A set of cores was prepared to allow examination of full-depth concrete cross sections of specimens from both the joint and mid-panel, outside of a wheel path, and both between and on vibrator trails. Examination of cores followed recommendations provided in ASTM C 856 [23], Standard Practice for Examination of Hardened Concrete, while procedures for preparation and examination may also be found in Walker [24].
Core numbers were logged upon receipt and core surfaces examined and described. Examination of the entire core allows description of the overall concrete condition, the coarse and fine aggregate characteristics, cement paste condition, qualitative assessment of entrapped and entrained air void system, cracking, and identification of any foreign objects. Photographing the prepared surfaces provided a record of the specimen condition.
Lapping provides a smooth surface necessary for examination of the concrete microstructure using a stereo microscope [23, 24]. Cracks were marked using India ink and a draftsman’s pen by touching the pen tip to the crack and allowing capillary suction to draw the ink along the crack plane [24]. This technique was also used to identify open aggregate cracks in the limestone coarse aggregate and shale in the sand. Selected cores were then marked for sampling for scanning electron microscopy (SEM). The cores were cut in half lengthwise and a slab approximately 15 mm was taken from the core center. The core slab was sectioned into four pieces and dried in an oven at 50 ºC. After 48 h, these sections were imbedded using a low-viscosity epoxy in a low vacuum to assist in infiltration of the pore structure, and the epoxy was cured at 60 ºC for 24 hours. The embedded specimens were then ground to expose the concrete face, and polished using a series of (6, 3, 1, and 0.25) µm diamond polish; the surfaces were cleaned using a lint-free cotton cloth. A thin carbon coating was deposited on the surface for conduction of excess SEM beam charge. The slabs were stored in a vacuum dessicator to protect them from laboratory humidity. Remnant core halves were lapped and polished similar to the procedure for petrographic analysis.