Mortar Strength vs. Masonry Strength and ACI 530 changes 1

[engjg]

My understanding is that although the minimum compressive strength of concrete masonry mortar of i.e. type S mortar per ASTM C270 is 1800 psi, one can achieve higher masonry strength than the mortar strength. In example, per ACI 530-13 by specifying block with 2000 psi cmu strength and using type S mortar one can use a f'm=2000 psi. My understanding of the explanation for this is that mortar strength has been found through research to not play a major role in the strength of the masonry and that one explanation may be the aspect ratio of the lab mortar cube sample versus the thickness of the mortar is underestimating its strength. I also found it is suggested weaker mortar may be better. I have also noted there is a lot of discussion on the proportion vs. property specification in ASTM C270 leading to question of lab vs. actual field mortar strength. Looking for confirmation of my understanding of these issues and recommendation on further references on the topic...

Also, I note there has been significant changes with f'm per the ACI 530-13 revision...what were the basis of these changes?

If a structure was designed and constructed under ACI 530-02 with masonry specified as type S mortar min. 1800 psi and block strength of 2000 psi could one now justifiably analyze this existing masonry with a min. f'm=2000 psi or would they have to follow older codes which would leave you with an f'm=1500 based on a 1900 psi block strength?

 

[MrHershey]

Part of the basis is just fabrication quality has gone up and higher strength is more in demand than it used to be. It's almost difficult to get 1500 psi in some areas of the country now. Know one of the primary masonry suppliers in our biggest market told us that we shouldn't bother specifying 1500 psi anymore. He's giving us 2000 psi regardless because that's the lowest he's got, so we may as well use the extra strength and be able to back off the design somewhere else.

Article (PDF)

 

[JLNJ]

I think the f'm of 1500 psi stems from the ASTM C90 absolute minimum block strength of 1900 psi with type M or S mortar (ACI unit strength method).

Most block strength is far stronger than 1900 psi so it's a no-brainer to go up at least to f'm of 2000 psi. Heck, you get 2500 psi f'm from 3750 psi net area compressive strength block.

I open my local guy's testing data and it is showing nearly 5000 psi net area compressive strength for "regular" 8" CMU. They didn't bat an eye when I specified f'm of 3000 psi.

 

[concretemasonry]

The block manufacturing equipment and manufacturing methods have caused the typical strengths to go up considerably. Now, the cheapest way to increase the strength is to add water to the typical mix, but other properties are affected. - In the past (when the basic ASTM standards were written), higher cement contents were needed to get the green molded block into the curing areas. Now, the handing equipment is faster and more gentle and the blocks do not have some of the bulges as older units. The current tolerances are still to loose for good masonry. - ASTM standards are very slow and cumbersome to be changed.

When I was in the block industry, I saw a problem in a water meter cause block to test at 8000 psi (net area hollow block) that gave 4800 f'm strengths (net area, 8" two block high prisms).

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dik]

Even though it's part of the design, mortar is vastly underrated for strength... with the confinement in a mortar joint the strength increases dramatically. I don't recall the paper, but, there was some work done about 40 years back on the effects of the mortar joint thickness.

In addition with the CMU's 'sucking' the moisture out, the strength improves more. Try running some mortar cube strengths using a non-porous mold and making a mortar cube using 4 CMUs and paper towel to keep it from bonding to the CMUs. Strength of the mortar cubes will increase 30-40% just from the removal of the water. If you've ever watched a CMU wall being filled with concrete/grout... you can see the outline of the fill level by the damp water line on the wall surface.

Dik

 

[concretemasonry]

dik had a good example regarding the research on mortar and grout and the relationship to masonry units.

The grout sample preparation does a good job of relating to actual results of full wall or panel tests. The preparation of a 4x4x8 grout sample is a very realistic method. - certainly better than a non-absorptive mold that can also suffer from some segregation that could come from moving/handing a smaller. - By the time it gets set enough to move the excess has been removed. - I have seen some international requirements that tried to test grout samples in 2:1 ratio samples and they were grossly misleading.

The small mortar cube samples are only good from lab work. It does not relate to actual field performance. That is also the reason the ASTM C270 mortar specification does not refer to any field sampling and later testing. The specification is by proportions. The compressive strength numbers are really for reference strength levels for types of mortar.

Some international testing can be very unusual. As an example, in the U.S. and most countries the standard of measure is a 2 block high hollow prism(f'm). At a technical bureau/university in Russia, where bigger is better, I saw a 4'x4'x8' (yes, feet) concrete block prism. I have no idea how they were going handle it or get it to failure unless they dropped it.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dik]

In Canada, and I'm not sure of elsewhere, we have a funny situation with our masonry code. It seems we have a prescribed statistical study of strengths to determine the mortar strength and then, arbitrarily, throw a safety factor of 10 (or whatever) on it...

Always wondered who decided to bring masonry from the 'dark ages' to current times... still scratch my head on that approach. The actual testing of materials bears little resemblance to the actual application.

Dik