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NON DESTRUCTIVE TESTING TECHNIQUES &
APPLICATIONS
by
B S RAO GROUP MANAGER
CENTRE FOR CONSTRUCTION DEVELOPMENT AND RESEARCH
NATIONAL COUNCIL FOR CEMENT AND BUILDING MATERIALS
DURABILITY OF CONCRETE
• A durable concrete is one that performs satisfactorily
in the working environment during its anticipated
exposure conditions during service.
• The materials and mix proportions specified and used
should be such as to maintain its integrity and, if
applicable, to protect embedded metal from
corrosion.
• One of the main characteristics influencing the
durability of concrete is its permeability to the ingress
of water, oxygen, carbon dioxide, chloride, sulphate
and other potentially deleterious substances.
FACTORS INFLUENCING DURABILITY
• The environment
• The cover to embedded steel
• The type and quality of constituent materials
• The cement content and water/cement ratio
of the concrete
• Workmanship, to obtain full compaction and
efficient curing.
• The shape and size of the member.
REQUIREMENTS FOR DURABILITY
• SHAPE AND SIZE OF MEMBER
• EXPOSURE CONDITIONS
• MINIMUM CONCRETE QUALITY
• EXPOSURE TO SULPHATE ATTACK
• CHLORIDE AND SULPHATE IN
CONCRETE
• ALKALI-AGGREGATE REACTION
ASSESSMENT OF DISTRESSED
CONCRETE STRUCTURES
VISUAL OBSERVATIONS
FIELD TESTS
INTERPRETAION
LABORATORY TESTS
TESTS STUDY OF RECORDS
AND
QUESTIONING
APPROACH
Further tests
ASSESSMENT OF CAUSE AND EXTENT OF DAMAGE
FORMULATION OF RECOMMENDATIONS FOR REPAIRS
TECHNIQUES ADOPTED FOR
ASSESSMENT OF DISTRESSED CONCRETE
STRUCTURES
• Rebound Hammer Test
• Ultrasonic Pulse Velocity Test
• Cover Measurement
• Core Extraction and Testing
• Carbonation Test
• Resistivity Test
• Half Cell Potential Test
• Chemical Analysis
REBOUND HAMMER TESTING
• In 1948, a Swiss engineer, Ernst Schmidt, developed a test
hammer for measuring the hardness of concrete by the rebound
principle.
• The Schmidt rebound hammer is principally a surface hardness
tester with little apparent theoretical relationship between the
strength of concrete and the rebound number of the hammer.
• However, within limits, empirical correlations have been
established between strength properties and the rebound
number.
• IS 13311-1992, part-II states, “As such, the estimation of
strength of concrete by rebound hammer method cannot be held
to be very accurate and probable accuracy of prediction of
concrete strength in a structure is ±25 percent.”
• The testing is conducted on smooth and uniform surface after
cleaning the surface with carborandum stone/grinding stone.
DIFFERENT TYPES OF REBOUND
HAMMER
Type of hammer Uses
L Type For testing Light weight
concrete
P Type For testing materials with low
hardness and strength such as
Plaster work and surfacing
N Type For testing concrete in ordinary
building and bridge
construction
M Type For testing the strength of
Mass Concrete
TYPICAL CORELATION FOR N TYPE
REBOUND HAMMER
FACTORS AFFECTING REBOUND
VALUE
• Type of Aggregates
• Degree of Compaction
• Age of Concrete
• Dryness/Wetness of The Surface
• Rigidity of The Member
• Surface Finish of Concrete- Moulded/ Troweled
• Maintenance of Rebound Hammer
• Inclination of The Rebound Hammer
• Type of Cement
• Carbonation
• Cover
APPLICATION OF REBOUND HAMMER
• Checking the uniformity of concrete quality
• Comparing a given concrete with a specified
requirement
• Approximate estimation of strength
• Abrasion resistance classification.
PHOTOGRAPHS SHOWING DIFFERENT
REBOUND HAMMER
N Type Rebound Hammer
M Type Rebound Hammer
ULTRASONIC PULSE VELOCITY
TESTING
• In this method, an ultrasonic pulse of longitudinal
vibrations is produced by an electro-acoustical transducer
which is held in contact with one surface of the concrete
member under test.
• The basic idea on which the pulse velocity method is
established is that the velocity of a pulse of compressional
waves through a medium depends on the elastic properties
and density of the medium
• Hence, comparatively higher pulse velocities are obtained
when the ‘quality’ of concrete in terms of density,
homogeneity and uniformity is good. In case of concrete of
poorer quality, lower velocities are obtained.
