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An Informational Series from NCMA Answering Frequently Asked Questions FAQ 11-14

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Last Revised June 9, 2010

NCMA Commentary Discussions to ASTM C90-09 Standard Specification for Loadbearing Concrete Masonry

Units Last Revised June 9, 2010

How to use this Commentary to ASTM C90:

This document provides background information and discussion on the requirements of ASTM C90 that

may be useful to designers, contractors, producers, or specifiers of manufactured concrete masonry

products. The provisions of ASTM C90 are shown shaded, with each section followed by commentary

discussion related to the provisions of that section. Where appropriate, the reader is referred to additional

discussion provided within other documents or resources for further information on a particular topic.

1. Scope

1.1 This specification covers hollow and solid (see 5.3 and 5.4) concrete masonry units made from

hydraulic cement, water, and mineral aggregates with or without the inclusion of other materials. There

are three classes of concrete masonry units: (1) normal weight, (2) medium weight, and (3) lightweight.

These units are suitable for both loadbearing and nonloadbearing applications.

Section 1.1 Commentary Discussion

General Requirements

Prior to reviewing the provisions and requirements that are included in ASTM C90, it is worth

discussing what requirements are not covered by ASTM C90. Because ASTM C90 is a minimum

product specification, it only addresses those physical attributes that are common to, or necessary

in, the majority of applications in which these units are used. As a result, designers and specifiers

must often stipulate requirements in addition to compliance with ASTM C90 if particular features

or properties are desired for a unique application. A few such attributes include:

Aesthetics – While ASTM C90 does include minimum finish and appearance

requirements, these provisions are oriented toward minimum chipping and cracking

common to all exposed units and do not address aesthetically-driven considerations such

as color, texture, or architectural features, which must be specified separately by the

purchaser. Refer to the commentary discussion provided under ASTM C90 Section 7 for

additional discussion related to finish and appearance.

Fire Resistance – Concrete masonry units are inherently noncombustible; indeed they are

used in many applications where direct exposure to heat and flame are anticipated.

Different concrete masonry units, however, will have varying fire resistance ratings

determined in accordance with ASTM E119, Standard Test Methods for Fire Tests of

Building Construction and Materials, or calculated in accordance with ACI 216.1/TMS

216, Standard Method for Determining Fire Resistance of Concrete and Masonry

Construction Assemblies. (The provisions of the ACI 216.1/TMS 216 standard are

referenced by and identical to the calculated fire resistance requirements contain in recent

editions of the International Building Code, Section 721 “Calculated Fire Resistance”.)

Concrete masonry assemblies have fire resistance ratings that can vary from 1 hour to 4

hours or more depending upon the type of aggregate(s) used in production, the volume of

concrete in the unit, and whether the cells of hollow units are filled with a material that is:

1) recognized by ACI 216.1/TMS 216 (or the International Building Code); or 2)

evaluated in accordance with ASTM E119 or an equivalent to this testing procedure.


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When a specific fire resistance rating is desired, it should be specified separately. For

more information on fire resistance ratings of concrete masonry assemblies see NCMA

TEK 7-1C, Fire Resistance of Concrete Masonry Assemblies.

Water Penetration Resistance – Refer to the commentary discussion provided in

Appendix X1.

Crack Control – Refer to the commentary discussion provided in Appendix X2.

Admixtures – Additives and admixtures are used in the production of concrete masonry

units to affect one or more plastic or hardened properties of the product. Other than

pigments for coloring, integral water repellents are the most commonly specified

admixture. Integral water repellents are added to concrete masonry units for two primary

purposes: 1) efflorescence control and 2) water repellency. Typically an integral water

repellent is introduced at different dosage rates depending upon its intended purpose. As

such, when specifying a concrete masonry unit containing an integral water repellent it is

important to clarify that the dosage rate meets the admixture manufacturer’s

recommended dosage rate for water repellency or efflorescence control, as appropriate.

Because the dosage rate for water repellency is often higher than it is for efflorescence

control, the higher dosage rate provides effective performance for both attributes. For

more information on water repellency characteristics of concrete masonry units, see

NCMA TEK 19-7, Characteristics of Concrete Masonry Units with Integral Water

Repellent.

