Non-Destructive Evaluation of the Interface in Reinforced Concrete Using Phase Measurement Interferometry
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After demonstrating the extent of the similarity between the two specimens, the wedge test is used to determine the unknown normal stress at the interface of the rebar specimen. Studying the behavior of these specimens is facilitated by accurate examination of the cracking of concrete. This is made possible with the aid of highly accurate displacement measurement techniques. An optical interferometric technique was used to obtain full field displacement measurements at the sub-micron level. In the second part of the study, a hybrid experimental-numerical technique is used as a more general approach for finding the unknown rebar interfacial normal stress, using the experimentally measured crack length and crack opening displacements (COD). Specimen Dimensions and Material Properties The concrete specimens are shown in Figure 1. The steel rebars are 9.5 mm in diameter with ribs at 90 0 to the bar axis. In order to induce pressure in the horizontal direction, and promote a vertical crack, the bar was partly milled along the top and bottom surfaces removing the lugs, as shown in the figure. The shear stress distribution along the length of the bar is assumed to be uniform based on the short embedment length.. Wedges used in the wedge pull-out specimens were made of mild steel, and they were milled on both sides at an inclination of 13 degrees relative to the wedge axis. Two layers of Teflon pads were used at the wedge bearing and specimen reaction bearing surfaces. The material composition of the concrete was 0.5,1,2,2 parts by weight of water, cement, sand, and gravel. The specimens were cured in 40% relative humidity for 28 days, achieving a compressive strength (f'c) of 33 MPa. The fracture toughness based on Jenq and Shah's [1 ] two parameter model was determined to be K1c=0.82 MPam. The tensile strength (fcr) was assumed to be 0.09f'c. Test Apparatus Figure 2 shows the experimental setup. The pull-out load was measured with a 25 KN Sensotec load cell, and the slip was measured using an LVDT. The crack length and crack opening displacement (COD) were also measured at the front face of the specimen along the entire length of the propagating crack. Full field surface displacement measurement was achieved using Phase Measurement Speckle Interferometry (PMI) [2]. The setup is a two dimensional interferometer, shown and used in its one dimensional mode as shown in the figure. Full field displacement map of an object can be obtained using standard Speckle pattern interferometery (SPI) [3]. The setup used in this experiment was made sensitive to in-plane horizontal displacements. The object was illuminated by two beams at 45 degrees to the specimen normal. The resulting scattered speckle pattern of the object in its undeformed state was stored in the computer image memory and subtracted from the speckle pattern corresponding to the deformed state. The result of the subtraction is an intensity fringe pattern which provides a contour map of horizontal displacement. The displacement information is carried by the phase in the
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