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Repointing mortars for older masonry
Trischuk, K. J.; Rousseau, M. Z.
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R e p o i n t i n g m o r t a r s f o r o l d e r m a s o n r y
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T r i s c h u k , K . ; R o u s s e a u , M . Z .
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Repointing Mortars for Older Masonry By Ken Trischuk and Madeleine Z. Rousseau
Masonry structures built between the early 1880s and 1940 make up much of the inventory of older buildings in Canada. They eventually require repointing to prolong service life (Figure 1).
Figure 1. Repointing is the process of removing the outer layer of (deteriorated) mortar from the mortar joints in existing masonry and replacing it with new mortar.
All aspects of repointing—from mortar mix selection and joint preparation to mortar installation and curing conditions—can have a permanent effect on an historic building’s visual and functional integrity. Poor repointing can detract from the masonry’s
appearance and lead to premature mortar deterioration.
Before a project is initiated, it is important to evaluate the structural integrity and condition of the masonry walls and the foundations. A building condition survey indicates the degree and location of damage (and likely causes), along with previous repair work. It also provides historical perspective on the building’s heritage character, ensuring repairs, maintenance, and conservation work can be planned and carried out for the best results.
For more important buildings, the design team should consider engaging a masonry conservation specialist to assist with issues including:
• historic authenticity;
• specifications for mortar mixes and application; • quality control and curing; and
• appropriate laboratory and on-site testing.
Mortar mix selection
A repointing mortar should be compatible not only with the masonry units and original mortar, but also with the building’s heritage character. Further, it must be resistant to agents of deterioration likely to cause premature failure (e.g. frost action).
No single mortar mix is suitable for all masonry assembly types across all Canadian climatic conditions. In some cases, more than one mix may be used on a single project to address different masonry and environmental loading characteristics because not all building envelope elements are exposed to the same environmental loads. Masonry repair should include interventions where necessary to reduce the moisture loads on masonry elements, which can significantly extend service life. The following sections provide guidance for matching repointing mortars with existing assemblies.
Compressive strength
The repointing mortar’s compressive strength should be lower than that of the existing masonry units, and similar to or lower than that of the existing bedding mortar. Using mortars that are too strong can lead to spalling of the masonry units or shortened mortar service life. Therefore, specifications for compressive strength should include both an upper and a lower limit. A lower-strength mortar tends to be ‘soft’ or more flexible, permitting its response to small amounts of differential movement without cracking.
Wetting and drying potential
Repointing mortar should have water absorption and vapour transmission rates similar to or greater than those of the bedding mortar and masonry units. A more permeable mortar promotes drying at the joints—a quite useful mechanism for avoiding moisture
accumulation in the masonry (especially in buildings with dense masonry units). As well, any salts in the masonry tend to migrate out through the mortar instead of the units.
Bond
Full contact between the repointing mortar, masonry units, and bedding mortar reduces water ingress into the joint and lowers the frost damage risk. Construction practices such as joint cleaning, mortar compaction, and curing also affect bond characteristics.
Appearance
The repointing product’s texture and colour should be matched to a reference mortar as required for historic authenticity and esthetics. Sand—the main ingredient in mortar— contributes most to colour and texture. If pigments are added, they should be inorganic oxides and the proportion added should not significantly alter the mortar properties.
Resistance to frost action
Adding an air-entraining agent to the mortar mix improves frost resistance and workability. Preferably, this additive should be premixed into one of the mortar ingredients to avoid errors onsite (e.g. SA hydrated lime is Type S lime with an air-entraining agent). Laboratory testing of the freeze−thaw resistance of small-scale
masonry specimens can provide useful data on the performance of different mortar mixes applied to specific masonry units.
Resistance to salts
Masonry adjacent to roads and sidewalks is prone to exposure to de-icing salts, which can be absorbed by the masonry units (depending on their porosity). The salts then crystallize and expand when the masonry dries, resulting in damage. When portland cement is used in the repair mortar, it must be sulphate-resistant. Remedial work should also include shielding the masonry from de-icing salts.
