Masonry Restoration refers to working with and repairing any form of masonry on an existing building that does not include actually building the structure. Some precursors to a restoration jobs might be cracks in mortar joints, damaged plasterwork, loose bricks/stones, deteriorating mortar, etc. Some causes for the deterioration might be capilary action causing rising damp conditions, differential building settlement, leaking roofs or gutters, etc, and these issues must be dealt with before the masonry restoration begins. One of the more common types of masonry restoration is ‘joint pointing’ (i.e. tuckpointing, repointing) which consists of removing and replacing the damaged or deteriorated mortar in the bed and head joints. ‘Pointing’ can improve the structural integrity of a masonry wall but the primary purpose is for waterproofing.
First the joint repair mortar must be chosen, often by analyzing existing historic mortar and sometimes even a lab analysis. The goal is to choose a mortar that matches the existing in color and texture. Finding sand similar to the type originally used is one of the most critical aspects of matching a good repair mortar. Also repair mortar must be more permeable than the masonry units and no harder or more permeable than the historic mortar. When repointing a masonry wall, the more recent trend has become a mortar with higher amounts of lime and lower amounts of cement. This creates a ‘softer’ more flexible bond which increases the sealing quality of the work. Also note that mortars higher in lime content than cement are more permeable.
A good restoration mason is seen as a bit of a magician, turning an old surface into one that appears new. And he typically knows the difference between a 50 year old building and a 100 year old building. Test panels are often required on larger projects on certain lower visibility areas of the building, allowing the architect or engineer to approve the work prior to startng the bulk of the project. The old mortar is commonly removed to 2 or 2-1/2 (rarely deeper than 1″ unless loose/disintegrated mortar is discovered deeper) times the width of the joint to achieve a quality bond and minimize possibility of popouts. One exception to this rule would be stone masonry with wide joints which may require removal of several inches. Mortar can be removed by way of hand chisels and mash hammers (traditionally) but more commonly is done with the use of power saws or grinders. The method used depends on how careful the moasons must be and how delicate the existing structure has become.
Often both hand tools and automatic tools are used in combination. Using power tools tends to be more acceptable on quartzite or granite hard stone than on Terra-Cotta or soft brock or stone. The joints are then typically rinsed with a water jet to remove loose particles. When filling the joints with mortar they should be damp but with no standing water. When preparing pointing mortar, the dry ingredients are commonly mixed first followed by half of the water required. Then small amounts of water are then added until the correct consistency is achieved. Mortar will typically need to be used within 30 minutes and retempering with water is not considered by most engineers a good practice as it may disrupt the intended mix and water cement/lime ratio. Other types of mortar used for pointing restoration is lime putty (Type “O” and “K”) referred to as roughage; mixed with sand, water already present) which will have lower strength than hydrated lime mortars, but will have a more workable consistency.
Lime putty may be premixed ahead of time as long as it is stored in sealed containers with a wet burlap cloth. Sometimes a portland cement may be spcified in conjunction with a lime putty. In this case the cement must be mixed with water to form a paste before mixed with lime putty to avoid clumping. When filling joints with mortar, the first areas to receive attention should be 1″ and deeper joints. To avoid mortar shrinking, these deeper joints should be packed and compacted in 1/4″ lifts while allowing each layer to harden (thumb print) before applying the next. Final surface finish tooling must be timed appropriately (thumb print hard). If tooled too early color may be lighter and hairline cracks are more likely too occur. If tooled too late, closure of the masonry-mortar connection may be compromised and also dark streaks could occur which are referred to in the field as “tool burning’. After tooling the joint, any excess mortar can be removed from the joint ledge by a bristle or brush.
Lime content mortars (Type “O” “K” and “L”) curing/hardening can take place quickly as water is lost to porous brick surfaces and evaporation. It is common for wetting or ‘misting’ the joints periodically when thumb print hard for a day or two after pointing. Some times this is done every hour initially and gradually reduced to every 3-4 hours. Covering walls with burlap for 3 days is often recommended. Plastic may also be used but should not directly touch the wall (tented). Either burlap or plastic lock moisture in for curing and protect the joints from direct sunlight. Lime mortars will ‘carbonate’ (strengthen) for many years until reverting. Pointed joints may take 2-3 months to fully cure. Because of numerous variables, matching mortar colors to existing is nearly impossible, and the only comfort is knowing that time will draw the similarities. Mismatched color issues may be imporved by cleaning non-repointed areas or staining new mortar, but are generally not recommended because of added weathering differences over time.
When pointing is executed properly, cleaning is often not required aside from removing excess waste mortar with a bristle or brush. If additional cleaning is necessary or required, it is best done with plain water and natural bristle or nylon brushes and a minimum of 30 days after pointing. If chemical cleaners must be used, they should only be applied to damp masonry, and be chosen and utilized with extreme caution and exactly as directed by manufacturer. When finished the walls should always be completely flushed of any chemical residue. Pressure washers are commonly used but should be set to 100psi max. “Bloom” or efflorescence typically occurs around the first few months of pointing and will typically disappear on its own. Hydrochloric (Muriatic) acid is more or less ineffective at cleaning effloresence and may stir up other salts and additional efflorescence..
In some cases, masonry unit deterioration is so severe, entire brick removal/replacements must be done.
Repointing work is intended to last a minimum of 30 years but preferrably 50-100 years
|Mortar Types (Measured by volume)|
|Designation||Cement||Hydrated Lime or Lime Putty||Sand|
|M||1||1/4||3 – 3 3/4|