.'.*~ ‘g 4... . i‘ g t El % E g l I 100 . , ~ h(DON w‘ ““_ THE CO£FHCIENTS Of THERMAL conoucnvm non ”mam mum“: AS 951mm» w W GUMDED HOT PLATE Mhtflacbcgmotfl. .-.S WSTATICOLLM VV'TIkthwmmflu 1949 IIIIIIIIIIIIIIIIIIIIIIIIIIIII 3 1293 01093 9753 PLACE IN RETURN Box to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. me campus-p14 THE COEFFICIENTS OF THERMAL CONDUCTIVITY FOR PAVEMENT MATERIALS AS DETERMINED BY THE GUARDED HOT PLATE A Thesis Submitted to The Faculty of MICHIGAN STATE COLLEGE ' Of AGRICULTURE AND APPLIED SCIENCE 5y W . T . Egnborough Candidate for the Degree of Bachelor of Science dune I949 THESIS C./ ACKNOWLEDG MENT The author would like to thank Mr. James T. Anderson of the Mechanical Engineering Department for his advice and assistance in the operation of the guarded hot plate, and Mr. Lawrence D. Childs of the Michigan State Highway Department for his advice and assistance in out-lining the initial pro- blem. TABLE OF CSNTENTS Page No. Introduction ----------- - ............................. I Description of Form--------------- ................... 2 Preparation of Concrete Specimens ....... ------------- 4 Preparation 0f ASphalt Specimens --------------- ---_--.5 Description of Apparatus ----------------------------- 7 Principle of Hot Plate ............................... 7 Principle of Cold Plates--—----------- ............... 8 Principle of Thermocouples --------------------------- 9 Typical Test Procedure ------------------------------- I4 Example of Data and Computations --------------------- 16 Mean Temperature Tests ------------------------------- I8 Tabulated Results ------------------------------------ 19 Mean Temperature vs. Coefficient Curves -------------- 20 Problems Encountered ----------- --------- ............. 21 Factors Affecting Results -------------------- --------23 Bibliography ----------------------------------------- 25 TABLE OF ILLUSTRATIONS Page No. Specimen Form --------------------------------------- - 3 Guarded Hot Plate---- --------------- - ................ II Hot Plate ----- - ----- ---------—-- ..................... II A.C. Power Circuit ----------------------------- 4 ------ I2 Thermocouples ------------------- - .................... I} Direct Current Circuit ------------------------------- 13 INTRODUCTION The problem of finding the coefficient of thermal conductivity for different pavement materials was origin- ally suggested by the Michigan State Highway Department Re- search Laboratory. I The coefficient of thermal conductivi- ty of a material is the number of B.T.U. that will be trans- mitted in one hour through one square foot of the material, one inch in thickness, where the Opposite faces are kept at a temperature difference of 1° F. The information was de- sired in conjunction with the experiments being conducted on the heating of pavements for the removal of ice and snow.' The necessary equipment for determining the coeffi- cient of thermal conductivity of a material was obtained from the Mechanical Engineering Department in the form of the guarded hot plate. The author decided to test the three most important types of pavement materials that are used by the Michigan State Highway Department. The materials tested were the standard concrete mix, the air-entrained concrete mix, and the asphalt cement mix. Each type of material was prOpor- tioned to meet the Michigan State Highway Department speci- fications. 51 Hausmann, Evich & Slack, Edgar P., Physics. D. Van Nostrand 00., I935, P.319. DESCRIPTION OF FORM The guarded hot plate takes two specimens for each test with the dimensions of each specimen being 12 inches by I2 inches by I inch. A steel form was constructed to hold two Specimens at a time. (See illustration, page_2_). This construction made it possible to place the two Specimens need- ed for each test from the same mixture, insuring consistent density. The bottom of the form was I/4 inch plate steel, while the sides were steel bar stock 1/2 inch by I inch. The bar stock was cut to the prOper length and placed on the steel plate. It was then drilled to pass the 3/8 inch stove bolts while the bottom plate was drilled and tapped to take the 3/8 inch stove bolts. The bolts were Spaced as seen in the draw- ing. This construction enabled the displacement of the