods
- Commonly used and Advanced
Concluding Remarks
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Visual observations
Documentations
Measurement of geometrical parameters
In-situ tests for evaluating material properties and member behavior
- Non-destructive testing (NDT)
- Partially destructives testing (PDT)
- Load tests
Interpretation and analysis of test results
Formulation of repair measures
Introduction
Condition assessment of structures consist of not only the evaluation of quality, integrity, strength, etc. but also the prediction of the cause of
deterioration and its future projection
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Visual Observations
Cracks
- width, depth, length
Spalling of concrete
Rust staining
Dampness
Drainage
Foundation
Past repairs (history)
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Types and pattern of cracking
Spalling
Delamination, Rust staining, Exposed reinforcement,
Honeycombing, Discoloration
Abnormal distress
History of construction
Analysis and design procedures with assumptions made
Types of materials used
Documentation
Photographs can be used to supplement the field notes and sketches
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Measurement of
Geometrical Parameters
Column, Beam, Slab dimensions
Vertical alignment
Deflections and Deformations, if any
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In-situ NDT & PDT
NDT&PDT techniques can be divided into those intended to
measure material condition, (i.e. the presence of flaws or
deteriorated areas), and those intended to measure mechanical
properties such as the material compressive strength and
deformability
The latter (mechanical properties) can be further divide into two
general categories:
(1) “indirect” tests in which mechanical properties are estimated via
correlations to nondestructive measurements, and
(2) “direct” physical measurements of mechanical properties
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Reinforced Concrete Masonry
Technique Category Type Technique Category Type
Impact-echo Indirect NDT Impact-echo Indirect NDT
Infrared Thermography Indirect NDT Infrared Thermography Indirect NDT
Ultrasonic Pulse Velocity Indirect NDT Ultrasonic Pulse Velocity Indirect NDT
Single Flat jack Test Direct PDT
Double Flat jack Test Direct PDT
Core Sampling Test Direct PDT Core Sampling Test Direct PDT Endoscope/Borescope (Hole Drilling Method) Indirect PDT
Endoscope/Borescope (Hole Drilling Method) Indirect PDT
Ultrasonic Pulse Echo Indirect NDT Ultrasonic Pulse Echo Indirect NDT
Ground Penetration Radar Indirect NDT Ground Penetration Radar Indirect NDT
Schmidt Hammer (Hardness test) Indirect NDT
Schmidt Hammer (Hardness test) Indirect NDT
Mechanical (Sonic) Pulse Indirect NDT Mechanical (Sonic) Pulse Indirect NDT
Cover Thickness NDT
Surface probe Indirect PDT
Pull-out & Pull-off Indirect PDT
Half-cell Potential & Corrosion Rate
PDT (de Vekey, 1988)
Resistivity Indirect NDT
In-situ NDT & PDT
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Load Tests: when member strengths/properties can not be adequately
determined from the results of above tests
Test Methods-Application
Rebound hammer test for assessing the quality and surface hardness
UPV test for assessing the integrity and quality of concrete
Core sampling and testing for estimating the density, water absorption
and compressive strength
Surface probe, Pull-out and Pull-off tests for strength estimation Carbonation test for the qualitative assessment of carbonation depth of
concrete
Half-cell potential and Resistivity measurements to assess the
activity of corrosion of rebars
Concrete powder samples for quantitative estimation of pH values
and chloride contents
GECOR, GPR, Impact-echo, Borescope etc. for various applications like
corrosion rate, flaw detection, its size, quality of concrete, type and
condition of structures
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Test method Application
Surface hardness
UPV
Permeability
Borescope/Endoscopy
Concrete quality, durability and
deterioration
Cores
Pull-out
Pull-off
Penetration resistance
Concrete strength
Half-cell potential
Resistivity
Cover depth
Carbonation depth
pH and Chloride concentration
Corrosion of embedded steel
Radar
Thermography
Impact - echo
Quality, Integrity and
performance
Test Methods – Application (Cont’d…)
(Bungey et al., 2006)
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Rebound Hammer Test
Striking the concrete at a defined energy and measuring the
hammer’s rebound of a spring loaded mass
All members to be marked with well defined grid points - spacing of
200 - 300 mm preferred Delamination of cover concrete can be identified with low and very low
rebound numbers
Larger the rebound number - harder the surface concrete
It is a qualitative test
Strength correlation quantifiable based on calibration charts
Testing Standards: ASTM C805, BS 1881:Part 202, IS 13311 Part 2
A statistical analysis gives indication of
overall quality and variability
Type
N (designed for testing concrete items
100mm or more in thickness)
L (designed for testing thin walled items
thickness between 50- 100mm)
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Rebound Hammer -
Quality of Concrete Cover
Average rebound
number
Quality of concrete
> 40 Very good hard layer
30 to 40 Good layer
20 to 30 Fair
10 to 20 Poor concrete
