Masonry Magazine October 1997 Page. 24
Mortar joints are typically removed with a chisel and hammer. In brick masonry or other masonry walls were the distance between the courses is less than three inches, vertical head joints should always be hand chiseled unless a grinding blade can be located that is smaller than the distance between the brick coursing. Grinders also may be more acceptable on hard stones, such as quartzite or granite, than for soft brick or stone. The test panel will determine the acceptability of the use of grinders. If power grinders will be permitted, the contractor should establish a quality control program to account for worker fatigue during the day and similar variables.
Mortar should be removed cleanly from the units, leaving square corners at the back of the cut. Before filling, the joints should be rinsed with a jet of water to remove all loose particles and dust.
Mortar Types
A mortar strong in compressive strength might be desired for a hard stone pier holding up a bridge deck, where as a softer more flexible mortar would be desired for a tall masonry chimney built from soft brick. If the two mortar mixes were switched, the bridge deck could collapse due to the soft flexible mortar being washed out from the water pressure of the river current and spalling and cracking of brick in the tall chimney could result as the high strength mortar restrains temperature and moisture related volume changes in the brick. The control of the mortar property, strong or soft, ultimately lay in the hands of the workers on the job site. If mixed with higher amounts of portland cement, harder mortar is obtained. The more lime that is added, the softer and more flexible the mortar becomes.
Five mortar types were established in 1954 to distinguish high strength mortar from soft flexible mortars. Type M (2,500psi), Type S (1,800psi), Type N (750psi), Type O (350psi) and Type K (75psi). The designation letters were taken from the two words, MASON WORK utilizing every other letter. The designation "L" in the chart below identifies a mix of straight lime and sand.
The sand ranges shown accommodate for different particle size distribution characteristics.
| Masonry Material | Selected Exposure Sheltered Moderate Severe |
|---|---|---|
| High Durable granite, hard core brick, etc. | O N S |
| Moderately limestone, durable stone, molded brick | K O N |
| Poorly Durable soft hand made brick, friable stone | L K O |
Curing Conditions In The Wall
At the time of filling, the joints should be damp but with no standing water present. For masonry walls that are extremely absorbent like limestone, sandstone and common brick, it is recommended to apply a continual mist of water for a few hours before repointing begins.
The preliminary hardening of high-lime content mortars (Type O, Type K, and straight lime sand mortar) takes place fairly rapidly as water in the mix is lost to the porous surface of the masonry and by evaporation. The initial hardening is accelerated in Type O and Type K due to the presence of portland cement. In straight lime, often called pure lime, the mix hardens by a process identified as carbonation where the lime putty absorbs the carbon dioxide from the air converting itself back to limestone. Recent research has indicated that the carbonation process may be significantly accelerated by periodic wetting of the work after placement.
Therefore it is recommended that high lime types be periodically wetted with a hand sprayer with a fine nossle once thumbprint hard and the finish tooling has been carried out. Generally this misting process is simple to execute for a day or two after the mortar has been placed. Local conditions will dictate the frequency of wetting, but may be as often as every hour initially, then increased to every three or four hours. As discussed earlier, if a high lime putty mortar (especially straight lime and sand) if left to dry out rapidly the carbonation process will be retarded resulting in chalking and poor adhesion and durability. Walls should be covered with plastic or burlap for the first three days after installation. This helps keep the walls damp and protects them from the direct sunlight. Once the carbonation of the putty has initialized, it will continue for many
| Mortar Type Designation | (measured by volume) Cement: Lime: Sand |
|---|---|---|
| M | 1: 1/4: 3-33/4 |
| S | 1: 1/2: 4-41/2 |
| N | 1: 1: 5-6 |
| O | 1: 2: 89 |
| K | 1: 3: 10-12 |
| L | 0: 1: 2-3 |
Successive repointing attempts over the past 100 years have left wall mis-matched (color) and brick structurally damaged due to incompatible ingredients.
Mason Jimmy Price shown beating the lime mortar pre-mix. Photo credit: Thomas Jefferson's Poplar Forest and Price Masonry.