Maximum Mortar Bed Thikness for Concrete Block

[connect2]

So interested here to know what maximum bed joint thickness would be permitted before compressive strength of the mortar might be affected.

TMS 602-13/ACI 530.1-13 3.3.B Placing mortar and units indicates that a 3/8" (9.5mm)thick bed is anticipated/typical. 3.3.F Site tolerances indicates that the bed joints between courses may vary by +/- 1/8" (3.2mm).

All issues of coursing aside and not including the first mortar bed on top of the foundation.

Questions:
Is anybody aware of what mortar bed thickness greater than that indicated above would cause structural concern?
For dumb reasons, not ours, there is a coursing issue here. The trade is proposing a 33/64" or 13mm as the standard mortar bed depth.
Although it doesn't make me comfortable would this 13mm bed depth as a maximum be acceptable?
Not keen on letting a site tolerance become interpreted as the new spec.

Comments/Thoughts?

thank-you.

 

[concretemasonry]

connecct2 -

For good construction, the the mortar bed thickness is not critical, since the intent is to be a structural element (a wall) that is economical and structurally reliable.

You are venturing into the world of myopic engineering when you look at the properties of specific different materials/elements used in a composite sample.

Just look at the ASTM standards for compressive strength of CMUs and mortar. For compressive strength, the mortar is lab prepared and tested in the form of a cube with a failure mode the is somewhat like a confined compression/crushing. The CMUs are tested in the form of a single unit and the typical failure is a pure shear failure.

The "magic" number for masonry structural design is f'm - the compressive strength of a 2 unit high hollow block prism laid with mortar. This represents a way to combine the different material properties. The first appendix note in the ASTM C270 mortar specs states that the most important property is the workability (not strength) of the mortar to be to support the wall during the construction process.

All of the basic codes and specifications (ACI 530) and design standards reference this (f'm) for wall design. - Solid grouted grouted or not.

Based on the decades of testing, this has been the basis because of agreement with wall section results. Which is very costly, unreliable and very, very slow.

If you look at the specifications for mortar there is no provision for site sampling and testing of mortar for strength and the numbers referred to are only for laboratory prepared mortar samples. I had some block and mortar standard prism tests done for a high rise project using a basic Type S mortar and high strength block. - The mortar was assumed to be 2200 psi and the block were 8400 psi and the two block prism strengths (f'm) was about 5000 psi. The failure mode was a diagonal shear failure through both the block and mortar as expected.

The company that I was responsible for made up to about 15,000,000 sf of CMUs annually and our most common complaint was from masons because the block were not within our 1/32" tolerance because of an effect on the labor cost and not for any visual purposes.

How do you even measure a wet mortar joint to a 1/32" tolerance when the mortar is consolidated by the units and the desired absorption of moisture from the mortar?

Dick





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

 

[dhengr]

Connect2:
Masons rarely use micrometers or a machinist calipers to measure mortar joints. Who, on the job site can actually measure the difference btwn. .5" and 13mm? You should probably watch the masons as they get started because that thicker joint will be a bit unusual as a std. without a specific local reason for the joint adjustment. Someplace on the job you are liable to have to match std. coursing and joint thickness, and there you may have an appearance problem.

 

[connect2]

k thanks for the replies. While it does seem miniscule, +/- 3mm from a base line mortar depth of 10mm with the variation being checked within any 3m distance it would appear that none the less this what you find in ACI 530 and CSA S304.1. It seems that as you follow the trail and papers that as the thickness and the mortar bed depth increase beyond 13mm the compressive strength of the joint decreases. The thinner the joint the greater the compressive strength. There are yes many variables in masonry for sure and why the capacity reduction factor in strength design for masonry = 0.6 Thanks again.

 

[concretemasonry]

The structural element is the wall panel. That element has been tested for decades and the mortar bed is just a small element for myopic analysis.

In the tests of walls it has not shown that the mortar has no real effect on a panel test report. That is why the folly of a field test for mortar strength has not been developed.

Dick

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

 

[hokie66]

This is not a structural question. It is purely about constructability, and may involve architectural aesthetics. But there are very good reasons why coursing in block walls is modular. For instance, in Australia, with blocks which are 190 high, 10 mm mortar joints are used, so that the coursing, and storey poles used by masons to control the coursing, are increments of 200. When you deviate from this, you create more problems than you solve. If the masons have to cut block at the top (or bottom), so be it.

 

[concretemasonry]

It is definitely an aesthetic question.

In my 40 years experience in masonry, I have never seen the need for a mason to cut down a block or two. It is easier/cheaper to throw the units away or reject a load entirely.

Usually, the wall height problems come from stiff mortar or variable masonry sand (stones). A mason will quickly send back a load of 1000 block rather than fight with poor quality control and height variations. If the block is outside of the standards there will be many thousands since the manufacturing process is automated. Height control is very easy to monitor and adjust. - This is fact for every major producer that uses the same basic manufacturing equipment and processes.

Fortunately, the height of units has no effect on the wall strength since the mortar efficiently distributes the loads while still maintaining the course modular dimensions. Unfortunately, the world-wide specifications for CMUs are very loose and the standards committees are reluctant to set higher standards. - This happens to be a problem with the effort to maintain a balance between users, manufacturers, producers and interested parties (little old ladies and environmentalists).

Dick

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