Table 3.

Survey of Commonly used NDE Methods for Inspection Building Materials

Method	Principle	Main Applications	Equipment Cost	User Expertise	Advantages	Limitations
1. Acoustic Emission	During crack growth or plastic deformation, the rapid release of strain energy produces acoustic (sound) waves that can be detected by sensors attached to the surface of a test object.	Continuous monitoring of structure during service life to detect impending failure; monitoring performance of structure during proof testing.	$10,000 for single pickup, up to $100,000 for multichannel pickup.	Extensive knowledge required to plan test and to interpret results.	Monitors structural response to applied load; capable of detecting onset of failure; capable of locating source of possible failure.	Requires means of loading structure; complex electronic equipment is required.
2. Acoustic Impact (Hammer Test)	Surface of object is struck with hammer (usually metallic). The frequency, through transmission time, and damping characteristics of the “ringing” can give an indication of the presence of defects.	Detect delamination or disbonds in composite systems; detect voids and cracks in materials, e.g. hammer technique detect defective masonry units; “chain drag” method to detect delaminations in concrete pavements. Can be used to measure the modulus of elasticity of wood.	Negligible for manual technique, $3000 for measuring devices.	Low level of expertise required to use, but experience needed for interpreting results.	Portable; easy to perform; electronic device not needed for qualitative results.	Geometry and mass of test object influences results; poor discrimination; reference standards required for electronic testing.
3. Cast-in-Place Pullout	Measure the force required to pull out the steel rod with enlarged head cast in concrete. Pullout forces produce tensile and shear stresses in concrete.	Estimation of compressive and tensile strengths of concrete.	$1000 to $6000	Low, can be used by field concrete testers and inspectors.	Only NDE method which directly measures inplace strength of concrete; appears to give good prediction of concrete strength.	Pullout devices must be inserted during construction. Cone of concrete may be pulled out, necessitating minor repairs.
4. Electrical Potential Measurements	Electrical potential of steel reinforcement measured. Potential indicates probability of corrosion.	Determining condition of steel rebars in concrete	$1000 to $2000	Moderate. User must be able to recognize problems.	Portable equipment; field measurements readily made; appears to give reliable information.	Does not provide information on rate of corrosion. Requires access to reinforcing bars.
5.1 Eddy Current	An electrically excited coil induces eddy current flow and an associated electromagnetic field in metal. Flaws alter induced electromagnetic field which in turn alters the impedance of the excitation coil. Change in coil impedance indicates presence of flaw or anomaly.	Inspection of metal parts for cracks, inclusions, seams and laps; measurement of thickness of non-metalic coating on metals; detection of improper alloy composition.	Minimum $3000	Moderate.	Extremely sensitive to change in properties and characteristics of metal; portable.	Requires calibrawith standards; limited depth of penetration; only applicable to metals; sensitive to geometry of part.
5.2 Magnetic Field Testing	An electrically energized primary coil is brought near test object, a voltage is induced in a secondary coil and its magnitude is compared to a reference standard. Magnetic properties of test object affect induced voltages.	Distinguishing between steels based on differences in composition, hardness, heat treatment, or residual stresses; locating hidden magnetic parts; measuring thickness of non-magnetic coatings or films.	$3000	Low to moderate depending on application.	Portable; rapid test; easily detects magnetic objects even if embedded in nonmagnetic material.	Applicable only to ferromagnetic alloys; reference standards and calibration may be required for some applications.
5.3 Covermeter	Presence of steel affects the magnetic field of a probe. Closer probe is to steel, the greater the effect. Principle of operation is similar to eddy current method.	Determination of presence, location and depth of rebars in concrete and masonry units.	$800 to $1500	Moderate. Easy to operate. Training needed to interpret results.	Portable equipment, good results if concrete is lightly reinforced.	Difficult to interpret results if concrete is heavily reinforced or if wire mesh is present.
5.4 Magnetic Particle Inspection	Presence of discontinuities in ferromagnetic material will cause leakage field to be formed at or above the discontinuity when the material is magnetized. The presence of the discontinuity is detected by use of ferromagnetic magnetic particles applied over the surface which form an outline of the discontinuity.	Usually used to detect fatigue cracks inservice metal components and inspection during production control. Applicable to inspecting welds.	Minimum $2000	Expertise required to plan non-routine tests. Moderate expertise to perform test.	Capable of detecting subsurface cracks if they are larger than surface cracks; size and shape of component poses no limitation; portable equipment available.	Non-ferromagnetic metal cannot be inspected; coatings affect sensitivity; demagnetization may be required after testing.
