NDT: A Tool for Condition Assessment of Structures by SK Singh

7/27/2019 NDT: A Tool for Condition Assessment of Structures by SK Singh

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NDT: A TOOL FOR CONDITION ASSESSMENT OF

STRUCTURES

S. K. Singh,

Principal Scientist, CSIR-Central Building Research Institute, Roorkee; [email protected]

abstract

Non Destructive Testing (NDT) techniques are being increasingly used for condition assessment and

quality assurance of civil engineering structures. These techniques are also extensively used for forensic

investigation of distressed structures. Nondestructive evaluation and testing also provides relevant means

for periodic health monitoring of structures in order to maintain the quality of constructed facilities. Thispaper presents different types of non destructive testing methods, its application, advantages and limitations

to take appropriate remedial measures to bring back the structures in the context of overall safety.

keywords:Non destructive testing, Condition Assessment, Quality assurance, Health monitoring

INTRODUCTION

To assess the anomalies in concrete structures, a number of non-destructive, partially destructive and

destructive techniques are available. Out of these techniques, Non-destructive tests are most suitable for the

condition assessment of concrete structures and for the prediction of the cause of distresses. With these

tests, it is possible to know the extent of damage & various causes of the distresses in structure precisely.

Based on the results of these tests, remedial measures to enhance the life of the structures can be suggested.

The very purpose of this paper is to disseminate the knowledge regarding suitability of NDT for condition

assessment of structure in terms of strength, quality and integrity. Non-destructive techniques, which are

quick to assess the condition of structure and relatively inexpensive, can be useful for following cases:

To remove uncertainties about the quality of the material supplied owing to apparent non-compliance

with specification.

To locate and determine the extent of defects and condition of reinforcement bars within a concrete

structure.

To determine the extent of concrete variability in order to help in the selection of sample locations

representative of the quality to be assessed.

To provide information for any proposed change of use of a structure.

To locate suspected deterioration of concrete resulting from factors such as earthquake, overloading,

fatigue etc and subsequent rehabilitation and retrofitting.

To assess the structure for seismic up-gradation.

To clear the doubt concerning the workmanship involved in batching, mixing, placing, compacting or

curing of concrete.

NON-DESTRUCTIVE TESTING

The objective of non-destructive tests is to obtain an estimate of properties of material by measuring certain

quantities which are empirically related to it. The accuracy of interpretation of results depends directly on


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the correlation between strength of material and measured properties. Thus, the user of NDT should have

an understanding of what quantity is measured by the tests and how this property/ quantity is related to the

strength & other properties of material.

The use of nondestructive testing techniques to test structural materials & system has been grown upsubstantially worldwide and equipment have moved from research stages to practical applications over the

past two decades. There are several different NDT techniques which are available and used for conditionassessment and health monitoring for the concrete structures. The application of stress waves,

electromagnetic waves, radiation and emitted thermal energy based techniques are also getting considerable

amount of attention nowadays. The efficiency and effectiveness of NDT for quality assurance, condition

assessment, distress diagnosis, repair and rehabilitation of concrete structures continue to increase.

Some of the most common NDT tests are:-.

Rebound Hammer

The application of Schmdits hammer has been shown in Fig.1. It works on the principle of measuring

surface hardness of concrete by measuring rebound of spring controlled mass, when plunger is pressed

against the surface of concrete. This test is a complex-problem of impact loading and stress wave

propagation. The energy absorbed by the concrete depends on the stress-strain relationship of concrete.Thus, a low strength, low stiffness concrete absorb more energy than high strength, high stiffness concreteand will give a lower rebound number. The impact energy required for rebound hammers for different

applications is given below:-

S.No Application Approx. Impact Energy Required for

Rebound Hammers (N-m)

1 For testing normal weight concrete 2.25

2For light weight concrete or small and impact

sensitive parts of concrete0.75

3 For testing mass concrete 30.00

1S:13311 Part 2-92 states the standard procedure for test and correlation between compressive strength of

concrete and rebound number. A typical calibration chart is shown in Fig.2.

In rebound hammer testing, only concrete in the immediate vicinity of plunger, has an influence on the

rebound value. Hence, the tests are sensitive to local conditions where the test is performed. To account for

these, minimum 8-10 rebound numbers should be recorded for a test. If an individual reading differs by

more than seven units from the average, that reading should be discarded and a new average must be

computed.

