TAM 224/CEE 210 3–1

3. Nondestructive Testing 3.1. Objective Six nondestructive testing (NDT) techniques will be demonstrated in this laboratory: (a) sound velocity measurements, (b) rebound hammer, (c) visual inspection of welds, (d) dye penetrant, (e) ultrasonic, (f) radiographic inspection. The first two techniques allow one to determine the strength of concrete; the last four permit the detection of internal or external discontinuities in metal structural elements.

3.2. Background How can one know the strength of a construction material? Performing laboratory or field tests on samples of the material is a reasonably sure way, particularly if one appreciates the statistical nature of

such information. However, there are many situations in which one cannot run destructive tests on the material in question or samples of it. In such circum-stances, one needs nondestructive means of deter-mining the strength or soundness of the material in a structure. In this laboratory, we will demonstrate several nondestructive test methods that allow the strength and soundness of concrete, structural steel and other construction materials to be determined both in the laboratory and in the field without the need to take a sample or to disturb the structure.

Two common methods used in the nondestructive evaluation or nondestructive testing (NDE or NDT) of concrete are the surface-hardness measurement (Schmidt rebound hammer), and ultrasound-velocity measurement (the V-meter). These methods are commonly used as a means of estimating the strength of in-place concrete. The Schmidt hammer (Fig. 1) is often used to determine when it is safe to remove the forms from concrete during construction. The V-meter can additionally be used to detect the presence of cracks and so is often used to assess the integrity of

old structures. Details of the two methods are given in the appended notes. Since neither method measures strength directly but a property that can be related to strength, both should be calibrated using concretes of different strengths. For most accurate results, the same kind of aggregate that will be found in the field concrete should be used in the calibration concrete.

The strength of concrete and other porous con-struction materials is determined by the level of

Fig. 1. Schmidt rebound hammer test for

compressive strength of concrete.

Porosity

Incomplete fusion

Toe crack

Slag entrapmentLack of fusion

Base metal

Weld metal

Fig. 2. Discontinuities in a weldment.

3–2 Behavior of Engineering Materials TAM 224/CEE 210

porosity or the distribution of the enormous number of flaws in the material. Structural metals contain far fewer flaws, but when these exceed a critical size they may drastically reduce the strength and ductility of the material and the component which it comprises. Structural metals do not usually contain large defects, but weldments can and often do contain discon-tinuities (such as those illustrated in Fig. 2) which can result in serious problems. (Undesirable or intolerable discontinuities are often termed “defects” or “flaws.”)

Defects or discontinuities can be classified as being planar (crack-like) or volumetric (rounded), and internal or external. The first classification based on shape relates to the severity of the defect; the second relates to its location and the ease with which it may be detected.

3.3. Materials and Equipment 6 x 6 x 21-in standard concrete prisms from Lab 1

Structural steel weldment

Ultrasonic pulse velocity measurement apparatus

Schmidt rebound hammer

Dye penetrant testing kit

Ultrasound pulse-echo testing equipment

X-ray radiography facility

3.4. Experiment 1. Calibration curves for both the Schmidt rebound

hammer and the V-meter will be obtained by measuring the rebound number and the ultrasonic velocity of concrete prisms having known strengths. The two (companion) standard cylin-ders also cast in Lab 1 were tested earlier on the day of the lab and the results will be given to you in the lab. The V-meter readings should be taken across the length of the prisms. Enter the time-of-transit readings in Table 1. The Schmidt hammer readings should be taken from the sides of the prisms. The prisms should be resting on the floor for these measurements. Avoid greasy areas for the rebound hammer readings, and avoid taking hammer readings in the same location twice. Enter the rebound hammer readings in Table 2. Every lab member should try each of the two techniques at least once.

2. The lab instructor will demonstrate examination of a dogbone-shaped welded steel plate using visual inspection, dye-penetrant inspection, ultra-

sonic inspection, and radiographic inspection. Plot the location of defects seen using each method on the sheets provided (pages 3-6 and 3-7).

3.5. Analysis of Results 1. Complete Tables 1 and 2.

2. Excluding data from Mix 1, plot compressive strength versus ultrasound velocity. This will be Fig. 1 of your lab writeup. (We will pretend that we don’t know the strength of Mix 1, that is, Mix 1 is the “unknown” mix.) Fit an appropriate line to the data. Graphically determine the strength of the “unknown” mix and compare this prediction with the measured value.

