DE102021004852A1 - Process and device for monitoring and determining the mechanical stress in concrete structures and structural parts - Google Patents

Abstract

1. Device for monitoring and determining the mechanical stress in structures and structural components made of concrete.2.1 The measuring method has the task of monitoring and determining mechanical stresses and stress redistribution in structures and structural components made of concrete in-situ and online.2.2. The object is achieved in that one or more ultrasonic sensors made of a PVDF film are attached to a measuring body and determine the transit time of the ultrasonic signals within the measuring body with a TDC circuit with high resolution in the concrete. According to the invention, the transit time and the temperature in the measuring body are measured in situ and compared with initial values. The change in the measured running times are proportional to the mechanical stresses that act.

Description

The invention relates to a method and a device for monitoring and determining the mechanical stress in buildings and parts of buildings, such as foundations, pillars, tunnels, avalanche galleries, halls, dams or bridge supports.

The monitoring of stress changes in existing structures can record the influences of aging, temperature, changes in load on the supporting structures or changes in the subsoil.

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.

Other modern electronic methods measure with ultrasound, for example.

Every solid has a specific speed of sound that depends on the density and elasticity of the solid. With the well-known sonic log, the propagation time of the sound pulse generated by a transmitter at the lower end of a probe through the rock surrounding the vertical borehole to one or more receivers at the upper end of the probe is measured. Solid bodies made of concrete can also be scanned with ultrasound.

A coupling medium is used to couple the ultrasonic transmitter and ultrasonic receiver.

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 the invention, the transit 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 provide a suitable structure monitoring device that independent of the anisotropies of the structure, enables the measurement of the stress within the structure and structures over a long period of time and, by comparing and analyzing the measured values with older values, detects possible damage at an early stage.

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.

The method according to the invention for measuring the mechanical stress for the purpose of early detection of structural damage through long-term observation and comparison of stress measurements within the structures using ultrasound is 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 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 metrologically recorded 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. $$\mathrm{\sigma }\left(\text{Tension}\right)=\text{E}\left(\text{E}-\text{module}\right)\times \mathrm{e}\left(\text{strain}\right)$$

Dabei gilt für L_T1 die Messtemperatur T₁ des Messkörpers und für L_T0 die Bezugstemperatur.
T₀ = 0 °C und die Bezugsspannung σ = 0.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.

Der thermische Faktor K_T ist für einen großen Temperaturbereich eine nichtlineare Funktion $${\text{K}}_{\text{T}}=\text{f}\left(\text{T}\right)$$

The temperature-compensated transit time L _T0 is determined from the measured transit time L _T and the correction factor K _T according to $${\text{L}}_{\text{T}0}={\text{L}}_{\text{T}}*{\text{K}}_{\text{T}}$$

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

Auf dem Sensorprüfstand wurde der akustoelastische Faktor K_σ, für die ausgewählte Metall-Legierung und Sensordicke, bestimmt zu $${\text{K}}_{\mathrm{\sigma }}=\mathrm{4,4585}{\text{ Mpa ns}}^{-1}$$

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

The thermal factor K _T of the transit time is determined according to (5) with the linear regression for the selected sensors $${\text{K}}_{\text{T}}=0.94684\text{ns}°{\text{C}}^{-1}$$

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 systems such as the TDC (time-to-digital converter) with resolutions of less than 50 ps can be used to record load shifts (or voltage shifts) of the order 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.

exemplary embodiments

• 1 eine schematische Darstellung einer Ausführungsvariante zur permanenten Messung der mechanischen Spannung in Beton

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 a schematic representation of a variant for the permanent measurement of the mechanical stress in concrete

In 1 is a schematic representation of an embodiment variant for measuring the mechanical stress in structures made of concrete. The measuring body (10) 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 in a component (20) determines the transit time.

The TDC (time-to-digital converter) with resolutions in the picosecond range are particularly suitable. If measured with a sufficiently high measuring 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.

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.

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

mechanical tension

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 [0010]
• DE 19830196 [0012]

Claims (4)

Method for monitoring and determining the mechanical stress in structures and parts of structures made of concrete, 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) the 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) the 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 resistant to the components (10), (11) and (12) and the concrete is neutral and incompressible, and e) which is connected via a cable connection (14), the piezoelectric component (11) to an ultrasonic unit (15), consisting of a component for generating a voltage pulse (16) and an amplifier (17), and in a component (20) determines the runtime f) the temperature measuring device (12) is connected to a computing unit (18), the computing unit (18) realtime 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 more 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. Device for monitoring and determining the mechanical stress in structures and parts of structures made of concrete, which includes: - at least one component consisting of a homogeneous, isotropic and elastic measuring body (10), preferably made of metal, which is embedded in the concrete in a non-positive and positive manner in all directions, and - at least one piezoelectric component (11), preferably a PVDF film, 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 mineral filling material, which serves for mechanical and chemical protection , whereby this is chemically neutral and incompressible in relation to the encased components (10), (11) and (12) and the concrete, and - a cable connection (14) which connects the piezoelectric component (11) to an ultrasonic unit (15), consisting of a component for generating a voltage pulse (16) and an 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 (19), preferably a digital 1-wire sensor with a 64-bit ROM ID, which uniquely identifies the measuring device and thereby permits unmistakable identification, 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 according to one of Claims 1 and 2 characterized in that the device (22) for data transmission is part of an RFID system. Device according to one of Claims 1 until 3 characterized in that the device is supplied with energy wirelessly, for example via electromagnetic waves.

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