non distructive methods

8/16/2019 Non Distructive Methods

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• The tests available for testing concrete in structures withhardened concrete from a long time range from the completelynon-destructive, where there is no damage to the concrete,through those where the concrete surface is slightly damaged,to partially destructive tests, such as core tests and pullout andpull off tests, where the surface has to be repaired after the test.The range of properties which can be assessed using non-destructive tests and partially destructive tests includesparameters as density, elastic modulus and strength, alsosurface hardness and surface absorption, reinforcementpos on, ame er an s ance rom e sur ace. n some casesit is possible to verify the quality of workmanship and structuralintegrity by detecting voids, cracking and delamination. Non-destructive testing can be applied to both old and newstructures. For new buildings, the principal applications are forquality control or the resolution of doubts about the quality of materials or construction (Knaze and Beno, 1985). If onlydestructive testing is used, for instance, by removing cores forcompression testing, the cost of coring and testing may onlyallow a small number of tests to be carried out that is notenough for big structures..


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Some situations where non-destructive testing can be use are, as

follows [185], [190], [191]:

• quality control of pre-cast units or construction in situ for confirm

the acceptability of the material supplied;

• confirming or not doubt concerning the workmanship implicated in

batching, mixing, placing, compacting or curing of concrete;

• monitoring of strength development during hardening in relation to

formwork removal, cessation of curing, pre-stressing, load

• location and determination in real time of the extent of cracks,

voids, honeycombing and any other defects within a concrete

structure;

• observing the concrete uniformity, possibly preliminary to core

cutting, load testing or other more expensive or disruptive tests;

• determining data referring to the position, quantity or condition of

reinforcement;

• increasing the confidence in a smaller number of destructive tests;


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• determining the extent of concrete variability forhelping the selection of sample locations which

are representative;• confirming or finding suspected deterioration of

concrete resulting from such factors asoverloading, fatigue, external or internal chemicalattack, fire, explosion, environmental effects;

• assessin the otential durabilit of the concrete

• monitoring long term changes in concretecharacteristics;

• providing data for any proposed change of usinga structure for insurance or for chaning of ownership.


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3.8.1. Basic methods for ndt of

concrete structures

• The following methods, with some typicalapplications, have been used for the NDT of polymer concrete [189], [190], [193], [194]:

•

Visual inspection, which is an essential precursor- .experienced civil or structural engineer may beable to establish the possible cause(s) of damageto a concrete structure and hence identify which

of the various NDT methods available could bemost useful for any further investigation of theproblem.


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• Half-cell electrical potential method, used to detect the

corrosion potential of reinforcing bars in concrete.

• Schmidt/rebound hammer test, used to evaluate the

surface hardness of concrete.

• Permeability test, used to measure the flow of water

through the concrete.

• Penetration resistance or Windsor robe test used to

measure the surface hardness and hence the strength of

the surface and near surface layers of the polymer

concrete.

• Covermeter testing, used to measure the distance of steel

reinforcing bars beneath the surface of the concrete and

also possibly to measure the diameter of the reinforcing

bars.


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• Radiographic testing, used to detect voids in the concreteand the position of stressing ducts.

• Ultrasonic pulse velocity testing, mainly used to measurethe sound velocity of the concrete and hence thecompressive strength of the concrete.

• Sonic methods using an instrumented hammer providingboth sonic echo and transmission methods.

• Tomographic modelling, which uses the data from

detect voids in concrete.

• Impact echo testing, used to detect voids, delamination andother anomalies in concrete.

• Ground penetrating radar or impulse radar testing, used todetect the position of reinforcing bars or stressing ducts.

• Infrared thermography, used to detect voids, delaminationand other anomalies in concrete and also detect waterentry points in buildings.


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3.8.2 Nondestructive test methods

•

Nondestructive test (NDT) methods are commonly used forquality control of various construction elements. Contrary todestructive methods, NDT techniques give information aboutmaterial properties without deteriorating materialmicrostructure and serviceability. The main advantages of NDT methods are: the possibility of on-site evaluation,

repeatability at the same place during structural service, andquick test results.

• By comparison to other construction materials like steel, thedevelopment of NDT methods for concrete-like compositeshas progressed at a slower pace because these kinds of composites are difficult to test. Concrete is heterogeneous,

intrinsically conductive (because the pore solution is an ionicelectrolyte), and also usually contains steel reinforcement. Forthese reasons, NDT techniques used with metals are not easyto implement for concrete and similar composites. Since it isdifficult to apply to concrete some of the techniques usedwith metals, alternative NDT methods have been considered.


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• The evaluation of structures prior to repair, aproper selection of repair materials, andquality control of the repairs should be doneto assure the effectiveness of the repairprocess. NDT methods are applied for fourmain reasons:

• ,

• unexpected problems with new construction,

• evaluation of existing material, including

evaluation prior to repair,• quality control of concrete repair.


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In general, considering the measured parameters, NDT

methods can be divided into the following categories:

•

rebound hammer,• acoustic methods – stress wave propagation (ultrasound,

acoustic emission, impact-echo, etc.),

• radiation methods (X-ray, gamma ray, neutron emission,

etc.),• electromagnetic methods,

• others – e.g. infrared thermography.

