Ap1_02.vp 1 Introduction The mechanical and physical properties of concrete deteriorate during the ageing process. Climatic conditions play a major role in this process [1]. The texture of concrete is not constant in time. When it is in a water saturated condi- tion and subjected to freezing and thawing cycles, concrete is susceptible to damage caused by hydraulic pressure generated in the capillary cavities of the cement paste as water freezes. Therefore, the compressive strength of concrete is definitely determined by modifications of its structure. Changes in the strength of concrete under the impact of cycles is studied in this article. Changes in porous cement paste texture are also investigated. 2 Material composition All studies were carried out on two concrete types: concrete from the Czech Republic, used for a containment structure at NPP TEMELÍN (T), and high-resistance PENLY concrete (P) (France). The composition of the concrete mix- tures for production of 1 m3 of ready material is shown in Table 1. The chemical composition of the cements (% by weight) is shown in Table 2. Both types of concrete are formed on the basis of Portland cement and have approximately the same water cement ratio (PENLY – 0.43 and TEMELÍN – 0.45). These are the only similar factors: TEMELÍN concrete belongs to class B 40, while PENLY concrete is prepared as so-called high-resist- ance concrete (the uniaxial compression strength is higher than 60 MPa) [3]. Acta Polytechnica Vol. 41 No. 2/2001 17 Effect of Thermal Cycling on the Strength and Texture of Concrete for Nuclear Safety Structures Š. Hošková, O. Kapičková, K. Trtík, F. Vodák The effect of thermal cycling (freezing and thawing) on the texture and strength of two types of concrete is studied: 1. Concrete used for a containment structure at NPP Temelín (Czech Republic) – so-called TEMELÍN concrete. 2. Highly resistant PENLY concrete, which was used as a standard because of its high quality, proved by the research carried out in a European Commission project. The results for the two samples of concrete are compared. Keywords: Concrete, thermal cycling, texture, strength, porosity. COMPONENT PENLY TEMELÍN Cement 290 kg (CPA) 499 kg (42.5R) Water 131 kg 215 kg Silica fume (Anglefort) 30 kg – Calcareous filler (Piketty) 105 kg – Superplasticizer (Resino CT) 10.62 kg – Retarder (Chrytard) 1.7 kg – Plasticizer: Ligoplast SF – 4.9 kg Aggregates river gravel crushed agg. crushed agg. 831 kg 0–5 mm 287 kg 5–12.5mm 752kg 12.5–25 mm 710kg 0–4mm 460kg 8–16mm 530kg16–22mm Table 1: Composition of the concrete mixtures for production of 1 m3 of fresh concrete Sample CP 52.5 France CEM I 42.5 Mokra Phase clinker composition C3S 53.7 68.5 C2S 26.7 11.6 C3A 4.4 7.4 C4AF 14.8 11.5 Cfree 0.4 1.0 Total 100.0 100.0 C3Seq 55.4 72.7 C2Seq 25.4 8.4 Components fraction in cement Clinker 97.0 95.0 Gypsum 2.9 3.5 Fly ash 0.1 1.3 Slag Trace 0.2 Total 100.0 100.1 Sample CP 52.5 France CEM I 42.5 Mokra Components fraction without gypsum Clinker 99.9 98.4 Fly ash 0.1 1.4 Slag Trace 0.2 Total 100.0 100.0 Table 2: Quantitative composition of cements (% by weight) 3 Experimental details An artificial ageing process was simulated by thermal cycling. The samples were completely saturated with water and were cooled down to a temperature of –20 °C. They were then warmed up to +25 °C in a HEREAUS HC 4020 conditioning chamber. The total time of one temperature cycle was 6 hours. Relative humidity of 95 % was maintained. A test of the compressive strength of the concrete was performed on the second fragment of the original beam. The [100 × 100] mm steel square plates, whose edges fitted with the face of the original beam, were placed on the body of the specimen. The test was performed in accordance with Czech National Standard ČSN 73 13 18. The pore- and cracks- distribution curve of the cement paste and the total specific volume of the pores were de- termined by mercury porosimetry. The porosimetric measurements were carried out at the Institute of Chemical Process of the Academy of Sciences of the Czech Republic. Table 3 shows the results of measurements of the com- pressive strength and volume of pores in dependence on the numbers of cycles for PENLY and TEMELÍN concrete. A graphical depiction is presented in Figs. 1 and 2. The compressive strength and the specific volume are presented here in percentages, where the value of 100% corresponds to a non-cycled sample (age 28 days). Table 4 relates the age of the samples and the number of climatic cycles. 