DEVELOPMENT OF A MICROCRACKING TECHNIQUE FOR MEASURING IN SITU STRESS AND STRAIN. REPORT 1. LABORATORY TESTS.
ARMY ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG MS
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The known relation between load and internal microcracking of hardened portland cement grouts was used to develop a rough, mechanical-type in situ strain gage. Different strength grout mixtures were proportioned, mixed, prepared, and tested. Static tests were conducted to determine compressive strength and to develop a method for confining the test specimens. Unconfined uniaxial dynamic compression tests were conducted at various increments of the ultimate stress. The specimens were then sawed, prepared, and photographed. The photographs were enlarged and examined to determine internal microcracking. The test results indicated that the total amount of internal microcracking per unit area of cross section increases with an increase in dynamic load. Neither of the methods tested for confining the cylinders was effective, and further investigation of this problem is recommended. It is concluded that the microcracking technique has possibilities as a field tool for measuring free- field strain in the in situ material adjacent to a high-yield explosion however, a full-scale field test should be conducted to further develop and determine the reliability of the technique under actual field conditions.
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