Fig 1: Application of Rebound Hammer Fig 2: Typical Calibration Chart


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The rebound hammer measures the hardness of surface, affected by following factors as given below:-

1. Mix characteristics

(a) Cement type(b) Cement content

(c) Coarse aggregate type

2. Member characteristics

(a) Mass

(b) Compaction

(c) Curing, age and rate of hardening

(d) Surface and internal moisture condition(e) Stress state and temperature

(f) Type of mould/forms

(g) Carbonation on concrete surface

Rebound hammer test is very simple and quick with negligible operating cost. The user needs to be well

versed with above factors which affect the tests results during estimation of concrete strength. If all factors

are taken into consideration, the strength of concrete in a structure may be estimated with an accuracy of15%. When little information is available about concrete then the possible error may be upto 25%.

Penetration Resistance

In the penetration resistance techniques, one measures the depth of penetration of a rod probe pin that is

forced into the hardened concrete by a driver unit. The probe penetration technique involves the use of a

specially designed gun to drive a hardened steel probe into the concrete (the commercial test system is

known as Windsor Probe given in Fig.3.). The probe penetrates into concrete until its initial kinetic energy

is completely absorbed by the concrete. The general shape of fractured zone in which most of probe energy

is absorbed is as given in Fig.4.

Fig 3: Windsor Probe

Fig 4: Fractured Zone in Concrete Fig 6: PNR TestFig 5: Curve


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The strength properties of both the mortar and the aggregates influence the penetration distance. This is

contrary to behaviour of normal strength concrete where mortar strength governs the strength of concrete in

a compression test. Thus, an important characteristic of coarse aggregate has strong effects on the

relationship between concrete strength and probe penetration, as given in Fig.5. For equal compressive

strengths, the concrete with the soft aggregate results in greater probe penetration than the concrete with theharder aggregate.

Because the probe penetrates into the concrete, test results are usually not affected by local surface

conditions such as texture and a harder surface layer, as would occur in rebound hammer test in trowel

finishing, can result in low penetrationvalues and excessive scatter of data. In addition, the direction of

penetration into the concrete is unimportant provided that the probe is driven in absence of reinforcing bars

within the zone of influence of penetrating probe. Thus, the location of the reinforcing steel should be

determined prior to selecting test locations.

The exposed length of the probe is measured by a calibration depth gauge. However, the fundamental

relationship is established between concrete strength and depth of penetration. Therefore, when assessing

the variability of test result, it is preferable depth rather than exposed length.

A pin penetration test device (PNR Tester), which requires less energy than the Windsor Probe system is

given in Fig.6. A spring loaded device is used in this system to drive a pointed 3.56mm diameter hardened

steel pin with tip machined at an angle of 22.50 into the concrete. The penetration by the pin creates a smallindentation (or hole) on the surface of the concrete. The pin is removed from the hole, the hole is cleaned

with an air jet, and the depth of hole is measured with a suitable depth gauge. The penetration depth is usedto estimate the compressive strength from a previously established calibration chart with penetration depth-

strength relationship. In the current test system, the maximum penetration is limited to 8.0mm.

The penetration resistance method is quick and relatively insensitive to operator techniques and factors

such as moisture content, cement type and curing. This method can not yield absolute concrete strength

values as it also measures the hardness. However, these measurements are over certain depth rather than on

the surface which is major advantage associated with it. Damage in the form of cracking may be caused to

slender members. A minimum edge distance and member thickness of 150mm are required. Estimation ofthe strength with an accuracy more than +20% may be possible.

Pull Out (LOK & CAPO) Test

This test is generally used for determining appropriate time for form work stripping and transfer of pre-

stressing force. This measure the ultimate load required to pullout an embedded metal insert (usually of

25mm diameter head) from a concrete specimen or structure. The pulling load is applied by a tension jack

or center hole ram, which reacts against the concrete surface through a reaction ring concentric with the

insert. As the insert is pulled out, a roughly cone shaped fragment of the concrete is also extracted. The

compressive strength, which is considered as indicator for the quality, is obtained from the calibration

curves which are prepared after carrying out extensive laboratory/field tests on concrete cubes. When the

test is conducted on inserts that have been embedded during construction is called LOK test (Fig. 7) whereas inserts installed in existing structure is called CAPO test (Fig. 8). The reliability of these tests is reported

to be good and provide an accurate estimation of strength.