Energy Efficiency – As an integral part of the exterior envelope, concrete masonry is

often relied upon to provide thermal efficiency to a completed building, which can vary

by climatic region and the intended use of the structure. The inherent flexibility of

concrete masonry construction allows insulation to be incorporated on the exterior,

interior, or integrally within the assembly; providing designers with nearly limitless

options. Additional information on insulation values and energy efficiency of concrete

masonry assemblies is available in NCMA TEK 6-1B, R-Values of Multi-Wythe Concrete

Masonry Walls and TEK 6-2B, R-Values and U-Factors for Single Wythe Concrete

Masonry Walls.

Sound Abatement – For mitigating sound, three different conditions can be considered

during design:

o Sound Transmission Class (STC), which is a measure of the sound loss from one

side of a wall to the other;

o Noise Reduction Coefficient (NRC), which reflects the percentage of sound

absorbed by a wall surface as opposed to reflected back; and

o Outside-Inside Transmission Class (OITC), which captures the sound loss

between the exterior of a building and its interior.

Additional information on STC, NRC, and OITC can be found in TEK 13-1B, Sound

Transmission Class Ratings for Concrete Masonry Walls, TEK 13-2A, Noise Control

with Concrete Masonry, and TEK 13-4, Outside-Inside Transmission Class of Concrete

Masonry Walls.

Moisture Controlled Units – In the 2000 edition, ASTM C90 was revised to remove the

two type designations for concrete masonry units: Type I, moisture-controlled units and

Type II non-moisture-controlled units. A comprehensive discussion on this change is

available at the following link:

http://www.ncma.org/resources/design/Technical%20FAQ/Type%20I%20and%20II%20

Designation%20in%20ASTM%2090.aspx

Grade of Units – In 1990, ASTM introduced another significant modification to ASTM

C90 by removing the unit grade classifications from the standard. Until that time two

grades were defined:

http://www.ncma.org/resources/design/Technical%20FAQ/Type%20I%20and%20II%20Designation%20in%20ASTM%2090.aspx

http://www.ncma.org/resources/design/Technical%20FAQ/Type%20I%20and%20II%20Designation%20in%20ASTM%2090.aspx


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o Grade N – For general use such as in exterior walls below and above grade that

may or may not be exposed to moisture penetration or the weather and for

interior walls and back-up.

o Grade S – Limited to use above grade in exterior walls with weather-protective

coatings and in walls not exposed to weather.

Due to their limited application, Grade S units were permitted to have a lower

compressive strength and higher water absorption compared to Grade N units. Over

time, market conditions as well as the logistical difficulty of stocking two different unit

grades resulted in the dropping of the Grade S classification from ASTM C90. While the

grade classification is no longer used by ASTM C90, the current minimum requirements

of this standard effectively require the physical properties of Grade N units.

Hollow versus Solid Units – At the same time as when the grade classifications were

removed from ASTM C90 the compressive strength requirements were converted from a

minimum gross area compressive strength to a minimum net area compressive strength.

Until this conversion was made, ASTM maintained two different standards for

loadbearing concrete masonry units: ASTM C90, for hollow units, and ASTM C145, for

solid units. The requirements of these two standards were effectively identical, with the

exception that the minimum compressive strength required was ‘calibrated’ to account

for the units being either solid or hollow. By revising the minimum compressive strength

requirements to be based on the net area of the unit, the need for two separate standards

covering hollow and solid units was no longer necessary and ASTM C145 was

withdrawn. Today, ASTM C90 applies equally to hollow and solid loadbearing concrete

masonry units.

In the context of ASTM C90 Section 1.1, hollow and solid units are implicitly referring

to unit configurations that are capable of being manufactured on a block machine. With

alternative technology, such as core bar pullers, unit configurations can be varied

significantly, but still with some limitations. Note that in accordance with established

ASTM definitions for hollow and solid masonry unit, solid masonry units need not have a

100% solid cross-section. In accordance with ASTM C1232-10, Standard Terminology

of Masonry, the definitions for hollow and solid masonry units are:

hollow masonry unit, n—unit whose net cross-sectional area in any plane

parallel to the surface containing cores, cells, or deep frogs is less than 75 % of

its gross cross-sectional area measured in the same plane.

solid masonry unit, n—unit whose net cross-sectional area in any plane parallel

to the surface containing cores, cells, or deep frogs is 75 % or more of its gross

cross-sectional area measured in the same plane.

