ࡱ; !"  %$#/.&'()*+,-9201436578:;<=>?@ABCDEFGHIJKLMNOPQRSTURoot Entry F5 CompObjbWordDocument ObjectPool   4@   !"#$%&'),-.13456789;>@ABCDEFGHIJ FMicrosoft Word 6.0-dokument MSWordDocWord.Document.6; ࡱ; LA| ࡱ; AnM r .1  ``&@; & MathTypeSymbol- 2 @ܥe=eJ IGTx"x""y"y"y"y"yy<y||| $|@|pyd|L|($}$}$}$}$}$}tvvv!GyT̈́>"y~&*$}$}~~~"y"y$}|~~~~"y$}"y$}t6y>tyh"y"y"y"y$}t~~ TID \@ "d MMMM " 27 November 1997 1^ Draft EVOLUTION OF Thermal properties 1. Introduction The analysis of the spatial and temporal temperature distribution in hardening concrete is carried out by solving the well known differential equation of Fourier. The calculation requires that thermal diffusivity, specific heat and their evolution during the hardening phase of concrete are known. Thermal diffusivity and specific heat are in relationship with the thermal conductivity and the density according to the following equation: thermal conductivity Thermal diffusivity = ____________________ density * specific heat In this chapter, general information will be given about existing data concerning the thermal properties of concrete and their evolution in the hardening phase. On that basis, the experimental research program within the sub-task is outlined. 2. Definitions, symbols and unit measurement Thermal conductivity, l [kcal/(h m C)]: the rate at which the heat flows through a unit area of homogeneous material of unit length along which a temperature gradient of one degree is applied. The conversion factor in SI units is 1 kcal/(h m C) = 1.163 W/(m C) Specific heat, c [kcal/(kg C)]: the heat quantity required to raise of 1C the temperature of the unit weight of a material. Heat capacity per unit volume, C [kcal/ m3 C]: the product between the specific heat and the unit mass g [kg/m3] of a material. It is defined as the heat quantity required to raise of 1C the temperature of a unit volume of a material. Thermal diffusivity, a [m2/h]: the ratio between the thermal conductivity and the heat capacity per unit volume. The coefficient of thermal diffusivity measures the capability of a material to support rapid changes in temperature. 3. SURVEY OF RELEVANT LITERATURE Concrete is an heterogeneous material essentially composed of cement, aggregate, water and air. The absolute values of the thermal properties will depend mainly on the thermal properties of the constituents of the mix, i. e. of the aggregate, the unhydrated cement, the amount of hydration products, the mixing water, the air volume, the temperature. This implies that, for a given concrete mix, changes in the thermal properties can be described as a function of porosity, moisture content, amount of hydration products and temperature. The first three parameters are subjected to changes during the hardening phase and, in turn, they can be expressed as a function of the degree of hydration, the temperature being a variable parameter that depends on the exchanges of heat with the environment and will affect the rate of hydration. As a matter of the fact, it will be possible to express the thermal properties as a function of the degree of hydration and evaluate separately the effects of the temperature on samples of hardened concrete. Published data on the thermal characteristics of hardening concrete appear to be scarce and often in contradictions among them. The main reason of that relies upon the difficulty to measure accurately these parameters at the early ages when the material is still plastic and with the complexity that the material itself develops an unknown quantity of heat in addition to that usually necessary to measure the thermal properties. For those reasons most of data from literature take into account the thermal properties of hardened concrete and their dependence on the other parameters. 3.1 Thermal conductivity. The coefficient of thermal conductivity of concrete depends on the moisture content, type of aggregate, porosity, density and temperature. For ordinary hardened concrete in a normal temperature and moisture state, the coefficient of thermal conductivity may vary between 1.2 and 3.0 [W/m/C]. Values beyond 3.0 W/m/C, even up to 3.5 W/m/C, are found as well (see Table 1). Table 1 - Coefficient of thermal conductivity of concrete: typical values. Type of aggregate Thermal conductivity [W/m/C] Quartzite 3.5 Dolomite 3.2 Limestone 2.6 - 3.3 Granite 2.6 - 2.7 Rhyolite 2.2 Basalt 1.9 - 2.2 3.1.1 Effect of the dry density. The influence of dry density go, which mainly depends on the content and the type of aggregate, is correlated with the conductivity lo measured in oven-dried samples. With some exception in concretes with barytes aggregate, the following relationship can be used:  INBDDA Equation.2  (2) where lo is expressed in W/(m C) and the dry density in kg/m3. 