NDT techniques such as rebound hammer testing, ultrasonic pulse velocity testing, and half-cell potential testing can evaluate the strength, durability, homogeneity, and corrosion risk of hardened concrete without damaging it. These methods provide estimates of properties based on correlations with standardized tests and can identify areas of poor quality concrete for repair. NDT is required for post-construction assessment of concrete structures to evaluate their current condition, residual life, and needs for repair or retrofitting.

1. What is NDT?
 Powerful technique for evaluating exiting concrete
structures with regard to their strength and durability apart
from assessment and control of quality of hardened
concrete without or partial damage to the concrete.
 Concrete is not loaded to the failure, therefore, the
strength inferred or estimated can not be expected to yield
absolute value of strength.
 NDT measures some other properties of concrete from
which an estimate of its strength , durability and elastic
properties are obtained.
1

2. WHY NDT INVESTIGATION IS REQUIRED
IN POST ENGINEERING?
 It is required to assess the quality control exercised by the
agency at the time of concreting and precaution can be taken if
found unsatisfactory.
 Environmental, manmade or natural impact may deteriorate
the concrete very fast in sustained loading condition.
 Ongoing corrosion activity can be trapped and stopped further.
 To evaluate structural strength at present and use it to increase
the load carrying capacity or reduce load to prevent collapse.
 Gives master print for repair and retrofitting work to be carried
out.
 To predict residual life of the structure for future planning.
 To get idea about extent of the damage occurred to the
structure due to a particular cause.
2

3. HOW NDT INVESTIGATION HELPS IN ASSESSMENT?
 Rebound Hammer Test: To estimate strength and hardness
of the concrete in the cover region (IS: 13311 Part II)
 UPV Test: To measure homogeneity, uniformity,
compactibility and presence of crack or void, thus representing
durability of concrete (IS: 13311 Part I).
 Core Test: To evaluate in situ compressive strength of
concrete (Represents real and actual picture of concrete) IS
516 & ASTM C42/41)
 Corrosion Assessment: Half Cell Potential (ASTM C876-
9), Resistivity and RCPT (ASTM C1202), to know about
extent and rate of ongoing corrosion activity.
 Bar Locater and Cover Meter: To know the structural
strength and protection to the reinforcement by cover concrete.
 Carbonation: To evaluate extent of carbonation for strength
3
and corrosion estimation.

4. Mechanism of Rebound Hammer

5. REBOUND HAMMER
• OBJECTIVES
• To assess quality of the concrete in relation to the standard
requirements
• To estimate compressive strength of concrete with the help of
suitable correlations between rebound index & compressive
strength
• To delineate regions of poor quality or deteriorated concrete
in a structure
• To assess quality of one element of the concrete in relation to
another

6. REBOUND HAMMER
FACTORS AFFECTING THE TEST RESULTS
• Type of Cement
• Type of Aggregate
• Surface Condition and Moisture Content of
Concrete
• Curing and Age of Concrete
• Carbonation of Concrete Surface

7. ULTRASONIC PULSE VELOCITY
 OBJECTIVES
 To assess uniformity and homogeneity of concrete
 To assess quality of concrete in relation to standard requirements
 Detection of the presence & approximate extent of cracks, voids &
other imperfection in concrete
 Measurement of changes occurring with time in the properties of the
concrete
 Correlation of pulse velocity & strength as a measure of concrete
quality
 Determination of the dynamic modulus of elasticity & Poisson's
ratio of the concrete

8. ULTRASONIC PULSE VELOCITY
FACTORS AFFECTING TEST RESULTS
 Surface Conditions and Moisture Content of Concrete
UPV α Moisture content
 Path Length, Shape and Size of the Concrete Member
 Min. Path Length–100 mm for Conc. with 20 mm down
150 mm-for Conc. With 20-40 mm
Grid of 30 Cm X 30 Cm for Results
Path Length Prob Minimum
KHz Transverse
Dimension of
Members
Upto 500 mm 150 25
500-700 ≥60 70
700-1500 ≥40 150
Above 1500 ≥20 300

9. ULTRASONIC PULSE VELOCITY
FACTORS AFFECTING TEST RESULTS
 Temperature of Concrete
50 – 300 Ideal
300 – 600 (Reduction in UPV upto 5%)
Below 50 (Increase upto 7.5%)
 Stress level in Concrete
At higher stress UPV reduces due to development of microcracks
Significant effect is observed when stress is higher than 60% of
its ultimate.
 Reinforcing Bars
UPV of R/f is 1.2 to 1.9 times than normal concrete