0 to 10 Delaminated
In summary, Rebound Hammer method is recognised as a useful tool for performing quick surveys to assess the uniformity of concrete
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Measuring the pulse travel time over a known path length
Method used to determine
- homogeneity & integrity
- presence of cracks, voids and any other imperfections
- quality of the concrete in relation to standard requirements
- the quality of one element of concrete in relation to another
- estimate the mechanical properties of concrete like modulus of elasticity
Ultrasonic Pulse Velocity (UPV) Test
Testing Standards: ASTM C 597, BS 1881:Part 203, IS 13311 Part 1
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Measurement of transit time can be made in three ways - Direct, Indirect or Surface transmission and Semi-direct
Whenever possible, the direct transmission shall be adopted, since the path length is clearly defined and also the pulse energy will be maximum
UPV Test - Transmission
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UPV – General Guidelines for
Concrete Quality
Velocity
(km/sec)
Concrete quality grading
Above 4.5 Excellent
3.5 - 4.5 Good
3.0 - 3.5 Medium
Below 3.0 Doubtful
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UPV Testing – Typical Photographs
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Rebar Locator
Also known as Profometer or Covermeter Works on the principle that the presence of steel affects the field of electromagnet
- Location of rebars
- Diameter of bars
- Depth of concrete cover
Testing Standards: BS 1881:Part 204
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Core Sampling and Testing
Core samples give direct evidence on - Visual examination
- Voids estimation
- Other parameters like carbonation depth, density, water absorption, etc.
- Strength determination
- Chemical analysis
Location of core sample can be based on Hammer, UPV and Cover meter
testing
Testing Standards:
ASTM C 42, BS 1881:Part 4,
120 and 122
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Pull-off Test
A metallic disk is glued either to the concrete surface or to the surface
of a partial core
Force required to pull-off the disk, causing tensile failure of concrete
is measured and correlated to compressive strength
The method is especially useful in measuring the bond between overlays
Testing Standards: DIN 1048 Part 2
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Test method Representativeness Reliability of absolute
strength correlations
General applications
Cores
Pull-out, Pull-off and
Penetration resistance
Comparative assessment
UPV
Surface hardness
Moderate
Near surface only
Good
Surface only
Good
Moderate
Poor
Poor
Strength Tests - Reliability
(Bungey et al., 2006)
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Chemical Tests
Carbonation Test
A simple phenolphthalein is sprayed, carbonated concrete
exhibits no colour change, uncarbonated concrete exhibits
pink colour
pH and Chloride Tests
Laboratory based titration methods
Essential for determination of - Extent of carbonation
- pH value to assess the corrosive environment
- Chloride content
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Tests for Reinforcement Corrosion
Four probe (wenner probe) resistivity meter
Half-cell potential and Resistivity readings are indicative of the probability
of corrosion activity of reinforcement
Generally, electrical methods are used - Half-cell potential method
- Resistivity method
- Corrosion Rate, GECOR
Half-cell potential method
Testing Standards: ASTM C876
GECOR
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Impact-Echo Method
It includes the following main components
- cylindrical handheld transducer unit,
- a set of spherical impactors,
- a portable computer,
- a high speed analog-to-digital data acquisition system, and - a software program that controls and monitor the test and displays the
results in numerical and graphical form
Application: location and extent of flaws
such as cracks, delaminations, voids, honeycombing, and debonding in plain,
reinforced, and post-tensioned concrete
structures
Testing Standards: ASTM D4580, ASTM 1383
Impact-echo is a method based on the use of impact-generated stress
(sound) waves that propagate through concrete and masonry and are
reflected by internal flaws and external surfaces
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Stress waves due to impact
Impact-Echo Principle
The arrival of these reflected waves at the
surface produces displacements which are
measured by a receiving transducer
If the receiver is placed close to the
impact point, the displacement waveform is
dominated by the
P-wave arrivals
The resulting displacement versus time
signals are transformed into the frequency
domain, using FFT and plots of amplitude
versus frequency (spectra) are obtained
Short duration pulse is introduced by the
mechanical impact on the surface
The P- and S- stress waves are
reflected by internal flaws or element
boundaries
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fp = Cp/2T or T = Cp/2fp
where, fp = frequency Cp = wave speed, and T = thickness
The waveform is periodic, and the period, t, is equal to the travel path
2T, divided by the P-wave speed (Cp)
Since frequency is the inverse of the period, the frequency, fp, of the
characteristic displacement pattern is
Impact-Echo Principle (Cont..)
