DE102021004840A1 - Process and device for monitoring the life cycle of structures made of monolithic concrete or concrete elements, preferably in tunnel construction - Google Patents

Abstract

1. Method and device for monitoring the life cycle of structures made of monolithic concrete or concrete elements, preferably in tunnel construction.2.1. The task is to create a suitable structure monitoring device that, independent of the anisotropies of the structure, over a long period of time, during the entire service life cycle, allows the measurement of the stresses of different directions of action within the structure and structures and by comparing and analyzing the measured values with Older values identify possible damage at an early stage. The monitoring consists of monitoring the stress states, the temperature and the humidity in the structure. The device according to the invention can already wirelessly record measured values online and in situ during the production of the structures or concrete elements. In tunnel construction, the complete life cycle can be recorded seamlessly, from the manufacture of the segments, to installation behind the TBM and throughout the entire service life of the structure. In addition, additional important influencing variables such as temperature and humidity should be recorded to monitor the life cycle of the structure. The power supply and the data transmission should be wireless2.2. The task is solved in that one or more ultrasonic sensors made of a PVDF film are attached to a measuring body and determine the radial, tangential and axial stress in different directions over the transit time of the ultrasonic signals within the measuring body with high resolution.

Description

The invention relates to a method and a device for monitoring the life cycle of structures made of monolithic concrete or concrete elements, preferably in tunnel construction.

The monitoring consists of monitoring the stress states, the temperature and the humidity in the structure. The device according to the invention can already wirelessly record measured values online and in situ during the production of the structures or concrete elements. In tunnel construction, the entire life cycle can be seamlessly recorded, from the production of the segments to installation behind the TBM and throughout the entire service life of the structure.

State of the art

In addition to the detour of determining geometric quantities with optical devices (strain measurement with fiber optic sensors or strain gauges) for detecting the movement of components, there are also length measurement methods with vibrating sides or fiber optic sensors for determining stress on buildings.

Strain gauge methods only measure geometric changes on the surface of structures.

If mechanical stresses are to be measured within concrete structures, these are unsuitable.

Methods with fiber optic sensors are complex and mechanically susceptible to damage to the fiber optic cable.

The state of the art also includes the general recommendation to carry out stress measurements using the slot unloading and compensation method.

Objects predestined for the process are unweathered rock surfaces, freshly excavated surfaces above or below ground, lining shells of tunnels and, in general, all types of structural parts. From the experimentally determined edge stresses, conclusions can be drawn about the degree of utilization of the building parts with regard to their load-bearing capacity and thus the safety of the geotechnical object.

mit:

• σn = Normalspannung am Ausbruchsrand des Gebirges oder Bauteils
• p = Öldruck im Kissen bei vollkommener Kompensation
• Km = Formkonstante des verwendeten Druckkissens
• Ka = AJ /AC (Verhältnis zwischen Kissenfläche AJ und Schlitzfläche AC)

The relief shifts due to the saw slot are compensated by the cushion pressure. When the original condition is reached again, the internal pressure of the pressure pad p is a measure of the local normal stress an perpendicular to the saw slot at the edge of the structural part. The test results are evaluated according to the equation: $$\mathrm{\sigma }\text{n}=\text{p}*\text{km}*\text{ca}$$

with:

• σn = normal stress at the fracture edge of the rock or component
• p = oil pressure in the cushion at full compensation
• Km = shape constant of the pressure pad used
• Ka = AJ /AC (ratio between pad area AJ and slot area AC)

• - Es ist ungeeignet zur Messung primärer (absoluter) Spannungszustände.
• - Es ist beschränkt auf direkt zugängliche Oberflächenmesspunkte.
• - Es ist ungeeignet für gestörtes, zerklüftetes, zerbrochenes, weiches und zur Verwitterung neigendes Gebirge.
• - Kein online Monitoring.
• - Es beschädigt das Bauwerk hinsichtlich der Dichtheit gegenüber Wasserzuflüssen.

This method has significant disadvantages:

• - It is unsuitable for measuring primary (absolute) stress states.
• - It is limited to directly accessible surface measurement points.
• - It is unsuitable for disturbed, jagged, broken, soft and weather-prone mountains.
• - No online monitoring.
• - It damages the structure in terms of tightness against water inflows.

