QUESTION

Mortar Bed Compressive Strength - An architect has requested that the mortar bed be at least 4000 PSI compressive strength. Do you know on average what a typical TCA F-111 would be with standard raw sand and
cement? Any information will be appreciated.

ANSWER

ANSWER - Laticrete 226/3701 and Mapei Mapicem are mortars the say they can achieve 5,000 psi compressive strength or greater.  Note that the reported values are based on laboratory results, not field testing.  Field testing is always lower.  You will want to point that out to the architect in advance.

ANSI doesn't have any compressive strength standards for mortar beds.  A118.7 modified grout has a 3,000 psi compressive strength.

I'm not sure of exactly what the compressive strength would be for a typical mortar.  I would guess that a regular mortar bed should get over 2,000 psi if compressed correctly depending on the cement to sand to water ratio.

The redimix concrete trucks can create a design mix at a certain value by using more cement and less water to reach higher strengths.

There are several key factors in achieving a high compressive strength.  Higher cement content adds more strength but don't go higher than a 1:3 mix of cement to sand.  The less water you use will achieve a higher strength down to a 25% moisture content, which wouldn't be a workable mix.  That is why a properly mixed dry pack mortar bed will give you more strength, which is the type referred to in ANSI as implied by requiring tamping the mortar bed to compress it.   I think the most important part is properly compressing the dry back mortar to achieve high compressive strength.

A large project had a problem with this because the installers where not compacting the mortar.  They were simply dropping the stones on the inconsistent volume of mortar and not fully beating in the stones.  They were using a 5,000 psi mortar, but because they didn't fully compress the mortar bed they were getting 1200 psi values from samples taken from the job.

What I normally do on our jobs is either have the installers pre-float and make sure the dry pack is well compressed, or when installing individual large format tiles one at a time wet set, we have the installer dry beat the tile into the fresh dry pack mortar to compress it and make sure it is in full contact.  Once the mortar is fully compressed and at the right height to adjacent tiles then butter the backs of the tiles and beat the tiles in.  This method gives a very dense and strong mortar bed.

Hope that helps.