When 100% solid concrete masonry units are desired, they should be explicitly specified

as such in the project documents. For additional information on the definition of a

concrete masonry unit refer to NCMA publication Definition of a Concrete Masonry

Unit.

Unit Density

For ease of specification and classification, ASTM C90 distinguishes concrete masonry units by

separating them into three different categories based on the density of the unit:

Lightweight Concrete Masonry Unit – having a density less than 105 lb/ft3;

Medium Weight Concrete Masonry Unit – having a density greater than or equal to 105

lb/ft3 but less than 125 lb/ft

3; and

Normal Weight Concrete Masonry unit – having a density of 125 lb/ft3 or more.


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Although ASTM C90 does not stipulate an upper or lower bound on density, as discussed in Note

3 of ASTM C90 the vast majority of unit densities fall within the range of 85 lb/ft3 to 145 lb/ft

3.

While a broad range of unit densities are available in each region across the U.S., some local

markets may gravitate toward a specific unit density to minimize the need to stock multiple unit

densities or to capitalize on locally available raw materials. Before specifying a concrete

masonry unit density, designers should contact local suppliers to determine which unit densities

are commonly available and which may be custom order products.

On occasion, one may hear the term “heavy weight” concrete masonry unit, which is used

interchangeably with normal weight concrete masonry unit in some regions of the country. Other

than the terminology used, there is no physical difference in “normal” and “heavy” weight

concrete masonry units.

Additional information on unit density and its affect on design variables is presented in TEK 2-6,

Density-Related Properties of Concrete Masonry Assemblies.

Code Application and Permitted Uses

More specifically, the permitted applications of concrete masonry units are defined by the

governing building code, which in turn may establish limitations on where and how a concrete

masonry unit (or concrete masonry assembly) can be used either directly by prescriptive means or

indirectly by design limitations. For example, a concrete masonry assembly can be used as a

loadbearing wall – but there are design limits on how much load can be applied to such a wall

based upon variables that are unique to each specific application as determined by the project’s

designer and/or the building official. In general, however, because of its physical attributes and

proven field performance there are very few limitations on the use of concrete masonry.

Section 1.1 of ASTM C90 specifically addresses the use of units complying with this standard as

being permitted in both loadbearing and nonloadbearing applications. The reason to distinguish

between loadbearing and nonloadbearing concrete masonry units is that ASTM C129, Standard

Specification for Nonloadbearing Concrete Masonry Units, specifically addresses the minimum

requirements for units used in nonloadbearing applications. The distinction between loadbearing

and nonloadbearing applications is often dictated by building code requirements, whereby

masonry members supporting axial loads greater than 200 lb/ft are considered loadbearing.

Because units complying with ASTM C90 for loadbearing applications can also be used in

nonloadbearing applications, most concrete masonry unit producers do not regularly manufacture

and stock both types of units.

Method of Production

Some manufactured concrete product specifications (for example ASTM C1372) reference a

method of production to further clarify and define the scope of the standard. ASTM C90,

however, does not – although the intent is to cover the minimum requirements for conventional

concrete masonry units. Additional discussion on defining concrete masonry units is provided in

the commentary to ASTM C90 Section 3.1.

Method of Curing

ASTM C90 does not directly address method of curing for concrete masonry units. Historically

curing has involved such methods as low-pressure steam curing, heat of hydration, autoclaving,

and the like using various and different equipment and technologies.


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1.2 Concrete masonry units covered by this specification are made from lightweight or normal weight

aggregates, or both.


Both lightweight and normal weight aggregates have defined specifications through ASTM C331,

Standard Specification for Lightweight Aggregates for Concrete Masonry Units, and ASTM C33,

Standard Specification for Concrete Aggregates, respectively, as covered in Section 4.2 of ASTM

C90. Note that heavyweight aggregates, as covered by ASTM C637, Specification for

Aggregates for Radiation-Shielding Concrete, are not addressed by ASTM C90, however,

concrete masonry units have been successfully produced using heavyweight aggregates. Varying

unit density is accomplished by using aggregates, or blends of aggregates, of varying density.

See TEK 2-6, Density-Related Properties of Concrete Masonry Assemblies, for more information.