Other modern electronic methods measure with ultrasound, for example.

The WO 2010/015248A2 describes a method and a device for determining rock stresses with which stress redistributions can be continuously recorded in-situ. In the patent, the object is achieved in that one or more ultrasonic sensors made of a PVDF film are attached to a measuring body and the propagation time of the ultrasonic signals within the measuring body is determined with high resolution in a borehole probe using a TDC circuit. According to this method, the running time and the temperature in the measuring body are measured in-situ and compared with initial values. The rock stresses and their changes are determined online from the change in the measured transit times, which is proportional to the mechanical stresses acting on them.

Methods that directly measure the ultrasonic velocity in structures (mostly concrete bodies) are very dependent on the properties of the inhomogeneous building material. This is how measurements are made of the strength and setting behavior of concrete. There are practically no applications for stress measurement in concrete in buildings. Mobile measurements on pillars or foundations fail because inhomogeneities in the structure make it impossible to compare the measurements.

In the patent DE 198 30 196 a method for determining the spatial rock and rock anisotropy and the rock stress state on test specimens is described. The method described therein for determining the rock stress state requires rock specimens with opposite end surfaces for attaching the ultrasonic transducers.

The above method is not suitable for the in-situ measurement of stress states and stress redistribution in or on structures.

Furthermore, the pressure dependence of the properties of the wave propagation in different media are known in the prior art. Direction-dependent pressures and stresses can be measured by measuring the propagation speed of compression and/or shear waves. Anisotropies, cracks, pore water, etc. significantly influence these measurements.

The metrological influence of changing porosity or moisture content can be far greater than the voltage-dependent portion of the measurement effect.

For the broad application of the measurement of propagation speeds of ultrasonic waves, the influence of changing concrete qualities and compositions must therefore be ruled out as far as possible.

task

The object of the invention is to create a suitable structure monitoring device which, regardless of the anisotropies of the structures, over a long period of time, throughout the entire service life cycle, allows the measurement of the stresses of different directions of action within the structures and structures and by comparing and analyzing the measured Values with older values detect possible damage early.

Flat-lying tunnels in soft soil, such as immersed tunnels through straits, require special attention when planning the monitoring.

The effect of ground movements on the tunnel structures has a major impact on the resulting forces and mechanical stresses within the tunnel segments and at the joints.

Furthermore, additional important influencing factors such as temperature and humidity are to be recorded in order to monitor the life cycle of the building.

The power supply and data transmission should be wireless

The solution to the problem is described in the characterizing features of claims 1 and 2.

The further claims reflect advantageous developments of the device for carrying out the method according to the invention.

A feature of the method according to the invention for monitoring the life cycle of concrete elements, preferably in tunnel construction, is the measurement of the mechanical stress through long-term observation and comparison of stress measurements within the structures using ultrasound, based on the acoustoelastic effect. The transit time of an ultrasonic impulse within inhomogeneous structures is measured in homogeneous and isotropic measuring bodies. The speed of sound of the ultrasonic waves depends on the elastic stresses within the measuring body.

If an elastic, non-compressible medium is used as the measuring medium, for example a solid of known composition and without anisotropies in the sound path, the mechanical stresses in the surrounding structure can be determined by determining the change in the speed of sound. The prerequisite for this measurement is the positive and non-positive connection of the measuring body with the building.

The measuring bodies can be connected using special cement, for example by embedding them in components, using force-locking and form-fitting connections, such as when components are supported or bridges are supported.

The medium for receiving the measuring body should be incompressible and homogeneous. This can be, for example, a plastic with a suitable strength, a metal or an artificial stone (concrete).

The problem with organic plastics is that many high polymers also show plastic behavior in addition to elastic behavior. A condition for the use of plastics is a sufficiently large modulus of elasticity and elastic behavior. Plastics capable of creep are unsuitable for measuring the speed of sound.

Furthermore, a low ultrasonic damping is required.

In order to achieve a high resolution, transmission pulses that are as short as possible and steep flanks of the reception pulse are required. This requires good ultrasonic conductivity of the measuring sections or measuring bodies.

