high test results for regular strength masonry grout

[msucog]

what is the downside of getting high strength test results out of "regular" strength grout and at what point do we really care? as a testing firm, should i really raise an uproar if said results are high and the EOR doesn't raise an eyebrow? i know some of the concerns...but would like to hear other's opinions.

 

[Ron]

The most prominent issue is lack of elasticity. It makes the mortar brittle and susceptible to more shrinkage/separation from the masonry units.

 

[concretemasonry]

msucog -

I would definitely report it to the EOR as being unusually high in a tactful manner.

I say that because I do not know what the specifications are or what the design methods were.

Grout is intended to be compatible with the materials it is used with. that means it should have similar strength and same physical properties for it to become a part of the structural element (the wall) that is being built and not treated as a microscopic independent element.

The grout is meant to transfer the load for the masonry units to the reinforcement.

A high strength grout usually has too much cement, which requires too much water to get the proper slump to properly fill the cells. The result is greater compressive strength, more shrinkage and inability to act in unison with the rest of the wall materials in that structural element.

The most amateurish step is to use high strength grout in grouted prisms made with lower strength masonry units to get a high f'm. This leads to the situation where lower strength units are used for the outer fibers, where the the units can incorrectly fail in compression. A bigger hammer (strength) in not the correct solution when you are dealing with a balanced design concept, based on years of research.

A good specification with give a minimum specified strength and hopefully, an maximum allowed strength or a percentage over-run. A bare minimum strength may be correct, but excesses should be reported to the EOR, in case he was not aware of the materials and methods that were being used and not caught by the "boilerplate".

I have had high strength hollow masonry prisms made using 8700 psi block that gave a 4500+ psi f'm with 2500 psi mortar, but no engineers had a need for the high strength. There a high strength grout could have been used, but never was required. However, it was good advertising and gave engineers the confidence that they could go a couple of steps beyond if they ever had the need.

Dick

 

[msucog]

thanks for the input. that's generally the thoughts i had...effectively translating that to the project team will be hard to not come across as "just being difficult". it's sort of like trying to explain why using higher strength concrete than specified to combat cold weather is not necessarily a good idea on floor slabs..."well it's stronger than it's supposed to be" sort of mentality that we're often stuck dealing with (but then they don't understand why the Ff/Fl are out of spec)--quite often it's a no win situation trying to look out for the best interests of the owner/engineer/contractor but such is the life of a testing firm

thanks again for the comments