ect i^f Heat on the Ci?il Esgineeriisg Bo S, ' 1 1 n',.r.,.wo.tc: y/iMi't.'i.jrX-'j: 1^ 4. ■ 1 Q^-^ .-^^v-^-' ^^^^^^^^ If Digitized by the Internet Archive in 2013 http://archive.org/details/effectofheatonstOObeck EFFECT OF HEAT ON THE STRENGTH OF CEMENT MORTAR BY Herbert Clinton Beck THESIS P'OR THE degrep: of Bachelor of Scienci: IN CIVIL ENGINEERING IN THE COLLEGE OF ENGINEERING UNIVERSITY OF ILLINOIS 1911 UmVERSITY OF ILLINOIS May 25, 1911 I recommend that the thesis prepared under my supervis- ion by HERBERT CLIFTON BECK entitled Effect of Heat on tne Strength of Cement Mortar be approved as fulfilling this part of tne require- ments for the degree of Bachelor of Science in Civil Engineering ► Instructor in Civil Engineering. Recommendation approved: Head of the Department of civil Engineering, Ei^TEOT OF IlEAT ON TIlE STRENGTH OF CSiO]NT MORTAR Table of Contents. Page. Introduction, l. Conditions of Tests, 2. Results: Discussion and Interpretation, . 4-. Tables and Plates, 7-15. Conclusion, 6. 1. EFFECT OF HEAT ON THE STRENGTH OP CEl^OT MORTAR. Intro ducr. lon. ¥lth the present extensive use of mortar and concrete as building materials and as flreprooflng materials their action un- der high temperature has hecome of much Interest. If a concrete structure he subjected to the Intense heat of a fire It Is Impera- tive to Know to what extent Its walls and floors have been damaged. The object of the tests herln described was to discover If possible the Initial temperature at v/hlch mortar begins to lose its strength and also to discover whether or not there Is a defi- nite relation between loss of strength and temperati:re. as a matter of convenience In heating '.ne tests were made on small cubes of cement mortar rather tnan on larger ones of concrete. When the tests were started practlcailly th' only ex- periments along this line were those performed by Mr. I. H. Wool- son In 1907. These experiments are of much value but leave som^ethlng to be desired, since temperatures below 700° F. (4-00° 0. ) are not considered and the variation of age of the test pieces Is small. Since the beginning of the tests described in the follov;- Ing page several articles on the subject have been published. The most notable of these are; a paper entitled, "Some Thermal Proper- ties of Concrete," by Mr. Chas. L. Norton of the Massachusetts Institute of Technology (Proceedings of the National Association of Cement Users), and papers by l/ir. ifoolson before the American Society for Testing Materials. Further reference to these exper- iments and comparison with the results there obtained will be made in the following discussion. 2, The experiments herein described consist of series of tensile and compressive tests on "briquettes and cubes respect- ively. iSeveral sets of each v/ere m.ade. Half of each set was heated before testing and half were tested without heating. The age of the test pieces and the degree of temperature ivere varied In the different tests. It was thought probable that there would be a somewhat sudden drop In the strength of the mortar at the temperature at which the v;ater of hydration was given off but the results do not seem to justify this belief. Conditions of T ests. The worKlng conditions for the tests here described will now be given In detail so that in comparing the results ?/lth those of other experiments the personal element may be reduced to a mini- mum. The cement used, Chicago AA Portland, was of good quali- ty, fine, and of moderate rate of setting. It was of uniform grade as shown by Vlcat tests of plasticity. The sand was natural banK deposit commonly Known as Wabash sand. It was screened to a maximum size of one tenth of an Inch. Plate I shows a sieve anal- ysis curve for the sand. The test pieces consisted of ordinary briquettes and 3-ln. cubes, made of mortar or normal plasticity. The briquettes were compacted by pressure of the fingers, but for the cubes a wooden rammer was used. The test pieces v/ere Kept In the molds under damp cloths for twenty- four hours and were then placed in wnte^.v, ThPi .spnnl- mens were dried in air at room temperature for one weeK before testing. In the tests the specimens were heated in a gas drying oven and for the higher temperatures in a gas Kiln. In no case was the flame allov/ed to touch the pieces. The temperature was measured with a mercury pressure thermometer and was raised at rate of about 100 C. in ten minutes. The maximum temperature was maintained for one hour and the specimens were then cooled slowly to room temperature, after which they were tested for strength, either at once or after standing for some time. The compression pieces were bedded in plaster of Paris to insure a plane bea^^ing surface. The load was applied with a Riehle testing machine of one hundred thousand pounds capacity. The load was centered by a ball bearing head and the specimen was compressed at the rate of l/l6 inch per minute. Each set of cubes consisted of six specimens and each set of briquettes of ten specimens. Interpi^tatlo n and Pis cu ssl on of Tests. One of the points V7hlch these experiments "bring out Is the lacK of uniformity of results of tests on heated specimens, or the unreliability as regards strength of mortar which has been heated to a high temperature. This effect of heat Is shown In both the tension and the compression tests but appears to be the greater In tension. The variation of the Individual specimens from the mean of the set Is much greater tn the heated