When metals are used, these requirements are largely met.

The introduction of force via the outer surfaces into the measuring body and the associated compression is measured as negative strain.

The measurement of the transit time of the ultrasonic pulses, i. H. the determination of the speed of sound is possible with one or more ultrasonic sensors. A sensor can work as a transmitter and receiver and evaluate one or more reflections of the ultrasound on the wall of the measuring section. Two or more ultrasonic sensors, i. H. separate transmitters and receivers can be used to advantage.

The acoustoelastic effect can be measured both by measuring the longitudinal (shear) wave and by measuring the transversal (shear) wave, or by evaluating the changes in both waves.

The reversibility between expansion and compression applies.

Hooke's law only applies to the elastic domain. $$\mathrm{\sigma }\left(\text{Tension}\right)=\text{E}\left(\text{E}-\text{module}\right)*\mathrm{e}\left(\text{strain}\right)$$

The metal ultrasonic conductors comply with Hooke's law.

The stress σ results from the temperature-compensated transit time L _T1 , the reference transit time L _T0 and the acoustoelastic factor of the measuring body material K _σ $$\mathrm{\sigma }=\left({\text{L}}_{\text{T1}}-{\text{L}}_{\text{T0}}\right)/{\text{K}}_{\mathrm{\sigma }}$$

The measuring temperature _T1 of the measuring body applies to L _T1 and the reference temperature to L _T0 .
T ₀ = 0 °C and the reference voltage σ = 0.

The temperature-compensated transit time L _T0 is determined from the measured transit time L _T and the correction factor K _T according to $${\text{L}}_{\text{T0}}={\text{L}}_{\text{T}}*{\text{K}}_{\text{T}}$$

The thermal factor K _T is a non-linear function over a wide temperature range $${\text{K}}_{\text{T}}=\text{f}\left(\text{T}\right)$$

The thermal factor K _T of the transit time is determined using linear regression for the selected sensors $${\text{K}}_{\text{T}}=0.94684{\text{ns}}^{\circ }{\text{C}}^{-1}$$

bzw. $${\text{K}}_{\mathrm{\sigma }}=\mathrm{4,4585}{\text{ Nmm}}^{-2}{\text{ ns}}^{-1}$$

The acoustoelastic factor K _σ for the selected metal alloy and sensor thickness was determined on the sensor test bench $${\text{K}}_{\mathrm{\sigma }}=4.4585{\text{Mpa ns}}^{-1}$$

or. $${\text{K}}_{\mathrm{\sigma }}=4.4585{\text{hmmm}}^{-2}{\text{ns}}^{-1}$$

The relative change in the wave speed due to the mechanical tension or stress is very small. The change in velocity of the longitudinal wave is a linear function.

There is a linear dependency in the examined range up to 350 MPa.

The change in the speed of sound depends not only on the mechanical stress that is applied, but also on the temperature.

In practice, the temperature equalization between the probe and the surrounding structure occurs sufficiently quickly. Larger temperature fluctuations are not to be expected in stationary installation in tunnels or foundations. For applications on bridge pillars or dams, where a changing ambient temperature is to be expected, temperature measurements for compensation are conceivable and can be easily implemented in the measuring body.

Due to the elastic behavior of the measuring section between the ultrasonic sensors, the length of the measuring section is also changed.

Since it is known that, for example, the change in the speed of sound due to the effect of mechanical stress (compression of the measuring section) is three times as great as the influence of the pure change in length (which occurs as a result of this stress or the effect of force on the measuring section) on the speed of sound by measuring the speed of sound, a sufficiently accurate determination of the stress in the surrounding structural parts can be made.

The acoustoelastic coefficients presented above are very small in relation to the absolute speed of sound.

The direct metrological evaluation by means of a conventional runtime measurement (time of flight) using only microprocessors is too imprecise, since the resolution here is not sufficient.

A direct pulse duration measurement via microprocessors is ruled out since the cycle time (computing cycle) is 1000 to 10000 times greater than the usable resolution required. Measuring bodies a few centimeters thick result in propagation times of the ultrasonic impulse of less than 10 µs with only one reflection.

If loads of only a few MPa are to be measured, the resolution must be below 0.1 ns.

