In this article, an innovative new method of non-destructive testing for estimating the sensitivity of limestone is presented. An advantage of the laser ultrasonic structuroscopy method is the time and the quality, due to which less tests are necessary for an analysis of rock properties. Limestone is measured with an innovative structural control method, which shows a good qualitative and quantitative agreement with the basic structural analysis

Baranowski , Paweł

Franek , Ondřej

Kravcov , Alexander

Kucewicz , Michał

Morozov , Nikolaj

Svoboda , Pavel

Tomaszewski , Michał

English

Digital Library of the Czech Technical University in Prague

DOI:10.14311/AP.2020.60.0410Acta Polytechnica 60(5):410–414, 2020 © Czech Technical University in Prague, 2020available online at https://ojs.cvut.cz/ojs/index.php/apLASER ULTRASONIC MEASUREMENT OF MECHANICALDECAY IN LIMESTONE CAUSED BY FREEZE-THAW CYCLESAlexander Kravcova,∗, Ondřej Franeka, Nikolaj Morozova,Michał Kucewiczb, Michał Tomaszewskib, Pavel Svobodaa,Paweł Baranowski ba Czech Technical University in Prague, Faculty of Civil Engineering, Department of Construction Technology,Thákurova 7, 160 00 Prague, Czech Republicb Military University of Technology, Faculty of Mechatronics and Aviation, Institute of Armament Technology,gen. W. Urbanowicza, 00-908 Warsaw, Poland∗ corresponding author: kravtale@fsv.cvut.czAbstract. In this article, an innovative new method of non-destructive testing for estimating thesensitivity of limestone is presented. An advantage of the laser ultrasonic structuroscopy methodis the time and the quality, due to which less tests are necessary for an analysis of rock properties.Limestone is measured with an innovative structural control method, which shows a good qualitativeand quantitative agreement with the basic structural analysis.Keywords: Exterior cladding material, civil construction, laser ultrasonic structuroscopy.1. IntroductionLimestone is one of the most widely used constructionmaterials in Central Europe. It has a fairly high porevalue and is flexible for work. Highly porous limestoneis especially susceptible to weathering.The study of porous limestone samples was aimedat studying the effect of an enclosed water on themechanical properties after an exposure to freezing [1].It is reliably known that the influence of a closedwater content on the mechanical properties of porouslimestone is quite similar as other lithologies [2].There are a large number of published articles withaccurate studies of mechanical parameters, such asbulk density, effective porosity, ultrasonic pulse veloc-ity and compressive strength [3]. It should be notedthat after a stable period when isolated microcracksappear, the destruction of the rock occurs in a smallnumber of cycles. The rapid growth of microcracks iscaused by the thermal expansion of pore water.It was found that the physico-mechanical proper-ties of the stone can be reduced over a wide rangetowards the initial values after freezing and thawingcycles. Nowadays, a study of the influence of tempera-ture differences, with the aim to predict physical andmechanical properties, is the topic of focus.Traditional mechanical tests are the most commonlyused, but they require a large volume of test samples,which, in turn, increases the costs. One way to re-duce this item of expenditure may be to partially re-place traditional destructive tests with non-destructivemethods of time-to-failure data. Ultrasonic structur-oscopy allows to obtain the relevant values of theelastic modules and their distribution in the arrayof samples. Taking into account the fact that therequirements for the geometry of the test samplesare lower, the use of ultrasonic methods to determineelastic moduli leads to a reduction in the requiredvolume of samples.Ultrasonic methods are used for the detection of sur-face and internal defects, measurement of the thicknessof layers in multilayer materials, and an evaluationof volumetric mechanical properties based on longitu-dinal and shear wave velocities. These applicationsusually use the frequency range from 50 to 500 kHz,which makes it possible to examine samples tens ofcentimetres thick with a spatial resolution of up to1 cm. The use of frequencies above 1MHz is limitedbecause of their strong attenuation in limestone [4–6].Spatial resolution can be higher, provided that theamplitude of the ultrasonic probe pulse is several or-ders of magnitude greater and the frequency is a fewtens of