The Influence of Negative Temperature son the Heat Conductivity Factor of Road-Building Materials

113 K O M U N I K Á C I E / C O M M U N I C A T I O N S 1 / 2 0 0 1 ● Posudzovanie vozoviek z hľadiska ochrany pred účinkami mrazu je dôležitým prvkom hodnotenia ich konštrukcie. Klimatické podmienky Slovenska spôsobujú počas zimného obdobia niekoľko cyklov premŕzania vozovky a jej podložia. Tento jav je nebezpečný z pohľadu deformácií jej povrchu a straty prevádzkovej spôsobilosti. Z toho dôvodu je pre návrh konštrukcie potrebné poznať teplotechnické charakteristiky materiálov jednotlivých vrstiev. Napriek tomu, že v procese premŕzania sú aktívne charakteristiky v zamrznutom stave, sú merané a hodnotené len v prirodzenom stave. Z toho dôvodu je príspevok zameraný na meranie koeficientu tepelnej vodivosti stavebných materiálov pri zápornej teplote metódou nestacionárneho tepelného toku. Pre analýzu boli vybraté asfaltové zmesi, používané na kryty vozoviek a niektoré druhy zemín podložia.


Fyzikálny princíp metódy
V stavebných materiáloch je teplo šírené takmer výlučne vedením. Z toho dôvodu je fyzikálnym základom metódy jednosmerné šírenie tepla, definované Fourierom: Appraisal of pavement from preservation against frost influence point of view is important point of its structure evaluation. The Slovak climatic conditions originate the same cycles of pavement and subgrade freezing during a winter period. This effect is dangerous from surface deformation and loss of serviceability point of view. Therefore is necessary to know the thermo-technical properties of layers' materials for structure design. The values in nature state are measured and evaluated only, although during freezing process the characteristics in frozen state are active. Therefore an article deals with measuring of a heat conductivity factor of building materials during negative temperature by method of non-stationary thermal flow. The asphalt mixtures used for pavement wearing courses and some type of subgrade soils were selected for analysis.

Introduction
The thermal characteristics of road-building materials are very important for design of pavement from protection against the frost penetration into subgrade point of view. The values in a natural state are measured and evaluated only, although during the freezing process the pavement materials and subgrade are in frozen state. Therefore, for a few years the attention has been paid to the measurement of heat conductivity factor of building materials by negative temperature.
Some standard methods for measuring the heat conductivity factor are used in Slovakia. The method of non-stationary thermal flow [1], which was verified many years in UTC Zilina and was based on many measurements within the framework of research work solution, was selected for measurement of materials in frost phase [2].
From (l) and (2) we can obtain an equation (3) describing a development of a temperature field, from which the heat conductivity factor can be calculated.
The initial conditions Numerical solution of equation (3) is based on principles of a heat conductivity factor measuring [3]. A sample is located to the heat-isolated vessel. A temperature on the bottom site of sample is registered while keeping a constant temperature on the top site. The next initial conditions were determined: The sample is divided to n-layers system with thickness of layer ⌬x. The time interval ⌬t between 10 % and 50 % decrease of initial temperature gradient on the bottom of the sample is calculated. Comparing theoretical and experimental values, the final equation for factor calculation was created: where all symbols are already known.

The border conditions
For measuring of material in a natural state the initial temperature of sample 20 °C and constant temperature on a top of sample 0 °C are used. The mixture of water and ice is used as a medium for assurance of the constant sample surface temperature. The problem arises in the case of measuring the material in the frozen phase. At first the temperature difference between sample and medium must be minimal 10 °C. From a physical point of view is not decisive if the sample is cooled or warmed, but from veľmi dôležité, aby nebola zasiahnutá hranica nulovej teploty. Nasiakavé stavebné materiály nemôžu počas merania prechádzať nulovou izotermou, maximálna teplota vzorky musí byť nižšia ako Ϫ3 °C. Predovšetkým v zeminách dochádza k fázovej premene vody už pri teplotách od Ϫ1 °C do Ϫ3 °C, čo môže mať rozhodujúci vplyv na namerané výsledky. Z tohto dôvodu sa pre meranie používa teplotný rozsah od Ϫ5 °C do Ϫ20 °C.
a latent heat standpoint is very important to not touch zero boundary. The absorptive building materials cannot proceed through zero point during measurement. The maximal temperature of sample must be below Ϫ3 °C. Above all the soils the water phase change occurs in temperatures from Ϫ1 °C to Ϫ3 °C, which has an ultimate influence to measure results. Therefore the temperatures ranging from Ϫ5 °C to Ϫ20 °C were used for measuring.
A very important problem of measurement was a medium for assuring the constant temperature of a sample top. After experiments with different types of non-freezing liquids classic salt water was selected. The 30 % salt solution that secures a permanent liquid state of medium in measuring range was selected. The medium is uninterruptedly mixed for equitable distribution of temperature on top of sample. For the these reasons the next variants were examined: a) temperature of sample and inside of climatic chamber Ϫ5 °C, temperature of medium Ϫ17 °C b) temperature of sample and inside of climatic chamber Ϫ18 °C, temperature of medium Ϫ5 °C The variant a) secures a constant temperature maximum 15 minutes, which is a short time for measuring. Therefore, the next temperatures were selected for routine measurements: -temperature of sample Ϫ18 °C Ϯ 2 °C. -temperature in climatic chamber Ϫ18 °C.
An important condition of measurement is the constant temperature of the whole system. The sample with measuring apparatus is placed for 24 hours into a climatic chamber in which a measurement is realised after this time, too. Warming apparatus localised immediately above a surface of sample ensures the constant temperature of medium. An electronic recorder that registered a temperature of medium during measurement controls the apparatus.

