The paper presents the evolution of equivalent thermal diffusivity with temperature of concretes with natural coarse aggregates and recycled concrete aggregates. Thermal diffusivity was evaluated based on the temperature measurements made on cylindrical samples using so-called "inverse technique". Thermal diffusivity parameter was estimated for concretes manufactured with different replacement ratios of natural by recycled coarse aggregate (0, 50 and 100 %) produced by crushing the worn-out concrete pavement that was dismounted after exploitation period. The diffusivity of concrete was tested up to 350 ◦C. The progressive decrease of thermal diffusivity was observed, and the lowest D values were obtained for higher recycled concrete aggregates content. The results were presented against the background of other authors results and referred to the D values proposed in EUROCODE 2

Fiala, Lukáš

Hager, Izabela

Kočí, Václav

Mróz, Katarzyna

Tracz, Tomasz

English

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

https://doi.org/10.14311/APP.2022.33.0207Acta Polytechnica CTU Proceedings 33:207–211, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licencePublished by the Czech Technical University in PragueEQUIVALENT THERMAL DIFFUSIVITY OF NATURAL ANDRECYCLED AGGREGATE CONCRETE AT TEMPERATURE UPTO 350 ◦CIzabela Hagera,∗, Katarzyna Mróza, Tomasz Tracza, Václav Kočíb,Lukáš Fialaba Chair of Building Materials Engineering, Faculty of Civil Engineering, Cracow University of Technology, 24Warszawska St. 31-155 Cracow, Polandb Czech Technical University in Prague, Department of Materials Engineering and Chemistry, Faculty of CivilEngineering, Thakurova 7/2077, 16629 Prague 6, Czech Republic∗ corresponding author: izabela.hager@pk.edu.plAbstract.The paper presents the evolution of equivalent thermal diffusivity with temperature of concreteswith natural coarse aggregates and recycled concrete aggregates. Thermal diffusivity was evaluatedbased on the temperature measurements made on cylindrical samples using so-called "inverse tech-nique". Thermal diffusivity parameter was estimated for concretes manufactured with different re-placement ratios of natural by recycled coarse aggregate (0, 50 and 100 %) produced by crushing theworn-out concrete pavement that was dismounted after exploitation period. The diffusivity of concretewas tested up to 350 ◦C. The progressive decrease of thermal diffusivity was observed, and the lowestD values were obtained for higher recycled concrete aggregates content. The results were presentedagainst the background of other authors results and referred to the D values proposed in EUROCODE2.Keywords: Fire, recycled concrete aggregates, temperature, thermal diffusion.1. IntroductionBy volume,construction and demolition waste(CDW) are one of the largest streams of waste inEurope with 850 million tons that are producedper year [1]. The waste separation process enablesto obtain good quality aggregates and their usein concrete is economically, environmentally andtechnically feasible with the respect of all technicalstandards. The progressive replacement of naturalaggregates by recycled concrete aggregates con-tributes to the implementation of the strategy forsustainable development of the field of constructionthrough the successful implementation of sustainablebuilding materials with a reduced carbon footprint.The concrete industry scepticism based on concernabout poor-quality recycled products needs tochange with the research projects showing successfulapplication of recycled concrete aggregates RCAwithout compromising on quality and contribute tothe decrease environmental impact. The detailedrecommendations concerning assessing the propertiesof recycled aggregates are given in [2], however theimitation concerning their replacement of naturalaggregate in the concrete mix is imposed [3].In the present research the properties of concretewith recycled aggregates were investigated in the viewof study their behaviour in fire. The fire behaviour ofconcrete with natural aggregate is well documentedin the literature [4–6]. Well known cases of fires inengineering facilities like Channel (1996); Mont Blanc(1999); Gotthard (2001) tunnels has drew attentionto one of the disadvantages of concrete that is con-crete spalling in a fire. The sudden loss of concretecover and concrete element