MEDIOS Y TÉCNICAS DE COMUNICACIÓN

Defects and voids within a concrete structure should always be recorded while investigat-ing the entire structure by visual inspection. Mainly old buildinvestigat-ings show distinct areas in which the concrete is not compacted properly, as depicted in Figure 3.3. Such areas may be visible from the outside. But if these areas are hidden and it is obvious or expected that a Figure 3.1 Assessment of a concrete structure under complicated conditions.

How to assess the status of a structure 39

structure contains such areas despite using boreholes, other nondestructive methods can be used to detect those areas. Most nondestructive methods are based on the measurement of acoustic or electromagnetic (radar) waves and are described in the following sections.

If both sides of the structure are accessible, the ultrasonic sound speed of the construc-tion material can be measured as depicted in Figure 3.4. The test method is standardised in (DIN) EN 12504-4:2004, which not only describes different test setup possibilities, but also provides information on the influences of the environment, e.g., temperature, relative humidity (rh), and cracks within the specimen, as well as the dimensions of the samples of this specific test method.

Through the cross section of the structure pulsed ultrasonic waves are sent by the trans-mitter and recorded by the receiver unit placed on the opposite side of the wall. The wall thickness, which has to be known, as well as the time difference between the sending and receiving of the signal, can be used to calculate the ultrasonic speed, which is a material con-stant. This material constant varies between the typical construction materials and between different types of concrete, so that this value always has to be determined and cannot be given exactly in advance.

Figure 3.2 Procedure for the assessment of the condition of concrete structures regarding reinforcement corrosion according to Raupach et al. 2013.

The ultrasonic speed can be correlated to a certain extent with the compression strength of the concrete, while changes of the ultrasonic speed can indicate areas with hidden defects and voids. The results of the measurements should always be correlated with destructive test methods—regarding the compressive strength using additional compression tests on cores taken out of the structure, regarding voids and defects, e.g., with endoscopic investigations.

If the thickness of the concrete element and the time shift between sending and receiving the signal is known, the following formula can be used to calculate the ultrasonic speed of the concrete:

v d

sound= t where

v_sound = Ultrasonic speed in m/ s

d = Thickness of the concrete element in m

t = Time shift between sending and receiving the signal in s Figure 3.3 Example of very distinct gravel pockets at the surface of a concrete wall.

d

Receiver Sending unit

Figure 3.4 Schematic drawing of how to measure the ultrasonic speed of a building material.

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As long as both sides of the structure are accessible, the previously shown method is suit-able for on- site investigations. If both sides of the construction element are not accessible, but still voids or defects within the cross section have to be detected, the impact- echo method can be used as depicted in Figure 3.5. The impact- echo method was developed for measuring the thickness of an element, but can also be used to detect irregularities in an element.

The basic idea is nearly the same as mentioned before, but it is not an ultrasonic wave that is induced into the structure but a wave introduced mechanically, e.g., by a hammer.

The induced sound waves are recorded by a receiver, which is usually mounted closely to the hammer. Because the sound wave is not reflected at a specific point, but the entire cross section around the measuring point, the receiver records a sound pattern. This sound pat-tern cannot be used for further analysis and has to be postprocessed. This is done by a fast Fourier transformation (FFT), which calculates a frequency spectrum (intensity over fre-quency) from the recorded sound waves. This allows the determination of the predominant frequency. This predominant frequency can then be used to calculate the thickness of the structural element according to the following formula:

v = 2·d·f where

v = Speed in m/ s

d = Thickness of the concrete element in m

f = Characteristic frequency of the multireflection in 1/s

Figure 3.6 shows the detection of voids by the impact- echo method on two different walls.