Both aggregate types may consist of natural or manufactured aggregates. Examples of normal

weight aggregates commonly used in the production of concrete masonry units include limestone

and granite. Types of lightweight aggregates include expanded, pellletized, or sintered clay, slate,

shale, or fly ash; quarried aggregates including, pumice, scoria, or tuff; and aggregates consisting

of end products of coal or coke combustion.

1.3 The text of this specification references notes and footnotes which provide explanatory material.

These notes and footnotes (excluding those in tables and figures) shall not be considered as requirements

of the standard.

Section 1.3 Commentary Discussion To provide additional background or explanatory information to users of this standard Notes are

included throughout the body of ASTM C90. Notes are considered to be non-mandatory, which

highlight information that should be considered, but is not required for compliance purposes.

Footnotes to tables and figures are mandatory and must be complied with.

1.4 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses

are mathematical conversions to SI units that are provided for information only and are not considered

standard.


In the U.S. the majority of concrete masonry units are manufactured to accommodate modular

construction using inch-pound units. For example, a concrete masonry unit with a nominal height

of 8 inch is manufactured to a specified height of 7 5/8 inches, so that when laid with a

3/8 inch

mortar joint the height of the course becomes 8 inches thereby facilitating the modular layout of

the structure. Conversely, hard metric units are manufactured to a 100 mm module, such that a

unit with a nominal 200 mm height is manufactured to a specified height of 190 mm to

accommodate a 10 mm mortar joint. While the differences in these modules are relatively small,

amounting to an approximate 1.6% difference, their cumulative impact on the layout of an

assembly can be significant. As such, incorporating both inch-pound and metric module units

into a single project is unfeasible. With proper planning, however, inch-pound units can be used

in projects designed using a metric module and vice versa. For additional information refer to

NCMA TEK 3-10A, Metric Concrete Masonry Construction or the NCMA Publication Metric

Design Guidelines available on the web site at:

http://www.ncma.org/resources/design/Pages/MetricDesign.aspx.

Because the U.S. market predominately used inch-pound units for design and layout, ASTM C90

addresses only inch-pound units, with soft metric conversations provided parenthetically for

information purposes.

http://www.ncma.org/resources/design/Pages/MetricDesign.aspx


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NOTE 1—When particular features are desired such as surface textures for appearance or bond, finish,

color, or particular properties such as density classification, higher compressive strength, fire resistance,

thermal performance or acoustical performance, these features should be specified separately by the

purchaser. Suppliers should be consulted as to the availability of units having the desired features.

Note 1 Commentary Discussion

This non-mandatory note highlights the nearly limitless versatility of concrete masonry aesthetics

and properties, however, not all specific features or properties are available in all locations.

When specific features or attributes are desired for a particular project, local availability should

be verified prior to specifying. See Section 1.1 Commentary Discussion “General Requirements”

for additional discussion.

2. Referenced Documents

2.1 ASTM Standards:

C 33 Specification for Concrete Aggregates

C 140 Test Methods for Sampling and Testing Concrete Masonry Units and Related Units

C 150 Specification for Portland Cement

C 331 Specification for Lightweight Aggregates for Concrete Masonry Units

C 426 Test Method for Linear Drying Shrinkage of Concrete Masonry Units

C 595 Specification for Blended Hydraulic Cements

C 618 Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete

C 989 Specification for Slag Cement for Use in Concrete and Mortars

C 1157 Performance Specification for Hydraulic Cement

C 1232 Terminology of Masonry

C 1314 Test Method for Compressive Strength of Masonry Prisms

E 519 Test Method for Diagonal Tension (Shear) in Masonry Assemblages

E 72 Test Methods of Conducting Strength Tests of Panels for Building Construction


Each of the standards cited in Section 2 is referenced within the body of ASTM C90. In some

cases compliance with the reference standard is required, in other cases the reference is for

informational purposes only. The context of the specific reference clearly indicates whether

compliance is mandatory or not.

3. Terminology

3.1 Terminology defined in Terminology C 1232 shall apply for this specification.


ASTM C1232-10, Standard Terminology of Masonry, provides specific definitions for terms and

products used within ASTM masonry specifications. At this time ASTM C1232 does not have a

definition for a generic concrete masonry unit. The introduction of such a definition has been

attempted multiple times, but continues to be a point of contention due to the broad use of the

term. ASTM C1232 does have several terms defined that may be pertinent to this guide:

concrete brick, n—a concrete masonry unit made from portland cement, water, and

suitable aggregates, with or without the inclusion of other materials. See Specification C

55.

dry-cast, adj—manufacturing concrete products using low frequency, high amplitude

vibration to consolidate concrete of stiff or extremely dry consistency in a form.

manufactured masonry unit, n—a manmade noncombustible building product intended

to be laid by hand and joined by mortar, grout, or other methods of joining.

masonry, n—the type of construction made up of masonry units laid with mortar, grout,

or other methods of joining.