ULTRASONIC PULSE VELOCITY
TESTING
• Types of testing method
a) Direct transmission
b) Semi direct transmission
c) Surface transmission
• Direct transmission method is the best but it
requires access to two opposite sides of concrete
member
DIFFERENT MODES OF PROPOGATING
ULTRASONIC PULSES
FACTORS AFFECTING PULSE
VELOCITY
• Degree of coupling
• Presence of reinforcement
• Concrete temperature
• Moisture content
• Mix proportion
• Age of concrete
• Stress level in concrete
• Concrete strength can be predicted within + 20%
provided calibration curve is established
VELOCITY CRITERIA FOR CONCRETE
QUALITY GRADING
[As per IS: 13311 (Part I) – 1992]
[UPV by cross probing method]
Sl No. Pulse Velocity by Cross-
Probing (km/sec)
Concrete Quality Grading
1 Above 4.5 Excellent
2 3.5 to 4.5 Good
3 3.0 to 3.5 Medium
4 Below 3.0 Doubtful
APPLICATIONS OF PULSE VELOCITY
METHOD
• Main application for assessment of concrete
uniformity
• To establish areas of deteriorated concrete
• Detection of cracks
• Calculation of dynamic young’s modulus
PHOTOGRAPH SHOWING ULTRASONIC
PULSE VELOCITY TESTING
COVER METER
• Cover is a very important parameter dictating durability
of concrete
• Conventionally provision of cover is checked prior to
concreting. Post Construction Assessment of cover is
possible through cover meters.
• Ferro Scanning and Profometer instrument is used to
assess the cover of reinforcement. The instrument is based
on the magnetic technique and is calibrated for different
purposes.
• They are not very effective in heavily reinforced members
or members with spiral reinforcement
• Size of the reinforcement bar is required to be known for
accurate assessment of cover
APPLICATION OF COVER METER
• Identification of location of reinforcement bar
with the following applications
– Helps in avoiding drilling into the
reinforcement
– To avoid reinforcement in pulse velocity
measurements
• To improve quality control during construction
• Assessment of residual time till initiation of
corrosion
• Rehabilitation planning
PHOTOGRAPH SHOWING
FERROSCANNER
CONCRETE CORE TESTING
Concrete cores of 60-mm diameter are extracted
from different structural members identified, to
estimate equivalent cube compressive strength of the
structure. Equivalent cube strength does not
indicate 28 days standard cube strength rather it
represents the in-situ cube strength, and is compared
vis-à-vis strength used in design calculation with
safety of the structure under load in mind.
VARIOUS STEPS FOR
CORE TESTING • LOCATION AND NO. OF CORES
– The points from which cores are to be taken and the number of cores required shall be at the discretion of engineer-in-charge. In no case, however, fewer than three cores shall be taken.
( Clause 17.4.1 of IS 456-2000)
• DIAMETER OF CORE (Clause 4.3 of IS 1199)
– A core specimen for pavement thickness shall have a diameter of at least 10 cm.
– A core specimen for compressive strength shall have diameter at least 3 times MSA
– In no case Dia shall be less than 2 MSA
25
VARIOUS STEPS FOR
CORE TESTING
• LENGTH OF CORE (L/D RATIO)
– Length of specimen, when capped shall be nearly as practicable twice its diameter ( Clause 4.3.1 of IS 1199)
– A correction factor according to the height/dia of specimen after capping shall be obtained from the curve in Fig 1 of IS 516
• CUTTING AND CAPPING
– Cutting the ends with masonry saw.
– The specimen are capped with sulphur compound
• Pure Sulphur : 3 Parts
• Inert filler (fire clay) : 1 Part
CORRECTION FACTOR FOR HEIGHT/ DIAMETER
RATIO OF A CORE
HEIGHT
DIAMETER RATIO-
CO
RR
EC
TIO
N F
AC
TO
R
VARIOUS STEPS FOR
CORE TESTING
• TESTING THE CORES FOR COMPRESSIVE
STRENGTH
– The core shall be placed in water at a temperature of 240
to 300C for 48 hours before testing
– After testing, the measured compressive strength shall be
multiplied by correction factor for L/D ratio between 1
and 2.
