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

Hošková, Š.

Kapičková, O.

Trtík, K.

Vodák, F.

Acta Polytechnica

English

Š. Hošková

O. Kapičková

K. Trtík

F. Vodák

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Effect of Thermal Cycling on the Strength and Texture of Concrete for Nuclear Safety Structures

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

1 IntroductionThe mechanical and physical properties of concretedeteriorate during the ageing process. Climatic conditionsplay a major role in this process [1]. The texture of concrete isnot constant in time. When it is in a water saturated condi-tion and subjected to freezing and thawing cycles, concreteis susceptible to damage caused by hydraulic pressuregenerated in the capillary cavities of the cement paste as waterfreezes. Therefore, the compressive strength of concrete isdefinitely determined by modifications of its structure.Changes in the strength of concrete under the impact ofcycles is studied in this article. Changes in porous cementpaste texture are also investigated.2 Material compositionAll studies were carried out on two concrete types:concrete from the Czech Republic, used for a containmentstructure at NPP TEMELÍN (T), and high-resistance PENLYconcrete (P) (France). The composition of the concrete mix-tures for production of 1 m3 of ready material is shown inTable 1. The chemical composition of the cements (% byweight) is shown in Table 2.Both types of concrete are formed on the basis of Portlandcement and have approximately the same water cement ratio(PENLY – 0.43 and TEMELÍN – 0.45). These are the onlysimilar 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 higherthan 60 MPa) [3].Acta Polytechnica Vol. 41  No. 2/200117Effect of Thermal Cycling on theStrength and Texture of Concrete forNuclear Safety StructuresŠ. Hošková, O. Kapičková, K. Trtík, F. VodákThe 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 ina European Commission project.The results for the two samples of concrete are compared.Keywords: Concrete, thermal cycling, texture, strength, porosity.COMPONENT PENLY TEMELÍNCement 290 kg (CPA) 499 kg (42.5R)Water 131 kg 215 kgSilica 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 kgAggregatesriver gravelcrushed agg.crushed agg.831 kg 0–5 mm287 kg 5–12.5mm752kg 12.5–25 mm710kg 0–4mm460kg 8–16mm530kg16–22mmTable 1: Composition of the concrete mixtures for production of1 m3 of fresh concreteSample CP 52.5 France CEM I 42.5 MokraPhase clinker compositionC3S 53.7 68.5C2S 26.7 11.6C3A 4.4 7.4C4AF 14.8 11.5Cfree 0.4 1.0Total 100.0 100.0C3Seq 55.4 72.7C2Seq 25.4 8.4Components fraction in cementClinker 97.0 95.0Gypsum 2.9 3.5Fly ash 0.1 1.3Slag Trace 0.2Total 100.0 100.1Sample CP 52.5 France CEM I 42.5 MokraComponents fraction without gypsumClinker 99.9 98.4Fly ash 0.1 1.4Slag Trace 0.2Total 100.0 100.0Table 2: Quantitative composition of cements (% by weight)3 Experimental detailsAn artificial ageing process was simulated by thermalcycling. The samples were completely saturated with waterand were cooled down to a temperature of –20 °C. Theywere then warmed up to +25 °C in a HEREAUS HC 4020conditioning chamber. The total time of one temperaturecycle was 6 hours. Relative humidity of 95 % was maintained.A test of the compressive strength of the concrete wasperformed on the second fragment of the original beam. The[100 × 100] mm steel square plates, whose edges fitted withthe face of the original beam, were placed on the body of thespecimen. The test was performed in accordance with CzechNational Standard ČSN 73 13 18.The pore- and cracks- distribution curve of the cementpaste and the total specific volume of the pores were de-termined by mercury porosimetry. The porosimetricmeasurements were carried out at the Institute of ChemicalProcess 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 onthe numbers of cycles for PENLY and TEMELÍN concrete.A graphical depiction is presented in Figs. 1 and 2. Thecompressive strength and the specific volume are presentedhere in percentages, where the value of 100% corresponds toa non-cycled sample (age 28 days).Table 4 relates the age of the samples and the number ofclimatic cycles.4 DiscussionOur experimental results in Fig. 1, 2 show that the be-haviour of both types of concrete (TEMELÍN, PENLY) isanalogous in the investigated parameters. Also, Fig. 1 showsthat the changes in compressive strength during thermalcycling are lower in the case of TEMELÍN. A decrease ofcompressive strength brings about 8.4 % of the initial value inthe case of TEMELÍN (after 400 cycles), and about 16.5 %in the case of PENLY already after 300 cycles. Additionalloading above 300 cycles results in the destruction of thePENLY samples.Both of the dependencies (Fig. 1, 2) present extremesafter about 100 climatic cycles: maximum values for com-pressive strength (Fig. 1) and minimum values for a specificvolume of pores (Fig. 2). It is a known fact that in solids thereis an inverse relationship between porosity and strength [2],[3]. It should be noted that the measurement of compressive18 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 41  No. 2/2001Number ofcyclesStrength[MPa]Specific volume of pores[mm3/g]PENLY0 64.4 35.1100 74.4 33.9200 65.2 37.7300 53.8 39.2TEMELÍN0 52.3 38.5100 57.7 31.0200 56.5 49.9400 47.9 57.2Table 3: Strength and volume of pores in dependence on thenumber of cyclesAge of concrete (days)NUMBER OFCYCLESPENLY TEMELÍN0 28 28100 84 86200 184 186300 281 –400 – 304Table 4: Age of samples and the number of climatic cyclesNumber of cyclesFig. 1: Dependence of the relative strength on the number ofcyclesNumber of cyclesÍFig. 2: Dependence of the relative specific volume of pores on thenumber of cyclesstrength was carried out on samples of concrete, but thespecific volume of pores were studied on samples of cementpaste. However, in concrete the porosity of the cement pasteusually determines the strength characteristic. Obviously, ourresults are in accordance with the assumed theoretical de-pendence between porosity and strength.After about 100 climatic cycles the strength in bothmaterials increased and the volume of pores decreased. Thisresult probably relates to the fact that during the first cyclesthe porous structure of the samples “matured”. Under idealmoisture conditions in the climatic chamber hydration isfurther in progress and causes a decrease in the volume of thepores and a refining of the porous structure. This processevidently prevails over cracking of the porous structure asa consequence of the temperature cycling. The change inthe development of the porous structure causes the cor-responding behaviour of the strength of the two samples(TEMELÍN, PENLY).AknowledgementsThis work was supported by the MŠMT ČR (contractNo. J04-098:210000004) and the Grant Agency of the CzechRepublic (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, PrenticeHall, 1993[3] Schneider, U.: Behaviour of concrete at high temperatures.Deutscher Ausschuss für Stahlbeton, Berlin, VerlagErnst & Sohn, 1982RNDr. Šárka HoškováDepartment of Physicsphone: +420 2 2435 4695e-mail: hoskova@fsv.cvut.czRNDr. Olga Kapičková, CSc.Department of Physicsphone: +420 2 2435 4696e-mail: kapickov@fsv.cvut.czDoc. Ing. Karel Trtík, CSc.Department of Concrete Structures and Bridgesphone: +420 2 2435 4626e-mail: trtik@fsv.cvut.czProf. František Vodák, DrSc.Department of Physicsphone: +420 2 2435 3886e-mail: vodak@fsv.cvut.czCzech Technical University in PragueFaculty of Civil EngineeringThákurova 7, 166 29 Praha 6, Czech Republic©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 19Acta Polytechnica Vol. 41  No. 2/2001

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Effect of Thermal Cycling on the Strength and Texture of Concrete for Nuclear Safety Structures

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