Frost Durability Testing of Stone Masonry Mortars: Restoring the Canadian Parliament Buildings

Abstract

Restoration of historic masonry is not only a matter of removing failing stone or mortar and replacing it with new. It is necessary to maintain the historic fabric of the structure and ensure that any intervention is reversible. It is also very important to ensure that the repointing effort has a high degree of durability. In much of northern North America and Europe, freeze-thaw durability is key to long-term service life. This is a particular challenge in climatic regions where winter thaws can result in temperature swings from -200C to '+5C and back to -200C in less than 24 hours. Driving rain is often associated with this type of weather pattern. Mortar failure in a single season is not uncommon under these severe climatic conditions. Ottawa, Canada, is in this type of region. Accelerated testing for frost durability of masonry is not a standard test. ASTM C-666 [1], which was developed for freeze-thaw durability testing of concrete, is commonly used for testing mortar; this is an unrealistic test. The durability of masonry is a complex interaction of the mortar and masonry unit. Recognizing this, the National Research Council of Canada (NRCC), Institute for Research in Construction (IRC) developed a frost durability test for masonry [2]. The development of the test was in direct response to the need of Public Works and Government Services Canada (PWGSC) to produce a guideline for specifying for the current, largescale restoration of the buildings that make up the Parliamentary Precinct of Canada's capital city, Ottawa. The results reported in this paper represents the 1994-95 test program of an ongoing investigation. Two phases of mixing were conducted. The first phase investigated a broad range of mix designs and the second a refinement of the favorable mix designs. A companion paper in this issue by Suter et al describes the mechanical testing protocols and details the results. The purpose of this paper is to describe the approach and outcome of frost durability testing of mortars and to discuss those physical properties that appear to relate to freeze-thaw durability. The work was conducted under the umbrella of the Masonry Retrofit Project, which is a strategic research focus of the Institute for Research in Construction.