The pullout strength is governed primarily by that portion of the concrete located adjacent to the conic

frustum defined by the inserts head and reaction ring. Commercial inserts have embedment depths of the

order of 25 to 30mm. Thus, only a small volume of the concrete is tested, and because of the inherent

heterogeneity of concrete, the typical average within-batch coefficient of variation of these pullout tests has

been found to be in the range of 7 to 10 percent, which is about two to three times more that of standard

cylinder compression tests.


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LOK Test:Generally an insert (steel disc) of 25mm diameter at a depth of 25 mm is pulled centrally against

a 55 mm dia. counter pressure ring bearing on surface. The pull out force (F) required to pull the inert is

measured. The concrete in the strut between the disc and counter pressure ring is subjected to a

compressive load. Therefore, the pull out load is directly related to the compressive strength. The general

correlation curve has been shown in Fig.9. The loading is performed either to a required force, in which the

case is nondestructive, or to the ultimate load, which results in slightly raised 55 mm circumferential crack

on the surface. The insert is cast into concrete either by attaching it to form work before placing concrete orby inserting it manually into the fresh concrete.

The two main limitations to this test are the preplanned usage and nature of the surface zone. Special care is

also required at the time of placement of inserts to minimize air voids below the disc.

CAPO Test: This test allows pullout tests on existing structures without the need of pre-installed inserts.

This test is similar to the LOK test and gives accurate estimates of strength. A recess is routed in the hole of

25mm diameter at a depth of 25 mm cored in the structure. A split ring is expanded in the recess and pulled

centrally against a 55 mm dia. counter pressure ring bearing on surface. The pull out force (F) required to

pull the inert is measured. The concrete in the strut between the disc and counter pressure ring is subjectedto a compressive load similarly to LOK test which provides direct compressive strength.

Advantages: Pullout test procedures a well defined failure in the concrete and easy to operate. It is superiorto the rebound hammer and Windsor probe test because of greater depth and volume of concrete tested. It is

also not affected significantly by properties of ingredients and concrete mix.

Pull off Test

The pull off test is near to surface method in which a circular steel disc is glued to the surface of concrete

with an epoxy or polyester resin adhesive. The force required to pull this from the surface, together with an

attached layer of concrete, is measured. Simple mechanical hand operated loading equipment has been

developed for this purpose. Partial coring may be used, if necessary, to eliminate the surface skin effects. In

this method a 50mm diameter disc is generally used. It is reported that a good correlation strength and pull

off strength.

Advantages and Limitations:This method directly measures a strength related property and requires onlyone exposed surface. It is suitable for use on members with a small section. There is insufficient evidence

available to enable detailed guidance to be given on the accuracy of the method under site conditions. Testresults are limited to the surface approx. 5mm below surface unless partial coring is used. It is used in

quality control, long term monitoring and in-situ strength assessment (particularly of high alumina cement

concrete and carbonated concrete using partial coring).

Break off Test

Fig 7: LOK Test Fig 8: CAPO Test Fig 9: F versus fckCurve


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Fig. 10 Schematic View

of Break off Test

Break off test determines directly the flexural tensile strength in plane

parallel to the concrete surface at a specified distance below the surface.

The break off strength also correlates well with the compressive

strength.

A schematic of the break off test is shown in Fig.10. For new

construction, the core is formed by inserting a plastic sieve into thesurface of the fresh concrete. The sleeves can also be attached to the sides

of formwork and filled during concrete placement. Alternatively, the tests

specimens can be prepared in hardened concrete by using a special corebit to cut the core and the counter-bore. Thus, the break off test can be

used to evaluate the strength of concrete in both new and existing

construction.

When the in-place compressive strength and special loading jacks are placed into the counter bore. A hand-

operated pump is used to supply hydraulic fluid to the jack which applies a horizontal force to the top of the

core as shown in Fig.10. The reaction to the horizontal forced is provided by a ring which bear against the

force on the core is gradually increased by operating the pump until the core ruptures at its base. Thehydraulic fluid pressure is monitored with a pressure gauge having an indicator to register the maximum

pressure gauge reading in units of 0.1 MPa is referred to as the break-off number of the concrete.