Modern measuring arrangements such as the TDC (Time-to-Digital-Converter) with resolutions of less than 50 ps can be used to detect load shifts (or voltage shifts) in the range of 100 kPa and smaller. A resolution in picoseconds is possible if measurements are taken at a sufficiently high measuring rate, which is more than 1000 or even 10000 measurements per second.

Another feature of the method according to the invention for monitoring the life cycle of concrete elements, preferably in tunnel construction, is the measurement of the moisture penetration of the concrete components.

One way of determining the degree of water saturation in concrete is to measure the relative humidity of a cavity in equilibrium with the concrete. Converting the relative humidity readings to standardized relative humidity values at one temperature can be obtained using the specific adsorption isotherm calculate the degree of water saturation of the concrete elements to be monitored for the concrete used.

The relative humidity is measured in a measuring chamber that is cast into the concrete element. A sealing cover that seals against the surrounding air prevents the measured value being falsified by the humidity in the surrounding air or by the inflow of water from outside.

• 1 schematische Darstellung einer Ausführungsvariante zur Überwachung des Lebenszyklus von Betonelementen vorzugsweise im Tunnelbau
• 2 schematische Darstellung einer Ausführungsvariante zur Überwachung der Radial- und Tangentialspannung im Tunnelbau

Further advantageous configurations of the invention emerge from the features of the dependent claims. The invention is to be described below using an exemplary embodiment. Show it:

• 1 schematic representation of an embodiment variant for monitoring the life cycle of concrete elements, preferably in tunnel construction
• 2 Schematic representation of a design variant for monitoring the radial and tangential stress in tunnel construction

In 1 is a schematic representation of an embodiment variant for monitoring the life cycle of concrete elements, preferably in tunnel construction. The measuring bodies (10) are in the concrete, each with the piezoelectric component (11), the temperature measuring device (12) and the component (19), preferably a digital 1-wire sensor with a 64-bit ROM ID. It is advantageous to use a 1-wire temperature sensor, which records the temperature with a resolution of 12 bits and provides a unique ID number to identify the device.

• - mechanischer und chemischer Schutz des Messkörper (10) mit dem piezoelektrischen Bauteil (11),
• - Übertragung der mechanischen Spannung vom Beton in den Messköper (10),
• - form- und kraftschlüssiger Verbund mit dem Beton.

The casing (13) has to fulfill several tasks:

• - mechanical and chemical protection of the measuring body (10) with the piezoelectric component (11),
• - Transmission of the mechanical stress from the concrete to the measuring body (10),
• - Positive and non-positive bond with the concrete.

The casing (13) is advantageously made from a mixture of mineral fillers and polyester or epoxy resins.

The mechanical properties of the polyester and epoxy resins are significantly improved. With this, increases in strength of 50% to 75% are achieved. The thermally induced shrinkage behavior is also reduced. A significantly better interlocking of the encased measuring body (10) with the concrete is also achieved.

Quartz sand is particularly suitable as a filling material. The encapsulation can also take place with several layers. For example: 1st layer of 1mm to 2mm grit 0-0.3mm; 2nd layer: grain size up to 2 mm.

The cover (13) should be chemically neutral and incompressible.

A cable connection (14) connects the piezoelectric component (11) to an ultrasonic unit (15) which consists of a component for generating a voltage pulse (16) and an amplifier (17), and
determines the running time in a component (20).

In the embodiment shown with two measuring bodies (10), the amplifier (17) is preceded by an analog switch (25). In an embodiment variant not shown here, a multi-channel analog switch (3 or more channels) can also be used. There are components for determining the transit time (20) that have two independent measurement channels. When using these components, the analog switch (25) can be omitted if only two measuring heads (10) are used. The TDC (time-to-digital converter) with resolutions in the picosecond range are particularly suitable. If measured with a sufficiently high measurement rate, which is over 1000 or even 10000 measurements per second, a stable resolution in picoseconds is possible. The fluctuations in the measured value are then essentially determined by the temperature measurement (12). The transit time is corrected via a linear dependency of the speed of sound on the temperature.

