f'm v mortar strength

[HSBNOLA]

No matter what the assembly strength (f'm) is, is the mortar (if type s) always 1,800 psi, per ASTM C270?

f'm = 3,000 psi uses a significantly less mortar strength - seems odd at first glance.

 

[EZBuilding]

I wish ACI was more explicit in describing this within the code.

Based upon my understanding, mortar strength does not have a substantial impact on the capacity of the masonry assembly.

 

[Aesur]

Take a look at TEK 18-01B5: Link. Per this TEK it implies the Unit Strength Method is based on the assembly which is made of the CMU block and is controlled by the block strength and mortar type, see Table 1 of the document. So for f'm = 3000 psi you would need to specify the blocks as 4800 psi with Type M or S Mortar.

 

[HSBNOLA]

thanks for the responses.

 

[masonrygeek]

The simplest way to explain f'm for masonry is that mortar is the weak link in a masonry assembly, and a combination of unit strength and mortar strength provide a strength higher than the weakest material. Each material provides some strength to the assembly, not just one material. There are charts in the TMS 602 Specification (previously ACI 530) that show the relationship. BTW, the old default of 1500 psi is gone with the 2013 version of TMS 402/602 and is now 2,000 psi. As Aesur mentions NCMA TEK talks about this.

One additional point is that TMS 602 doesn't differentiate between Type M and Type S mortar even though their material strengths are different. A whole bunch of tests showed that their use in a masonry assembly showed they provide the same strength to the assembly. A little confusing, but that's what the data showed.

 

[JLNJ]

A mortar cube breaks in a different manner than mortar fails in a prism assembly. I don’t even know if failure of the mortar is common for prism breaks.

Similarly, a prism breaks at a different strength than the masonry unit alone.

In flexural tension, the mortar is definitely the weak link.