The first example (a) shows a homogeneous wall section. With a known sound velocity of 3.42 km/ s, the characteristic frequency of the multireflection is determined to be 3.42 kHz, and by using the given formula, the wall thickness can be calculated to 50 cm. The second example (b) shows the same wall with a void within the cross section. If the material is iden-tical to the first wall, the sound velocity does not change, but the fast Fourier transformation calculates a characteristic frequency of 7.32 kHz, which leads to a thickness of 23 cm. This is equivalent to the depth of the void within the cross section.

Figure 3.7 shows an impact- echo device during operation on site. The different hammers are selected in dependency of the desired impact. The hammer itself is trigged manually, and the sound is recorded by the microphones in the silver cylinder. Not shown is the recording and processing unit.

Voids or irregularities resolve in a frequency shift in the resulting spectrum, as well as the predominant frequency, and thus indicate lower or greater thicknesses of the

Impact Receiver

d

Figure 3.5 Schematic drawing of the principle of the impact- echo method.

element, which then means that voids or defects are hidden in the structure. As mentioned before, a destructive verification of the test results is also recommended, e.g., by endo-scopic investigations. (See Figure 3.8.)

Figures  3.9 and 3.10 were taken during the inspection of the mounting construction of concrete façade elements. The mounting elements are able to adjust the position of the façade element during mounting, as well as bear compressive and tension forces between the façade elements and the substructure.

Figure 3.7 Left: Impact- echo device placed on the concrete surface. Right: Impact- echo device during operation.

Figure 3.6 Impact- echo frequency analysis—example of two different measurement tasks.

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For large areas the previously mentioned methods of detecting hidden voids and defects will lead to an enormous amount of work, which can be very tedious, especially if ceilings have to be investigated. In this case as well as for large areas, radar, e.g., ground- penetrating radar (GPR), can be used.

Figure 3.11 shows a steel- reinforced arch below railway tracks. The shown areas without any concrete within the structure were detected accidentally. Due to the railway traffic there were no possibilities to perform time- consuming investigations of the arch from the upper side. So in addition to boreholes and endoscopic investigations from the lower side, GPR measurements were carried out from the upper side of the construction.

GPR was originally designed for geophysical measurements in order to detect buried objects or soil boundaries. It is also based on sending and receiving electromagnetic radia-tion, which are in the microwave band (UHF and VHF frequencies) and not introduced into the structure by a mechanical device but a complex sending unit.

Figures 3.12 to 3.16 show the basic principle, a handheld GPR device, as well as various results of different ground- penetrating radar measurements. The pictures clearly show that the thickness as well as the position of the reinforcement can be measured at once. Additionally, voids or changes in the concrete cover are also detected. Generally, GPR measurements pro-duce results that have to be investigated carefully and require some experience.

Figure 3.8 Inspections through a borehole with a rigid endoscope with fluorescent light source.

Figure 3.14 shows a GPR recording of a defect- free concrete slab with a regular concrete cover as well as a constant thickness. It can be seen that the interpretation of this com-paratively easy measurement task still requires some experience with GPR measurements because the readings themselves have to be interpreted by the operator.

Figures 3.15 and 3.16 show GPR measurements with a void as well as varying concrete cover. Like always, the measurements should be confirmed on a random basis with destruc-tive tests.

Depending on the frequency used for GPR measurements, the maximum measurement depth as well as the resolution varies; see Table 3.1.

Numerous investigations regarding the advantages and disadvantages of the mentioned measuring techniques reveal that none of the different methods are capable of detecting all different types of possible defects or voids. Generally these investigations lead to the conclu-sion that the water content of the construction material mainly affects the reliability of the different measuring techniques. Comparatively, young and wet concrete can be best inves-tigated with impact- echo or ultrasonic measurements. GPR measurements lead to reliable results, as long as the concrete is sufficiently dried out (Müller and Fenchel 2006).

As a conclusion for on- site investigations, it can comprehended that the various non-destructive methods can be combined in order to be able to detect voids or defects. Also, the nondestructive methods should always be calibrated and verified with destructive investiga-tional methods, such as endoscopic investigations.