Mortar Study of Mechanical Properties for the Repointing of the Canadian Parliament Buildings

Abstract

This paper contributes to the ongoing conservation work for stone masonry structures on the Parliamentary Precinct in Ottawa, Canada. A major part of the work focuses on deep repointing of Nepean sandstone masonry. To help define durable repointing mortars, two related test programs were commissioned by Parks Canada/Environment Canada/Public Works and Government Services Canada (PWGSC) in 1994-95; these programs essentially dealt with mechanical properties and frost durability. While work on the mechanical properties is the subject of this paper, a companion paper deals with the frost durability testing [1]. Together they represent the first comprehensive test program on repointing mortars where both mechanical properties and frost durability are examined together. Previous work carried out in the early 1990s concentrated on selected mechanical properties, including the bond between various mortars and masonry units [2]. Throughout the current test programs two basic mortar types were used: masonry cement (MC) and portland cement-lime (PC-L) mortars. Mortarmaterial variables included the type of lime (hydrated types N or S, lime putty, and hydraulic lime) and partial sand replacement by means of crushed clay brick and clay brick dust. At the outset 26 trial mixes were prepared to determine their plastic properties and help select workable mixes for later specimen construction. The trial-mix work was followed by two phases of mortar mixes, including specimen construction for mechanical property and frost-durability testing. While phase 1 involved 17 mixes over a wide range of mortar material variables, phase 2 concentrated on 16 mixes within a narrower range of material variables. Recommended repointing mixes for the conservation work on the Parliamentary Precinct structures came from the refined phase 2 mixes. Because conservation work typically requires relatively low-strength, softer mortars, mortar proportions corresponded to type N or weaker mixes throughout the test programs. The determination of mechanical properties included the mortar's compressive and split-tensile strengths, as well as its modulus of elasticity, and the masonry's flexural bond strength. The test results were judged against a set of performance criteria that represent a first attempt to delineate such criteria for repointing mortars. This paper provides an overview of the mechanical test program for Nepean sandstone mortars, summarizes key results, and gives recommendations for suitable mortar mixes and future work. Further details are presented elsewhere [3].