Characterization of the interface between brick and mortar

Chase, Gerald
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Civil, Construction, and Environmental Engineering

Although mortar has been performing its role of holding masonry structures together for hundreds of years, masonry does fail, and the majority of cases of failure occur at the interface between the mortar and the masonry unit. The purpose of this investigation was to examine that interface in more detail, using a combination of crossed-couplet bond tests, the scanning electron microscope, and X-ray diffraction. Several brick pretreatments were also evaluated. Previous investigations of tensile bond strength have shown that bond strength is affected by brick suction, brick surface texture, mortar flow and composition, and other factors. But they did not examine the fundamental mechanism of bonding, nor the chemistry at the interface;The crossed couplet testing done as a part of this investigation examined the effect of NaOH, limewater, and H(,3)PO(,4) pretreatments applied to the brick. For type S mortar, these pretreatments have a significant beneficial effect on tensile bond strength, when compared to water only, or dry brick. Pre-wetting the brick was shown to have a beneficial effect on the bond strength;Scanning electron microscope (SEM) photographs of the interface region show that the preponderant materials present for untreated brick are calcium-silicate-hydrate (C-S-H) and calcium hydroxide. Micrographs of brick treated with NaOH and H(,3)PO(,4) show the presence of complex crystals at the interface, which may account for the increased tensile bond strength observed with these treatments. Energy dispersive X-ray analysis (EDXA) performed on the SEM samples provides clues as to the possible composition of interfacial compounds, but additional means are needed for positive identification;X-ray diffraction of cement and lime paste and untreated brick shows that there is evidence of an increase in crystalline Ca(OH)(,2) beyond that which would be expected. X-ray diffraction confirms the presence of a reaction product between paste and brick treated with H(,3)PO(,4) or NaOH. However, positive identification has not been made;Additional investigations are recommended to determine the identity of the reaction products mentioned above, to test the durability and water resistance of chemically treated brick assemblages, and to perform more extensive couplet testing using mortars of different composition and flow.