Indian Journal of Engineering & Materials SciencesVol. 5, February 1998, pp.43-48

Compressive strength and tensile strength of rocks atsub-zero temperature

R D Dwivedi, P K Singh, TN Singh & D P Singh

Centre of Advanced Study, Department of Mining Engineering, Banaras Hindu University,Varanasi 221005, India

Received 21 November 1996; accepted 2 June 1997

The present paper deals with the uniaxial compressive and tensile behaviour of Chunar sandstoneand Makrana marble at sub-zero temperature ( O°Cto -20°C) in dry and wet conditions. It was found thatuniaxial compressive and tensile strengths of these rocks increased with decrease in temperature belowO°e. Both strengths in general and tensile strengths in particular of wet rocks increased more rapidly incomparison to dry rocks.

Tunnels and underground caverns are integral partsof any hydel power projects. Some ofIndia's hydelpower projects are located in Himalayan regionwhere temperature below O°C is quite common.Also defence deployment in cold mountaneousregions, i.e., Jammu & Kashmir, HimachalPradesh, some part of Utter Pradesh and North -East part of India, require large excavations forvarious purposes such as underground air-bases,underground missile and ammunition stations,deep trenches for protection against enemy.

On the other hand, availability of rich oredeposits in Himalaya and gradual depletion of orereserves in plane area is compelling our country todo mining in Himalayan region to meet the demandof mineral resources.

In developed countries, scarcity of land in planeareas are forcing those countries to considerconstruction of laboratories, store houses, airbases, nuclear disposal chambers. in hilly areas(sub-zero temperature region). Japan is consideringthe construction of underground chambers for thestorage of liquified natural gas due to problem ofland'.

The basic knowledge of physico-mechanicalproperties of rock, in sub-zero temperature range,is essential for efficient rock excavation andassessing the stability of the tunnels and rockcaverns in cold regions.

Out of numerous physico-mechanical propertiesof rocks, the knowledge of strength properties(compressive, tensile and shear strength) are more

important for safe design of rock structures.Therefore, the present investigation is carried outto determine the uniaxial compressive strength andtensile strength of Chunar sandstone and Makranamarble at sub-zero temperatures and to observevariation in rock strength with temperature (belowO°C ).

The Chunar sandstone and Makrana marblewere selected for the present investigation due totheir homogeneous composition and isotropicstrength behaviour.Experimental Procedure

Preparation of specimen-Cylinders of SOmmdiameter and 125mm length and discs of 50mmdiameter and 25mm thickness were prepared fromthe same rock block for uniaxial compression testand Brazilian tensile test respectively. Rock cores.which were used for preparing cylindrical and discspecimens, were obtained by using NX size(54mm) diamond drill bit. Specimens wereprepared in accordance with ISRM specifications.

The specimens were treated in followingdifferent ways:

(i) Twenty (ten cylindrical and ten discspecimens) specimens each of Chunar sandstoneand Makrana marble were dried in oven at105±5°C for four days. These were termed as dryspecimens. The specimens were dried only for fourdays because after that there was no further loss inweight of the specimens.

(ii) The same number of specimens weresubmerged in water for only 7 days because after

44 INDIAN 1. ENG. MATER. set, FEBRUARY 1998

that there was no increase in the weight of thespecimens.

Specimens of both type (dry and wet) wrappedin polythene, were kept in deep freezer at -20°Ctemperature for 10 days. The specimens werewrapped in polythene for protecting the specimensfrom any loss of moisture.

Determination of moisture content-Fordetermining moisture content of the specimens(dryor wet) , some specimens of the set wereground(pulverized) in a ball mill. To minimise thepossible moisture loss in the ball mill, minimumpossible number of balls were used in the ball mill.100 g of the ground material was kept in oven at105+ 5°C temperature for 24 h. The % of moisturecontent was calculated by using the followingformula:Moisture content of material = (100- w) x 100

wwhere, w = weight of the material after ovenheating, g.

The moisture content of dry and wet Chunarsandstone were found to be 0.35% and 1.25%respectively whereas moisture content of dry andwet Makrana marble were 0.2% and 0.65%respectively.

