ASTM C90 revisions reflect environmental advantages,energy efficiency and mason productivity

Smaller Webs = Higher R-Value

by Jason Thompson

It isn’t often that I recommend a book to read, and even less frequentlyon such topics as the history of concrete masonry. But if you are readingthis article you will likely find From the Carriage Age…to the Space Age,The Birth and Growth of the Concrete Masonry Industry (Ref. 1) fascinating.Not because it is well written (it isn’t), nor because it presents an other -

wise dry topic in an entertaining light (it doesn’t). What From the CarriageAge…to the Space Age does do very well is provide background, insight andreasoning behind the culture and products that define the concrete masonryindustry as we know and understand it today. One passage in particular isnoteworthy and relevant:

“In the early 1920s, the concrete block industry was in a chaotic state withrespect to the sizes of concrete block. In that year, block were manufacturedin 30 different lengths, 20 different widths and 26 different heights. Architectswere severely handicapped because they had no way of knowing what sizeswould be delivered to the job until the block actually arrived. They were,therefore, unable to dimension their buildings with any assurance that theunits actually delivered would correspond to the dimensions.”

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Photo by CMACN, courtesy of NCMA

LEARNING OBJECTIVES

Upon reading the article you will be able to:

1 Identify changes to ASTM C90 2011b.

2 Summarize potential benefitsto designers, owners andcontractors as a result of the changes.

3 State the effect web size andconfiguration many have ongrout application in CMU.

4 Explore ideas for industryadvancement as a result ofASTM C90 changes.

This article originally appeared in MasonryEdge/theStoryPole Vol 7 No 2.

The book goes on to say that by 1924 block sizeswere standardized and by 1927, 95% of allconcrete block were produced in conformity withthis standard. Shortly thereafter, the first versionof ASTM C90, Standard Specifi cation for HollowLoadbearing Concrete Masonry Units, waspublished in 1931.

Little did these early pioneers know at the timethat these monumental efforts would comeunder scrutiny in 2011 as the concrete masonryindustry contemplated, debated and ultimatelyapproved revisions to the prescriptive webconfiguration requirements that have been a part of ASTM C90 for nearly a century. Thesechanges, which appear in the 2011b version ofASTM C90, Standard Specification for LoadbearingConcrete Masonry Units (Ref. 2), are shown inTable 1 and summarized as follows:1. The equivalent web thickness, which is the

sum of the individual web thicknesses per footof block length, has been removed.

2. The minimum thickness of each web cannotbe less than ¾'' (19 mm).

3. The total minimum cross-sectional area of thewebs connecting the face shells of the unitcannot be less than 6.5 in2/ft2 (45,140mm2/m2), which translates to 5.8 in2 (40,280mm2) for a unit with 8" X 16" (203 X 406 mm)nominal face dimensions.

Meeting Evolving MarketInterests Probably most importantly, web configuration changes recently introducedinto ASTM C90 require no change to webconfigu ra tions or unit properties. They do,however, permit the webs of units to beconfigured differently to meet evolving marketneeds. Some regions will view these changes toASTM C90 as an opportunity to supply moresustainable products to the marketplace by usingfewer raw materials and less energy duringproduction. Others will see the ability to reduceunit weight and thereby increase mason

productivity and reduce injury and fatigue.Perhaps the greatest opportunity these revisionsoffer is the chance to increase the energyefficiency of concrete masonry construc tion –particularly single wythe construction. In mypersonal opinion, however, the market that cancapture all of the nuances, as well as others relatedto production efficiency, fire resistance and otherintrinsic properties of concrete masonryconstruction has the chance of greatest success.

Energy Efficiency of theFinished Assembly While there aremany aspects and opportunities this change toASTM C90 offers that are worthy of review, thisdiscussion focuses primarily on those related toenergy efficiency of the finished assembly. Othertopics, such as fire resistance, sound abatementand structural design, will continue to be visitedin future articles and discussions.

Modeling of thermal properties of concretemasonry units (CMU) is accomplished using the series-parallel (also called isothermal-planes)method as described in ACI 122R-02 Guide toThermal Properties of Concrete and MasonrySystems (Ref. 4) and ASHRAE 90.1-10 EnergyStandard for Buildings Except Low-Rise ResidentialBuildings (Ref. 5). Using this code-recognizedmodeling procedure, thermal properties (R-valueand U-factor) of a concrete masonry assembly

are influenced by the presence and placement of insulation, grouting, unit density andconfiguration of the webs and face shells.Further, and possibly most importantly, becausewebs provide the direct means of transferringheat through a CMU as shown in Figures 1a and 1b,changing the configuration or size of the webscan have a significant and direct impact onresulting thermal properties of an unfinished,single wythe concrete masonry assembly. Unlessthe resulting assembly is to be solidly grouted, in which case the web configuration is irrelevantregarding thermal efficiency, modeling resultsare simple: smaller webs equal higher R-values.

