By: Jimmy Monahan
In the construction world, there are thousands of manufacturers who provide countless types of building materials. When it comes to façade restoration work, selecting materials from a large variety of brick, stone, terra cotta, and mortar materials can be overwhelming. A considerably less understood component is mortar. Mortar may seem like a trivial component of a building; however, it is extremely important, as bonds the structure together and provides structural integrity. This article will briefly outline the various types of masonry mortar and their best uses.
The 6 most common types of mortar are: Type M, Type N, Type O, Type S, Type K, and Specification Mix mortar. Each type of mortar has specific material properties that are tailored to the design requirements of the built structure. The selection criteria of mortar is based primarily on compressive strength, which is measured in pounds per square inch, also known as “psi.” Compressive strength is the resistance capacity that a material can withstand before breaking under pressure. Selecting the appropriate compressive strength is imperative when selecting mortar because it can affect all surrounding materials. If the mortar is “too soft,” it can easily crumble and fail, and if the mortar’s comprehensive strength is too high, it can cause the adjacent masonry to fail.
The strongest mortar available is type M, which offers the highest compressive strength available and has a minimum psi of 2,500. Since it can sustain higher gravity and lateral loads than its mortar-type counterparts, this mortar is best suited for below-grade application. Type M mortar is often used with stone or masonry units for the construction of retaining walls because the mortar must resist lateral pressures as well as vertical loads. The type S variety is a medium-strength mortar that is commonly used for below-grade or at-grade applications; this mortar variety is frequently utilized in the construction of exterior walls and patios. Type S mortar is stronger than the more common type N mortar, is best suited for more moderate loads, and has a minimum compressive strength of 1,800 psi.
The most commonly used mortar is the general purpose, type N. This medium-strength mortar is known for its above-ground applications, especially in the construction of interior and exterior walls, and has a compressive strength of approximately 750 psi. Type N mortar is typically specified for clay brick masonry or concrete masonry wall construction. It is the standard mortar Sullivan Engineering specifies for the majority of our façade related projects. Type N mortar is “softer” than its masonry unit counterparts and will not cause the face of masonry units to crack. If the mortar strength is greater than the compressive strength of the masonry unit, the bricks will begin to crack and spall, eventually resulting in masonry unit failure.
Type O mortar is even softer than type N, as its compressive strength is a mere 350 psi. Type O is often specified for interior nonstructural applications and is commonly used in sandstone or brownstone construction. The mortar is soft and shares similar physical properties to sandstone and brownstone; however, it is not recommended for use in any load bearing construction. Type K mortar is no longer specified, as it has a very low compressive strength and cannot be used in structural application. It is used solely during historic preservation work because it is extremely soft and will not damage the existing stone and masonry. In historic preservation projects, it is crucial that the mortar being utilized is softer than the existing masonry or stone; if the mortar is too strong, it will apply undue pressure and damage the stone or clay fired masonry.
The final mortar is known as specifying mortar, which is essentially a custom mortar that is project specific; this mortar may be used in cases where the design engineer requires specific properties that cannot be met by the standard mortar types. A structural engineer will curate a unique list of required properties for its application, and the mix is created and tested in a laboratory. The list can specify qualities such as compressive strength, water content, workability, air content, stiffening, and hardening. Once the proper ingredients and proportions are known, the mortar can be prepared in the field by the Contractor.
When selecting or specifying a mortar, it is crucial to know what it will be used for. Each mortar type has its own purpose and will function under the appropriate application. In the event you are unsure of the correct material properties required for your restoration project, please always consult with a structural engineer or architect to get the correct information – it will save time, money, and most importantly, the integrity of your building for years to come.