The break-off test is not recommended for concrete having a maximum nominal aggregate size greater then

25mm. There is evidence that variability of the break-off number increase for large aggregate size. Sleeve

insertion must be performed carefully to assure good compaction around the sleeve and a minimum of

disturbance at the base of the formed core. It is reported that the beam strength for practical purpose may be

approximately 78% of the break-off strength. In more recent applications of this method, break off numberis related directly to the compressive strength. It has been also reported that the computed flexural strength

based on break off test is about 30% greater than the modulus of rupture obtained by standard tests of

beams. This difference is probably due to different specimen sizes in the break off and modulus of rupture

tests.

Advantages and Application: It is handy, very rapid and easy to operate and results are little affected by

local shrinkage and temperature forces and the method gives satisfactory correlation with strength. It is

used in quality control of concrete pavements and estimation of in-place compressive strength. The method

is also used to evaluate bond strength between concrete and overly materials.

Resonant Frequency Method

This method is based upon the determination of the fundamental resonant frequency of vibration of a

specimen whereas the continuous vibration being generated electro mechanically. The equipment

essentially consists of vibration generating section and vibration sensing section. This method is used to

determine the dynamic modulus of elasticity, Ed = constant x density of concrete x (frequency)2 . It is also

used in detecting voids and delamination. This method is applicable only on laboratory specimens.

Ultrasonic Pulse Velocity

The above tests indicate the quality of concrete only near the surface where as the UPV test gives the

quality over a depth, through which the pulses are transmitted. It is, generally, used for the measurement of

concrete uniformity, determination of cracks & honeycombing, strength estimation and relative quality

between members or with in a member. The operational principal of modern test equipment is illustrated inFig.11. UPV test determines the propagation velocity of a pulse of vibration energy through a concrete

member. In concrete, this test consists of transmitting electro-acoustic pulse through the concrete medium

from one side, receiving the signal in different ways (Fig. 12), and measuring the transit time over a known

travel distance. Transit time depends mainly on elastic modulus, density and Poissons ratio of concrete.


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The transducer contact with the surface of concrete is made with grease or petroleum jelly to avoid any

entrapped air, thus causes loss of acoustical energy at the interface. The average velocity (V) of wave

propagation, V = Path-length traveled (L)/ Time elapsed between the transducers (T);

A suitable type transducer operating within a frequency range of 20 kHz to 150 kHz may be used.However, commonly used frequency transducer for testing of normal member is 54 kHz. It has been well

established that the relation between ultrasonic pulse velocity and concrete strength, in general, is notreliable enough for practical purpose.

Natural Frequency of Transducers for Different Path Length

Path lengths(mm)

Natural frequency of

Transducer (KHz)

Min. Transverse Dimensions ofMembers (mm)

Upto 500 150 25

500-700 >60 70

700-1500 >40 150

Above 1500 >20 300

The direct method of testing, in which transmitting and receiving points are on the opposite faces, is the

most reliable from the point of view of transit time measurement, as maximum pulse energy is transmitted

at right angles to the face of transmitter.

Direct Method

In direct Method

Semi direct Method

Fig. 11: Schematic View of UPV Test Equipment


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Fig. 12 Ultrasonic Pulse Velocity Method

This method is considered to be a valuable and reliable method of examining the interior of a body of

concrete in a non-destructive way. However, the operator must be well trained and the results should be

properly evaluated and interpreted by the experienced engineers. The detection of flaws within the concreteis not reliable in wet conditions by this method.

Principles of Pulse Propagation through Concrete

Three types of waves such as surface, transverse and longitudinal waves are generated by an impulse

applied to a solid mass. Longitudinal waves (compression wave) with particle displacement in the direction

of travel are the fastest and provide useful information. The wave velocity depends upon the elastic

properties and mass of the medium. Therefore, if mass and velocity of propagations are known, it is

possible to assess the elastic properties. For an infinite, homogenous, isotropic elastic medium, thelongitudinal wave velocity V:

where E= Dynamic modulus of elasticity (MPa)

= Density of concrete (Kg/cum)

= Dynamic Poisson's ratio

In this value of K is insensitive to dynamic Poisson's ratio, hence we can get a reasonable value.

Density can also be known. Hence, value of E can be measures using wave velocity, V.

Factors Affecting The Pulse Velocity

Surface Condition: For most concrete surface, the finish is usually sufficiently smooth. To ensure good

acoustical contact, use a coupling medium. In case surface is very rough, use the point probes.

Temperature of concrete: There is no significant variation in pulse velocity between temperatures 50C to300C. But it reduces by 50% with increase in temperature from 20 0C to 600C. Therefore, increase in 40C

temperature, the velocity decrease up to 7.50%.

