Non-destructive testing in construction

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Due to the increasing aging of buildings, non- destructive testing in construction (NDT) is becoming more and more important. In the classic sense, your task is to examine the existing building fabric for damage. But NDT is also becoming increasingly important in quality assurance during the construction phase.

history

To this day, mainly destructive material testing methods are used to examine structures for damage . These include, among other things, hardness tests on concrete pressure cubes and cylinders, removal of drill cores , tensile and bending tests, etc. Due to the need to assess the stability and remaining service life of existing structures , test methods known from seismology , medical technology and mechanical engineering have been established according to the requirements further developed in the construction industry, or new processes developed for building-specific areas of application.

Basics

Procedure

In general, NDT distinguishes between active and passive test methods. With active processes, a signal is sent through the component and then evaluated with regard to the material and building material properties. With passive processes, the component generates the signal itself. This is then evaluated for location and type of origin.

Elastic waves

The waves used in NDT ( sound waves ) can propagate in different ways in the material to be examined. The most important types of waves are longitudinal waves (primary or compression waves), transverse waves (secondary, shear waves) and surface waves (Rayleigh waves ). Other types of waves that can occur are love waves , stretching or bending waves and torsion waves , but these only play a subordinate role here. For a more detailed description of the latter waves, see Seismic waves .

The speed of the waves depends on Young's modulus , density and Poisson's ratio ( Poisson ) from the transmitting medium. In solid, homogeneous, isotropic, infinitely extended media, the following applies: ${\displaystyle E}$ ${\displaystyle \rho }$ ${\displaystyle \mu }$

${\displaystyle v_{\mathrm {prim{\ddot {a}}r} }={\sqrt {\frac {E\cdot (1-\mu )}{\rho \cdot (1-2\mu )(1+\mu )}}}~\left[{\frac {\mathrm {m} }{\mathrm {s} }}\right]}$

${\displaystyle v_{\mathrm {sekund{\ddot {a}}r} }={\sqrt {\frac {E}{2\rho \cdot (1+\mu )}}}\approx {\frac {v_{\mathrm {prim{\ddot {a}}r} }}{\sqrt {3}}}~\left[{\frac {\mathrm {m} }{\mathrm {s} }}\right]}$

${\displaystyle v_{\mathrm {Rayleigh} }\approx 0{,}9194\cdot v_{\mathrm {sekund{\ddot {a}}r} }~\left[{\frac {\mathrm {m} }{\mathrm {s} }}\right]}$

Wave speed in concrete

In concrete , the primary wave speed is between 3500 and 4600 (C12 / 15 to C100 / 115). If the concrete compressive strength is unknown, a first approximation of = 4000 can be assumed. The relationship between wave speed, wavelength and frequency is given by: ${\displaystyle \textstyle {v_{\mathrm {prim{\ddot {a}}r} }}}$ ${\displaystyle \textstyle {\frac {\mathrm {m} }{\mathrm {s} }}}$ ${\displaystyle \textstyle {v_{\mathrm {prim{\ddot {a}}r} }}}$ ${\displaystyle \textstyle {\frac {\mathrm {m} }{\mathrm {s} }}}$

${\displaystyle f={\frac {v}{\lambda }}~\left[\mathrm {Hz} ={\frac {1}{\mathrm {s} }}\right]}$

${\displaystyle \lambda ={\frac {v}{f}}~\left[\mathrm {m} \right]}$

reflection

If a sound wave hits an interface, it is reflected or refracted there ( Snellius' law of refraction ). The acoustic impedance determines the proportion of the reflected wave energy . The greater the difference in impedance, the more energy is reflected. The impedance Z is determined by the density of the material and the corresponding sound wave speed : ${\displaystyle \textstyle {\rho \left[{\frac {\mathrm {kg} }{\mathrm {m} ^{3}}}\right]}}$ ${\displaystyle \textstyle {v\left[{\frac {\mathrm {m} }{\mathrm {s} }}\right]}}$

${\displaystyle Z=\rho \cdot v\left[{\frac {\mathrm {kg} }{\mathrm {m} ^{2}\,\mathrm {s} }}\right]}$

