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RE: Masonry mortar: Low Compression Results

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Constituent sample strength is a poor indication of masonry strength in the field. If your low strength was a laboratory mixed value, you probably have problems. And the mix should have never gone to the field. If your low strength was from a field mixed sample, don't worry about it.

Special inspection, verifying the mortar proportions, and monitoring consistancy is much more important than the compression strength of constituent samples. The only way to verify proportions in the hardened mortar is a petrographic analysis. That is very expensive to do, and probably not necessary.

References:
1.  PCA:
http://www.portcement.org/pdf_files/IS040.pdf

2.  BIA Tech Note 3a and 8 on the BIA web site:
http://www.bia.org/html/frmset_thnt.htm

3. Minnesota Concrete Masonry Association "Recommended Practices for Specifying and Testing Masonry Mortars for Concrete Masonry" that is particularly good with the commentary.

In summary of the above articles. Don't use the compression strength of field collected constituant samples as an acceptance criteria. If in fact strength is an important criteria, do a prism test. We structural engineers get hung up on strength as a performance criteria. In some areas, that concern is justified. In some other areas (like masonry components) strength is NOT a good criteria. The f'm of the ASSEMBLED components (prisms) is what we want.

By the way, the differences in component vs. prisms are even more pronounced in CMU construction.

If you read the specs like the ACI 530, you will see that you can not use component strength of field collected samples as an acceptance criteria. Thus if you make the contractor take remedial action, the owner will have to pay for it. And it is probably not necessary. If strength is THE issue, you should have required field assembled prisms.

Regards,
Harold Sprague




From: hadiprawira djohan <hadiprawira(--nospam--at)yahoo.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Masonry mortar: Low Compression Results
Date: Tue, 12 Oct 2004 15:06:48 -0700 (PDT)


Fellow Engineers,



Recently, I received many low compressive results for masonry mortar testing. Most of the low results were found with the brick mortar. I specified Type S mortar, which should have 1800 psi strength, yet the result came back as low as 900 psi.

This project is an 8 story CMU structure with a brick façade. There is a relieving angle placed at each floor to carry the brick load.

The following are my questions:

Since the brick is supported at every floor, should I settle with Type N mortar (750 psi)? What is the standard mortar type for a one-floor brick wall structure? Is there any impact of the low strength compression in the long run? Will lower mortar strength cause premature repointing? The material testing company used a plastic form, which doesn?t allow any water absorption compared to brick, thus the plastic form resulted in a higher water content than the actual condition. Is it possible that the cause of the low compression is due to the method of the sampling? Is there any insitu testing that can be performed to verify mortar compressive strength other than winsor-pro? Did anyone encounter this situation before, and what was your course of action as the EOR?

 Any thought counts! Thank you as always?

Djohan
Brooker Engineering, NY.

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