Concrete Masonry Units Are Being Made with Smaller Cross Webs – This is Better, but “On the Other Hand…”

Posted by in Masonry | February 20, 2013

It has been over a year since the American Society of Testing Materials approved ASTM C90-11 which allows thinner cross webs in concrete masonry units (CMU).  The new standard allows cross webs to be ¾ inches thick instead of 11/8 inches.  The masonry industry is touting the potential benefits of this change, especially the “sustainability” benefits:

  1. The block manufacturers will be processing and selling less material.
  2. The teamsters will be hauling lighter block to the construction suppliers and the construction site so they will be using less fuel.
  3. The masons will be lifting lighter block and be having a lesser risk of injury.
  4. Thermal resistance of the block increases due to the smaller width of the thermal bridge (the cross web).
  5. The hollow cores are larger, thus, providing less congestion and more room for reinforcing steel.

But “on the other hand,” from what I remember in my Strength of Materials class, the reduction of the cross web will reduce the area (reducing the inward shear strength and the axial/vertical load strength), the section modulus (reducing the bending strength), and the moment of inertia (reducing the stiffness).  And to the credit of the masonry industry they do admit that the stiffness and the strength of an unreinforced block wall assembly will be reduced some and that it needs to be accounted for.  Thus, this reduced will make unreinforced block walls weaker and more flexible and susceptible to cracking from bending and axial loads.

It’s like my local baker telling me that the new, smaller donuts are better for me because:

  1. He won’t have to buy as much flour and sugar.
  2. I will save fuel because the dozen of donuts I bring home will be lighter.
  3. The donuts will be easier to lift to my mouth.
  4. I will have less blockage in my arteries from cholesterol.
  5. I will lose weight because of my lower caloric intake.
  6. The donuts won’t be touching each other in the box, and thus, won’t be “contaminating” each other with icing and jelly.

But “on the other hand,” I get fewer bites per donut.

It will be interesting to see if this change results in an increase in the number of unreinforced block wall failures or serviceability issues.  Fortunately, unreinforced masonry construction is becoming less common, and thus, this  concern may become a non-issue.  But then “on the other hand,” all the benefits of the thinner cross webs will be totally offset by the increased material and fuel usage in manufacturing, transporting, and installing grout, which is much heavier than unreinforced block, to fill the hollow cores.

As Gilligan (on Gilligan’s Island) once said, “It’s always that ‘other hand’ that gets us in trouble, Professor.”