The impedance of air at 15 ° C is thus and the impedance of concrete (C30 / 37) . This means that at a shift change between concrete and air and vice versa, most of the transmitted sound energy is reflected. On the one hand, defects in the structure can be easily identified, on the other hand, it is hardly possible to see what lies behind such shift changes. ${\displaystyle \textstyle {417{,}0\,{\frac {\mathrm {kg} }{\mathrm {m} ^{2}\,\mathrm {s} }}}}$ ${\displaystyle \textstyle {9\,223\,159\,{\frac {\mathrm {kg} }{\mathrm {m} ^{2}\,\mathrm {s} }}}}$

resolution

Depending on the purpose of the measurement, the resolution and thus the wavelength of the test pulse must be adapted. However, the higher the resolution, the smaller the possible penetration depth. This is due to the scattering and absorption of the waves in the component. The difficulty lies in choosing the resolution so high that z. B. the reinforcement can be seen, but the aggregates in the concrete do not already scatter the waves. All structures that are larger than half the wavelength are resolved:

${\displaystyle \Delta l\approx {\frac {\lambda }{2}}={\frac {v}{2f}}}$

Sender and receiver

Piezoelectric transmitters , mechanical impacts or NDT-specific transmitters, such as the Hsu-Nielson source (broken pencil lead), are used to generate the sound waves . The appropriate source is selected depending on the requirements for pulse strength, pulse duration and frequency content. Accelerometers ( piezoelectric sensors ) are mainly used as receivers . These are divided into resonant, multi-resonant and broadband sensors, whereby the transitions are fluid. They differ with regard to their frequency response function and sensitivity and are selected according to the task area.

Electromagnetic waves

Electromagnetic waves are differentiated according to their wavelength or their frequency (see electromagnetic spectrum ). The relationship between the frequency f and the wavelength λ is described by: where is the phase velocity. ${\displaystyle v=f\cdot \lambda }$
${\displaystyle v}$

reflection

If an electromagnetic wave hits an interface, it is reflected there. The dielectric constant determines the proportion of the reflected wave energy . The higher the dielectric constant (depending on the material), the more energy is reflected. The impedance Z is determined by the magnetic permeability of the material and the dielectric constant : ${\displaystyle \textstyle {\mu }}$ ${\displaystyle \textstyle {\epsilon }}$

${\displaystyle Z={\sqrt {\frac {\mu \cdot \mu _{0}}{\epsilon \cdot \epsilon _{0}}}}}$

Magnetic permeability is the same for everything but ferromagnets . The dielectric constant of water is approximately at room temperature . Since this value is very high, the dielectricity of building materials depends heavily on their moisture. The dielectric constant of concrete is between 4 and 14. ${\displaystyle \mu \approx 1}$ ${\displaystyle \epsilon \approx 81}$

resolution

The penetration of electromagnetic waves increases with increasing wavelength, while the resolution decreases. According to Fresnel's theory, radar waves have a resolution of

${\displaystyle r={\sqrt {{\frac {{\lambda }\cdot d}{2}}+{\frac {\lambda ^{2}}{16}}}}}$ where r is the largest dimension of the object and d is the distance to the object.

According to this, objects with a size of at least 6.4 cm in a depth of 10 cm can be located in dry concrete with a 2.5 GHz radar antenna. With a 300 MHz radar antenna, objects 20 cm in size can be located in dry concrete at a depth of 10 cm.

Test procedure in NDT

Ultrasonic transit time method

Frequency range: 20 - 250 kHz Procedure: active Accessibility: 2 sides

→ Main article : Ultrasonic testing

A congruent measuring grid is measured on both sides of the component to be examined . The transit time of the ultrasonic pulse is then determined for each measuring point. The sound has to run around imperfections, which results in a longer running time. If necessary, the frequency spectrum or the intensity of the received signal can also be measured.

Application area:

• Determination of the homogeneity : a small spread of the running times means a high uniformity of the execution.
• Minimization of destructive component tests: in the area of ​​longer running times, the measuring grid is refined and the weak point is thus limited. A sample can then be specifically taken there.

Remarks:

• The component must be accessible from both sides.
• It takes a lot of time because each transmitter and receiver has to be coupled to the component.
• Statements about the intensity are difficult because the couplings are practically not reproducible.

Ultrasonic echo method

Frequency range: 40–200 kHz Procedure: active Accessibility: 1 page

→ Main article : Ultrasonic testing

Ultrasonic waves are sent into the component on a measuring grid and the reflections are registered. Similar to the ultrasonic transit time method, transit time and, if necessary, intensity and frequency spectrum are measured.

