The Evolution of Concrete Masonry
- Published: Wednesday, 13 June 2012 17:22
- Written by CP Staff
In 2010 while catching up with her on some circa-1930s Popeye cartoons, my (then) three-year-old daughter made the astute observation that Bluto had selected “concrete masonry” in an attempt to slow down a spinach-juiced Popeye saving Olive Oyl from imminent peril.
I had two reactions. The first being that of fatherly pride, whereby my young daughter not only noticed, but focused on the (albeit unconventional) use of concrete masonry amongst all the other distractions and excitement playing out before her. (Admittedly, such knowledge is foregone in my house.) The second was the realization that the iconic shape of a block, gray, consisting of two open cells and three webs, was as immediately recognizable today as it was nearly 80 years ago when this cartoon debuted.
So how was it that the concrete masonry industry converged on this “standard” unit configuration nearly a century ago? Among many factors it was driven by the need to maintain interchangeable and modular units, address production and handling limitations, allow units to be laid with one hand, vertically align cells for the placement of grout and reinforcement, ensure structural stability well before engineering analysis was commonplace, or most likely a combination of all these traits as well as many others. Certainly there are examples of proprietary and non-proprietary unit configurations that exist today to meet specific market needs, such as open-ended units used on the West Coast to accommodate heavy reinforcing detailing and reduced-web units used in combination with insulation to increase the energy efficiency of the assembly. Yet the standard two-cell, three-web unit configuration has prevailed.
But why? What makes this single-unit configuration the go-to choice for a nearly limitless number of applications across the globe? In discussing this question with producers, mason contractors and designers, I was surprised by the often received response, which basically boiled down to: “Because this is what ASTM requires.”
Contrary to this common misconception, ASTM does not contain, nor has it ever contained, requirements for unit configuration. ASTM C90 Standard Specification for Loadbearing Concrete Masonry Units has historically spelled minimum thickness requirements for face shells and webs, as well as a minimum requirement for the total equivalent web thickness, but never has it included requirements for how the webs of a unit are configured. Unit configuration—specifically how and where the webs of a unit are located—is market driven.
It was this realization that prompted National Concrete Masonry Association’s Masonry Technical Subcommittee to discuss alternative web configuration strategies in early 2011 and identify benefits that could be realized by revisiting a century-old icon. As a result of these discussions, a series of changes was proposed to modify the minimum web requirements in ASTM C90 to provide more explicit flexibility in unit configurations to meet continuously evolving market demands. In December 2011, these changes were approved by ASTM, and a new version of ASTM C90 will be available by mid-year.
In a nutshell, these changes modify the web requirements in ASTM C90 in three significant ways:
- The equivalent web thickness, which is the sum of the individual web thicknesses per foot of block length, has been removed;
- The minimum thickness of each web cannot be less than ¾ in.; and,
- The total minimum cross-sectional area of the webs connecting the face shells of the unit cannot be less than 6.5 in.²/ft.², which translates to 5.8 in.² for a unit with 8- x 16-in. nominal face dimensions.
With these changes now in place, the question on everyone’s mind is “What’s next?” How quickly will the industry (producers, contractors, and designers alike) adapt to and adopt this new flexibility into the marketplace? What factors will drive the use of one particular unit configuration over another? What impact, if any, does this change have on how concrete masonry is constructed or how it is designed for fire, sound, energy, water penetration, or structural loads? These questions, and likely many others, will continue to be debated in the months and years to come as each region and market application settles in on a unit configuration that best fits their needs. While this transition unfolds, I offer the following insights and predictions, each of which is certainly open to debate.
The immediate impact of these changes to ASTM C90 is somewhat nuanced, but important nonetheless. Currently, there are a myriad of units commonly used that have web configurations that do not comply with the old ASTM C90 requirements, but would comply with the new ASTM C90 requirements. Examples include open-ended units such as H-block and A-block, which although these units have been used successfully in structural loadbearing applications for years, they are essentially limited to grouted and reinforced applications to address the reduced webs provided by these units. Hence, while I foresee no immediate shift in the increased or decreased use of specialty configuration units, I do see reduced confusion in the field as to whether these units can be used in loadbearing applications.
In the short term, I suspect the changes to unit configurations will initially be subtle, but tangible. For example, under the old ASTM C90 web requirements a standard 8-in. unit is manufactured with three webs each measuring nominally 1 in. in thickness. By reducing the thickness of these three webs to ¾-in. each, as would be permitted under the new ASTM C90 web requirements, a slight material and weight savings can be realized. Not much for an individual unit, but for a facility that produces 1 million units in a year this savings can add up to more than 620 cu. yd. and 2 million lbs. of material. Virtually identical unit configuration, yet with real, significant savings that will yield a unit that is more sustainable and cost effective to produce, easier to transport, and lighter to install.
Looking even further ahead into the future, the real challenges facing the industry will be to offer cost effective solutions to meet continuously increasing energy efficiency requirements imposed by building codes. In my opinion, this is where the flexibility in unit configurations available under the new ASTM C90 requirements can be the most beneficial when coupled with innovation and creativity. Because the webs of concrete masonry units are the paths by which heat transfers across the assembly, we can realize potential increases in energy efficiency for single wythe R-values two to three times higher by simply removing material from the web.
Where and how we move forward with these options has yet to be determined. They will be a central theme of discussion at NCMA gatherings as we identify new design resources to develop, pending revisions to design codes and standards, and tools to educate the entire industry on how to successfully manufacture, install, test, and design using unit configurations that may not yet exist in the marketplace.
Reengineering the iconic concrete masonry unit to use less material, be more energy efficient, and maintain its inherent structural, fire safety, sound abatement, and durable characteristics isn’t without its challenges, but I am convinced this industry is ready to take these challenges on in supplying the marketplace with the next generation block.
Jason Thompson, National Concrete Masonry Association Vice President of Engineering, helped spearhead preliminary discussion and subsequent balloting of ASTM C90 revisions allowing the single-web CMU.