Micro Cracks in Concrete: The development of micro cracks in the concrete due to abnormal high stress or

other reason may reduce the pulse velocity. This influence is more predominant if the pulse path is normal

to the micro cracks.

Water Cement Ratio: The pulse velocity decreases by increase in w/c ratio that is concrete strength is

highly dependent upto the micro-porosity of concrete.

Age of Concrete: The pulse velocity in concrete is not sensitive to development of strength at later ages.

This is due to the presence of micro cracks when concrete tends to dry.

Presence of Steel Reinforcement: The pulse velocity through steel is about 40% greater than the concrete.

Hence, pulse velocity through heavily reinforced concrete member may be greater than through one with

little reinforcement. This is especially concerning when reinforcing bars are oriented parallel to the


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Fig. 13 Crack Measuring Instrument

direction of pulse, which may be refracted into the bars and transmitted to the receiver. This must be taken

into account during testing.

Aggregate type, size, grading & content: These parameters of aggregate of concrete mix have a much

greater effect on the pulse velocity than on the strength. In general, the pulse velocity in cement paste islower than the aggregate. IS-13311-92 (Part 1) recommends minimum path length of 100mm and 150mm

for maximum aggregate size up to 20mm and 20-40mm respectively.

Moisture: The pulse velocity in saturated concrete was found more than the air dried concrete. This effect

is more predominant in low grade concrete having higher porosity.

The test results are relatively insensitive to the normal heterogeneity of concrete. For this reason, the test

method has been found to have an extremely low within batch coefficient of variation. However,predictions are necessarily highly reliable.

The velocity criterion for concrete quality as per IS 13311-92 (Part I) in terms of uniformity, absence of

internal flows, and segregations etc. is given below [13].

Pulse Velocity (km/sec.) Concrete quality

>4.5 Excellent

3.5-4.5 Good

3.0-3.5 Medium

>3.0 Doubtful

Crack Width Measurement

The width of surface cracks as well as its progression is accuratelymeasured with crack measuring instrument ( Fig. 13 ). Usually, the

magnifying capacity of crack measuring instrument is 25 to 35

times. It can allow measuring the crack width as low as

0.0025mm. This is very useful instrument for initial appraisal of

structure. In case of in-accessible area, the measurement of width

of cracks is being done through bore scope.

Core Test

The main purpose of measuring the strength of concrete test specimens is to estimate the strength of

concrete in the actual structures. The emphasis is on the word estimate, and indeed it is not possible toobtain more than an indication of the strength of concrete in a structure as this is dependent, on adequacy of

compaction and curing whereas the strength of a test specimen depends on its shape, proportions and size

so that a test result does not give the value of the intrinsic strength of the concrete in structure. If strength of

found to be below the specified minimum then either the concrete in the actual structure is too weak, or else

the specimens are not truly representative of the concrete in the structure. The latter suggestions are often

put forward in disputes on the acceptance, which is often resolved by testing a sample of concrete taken

from the suspected member. This test is to determine the potential strength of the concrete mix used so that

corrections for the actual conditions have to be applied. Cores can also be cut in order to determine theactual strength of concrete in the structure. Usually, a core is cut by means of rotary cutting tool with

diamond bits (Fig. 14). In this manner, a cylindrical specimen is obtained, sometimes containing embeddedfragments of reinforcement, and usually with end surface far from plane and square.


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The influence of the height/diameter ratio (Fig 15) of the core on the recorded strength was considered in

failure of compression specimens. If the strength of the cores is to be related to the strength of the standard

cylinders (height/diameter ratio of 2) then in the core also, this ratio should be near 2. When cubes are thestandard test specimen, there is some advantage in using core with a height/diameter ratio of 1, as cylinderswith this ratio have very nearly the same strength as cubes. For values of the ratio between 1 and 2, a

correction factor has to be applied; recent work shows that the factor is the same for wet and dry-tested

cores. Cores with height/diameter ratios lower than 1 yields unreliable results, and BS 1881: Part 4

prescribes a minimum value of h/d ratio 0.95. The standard specifies the use of 150mm or 100mm cores;

however, cores as small as 50mm have been successfully used and are permitted in different International

Standards. The latter assumes that a 50mm core with a height/diameter ratio of 1has a strength of 10

percent higher than a 200mm cube. Very small cores exhibit more variability than larger ones and therefore

very small cores are recommended only if, it is unavoidable.