10. ULTRASONIC PULSE VELOCITY
EVALUATION OF DYNAMIC MODULUS OF ELASTICITY
Ed = ρf(μ)v2
 Where ρ is density of concrete in Kg/m3,
 v is pulse velocity in m/sec,
f(μ) = (1+ μ)(1-2μ)/(1- μ)
where μ is dynamic poisson’s ratio of concrete
Es = 1.25Ed-19
Es is static modulus of elasticity

11. Methods of Conduct of UPV Test

12. Detection of Flaws and Cracks by
UPV Test
UPV value in Indirect method lowers by 5-20%
compared to direct method (velocity is increased by
1km/sec.)

13. ULTRASONIC PULSE VELOCITY
INTERPRETATION OF THE TEST RESULTS
Velocity criteria for concrete quality grading
Sr. No. Pulse Velocity ( km / sec ) Concrete Quality
Grading
1 Above 4.5 Excellent
2 3.5 to 4.5 Good
3 3.0 to 3.4 Medium
4 Below 3.0 Doubtful

14. OBJECTIVES OF HALF CELL
POTENTIAL TEST
 To evaluate potential of concrete in promoting
corrosion activity of reinforcement by Half Cell
Potential test.
 To evaluate reinforcing steel in concrete that has
carbonated to the level of the embedded steel
 To evaluate indoor concrete that has not been
subjected to frequent wetting unless it has been
protected from drying after casting
 To compare corrosion activity in outdoor
reinforced concretes of highly variable moisture or
oxygen content, or both, at the embedded steel

15. Mechanism of Chloride Attack
 Ingress of Cl- and O2 , Formation of OH-
 Preferential migration of Cl- to neutralize Fe++
formed within the pit and formation of FeCl2.
 Hydrolysis of FeCl2 and acid formation ( HCl )
which accelerates further attack on metal.
 Intensive localized corrosion within the pit
leading to failure.

16. Corrosion Mechanism
 Corrosion of Steel in Concrete is an Electrochemical
process.
 In Steel, one part becomes anode and other part
becomes cathode connected by electrolyte in the form of
pore water in the hardened cement paste.
 Positively charged Fe++ at the anode passes into
solution.
 Negatively charged free electrons e- passes through the
steel into cathode where they are absorbed by the
constituents of the electrolyte and combine with water &
oxygen to from hydroxyl ions (OH)-.
 (OH)- travel through the electrolyte and combine with the
ferrous ions to form ferric hydroxide and by further
oxidation converted to rust.

17. HALF CELL POTENTIAL
• FACTORS AFFECTING TEST RESULTS
o WETTNESS OF SURFACE
o CARBONATION
o ELECTRIC CURRENT

18. HALF CELL POTENTIAL
• INTERPRETATION OF TEST RESULTS
If potentials over an area are more positive than -0.20 V
CSE, there is a greater than 90% probability that no
reinforcing steel corrosion is occurring in that area at the
time of measurement.
 If potentials over an area are in the range of -0.20 to
-0.35 V CSE, corrosion activity of the reinforcing steel in
that area is uncertain.
 If potentials over an area are more negative than -0.35 V
CSE, there is a greater than 90 % probability that
reinforcing steel corrosion is occurring in that area at the
time of measurement.

19. CONCRETE CORE
• OBJECTIVES
 To assess the compressive strength of concrete on
samples by cutting core with help of suitable correlation
between L/D ratio.
 To assess the quality of concrete in relation to
standard requirements.

20. PROFOMETER REBAR
DETECTION
 The measuring method is based on the eddy current principle
with pulse induction. The basic unit can perform the functions
such as: (i) locating rebars, (ii) measuring concrete cover, (iii)
storing individual cover values and statistical evaluation, and
(iv) determining the bar diameter. It measures more accurately
cover in the range of 60 mm – 150 mm, and bar diameter in the
range of 10 – 26 mm with bar spacing in the range of 70 mm –
200 mm.
 The instrument has Neighboring bar correction and 2-Layer
correction facility. Neighboring bar correction mainly used for
determination of the bar diameter in close spaced parallel bar
arrangements. 2-Layer Correction mode helps in congested
standard orthogonal bar arrangements. The congested bars
induce a too strong measuring signal which can be corrected.

21. OBJECTIVES OF PROFOMETER
REBAR DETECTION
 Formeasurement of concrete cover.
 To find the diameter of bars in the
concrete.
 To find the spacing between the bars.