Important Differences: Density vs Strength
It is important to understand the difference between the “Density” and “Strength” of a concrete masonry unit, as the two terms are often misunderstood and incorrectly assumed to be the same. In short, all three “Density” categories of CMU meet the minimum “Strength” requirements as defined by ASTM C90. To avoid any confusion, it is important to know that every mix design has both a density, and a strength. Higher density does not necessarily mean higher strength... in fact, you can select a Lightweight, High Strength mix design, which is stronger than a Medium Weight, Regular Strength mix design. Are you confused yet? The following should help clear things up...
“Density” is another word for “weight”. A concrete “mix design” is measured in pounds per cubic foot, (“lbs/ft3”or “pcf”). In other words, “How much does one cubic foot of a given mix design weigh, when paced on a scale?” Most mix designs for CMU range between 85pcf and 140pcf+. Standards applicable to CMU include three classifications of weight: Lightweight (“LW”), Medium Weight (“MW”) and Normal Weight (“NW”). The term “Normal Weight” means “Heavy Weight”, because most aggregates in the U.S. are heavy. In Northern California, most local aggregates are light and medium, therefore, most CMU in Northern California is produced as Lightweight and Medium Weight, while Normal Weight is used less frequently.
Industry Classification of Density (weight) for Concrete Masonry (as determined by the weight of one cubic foot of the mix design used in a block):
· Lightweight (LW) CMU: Weighs less than 105lb/ft3 (1,680 kg/m3)
· Medium Weight (MW) CMU: Weighs 105lb/ft3 to less than 125lb/ft3 (1,680-2,000kg/m3)
· Normal Weight (NW) CMU: Weighs 125lb/ft3 (2,000kg/m3) or greater.
Note that CMU “Weight” does NOT equal CMU “Strength”.
“Strength,” as related to masonry systems, is expressed in two distinct ways: Compressive Strength of the CMU, and the Specified Compressive Strength of the Masonry System (masonry units + mortar + grout).
The Compressive Strength of CMU is measured in “pounds per square inch”, (“lbs/in2” or “psi”). Among the combined strengths of the aggregates, sand and cement, the more important influence on overall compressive strength is the quantity of cement used in the mix, and the time allowed for the block to cure. Generally, the higher the cement content, the higher the strength, and, the longer the CMU cures, the stronger it gets, until it reaches its maximum designed strength (usually about 28 days).
Industry Classification of the compressive strength of CMU, (as determined by the psi calculated from the net area of the individual unit), must meet at least the minimum average net compressive strength, per ASTM C90, (which, as of the 2014 edition, is 2,000 net psi).
CMU that meets the ASTM C-90 minimum of 2,000psi is considered “Standard Strength”. CMU is also available in High Strength (“HS”) and Ultra High Strength (“UHS”), and the higher strengths are typically achieved by simply adding more cement to a standard strength mix design.
The Specified Compressive Strength of the Masonry System, (known as f’m), is the value used in the design of the masonry wall, (the combined assembly, including the CMU, Grout, Mortar, and Steel). The California Building Code Sections 2105.1 and 2015A.1 refer to the TMS 602 for compliance with the specified f’m value. Testing methods used include the Unit Strength Method, and Prism Testing. More information on these testing methods is available in TMS 602.
Industry Classification of Strength for Concrete Masonry Systems, (as determined by the compressive strength of the wall assembly, [CMU + (TYPE M OR S Mortar) + Grout], include:
Standard Strength: f’m 2,000 net psi (min) met with 2,000 net psi CMU
High Strength: f’m 2,250 net psi (min) met with 2,660 net psi CMU
Ultra High Strength: f’m 2,500 net psi (min) met with 3,250 net psi CMU
Super Ultra High Strength: f’m 3,000psi (min) met with 4,500 net psi CMU
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