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Ground Penetrating Radar (GPR)
GPR includes the following main
components
- an antenna, a crucial element of RADAR
- a transducer, - a control unit,
- a display device, and
- a storage device
Testing Standards: ASTM D4748
[Radio detection and ranging]
Also known as Ground Probing Radar, Georadar, Subsurface radar, Earth
sounding radar, Impulse Radar
GPR works by sending a pulse of energy into a material and recording the strength and the time required for the return of any reflected signal
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• An antenna sends a short duration pulse of electromagnetic waves
• When the pulse encounters an interface between dissimilar materials, some of
the energy is reflected back toward the antenna as an echo
• Antenna receives the echo and generates an output signal
GPR – Operating Principle
• The amplitude of reflection at an interface
depends on the difference between the relative
dielectric constants of the two materials
2r1r
2r1r
2,1
2,1 = reflection coefficient
= relative dielectric constant of material 1, and
= relative dielectric constant of material 2
1r
2r
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GPR - Applications
For condition assessment, GPR can be effectively used for finding the following
• Measurement of structural elements thickness
• Location and spacing of reinforcement bars
• Measurement of cover depth to rebars
• Mapping of cracks in phase of concrete surfaces scanned
• Foundation materials shift or settlements
• Detects presence of delamination, voids, honeycombing etc.
• Detects corrosion indirectly, as the strength of reflections is decreased
• Detects position and profile of pre-stressing cables
• Detects different layers of materials if used in construction
• Show locations without reinforcing bars, if drilling is required
One can always cross check and validate the structural drawings using GPR. In
the absence of structural drawings, GPR can be used to assist in developing the
as-built drawing
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Depth range
(approximate)
Primary
antenna
choice
Secondary
antenna
choice
Appropriate application
0-0.5m 1600 MHz 900 MHz Structural concrete, Roadways,
Bridge decks
0-1m 900 MHz 400 MHz Concrete, Shallow soils,
Archaeology
0-9m 400 MHz 200 MHz Shallow geology, Utilities,
Archaeology
0-20m 200 MHz 100 MHz Geology, environmental, utilities,
archaeology
Antenna frequency determines data quality, range, resolution, approximate
depth of penetration and appropriate application
GPR - Antennas
(GSSI User Manual, 2006)
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Ultrasonic Tomograph
http://www.acsys.ru/
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Applications
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Slab size: 3m x 3m;
different thicknesses: 200mm, 300mm and 400mm
Concrete Slab
Thickness Determination
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388
287
199
Frequency Plot
ACTEL Ungrouted Plastic Duct
Ungrouted Plastic Duct
with 12 mm dia. of
HYSD bar
1500mm
117mm
350mm
200m
m
75mm
Fully Grouted Plastic
Duct with 12 mm dia.