The strength of cores is generally lower than that of standard cylinders, partly as a consequence of thedrilling operation and partly because site curing is almost invariably inferior to curing prescribed for

standard test specimens and high risk exist due damage during drilling. The effect appears to be greater instronger concrete. Malhotra' suggests that the reduction in strength can be as high as 15 percent for 40 MPa

concrete. A reduction of 5 to 7 percent is considered reasonable by the Concrete Society.

If a core contains transverse reinforcing steel some effect on strength could be expected, but the

information on this is contradictory. Malhotra reports two investigators who found no effect and one who

found the steel to reduce the strength by 8 to 13 percent compared with steel-free core. The ConcreteSociety also reports a reduction in strength as a function of the steel. The effect is greater if it is found near

the end of the core. In view of the above, it is reasonable to assume that transverse steel causes a modest

loss of strength, say 5 to 10 percent. The presence of steel parallel to axis of the core is undesirable.

The exact curing history of the structure is usually difficult to determine so that the effect of curing on the

strength of cores is uncertain. For structures cured in accordance with the recommended practice, it was

found that the ratio of core strength to standard cylinder strength (at the same age) is always less than 1,

and decreases with an increase in the cylinder strength level. Approximate values of this ratio are just under1 when the cylinder strength is 20 MPa and 0.7 when it is 60 MPa.

It is sometimes argued that cores taken from concrete many months old should have a higher strength than

at 28 days. This appears not to be the case in practice and there is evidence that in situ concrete gains little

strength after 28 days. On the other hand, it was suggested that, for average conditions, the increase in

strength over that at 28 days is 10 percent at three months and 15 percent over the age of six months. The

effect of age is therefore not easy to deal with but, in the absence of definite moist curing, no increase in

strength should be expected.

Fig. 14: Core Cutting & Preparation of Core Fig. 15: Influence of h/d ratio on Strength


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Fig. 16 Schematic Diagram of Impact Echo Test Method

A further factor in the strength of cores is the position of the cut-out concrete in structure. Cores usually

have the lowest strength near the top surface of the structure, be it a column, a wall, a beam or even a slab.

With an increase in depth below the top surface, the strength of cores increases, but at depths greater than

about 300mm there is no further increase. The difference can be 10 percent or even 20 percent. In the caseof the slabs, poor curing increase this difference. Compressive and tensile strengths are affected to the same

degree.

Cores are cut from in-place concrete for a variety of reasons, whereas almost in all the cases the concern is

assessment of the integrity of structures. The most common requirement for core testing is when the results

of standard specimen fail to comply with the specified strength and in the case of old structures under

investigations.

The provisions specified in Indian codes tend to be very cautions in requiring the core diameter to be not

less than 100mm or preferably 150mm and a slenderness ratio 2. Cores can also be used to petrographic

examination and most reliable method enabling visual inspection of the interior regions & direct estimation

of the strength. It can in also used to detect segregation, honeycombing or to check the bond at construction

joints.

Impact Echo Method

The use of traditional stress wave method (UPV etc.) requires access to both the faces of member for

identifying the presence of irregularities in structures. Furthermore, the depth of irregularities can not beassessed. These drawbacks are eliminated in Impact echo method where reflected stress waves are

monitored on the surface through a displacement transducer located close to the impact point thus enabling

to obtain the information on the depth of the internal reflecting interface (fig.16).

This is very versatile and portable system and used for detecting very large structural cracks in concrete

dams, piles, caissons and piers, to locate voids & honey combed concrete, measurement of thickness of

pavements, asphalt overlays, quality of grouting in post tensioned cable duct, delamination surveys of

bridge decks, piers, cooling tower etc.

In impact echo test method, impact generates three types of stress waves that propagate away from thepoint. A surface wave( R-wave) travels along the top surface, where P & S waves travel into member. The

P-wave is used to obtain the information about the integrity of structure. The displacement wave form is

transformed into the frequency domain, by which the frequency of P- wave arrival is measured. The

thickness (T) of member is related to thickness frequency () and wave speed (Cp) by equation:-

T = Cp / 2. The alternatively Cp can be determined by using two transducers placed at a known distance on

the surface, through the thickness of member can be establish with great accuracy.

The limitation of equipment encompasses as below:-


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Fig. 17 SASW Test Diagram

High degree of experience required.