3.1.2 Effect of the temperature. Data concerning the influence of the temperature on thermal conductivity are not sufficient to postulate any relationship. However, they agree in indicating a decrease in thermal conductivity with the increasing in temperature. As order of magnitude, it could be assumed a decrease of about 0.04 kcal/(h m C) for a rise in temperature of 10 C. 3.1.3 Effect of the moisture content. The thermal conductivity of concrete increases with increasing of the moisture content. ACI recommends the use of the following rule-of-thumb:  INBDDA Equation.2  (3) with: kw = a factor equal to 6 for light weight concrete and to 9 for normal concrete w = water content [kg/m3] 3.1.4 Effect of the degree of hydration. With the progress of hydration the amount of free capillary water decreases whereas the amount of solid substance increases. Since the conductivity of the solid substance is greater than that of water an increase of thermal conductivity would be expected. On the other hand, the gradual reduction of the free moisture content would give rise to a reduction of the bulk thermal conductivity. Most of the findings given in literature report an increase of the conductivity with the age but a decrease by 20 to 30% has also been found. It is not completely clear which of the two described effects would dominate. This is a reason why in numerical analysis it is common practice to adopt constant values of thermal conductivity. 3.1.5 Measurement method. No standard testing method has been suggested to measure the thermal conductivity of concrete. The main reason is that concrete is a complex system constituted from a solid skeleton and pore space, partially filled with water, which can be closed or interconnected. As far as a thermal gradient is applied, migration of the water through the interconnected pore spaces takes place. This alters the hygrometric conditions of the sample to be tested, the higher the thermal gradient and the longer the duration of the test, the higher the degree of disturbance is. Standard testing methods currently used for measurements on insulating and dry materials, such as the hot guarded plate method, can be applied to the concrete only if the samples are in oven-dried conditions. The alternative way is to use transient methods. The most popular among them is the so-called linear heat source method which consists in introducing inside the material a special thermal probe, heated at constant power, and in measuring the temperature rise during the transient time. Data reduction according to the linear heat source theory allows to determine the thermal conductivity. 3.2 Specific heat. The specific heat of concrete can be determined from the values of the component parts of the mix according to the following relationship:  INBDDA Equation.2  (4) with: gi = fraction of content by mass of the mix components ci = specific heat of the mix components: The range of specific heats of the constituents is: aggregate: ca = 0.17 - 0.22 kcal/kg/C cement: cc = 0.20 kcal/kg/C water: cw = 1 kcal/kg/C Typical values of the specific heat for ordinary concrete vary between 0.20 and 0.27 kcal/kg/C. 3.2.1 Effect of the moisture content. Because of the high specific heat of water there is a strong correlation between the specific heat of a mix and the amount of free water content. The specific heat of concrete increases with the increasing moisture content. Eq. (4) can be applied for calculations. 3.2.2 Effect of the temperature The specific heat increases with the temperature: an increase of about 10% for an increase in temperature from 10C to 66C was found but the relationship should be studied. 3.2.3 Effect of the degree of hydration. The available results dealing with the evolution of specific heat during hydration seem to indicate a decrease during hardening, in agreement with the gradual reduction of free moisture content. The decrease seems to be a linear function of the degree of hydration but the total lowering is still an opened question. Data from literature would indicate a decrease ranging from 1% up to 20%. 