PDB Debrief: Mortar
Mortar is the element that bonds bricks or other masonry units together and provides structural capacity to the wall or other structure. There are four main types of mortar mix: N, O, S, and M. Each type is mixed with a different ratio of cement, lime, and sand to produce specific performance characteristics such as flexibility, bonding properties, and compressive strength. The best mortar mix type for any project depends on the application and the various design specifications for the masonry construction.
The Contract Documents should always say which type of mortar to use for each application, and if they do not the A/E needs to make the selection. Also, if the type specified is materially different than these descriptions, it is at least worth
a question to the A/E to make sure there is not an error.
Type N Mortar Mix — Type N mortar is usually recommended on exterior and above-grade walls that are exposed to severe weather and high heat. Type N mortar mix has a medium compressive strength and it is composed of 1 part Portland cement, 1 part lime, and 6 parts sand. It is considered to be a general-purpose mix, useful for above grade, exterior, and interior load-bearing installations. It is also the preferred mortar mix for soft stone masonry. Type N is the mortar most often used by homeowners and is the best choice for general application. It typically achieves 28-day strength in the range of 750 pounds per square inch (psi).
Type O Mortar Mix — Type O mortar mix has relatively low compressive strength, at only about 350 psi. It is used primarily interior, above-grade, non-load–bearing walls. Type O can be used as an alternative to Type N for some interior applications, but its exterior use is limited due to its low structural capacity. It is not recommended in areas subjected to high winds. However, type O mortar mix is ideal for repointing and similar repair work on existing structures, due to its consistency and ease of application.
Type S Mortar Mix — Offering a high compressive strength of over 1,800 psi and a high-tensile bond strength, type S
mortar is suitable for many projects at or below grade. It performs extremely well to withstand soil pressure and wind and seismic loads.Type S is the common choice for many below-grade applications, such as masonry foundations, manholes, retaining walls, and sewers, as well as at-grade projects like brick patios and walkways. Although type S mortar must have a minimum compressive strength of 1,800 psi, it is often mixed for strengths between 2,300 and 3,000 psi.
Type M Mortar Mix — Type M mortar mix has the highest amount of Portland cement and is recommended for heavy loads and below-grade applications, including foundations, retaining walls, and driveways. While type M mortar provides at least 2,500 psi of compressive strength, it offers relatively poor adhesion and sealing properties, making it unsuitable for many exposed applications. Type M is preferred for use with natural stone because it offers similar strength to that of stone.
Type K Mortar Mix — Type K mortar is rarely used for new construction but may be specified for restoration or other specialty applications. It offers a very low compressive strength of only about 75 psi. Because of its softness, type K is primarily used for restoring the masonry on historic or ancient buildings that require a special mix that is not significantly stronger than the existing masonry.