Mechanical behaviour of masonry assemblages built with mortar containing anti-freeze admixtures subject to sub-freezing curing temperatures

Cold weather masonry construction is a major concern for contractors in North America as well as other geographic locations. When exposed to cold conditions during construction and curing, the performance of masonry assemblies may be affected. Because of the vulnerability of mortar joints to considerable delay in setting time and low strength development rate in cold temperatures, the construction industry has been forced to follow extraordinary building methods. These methods focused primarily on protecting the freshly mixed and placed mortar from freezing for a suitable curing time. This can lead to loss of productivity rate and postponements in construction plans with related additional costs. This research evaluates the performance of mortars that contain nanocellulose and sodium nitrite and as a cold-weather admixture in masonry assemblages under sub-freezing curing temperatures. The effect of sodium nitrite with a dosage of 12% by mass of mixing water and nanocellulose with a dosage of 0.3% by mass of cement on Type S masonry mortars cured at -10°C and room temperatures was investigated. Flowability, air content, and setting time of fresh mixtures were determined. Moreover, for the hardened mortar state, the 28-day compressive strength of mortar cubes and the air content of mortar cylinders were determined. For the purpose of this study, two types of experiments were performed on masonry prisms constructed with four different kinds of mortars and cured under both sub-freezing and room temperatures. The first test was the compressive test to assess the design compressive strength and failure behaviour of the assemblies. A total of 72 prisms of six hollow concrete bricks stacked vertically with full bedding were built and tested in eight separate sets of nine prisms each. The second test was the flexural bond strength test using the bond wrench method described in CSA A3004-C9. For this test, 40 prisms were built, each one six units in height with five mortar joints for a total of 200 joints. These were divided into eight groups based on the kind of mortar used and the curing temperature. Test results indicated that the addition of sodium nitrite and cellulose nanocrystals affected the properties of the fresh and hardened mortar mixtures. The admixtures generally increased the flowability, reduced the air content, and sped up the hydration of cement. The mechanical property tests on masonry prisms showed that the compressive and flexural bond strengths of the prisms were affected considerably by the addition of these admixtures when cured at room temperature. At room temperature, the incorporation of nanocellulose in the mortar resulted in an increase in the compressive strength of the brick prisms by 15% compared to the control sample, while the use of sodium nitrite in the mortar resulted in an 11% increase in the compressive strength compared to the control prisms. For the flexural bond strength, the average results for prisms cured at room temperature indicate that the addition of sodium nitrite lowered the flexural bond strength by 14%, while incorporating nanocellulose separately or in combination with sodium nitrite improved the flexural bond strength by 13% and 14%, respectively. However, at -10°C, only samples with sodium nitrite reached acceptable compressive and flexural bond strengths (21.2 MPa and 0.404 MPa, respectively), as defined by the relevant Canadian standards. Generally, test results showed that sodium nitrite can be used successfully to minimize the adverse effects of freezing temperatures as low as -10°C on strength development of Type S mortar joints by lowering the freezing point of the mixing water, which opens a new approach in mitigating the problems associated with cold weather masonry construction. On the other hand, the use of nanocellulose at normal curing temperatures (22±2°C) resulted in a considerable improvement in the mechanical properties of masonry prisms. In order to improve the mechanical properties of the masonry assemblages, the incorporation of nanocellulose with sodium nitrite resulted in the highest compressive and flexural bond strengths at both normal and subfreezing curing temperatures. From the results discussed in this work, it is shown that the addition of sodium nitrite in masonry mortar mixtures can speed up masonry construction during the winter, with the need for only 4-8 hours of pre-curing at room temperature before being exposed to subfreezing temperatures, which is much relaxed compared to the 48 hours of protection currently required by the Canadian CSA A371 standard.

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