Like many other ASTM standards, ASTM C1232 is under continuous maintenance, revising or

adding definitions as necessary. In applying the requirements of ASTM C90, one should be


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familiar with the terms defined in ASTM C1232 as they may have unique application or meaning

within the context of ASTM specifications.

4. Materials

4.1 Cementitious Materials—Materials shall conform to the following applicable specifications:

4.1.1 Portland Cement—Specification C 150.

4.1.2 Modified Portland Cement—Portland cement conforming to Specification C 150, modified as

follows:

(1) Limestone—If calcium carbonate is added to the cement, the CaCO3 content shall not be less

than 85%.

(2) Limitation on Insoluble Residue—1.5%.

(3) Limitation on Air Content of Mortar—Volume percent, 22% max.

(4) Limitation on Loss on Ignition—7%.

4.1.3 Blended Hydraulic Cements—Specification C 595.

4.1.4 Hydraulic Cement—Specification C 1157.

4.1.5 Pozzolans—Specification C 618.

4.1.6 Blast Furnace Slag Cement—Specification C 989.


The most common cementitious material used in the production of concrete masonry products

remains portland cement, however, a number of alternative cementitious materials are also used

as partial or full replacement of the portland cement. Typically designers and specifiers are not

concerned with the type of cement used to manufacture a unit provided the unit’s physical

properties meet those specified for a project. Occasionally however, a specific cement type may

be more desirable due to its sustainable attributes or lower embodied energy. In either case, it is

the producer’s responsibility to ensure that the type of cement used is proportioned and batched

correctly to produce the desired physical unit properties.

4.2 Aggregates—Aggregates shall conform to the following specifications, except that grading

requirements shall not necessarily apply:

4.2.1 Normal Weight Aggregates—Specification C 33.

4.2.2 Lightweight Aggregates—Specification C 331.


Refer to Section 1.2 commentary discussion.

4.3 Other Constituents—Air-entraining agents, coloring pigments, integral water repellents, finely ground

silica, and other constituents shall be previously established as suitable for use in concrete masonry units

and shall conform to applicable ASTM standards or shall be shown by test or experience not to be

detrimental to the durability of the concrete masonry units or any material customarily used in masonry

construction.


Originally Section 4.3 was intended to capture and permit the use of materials, such as

admixtures and pigments, which are commonly introduced into concrete masonry units during

production but may not otherwise fall into one of the other material classifications of Sections 4.1

or 4.2. In recent years the term ‘other constituents’ has evolved from its original meaning to

encompass virtually any material that could conceivably be incorporated into a concrete masonry

unit.

As stated in this section, these materials are required to “be shown by test or experience not to be

detrimental to the durability of the concrete masonry units or any material customarily used in

masonry construction”. This requirement, applied specifically to this commentary discussion and

globally to this overall guide, is interpreted to mean that any material not defined by Sections 4.1


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or 4.2 – when part of a concrete masonry unit – shall result in a unit that has the same or better

performance when in service. For additional information on assessing the performance attributes

of concrete masonry unit manufactured with other constituents refer to NCMA’s publication

Guide for Ascertaining Performance Requirements for ASTM C90.

5. Physical Requirements

5.1 At the time of delivery to the purchaser, units shall conform to the physical requirements prescribed in

Table 1 and Table 2.


A common mistake in the application of the requirements of ASTM C90 is that the physical

properties of concrete masonry units must be met following 28 days of curing. Instead, ASTM

C90 only requires that the minimum specified requirements be met “at the time of delivery”.

Different producers of concrete masonry products may opt to employ varying curing targets prior

to shipping products, which could be shorter or longer than 28 days, depending upon the product

they are manufacturing, their curing/production equipment, or the mix design.

NOTE 2—Higher compressive strengths than those listed in Table 2 may be specified where required by

design. Consult with suppliers to determine availability of units of higher compressive strength.