6. Leak Testing	Telltale substances added to piping system under pressure reveal presence of leaks. Sound amplification to detect leak.	Detection of leaks in pipes carrying fluids.	Wide range depending on detection method. $100 to $5000.	Low to high depending on application.	Can locate leaks too small to be found by any other NDE method.	Difficult to determine position of leaks in pipes hidden in wall or floor cavities.
8.1 Moisture Meter-Electrical Resistance Probe	Electrical resistance between two probes inserted into test component is measured. The resistance decreases with increased moisture contents.	Measurement of moisture contents of timber, roofing materials and Boils.	$300 to $1000	Low	Equipment is inexpensive, simple to operate and many measurements can be rapidly made.	Not reliable at high moisture contents; needs to be calibrated; precise results are not usually obtained.
8.2 Moisture Meter-Capacitance	Water affects die-electric constant and the dielectric loss factor of materials. Measurement of either property can be used to estimate moisture contents.	Measurement of moisture contents of timber and roofing materials.	$2,000 to $5,000	Low level of expertise required to use but experience needed to plan test.	Portable; simple to operate; effective over a wide range of moisture contents.	Measurement is only of surface layer; calibration required; results affected by roofing aggregates; other factors affect accuracy.
8.3 Moisture Meter-Neutron	Fast neutrons are slowed by interactions with hydrogen atoms. Backscattered slowed neutrons are measured, the number of which are proportional to the amount of hydrogen atoms present in a material.	Moisture content measurements of soil and roofing materials.	$4000 to $6000	Must be operated by trained and licensed personnel.	Portable; moisture measurements can rapidly be made on in-service materials.	Only measures moisture content of surface layer (50 mm); dangerous radiation; hydrogen atoms of building materials are measured in addition to those of water.
8.3 Nuclear Density Meter	Gamma rays are used to measure mass density. The energy loss of the emitted gamma rays is proportional to the mass density of the material through which the rays pass.	Measurement of density of soils.	$4000 to $6000	Must be operated by trained and licensed personnel.	Portable; density measurements can be made without disturbing the soil.	Calibration necessary; dangerous radiation; only measures density of surface layers.
9. Tooke Gage	A V-groove is cut into the coating and an illuminated magnifier equipped with a reticle in the eyeplace is used to measure the number and thickness of the films.	Measurement of the number and thickness of paint layers.	$1300	Low	Simple to operate, portable; measurement can be made with any type of substrate.	Small scratch is made in coating and the substrate is exposed.
10. Pin Hole (Holiday) Meter	An earth load is connected to a conductive substrate and a probe (a moistened sponge) is passed directly over a coating. An alarm is sounded when the circuit is completed resulting from the probe contacting a pin hole (holiday).	Determining the presence of pin holes in non-conductive coatings over metals.	$200 to$1000	Low	Simple to operate; portable.	Results are qualitative, e.g., there is no measure of the pin hole size.
11. Proof Loading	Structure or system is subjected to loads and respond is measured.	Determining safe capacity and integrity of structures. Leak testing of pressure vessels and plumbing.	Wide, depending on application; often high.	Depends on nature of tests; can be high.	Entire structure can be tested in its “as-built” condition.	Can be very costly; instrumentation required to measure response; careful planning required; can damage structure.
12. Windsor Probe	Probe fired into concrete and depth of penetration is measured. Surface and subsurface hardness measured.	Estimations of compressive strength, uniformity and quality of concrete.	$1000 plus cost of probes.	Low, can be operated by field personnel.	Equipment is simple and durable; good for determining quality of concrete.	Slightly damages small area; does not give precise prediction of strength.
13.1 X-ray Radiography	Similar to gamma radiography, except X-rays are used.	To identify hidden construction features in wooden structures.	Field Equipment in excess of $5000	Should be operated by trained personnel because of radiation.	Portable equipment available, intensity of radiation can be varied.	Dangerous radiation; portable units have low intensities and field applications limited to wooden and thin components; opposite surface of component must be accessible.
