A.H.P. Maurenbrecher¹, K. Trischuk², M. Subercaseaux³ & G.T. Suter⁴

1. Research Officer, ² Technical Officer, Institute for Research in Construction, National Research Council, Ottawa, ON. K1A 0R6, paul.maurenbrecher@nrc.gc.ca, ken.trischuk@nrc.gc.ca
2. Conservation Architect, Heritage Conservation Directorate, Public Works & Government Services Canada, Gatineau, QC, K1A 0S5, maria_subercaseaux@pch.gc.ca
3. Principal, Suter Consultants Inc, Victoria, BC, gsuter@telus.net

ABSTRACT

This paper assesses whether the 28-day protection period for a low strength Portland cement/lime repointing mortar before exposure to freezing temperatures could be reduced. This requirement, by the Heritage Conservation Directorate at Public Works & Government Services Canada, was used for repointing mortars in the conservation of historic masonry.

The performance of the mortar was evaluated for frost durability in accelerated laboratory testing as well as some testing at an outdoor exposure site. The mortar was tested as part of small sandstone masonry prisms to more realistically simulate practice.

An interim recommendation is that the protection period can be reduced to 7 days. During the first three days, the mortar should be damp cured by using damp burlap covered with plastic at a masonry temperature above 10°C. This is followed by four additional days of protection from wind and precipitation with the masonry temperature above 0°C. Further tests to assess lower curing temperatures and possible longer protection periods against precipitation, and field experience with repointed mortar joints are needed before firmly adopting these recommendations. They should not be taken as a green light for construction in winter. Repointing of mortar joints during periods when freezing conditions may occur should be discouraged, but can be inevitable if projects run late or unusual weather conditions occur.

KEYWORDS: repointing, mortar, cold weather construction, freezing, curing.

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