MHz. This can be attained [7, 8] by meansof laser-based excitation of acoustic pulses and recep-tion of ultrasonic energy by a broadband piezoelectrictransducer [9]. The use of a Q-switched laser gener-ating pulses with a duration of around 10 ns and en-ergy of several mJ, which in turn generates ultrasonicpulses with less than 70 ns duration and a pressureamplitude of up to 1MPa, allows achieving an excel-lent spatial resolution of about 50 µm [10]. Therefore,it is possible to detect a variety of microstructuresand heterogeneities and assess their dimensions (sizeof crystals, porosity, and degree of weathering) inpulse-echo mode.This paper describes a study of local physical andmechanical characteristics after freeze-thaw cycles oflimestone samples by laser ultrasonic spectroscopy,which is able to define local elastic modules of materi-als.410vol. 60 no. 5/2020 Laser ultrasonic measurement of mechanical decay. . .Figure 1. Schematic diagram of optoacoustic genera-tor.2. Materials and methods2.1. Optoacoustic equipment andmeasuring techniqueLaser ultrasonic structuroscopy is similar to tradi-tional ultrasonic systems operating in echo-pulse mode.The generation of ultrashort high-power ultrasonicpulses with a strictly controlled form occurs in anoptoacoustic cell based on the generation of ultrasonicpulses by thermal expansion. A pulse generated by alaser is transmitted to the front side of a special optoa-coustic generator (OAG). Figure 1 shows a schematicdiagram of this cell for the diagnostics of materials atone-side access.The pulse energy enters the sample and is scatteredby its heterogeneities and reflected from the backside of the sample. In the case of sufficiently strongscattering (which is indicative of a high degree ofheterogeneity), the reflection from the back side maynot be observed [11, 12]. All scattered and reflectedsignals are recorded by the piezoelectric transducerand processed by a processing system. The surfaceof the sample is scanned with a 1mm step. Providedthat the reflection from the back side of the sampleis distinctly recorded (no defects and low porosity),the phase velocity of the longitudinal ultrasonic wavescan be evaluated by the difference between the arrivaltimes of the minimum of the reflection from the backside and the maximum of the reflection from the OAG-sample interface [13].Note that an S-wave pulse is recorded in the intervalbetween the first and second reflections of P-wavesfrom the back side; the time delay of the S-wave’sarrival can be used to calculate its velocity. In thecourse of scanning, local velocities are determined.The velocities dependent on density are related tolocal modules of elasticity; they can be calculated asfollows:E = ρC2t[3 − 1x2 − 1](1)ν = x2 − 22(x2 − 1) (2)G = ρC2t (3)where: E is Young’s modulus, G is shear modulus, νis Poisson’s ratio, ρ is the calculated density of thesample, Ct is the measured value of shear acousticwave velocity in the sample, m/s; Cl is the measuredvalue of longitudinal acoustic wave velocity in thesample, m/s; x = Cl/Ct is the ratio of the longitudinaland shear wave velocities.Based on the above-described, it is possible to mapultrasonic wave velocities, elastic moduli, and porosityP using the expression.2.2. SamplesAccording to mineralogical features, the tested rocksamples are medium-grained porous limestone; in mostcases, they contain interlayers of marl.Using electron microscopy, the pore sizes, their lo-cation in space, and the bulk porosity of the sampleswere studied. Figures 2a and 2b present electronicimages of samples with pores and structural inhomo-geneities characteristic for these samples. The sizeof grain ranges from 50µm to 400 µm; most grainsusually have a size of 200 µm, porosity varies from 2to 7%.The physical and mechanical properties of the sam-ples are as follows: the average density is 2 827 kg/m3,the ultimate compression strength varies from 85 to140MPa, average water absorption is 2.76%. The sam-ples studied are rectangular parallelepipeds with thecharacteristic dimensions 65mm, 65mm, and 5mm.3. ResultsThe study of samples took place in 2 stages. In the firststage, the elastic moduli and porosity of the sampleswere determined. Then, the samples were exposed to20 cycles of freezing based on EN 12371 recommenda-tions for Natural Stone tests methods: determinationof frost resistance, and thawing in temperatures rang-ing from 300K to 253K. After the freeze–thaw cycles,the samples display clear fractures