3.3 Thermal diffusivity. The main factor of influence on the thermal diffusivity is the type of aggregate. Table 2 gives typical values of thermal diffusivity of concrete for different types of aggregate. As for thermal conductivity, diffusivity will depend on the dry density, moisture content and temperature as well. The dependency follows from the effects of these parameters on conductivity and specific heat. Table 2 - Thermal diffusivity of concrete for different types of aggregate. Coarse aggregate Thermal diffusivity [m2/h] Quartzite 0.0054 Limestone 0.0047 Dolomite 0.0046 Granite 0.0040 Rhyolite 0.0033 Basalt 0.0030 3.3.1 Effect of the dry density Very little data are given about the influence of the dry density on the thermal diffusivity. They seem to indicate a linear increase of the diffusivity of the order of 4% for an increase in dry density of 100 kg/m3. 3.3.2 Effect of the moisture content Since both the conductivity and the specific heat increase with increasing moisture content, the net result on the coefficient of thermal diffusivity is only marginal. Data reported in literature do not seem to indicate a clear dependence of diffusivity on the moisture content. 3.3.3 Effect of the temperature The influence of temperature on the thermal diffusivity depends upon the corresponding influence on both the thermal conductivity and the specific heat. Since with the increasing temperature the conductivity decreases and the specific heat increases, the thermal diffusivity will decrease at a greater rate (up to about 19% in the temperature range from 10C to 90C) than thermal conductivity. 3.3.4 Effect of the degree of hydration. Thermal diffusivity is usually obtained from the relationship with the conductivity and specific heat. Since it is not completely clear the dependency of the thermal conductivity on the degree of hydration, this prevents us from making final statements as regards the dependency of diffusivity on the degree of hydration. 3.3.5 Measurement method. Even more difficult than for conductivity is the problem to measure the coefficient of thermal diffusivity of concrete. The main reason relies upon the fact that the heterogeneous nature of the material requires large samples to be tested. Currently testing methods, such as the laser flash method, works on little samples whose dimensions cannot be representative of a concrete. So far, most of the testing methods to determine the thermal diffusivity consists in measuring the temperature variation in different points of large concrete sample samples, or directly inside a structure, under dynamic thermal conditions. Through a careful data reduction which takes into account the phase shift of the thermal wave in the different points, thermal diffusivity can be estimated. 4. EVALUATION From the analysis of the literature survey it appears clear that many points are still to be studied. Numerical analysis of temperature distribution in actual concrete structures performed with different values for the thermal conductivity have revealed that only marginal differences in the calculated temperatures have to be expected if moisture movements between the structure and the environment are prevented. This condition is met only in massive structures for the greater part of the cross section. Similar analysis carried out with different values of specific heat have put in evidence that the differences on the temperature distribution, especially on the peak temperatures in hardening concrete, are more pronounced. 5. PROPOSAL FOR RESEARCH The experimental research within the IPACS project will be focused to study the evolution of the thermal properties during the hardening phase and the influence of the constituents. The partners involved in the sub-task # 2.3 are ENEL Spa and IBMB. 5.1 Research program at ENEL Spa. ENEL Spa will perform experimental tests during the hardening phase by an innovative transient measuring technique based upon the linear heat source method. The method, the so-called Two Linear and Parallel Probe method (TLPP), allows to measure simultaneously the coefficients of conductivity and thermal diffusivity. The heat specific will be determined according to the relationship with conductivity and diffusivity. The experimental research program will take into account two equal concrete mixes with a different type of the cement. The type of aggregate is crushed limestone and the cement types are 42.5 II/A-L and 32.5 III/A. The following mixing constituents will be used: Aggregate: 1976 kg/m3 Cement: 300 kg/m3 w/c ratio: 0.58 The tests will be carried out in concrete samples under semi-adiabatic conditions. The measurements will start right after the pouring of cylindrical samples, 16 cm in diameter and 32 cm in height, they will be performed at intervals of about 2 hours and will be stopped when no significant changes in the thermal properties will be detected. A complete set of tests should take about 120 hours, for a total of about 60 tests per sample. Data reduction will be devoted to study the correlation between the thermal properties and the degree of hydration. To take account of the temperature effects on the measurements performed during the hardening phase, the same hardened samples will be subjected to TLPP tests inside a controlled climatic chamber in order to determine the changes in conductivity, _905339594 F  Ole PIC LMETA l 2 gTimes New Roman- 2 3oTimes New Roman- 2 @ e 2 ZoSymbol- 2 @= 2 @ ` 2 `Times New Roman- 2 @0 2 @072 2 2 02 : 00125 2 @.