Abstract

A study of the point-load breaking strength and the three-point bending strength of three rock types has been made at room [75F] and liquid nitrogen [-320F] temperatures. A factorial design experiment, using three variables at two levels, was made on a limestone rock to evaluate the effects of these variables on the mechanical properties of this rock. The factorial design offered simplicity and economy in this type of testing. The same mechanical tests were applied to basalt and charcoal granite specimens supplied by the USBM. In general, average increases in the point-load breaking strength at the low temperature level were 46 per cent for the limestone, 52 per cent for the basalt, and 21 per cent for the granite. The average modulus of rupture strength increased 59 per cent for limestone, 49 per cent for basalt and 19 per cent for granite. analysis of the data from the factorial experiment indicated that the main effect was temperature, the intermediate effect was sample preparation, and no effect was observed for the two rates of loading used. No interaction between variables was evident. A determination of Young's modulus at the two temperature levels was made on the limestone. The increase in the modulus at liquid nitrogen temperature was approximately 2.5 times that at room temperature.

Introduction

The advent of lunar exploration is creating interest in the physical properties of rock at very low temperatures. This interest extends not only to the various engineering and space-oriented groups but also to those with an interest in the general field of rock mechanics. Penn and Gaudin indicated that the thermal environment on the lunar surface varies from approximately -250F to +250F. Associated with the temperature effect are the additional complicating environmental factors of reduced gravity in a vacuum. Our discussion, while dealing with only room and liquid nitrogen temperatures, indicates some interesting changes in the point-load breaking strength and modulus of rupture of three rock types. A search of the literature failed to turn up any information on the behavior of rocks at very low temperatures. The most related paper is that of Monfore and Lentz who studied the effects of low temperatures on several different. concrete mixes. Their measurements included compressive strength, splitting strength, Young's modulus, Poisson's ratio, thermal contraction and freeze-thaw resistance over a temperature range from, room to -250F. Of particular interest are the effects-of temperature, the compressive and splitting strengths of concrete. In general, the various concrete samples reached a maximum compressive strength at approximately -150F and decreased with further decreasing temperature. The splitting strengths followed a similar pattern but rather sharply at the freezing joint and a maximum, at -175F, somewhat higher than that observed for compressive strength specification. The lack of data on low temperature effects on rock properties prompted this investigation. The testing program consisted of the point-load breaking strength test after -Reichmuth and the three-point bending [modulus of rupture] test. Test specimens used were limestone obtained from a quarry near Valders, Wis., and basalt and charcoal granite supplied by the Twin Cities Mining Research Center of the USBM. In addition to the above tests, which were run at both room and liquid nitrogen temperatures, Young's modulus was determined for the limestone at the two temperature levels.

EXPERIMENTAL PROCEDURE

Bulk samples of the limestone as obtained from the quarry were randomly cored using 1/2-in. and 1-in. diameter diamond bits.

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