– Equivalent cube strength of the concrete shall be
determined by multiplying the corrected cylinder strength
by5/4
VARIOUS STEPS FOR
CORE TESTING
• ACCEPTANCE CRITERIA (AS PER IS 456-2000)
Concrete in a member represented by a core test shall be considered acceptable if the average equivalent cube strength of the cores is equal to at least 85 percent of the cube strength of the grade of concrete specified for the corresponding age and no individual core has a strength less than 75 percent
PHOTOGRAPH SHOWING CORE
EXTRACTION
CARBONATION DEPTH
• Carbonation is chemical reaction between Ca
(OH)2 and CO2 of the atmosphere
• Carbonation destroys passive protection
provided by concrete to the reinforcement
• Carbonation proceeds from the surface into the
concrete
• When depth of carbonation equals concrete
cover reinforcement corrosion is imminent
CARBONATION DEPTH
• Method of Assessment
– Drill into concrete
– Spray phenolphthalein solution
– Uncarbonated concrete will show bright pink
stain
– Carbonated concrete will not change colour
– With the above visual indication depth of
carbonation can be easily measured
PHOTOGRAPH SHOWING MEASURED
CARBONATION DEPTH AT SITE
ELECTRICAL RESISTIVITY TESTING
• Concrete resistivity is geometrical independent
material property that indicates the ratio between the
applied voltage and resulting current in a unit cell.
• The resistivity of concrete impacts the current flow
between the cathodic and anodic regions of the
concrete.
• The higher the concrete resistivity, the lower the
current flowing between anodic and cathodic areas
will be, and therefore lower the corrosion rate.
• The proceq RESI Resistivity Meter i.e. a four-point
Wenner probe resistivity meter permits a rapid and
non-destructive measurement of the quality of
concrete with respect to its resistivity.
PHOTOGRAPH SHOWING RESISTIVITY
METER
HALF CELL POTENTIAL
MEASUREMENT
• This test method covers the estimation of electrical
Half Cell Potential of uncoated reinforcing steel, to
determine corrosion activity using reference electrode
copper; copper sulphate half-cell.
• It is not possible to expose all the reinforcements in the
structural element and observe the extent of corrosion.
So, this method has been very convenient to assess the
condition of the entire length of a member by exposing
a portion of the reinforcement at a suitable location,
which measures the half-cell potential on the entire
length, by placing the reference electrode on the wet
concrete surface.
HALF CELL POTENTIAL
MEASUREMENT
• The Half-Cell Potential measurement is based on
the principal of the corrosion, being an electro-
chemical process, induces certain voltage to the
reinforcement steel that is corroding.
• The wetting of the concrete is required to make
the portion between the concrete surface and the
reinforcing bar as electrolytes.
CRITERIA FOR DECIDING THE STATUS
OF CORROSION
[According to ASTM C – 876]
Phases of Corrosion
Activity
As measured By Copper
– Copper Sulphate Half
Cell
Initial phase-corrosion
activity not taking place
< - 200 mV
Transient phase-corrosion
activity uncertain
- 200 mV to –350 mV
Final Phase – corrosion
occurring positively
> - 350 mV
PHOTOGRAPH SHOWING HALF CELL
POTENTIAL MEASUREMENT
CHEMICAL ANALYSIS
• For analyzing Chloride content and pH of concrete,
concrete powder samples were extracted from 0-20mm, 20-
40mm & 40-60mm depths at identified locations and then
tested as per IS:14959(Part 2) -2001 (Determination of
water soluble and acid soluble Chlorides in Mortar and
Concrete – Method of Test).
• Corrosion of reinforcing steel due to chlorides in concrete
is one of the most common environmental attacks that lead
to deterioration of concrete structures. Whenever there is
chloride in concrete there is an increased risk of corrosion
of embedded metal. Chloride content is then expressed in
kg per cubic meter of concrete and compared with the
values of limits of chloride contents of concrete (Table 7 of
IS: 456–2000).
CHEMICAL ANALYSIS
• Sulphates (SO3) are present in most cements and in some
aggregates; excessive amounts of water-soluble sulphate
from these or other mix constituents can cause expansion
and disruption of concrete. To prevent it, IS: 456-2000
clause-8.2.5.3 states that the total water-soluble sulphate
content of the concrete mix, expressed as SO3, should not
exceed 4 percent by mass of the cement in the mix. The
sulphate content should be calculated as the total from the
various constituents of the mix.
• The pH value of the concrete should be above 11.5 to
maintain alkalinity of concrete surrounding the embedded
steel. A reduction in the pH value of concrete indicates loss
of passive layer around the reinforcement which protects
the steel from distress.
THANKS