Note 2 Commentary Discussions

Table 2 only requires a minimum average compressive strength of 1,900 psi, with no individual

unit having a compressive strength of less than 1,700 psi. Because ASTM C90 is a minimum

specification, designers and specifiers always have the option of specifying units with a higher

average compressive strength. The range of available unit compressive strengths that are

routinely manufactured, or may be a special order item, vary by region. Local manufacturers

should be consulted for available options prior to specifying a unit with an average compressive

strength greater than 1,900 psi.

NOTE 3—Oven-dry densities of concrete masonry units generally fall within the range of 85 to 145

lbf/ft3 (1360 to 2320 kg/m

3). Because available densities will vary, suppliers should be consulted before

specifying project requirements.


Refer to Section 1.1 commentary discussion.

TABLE 1 Minimum Thickness of Face Shells and WebsA

Nominal Width (W)

of Units, in. (mm)

Face Shell Thickness

(tfs), min, in. (mm)B,C

Web Thickness (tw)

WebsB,D,C

min,

in. (mm)

Equivalent Web Thickness, min,

in./linear ftE (mm/linear m)

3 (76.2) and 4 (102) 3/4 (19)

3/4 (19) 1

5/8 (136)

6 (152) 1 (25) 1 (25) 2 1/4 (188)

8 (203) 1 1/4 (32) 1 (25) 2

1/4 (188)

10 (254) and greater 1 1/4 (32) 1

1/8 (29) 2

1/2 (209)

A Average of measurements on a minimum of 3 units when measured as described in Test Methods C 140.

B When this standard is used for units having split surfaces, a maximum of 10% of the split surface is

permitted to have thickness less than those shown, but not less than 3/4 in. (19.1 mm). When the units are

to be solid grouted, the 10% limit does not apply and Footnote C establishes a thickness requirement for

the entire face shell. C When the units are to be solid grouted, minimum face shell and web thickness shall be not less than

5/8

in. (16 mm). D The minimum web thickness for units with webs closer than 1 in. (25.4 mm) apart shall be

3/4 in. (19.1

mm).


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E Equivalent web thickness does not apply to the portion of the unit to be filled with grout. The length of

that portion shall be deducted from the overall length of the unit for the calculation of the equivalent web

thickness.

Table 1 Commentary Discussions

Table 1 outlines minimum face shell, web, and equivalent web thickness requirements based on

the each nominal unit width. The requirements of Table 1 directly and indirectly address two

distinct performance attributes:

1) These minimum thicknesses facilitate the manufacturing and handling of concrete masonry

units to minimize cracking and breakage; particularly during the first hours following

production.

2) The minimum thicknesses also provide a minimum level of structural redundancy and

robustness of the installed product against loads and impact. They also provide a minimum

specified value upon which structural engineers can base their design calculations.

The majority of the key information to Table 1 resides in the table’s footnotes:

Footnote A clarifies that the minimum face shell, web, and equivalent web thicknesses are

determined in accordance with ASTM C140.

Footnote B allows face shell thicknesses of an ungrouted, split face unit to be as small as 0.75

inches over no more than 10% of the surface area of the unit. This option recognizes that

split surfaces are intentionally variable and may in some cases be less than the required face

shell thickness of Table 1. In cases where the unit is to be grouted solid, Footnote C applies.

Footnote C permits both face shell and web thickness to be reduced to 0.625 inches in cases

where the unit is grouted solid. In solid grouted units, the grout acts as the binding agent to

provide composite action across the entire unit cross section. The 0.625 inch minimum web

and face shell requirement is considered a reasonable minimum for manufacturing, handling,

and transportation purposes.

Footnote D addresses a specific type of concrete masonry unit manufactured with two,

smaller webs located adjacent to one another in the middle of the unit. The two webs permit

the unit to be field-split to product two half-length units. Refer to the commentary provided

for Note 5 for additional discussion on web thickness requirements.

Footnote E, like Footnote C, addresses situations where the cells of units are filled solid with

grout. The equivalent web thickness is stipulated to ensure that there is sufficient material

connecting the two face shells of a unit. In cases where the unit is grouted solid, the grout

supplements the webs in physically connecting the face shells.

