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: bhaskar@serc.res.in

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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