Non Destructive Testing

NDT OF CONCRETE

By: Bhaskar Joshi

What 

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is Non Destr Destructive uctive Testing Testing ???

Acc Accordi ording ng to the the AMER AMERIICAN CAN St Sta anda ndards of Non Destruc ructive Testing (ASNT), NDT is said to be comprising those test methods used to examine an obje ob ject ct,, ma mate teri rial al or syste ystem m with withou outt impa impair irin ing g its its future usefulness. As per the INDIAN Standards of Non Destructive Testi esting ng (ISNT ISNT), ), NDT NDT is a way of te tessting ting with withou outt desstroying de ying.. This This mea eans ns that hat the comp compon onen entt ca cann continue to be used and that the non destructive testing method has done no harm.

Why y

y

y

Non Destr Destructive uctive Testing Testing ??????

Manner of preparation and use is poor, mainly due to lack of proper quality control & supervision during the course of construction. Often safety & durability of structures are given the go-by go-by by permitting poor quality of construction. At present the test used mainly as a basis of quality control is compression testing of cubes & it represents the potential strength of the concrete used.

Why

Compression Compress ion testing of cubes is inadequate ?????????? y

Quality of concrete -- The three ´Cµ phenomenon.

y

Deliberate Deviation ?

y

y

y

Not a True True Representation Representation of the structure due to presence of Steel, Voids, Cracks. Can only be performed during the construction stage. All of the above reasons make NDT helpful as well as very useful.

Objectives of Non Destructive Testing !!!! 

To establish homogeneity of concrete.

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Detection of cracks, voids and other imperfections. imperfections.

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Monitoring changes in concrete with passage of time.

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To establish quality of one element w.r.t. another.

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Assessment of existing structure for rehabilitation planning.

Types of Non Destructive Testing as per the ISNT !!!! 

Rebound Hammer test.

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Ultrasonic pulse velocity test.

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Electrical Resistivity test.

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Half cell potential test.

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Cover meter test

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Carbonation depth measurement

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Chloride determination test

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Core test ( Partially Partially non no n destructive )

Rebound Hammer test !!!!!!!!!!!

About Rebound Hammer !!!!!!!!!!!!!!! 

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This is basically a surface hardness test & should be used only on concrete where the surface has not carbonated as the results tend to be very high & unrealistic on a carbonated surface. Hence it should be used for younger concrete than for older one. In case of old concrete the carbonated layer should be chi chipped off or grounded to expose the core concrete & then the hammer test should be conducted. The rebound is affected if the surface is moist.

Cont«««««««««««« ««.. 

CONDUCT ON SMOOTH AND UNIFORM FACE

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AVOID ROUGH SPOTS, S POTS, HONEY COMBS COMB S

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AVOID TROWELLED SURFACES

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THIN SECTIONS (< 100 mm) SHOULD BE BA BACKED UP TO AVOID AVOID DEFLECTIONS DEFLEC TIONS TAKE ATLEAST ATLEAST 15 REBOUND R EBOUND READINGS IN ANY AN Y ONE TEST

Ultrasonic Pulse Velocity Test !!!!!!!!!!

About Ultrasonic Pulse Velocity !!!!!! 

TYPES OF TESTING METHOD 

DIRECT TRANSMISSION

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SEMI DIRECT TRANSMISSION SURFACE SURFACE TRANSMISSION TRANSMIS SION

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To assessing the quality & integrity of concrete by passing ultrasound waves through the specimen / RCC member under test. This test can also be used to determine the presence of hone honey ycomb combss, voids oids,, crack crackss et etc. c. The The inst instrum rumen entt consists of a transmitter & a receiver (two (two probes).

Cont«««««««««««« ««««. 

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The distance between the two probes (path length) can be physically measured. The time of travel for the wave to pass from the transmitter to the receiver is recorded in the ultrasonic instrument. Hence Ultrasonic Pulse Velocity = Path length / Time This velo velocit city y in concret concrete e can then then be relate related d to the uniformity of concrete.

Cont«««««««««««« ««« Quality

Pulse Velocity ( Km/sec )

Excellent

> 4.6

Good

4.6 ² 3.7

Fair

3.7 - 3.0

Poor

3.0-2.1

Very Poor

< 2.1

Electrical Resistivity Test !!!!!!!!!!!!!!!!!

About Electrical Electrical Resistivity Test !!!!!!! 

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This test is carried out to assess the quality / uniformity of concrete at various depths. The The ins instrum trumen entt con consis sists of a four our prob probe e de devi vicce. Electrical current is passed through the outer probes & the pot oten enttial drop is mea eassure ured by the inn inner probes. From the current & voltage drop measurements, the resistivity of concrete can be measured. This resistivity can be related to quality of concrete.

Cont«««««««««««.. 

Electrical resistivity = 2(pi)aE/i (in kilo-ohm cm)

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where pi = 22 / 7 = 3.14

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a = distance between probes probes

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E = potential difference between inner two probes in mV i = current passed through outer probes in microamp The penetration of current depends upon the distance bet etw ween the probes. es. Various ous sets of readings are taken.

Cont«««««««««««« « 

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When

the RCC members to be tested are covered with costly finishes like marble, granite etc. This test can be used without having to remove such finishes.

Finally, the values of the resistivity are compared against Cor rosion Ra te

Resistivity

Low

> 20 Kilo ohm cm

Low to moderate

10 ² 20 Kilo ohm cm

High

5 ² 10 Kilo ohm cm

Very High

< 5 Kilo ohm cm

Half Cell Potential Test !!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!

About Half Cell Potential Test Test !!!!!!!!!! !!!!!!!!!! 

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A Half cell is a piece of metal in a solution of its own ions ( copper in copper sulphate). When connected to another half cell ( Silver ), a voltage is set up. Thi This test can give the prob oba abilit lity of corr orrosi osion activity taking place at the point where the measurement of potentials are taken. An electrical contact is established with the exposed steel & the half-cell is moved across the surface of concrete for measuring the potentials. By convention, +ve end of voltmeter is connected to

Cont«««««««««««« «««««. Copper/Copper

sulphat ate e

Silver / Si Silver ch chloride

Corrosion Condition

> -200 mV

> -106 mV

Low ( 10% chances )

-200 to -350 mV

-106 to -256 mV

Intermediate

< -350 mV

< -256 mV

High (