Mortar Compressive Strength 2

[BSVBD]

Upon reviewing masonry submittals, I notice that the Compliance Test Results for Concrete Masonry Unit Compressive Strength of 2,225 psi exceeds the specified requirement of 1,900 psi. However, the Compliance Test Results for Mortar of 1,140 psi is less than the 1,900 psi for the concrete masonry units.

I understand that I did not specify a minimum requirement for Mortar, but, I believe it should understood that the mortar should also equal the specified minimum for the units. How else could we have an acceptable wall if the mortar is not as strong as the units?

I realize I can reject the submittal, but, why would a supplier submit a mortar with a tested compression strength of less than the masonry units specified minimum compression strength?

I also recall similar discussion regarding unit strength versus mortar strength, but, I do not recall the verdict and I want to make sure I understand something that I, perhaps, do not, if I need to.

Please advise.

Thank you!

 

[Ron]

BSVBD....the mortar is never as strong as the unit. Under ASTM C270, there are two approaches for masonry mortar. One is the proportion specification where the compressive strength of the mortar is somewhat irrelevant and there is the property specification which is based on the mortar type; either type M, S, N or O. Each has its own properties and they are related to compressive strength; however, ASTM does not subscribe to field testing mortar for compressive strength for acceptance or rejection.

I would suggest you either follow one of the Type specifications in C270 or is you suspect the mortar/mixing process to be used in the field then go through the construction pre-qualification procedure for mortar under ASTM C780.

In your specification I would recommend that your require a volumetric measurement of the sand and cement to be used. What I have done in the past is require that the masonry contractor or general contractor construct a plywood box that is one cubic foot in volume and that the box be used for measuring all solid items to go into the mixer. Most masonry subs will measure the sand by using a count of shovels full of sand per bag of masonry cement. Keep in mind that volume changes during the day. For example, in the morning when the sand is damp, the shovel will have more sand on it and thus the mix will tend to be oversanded and lower in strength. In the afternoon as the sand dries out, there will be less sand on each shovel and the mix will tend to be undersanded and higher in strength. When you use a volumetric box for both the cement and sand, these variations are minimized. This will help them comply directly with the proportion specification of ASTM C270. Yes, the contractor will complain. Stick to your guns and make them do it.

Look at the design provisions of ACI 530, particularly with respect to inspected on non-inspected provisions of the masonry construction.

 

[Aesur]

Well said @Ron. I encountered this years ago when we started designing for 2000 psi in lieu of 1500 psi based on the updated Tek note 18-1C. I spent a good amount of time researching this to realize the that when we specify and design for 2000 psi, the mortar usually comes out as 1900 psi, but the assembly is still 2000 psi when using the correctly specified mortar, in our case Type S Portland. The contractor on that particular project didn't feel comfortable and ended up doing a prism test and found the assembly had strength much higher than specified.

 

[dik]

Further to Ron... you don't want the mortar to be stronger than the unit. This is true for millenia. You want the failure due to differential movement to occur at the mortar joint and not the masonry unit. It's better to re-point the mortar joints.

For a real eye opener, next mortar cube you make, put 4 cinder blocks in a pinwheel pattern and line the interior space with paper towel and cast your mortar into the space. The moisture leaves the mortar being 'sucked' into the cinder blocks. There is a significant increase in strength that is unaccounted for with a normal mortar cube.

I used to have some literature about mortar strength and joint thicknesses. Samples were made using dolomitic limestone with joints of varying thicknesses. As the joints got smaller, the strength increased significantly.

Just some info...



Dik