A through knowledge of interpretation of results required.

A very sophisticated time measuring device required.

Spectral Analysis of Surface Waves (SASW) Method

This method (fig. 17) based upon the theory of stress wave

propagation in layered elastic media and field measurement ofsurface wave velocity as a function of wavelength and subsequent

theoretical modeling to determine the shear wave velocity versus

depth profile which matches the field data. The SASW method is

capable of determining the shear wave velocity profile of a

structure without coring.

The ratio of surface wave velocity to shear wave velocity varies

slightly with Poissons ratio, but can be assumed to be within 5

percent of 0.90. Measurement of the surface wave velocity of theSASW method similarly allows calculation of compression (P

wave) velocity. To obtain increasingly deeper data, several tests

with different receiver spacing are performed by doubling the

distance between the receivers about the imaginary centerlinebetween the receivers. Signal analyzer, digitize the analog receiver outputs and records the signal for

spectral (frequency) analysis to determine the phase information of the cross power spectrum between two

receivers for each frequency. Surface wave velocity (Vr) = Spacing between receivers (X) / Travel time (t).

The wavelength (Lr) is related to surface wave velocity and frequency. Lr = Vr/ . By using a computer to

repeat the above procedure for every frequency, the surface wave velocity corresponding to each wave

length is evaluated, and the dispersion curve is determine.

Acoustic Emission Technique

An acoustic emission (AE) is a localised rapid release of strain energy in a stressed material. This energy

release causes stress waves to propagate through the specimen. These AEs can be deducted at surface and

analyzed to deduce the nature of damage such as micro-cracking, progression of freeze thaw, alkaliaggregate reaction etc. The method is useful in monitoring progressive cracking.

Shear Wave 3D Tomography

PreamplifierFilter Processor

Recorder, Printeror computer

PiezoelectricTransducer


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The measurement of maturity is a simple non-destructive technique which takes account of the temperature

history within the concrete during adverse weather conditions when knowledge of strength development is

important. However, these measurements relates only to the individual test points, and for a major pour it

will thus be necessary to take measurement at several points simultaneously to account for variations within

the concrete. This can involve considerable expense if used on a regular basis. Correlations betweenmaturity and strength will only apply to the particular mix and circumstances for they have been

developed.

Pile Integrity Tests

This method is limited to testing the integrity of the pile and not intended to replace the use of static load

testing. The method does not indent provide all imperfections in a pile, but gives information with great

accuracy about continuity, defects such as cracks, necking, soil incursions and changes in cross section andapproximate pile length. The following information is generally required to carry out an Pile Integrity Test:

Location site, casting date of each pile

Number of piles tested

Length and diameter of piles

Details of bore log/driving of piles

Density of strength of concrete Depth of water table

Abnormal conditions noted while driving/boring and concerting the piles.

There are four popularly known NDT systems for the integrity testing of piles:

Transient Stock or Pulse Echo Test: This is a system of assessing the piles by the use of a low stress wave

being set up in the pile shaft. A small metal/hard rubber hammer (1/4 kg) is used to produce a light tap

(from height of 15 to 30 cm). The shock traveling down the length of the pile is reflected back from the toeof the pile, recorded through a suitable transducer (also held on the top as close to the computer disc or

diskette for later analysis. The shock wave which travels down the length of the shaft is reflected from the

toe by the change in density between the concrete and substrata. However, if the pile has any imperfection

or discontinuities within its length these will set up secondary reflections which will be added to the

returning signal. By a careful analysis of the captured signal and knowledge of the condition of the ground,age of concrete etc. a picture of the locations of such problem can be built up. Some systems filter down the

signals i.e. the secondary reflections are filtered out while the signal is being recorded. There are other

systems which is use an unfiltered signal and this is advantageous as it allows for a greater degree offlexibility in the analysis and using fast Fourier transform (F.F.T) enables the high and low frequencies to

be separated with the necking, instructions, cracks and building causing these located.

Sonic Echo Test: The sonic echo test is a variation of the transient shock method and the main difference

lies in filtering of the signal obtained. This can be done by using a special transducer responsive to low

frequency signals or these are filtering in the computer itself. Due to filtering of the high frequencies the

extraneous effects have been deleted allowing the response of the pile to become easier to understand.

