Mortar Evaluation 2

[jike]

Why does the ACI 530, 3.7B, require only lab compressive strength testing (ASTM C270) for acceptance of mortar? Field testing (ASTM C780)is only to verify consistency of materials and procedures, NOT FOR MORTAR STRENGTH. Yet, the ASTM C780 tests for compressive strength.

This seems like the mason and mortar suppliers have lobbied hard for this provision to remove any checks and balance on their in place work. Other than a prism test, what other basis do we have to evaluate the strength of mortar (really interested in the strength of the entire assembly) that we are actually getting in the field?

 

[casseopeia]

jike,

Compressive strength for mortar is not usually your primary concern. No reason to have the mortar stronger than the masonry unit (generally around 1800-2200 psi). In fact, higher compressive strengths in mortar works against the overall integrity of the wall by reducing bond strength, weatherability, etc.

Specify ASTM C1072, bond wrench test for flexural bond strength. Check out this link to Brick Intitute's tech notes on mortar. Lots of good material:

http://www.bia.org/BIA/technotes/t8.htm

 

[jike]

Casseopeia:

Thanks for the suggestion of using ASTM C-1072.

I have a project where the mortar strength test per ASTM C-780 is 880 psi vs. 1800 psi. It seems that something is possibly wrong. This is pretty low.

 

[UcfSE]

Sounds like it might be time to grout the wall solid and move on with life

 

[Ron]

jike...your results are not unusual. When pre-bagged masonry cement is used, the strengths are often lower than using the true proportion specification of C270. This is probably what was used as most subcontractors prefer it.

Further, unless you specify controls on the mixing, you'll get results all over the board. If they are using a shovel to measure the mixture, the results will be different in the morning compared to the afternoon....sand dries out during day...less stays on the shovel....so, higher relative strength given the same cement quantity.

Use C780 and pre-qualify the mortar. Then use C780 or prisms to get relevant field data. I suggest small cylinders rather than cubes for field use.

 

[haynewp]

Even if the mortar strength is low you may still combine it with the actual strength of the cmu unit and come out with a good compressive strength of masonry.

I had this problem before on a project. The mortar tested low, more like a type 'N' mortar instead of type 'S'. I got the information for the actual block unit strength from the mfr. The average 'unit' strength according to them could be estimated at around 2500psi. When I combined the low mortar with the high unit strength per table 2 of ACI 530, I still got my total f'm to be 1500 psi. But like you stated, the field test is more for consistency anyway.