The temperature measuring device (12) is connected to a computing unit (18), with the previously determined transit time of the ultrasonic wave and the timely determined temperature converting this transit time into a later comparison of the transit times of the ultrasonic waves determined at different times by one or more measuring devices , into suitable temperature-normalized running times and these in turn are converted into mechanical stress.

The cable connection (14) is laid along the reinforcement in the concrete element (tubbing) using cable ties.

A component (19) is part of a digital 1-wire sensor with a 64-bit ROM ID, which uniquely identifies the measuring device and thus permits unmistakable identification. The measurement of the mechanical stress (26) is in the direction of the ultrasonic wave (21).

A component for the wireless power supply (24) includes a receiving coil for the resonant induction, not shown here. When inspecting the tubbings, energy can be transmitted to the device according to the invention with a transmission coil attached to a boom of the inspection vehicle. The measurement results are made available via the data transmission device (22). The measurement results can be evaluated in different ways.

In the simplest case, the data stream can be automatically evaluated immediately on site. Since each measuring point is assigned a unique ID, changes are recognized immediately and the component can be inspected and marked immediately if necessary.

The components (15) to (18), (20) and (22) to (25) are advantageously combined in a watertight cast electronic unit (27). This watertight electronics unit (27) partially fills out the box for receiving the electronics unit (29), which is firmly embedded in concrete in the tubbing. The component for relative humidity measurement is arranged on the outside of the watertight electronic unit (27) in the remaining cavity for humidity measurement (28).

A component for water vapor diffusion (30) is located in the wall to the concrete of the tubbing. This component can be designed as a semipermeable membrane. This prevents the cavity for moisture measurement (28) from filling up when the tubbing is cast in concrete. In the simplest case, the penetration of liquid during the production of the tubbing is prevented by a cover. This cover must be removed after the concrete has hardened.

In 2 is the schematic representation of an embodiment variant for monitoring the radial and tangential stress in tunneling. The box (29) for receiving the watertight electronics unit (27) is located in the tunnel inside of the tubbing (36).

A cover (38) closes the box (29) so tightly that the saturation equilibrium between the relative humidity and material moisture of the concrete can be established in the cavity for measuring moisture (28) without the relative humidity of the tunnel interior (37) falsifying the measurement. The box (29) is advantageously installed in the tubbing (36) in such a way that the component for the diffusion of water vapor (30) points downwards. The sensor (32) for the radial stress placed close to the outer wall of the tubbing (36) to the rock (38). The sensors (34) for the tangential stress (33) can be arranged as required.

The arrangement of measuring sensors for the axial stress is not shown. Measuring axial stress can be important for immersed tunnels where ground movement is expected.

The monitoring of the life cycle of concrete elements can also be carried out with a stationary power supply and data transmission with cables.

The sensors and the box (29) can also be used in buildings and tunnels with a monolithic construction.

Reference List

10

measuring body

11

Piezoelectric component

12

temperature measuring device

13

wrapping

14

cable connection

15

ultrasonic unit

16

Component for generating a voltage pulse

17

amplifier

18

unit of account

19

component for identification

20

Component for determining the runtime

21

ultrasonic wave

22

Device for data transmission

23

Component for relative humidity measurement

24

Wireless power supply component

25

analog switch

26

mechanical tension

27

Waterproof electronic unit

28

Cavity for moisture measurement

29

Box for accommodating the electronics unit

30

Component for water vapor diffusion

31

radial tension

32

Radial tension transducer

33

tangential stress

34

Sensor for tangential stress

35

mountains

36

Tunnel wall or tubbing

37

tunnel interior

38

Lid

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2010015248 A2 [0012]
• DE 19830196 [0014]

Claims (5)

