Stronger Isn't Necessarily Better: The Importance of Historic Mortar Analysis


Stronger Isn't Necessarily Better: The Importance of Historic Mortar Analysis

Damaged face shells and mortar joints.

Repointing joints, in damaged existing masonry, involves removal and replacement of the deteriorated outermost layer of mortar in masonry walls. In this blog, Hayder discusses why a stronger repointing mortar is not necessarily better for use with old masonry buildings.   

Prior to the early 1900s, masonry mortars consisted primarily of lime and sand. Portland cement was not commonly added to mortar mixes until later. Lime based mortars containing little or no cement content have lower compressive strength than cement based mortars.  Although not as strong as cement-based mortar, these “Father Time” lime based mortars have centuries-old successful durability.  They possess beneficial qualities from the lime, including vapor permeability (breathability), “flexibility” and good adherence (bond) to masonry units.  They also have an ability to heal (mortar ability to reconstitute and reform) itself to repair minor cracks. 

Cement based mortars are stronger but less breathable and less malleable than lime based mortars. Industry studies and practical observations have shown repointing lime based mortar joints with modern, cement based mortar often lasts only for several years and may lead to damaged walls, such as:

  • Freeze-thaw damage due to freezing of entrapped moisture behind cement based replacement mortar.  Breathability of lime based mortars and masonry units allows an exterior wall to dry out rapidly after being exposed to rain making them less prone to freezing.
  • Spalling damage can result from stronger repointing mortar  that can create stress concentrations in existing historic masonry. Use of a weaker and more breathable mortar for repairing older masonry can avoid this problem.

Applicable industry standards for repointing mortar joints suggest the following qualities for appropriate repointing mortar:

  • Match the historic original mortar in mix proportions, color and texture.
  • Have a greater vapor permeability and be softer (measured as compressive strength) than the historic original mortar.

SME’s Materials team routinely helps restoration contractors by providing historic mortar analysis in accordance with ASTM C 1324-15, “Standard Test Method for Examination and Analysis of Hardened Masonry Mortar”, and ASTM C 856/856M-20, “Standard Practice for Petrographic Examination of Hardened Concrete”. This includes microscopic evaluation and determination of the volumetric formulation of cementitious proportions, cementitious matrix classification, void classification, mix ratios and other characteristics.

Upon completion of the analysis, a detailed report is provided. The report gives original mix proportions information, physical properties and other relevant characteristics of the mortar.  It also provides recommendations on aggregate selection, binder proportioning, and pigment coloring, required to achieve a compatible mortar match for restoration work that can avoid the pitfalls of using the wrong repointing mortar.

Contact Hayder Al-Hilal, PE for more information. 


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