ASTM Requirements for Mortar

Mortar performs many functions: it fills gaps between units, bonds the units together, provides weather protection. Simply changing mortar color or joint tooling provides opportunities for varying architectural treatments. Mortar specifications contained in ASTM C 270, Standard Specification for Mortar for Unit Masonry, spell out material requirements and typical formulations.

Mortar Types: by Proportion, by Property

Mortar types are designated by letter as Types M, S, N, O, and K (every other letter in the word MaSoNwOrK). ASTM C 270 contains two alternative specifications of masonry mortars, property or proportion. Mortar can specified by only one of these methods, not both. Under the proportion specification the type of mortar is created using volumetric proportions of cementitious materials and aggregates as set forth in Table 1 of ASTM C 270. No physical requirements are required of the mortar with this method. The property specification requires pretesting of the mortar mix designs in the laboratory to establish compliance with Table 2 of ASTM C 270. The mortar produced using the property specification must meet the required properties of compressive strength, water retention and air content. Factory blended mortar generally is manufactured to meet the property requirements of ASTM C 270.

Mortar Usage

A wide range of mortar types are available for use, depending on durability requirements, unit types, and the initial rate of absorption (suction) of masonry units. Mortars with a high cement content (Type M and S, for example) are very strong but are not as “workable” as mortar with a lower cement content (Type N or O). A good rule of thumb is to not use excessively strong mortars, with a high cement content, as these mortars are less able to retain water and can be prone to shrinkage cracking. Type S and N mortars are commonly used for veneer and structural applications, although Type N mortar is prohibited in the higher seismic design categories.

Mortar Joint Filling

Typical mortar joint thickness is 3/8” for most applications. Mortar joints must be completely filled to resist water penetration and for proper bond. When building with solid units, mortar must completely fill all bed (horizontal) and head (vertical) joints.

For hollow masonry units used in reinforced masonry construction, bed joint mortar is placed only on the face shells. Head joints are filled at each wall face, to a depth equal to the face shell thickness. The exception to this is at columns and pilasters, where all face shells and cross webs are typically filled to resist structural loads. The bed joint at the starting course on the foundation also needs to be solidly filled. The Building Code Requirements and Specification for Masonry Structures Specification (Section 3.3 B. 1) requires the starting bed joint to be between ¼ and ¾ inch thick.

In multi-wythe construction, collar joints (less than 1” wide) or reinforced spaces should be completely filled with grout. Drainage cavities, on the other hand, should be free and clear of mortar obstructions that inhibit water drainage.

Pigmented Mortar

Adding pigments to mortar gives the architect an opportunity to change the wall’s appearance with little extra effort. Pigmented mortars are more expensive and can be tricky to batch, place, and tool to get a consistent color throughout. Small variations in mortar color will normally fade after final cleaning or with several months’ weather exposure.

Mortar Joint Profiles

The final act of laying mortar is tooling, with a metal sled or runner, after the mortar is fairly hard (often called “thumbprint hard”). Joint tooling compresses the surface of the joint and brings cement fines to the surface to provide a durable weather surface free of cracks or delaminations. Concave tooled joints are most resistant to weather penetration. Raked or struck joints are sometimes used to create shadow lines for architectural effect, but this practice creates ledges where rain and snow collect, often leading to staining or increased water penetration.

Mortar Joints Diagram

Specifying Mortar

Mortar Quality Assurance & Testing