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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" <ar) 


U IlilC d CU. 


1 >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 




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U ur I S. S. FORM 3