sides for removing the Specimens and cleaning the form. The steel construction insured accurate dimensions, a smooth finish, and a guide for striking off the tops of the Specimens. Eco... Zuznkun Wm PREPARATION OF CONCRETE SPECIEENS Since the required specimens are 1/6 of a cubic foot, .2 of a cubic foot of the standard concrete and the air-en- trained concrete was mixed at one time. The proportions for the concrete mixes used are as follows: Standard Concrete Mix Proportions Volume Cement F.A. C.A. ‘ Water (cu.ft.) (1b.) (1b.) (1b.) (1b.) Chart 4.909 94.00 232.00 373.00 41.60 Batch 0.200 3.84 9.46 15.2I 1.70 Air-Entrained Concrete Mix Proportions Volume Cement F.A. C.A. Water (cu.ft.) (1b.) (1b.) (1b.) (1b.) Chart 4.909 94.00 2I0.00 375.00 48.60 Batch 0.200 3.83 8.55 I5.20 1.98 The various materials which make up each concrete mix were accurately weighed and placed in a mixing pan. The con- crete was mixed by hand and transferred to the oiled form. After placing the concrete in the form, the material was set- tled and entrapped air removed by pounding on the sides of the form. Then the tOps of the concrete Specimens were struck off even with the form and finished with a hand trowel. The specimens were left in the form for two days after placing, to gain sufficient strength for removal without damage. Dur- ing this time, the Specimens were kept covered with wet burlap sacks. At the end of two days, the Specimens were removed from the form and placed in the curing room for a minimum of fourteen days. After fourteen days, the concrete Specimens were re- moved from the curing room and placed in the drying oven for at least three days, or when ready for testing. Just prior to testing, the specimens were removed from the drying oven and the tops were ground smooth with a carburundum block to insure a good fit for all Sides in the guarded hot plate. PREPARATION OF ASPHALT SPECIMENS The asphalt cement mixture was supplied by the Mich- igan State Highway Department. The mix was made at Ann Arbor, Michigan and sent to the East Lansing laboratory. The asphalt mixture was Just as it came from the mixing plant in Ann Ar- bor, and compression in the form was necessary to meet the Michigan State Highway Department specifications of I40 pounds per cubic foot. The asphalt cement supplied is designated in the Mich- igan State Highway Department as 26A bituminous capping mix- ture. The bitumen is asphalt cement of 85 tOIOO penetra- tion with components of the mix as follows: 5.5 % Bitumen. 5.5 % Fine Aggregate passing a #200 sieve. 55.0 % Aggregate retained on a #10 Seive. The remainder of the aggregate between these limitl. The aSphalt cement mix was compressed in the form in the following manner. The mix was heated in a pan placed over a large gas burner to a temperature of 260 degrees P., as measured by a grarded thermometer. When the desired tem- perature was reached, the mix was Spread in the oiled form and compressed in layers by pounding with a hand sledge. This was repeated until the form was full and level. In this case, the steel form furnished the necessary strength for the compression of the material. The Specimens were re- moved from the form after cooling and were ready for use. The smoothness obtained on the surface of the concrete Spe- cimens was impossible to obtain on those of the asphalt. 'DESCRIPTICN 0F APPARATUS The main unit of the testing apparatus is the guarded hot plate which is installed in a cork-insulated box. The guarded hot plate is composed of the hot plate, Specimens, cold plates, and thermocouple leads. (See illustration, page_l;_). All of the other units are supplementary to the guarded hot plate. There are three main supplementary units. These are the direct current circuit for measuring the poten- tial in the thermocouples for determination of Specimen sur- face temperatures, the pump and water supplies for transfer- ring heat from the cold plates, and the alternating current supply with controls for the hot plate. PRINCIPLE 0F HOT PLATE The hot plate is actually composed of two separate u- nits, the central heater and the guard heater. (Seeillus- tration, page_;;_). Each unit is constructed of cOpper plates with heating coils inside. The central heater has a total surface area in contact with the specimens of .889 square feet and is metered while the guard heater is not. The 1/8 inch air space between the central heater and the guard heat- er is spanned by several differential thermocouple wires. Thus, the differential in temperature of the two heating u- nits can be measured. The power for the heating units comes from the regular IIO-volt alternating current source. (See illustration, page I2 ). The current is first put through a voltage regu- T lator since there are fluctuations in the voltage from the source. The power is controlled by a Slide wire rheostat be- fore going through the heating units which are wired in par- allel. The power to the heating units is further controlled by slide wire rheostats with an additional fine adjustment slide wire rheostat on the guard heater. After the desired initial power input has been regu- lated, the differential of temperature of the two heating u- nits can be measured with the differential thermocouple as observed on the galvanometer in the circuit. Then the slide wire rheostats on the units can be adjusted to obtain the de- sired zero value of differential temperature, since the tem- perature will vary as the power input. When the differential of temperature of the two units is zero, theoretically, there will be no transfer of heat from one unit to the other. Therefore, all the heat from the metered central unit must pass through the Specimens. Thus the quantity of heat put through the Specimens can be com- puted from the metered power input for the final computations. The insulated box enclosing the main unit and the thickness of the Specimens, being prOportioned to the width of the guard ring, are insurance that there will be no trans- fer of heat to or from the main unit and that the heat trans- fer lines through the specimens will be straight. PRINCIPLE OF COLD PLATES The cold plates are constructed of copper sheets in the form of a box, leaving the center empty. Connections 8 are fixed to the outside so water can be pumped through the cold plates to remove the heat transferred through the speci- mens. The water was pumped from a barrel at room temperature and from a water cooler containing ice in alternate tests by a centrifugal pump. PRINCIPLE 0F THEPMOCOUPIES There are two thermocouple leads between each speci- men and adjoining plate, totaling eight thermocouples. The thermocouples have one connected end at the main unit and one at the reference Junction. (See illustration, page_;2_). A sheet of blotting paper the Size of the plates was placed between each specimen and its adjoining plate. The main unit thermocouple connections were placed between the blotting paper and the Specimens so their recorded tempera- ture would be that of the Specimen surface and not the adja- cent copper plate. The reference junction connections were placed in a thermos jug of cracked ice and water. A direct current circuit is used for measuring the potential set up in the thermocouple wire by the differential in temperature. This direct current circuit is composed of a two-volt battery, a standard cell, a galvanometer, and a resistance box. (See illustration, page_;2_). The output of the two-volt battery is regulated to one volt with the standard cell by entering a resistance in the circuit as observed on the galvanometer. The one-volt circuit is then connected to the thermocouples and a measured resistance put in the circuit to decrease the one volt until it is equal to the thermocouple potential as observed on the galvanometer in the circuit. A table, published by Leeds & Northrup Company, "Copper vs. Constantan Thermocouple with Reference Junction at 32 degrees Fahrenheit", makes it possi- ble to obtain the temperature in degrees Fahrenheit from the amount of resistance necessary in reference to one volt. IO THERMOCOUPLE LEADS — —— u—q P My MP W PM: .( 2|- 2 < .I u( m J a 551 z a o ‘ - .1 3|- 3 3 0 (L0 a 0 U ”I m u __ -‘ _ fl r I4 GUARDED HOT PLATE GUARD HEATER r CENTRAL HEATER T HOT PL ATE II IIO A.C. CONSTANT VOLTAGE REGULATOR POWER RHEOSTAT CENTRAL RHEOSTAT W ”LI L——./vvw GUARD RHEOSTATS t__ GUARD COIL ’VVVV f— wvw AMMETER CENTRAL COIL I | WA, L____[ VOLTMETER A.C. POWER CIRCUIT I2 >1ukk<0 uU2(Pm_num 50> m 50> _ III— ._ \/>\ hi T 32523”. .33 3522.. 9.325» muaaou I g zfizfimzou I.. l muhuzoz<>u