CMU wall mortar strength, field test data

[dcceecy]

We have specified mortar mix strength of 1800 psi. But for several times, the field test data shows those mortar sample has only about 1200 to 1500 psi per ASTM C780. I checked online it seems a quite common situation.
Can anybody have some reference for that? What is minimum strength we can accept?

Thanks.

 

[strucguy]

Please refer to the link below. They have a pretty good explanation.

http://www.cement.org/masonry/compressive_strength.asp

 

[concretemasonry]

What is the need and basis for the mortar strength?

ASTM 780 is for pre-construction approvals and not site testing.

Did you specify the mortar based on EITHER proportions or performance (lab tests)? - You cannot use a combination!

If you look at ASTM 270 (Appendix note #1), you will find a recommendation to use the weakest mortar necessary to carry the "structural" loads because there are many properties for mortar (workability and others) that are more important than a simplistic lab compressive strength. The big challenge is to produce a field sample that can be cured and tested in a lab that is representative of actual construction.

From a structural strength, mortar strength is really not important in the strength of a wall. I have made hollow block prisms with 1800 psi mortar than tested at over 4800 psi (f'm) because the masonry unit controls the compressive strength and not the mortar.

The PCA reference noted earlier is fairly good in that it alludes to the aspect ratio of a specimen (cube with shear as the failure measurement) in comparison with a real life situation of a 3/8" joint that is much wider that is put into a situation of direct compression with stronger(intentionally) units with a different failure mechanism.

Dick

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