Now a concert venue, Almonte, Ont.’s Old Town Hall benefited from an expansion that relied on effective masonry repointing.
Typical mortar mixes
Mortar mixes are defined by the binder in the mix: (hydrated) lime mortars, hydraulic lime mortars, and cement/lime mortars. The current industry standard is to express a mortar mix in terms of the volume of the main ingredients. For instance, a 1:2:9 mix contains one volume of cement, two volumes of lime, and nine volumes of sand. The ingredients can be mixed onsite or obtained as a pre-mix (either proprietary or as specified for the project).
Figure 2 presents information on the different mortar types currently used in
conservation. Typical mix proportions for cement/lime mortars can be found in Canadian Standards Association (CSA) A179, Mortar and Grout for Unit Masonry.
Pre-construction testing can analyze the existing mortar’s characteristics, identifying a suitable product mix. Analysis of existing mortar samples provides a benchmark for selecting a compatible new repointing mortar. Compressive strength, sand grading, and flexural bond strength can be determined using standard tests (i.e. CSA A179).
Figure 2
Characteristics of Mortar Mixes
Lime mortar Hydraulic lime mortar Portland cement/lime mortars*
Compositi on
Hydrated lime (powder) or lime putty and sand in a 1:2−3 mix
Hydraulic lime and sand in a
1:2−3 mix. Type N: 1:1:5−6 Type O: 1:2:8−9
Type K: 1:3:10−12 (see CSA A179, Annex A)
Properties
• gains strength by reaction with carbon dioxide and moisture in the air
• low compressive strength (0.5−2 MPa)
• high vapour- and water-transmission properties • capability for self-healing of fine cracks
• very flexible and soft (can accommodate small
movements without cracking) • low shrinkage
• gains strength by reaction with carbon dioxide as well as directly with water
• hydraulic limes can be feebly, moderately, or eminently hydraulic
• broad range of compressive strength (1−10 MPa)
• gains strength at a faster rate than lime/sand mortars, but at a much lower rate than portland cement-based mixes
• properties of mixes can vary widely between different manufacturers of hydraulic lime
• gains nearly all strength by reaction with water
• faster rate of strength gain • broad range of compressive strength (2−15 MPa)
• a mix richer in portland cement exhibits higher compressive strength, more shrinkage, lower vapour transmission, and lower workability than a mix rich in lime
Uses
• sheltered areas with low exposure to wetting and frost action
• locations where lime mortar provided satisfactory
performance previously, assuming environment has not changed
• experience and research gained on a few historic properties in Canada showed they can exhibit adequate frost resistance in moderate exposure when site quality control is rigorous
• considerable experience and research in Canada indicate a Type O mortar with an air-entraining agent (compressive strength of 3−9 MPa) can provide satisfactory performance in a broad range of conditions.
Cautions
• gains strength very slowly • low initial frost resistance • controlled curing is
essential for performance and to prevent leaching of lime and staining of masonry • a given volume of lime putty contains significantly more lime than the same volume of hydrated lime
• gains strength slowly
• little documented performance in Canadian climate
• involvement of a masonry conservation specialist is recommended
• quality control on the execution and curing must be rigorous
• can result in too high a compressive strength for the hardness of the masonry units, resulting in damage to the units • where historic authenticity is important, these mortars may not be appropriate
* An alternative to portland cement/lime is masonry or mortar cement often used in modern masonry construction (these already incorporate an air-entraining agent).
Conservation practitioners who want to know all the ingredients in their mortar mixes do not use them because the ingredients are proprietary and are subject to change.
Construction
The best mortar mix for a repointing project may perform poorly because of inadequate execution and curing practices. Before construction begins, there are several important considerations.
Selection of a qualified contractor
It is crucial to select contractors experienced in the conservation of older masonry. Their accreditation gives assurance the workers executing the job have received appropriate training by a recognized organization. (The quality of similar jobs could also help assess suitability.) Some colleges and historic masonry conservation organizations offer training courses on masonry restoration. Mortar manufacturers also offer hands-on training for use of their products.