Frequency Response: In this method a variable frequency vibrator is fixed to a plate on the top of a pile. In

addition, a transducer clamped on the top of the plate measures pile head response as the head vigorouslyvibrated at frequency varying between 20 and 5000 Hz. The results of the pile head response for each

frequency are obtained and analyzed by computer aided transformation techniques. This process is fairly

lengthy and time consuming and is rarely employed.

Sonic Coring: Since this method requires the use of three cast in tubes or three full length cores which

makes this expensive. It is used on large diameter piles, but not normally on piles less than 1 meter in

diameter. The test is based on measuring the propagation time of a signal between a sonic emitter and

receiver placed in adjacent tubes.


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Fig. 20 Dynamic Load Test

Dynamic Load Testing

This method (fig 20) is used in determine the bearing capacity of an

installed pile foundation. It can also give the information about the integrity

of the pile shaft of the load of the same order as the designed load. In thismethod, two identical sensors are connected to the side of the pile opposite

to each other near the pile head. A heavy, specially prepared guided block

is dropped into the pile head. The generated compression wave travels

down the pile and reflects from the pile toe upward. The waves which are

picked up by the sensors are processed and stored in the field by the FPDS-

2 computer. A pile driving hammer as well as a drop weight can be used to

apply a dynamic load. The range of drop weights which vary in mass from

200 kg to 20,000 kg and these may be used in loads piles with a capacityupto 20 MN. To assess the static performance by this method, it is

necessary to establish a correlation between the static and dynamic pile

performance. When the results integrity test on a pile causes doubts

concerning the competence of that pile to carry its design load, then pile can

be subjected to dynamic load testing. It is a reliable method for comparing the piles.

Corrosion Analysis Test

Corrosion analyzer is based on electro-chemical process to detect corrosion in the reinforcement bars of

structure. It represents a galvanic element similar to a battery, producing an electrical current, measurable

as an electric field on the surface of concrete. The potential field can be measured with an electrode known

as half cell. The electrical activity of the steel reinforcement and concrete leads them to be considered as

one half of battery cell with the steel acting as one electrode on the surface. For conducting this tests

access to the reinforcement is must. The method cannot be applied to epoxy coated reinforcement or

concrete surfaces. The concrete should be sufficiently moist for conducting this test. This test only indicatesthe likelihood of corrosion activity at the time of measurement. It does not furnish direct information on the

rate of corrosion of the reinforcement.

Resistivity Test

It is used to measure the electrical resistance of the cover concrete. Once the reinforcement bar loses its

passivity, the corrosion rate depends on the availability of oxygen for the cathodic reaction. It also depends

on the concrete, which controls the ease with which ion migrates through the concrete between anodic and

cathodic site. Electrical resistance, in turn, depends on the microstructure of the paste and the moisture

content of the concrete. The corrosion of steel in concrete is an electrochemical process, which generates a

flow of current and can dissolve metals. The lower the electrical resistance, the more readily the corrosion

current flows through the concrete and greater is the probability of corrosion. The resistivity is numerically

equal to the electrical resistance of a unit cube of a material and has units of resistance (in ohms) timeslength.

The method is slow because it covers small area at a time. The system should not be used in isolation

because it gives better indication of corrosion in reinforced concrete if used in combination with half-cell

potentiometer.

Visual Inspection


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Visual inspection provides valuable information to experienced professional. The tests programme largely

depends upon the visual inspection and study of drawings, tests data etc. This may be correlated to

workmanship, Structural serviceability, damages to materials and structures. Visual inspection is not

confined to the surface, but may include examination of drawings, documents and preliminary tests on

concrete structures, evaluation of safety of structures against provisions in codes.

CONCLUSION

The condition assessment of structure for suggesting appropriate remedial measures. Various NDT

methods have been discussed in the chapter to assess the strength, quality, durability and long term

monitoring of structures. It is well established that NDT methods are advantageous in determining the in

place strength, integrity and relative performance of structures It is important to note that almost all the

NDT methods indirectly estimate the concrete strength and strength obtained by these methods, in most of

the cases, is comparable. Even then, no single method can be said to be fully reliable and therefore, the user

must consider the relative importance of each method in selecting the most appropriate technique for a

particular application and more than one method should be performed to correlate the results. It is alsosuggested that NDT tests should be carried by the skilled operators whereas interpretation of the results

must be done by the experts, having experience and knowledge of application of such NDT tests.

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Documents

NDT: A Tool for Condition Assessment of Structures by SK Singh