The relationship between web configuration andassembly R-value is illustrated in Table 2 for 8"concrete masonry assemblies and again in Table 3for 12" concrete masonry assemblies. The middletwo columns of each of these tables comparesthe steady-state R-values of a conventional three-

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TABLE 1 Minimum Face Shells and Web RequirementsA

Nominal Width (W) of Units,in. (mm)

Face Shell Thickness (tfs),min, in. (mm)B,C

Webs

Web ThicknessC (tw),min. in. (mm)

Normalized Web Area (Anw) min, in.2/ft2 (mm2/m2)D

3 (76.2) and 4 (102) ¾ (19) ¾ (19) 6.5 (45,140)

6 (152) 1 (25) ¾ (19) 6.5 (45,140)

8 (203) and greater 1¼ (32) ¾ (19) 6.5 (45,140)

Courtesy of NCMA

AAverage of measurements on a minimum of 3 units when measured as described in Test Methods C140.BWhen this standard is used for units having split surfaces, a maximum of 10% of the split surface is permitted to have thickness less thanthose shown, but not less than ¾ in. (19.1 mm). When the units are to be solid grouted, the 10% limit does not apply and Footnote Cestablishes a thickness requirement for the entire face shell.

CWhen the units are to be solid grouted, minimum face shell and web thickness shall be not less than 5/8 in. (16 mm).DMinimum normalized web area does not apply to the portion of the unit to be filled with grout. The length of that portion shall be deducted fromthe overall length of the unit for the calculation of the minimum web cross-sectional area.

Figure 1a – Heat Flow Through CMU Figure 1b – Series-Parallel Model

Masonry construction goes beyondsimply providing steady-state

thermal resistance by enhancingthe less quantifiable user-comfortcharacteristics of energy efficiency.

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web, two-core CMU manufactured to complywith historical ASTM C90 requirements to the R-values that would result if this same assemblywere constructed using units manufactured tocomply with new ASTM C90-11b minimum webrequirements. Relative differences between the R-values for these assemblies are impressive,increasing the steady-state R-value by an averagemultiple of 2.7 for the 8" assembly and 3.2 forthe 12" assembly.

While illustrative of the concept, R-valueincreases shown in Tables 2 and 3 do not,however, capture the entire array of production,construction and design variables that must befactored into each project. For example, manu -factur ing units with the minimum web areapermitted under ASTM C90-11b can be challengingwhile ensuring the resulting product is ofconsistent quality and can be handled withoutdamage, particularly when architectural finishessuch as split surfaces are desired. Whenincorporated into the exterior envelope of a building, single-wythe concrete masonry

construction serves the dual role of providingboth enclosure as well as structural strength to the system. As such, these assemblies almostalways contain reinforcement and grout. Whilereinforced cells of an assembly increase thestrength of the system, the presence of groutprovides for a larger area for heat to pass,creating a larger thermal short within theassembly. A further complication is that singleweb units (right) cannot be used in partiallygrouted construction as there is no means ofconfining grout discrete cells within the assembly.The net result is a decrease in steady-state R-values as shown in the last column of Tables 2and 3 whereby grout is assumed to be spaced at48'' on center. Although not shown in the tables,using less grout in the assembly would increasethe resulting R-value, while using more groutwould have the opposite effect.

Not the whole story…Everyone understands that higher R-valuesequate to higher energy efficiency, but endingthe comparison there can lead to gross miscon -

ceptions regarding the true energy efficiency andenergy use of a given building. Focusing solely onthe R-value of the opaque portion of the buildingenvelope only provides a very narrow view of a building’s energy use. In reality, masonryconstruction goes beyond simply providingsteady-state thermal resistance by enhancing theless quantifiable user-comfort characteristics ofenergy efficiency. This includes such attributes asdampening temperature swings through thermalmass and providing a draft-free environmentwith an air-tight enclosure. These topics,however, have been thoroughly covered in 2009ASHRAE Handbook of Fundamentals.

What does the future hold?We all recognize that the entire industry is notgoing to immediately retool to begin producingunits of alternative or unique configurations inresponse to the revisions incorporated intoASTM C90. Instead, use of alternative unitconfigura tions and associated technologies willlikely be a slow transition driven by local marketdemands and needs of producers, contractors,designers and owners – a process that may takeyears, if not decades.