3. Plot compressive strength versus average rebound hammer reading in Fig. 2 of your lab writeup. Indicate the range in values with an error bar through each of the plotted average values to indicate the amount of scatter in the rebound hammer data. Fit an appropriate line to the data. Graphically determine the strength of the “unknown” mix and compare this prediction with the measured value.

4. Complete Fig. 3 of your lab writeup by sketching in the indications obtained using each of the nondestructive testing techniques. (See pages 3-6 and 3-7.)

3.6. Topics for Discussion Note.—Your lab instructor will indicate which of the

following questions are to be addressed in your report.

1. Discuss the physics of the dye-penetrant test on the weldment. What is the reason for the initial cleaning of the surface in terms of the physics of wetting and spreading? Why is the ink drawn into a surface defect? How is the defect made visible?

2. Discuss the physics of the radiographic inspection of the weldment. Why does any sizable internal defect appear as a dark shadow on the radio-graph?

3. Discuss the physics of the ultrasonic inspection of the weldment. How is an internal defect detected in a weldment using this technique?

4. Compare and contrast the type of defects which can be located in a metal component using visual, dye-penetrant, ultrasonic, and radiographic inspection.

TAM 224/CEE 210 Nondestructive Testing 3–3

5. Compare the level of operator skill required in using visual, dye-penetrant, radiographic, and ultrasonic nondestructive testing.

6. Discuss the physical reasons for the observed correlation between rebound hammer number and the compressive strength of concrete.

7. Discuss the proper test procedures to be used in performing the rebound hammer test to determine the compressive strength of concrete.

8. Compare the significance of a single rebound hammer reading and a single ultrasonic wave velocity measurement as regards the determina-tion of the mechanical properties of a concrete mix.

9. Discuss the physics of the rebound hammer test. How is the development of cement microstructure reflected in the rebound hammer reading?

10. Is it reasonable to fit a straight line to the rebound hammer data, given the shape of the curves shown in Fig. A4 in the lab manual (page 3-11)? Can you suggest any physical reason or reasons for the shape of the curve shown in Fig. A4?

11. Discuss the physical reasons for the observed correlation between ultrasound wave velocity and the compressive strength of concrete.

12. Discuss the proper test methods to be used in performing the ultrasound wave-velocity test to determine the compressive strength of concrete.

13. Discuss the physics of the ultrasound wave veloc-ity. How is the development of cement micro-structure reflected in the ultrasound wave velocity?

3.7. References American Society for Testing and Materials (ASTM)

standards: E273, E317, E428, E214, E588, E587, E164, E494 (ultrasound); E142, E390, E94, E592 (radiography); C597, C805 (concrete NDT).

Callister Jr., W. D. 2000. Materials Science and Engi-neering—An Introduction, 5th ed. New York: Wiley, 201.

Mindess, S., and J. F. Young. 1981. Concrete. Engle-wood Cliffs, N.J.: Prentice-Hall, 440–449.

Notes

3–4 Behavior of Engineering Materials TAM 224/CEE 210

Ref

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Fig. 3(a). Visual inspection of weldment.

Ref

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Fig. 3(b). Dye-penetrant inspection of weldment.

TAM 224/CEE 210 Nondestructive Testing 3–5

Ref

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Fig. 3(c). Ultrasonic inspection of weldment.

Ref

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Fig. 3(d). Radiographic inspection of weldment.

3–6 Behavior of Engineering Materials TAM 224/CEE 210

Table 1—Ultrasound wave velocity data

Mix # 1 2 3 4 5

Description Typical Low w/c High w/c Air-entrained Low w/c

water reduced

Prism length (mm) _______ _______ _______ _______ _______

Time (µsec) _______ _______ _______ _______ _______

Time (µsec) _______ _______ _______ _______ _______

Velocity (m/s) _______ _______ _______ _______ _______

Compressive strength (MPa) _______ _______ _______ _______ _______

Table 2—Rebound hammer data

Mix # 1 2 3 4 5

Description Typical Low w/c High w/c Air-entrained Low w/c

water reduced

Reading #1 _______ _______ _______ _______ _______

Reading #2 _______ _______ _______ _______ _______

Reading #3 _______ _______ _______ _______ _______

Reading #4 _______ _______ _______ _______ _______

Reading #5 _______ _______ _______ _______ _______

Average _______ _______ _______ _______ _______

Compressive strength (MPa) _______ _______ _______ _______ _______

TAM 224/CEE 210 Nondestructive Testing 3–7

Appendix NONDESTRUCTIVE TESTING OF CONCRETE

Ultrasonic Pulse Velocity This method is based on the fact that the velocity V of sound in a material is related to the elastic modulus E by the expression

V E=

ρ,

where ρ is the density of the material. Since the pulse velocity depends only on the elastic properties of the material and not on the geometry, this is a very convenient technique for evaluating concrete quality.