4 Discussion Our experimental results in Fig. 1, 2 show that the be- haviour of both types of concrete (TEMELÍN, PENLY) is analogous in the investigated parameters. Also, Fig. 1 shows that the changes in compressive strength during thermal cycling are lower in the case of TEMELÍN. A decrease of compressive strength brings about 8.4 % of the initial value in the case of TEMELÍN (after 400 cycles), and about 16.5 % in the case of PENLY already after 300 cycles. Additional loading above 300 cycles results in the destruction of the PENLY samples. Both of the dependencies (Fig. 1, 2) present extremes after about 100 climatic cycles: maximum values for com- pressive strength (Fig. 1) and minimum values for a specific volume of pores (Fig. 2). It is a known fact that in solids there is an inverse relationship between porosity and strength [2], [3]. It should be noted that the measurement of compressive 18 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 41 No. 2/2001 Number of cycles Strength [MPa] Specific volume of pores [mm3/g] PENLY 0 64.4 35.1 100 74.4 33.9 200 65.2 37.7 300 53.8 39.2 TEMELÍN 0 52.3 38.5 100 57.7 31.0 200 56.5 49.9 400 47.9 57.2 Table 3: Strength and volume of pores in dependence on the number of cycles Age of concrete (days) NUMBER OF CYCLES PENLY TEMELÍN 0 28 28 100 84 86 200 184 186 300 281 – 400 – 304 Table 4: Age of samples and the number of climatic cycles Number of cycles Fig. 1: Dependence of the relative strength on the number of cycles Number of cycles Í Fig. 2: Dependence of the relative specific volume of pores on the number of cycles strength was carried out on samples of concrete, but the specific volume of pores were studied on samples of cement paste. However, in concrete the porosity of the cement paste usually determines the strength characteristic. Obviously, our results are in accordance with the assumed theoretical de- pendence between porosity and strength. After about 100 climatic cycles the strength in both materials increased and the volume of pores decreased. This result probably relates to the fact that during the first cycles the porous structure of the samples “matured”. Under ideal moisture conditions in the climatic chamber hydration is further in progress and causes a decrease in the volume of the pores and a refining of the porous structure. This process evidently prevails over cracking of the porous structure as a consequence of the temperature cycling. The change in the development of the porous structure causes the cor- responding behaviour of the strength of the two samples (TEMELÍN, PENLY). Aknowledgements This work was supported by the MŠMT ČR (contract No. J04-098:210000004) and the Grant Agency of the Czech Republic (grant No. 103/99/0248). References [1] Pachner, J. et al.: Concrete containment buildings. IAEA – TECDOC – 1025, Vienna, IAEA, 1998 [2] Mehta, P. K. – Mouteiro: Concrete. New Jersey, Prentice Hall, 1993 [3] Schneider, U.: Behaviour of concrete at high temperatures. Deutscher Ausschuss für Stahlbeton, Berlin, Verlag Ernst & Sohn, 1982 RNDr. Šárka Hošková Department of Physics phone: +420 2 2435 4695 e-mail: hoskova@fsv.cvut.cz RNDr. Olga Kapičková, CSc. Department of Physics phone: +420 2 2435 4696 e-mail: kapickov@fsv.cvut.cz Doc. Ing. Karel Trtík, CSc. Department of Concrete Structures and Bridges phone: +420 2 2435 4626 e-mail: trtik@fsv.cvut.cz Prof. František Vodák, DrSc. Department of Physics phone: +420 2 2435 3886 e-mail: vodak@fsv.cvut.cz Czech Technical University in Prague Faculty of Civil Engineering Thákurova 7, 166 29 Praha 6, Czech Republic © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 19 Acta Polytechnica Vol. 41 No. 2/2001 << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles true /AutoRotatePages /None /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Error /CompatibilityLevel 1.4 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends false /DetectCurves 0.0000 /ColorConversionStrategy /CMYK /DoThumbnails false /EmbedAllFonts true /EmbedOpenType false /ParseICCProfilesInComments true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams false /MaxSubsetPct 100 /Optimize true /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveDICMYKValues true /PreserveEPSInfo true /PreserveFlatness true /PreserveHalftoneInfo false /PreserveOPIComments true /PreserveOverprintSettings true /StartPage 1 /SubsetFonts true /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile () /AlwaysEmbed [ true ] /NeverEmbed [ true ] /AntiAliasColorImages false /CropColorImages true /ColorImageMinResolution 300 /ColorImageMinResolutionPolicy /OK /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 300 /ColorImageDepth -1 /ColorImageMinDownsampleDepth 1 /ColorImageDownsampleThreshold 1.50000 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /CreateJDFFile false /Description << /ARA /BGR /CHS /CHT /DAN /DEU /ESP /ETI /FRA /GRE /HEB /HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. 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