Energy-Microcrack Growth Measurments for Mortar Cylinders in Compression
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Mat. Res. Soc. Symp. Proc. Vol. 503 ©1998 Materials Research Society
Downloaded from https://www.cambridge.org/core. Simon Fraser University Library, on 04 Nov 2019 at 12:42:05, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-503-201
SLoad
start of scan ,endof scan
total work of load •
creep/relaxation ,elastic recovery
~N Deformation Figure 1. Illustration of terms used in work-of-load calculation looking at the entire area under the load-displacement curve between two points, we consider only the portion of the area that is not creep/relaxation area or recovered (elastic) area. The recoverable energy is determined by unloading the specimen after each scan. The different areas considered are illustrated in Figure 1.Using the quantities illustrated in this figure, the work of load assumed to cause cracking was calculated as the total work of load minus the creep and the elastic recovery. EXPERIMENTAL PROCEDURE The specimens were 4 mm diameter by 4 mm high mortar cylinders having a mix proportion of 1 to 2 to 0.6 parts by weight of type I portland cement to sand (.425 mm max. size) to water. The mix was proportioned to correspond roughly to the mortar phase of a conventional concrete. The microtomographic scans were conducted at the National Synchrotron Light Source (beamline X2B) at Brookhaven National Lab. Beamline X2B was developed by Exxon Research specifically for microtomography. [1,2] More detailed information on the applications to cementbased materials can be found in reference [4]. The general procedure was to place the specimen in the loading frame and mount the loading frame on a rotation stage in the x-ray path. An initial tomographic scan was taken of the undamaged specimen. A compressive load was then applied to the specimen, with the load and displacement recorded, and another scan taken. The cylinder was then unloaded and reloaded to a higher load for another scan. This cycle was continued until after failure. A total of six scans were taken during the loading process, including the baseline case. The microtomography set-up is illustrated in Figure 2. The x-ray beam exits the synchrotron loop and is monochromated with a silicon crystal. The narrow energy band x-ray then passes through the sample where it hits a phosphor screen and is converted to visible light. The light is magnified by a microscopic objective. In our case we used 2.5X lens system. The light then hits a CCD which sends the digitized through transmission images to a computer for reconstruction. A total of 720 images over 180* were taken for each scan. A special loading frame was designed for the project. Figure 3 shows the details of the frame, including the thumb screw load mechanism, the tension rods which pull down the top platen against the specimen, the in-line load cell and the proximity sensor which measured top platen motion. It was assumed that the rest of the loading frame was stationary which meant that top platen motion was the same as plat
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