Application area:

• Localization of construction elements and flaws: a short time of flight of the impulse means a small depth up to the impedance jump (reflection point). What it is about cannot be said directly from an individual measurement, but it can result from a comparison with the other measurement points.
• Wall thickness measurement

Remarks:

• Measurement only works up to the first impedance jump, so it is unsuitable for highly reinforced components.

Fresh concrete measurement

Frequency range: 1–250 kHz Procedure: active Accessibility: 1 page

→ Main article : Ultrasonic testing

In a geometrically defined measuring container, sound is transmitted through fresh concrete and the sound propagation time, energy, frequency spectrum and temperature are recorded over time. Conclusions about the progress of solidification can be drawn from this data.

Application area:

• Optimization of the workflow in time-critical construction processes
• quality control
• Development of new building materials

Remarks:

• Much closer to the construction process than conventional methods
• Good reproducibility
• Material properties are recorded as a function of time, not just selectively

Impact echo method

Frequency range: 1–80 kHz Procedure: active Accessibility: 1 page

The component is excited to vibrate by a mechanical shock. Standing waves form between the interfaces (e.g. transmitter on the surface and imperfection) . These natural frequencies can be determined using a Fourier analysis of the received, i.e. the reflected, signal . The lowest natural frequency is , the distance d from the transmitter to the reflection point results from the relationship: ${\displaystyle \textstyle f_{R}}$

${\displaystyle d={\frac {v}{2f_{R}}}~\left[\mathrm {m} \right]}$

Application area:

• Localization of construction elements and imperfections
• Wall thickness measurement

Remarks:

• Due to the relatively low test frequencies, only elements larger than 2.5 cm are resolved. This means that this method is unsuitable for finding slack reinforcement, but all the more for measuring wall thickness.

Acoustic emission analysis

Frequency range: 10-300 kHz Procedure: passive Accessibility: 1 page

→ Main article : Acoustic emission testing

The test object emits sound signals under load. The reason for this is the internal cracking. The transit time and the frequency spectrum of the signal are measured with at least four receivers. Three receivers are required to determine the point in the room at which the signal was sent, the fourth receiver is used to determine the time of transmission.

Operation area:

• Localization of damage
• Component monitoring ( Monitoring )

Remarks:

• Even very small cracks can be detected.
• Evaluation can be very difficult due to the continuous background noise.

radar

Frequency range: 0.3-30 GHz Procedure: active Accessibility: 1 page

→ Main article : Microwave testing

An electromagnetic wavelet is radiated into the component and the reflections are recorded. These occur particularly on conductive materials and dielectric structures.

Operation area:

• Localization of reinforcement and tension channels
• Localization of damp spots

Remarks:

• Very little measurement effort
• No coupling problem
• Only the topmost reinforcement layers can be recorded, i.e. H. If the degree of reinforcement is high, the ultrasonic echo method is more advantageous
• The radar test is a special case of the microwave test .

Infrared thermography

Frequency range: 25–150 THz Procedure: passive Accessibility: 1 page

→ Main article : Thermography

The object to be examined emits thermal radiation, which can be recorded by appropriate cameras and converted into false color images.

Application area:

• Finding thermal bridges
• Finding hidden structures (e.g. trusses or underfloor heating)
• Localization of moisture and defects

Remarks:

• Little measurement effort, as entire structures can be measured at once
• High thermal contrast required
• Metals reflect thermal radiation, which can lead to incorrect results

Further procedures

• Visual inspection , optical control for errors.
• Reinforcement location, determination of the position of individual reinforcement bars.
• Prestressing steel fracture location, location of cracks in prestressing steel.
• Potential field measurement, evaluation of the corrosion status of reinforcing steel.
• Rebound hammer , test of uniformity of compressive strength.

literature

• Michael Sackewitz (Ed.): Guide to image processing in non-destructive testing (Volume 18). Fraunhofer-Verlag Stuttgart, 2018, ISBN 978-3-8396-1380-1

Web links

• German Society for Non-Destructive Testing (DGZfP)
• Brief overview of the NDT ( Memento from September 29, 2007 in the Internet Archive ) (PDF file; 36 kB)
• Wikipedia about Nondestructice Testing (NDT) - English
• European Federation for Non-Destructive Testing - English
• The American Society for Nondestructive Testing - English
• Canadian Institute for NDE - English
• The British Institute of Non-Destructive Testing - English -