Seasonal considerations
Low-strength mortars are sensitive to extreme climate conditions during application and curing. For example, freezing conditions or high temperatures leading to rapid drying have a detrimental effect on mortar durability. In northern climates, the window of opportunity for repointing can be quite short. In these cases, temporary enclosures have to be set up to protect the walls during repair work.
Test panels
The contractor should prepare representative test sections to set the benchmark for an acceptable standard of quality as work proceeds. These test panels can assess the mason’s skill and confirm the mortar colour, texture, and joint finishes selected for the job.
Supervision
Site supervision is critical as lower strength mortars are less forgiving of poor construction practices than their counterparts used in modern masonry.
Joint preparation
Joint preparation is critical for enabling good compaction of the fresh mortar into the joint and proper bonding to both the masonry units and the bedding mortar. Incorrect procedures, improper tools, or inadequate skills can result in damage to the edges of masonry units—this not only affects their visual character, but can also increase the chance of weather damage.
A clear, rectangular space must be raked in the joint for placing the repointing mortar (Figure 3). The depth of the raking should be twice the joint thickness. To avoid damaging the masonry, only deteriorated areas of narrow mortar joints should be removed. The deeper the joint, the more difficult it is to ensure contact between the old mortar and the repointing one. For joints needing deep raking, wall stability must be carefully assessed.
Figure 3. Correct and incorrect ways of raking joint in preparation for repointing. Hand chisels or small pneumatic tools for removing old mortar minimize the risk of damage to the masonry units. For masonry with narrow or vertical joints, power saws and grinders should not be used due to the elevated chance of damage to the unit edges. However, on wide and uniform horizontal joints, careful use of a power grinder or saw in the middle of the joint may be acceptable. Before power tools are employed, a test should be carried out to confirm the procedure and the operator’s experience.
After raking, the joints should be brushed or vacuumed, and then washed with a water jet from the top of the masonry area down to remove any remaining debris and dust.
Mortar mixing
While batching (i.e. measuring) mortar ingredients by weight gives much better
uniformity, it is still common site practice to batch by volume. The specified mortar mix may be based on either dry or ‘damp’ sand. (CSA A179 assumes sand in the mix
proportions is in a damp state.) This specification must be taken into account when batching mortar ingredients onsite.
It is advisable to check before and during construction to determine the moisture content of the sand so mix proportions can be adjusted. To obtain a consistent degree of moisture content, sand should be protected from direct exposure to the elements (e.g. stored in a box or protected from sun and rain). Bagged pre-mix mortars, which require only the addition of water, offer one solution to the variability of onsite batching.
Mix procedures should be specified before work begins. Mechanical mixing is preferred, using a standard paddle mortar mixer or a mortar mill (normally employed for mortars with lime putty as the main binder). Small amounts of mortar can also be mixed (no more than needed at one time) using a hand drill with a whisk attachment or by hand.
Plasticity improves with mixing time, but care must be taken if an air-entraining agent has been added—too much mixing may introduce excessive air. (A maximum of five minutes of wet-mixing is recommended.) It is good practice to allow the mortar to stand
for a short while afterward to allow good wetting of the ingredients, especially if dry sand has been used.
The total volume of water necessary for a mortar mix depends on weather conditions. Repointing mortars require less water than those used for new construction. A drier mortar is cleaner to use (i.e. less risk of staining the units around the joints), can be compacted into the joint better, and will be less prone to shrinkage during curing. However, a too-dry mortar may not activate air-entraining agents, negatively affecting performance. A Vicat cone test (i.e. ASTM International C 780, Standard Test Method
for Pre-construction and Construction Evaluation of Mortars for Plain and Reinforced Unit Masonry) can help ensure consistent workability for a particular mix (Figure 4).
Repointing mortars should have a Vicat cone reading of approximately 15 to 30 mm (0.6 to 1.2 in.), depending on mortar type and site conditions.