as the main weath-ering pattern. After the freezing and thawing cycles,the samples were re-scanned by the laser-ultrasounddevice. The samples were scanned with a step of300µm to 1mm.Defect-free limestone samples were selected to befurther examined. Figures 3, 4a and 4b show maps ofelastic wave velocities, elastic modules and porosityfor one of the samples. Clearly, this limestone sampleis quite homogeneous, longitudinal and shear wavevelocities vary from 4 710 to 4 830m/s and from 1 640to 1 675m/s, respectively; Young’s modulus and Pois-son’s ratio vary from 38.0 to 40.5 and from 0.403 to0.408GPa, respectively. Porosity has a medium value,about 2.7%.After 20 freezing and thawing cycles, samples dis-play clear fractures and the porosity of the samplesincreased significantly, which led to a decrease inYoung’s modulus by an average of 20% (Figure 5aand 5b).411A. Kravcov, O. Franek, N. Morozov et al. Acta Polytechnica(a). (b).Figure 2. Phenom ProX3 SEM images of polished sections.Figure 3. Poisson’s ratio map of limestone sample.(a). (b).Figure 4. Poisson’s ratio map of limestone sample.412vol. 60 no. 5/2020 Laser ultrasonic measurement of mechanical decay. . .(a). (b).Figure 5. Young’s modulus and Porosity maps of limestone sample after of freezing and thawing.4. ConclusionThe loss of durability of medium-grained porous lime-stone is related to the creation of new void volume dueto an expansion of water closed in pores during freeze-thaw cycles. Ice crystals crack the internal structure,forming an additional water filling systems. Affectedsamples absorb more water due to the newly createdpore volume. As expected, the diminution of adhesionof grain bonders and the increase of pore capacityvastly reduce the obtained load resistance after 20freeze-thaw cycles.The degree of strength reduction depends on theporosity and the natural fissuring of the rocks. Inthe experiments, it was observed that with increasingporosity, the compressive strength of rocks decreases.The compressive strength of the samples fell by 30%after the exposure to 20 freeze-thaw cycles.AcknowledgementsThis study was financially supported by the National Min-istry of Education of Czech Republic (No. 027/0008465)and Warszawa War Studies Academy endowment. Theauthors are grateful to the Czech Technical University inPrague, they would also want to thank Prof. Jerzy Mala-chowski from the Military University of Technology andLt.Col. Paweł Maciejewski from War Studies Academy,both in Warszawa, for their advices.References[1] B. Vásárhelyi. Statistical analysis of the influence ofwater content on the strength of the miocene limestone.Rock Mechanics and Rock Engineering 38:69 – 76, 2005.doi:10.1007/s00603-004-0034-3.[2] B. Vásárhelyi, P. Ván. Influence of water content onthe strength of rock. Engineering Geology 84(1):70 – 74,2006. doi:10.1016/j.enggeo.2005.11.011.[3] J. Martínez-Martínez, D. Benavente, M. Gomez-Heras,et al. Non-linear decay of building stones duringfreeze–thaw weathering processes. Construction andBuilding Materials 38:443 – 454, 2013.doi:10.1016/j.conbuildmat.2012.07.059.[4] J. Štoller, E. Zezulová. Use of ultrasound — Theultrasonic pulse velocity method for the diagnosis ofprotective structures after the load of TNT explosion.In ICMT 2017 - 6th International Conference onMilitary Technologies, pp. 230 – 235. 2017.doi:10.1109/MILTECHS.2017.7988761.[5] J. Štoller, P. Dvořák. Ultrasound diagnosis ofprotective structures after contact explosion. InTransport Means 2014, pp. 264 – 267. KaunasUniversity of Technology, Lithuania, 2014.[6] S. N. Iliopoulos, L. De Smet, D. G. Aggelis.Investigating ultrasonic wave dispersion and attenuationin fresh cementitious materials: A combined numerical,analytical, and experimental approach. NDT |& EInternational 100:115 – 123, 2018.doi:10.1016/j.ndteint.2018.09.005.[7] D. Ciofini, A. A. Mencaglia, S. 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LASER ULTRASONIC MEASUREMENT OF MECHANICAL DECAY IN LIMESTONE CAUSED BY FREEZETHAW CYCLES

Kravcov, Alexander

Franek, Ondřej

Morozov, Nikolaj

Kucewicz, Michał

Tomaszewski, Michał

Svoboda, Pavel

Baranowski, Paweł

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

LASER ULTRASONIC MEASUREMENT OF MECHANICAL DECAY IN LIMESTONE CAUSED BY FREEZE-THAW CYCLES

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LASER ULTRASONIC MEASUREMENT OF MECHANICAL DECAY IN LIMESTONE CAUSED BY FREEZETHAW CYCLES

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