In essence, an apparatus such as that shown schematically in Fig. A1 is used to determine the pulse velocity through a known thickness of concrete, using

the procedure outlined in ASTM C597. A number of commercial devices are available which meet these requirements, with an accuracy of measurement of about ±1% . Typical results are shown in Fig. A2.

Accuracy of pulse-velocity measurements and the correlation between pulse velocity and strength

depend on several specific parameters of the test procedure and the material being tested:

1. If the contact surfaces are not reasonably smooth, they should be ground smooth; a coupling medium such as grease must also be used to ensure good contact between the transducers and the concrete.

2. The pulse velocity seems to depend on the path length, decreasing somewhat as the path length is increased.

3. The pulse velocity is not sensitive to temperature in the range 5 to 30°C. At higher temperatures, the pulse velocity is decreased, and at tempera-tures below freezing, it is increased.

4. Pulse velocity increases with moisture content.

5. The presence of steel bars will tend to increase the pulse velocity.

Pulsegenerator

Timemeasuring circuit

Fixeddelay

Mixingcircuit

Trigger

Transducer(transmitter)

Transducer(receiver)

Cathode-rayoscilloscopedisplay unit

Specimen

Preamplifier

Fig. A1. Schematic diagram of pulse velocity testing

circuit. (Adapted from ASTM C597.)

17,000

16,000

15,000

14,000

0 2 4 6 8 10

0

5100

(m/s)

(ft/s)

Puls

e ve

loci

ty

Cube compressive strength

(MPa)

(ksi)

1:1.5:3 Rounded gravel

1:1.5:3 Crushed limestone

1:1.5:3 Crushed granite

4900

4700

4500

4300

10 20 30 40 50 60 70 80

Fig. A2. Correlation of pulse velocity with compressive

strength. (Adapted from R. Jones, Non-destructive Testing of Concrete, Cambridge University Press, 1962.)

3–8 Behavior of Engineering Materials TAM 224/CEE 210

6. For a given pulse velocity, the compressive strength is higher for older specimens.

Rebound Hardness Probably the most common nondestructive test is the rebound test, using a Schmidt rebound hammer. This device (Fig. A3) was developed in 1948, and is

universally used because of its simplicity. The test measures the rebound of a hardened steel hammer impacted on the concrete by a spring.

Again, although there is no theoretical relation-ship, empirical correlations between rebound hard-ness and strength can be obtained (Fig. A4). (In Fig. A4, five hundred standard 150 x 300 mm (6 x 12 in.) cylinders tested SSD at 28 days were used.) This method is described in detail in ASTM C805 (BS 4408: Part 4) and suffers from the same limitations as the

surface hardness method, that is, the results will be affected by:

1. The surface finish of the concrete being tested; trowelled surfaces give higher values than formed surfaces, and ground and unground surfaces cannot be compared.

2. The moisture content: dry concrete gives higher values than does wet concrete.

3. Temperature: frozen concrete will give very high values, and must be thawed before testing; the temperature of the hammer will also affect the rebound number.

4. The rigidity of the member being tested.

5. The carbonation of the surface, which can increase the hardness values by as much as 50%.

6. The direction of impact (upward, downward, horizontal).

The general view held by many users of the Schmidt rebound hammer is that it is useful in checking the uniformity of concrete and in comparing one concrete against another, but that it can be used only as a rough indication of the concrete strength in absolute terms. Printed 1/1/02

Compressionspring

Impactspring

Latchingmechanism

Hammermass

Housing

Hammerguide bar

Impactplunger

Specimen

Windowwith scale

Fig. A3. Schmidt rebound hammer.

Longitudinal section of the Type N concrete test hammer (condition on impact).

Adapted from Mindess and Young (1981).

1

2

3

4

5

6

020 25 30 35 40

15 20 25 30 35

Dry

SSD

Rebound readings on concrete, xC

ompr

essi

ve s

tren

gth

(ksi

)

Mean curve

Stand. dev. upper limitStand. dev. lower limit

+15%

−15%

Fig. A4. Calibration chart for concrete made with crushed

limestone and natural sand aggregates.

(Adapted from Mindess and Young (1981), after N. Zolders, Journal of the American Concrete Institute 54(2),

1957, 161-165.)