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    New technology for non-destructive testing of concrete strength in railway tunnels

    New technology for non-destructive testing of concrete strength in railway tunnels

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    Mainlyforrailwaytunnelliningandlarge-scaleconcretestructures,fastandaccuratestrengthnon-destructivetestingisperformed.Theproductuseselasticwavesasamediumtodetecttheinternalstrengthofthestructure,andhasunparalleledadvantagesofultrasonicdetectorsandreboundmeters. First,thebasicprinciplesofelasticmodulusandstrength Theimpactelasticwaveisusedasthedetectionmedium.Thedynamicelasticmodulusofthematerialiscalculatedbydetectingthewavevelocityoftheelasticwave.Thecompressivestrengthoftheconcreteiscalculatedbasedonthecorrelationbetweenthedynamicelasticmodulusandthecompressivestrength.Thedynamicelasticmodulus. Thedynamicelasticmodulusofconcretenotonlyaffectsthedeformationofthestructure,butalsoanimportantindicatorofthequalityanddurabilityofconcrete: 1.Itcanreflecttherigidityofthematerialandisanimportantparameterinthecalculationofthedeformationofthestructure.Especiallyforhigh-strengthconcrete,themethodofsimplyusingcompressivestrengthtopushbacktheelasticmoldoftenhasalargeerror; 2.Theagingofconcretematerialsusuallystartswiththereductionofthespringmold,andthepoorconstructionofnewstructureswillalsoappearinthespringmold. Thedetectionofshotgunmodeismainlythroughthedetectionofwavespeed. Fora1-dimensionalhomogeneouselastomer,therelationshipbetweenthedynamicelasticmodulusEdandtheelasticwavePwavevelocityVp1canbeexpressedas:  Amongthem,ρisthedensityofthematerial.Forconcrete,ρisgenerallyabout2400kg/m3.Whenthedetectionobjectistwo-dimensionalorthree-dimensional,thePwavevelocitychangestosomeextent.  2D:  3D:  Forsurfacewaves(Rayleighwaves),therelationshipcanbeexpressedas: Rayleighwave:  Ingeneral,thedynamicPoisson'sratioofconcreteis0.2to0.25.Thecomparisonbetweendifferentwavevelocitiesisasfollows:  Accordingtothecalibrationdata,thecorrespondingrelationshipbetweenthedynamicelasticmodulusandthestrengthisestablished,andtheconcretestrengthvalueiscalculated. Two,simplecalibrationmethodofmodulustostrengthcurve Formoststructuralconcrete,itscompressivestrengthis20~60MPa.Inthisinterval,theinfluenceofdifferentshapedeterminationcoefficientsontheregressioncurveisnotlarge(asshowninthefigurebelow).Therefore,settingittoaconstant,suchas5.5,doesnotsignificantlyaffecttheaccuracyofthefittedcurve. Therefore,thereisonlyoneEAfortheparametersthatneedtoberegressed.Then,accordingtotheleastsquaremethod,itiseasytoperformregressionfittingonEA:   Calibration-Establishingtherelationshipbetweenelasticmodulusandstrength Thestrengthtestrequiresthreesetsof150mm×150mm×150mmconcretestandardtestblockswiththesameproportions(especiallyaggregates)onthesite.First,testthedynamicelasticmodulusoftheconcreteofeachtestblock,andthenperformthecompressiontesttoobtainThestrengthofeachtestblockisusedforcalibration,thecorrespondencebetweentheelasticmodulusandthecompressivestrengthisestablished,theregressioncurveisfitted,andthecalculationformulaisderived. UsingtherepeatedreflectionmethodtotestthePwavevelocityofconcreteofeachtestblock,thespecificprocessisasfollows: ΗThethicknessofthetestblockisknown.Theshockelasticwaveisexcitedonthesurfaceofthestructure.BymeasuringthetimeTatwhichtheelasticwavereflectsatthebottomofthestructure,theshockelasticwavevelocityVpiscalculated. Fourth,thesurfacewavemethod(Rwavemethod) Whenthethicknessoftheconcreteislarge,thetwosensorsarearrangedinastraightlineatacertaindistance,andthevibrationdeviceisusedtoexcitethevibrationatacertaindistancefromthefirstsensortogenerateasurfacewave.Thesurfacewavevelocityismeasuredtocalculatetheelasticmodulusoftheconcrete,andthestrengthindexoftheconcretecanbeestimated.   Thesurfacewavemethodhasaninfluencedepthrangeof1.5timestheexcitationwavelengthrange,andcancomprehensivelyreflectthestrengthoftheconcreteintheentireliningthicknessrangeatthetestposition. 5.Testsiteandresultcomparison  Testsitemap▲  Testsitemap▲ Intheactuallargenumberofrailwaytunnelinspections,theaccuracyandapplicabilityofthistechnologyhavebeenverifiedwithnon-destructivetestingandcoresampling.Theresultsshowthatthismethodcanbetterestimatethestrengthofconcrete(surfacelayerandinterior),andissensitivetodamagesuchascracks,anditsestimationaccuracyisfarbetterthanthatofreboundmeter. Asshowninthefigure,theteststrengthofthereboundmetercannotreflecttheactualconcretestrengthatall.   Comparisonofteststrengthandcoreballaststrengthofthistechnology▲  Comparisonofteststrengthandcoreballaststrengthofthistechnology▲  Comparisonofreboundtesterstrengthandcoreballaststrength▲  Distributioncurveofconcretestrengthalongdepth▲