Low temperature arrangement-A cubical caseof 20cm size, made of 6mm thick perspex sheetand open at two opposite ends, was fabricated inthe departmental workshop. It served as a lowtemperature chamber for the specimen duringtest(Fig. I).

/

»". (fo(l:'

,~'~

Fig.I--Low temperature chamber of perspex sheet

A special thermometer of temperature range-20°C to 150°C was used for temperaturemeasurement. For facilitating the measurement oftemperature just before the loading of specimen, ahole of Smm dia was provided in one side ofcubical case for inserting the thermometer.

Testing procedure-The cubical casing, filledwith the mixture of ice and salts (sodium chlorideand calcium chloride), was kept at the lower platenof the universal testing machine (UTM) of 40 tonscompressional load capacity. The specimen waskept at the center of the lower platen and inside thecasing (Fig. 2). The mixture of ice and salts wasused to maintain the low temperature of thespecimen during the experiment. The upper platenwas brought in contact with the specimen and loadwas gradually applied at the rate of 0.5 MPais and200 kN/s for uniaxial compression test andBrazilian tensile test respectively (as per thesuggestion of ISRM) till the specimen fails.

The temperature of the specimen was measuredby keeping the bulb of the thermometer in contactwith the specimen for five minutes.

Determination of compressive and tensilestrengths of rock-The uniaxial compressivestrength of the rock specimen was calculated bythe following formula3

:

pcr = -- ... (1)

C A

where, crc = compressive strength, MPa,P = failure load in uniaxial compression test, NandA = initial cross-sectional area of the specimen,mrrr'.The tensile strength of the specimen was calculatedby using the following formula":

Fig. 2-Uniaxial compressional loading at sub-zerotemperature

DWIVEDI et al.: COMPRESSIVE AND TENSILE HEHA VIOUR OF ROCKS 45

2Pa =--t 1t Dt

... (2) t = thickness of the specimen, mm.

where, at = tensi Ie strength of the specimen, MPa,P = failure load in Brazilian test, N,D = diameter of the specimen, mm, and

110r-----------------------------------------------------~t05 x-Dry

o 100 • - Wet0..~•• 9S

Results and DiscussionCompressive strength-- The variation in

compressive strength with temperature are shownin Figs 3 and 4 for Chunar sandstone and Makrana

J:....01 90cGI\..

85-11\

•• 80>11\11\ 7SII...a.E 700u

S0

0 -1 -2

------II

-3 -4 -S -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18•Temperature C

Fig.3~ompressive strength of Chunar sandstone at sub-zerotemperature120~------=~==~------------------------~A!---------,

x - Dry.-W~t

70

115

110L__ -.---------~------~------.--e 105~•...100J:...g' 95II...tii 90II

.~ 85III

~ 80~------------~------Eo 75u

5OL-J--L__L-J--L__L-~_L __L_~~ __ ~~~ __ ~~~ __ ~~~

o -1 -2 -3 -4 -5 -6 -7 -8 ·9 -10-11-12-13 -14 -15 -16-17-18-19-20T.mp.ratur. , ·C

Fig.4-Compressive strength of Makrana marble at sub-zerotemperature

46 INDIAN J. ENG. MATER. SCI., FEBRUARY 1998

marble respectively. It is quite evident from Figs 3and 4 that compressive strength increased withdecrease in temperature. The following reasons canbe attributed to this phenomenon; as thetemperature of the specimen was reduced, mineralgrains of rock shrunk. This shrinkage of mineralgrains produced tri-axial stress condition in thespecimen which resulted in higher compressivestrength. Also, the pore water became ice belowOcCand strength of th is ice increased with decreasein temperature. Consequently, the strength of rocksincreased with decrease in temperature.

It is also evident from Figs 3 and 4 thatcompressive strength of wet specimen was lowerthan dry specimen of same type of rocks at anyparticular sub-zero temperature. This lowering ofwet rock strength may have resulted due toenlargement and extension of micro-cracks presentin the rock specimen at the time of cooling. Theintensity of enlargement and extension of micro-cracks depends on pore water % of the rock

2019 x- Dry

18 e- Wet

17

16

"0

11.c,:l: 13•..s:- 12C7IcCII\" 1 1~.!! 10.~cCII 9•....