than In the unheated sets. The value of mortar or concrete as a fire-proof building material therefore probably lies In the low thermal con- ductivity of the material rather than in Its frequently supposed Immunity from loss of strength by fire. Prof. iToolson's tests show no reduction In strength for a temperature of less than 750 F. (4-00 c). The experiments performed by the writer shovi a reduction for lov7 temperatures for ages of four weeKs or under, although above this age the loss in strength is not great. Prof, woolson's experiments on limestone cubes showed that the cubes disintegrated on standing after heating The tests described In this thesis, performed on 1 - 3 mortar vary somewhat from this rule. For low temperatures there was a gain in strength on standing after heating, the specimens being weaKest while still warm. For higher temperatures the specimens were wealc while warm, gained slightly In strength immediately after cooling, and then gradually lost strength with increasing age. For cubes over twenty-eight days old there was little reduction of strength for temperatures under 300° 0. Above this age the strength in some Instances was increased by application of 5. heat at low temperature. For ages over twenty-eight days and temperature between 300° and 500° C. the loss In strength was very marKed, the reduction being as high as 20 per cent In some cases. It Is probable that above forty-two days an Increase In the ago v^ould effect the results very little, while an Increase In temper- ature over 500 0. would cause a marKed reduction in strength. In no case did the heating cause cracKs In the specimens or otherwise change their appearance. When heated to temperatures above 300 C, hov/ever, the specimens when strucK together gave out a metallic sound llKe that made by well burned brlcK. One Interest- ing feature of the experiments was the apparent change In Internal structure of the heated cubes. Instead of being somewhat crystal- line as In an unheated specimen, the Interior of the heated cubes seemed to have disintegrated, leaving a surface from which granular particles could be crumbled as from rotten sandstone. It Is sug- gested that this may account for the loss of strength, the varying amounts of thermal expansion of the components of the mortar des- troying the bond between the unit particles. Con clusion^ In Interpreting the results of these experiments it must be remembered that the tests are of necessity too few in number and cover too limited a range of temperatures to prove definitely the conclusions given above. The short time of heating also may give results from which wrong conclusions might be drawn. However, the results may safely be taKen as indicators of the probable action of mortar or concrete when subjected to high temperatures. I TABLE I. COi^iPKESSIVE STRMGTIi OF 3-IN. CUBES - HEATED AW UN- HEATED. 1-3 Portland Cement Mortar. 1 — ■ Time Heated Cubes , Unheated Cubes Set ITO. Age Before Heating days Between Heating and Testing days Temp. Degrees Cent . Ultimate S trength It . per sq.ln. Average Strength for Set Vo . per sq. In. Ult Imate trength It), per sq , In. Average s t re ng t h for Get It). per sq . In. 1 m- 1 150 3060 2990 31^4-0 3I6O 3200 354-0 3290 T "7 c r\ 3350 2 15 1 185 3300 35 60 3490 3^50 3540 2890 3280 3230 3 15 1 350 M-lOO 4-14-0 3990 4080 4440 4970 4700 1 ( "7 r\ 4700 4- 21 1 100 4-010 4-150 34-^0 3870 4070 4-4-30 4160 4220 5 21 1 150 3430 3670 367c 3590 4710 4540 H-^O u 6 2S 1 150 4-600 4-540 4-4-40 4520 5400 5240 5050 7 n-2 1 150 4970 5370 4810 5050 4530 4830 47 60 4700 8 4-2 1 350 5260 5030 38 60 4730 5570 49 60 5050 5200 9 42 1 500 3090 3000 3740 3280 4980 5750 5450 5400 TABLE 2. C0.\iPRE8SIVE STRENGTH OP 3-IN. CUBES HEATED. 1-3 Portland Cement Mortar. c c m p r e s s 1 V ? 3 Strength 1/2 day after Heating 7 days after heating Set No. Age TThen Heated Temp. Degrees Cent . Ultimate Strength Average Strength for Set Ultimate Strength Average 8 trength for Set days lb .per sq. In. lb. per sq.ln. lb. per sq. In. lb. per sq.ln. 1 30 150 4110 3860 4080 4010 4600 4530 43 60 4500 2 44 150 4460 45 60 4920 4650 53 60 4980 5310 5220 5 27 350 3440 3780 3510 3570 3 600 37^ 2960 3^50 41 500 4 650 4470 4080 4210 4090 3940 4050 4030 TABLE 3. COiAPRESSIVE STRENGTH OF 3-IN. CUBES. HEATED' AND UimEATED. 1-3 Portland Cement Mortar. Time Temp. Heated Cuhes. Unheated Cubes. ' Set NO. Age Before Heating days Between Heating and Testing days Degrees C en t . Ultimate Strength It), per sq. In. Average Strength for Set liD.per sq. In. Ult Imate s trength lb. per sq.ln. Average Strength for Set lb. per sq. In. 1 13 1/2 100 2170 2850 3190 2730 3660 3 680 3430 3590 2 1/2 150 41 60 3900 4-24-0 4-100 5260 5310 5 4-80 5350 ^Heated cubes tested before cooling. 10. TABLE 4-. TENSILE STRENGTH OF STANDARD BRIQUETTES - HEATED AITO UNHEATED. 1-3 Portland Cement Mortar. Set riu • Age J IC II Tested U. djr T emp * cryi 00 Cent . S t r e n g t h Per cent T . "I r"i IjU LLl Strength Q f" i 10 V XL). ptJi sq.ln. 1 "K "n ra'io X U . CI sq.ln. 1 14- 50 4-4-1 4-40 0.0 2 14- 75 4-12 44-0 6.4 3 14- 100 29s 388 23.2 4- 14- 100 309 40 6 23. 8 5 14 150 278 376 26.0 1 28 50 276 289 ^.5 2 28 75 305 432 29.4 3 28 100 301 362 16.8 28 100 356 -12.6 • 5 28 150 413 409 0.0 6 28 150 376 40 6 7.4 01^ 28 100 4-14- 321 -29.0 02^ 28 100 392 336 -16.7 Ottawa sand. N.B. Each value given Is the mean or five tests. u. or I. 8. s. roHM 3 u- or I. 8. %. rOHM 3 0> Of L'- OF I. 8. S. rONM 3 U. Of- I. ti. S. FOHM a U ur I S. S. FORM 3