     Mainly for railway tunnel lining and large-scale concrete structures, fast and accurate strength non-destructive testing is performed. The product uses elastic waves as a medium to detect the internal strength of the structure, and has unparalleled advantages of ultrasonic detectors and rebound meters.

    First, the basic principles of elastic modulus and strength
    The impact elastic wave is used as the detection medium. The dynamic elastic modulus of the material is calculated by detecting the wave velocity of the elastic wave. The compressive strength of the concrete is calculated based on the correlation between the dynamic elastic modulus and the compressive strength. The dynamic elastic modulus.
    The dynamic elastic modulus of concrete not only affects the deformation of the structure, but also an important indicator of the quality and durability of concrete:
    1. It can reflect the rigidity of the material and is an important parameter in the calculation of the deformation of the structure. Especially for high-strength concrete, the method of simply using compressive strength to push back the elastic mold often has a large error;
    2. The aging of concrete materials usually starts with the reduction of the spring mold, and the poor construction of new structures will also appear in the spring mold.
    The detection of shotgun mode is mainly through the detection of wave speed.
    For a 1-dimensional homogeneous elastomer, the relationship between the dynamic elastic modulus Ed and the elastic wave P wave velocity Vp1 can be expressed as:

      Among them, ρ is the density of the material. For concrete, ρ is generally about 2400kg / m3. When the detection object is two-dimensional or three-dimensional, the P wave velocity changes to some extent.



      For surface waves (Rayleigh waves), the relationship can be expressed as:

    Rayleigh wave:

      In general, the dynamic Poisson's ratio of concrete is 0.2 to 0.25. The comparison between different wave velocities is as follows:

      According to the calibration data, the corresponding relationship between the dynamic elastic modulus and the strength is established, and the concrete strength value is calculated.

    Two, simple calibration method of modulus to strength curve
    For most structural concrete, its compressive strength is 20 ~ 60MPa. In this interval, the influence of different shape determination coefficients on the regression curve is not large (as shown in the figure below). Therefore, setting it to a constant, such as 5.5, does not significantly affect the accuracy of the fitted curve.
    Therefore, there is only one EA for the parameters that need to be regressed. Then, according to the least square method, it is easy to perform regression fitting on EA:

      Calibration-Establishing the relationship between elastic modulus and strength

    The strength test requires three sets of 150mm × 150mm × 150mm concrete standard test blocks with the same proportions (especially aggregates) on the site. First, test the dynamic elastic modulus of the concrete of each test block, and then perform the compression test to obtain The strength of each test block is used for calibration, the correspondence between the elastic modulus and the compressive strength is established, the regression curve is fitted, and the calculation formula is derived.
    Using the repeated reflection method to test the P wave velocity of concrete of each test block, the specific process is as follows:
    ΗThe thickness of the test block is known. The shock elastic wave is excited on the surface of the structure. By measuring the time T at which the elastic wave reflects at the bottom of the structure, the shock elastic wave velocity Vp is calculated.

     Fourth, the surface wave method (R wave method)

    When the thickness of the concrete is large, the two sensors are arranged in a straight line at a certain distance, and the vibration device is used to excite the vibration at a certain distance from the first sensor to generate a surface wave. The surface wave velocity is measured to calculate the elastic modulus of the concrete, and the strength index of the concrete can be estimated.

      The surface wave method has an influence depth range of 1.5 times the excitation wavelength range, and can comprehensively reflect the strength of the concrete in the entire lining thickness range at the test position.


    5. Test site and result comparison

      Test site map ▲

      Test site map ▲

    In the actual large number of railway tunnel inspections, the accuracy and applicability of this technology have been verified with non-destructive testing and core sampling. The results show that this method can better estimate the strength of concrete (surface layer and interior), and is sensitive to damage such as cracks, and its estimation accuracy is far better than that of rebound meter.

    As shown in the figure, the test strength of the rebound meter can not reflect the actual concrete strength at all.


      Comparison of test strength and core ballast strength of this technology ▲

      Comparison of test strength and core ballast strength of this technology ▲

      Comparison of rebound tester strength and core ballast strength ▲

      Distribution curve of concrete strength along depth ▲