HEY HEIDI – The Dear Abby of Concrete Masonry
ALBIE asks what the performance variations for different densities are, and if the compressive strength of concrete masonry is effected by density.
Q: Is the compressive strength (PSI) of Concrete Masonry Units effected by density and what are the main performance variations associated with the different densities?
–Are Lightweights Better In the Envelope?
A: Dear ALBIE,
For architectural units, aesthetics are a major consideration. The light weight aggregate available in the Northeast is a very dark grey, which is visible in ground face & polished units. For standard grey block, mix designs can have different densities, depending on the amount of light weight aggregate used in the mix. ASTM C-90 defines lightweight as less than 105 lbs/cubic foot. Usually lightweight units are between 100 & 105. Medium weight ranges from 105 to 125 lbs/cubic foot and normal weight is 125 lbs/cubic foot and above. Some of the properties effected by the use of light weight aggregate in the CMU mix are energy efficiency, fire resistance and STC ratings. The lighter weight the concrete is, the less heat is able to transfer through the unit. This increases fire ratings and R-Values. The denser the unit, the better the STC ratings. Another consideration is the productivity of the mason contractors. Many masons like to use units that are lighter to increase their productivity. One property that is NOT effected by the density is compressive strength. For Concrete Masonry Units, strength is a variable of 2 major factors: the manufacturing process and the percentage of cement used. CMU are manufactured by a block machine which compacts and vibrates zero slump concrete into a mold. This, along with the amount of cement, determines what the PSI of the unit will be. Density has very little to do with it. Contrary to the name “lightweight”, lower density units are just as strong as their normal weight cousins.