The measuring of the heat capacity
The heat capacity 'c' as a basic input parameter for heat conductivity factor calculation is a determinate for each sample. The calculation of frozen material value is not possible from value of dry natural material. An analogy with calculation the heat capacity of wet material during positive temperature is not confirmed by research.
The heat capacity is measured on the base of physical definition (6). The transmitted heat is determined by calorimetric method, which assumes the heat-isolated calorimeter with sample and medium. The specific heat capacity is determines by calorimetric equation after achievement of temperatures' balance.
where: m -mass of the sample, kg ⌬q -transmitted heat to sample, J ⌬T -change of temperature for transmitted heat, K.

The measuring of the heat conductivity factor
The article treats the heat conductivity factor of selected road building materials. The first material is asphalt mixtures, second is soil of pavement subgrade. Other materials are analysed in frame of the grant research project in this time.

Asphalt mixtures
The asphalt mixtures containing the modified asphalt (sample 1, 2) and classic asphalt (sample 3) were selected for measuring. The mixtures with modified asphalt are using more in this time than with classic asphalt for better deformation characteristics. The comparison of thermal-technical properties is one from aims of recent research activities of our department. The basic composition of measured mixtures is described in Table 1.
The measurements have sustained irrelevant differences between thermal-technical characteristics of dry and frozen materials. In terms of physical essence of mixture it was determined that it is not necessary to regard the temperature influence on the heat conductivity factor in winter conditions.
In terms of the used binder the measurements showed that modified asphalt doesn't essentially influence the heat conductivity factor of asphalt mixture. The maximal difference is about 10 %. The comparison of measured and standard values indicates that standard values for mixtures AKM is necessary to be more accurate. Presented values of heat conductivity factor 1.40 W.m Ϫ1 .K Ϫ1 were exhibited for mixture number 1 only. The values of other test mixtures exceeded 25 -80 %, which are not negligible values. The results are presented in Table 2 and on Fig. 1.
The heat conductivity factor of asphalt mixtures Table 2 Zmes

The soils of subgrade
The four types of subgrade soil were selected for analysis. The option includes the most used types of soils from different localities in Slovakia. The basic parameters of materials are presented in Table 3.
The values of observed characteristics were determinated on samples in the dry phase, in phase of natural moisture and after freezing. Obtained values of the heat capacity and the heat conductivity factors are presented in Table  4 and on Fig. 2. The soils 1-3 were very similar, coherent soils; soil 4 was non-coherent gravel loam with different grading. It follows the different thermal-technical properties, too.
The values of the heat conductivity factor of frozen samples are mentioned approximately about 300 -400 % upper than values of non-frozen samples. The measurements have showed that the value of the heat conductivity factor of frozen soils approaches the value of moist soil but doesn't exceed it. The fact is different only in soil 4, which is a little absorptive. The values for dry and frozen sample are very close.

The total evaluation
The obtained values of the heat conductivity factors advert to the reserve of the appraisal methodology of the pavement structure from thermal resistance point of view. The standard values of the heat conductivity factor don't respond to the most unfavourable conditions in the road structure, and they are, many times, underestimated. The determined values respond designed values in full-scale for subgrade soils only, for asphalt mixtures considerable differences were found. The pavement's status of road network in Slovakia support this sentences, mainly after wintertime.

Conclusions
The comparison of measured and design values conforms their exact estimate for subgrade soil. The thermal-technical characteristics are significant parameters for subgrade soils. In consequence of this the road deformations originate and road serviceability and performance is lost. The attention was devoted to the relation between the heat conductivity factor and moisture. The increase of the heat conductivity factor was confirmed sometimes more than 800 %. Present results verify the weight of moisture influence of subgrade soil on the deformations incipient in wintertime and springtime.
The described method of evaluation of the heat conductivity factor method is economically and timely undemanding. The method determines conditions that correspond to real conditions in pavement structure. In this time the measurements of the heat conductivity factor of sub-base materials are realised. Attention is also paid to the determination of relation between specific heat capacity and a temperature of the sample. That relation can have an influence to the immediate value of coefficient at a definite negative temperature. We do not assume an influence that will have an expressive effect to present values. The thermal-technical properties of soils Table 4 Súčiniteľ tepelnej vodivosti zemín The heat conductivity factor of solls