S

7

6

specimen. Since more pore water was present inwet specimen than in dry specimen, enlargementand extension was more in case of wet specimen.This resulted in lower .compressive strength for wetspecimen.

Tensile strength--The variation of tensilestrength with temperature for Chunar sandstoneand Makrana marble are shown in Figs 5 and 6respectively. It is obvious from figures that tensile

10.---------------------------,~ x- DryZ 9 .- W••t

~ .,r•• 7to

1 ,~..; I

!

~/-:.-3 -4 -5 -& -7 -8 -9 -10-11-12-13-14-15-Iii -17

T.mp.rotur. , ·C

Fig.5- Tensile strength of Chunar sandstone at sub-zerotemperature

OL--L __~~ __L-~ __ ~~ __ ~~ __ ~ __ ~~ __ L__L __ ~~ __ ~~ __ ~~

-0 -1 -2 - 3 -4 - 5 - 6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19 -20T.mp.rolur. , ·c

Fig.6- Tensile strength of Makrana marble at SUb-ZL70

temperature

DWIVEDI et al.: COMPRESSIVE AND TENSILE BEHA VIOUR OF ROCKS 47

strength increased with decrease in temperature.The same reason can be extended from thisphenomenon as was given in case of compressivestrength.

It can also be seen from Figs 5 and 6 that ini-tially tensile strength of the wet specimen was lessthan that of dry specimen of same rock but below-15°C, tensile strength of wet specimen exceededthat of dry specimen. We can consider followingreasons behind this phenomenon; as thetemperature dropped below O°C, pore waterchanged into ice. The strength of ice increasedwith decrease in temperature below O°C.

The compressive strength of ice, i.e., 1.08 to1.96 MPa at O°C and 3.92 to 8.04 MPa at -50°Cwas quite less in comparison to compressivestrength of the rocks tested. But tensile strength ofice, i.e, 1.37 MPa at O°C and 2.45 MPa at -SO°C isquite near to tensile strength of rocks tested",Therefore, it can be said that ice contributed to thetensile strength but not to the uniaxial compressivestrength of rocks. Besides this, ice also acted as an

Fig. 7-;- Tensile fracture in Chunar sandstone during uniaxialcompression test

Fig.8-Cone shear fracture in Makrana marble during uniaxialcompression test

adhesive material and contributed to the tensilestrength of the rocks.

As ice content of the wet specimen is more thandry specimen of the same rock, the tensile strengthof wet specimen increased at a faster rate withdecrease in temperature than that of dry specimenand exceeded the tensile strength of dry specimenbelow -IS°C.

Fracture patterns observed during uniaxialcompression test (Figs 7 and 8) and Braziliantensile test (Figs 9 and 10) were same as a fracturepatterns observed at the normal temperature(25°C).

ConclusionsFollowing important conclusions can be drawn

from the present study:(i) The compressive strength of both rocks (Chunar

sandstone and Makrana marble) increased withdecrease in temperature.

(ii) The compressive strength of the wet specimenwas less than. that of dry specimen for bothtypes of rock

Fig.9-Fracture pattern in Chunar sandstone

Fig.IO--Fracture pattern in Makrana marble during Braziliantest

48 INDIAN 1. ENG. MATER. sci., FEBRUARY 1998

(iii) The tensile strength of both rocks increased withdecrease in temperature.

(iv) Initially, the tensile strength of the wet specimenwas less than that of dry specimen of the samerock but it became more below -1 SoC.

(v) Compressive and tensile strengths of Makranamarble were higher than those of Chunarsandstone.

AcknowledgementThe Contribution of Centre of Advanced Study,

Department of Mining Engineering Institute ofTechnology, Banaras Hindu University (BHU), forproviding financial assistance and techriical supportfor carrying out experiments, is gratefully anddeeply acknowledged. We would like to thank Dr.H.D Khanna, Head, Department of Bio-physics,[MS, BHU for providing deep freezer and other 10accessory facilities.

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