ACI 530.1 05 Table 2 1

[Hoboneer]

ACI 530.1-05 Table 2
Why are there different strength units listed under Type M or S and Type N mortar?
What does this mean? That 2150 psi units can only be used with type N mortar?

 

[mike20793]

This table is for the Unit Strength Method to achieve the listed f'm rather than doing a Prism Test (which is the preferred method).

 

[JAE]

We never use prisms at all....only unit strength method.

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[dik]

and if you make a prism using paper towels and CMU blocks as a form, you will end up with a much higher strength because of the water absorbed by the CMU blocks.

Dik

 

[Hoboneer]

I suppose I didn't phrase my question clearly. Let me try again. ACI 530.1 Table 2 has 3 columns. The first column lists various psi block (1,900 2,800 3,750 4,800) combined with type M or type S. The second column lists DIFFERENT various psi block (1,900 2,125 3,050 4,050 5,250) combined with type N. So the question is why the different psi block for type N? What if I want to combine 2,150 with type S, for example?

 

[JAE]

Well this table is labeled Table 3 but it has the same numbers you reference:

This table lists in the left two columns several types of BLOCKS (or as the table states - UNITS) and their associated net compressive strength. These
are the compressive strength of ONLY the block itself - not the MASONRY which combines blocks with mortar.

The far right column is the design compressive strength f'm of the MASONRY (i.e. blocks combined with mortar).

So if you are using blocks that provide 1900 psi strength (block only strength) and you use type N mortar, your design f'm would be 1350 psi.

So to your question - why the different types of BLOCKS in the left two columns? Probably due to limitations on testing perhaps? You are right - it doesn't make sense.

If you had a block that met the 2800 psi level, and you used Type N mortar - I would think that you'd have to use f'm = 1500 unless ACI 530 mentions interpolation here...which I doubt.

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[Hoboneer]

Thanks JAE. I swear I've been looking at this damn table every now and then for years and was never able to come up with a satisfying answer. I'm glad I'm not the only one that thinks it doesn't really make sense.

 

[JAE]

Staneer - one thing that "might" explain it a little - if you read the commentary in 530.1-11 they show a graph plot of various points showing the relationship between compressive strength of units vs. compressive strength of masonry separated by mortar type. (see Figure SC-2 on page S-17). This plot creates two separate lines - one for M and S mortar and one for N mortar.

To define the lines, Table 2 must have simply used numbers from that plot to define the relationships, perhaps rounding up or down to get close to "typical" block strengths.

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