Method for monitoring the life cycle of structures made of monolithic concrete or concrete elements, preferably in tunnel construction, which includes: a) at least one component consisting of a homogeneous, isotropic and elastic measuring body (10) which is embedded in the concrete in a non-positive and positive manner in all directions, and b) at least one piezoelectric component (11) emits and receives an ultrasonic wave (21) in a homogeneous, isotropic and elastic measuring body in the direction of the mechanical stress to be measured, and c) a temperature measuring device (12) which determines the temperature of the homogeneous, isotropic and elastic measuring body in real time for the transmission and reception of the ultrasonic wave, and d) at least one component (13) encapsulates the measuring body with the piezoelectric component and the temperature measuring device for mechanical and chemical protection, this enclosing component (13) being chemically neutral with respect to the components (10), (11) and (12) and the concrete and is incompressible, and e) via a cable connection (14), the piezoelectric component (11) with an ultrasonic unit (15), consisting of a component for generating a voltage pulse (16) and an amplifier (17) is connected, and in a component (20) the Running time determined, if necessary an analog switch (25) in front of the amplifier (17) switches on further piezoelectric components (11) individually, f) the temperature measuring device (12) is connected to a computing unit (18), the computing unit (18) promptly calculates the prevailing temperature from the previously determined transit time of the ultrasonic wave, this transit time into a for later comparison of the transit times of the ultrasonic waves determined at different times, one or several measuring bodies, into suitable temperature-normalized running times and these in turn converted into mechanical stress, and g) a component (19) which uniquely identifies the measuring device and thereby permits unmistakable identification, and h) the transmission of the data with a device (22) at a location suitable for evaluation, with the data being transmitted wirelessly or by wire, i) a component for relative humidity measurement (23), which is located outside the watertight electronic unit (27) in a cavity for humidity measurement (28) which is sealed off from the tunnel interior (37) by a cover (38) and which has a component for water vapor diffusion ( 30) is connected to the concrete of the tunnel wall (36), determines the equilibrium moisture content, the computing unit (19) forms a relative humidity from the equilibrium humidity and the temperature, j) the computing unit (19) uses the specific adsorption isotherm for the concrete used to calculate the degree of water saturation of the concrete elements to be monitored, k) forms at least one of the stresses, such as tangential stress, radial stress, axial stress or principal stress, from measurements with differently oriented sensors, and I) evaluates the course of the life cycle in a time series. Device for monitoring the life cycle of structures made of monolithic concrete or concrete elements, preferably in tunnel construction, which comprises: - at least one component, consisting of a homogeneous, isotropic and elastic measuring body (10), preferably made of metal, which is positively and non-positively connected in all directions Concrete is embedded, and - at least one piezoelectric component (11), preferably a PVDF foil, which emits and receives a longitudinal ultrasonic wave (21) in a homogeneous, isotropic and elastic measuring body (10) in the direction of the mechanical stress to be measured, and - a Temperature measuring device (12) with at least 12 bit resolution, and at least one component (13) encasing the measuring body with the piezoelectric component and the temperature measuring device, preferably made of a mixture of epoxy resin with or without mineral filling material or of a mixture of polyester resin with or without a mineral filling material, we Is used for mechanical and chemical protection, being chemically neutral and incompressible with respect to the encased components (10), (11) and (12) and the concrete, and - a cable connection (14) which connects the piezoelectric component (11). an ultrasonic unit (15), consisting of a component for generating a voltage pulse (16) and an amplifier (17), or an analog switch (25) additionally connected upstream of the amplifier (17), each with a component for determining the transit time (20), preferably a TDC circuit, and connects the temperature measuring device (12) to a computing unit (18), and - a component for measuring moisture (23), and - a closed cavity (28) with a component for water vapor diffusion (30) to form the equilibrium moisture , and - a component (19), preferably a digital 1-wire sensor with a 64-bit ROM ID, which uniquely identifies the measuring device and thus an unmistakable identification selbar permitted, and -a component for wireless power supply and, - a device (22) for transmitting the data to a location suitable for evaluation, the data transmission being able to take place wirelessly or by wire. Device for monitoring the life cycle of concrete elements, preferably in tunnel construction according to one of Claims 1 and 2 characterized in that the device (22) for data transmission is part of an RFID system. Device for monitoring the life cycle of concrete elements, preferably in tunnel construction according to one of Claims 1 until 3 characterized in that the device is supplied with energy wirelessly, for example via electromagnetic waves. Method for monitoring the life cycle of concrete elements, preferably in tunnel construction for immersed tunnels in areas endangered by ground movement according to one of Claims 1 until 4 characterized in that at least one of the stresses, such as axial stress, radial stress or tangential stress, is measured and its progression over time is evaluated.

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