cross-section reductionmay jeopardise the overall load-bearing capacity ofthe structural element [7]. It has been confirmed thatthe composition of concrete, as well as boundary con-ditions, affects the concrete propensity to spall in afire. One of the main parameters governing concretespalling is permeability. In denser and less perme-able concrete, the spalling risk is higher [8]. Concretepermeability is a derivative of the type of aggregateused, transport properties of the cement paste andthe permeability of interfacial transition zone ITZ be-tween the cement paste and the aggregate. The per-meability of recycled concrete aggregate differs fromnatural aggregate, thus transport properties of thoseconcretes may be higher than in traditional concretes[9], thus lowering the risk of spalling behaviour. Inview of studying the RCA concrete behaviour in thefire the first step that was undertaking was the ob-servation of thermal properties evolution presented inthis paper.The aim of this project is to examine the impact ofconcrete composition, in particular the recycled con-crete aggregate amount RCA on the thermal prop-erties of concrete. As the heat transfer in concreteplays a crucial role in fire resistance evaluation, the207I. Hager, K. Mróz, T. Tracz et al. Acta Polytechnica CTU ProceedingsFigure 1. Thermocouples arrangement in the concrete cylinder, and the furnace heating rate.thermal properties that change with the temperatureare essential and need to be provided to the calcu-lation model. Thermal diffusivity (D) is a specificproperty of the material and allows you to determinehow quickly a given material reacts to temperaturechanges. This is a property that measures the rate ofheat transfer from the exposed surface of the materialto the inner layers. The higher the value of thermaldiffusivity, the faster the temperature increase at acertain depth. To predict the behaviour of concreteduring a fire and carry out simulations determiningthe temperature development in a heated element,one need to know the value of thermal diffusivity.Thermal diffusivity can be evaluated directly from itsdefinition or its evaluation can be based on the tem-perature measurements made on cylindrical samples,called an "inverse technique".2. Experimental methodThe thermal diffusivity (D) of concrete depends onits thermal conductivity (λ), density (ρ) and specificheat of the material (cp). Thermal diffusivity canbe determined (Eq. 1) by directly determining thesephysical properties by experimental methods or byusing an inverse method leading to the determinationof D. The unit of thermal diffusivity is [J/kgK], but[mm2/s] or [m2/s] are also used in the literature.D = λρ cp!m2s"(1)The method determining the equivalent thermaldiffusivity of concrete was recently described by Fe-licetti [10, 11] showing good compliance with exper-imental results from standard fire tests. The mainadvantage of the "inverse technique" is the determina-tion of thermal diffusivity without the need for cum-bersome measurements of thermal conductivity andspecific heat values as well as density change evalua-tion, knowing that all those values vary with temper-ature increase.The inverse method is quite sensitive to moisturepresence in concrete and other physicochemical pro-cesses that occur when the material is exposed tohigh temperature. It consists in heating the cylin-drical sample (diameter 100 mm; height 200 mm) inthe electric furnace and measuring temperature in-crease in the centre of the sample (Tc - centre) andon 2 mm from cylinder lateral surface (Te - exterior)using K-type thermocouples, Figure 1. The temper-ature differences are measured on the central cross-section where the field of temperature is considereddependant only to distance from the cylinder centre.The temperature increases with the constant heatingrate 1 ◦C/min up to 350 ◦C. The test result is thetemperature difference Te − Tc in function of time.during the test. In the determination of thermaldiffusivity by the inverse method, the initial assump-tions is adopted that the cylinder is axis-symmetricand infinitely long, so the analyse is related to a dif-ferential equation for transient heat conduction. Todetermine the diffusivity that characterises the tran-sient process of heat exchange inside a constant masswe use Fourier’s Law (Eq. 2):ρ cp∂T∂t+ div (q) = 0 (2)Assuming a cylindrical coordinate system (θ, r, z)(Eq. 3):∂T∂t= D!d2Tdr2 +1r∂T∂t"(3)and using Laplace transformation, we get a formulafor the temperature difference between the edge andthe centre of the sample (Eq. 4):208vol. 33/2022 Natural and Recycled Aggregate Concrete Thermal DiffusivityComponent Units OC RAC 50% RAC 100%CEM I 42.5 R kg/m3 414.2 414.2 414.2water kg/m3 186.4 186.4 186.4w/c − 0.45 0.45 0.45river sand 0/4 mm Dwudniaki kg/m3 615.1 280.6 569.2coarse agg. 2/8 mm Dwudniaki kg/m3 579.9 280.6 −coarse agg. 8/16 mm Dwudniaki kg/m3 562.3 264.1 −recycled concrete agg. 4/16 mm kg/m3 − 825.3 1057.2plasticizer BASF BV 18 % mc 0.9 0.9 0.9Basf Glenium SKY 591 % mc 1.4 1.4 1.4cement paste content dm3/m3 320 320 320mortar content dm3/m3 542 511 540density kg/m3 2248.5 2236.8 2221.3water content % 5.2 5.8 6.7water absorption % 6.9 7.3 7.828 days compressive strength MPa 46.1 56.8 54.528 days tensile splitting strength MPa 3.3 3.6 3.4Table 1. Mix design and properties.T (r, t) = R!t − a2 − r24D"(4)From here we get the formula for D - equivalentthermal diffusivity (Eq. 5):D = Ra24∆T (5)where:• R - heating rate [◦C/min],• a - sample radius [m],• ∆T - temperature gradient [◦C/cm],• D -thermal diffusivity [mm2/s].3. MaterialsThe measurements were carried out for concretemade of CEM I 42.5 cement, riverbed coarse aggre-gate Dwudniaki from Dunajec River southern Poland(grain size 2/8 mm and 8/16 mm) and crushed con-crete from the worn-out concrete pavement with grainsize - 4/16 mm. Before being crushed the mechanicalproperties of concrete from the pavement was deter-mined on drills. The compressive strength of 66.8MPa, modulus of elasticity 41.2 GPa and apparentdensity of 2290 kg/m3 characterised the recycled con-crete.For all 3 concretes the same water-cement ratio(w/c) and cement paste volume was maintained. Ad-ditionally, a plasticiser was used to obtain good work-ability and water content reduction. The mix compo-sition of the concretes is shown in Table 1. The con-cretes were labelled: the reference ordinary concrete(OC) made of natural riverbed aggregates; RAC 50%is concrete made of recycled pavement aggregate with50% of natural coarse aggregate replacement with re-cycled one; RAC 100% recycled aggregate concretemade of recycled concrete coarse aggregates. In allconcretes - OC, RAC 50% and RAC 100% riverbedquartz sand was used.The concretes specimens were cast and stored inplastic cubic or cylindrical moulds for the first 24hours. After preliminary curing, water evaporationwas prevented by covering them with plastic lids for7 days. During the next 28 days, they were stored innatural air-drying conditions at the temperature of20±5◦C and relative humidity of 50±5 %. The com-pressive strength and splitting tensile strength of con-cretes after 28 days were determined on 10 cm3 cubes.All basic physical (density, water content, water ab-sorption) and mechanical (compressive and splittingtensile strength) performances are summarised in Ta-ble 1.4. Results and observationsThe temperature evolution in time measured withtype K thermocouples was presented in Figure 2. Thesolid line is showing the heating programme. The pre-sented results were used to determine effective ther-mal diffusivity as a function of temperature, obtainedfrom the Eq. 5 and expressed in [mm2/s]. The resultsare shown in Figure 3.Due to thermal gradient value close to zero at thebeginning of the test, when the furnace starts to heat,the effective thermal diffusivity values are very high.This phenomenon was previously observed by [10],and the correction factor was proposed to reduce thisunwanted effect so that the suggestion to compute theinstantaneous heating rate was proposed [11]. Theequivalent thermal diffusivity decreases with increas-ing temperature in the range up to 350 ◦C. In heated209I. Hager, K. Mróz, T. Tracz et al. Acta Polytechnica CTU ProceedingsFigure 2. Time - temperature curves for OC, RAC 50% and RAC 100%.Figure 3. Equivalent thermal diffusivity for OC, RAC 50% and RAC 100%compared with the results of Felicetti[11] and EC 2 proposed values.with 1◦C/min cylinders the values of equivalent ther-mal diffusivity reaches at 100 ◦C 0.76 mm2/s, 0.61mm2/s and 0.45 mm2/s respectively for OC, RAC50% and RAC 100%. The highest value obtained forOC results from the density of this material. With anincreasing proportion of