13.2 Gamma Radiography	Gamma radiation attenuates when passing through a building component. Extent of attenuation controlled by density and thickness of the materials of the building component. Photo-graphic film record usually made, which is analyzed.	Locating internal cracks, voids and variations in density and composition of materials. Locating internal parts in a building component, e.g. reinforcing steel in concrete.	$5000 to $10,000	Must be operated by trained and licensed personnel.	Portable and relatively inexpensive compared to X-ray radiography; internal defects can be detected; applicable to a variety of materials.	Radiation intensity cannot be adjusted; long exposure times may be required; dangerous radiation; two opposite surfaces of component must be accessible.
14. Seismic Testing	Integrity of material evaluated by analysis of shock wave transmission and effects. Shock wave induced by explosive charges and transmission detected by transducers.	Determination of soil densities and variation in densities. Also vibrational characteristics of buildings can be determined.	Wide, depending on amount of information desired.	Experience required to plan test and to interpret results.	Large area of soil and entire structure in its “as-built” condition can be tested.	If incorrectly placed, explosive charge could damage structure; care must be exercised in handling explosives.
15.1 Indentation Hardness Test	Pointed probe is mechanically forced into surface of a material, usually a metal, under a specified load. The depth of identification is measured and strength of materials may be estimated.	Determination of effectiveness of heat treatment on hardness of metals. Estimating tensile strength of metals.	$600 to $4000	Low.	Portable equipment available; fast and easy test to perform.	Conversion tables give only approximate tensile strengths; feasibility of testing limited by size and geometry of component.
15.2 Rebound Hammer	Spring driven mass strikes surface of concrete and rebound distance is given in R-values. Surface hardness is measured.	Estimation of compressive strength, uniformity and equality of concrete.	$250 to $600	Low, can be readily operated by field personnel.	Inexpensive; large amount of data can be quickly obtained; good for determining uniformity of concrete.	Results affected by condition of concrete surface; does not give precise prediction of strength.
16. Thermal Inspection	Heat sensing devices are used to detect irregular temperature distributions due to presence of flaws or inhomogenities in a material or component that have different impedances to heat flow. Contours of equal temperature (thermography) or temperatures (thermometry) are measured over the test surface with contact or noncontact detection devices. A common detection device is an infrared scanning camera.	Detection of heat loss through walls and roofs; detection of moisture in roofs; detection of delamination in composite materials.	$30,000 for infrared scanning camera. Less expensive hand held equipment becoming commercially available.	Moderate to extensive depending on nature of test.	Portable; permanent record can be made; testing can be done without direct access to surface and large area can be rapidly inspected using infrared cameras.	Costly equipment; referenced standards needed; means of producing thermal gradient in test component or material is required.
17.1 Ultrasonic Pulse Velocity	Based on measuring the transit time of an induced pulsed compressional wave propagating through a material.	Estimation of the quality and uniformity of concrete.	$4000 to $6000	Low level required to make measurements.	Excellent for determining the quality and uniformity of concrete; test can be quickly performed.	Does not provide estimate of strength; skill required in analysis of results; moisture variation can affect results.
17.2 Ultrasonic Pulse Echo	Pulse compressional waves are induced in materials and those reflected hack are detected. Both the transmitting and receiving transducers usually are contained in the same probe.	Inspecting metals for internal discontinuities. Some work has been performed on the use of the pulse echo method to inspect concrete.	Minimum $3000	High level of expertise required to interpret results.	Portable; internal discontinuities can be located and their sizes estimated.	Good coupling between transducer and test substrate critical; interpretation of results can be difficult; calibration standards required.
18.2 Fiberscope (Endoscope)	Bundle of flexible, optical fibers with lens and illuminating systems is inserted into small bore hole thus enabling view of interior of cavities.	Check condition of materials in cavity, such as thermal insulation in wall cavities, pipes and electrical wiring in cavity walls; check for unfilled cores in reinforced masonry construction; check for voids along grouted stressed tendons.	$3000 to $6000	Low.	Direct visual inspection of otherwise unaccess ible parts is possible.	Probe boles usually must be drilled; probe holes must connect to a ' cavity.
18.3 Liquid Penetrant Inspection	Surface is covered with a liquid dye which is drawn into surface cracks and voids. Developer is applied to reveal presence and location of flaws.	Detection of surface cracks and flaws. Usually used to inspect metals.	$50 to $250 per 100 linear feet of inspection.	Low.	Inexpensive; easy to use; can be applied to complex parts; results are easy to interpret	Detects only surface flaws, false indications possible on rough or porous materials; surface requires cleaning prior to testing.