of HYSD bar
Ungrouted Steel Duct
with 12 mm dia. of
HYSD bar
Fully Grouted Steel
Duct with 12 mm dia. of
HYSD bar
Steel Plate C/S
350*250*4 mm
8mm dia. of HYSD bar
Slab size: 2 m x 1.5 m x 0.2 m
Materials: Concrete-M35 Grade; Steel-Fe415 Grade
Ducts of 50 mm Dia. (Steel and Plastic)
Thickness Measurement and
Detection of Tendon Ducts
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Impact Scanning
Scanning has been carried out on a 2 m X 1.5 m area
50 mm x 50 mm grid marking
A wave speed of 4150 m/s is used
Scanning carried out systematically along each line and
average of two impacts that are repeatable in response
have taken at each grid point
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Thickness Measurement The recorded waveform data is transformed into frequency spectra
The thickness of the slab can be obtained by using the Eq.
Typical frequency spectra of a point
in the solid slab region
fp = Cp/2T or
T = 4150/2x10.53 = 197 mm
Typical waveform
The determined/measured thickness is 197 mm which is almost equal to the actual thickness (i.e., 200mm) of slab and the difference is around 2.0% only
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Typical B-Scan Image
for the Solid Portion
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B-scan Image across Fully Grouted
and Ungrouted Steel Ducts
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B-scan Image of Fully Grouted and
Ungrouted Plastic Ducts
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Amplitude Spectrum for Fully Grouted Steel Duct
212.597662
4150T
Apparent thickness at fully grouted steel duct,
Detection of Tendon Ducts
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Apparent Increase in Thickness
at Duct Locations
Description Frequency
(Hz)
Measured
Thickness (mm)
Apparent
increase in thickness
Solid Portion 10530 197.0 ------
Fully Grouted
Steel Duct
9766 212.5 6.3%
Ungrouted
Steel Duct
8392 247.4 23.7%
Fully Grouted
Plastic Duct
8850 234.6 17.3%
Ungrouted
Plastic Duct
8390 247.5 23.7%
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Thickness Evaluation and Identification of Defects in Masonry Elements
IE Scanning System (olson instruments, 2008)
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Thickness side Length side Thickness
Longer side
Thickness Evaluation
Solid brick masonry wall with grid lines
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Point selected
Thickness shown for the
selected point
A1
A20
Thickness Evaluation (Cont’d...)
B-scan image of typical line A; amplitude-frequency and
waveform of grid point, A2
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Thickness side Length side
Defect Identification
Brick masonry wall with internal defects
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4TH LAYER
11TH LAYER
10TH LAYER
9TH LAYER
8TH LAYER
7TH LAYER
6TH LAYER
5TH LAYER
3RD LAYER
2ND LAYER
1ST LAYER
Thermocol (inside)
Cavity/void (inside)
Defect Identification (Cont’d...)
Internal details
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a) Line G:Wood b) Line EF: Void c) Line DE: Cavity
(d) Line D: Thermocol & Cavity (e) Line BC: Stone (f) Line B: Thermocol
Void
Thermocol
Cavity
Stone
Cavity
Thermocol
B-scan images at different scan lines
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Ultrasonic Pulse Echo
Shear wave technique, useful for identification of thickness
and defects like delamination, honeycoming, etc.
A 1220 Monolith PUNDIT-PL 200PE
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B, C and D Scan Images
C-scan D-scan
B-scan A- scan
Ducts
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Thermography is a technique for producing a visible
image of the invisible heat energy (infrared radiation)
emitted from the surface of an object, through a non-
contact thermal imaging device
Infrared thermography
VarioCAM
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heat transfer around the window
Thermal images
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Just before charging 15 min after charging 30 min after charging
Thermal images – RC Waterproof wall
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Ground Penetrating Radar (GPR)
GSSI
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GPR Images
Thickness
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Presence of stone Presence of thermocol & cavity
GPR Images – Defect Identification
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GPR image on RC Specimen
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An understanding of the physical principles of the test method and
information about the structure being tested are both necessary for
successful quality assessment of concrete/masonry structures
Concluding Remarks
A variety of in-situ methods are available for damage detection and
condition assessment, some of them are described
Radar has an added advantage that large portions of a structure can
be scanned in a short time
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