recycled concrete aggregates,the D values are reduced. The similar tendencies areobserved at 200 ◦C and 300 ◦C.From the graph, we can see the water migration ef-fect occurring in the lower temperature in case of con-cretes with recycled concrete aggregates. It confirmsthat those materials enable faster water migrationdue to their higher permeability. The observed wa-ter migration perturbance on Figure 3 is linked withspecific heat significant peak at a temperature closeto 200 ◦C.The comparison with the D range proposed by EU-ROCODE 2 (EC2) [12] for both samples with re-cycled aggregates RAC 50% and RAC 100% showsthatthe thermal diffusivity values are mostly belowboth the upper and lower limits given in EC2. Theresults remain similar to the experimental results ofFelicetti [10, 11].5. ConclusionsDetermination of thermal diffusivity is a vital param-eter for the correct modelling of heat flow throughheated concrete. Thanks to the inverse method, wedetermine diffusivity for much higher temperaturesthan other stationary and non-stationary methods.Non-stationary transient temperature methods, de-spite their broad application, require the use of spe-cialised and often costly measuring equipment com-plex in mathematical and numerical terms. In theinverse method, we can use widely available and easy-to-use laboratory equipment to perform the tests. Al-though the technique does not allow for specific pa-rameters to established: ρ, cp and λ but their ratio210vol. 33/2022 Natural and Recycled Aggregate Concrete Thermal Diffusivityexpressed in diffusivity definition D = λ/ρ cpis as-sessed.AcknowledgementsThis research was supported and funded in part fromthe EMMAT project E-mobility and sustainable mate-rials and technologies PPI/APM/2018/1/00027 financedby the Polish National Agency for Academic Exchange(NAWA).References[1] Global Recycling. Recycled Construction Waste inEurope.https://global-recycling.info/archives/2964.[2] RILEM TC 121-DRG. Specifications for concretewith recycled aggregates. Materials and Structures27(173):557-59, 1994.[3] EN 12620:2004 + A1:2010. Aggregates for concrete,2004.[4] J.-C. Mindeguia, P. Pimienta, H. Carré, et al. On theinfluence of aggregate nature on concrete behaviour athigh temperature. European Journal of Environmentaland Civil Engineering 16(2):236-53, 2012.https://doi.org/10.1080/19648189.2012.667682.[5] Z. Xing, A.-L. Beaucour, R. Hebert, et al. Influenceof the nature of aggregates on the behaviour ofconcrete subjected to elevated temperature. Cementand Concrete Research 41(4):392-402, 2011. https://doi.org/10.1016/j.cemconres.2011.01.005.[6] I. Hager, T. Tracz, J. Śliwiński, et al. The influenceof aggregate type on the physical and mechanicalproperties of high-performance concrete subjected tohigh temperature. Fire and Materials 40(5):668-82,2016. https://doi.org/10.1002/fam.2318.[7] K. D. Hertz. Limits of Spalling of Fire-ExposedConcrete. Fire Safety Journal 38(2):103-116, 2003.[8] I. Hager, K. Mróz. Role of Polypropylene Fibres inConcrete Spalling Risk Mitigation in Fire and TestMethods of Fibres Effectiveness Evaluation. Materials12(23), 2019. https://doi.org/10.3390/ma12233869.[9] M. Choinska, A. Khelidj, G. Chatzigeorgiou, et al.Effects and interactions of temperature and stress-levelrelated damage on permeability of concrete. Cementand Concrete Research 37(1):79-88, 2007. https://doi.org/10.1016/j.cemconres.2006.09.015.[10] R. Felicetti. Thermal Diffusivity and ResidualStrength of High-Performance Light-Weight ConcreteExposed to High Temperature. Proc. 6th Int.Symposium on the Utilisation of High-Strength/HighPerformance Concrete 2, Leipzig (Germany) 935-48,2002.[11] R. Felicetti. Assessment of the equivalent thermaldiffusivity for fire analysis of concrete structures, Proc.Fib Task Group 4.3 Workshop Fire Design of ConcreteStructures, Coimbra 149-58, 2007.[12] EN 1992-1-2. Eurocode 2: Design of ConcreteStructures - Part 1-2: General Rules - Structural FireDesign 2004, 1992.211

Equivalent thermal diffusivity of natural and recycled aggregate concrete at temperature up to 350 ⁰C

https://ojs.cvut.cz/ojs/index.php/APP/article/download/7999/6226

Equivalent thermal diffusivity of natural and recycled aggregate concrete at temperature up to 350 ⁰C

Abstract

Similar works

Full text

Available Versions

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