` 2  .` & "System-GreekSymbolVectorNumberUࡱ;  FMicrosoft Equation 2.0 DS Equation Equation.2ࡱ; ࡱ; #ԠDCx C  e e el e@o e=0.072e  e0.00125goCompObj ZObjInfo Equation Native _905339733 F5 5 diffusivity and heat specific during a variation of temperature from 10C up to about 90C. 5.2 Research program at IBMB. To study the influence of the constituents of concrete, IBMB will perform tests on hardened concrete samples with quartzite aggregate. Experimental tests will be carried out to measure specific heat and thermal conductivity. Two different cement types will be considered, namely CEM I 32.5 R and CEM III A+B/32.5. Admixtures of fly ash and superplasticizer will be used. Partner: ENEL Spa Experimental research - Inventory form Sub-task No: Sub-task title: 2.3 Thermal propertiesDate:...........................Research topic 1)Thermal conductivity, thermal diffusivity, specific heatType of testTwo-linear-parallel-probe testAim of the testsEvolution of the thermal properties at early ages.Mix propertiesType of cement42.5 II/A-L 32.5 III/A Type of aggregateCrushed limestoneAdmixturesw/c-ratio0.58Initial mix temperature 20CNumber of testsAbout 60Number of samples2Curing conditionsTemperature regime (adia., iso., semi-adia.)Semi-adiabaticMoisture condition (sealed, saturated)SealedModellingBasic materials modelsEngineering modelsc, l, and a as function of the degree of hydrationImplementationCalculation of temperature distribution T(t)Additional information 1) Research topics (see also Baseline Report, Draft 1, 25 July 1997): - Hydration process (heat of hydration, kinetics, microstructure, modeling) - Activation energy - Thermal properties (conductivity, specific heat, diffusivity, thermal dilatation) - Autogenous deformations (shrinkage, swelling) - ................................... Partner: ENEL Spa Experimental research - Inventory form Sub-task No: Sub-task title: 2.3 Thermal propertiesDate:...........................Research topic 1)Thermal conductivity, thermal diffusivity, specific heatType of testTwo-linear-parallel-probe testAim of the testsInfluence of temperature on the thermal properties.Mix propertiesType of cement42.5 II/A-L 32.5 III/A Type of aggregateCrushed limestoneAdmixturesw/c-ratio0.58Initial mix temperature 20CNumber of testsAbout 15Number of samples2Curing conditionsTemperature regime (adia., iso., semi-adia.)IsothermalMoisture condition (sealed, saturated)SealedModellingBasic materials modelsEngineering modelsc, l, and a as function of temperatureImplementationCalculation of temperature distribution T(t)Additional information 1) Research topics (see also Baseline Report, Draft 1, 25 July 1997): - Hydration process (heat of hydration, kinetics, microstructure, modeling) - Activation energy - Thermal properties (conductivity, specific heat, diffusivity, thermal dilatation) - Autogenous deformations (shrinkage, swelling) - ................................... Partner: IBMB (Braunschweig) Experimental research - Inventory form Sub-task No: Sub-task title: 2.3 Thermal propertiesDate:...........................Research topic 1)Specific heat, thermal conductivityType of testTest on hardened concreteAim of the testsNeeded for model of degree of hydrationMix propertiesType of cementCEM I 32.5 R, CEM III A+B / 32.5 Type of aggregateQuartziteAdmixturesFly ash, + superplasticizerw/c-ratio0.5 to 0.6Initial mix temperature Number of testsCuring conditionsTemperature regime (adia., iso., semi-adia.)Moisture condition (sealed, saturated)ModellingBasic materials modelsEngineering modelsc, and l as function of constituentsImplementationfor T(t); field calculationAdditional information 1) Research topics (see also Baseline Report, Draft 1, 25 July 1997): - Hydration process (heat of hydration, kinetics, microstructure, modeling) - Activation energy - Thermal properties (conductivity, specific heat, diffusivity, thermal dilatation) - Autogenous deformations (shrinkage, swelling) - ................................... IPACS / Task #2 / Sub-task # 2.3 pag.  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