PRACTICAL APPLICATION OF THE MICROWAVE OVEN IN THE GEOTECHNICAL LABORATORY

Abstract

K e y w o r d s : Clay; Conventional oven; Drying time; Gravel; Microwave oven; Moisture content; Sand; Specimen mass; Specimens number. 2/2019 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 91 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 2/2019 d o i : 1 0 . 2 1 3 0 7 / A C E E 2 0 1 9 0 2 6 M . J a s t r z ę b s k a All publications ranging from the oldest (e.g. Creelman and Vaughan 1966), to the most recent (e.g. Jalilian et al. 2017) focus on two basic issues related to microwave drying. The former is the compatibility of moisture measurements between two types of ovens: convection drying performed in conventional ovens and microwave drying carried out in commonly available microwave ovens. Convection drying is an air-to-air method in which, in the case of ovens used in a geotechnical laboratory, hot dry air is the factor that transfers the heat to the soil and drains the moisture away. The air flows around the specimen in a natural or forced way. In turn, microwave drying is associated with electromagnetic radiation, which induces vibrations of water molecules in the specimen forming an internal source of heat. Despite the different drying mechanisms researchers have always confirmed the consistency of results obtained by two methods. Among others, Routledge and Sabey (1976) and Balscio (1992) showed very high compatibility using the coefficient of determination that is R2 > 0.980; which was determined by Gilbert (1991) as R2 > 0.994; and by the author R2 = 0.999. Moreover the results of literature study proved good correlation between results obtained from both methods as the variation in the majority of results is less than 1% (e.g. Gilbert 1988; Hagerty at al. 1990b; Chung and Ho 2008). The second issue is the development of the proper microwave drying procedure which is greatly influenced by: soil type, specimen mass, number of specimens dried in a microwave oven and the way they are arranged (inside or around the plate), drying time, microwave power, container material type (containing the tested soil). The problem with developing a clear procedure results from the basic difference in the nature of the drying processes. Conventional drying means that the heat is delivered from outside through the surface of the material which is why the surface has the highest temperature. On the other hand, microwave drying is characterized by the fact that the microwaves penetrate the interior of the material heating up the entire volume from the inside. All the factors mentioned above affect the quality and efficiency of microwave drying. The existence of so many types of soils with diversified structure and mineralogical composition makes it necessary to create extensive database of results which would enable the development of universal procedures that have not yet been created or standarized in the majority of countries. For this reason, the author of this paper completed a comprehensive research plan for three selected soils (FSa – SP, saGr – GW and siCl – CH) which includes the influence of moisture on the drying process, specimen mass, number of dried specimens and their distribution in the microwave for the duration of drying. The results of the microwave drying were confronted with the results of the conventional method. The choice of soils was determined by the will to compare various grain sizes of soils common in the south of Poland. In addition, siCl (CH) is a swelling soil and as far as author’s knowledge is concerned, such soils have not yet been the subject of research in the determination of moisture by microwaves. The presented research cycle will be continued in the future in order to determine the impact of microwave drying on the soil structure and further on its mechanical parameters. 2. LITERATURE REVIEW The first attempts of defining the soil moisture content by using microwave oven were undertaken in the 1960s by Creelman and Vaughan (1966), Algee, Callaghan, and Creelman (1969), Ryley (1969). Later the research results were published by Routledge and Sabey (1976), Carter and Bentley (1986), Hagerty et al. (1990a, b), Gilbert (1988, 1991), Balscio (1992), Gaspard (2002), Chung and Ho (2006). However, the latest research was published between 2012 and 2017 by Berney, Kyzar, and Oyelami (2012), Daod (2012), Cormick (2015), Kramarenko et al. (2015, 2016), Jalilian, Moghaddam, and Tagizadeh (2017). During over 60 years a number of research were published concerning the guidelines for determining moisture content in terms of soil type, specimen mass, number of specimens dried in a microwave oven at the same time and their distribution (inside or around the perimeter of the plate), drying time, type of the microwave oven, the type of container material. Everyone unanimously confirmed the effectiveness of the laboratory microwave drying and compatibility of its results with the results of standard drying. Among others, Gaspard (2002), who tested 4 different drying devices, proved the effectiveness of a microwave oven regarding the economical aspect of the method (saving time and energy) and its accuracy. Similarly, Berney et al. (2012) in an extensive report proved the above theory first and then recommended a compatible method for use in situ, i.e. the soil density gauge and the gas stove with pan technique. For the purpose of their research 9 devices with different operating mechanisms were tested. 