Another classification takes into account the aim of

nondestructive evaluation. Two main categories can be

recognized:

• – evaluation of concrete strength and its homogeneity

(e.g. rebound hammer, ultrasonic pulse velocity),


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• – evaluation of structural integrity – detection of

various types of defect in polymer concrete,detection and evaluation of steel reinforcement(e.g. visual inspection, stress wave propagation,impact-echo, infrared thermography, radiationmethods, electromagnetic methods) [170], [171],

[180].,

methods, especially the ultrasonic pulse velocitymethod, are still used for the evaluation of concrete.


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• – evaluation of structural integrity – detection of various types of defect in polymer concrete,detection and evaluation of steel reinforcement(e.g. visual inspection, stress wave propagation,impact-echo, infrared thermography, radiationmethods, electromagnetic methods) [170], [171],

[180].• ,

methods, especially the ultrasonic pulse velocitymethod, are still used for the evaluation of concrete [172], [187], [193].


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Rebound hammer test

• This is one of the oldest non-destructive tests andit is still widely used. It was devised in 1948 byErnst Schmidt and is therefore known also asSchmidt hammer or sclerometer test.

• quite different from the hardness determined intests on metals.

• The rebound hammer test is based on the

principle that the rebound of an elastic massdepends on the hardness of the surface againstwhich the mass impinges.


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• However, despite its apparent simplicity, the

rebound hammer test involves complexproblems of impact and associated stress-wave propagation. In the test a spring-loadedmass has a fixed amount of energy imparted

to it by extending the spring to a fixed

plunger against the surface of the concreteunder test. Upon release, the mass rebounds

from the plunger, still in contact with theconcrete surface, and the distance travelled bythe mass, expressed as a percentage of theinitial extension of the spring, is called the

rebound number.


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Sclerometru: 1- piston; 2-beton; 3-carcasa tubulara; 4-cursor indicator;

5-scala; 6- ciocan; 7- buton de declanșare; 8,9-resort; 10-știft de oprire


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REBOUND HAMMER

Limitations:

- evaluation of near-surface properties only,- results depend on surface roughness,

- reference curve needed for strength estimation,

apparatus,


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Ultrasonic pulse velocity test

This is a non-destructive test method whichdetermines the velocity of longitudinal waves. Thisdetermination consists of measurement of the time

taken by a pulse to travel a measured distance. The apparatus includes transducers which are

placed in contact with the concrete, a pulsegenerator with a frequency of between 10 and 150

Hz, an amplifier, a time measuring circuit, and adigital display of teh time taken by the pulse of longitudinal waves to travel between transducers.


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• The velocity of the ultrasonic pulse through

the concrete is the the outcome of the time

taken by the pulse to travel through the

hardened cement paste and through the

aggregate. The modulus of elasticity of

aggregates varies considerably, so that the pu se ve oc ty o t e concrete epen s on t e

modulus of elasticity of the actual aggregate

and on the aggregate content of the mix.

• With some limitations the ultrasonic pulse

velocity test can be used to assess the

strength of concrete.


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a) b) c)

For determining the compressive strength of

concrete some reference curve can be used.

,

b) semi-direct method, c) indirect (surface) method


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• The ultrasonic pulse velocity test has the

considerable merit of giving information about

the interior of a concrete element.

• The test is used to detect cracking (but not

parallel to the direction of the pulse), voids,

deterioration due to frost or fire and theuniformity of concrete in similar elements.

• The test can be used for the purpose of

following changes in a given concreteelements, for instance, due to repeated cycles

of freezing and thawing.


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The test can also be used to asses the strength of concrete at very early ages, from about 3 hoursonwords. This is of interest in precasting or as an aid indeciding on removal of formwork, including steam-cured concrete.

An echo type of ultrasonic pulse technique makes it

possible to measure the thickness of concrete roads.

The method consists in generation of a short impulseof the ultrasonic wave by the transmitting transducer(Figure 3.20). After reflection by the material’s

structural heterogeneity or by the limiting surface, theimpulses are recorded by the receiving transducer(dual transmitting-receiving transducers are alsoavailable).


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Testing the concrete by ultrasonic echo method: a)

transmitting-receiving transducer, b) double transducer

Part of the ultrasonic wave is reflected by the material

defect, returns to the receiving transducers and isrecorded as the defect’s echo. Another part of the wave

passes by the defect and reaches the opposite wall of the

tested material, where it is reflected and returns to the

receiver with some delay as the bottom echo.


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• The depth of the defect or the reflectingsurface is determined on the basis of the

travel time of the impulse and the ultrasonicwave velocity. A small grain size of the testedmaterial is necessary for the echo method tobe efficient. The grain size should be

si nificantl smaller than the searched-fordefects; if not, then any defect echo will beoverlapped by the echoes formed by the grainboundaries.


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Test cores

The fundamental purpose of measuring the

strength of concrete test specimens is to

estimate the strength of concrete in the actual

.

The selection of the location of cores depends

on the purpose of testing.


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• This may be: to estimate the strength of a criticalpart of a structure, or of a part suspected of

having been damaged, for example by frost; oralternatively, to estimate a representative valuefor the entire structure, in which case a randomselection of locations is appropriate.

•

Cores can also be used to detect segregation of construction joints or to verify the thickness of pavement.

•

Cores are cut by means of a rotary cutting toolwith diamond bits. In this manner, a cylindricalspecimen is obtained, sometimes containingembedded fragments of reinforcement.


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non distructive methods

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