Repointing Shortcuts Can Cause Coating Failures, but New Mortar System Offers Relief


By Michael P. Edison, Edison Coatings, Inc.

Although properly proportioned, installed and cured masonry repointing mortar should be expected to provide at least 50 years of service, actual results too often fall far short of that. One reason involves shortcuts taken in the repointing process, including omission of labor-intensive steps required to minimize shrinkage and maximize bond strength.

Because mortar is a vital, functional component of a masonry building's waterproof envelope, premature mortar failures on certain structures can lead to moisture infiltration problems and premature coating failures on exterior and/or interior surfaces. Ideally, owners or their representatives will enforce the use of best industry practices to ensure the full performance and durability of their repointing work, but the costs of doing the work properly can be substantially higher than what many owners expect — or are willing — to pay.

While this doesn't justify poor workmanship, it does highlight the need for more time- and cost-effective solutions. A newly released mortar system simplifies the repointing requirements, ensuring reliably high performance while reducing labor requirements. The two-component mortar system consists of a pre-packaged dry mix based on natural cement — rather than Portland cement — and a liquid polymer that is used in place of water when mixing the mortar. Manufactured by Edison Coatings, Inc., the system is available in a wide range of strengths and custom colors.

Repointing is Labor Intensive

Mortar is considered to be a sacrificial component in masonry construction, and periodic partial-depth replacement of failing mortar is an expected maintenance requirement. Modern mortars based on mixtures with Portland cement carbonate with age, rendering them brittle and prone to cracking and/or delaminating over time. Just a 0.008-inch width crack will leak when exposed to rain.

As Portland cement-based mortars age and carbonate, they embrittle and may delaminate from masonry units. Joints with openings as narrow as 0.008 inch will leak when exposed to rain and must be repointed.

The process of removing failing mortar to a proper minimum depth, preparing cavities for repointing, and then properly placing, compacting and curing the new mortar is labor-intensive. In the competitive world of commercial building maintenance and restoration, a project's successful low bidder will have often planned on taking shortcuts, skipping certain steps to reduce their costs.

Proper Repointing

The rules for proper repointing are not secret, and useful guides have been published, such as Preservation Briefs #2 by the National Park Service and the appendix of ASTM C270, among others. But how often are these rules actually followed?

  • Old mortar must be removed to a minimum depth, typically specified as either 2 1/2 times the joint width, or a minimum of some specified depth between 1/2 inch and 1 inch, or to the depth required to reach sound mortar, whichever is greater. Superficial striping over old mortar is not effective.
  • Joints must be properly prepared for repointing after old mortar removal, typically involving washing out all dust and debris to achieve a clean bonding surface.
  • Masonry must be pre-dampened before repointing, to assure that the new mortar does not dry out too quickly.
  • New mortar must be correctly selected, proportioned and mixed to the proper consistency. Proper repointing consistency is the driest paste that is workable. Making wetter mixes to facilitate use of grout bags or pointing guns results in smearing and excessive shrinkage.

RILEM tube testing can be performed on cured repointing mortar to confirm that the new mortar is functioning properly as a component of a building's waterproof envelope.

So far, the list includes nothing extraordinary, though even these basic steps are often compromised. The costlier steps revolve around proper placement, compaction and curing.

  • Cement-based mortars are supposed to be pre-hydrated for 30 minutes after they are mixed, and then remixed and adjusted to proper consistency before use.
  • Mortar is supposed to be placed in "lifts." This includes placing 1/4 inch of mortar into the joint cavity, compacting the mortar with a slicker that is narrower than the joint, and waiting for the mortar to achieve "thumbprint hardness" before placing the next lift. While guidelines vary somewhat, at least two or three lifts are typically required. Often, lifts are restricted to a quarter-inch depth.
  • Once the final lift is placed and achieves thumbprint hardness, final tooling is performed. Then the mortar must be misted for a minimum of 72 hours to achieve proper initial cure.

These steps are sometimes skipped, because they are time-consuming and expensive — yet their omission can have a major impact on shrinkage, bond strength and general property development. The curing requirements are most problematic, as a laborer must come to the site on weekends to mist mortars placed late in the week, unless an automatic irrigation system is set up.

New materials technologies created to overcome some of these burdensome requirements include fast-setting mortars based on natural cement, conforming to ASTM C10/10M, that rapidly pass through the period of vulnerability to plastic shrinkage. This allows them to be placed in one continuous operation, rather than in separate lifts that may require several hours of cure between applications and several return trips to the same area by the installing mechanic. Edison Coatings produces a wide range of natural cement mortars under its Rosendale and Translantic 12M labels.

A series of hoses and spray nozzles are attached to the scaffolding on this project, connected to a timer to automatically achieve proper moist curing of new mortar for 3 days after placement.

While natural cement alone typically requires a minimum period of wet curing, Edison's ICE-9RL mortar admixture, based on hydrolysis-resistant polymer modifier, can convert mortars to dry-curing systems. This eliminates the need to keep mortars damp beyond the first day. In testing with a variety of mortars based on various binders, including Portland cement and lime, natural cement and natural hydraulic lime, the admixture effectively eliminated wet-curing requirements, reduced shrinkage, increased bond strength, maintained 75 percent of the mortar's "breathability" and reduced water absorption by 80 percent. Dramatic improvements in freeze-thaw and salt-scaling resistances were also documented.

Test panel of Rosendale 12M natural cement mortar with ICE-9 admixture exhibits no scaling after 50 freeze-thaw cycles beneath 1/4-inch ponded calcium chloride solution.

Long-Term Value

Although the system increases mortar material costs, masonry repointing is not a material-intensive operation. Labor, access, equipment and mobilization costs may represent 97 to 99 percent of the cost of a typical repointing project, while materials represent just 1 to 3 percent of total cost. The new system raises the material costs to perhaps 5 to 10 percent of project cost but provides a much higher level of performance, ensuring the achievement of full mortar potential. The much higher labor costs associated with traditional mortar placement and curing are also eliminated.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, Durability + Design or its editors.

Michael P. Edison, Edison Coatings, Inc.

Michael Edison is a chemical engineer and the founder and president of Edison Coatings, Inc., in Plainville, Connecticut, which develops and manufactures specialty repair and coating systems for stone, masonry and concrete. The company also produces natural cement. Over the past 37 years, Edison Coatings has provided specialty materials and support services for approximately 30,000 projects.




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