Looking ahead, real challenges facing the industry will be to offer cost effective solutions to meet continuously increasing energy efficiencyrequirements imposed by building codes. This iswhere flexibility in unit configurations availableunder new ASTM C90 requirements can be mostbeneficial when coupled with innovation andcreativity. Specifics of how the industry movesforward with these options is yet to be deter -mined, but will continue to be a central theme of discussion for years as the entire industrycontemplates ways to successfully manufacture,install, test and design using unit configurationsthat may not yet exist, while meeting evolving

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Table 2 – Comparison of 8" Concrete Masonry Assembly R-Values (hr-ft2-°F/BTU)1

Unit DensityConventional Three-Web,

Two Core Unit2,3

Minimum One-Web Permitted by ASTM C90-11b3

ASTM C90-11b CMU Grouted at 48” oc3,4

Two-Webs

85 lb/ft3 7.9 18.0 7.9

95 lb/ft3 6.7 16.3 7.4

105 lb/ft3 5.6 14.7 6.8

115 lb/ft3 4.7 13.1 6.4

125 lb/ft3 4.0 11.6 5.9

135 lb/ft3 3.4 10.2 5.4

1Single wythe assembly without finishes. 2See Reference 3, Thermal Catalog of Concrete Masonry Assemblies, for modeling procedure anddesign assumptions.

3Values assume all ungrouted cells contain foam-in-place insulation and moisture-corrected valuesfor thermal conductivity.

4Assembly constructed using two-web A-block at grout locations to confine grout.

Table 3 – Comparison of 12" Concrete Masonry Assembly R-Values (hr-ft2-°F/BTU)1

Unit DensityConventional Three-Web,

Two Core Unit2,3

Minimum One-Web Permitted by ASTM C90-11b3

ASTM C90-11b CMU Grouted at 48” oc3,4

Two-Webs

85 lb/ft3 11.9 30.9 10.7

95 lb/ft3 9.9 28.0 10.1

105 lb/ft3 8.2 25.1 9.5

115 lb/ft3 6.8 22.3 8.9

125 lb/ft3 5.7 19.7 8.3

135 lb/ft3 4.7 17.3 7.7

1Single wythe assembly without finishes. 2See Reference 3, Thermal Catalog of Concrete Masonry Assemblies, for modeling procedure anddesign assumptions.

3Values assume all ungrouted cells contain foam-in-place insulation and moisture-corrected valuesfor thermal conductivity.

4Assembly constructed using two-web A-block at grout locations to confine grout.

Smaller Webs = Higher R-Values

Photos courtesy of NCMA

H-block (above) has only one web and, as a result,

no ability to confine grout laterally. A-block (below) with two webs can be

partially grouted.

Jason J Thompson isthe vice president ofEngineering for theNational ConcreteMasonry Association(NCMA). A structuralengineer, he manages NCMA’s technical andresearch activities.

Thompson is active in many masonryindustry organizations and has held manyleadership positions, including chair of the Masonry Alliance for Codes andStandards and secretary of the MasonryStandards Joint Committee (MSJC). He has been recognized through severalawards including ASTM’s Alan H YorkdaleMemorial Award for the best technicalpaper regarding masonry on four separateoccasions and The Masonry Society’sService Award. He holds a Bachelor andMaster of Science in Civil Engineeringfrom Washington State University.jthompson@ncma.org | 703.713.1900

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market-driven needs to use less material, be moreenergy efficient and maintain its inherentstructural, fire safety, sound abatement anddurability characteristics.

I invite each of you to share your thoughts and insights with me as to what your crystal ball tells you the future holds for the concrete masonry industry. I can be reachedat jthompson@ncma.org or 703-713-1900. � � �

References

1. From the Carriage Age…to the Space Age, The Birth and Growth of the Concrete MasonryIndustry, National Concrete MasonryAssociation, Herndon VA, 1969, Edited byJoseph Bell, (ncma.org/Pages/AboutNCMA.aspx).

2. ASTM C90-11b, Standard Specification forLoadbearing Concrete Masonry Units, ASTMInternational, West Conshohocken PA, 2011,(astm.org).

3. Thermal Catalog of Concrete MasonryAssemblies, National Concrete MasonryAssociation, Herndon VA, 2010,(ncma.org/resources/design/Pages/default.aspx).

4. ACI 122R-02, Guide to Thermal Properties ofConcrete and Masonry Systems, AmericanConcrete Institute, Farmington Hills MI, 2002.

5. ASHRAE 90.1-10, Energy Standard for BuildingsExcept Low-Rise Residential Buildings,

American Society of Heating,Refrigeration and Air-Conditioning Engineers,Atlanta GA, 2010.

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