92 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 2/2019 P R A C T I C A L A P P L I C AT I O N O F T H E M I C R O WAV E O V E N I N T H E G E O T E C H N I C A L L A B O R AT O R Y Eventually they did not recommend the field microwave oven powered by batteries due to the loss of power during drying. In turn, Gilbert (1988) proposed a special place for automatic microwave drying. His work is extremely valuable due to a number of comments regarding the safe use of microwave for the soil drying. Among others, as one of the few, Gilbert (1988) wrote that “undisturbed, low permeability soils and rock, gravel, shale and even brittle undisturbed clay fragments subject to microwave heating may explode”. The author of the paper also encountered a similar phenomenon and the instructions on how to proceed in this situation will be listed later in the following sections of the paper. Gaspard (2002) also tested the computer-controlled position (CMWO – Computer Controlled Microwave Oven). He confirmed the effectiveness of the above and Standard Microwave Oven (SMWO) methods but due to high costs he recommended the latter. The author shares the opinion of Gaspard that the economical factor is fundamental for the use of microwave radiation. Time, simplicity and availability of microwave ovens are the most important criteria. Although the microwave soil drying has been the subject of research for many years, so far only in the United States (ASTM D4643-00; ASTM D4643-08), Australia (AS 1289.2.1.4-2015; AS 1289.0:2014) and France (NF P 94-049-1), standardized guidelines of different level of detail have been published regarding the determination of soil moisture in microwave ovens. In addition, a number of national reports created as a result of research can be found, e.g. in Canada (ATT 15/96) or in Hong Kong (Chung and Ho 2008). However, it is worth noting, that the indicated guidelines do not take into account all variants related to soil heterogeneity and research methods. The selection of the specific heating power and specimen mass is still generally an arbitrary decision of the researcher. In literature sources, the power of the microwave oven adopted by the authors ranges between 700 W (ASTM D4643-00; Chung and Ho 2008; Gaspard 2002) and 1700 W (Chung and Ho 2008). The majority of researchers chose 800 W. In this respect, the author also suggest the power of 800 W. However, for some soils it is believed that the power should be reduced due to the sintering of the tested specimens. Thus, they share the opinion of Jalilian, Moghaddam and Tagizadeh (2017), who based on a series of tests carried out at different powers: 90-180-360-600 and 900 W in heating cycles of 5 and 10 min on specimens of 50 g, concluded that the optimal system is 600 W and 10 min of drying. With regard to the mass of the specimen, it is even more difficult to find a specific rules. Generally, each researcher formulates their own guidelines. Some authors agreed that too small amount of specimen should not be used due to the possibility of its overheating (ASTM D4643-00; Chung and Ho 2008; Gołębiewska, Połoński, and Witkowski 2003). In turn, the author of this paper believes that both too small and too large specimens in the so-called compacted mass are not recommended because there is a risk of scorching. Another controversial issue is the recommended drying of the specimens. Considering the fact that it depends on the size of the specimen (that is different in each guidelines), the recommendations cannot be unambiguously determined. In literature sources it ranges between 2 min (Grymowicz and Jastrzębska 2015) and 50 min (Chung and Ho 2008). In addition, it is still necessary to determine the frequency in which the specimen will be weighed during the entire drying process. In comparison, according to the latest ASTM 2008 guidelines, constant heating is recommended based on the time determined by a nomogram. On the other hand, other scientists, based on their own research, suggested drying at different intervals. Thus, the authors formulated their own guidelines (Appendix B). It is worth noting that the Cormick’s doctoral thesis (2015) is a good summary and description of previous works related to drying of soil using microwaves. Based on work of other researchers and his own, Cormick (2015) approved soil drying for all soil types except for soils with organic content above 10% and bentonites with mois