Figure 4. The Vicat cone is dropped into a cylinder of mortar and the depth of penetration into the mortar provides an indication of the consistency of the mortar.
In warm weather, the mortar container should be covered with a damp cloth to prevent drying. When air temperatures are likely to exceed 27 C (81 F), repointing should be curtailed.
Mortar application
The final tooling of the mortar joint affects its water-shedding ability (Figure 5). Thin fins of mortar should not extend onto the surface of the masonry units, as they easily spall and collect water (Figure 5c). Standard finishes range from concave (5a—best
compaction and weather tightness) to raked (5b—worst). If the masonry units have worn, rounded edges, it is best to slightly recess the mortar away from the face. Texture can also be adjusted by using a stiff brush to tap the mortar after its initial set.
Natural bristle or nylon brushes can remove small pieces of mortar at the repointed joint’s edge after the mortar has become thumbprint hard. If the work is carefully executed, there is no need for any other cleaning. Washing the masonry can lead to leaching of the lime out of the mortar and staining.
Figure 5. Examples of different types of tooled mortar joints.
Mortar curing and protection
Fresh mortar joints should be protected from rain, wind, and sun and kept damp for three to seven days. Rapid drying should be avoided because it can lead to shrinkage cracks. Mixes with slower-hardening hydraulic binders (e.g. hydraulic lime) should be damp-cured for at least seven days. Three to four days can be sufficient for portland cement mixes.
Damp-curing can be achieved by installing a wet burlap fabric (covered by plastic sheet) onto the masonry with regular re-wetting of the burlap before it dries. (It should be kept a short distance away from the masonry to avoid mortar staining.) Regular misting of the masonry alone is unreliable as evaporation from the surface can occur quite quickly in warm, dry, and windy weather. If misting is used, water should not run off the joints, because staining may result from lime leaching out of the joint.
Repointing should be done well before or well after freezing weather. If work has been unavoidably delayed to the cold season, a temporary heated enclosure should be used. The recommended curing for 1:2:8 portland cement/lime and stronger mortars is as follows:
1. For the first three days, the masonry should be maintained at a temperature above 10 C (50 F) while the damp-curing takes place. This should extend for seven days in the case of weaker mortars.
2. For the next four days, the masonry should be protected against wind and rain while keeping the temperature above 0 C (32 F)—the damp burlap and plastic should be removed at this point. This allows the mortar to partially dry before exposure to frost. However, excessive mortar drying (heating cold air has the effect of lowering its relative humidity [RH]) should be avoided; a high relative humidity level should be maintained within the heated enclosure. For mortars with very low strength, a longer protection from precipitation may be needed.
Wall repair at the Halifax citadel – proper restoration work requires knowledge of the historical project.
Quality control and future maintenance
The supervision and quality control inspector should visit the site more regularly than in the case of modern masonry projects because workmanship is much more important with low-strength mortars. Inspections should take place during joint preparation, prior to mortar placement, and during repointing.
Quality control tests should be made to check the compressive strength of mortar cubes, the change in sand volume due to moisture, the air content of the fresh mortar, the binder/sand ratio, and the mortar consistency using the Vicat cone test.
After completion of the work, a maintenance manual should be produced to document the evaluation of the masonry carried out before the work started, the materials used for the repointing, and the application practices and the inspection and maintenance schedule. The repointing of older and historic masonry is a specialized construction requiring care in mortar mix design, joints preparation, and mortar installation/curing.1
Notes
1
The NRC-IRC website provides further information about repointing. Visit irc.nrc-cnrc.gc.ca/bes/hmpe/masonry/index_e.html.
Ken Trischuk is a technical officer in the Building Envelope and Structure Program of the National Research Council’s Institute for Research in Construction (NRC-IRC). He can be contacted via e-mail at ken.trischuk@nrc-cnrc.gc.ca.
Madeleine Z. Rousseau is a research council officer in the same program. She can be reached at madeleine.rousseau@nrc-cnrc.gc.ca.
