Am I supposed to specify mortar type M, S, N, or O? 1

[Mike Mike]

Typically for CMU I just specify wall assembly compressive strength f'm on my structural drawings, which determines permissible combinations of block strength and mortar per TMS602-22 specification table 2.

But ASTM C270 and other sources such as the article below indicate an inverse correlation between portland content and bond strength/moisture permeability, and recommend specifying type N in exterior walls if structural design allows for the decreased allowable compressive strength.

https://www.structuremag.org/wp-content/uploads/2022/04/292205-C-StructuralComponents-Chrysler.pdf

International Masonry Institute has been pushing structural engineers to specify higher f'm, switching from default 1500psi to 2000psi or 2500psi, which seems to run somewhat counter to the push for type N mortar.

 

[masonrygeek]

Unfortunately, the article was meant to discuss reducing the embodied carbon of the mortar, but may have confused designers (as noted in your post). The most basic of all mortar recommendations is still Type S for strong or structural, and Type N for normal. So for reinforced masonry Type S should be used (TMS 402 lumps Type M and S together and since Type M is hard to work with, Type S is preferred). For masonry veneer Type N is appropriate. While you can use a Type N mortar for low seismic zones, I think most designers will stick to Type S. And as you say, using a higher f'm is a way to make masonry more economical. And if designers have been paying attention, a default of 1500 psi is no longer used and now a default f'm of 2000 psi is the proper strength. Using an f'm of 2500 psi can help push bars farther apart and maybe make the walls thinner. Engineers are being asked to balance strong and durable structures with low embodied carbon. That can be difficult, but making masonry more efficient does both.

 

[phamENG]

It also depends on the block available to the project. If you're only going to get bare minimum C90 blocks, type N would not be appropriate if you're designing to f'm=2,000psi. But if the supplier is providing blocks that are testing at 2,650psi or above, you can go down to type N if you don't have other reasons to use S or M. For small projects where verification of block strength may not actually happen, best to stick with type S. For large projects with sufficient oversight, type N may be acceptable.

O or 'K' mortar is really just for work on old structures where the cement content in the mortar can cause damage to older, softer brick walls.

masonrygeek - I'm curious, where did you get the impression that they are trying to reduce embodied carbon? I didn't see a mention of that anywhere.

 

[masonrygeek]

I guess I don't see why you would design to a lower strength except to lower the cement content (i.e. lower embodied carbon). Flexural bond strength is actually higher with a Type S mortar (especially PCL mortars) than Type N and both Types N and S can provide for good water penetration resistance. I like that idea of designing for low carbon content, but I also don't want engineers to underutilize the full strength of masonry. To a certain extent, designers have already given up the potential of loadbearing masonry by using to brick veneer (and now thin brick veneer), and by recommending using lower strength CMU walls only pushes masonry into a more non-loadbearing usage. But that's just my take and I could be wrong.

 

[phamENG]

Got it. It read more like an intro to advanced masonry design to me; an attempt to expose engineers to the nuances of masonry design and understanding that a basic strength based approach to assessing the system as a sum of its parts may work but isn't the best way to go about it. Masonry can be used for very high strength applications (not much around here where I live and practice, but I always admire the stuff going up in my home state of Florida when I visit...those masons are magicians), but it's good to understand that other benefits can come from being more intentional in specifying the system components.

So meet strength requirements first, then consider the whole design and make decisions that are informed.

There's also something to be said for not getting masons too used to always using high strength mortar. I showed up to a restoration of a historic brick building (1880s) a few months back. They were halfway through repointing. With type S masonry cement. The mason got a deer in the headlights look when I challenged the contractor on use of incorrect mortar.

 

[Mike Mike]

phameng, I think you're saying some masonry wall costs are sensitive to f'm variation, and other walls are not, right? I think you're saying if we as designers see an opportunity for savings thru specification of type S instead of N, we should weigh the various details and specifics of the project and consider specifying type S, even though this increases the likelihood of moisture intrusion issues? Do you typically specify mortar type on your structural drawings or specifications?

masonrygeek, as stated in my post and multiple times in the article, the reason for specifying lower strength is "mortar with less cement content provides a better bond to the masonry unit, mitigating or eliminating moisture intrusion into the masonry system". Does not appear we're on the same page here. Different planet perhaps.

 

[phamENG]

Mike Mike - it's not really about cost. It's about understanding how the system functions. In a modern wall assembly, you likely have independent structure and building envelope components. Perhaps a CMU wall with a painted on moisture barrier. So you can go ham on the strength of the wall without regard to the bond and water-tightness because the structure isn't keeping the water out - the moisture barrier is.

In a historic building, you get 2 or 3 wythes of brick, some lathe, and some plaster. That's it. The brick needs to keep most of the water out, and then needs to be able to dry properly in between. Add in modern HVAC in that historic structure to make the interior nice and dry, and you'll be sucking water through that wall like a straw. In some cases, a modern wall is built in a similar fashion. So even though you have modern bricks or block that can use high strength mortar, it may not be the best choice for the performance of the building envelope.

 

[masonrygeek]

There are two properties here that shouldn't be interchanged - bond strength and extent of bond. A third type of strength - compressive strength - shouldn't be brought into the discussion because it doesn't necessarily correlate with the important properties of a wall assemblage. Bond strength is important when discussing the structural capacity of the wall. Extent of bond is important when discussing water penetration resistance. As noted in this PCA document on bond strength (

https://bit.ly/3Vd2gqp)

"Increased portland cement content of mortar generally provides increased bond strengths", so I think the article is misleading. It should have said "mortar with less cement MAY provide for a better EXTENT of bond than a higher strength mortar". However, the PCA document goes on to say, "Research has shown that the extent of bond and bond strength do not necessarily relate to one another". In other words, high bond strength doesn't mean that the wall won't leak, and vice versa. It's the compatibility between the unit and mortar that is most important. In general, use Type N mortars for brick veneer (to get good water penetration resistance) and Type S for reinforced block walls (for loadbearing strength). There will be times when Type N is fine for a block wall, but in most cases we are looking for higher f'm which is achieved with Type S, not Type N. And as Pham mentioned for any historical masonry, use low strength mortar so as not to damage the surrounding brick.

 

[Mike Mike]

masonrygeek, yes I think we're on the same page now. I agree the article could have done a better job of distinguishing between bond strength and extent of bond. Do you agree type N provides superior extent of bond? Would you consider using type N where your design was less sensitive to f'm?

phameng, agreed, there's probably no need for increased water resistance if our system has a water barrier

should we be specifying mortar type on our drawings? for typical projects where structural engineers and architects are too busy to give a rat's ass, I assume field masons just select the cheapest mortar that gets them the f'm specified on our drawings, regardless of how much a system may benefit from better extent of bond?

 

[phamENG]

should we be specifying mortar type on our drawings?

This is going to depend on the job. On a big project with a really good and experienced mason - sure. Give them the latitude to come up with their own wall system design and do prism tests to confirm it will work. There's a lot of cost savings to be had there on the right job.

Smaller jobs or jobs where it's reasonable to assume you're just going to get a residential foundation contractor trying his hand at commercial work? Then you should probably go with the Unit Strength Method. If I remember correctly, the tables stating how to meet that have been removed from the body of the building code and are now in design standards - meaning it's no longer reasonable to assume a mason has knowledge of it or even access to it. So, you would need to spell out the minimum requirements for each component to get to your desired f'm.