TABLE 2 Strength, Absorption, and Density Classification Requirements

Density

Classification

Oven-Dry

Density of

Concrete, lb/ft3

(kg/m3)

Maximum Water Absorption,

lb/ft3 (kg/m

3)

Minimum Net Area Compressive

Strength, lb/in2 (MPa)

Average of 3

Units

Average of 3

Units

Individual

Units

Average of 3

Units

Individual

Units

Lightweight Less than 105

(1680)

18 (288) 20 (320) 1900 (13.1) 1700 (11.7)

Medium

Weight

105 to less than

125 (1680–

2000)

15 (240) 17 (272) 1900 (13.1) 1700 (11.7)

Normal Weight 125 (2000) or

more

13 (208) 15 (240) 1900 (13.1) 1700 (11.7)

Table 2 Commentary Discussions


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Table 2 covers average and individual maximum absorption and minimum compressive strength

requirements for lightweight, medium weight, and normal weight concrete masonry units. The

minimum compressive strength requirements are the same regardless of unit density. Conversely,

the maximum water absorption is permitted to vary based on unit density. Refer to the

commentary provided for Section 1.1 for additional discussion on unit density.

5.1.1 When higher compressive strengths than those listed in Table 2 are specified, the tested average net

area compressive strength of three units shall equal or exceed the specified compressive strength, and the

tested individual unit net area compressive strength of all three units shall exceed 90% of the specified

compressive strength.

Section 5.1.1 Commentary Discussions

When unit compressive strengths are specified that are higher than the minimum values, no

individual unit is permitted to have a net area compressive strength less than 90% of the specified

compressive strength.

5.2 At the time of delivery to the purchaser, the linear shrinkage of units shall not exceed 0.065%.

Section 5.2 Commentary Discussions

A maximum linear drying shrinkage value is required by ASTM C90 to mitigate the development

of shrinkage cracking following construction. Refer to the commentary provided for Section 5.1

for additional discussion on requirements “at the time of delivery”. Refer to the commentary

provided for Section 8.3 for additional discussion on linear drying shrinkage requirements. See

also NCMA TEK 10-1A, Crack Control in Concrete Masonry Walls, for additional information

on shrinkage crack mitigation.

NOTE 4—The purchaser is the public body or authority, association, corporation, partnership, or

individual entering into a contract or agreement to purchase or install, or both, concrete masonry units.

The time of delivery to the purchaser is FOB plant when the purchaser or the purchaser’s agent transports

the concrete masonry units, or at the time unloaded at the worksite if the manufacturer or the

manufacturer’s agent transports the concrete masonry units.


Note 4 provides additional non-mandatory information regarding the standard’s intent of “time of

delivery” and “purchaser”.

5.3 Hollow Units:

5.3.1 Face shell thickness (tfs) and web thickness (tw) shall conform to the requirements prescribed in

Table 1.


Refer to the commentary provided for Table 1 for additional discussion on these requirements.

NOTE 5—Web thickness (tw) not conforming to the requirements prescribed in Table 1 may be approved,

provided equivalent structural capability has been established when tested in accordance with the

applicable provisions of Test Methods E 72, C 1314, E 519, or other applicable tests and the appropriate

design criteria developed is in accordance with applicable building codes.


One of the great features of concrete masonry construction is the inherent flexibility of the system

afforded through varying the configuration of the individual units. The configuration of units can

be altered to achieve desired aesthetic effects, structural capabilities, construction productivity,

energy efficiency, or any number of other intrinsic properties inherent to concrete masonry

construction.


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The creativity and imagination of the marketplace in designing and developing novel unit

configuration far exceeds the ability of prescriptive national standards to keep pace. Case in

point, ASTM C90 addresses the minimum physical properties for loadbearing concrete masonry

units. The requirements of ASTM C90 and the companion testing standard ASTM C140 are by

necessity generic in nature – applying to the majority, but not all, configurations of units. As

special or proprietary units are introduced to the market, they may not be able to be evaluated

consistently or accurately under ASTM C140, or may not have all of the relevant features for

strict compliance with ASTM C90.

One such feature is the equivalent web thickness – which is a measurement of the amount of

concrete material connecting the two face shells of a unit per linear foot. To increase the energy

efficiency of a unit, to accommodate varying reinforcement schedules, increase construction

productivity, or any number of other reasons, the webs of a concrete masonry unit may be

reduced. Because webs can be reduced in a near unlimited number of ways and degrees, neither

ASTM C90 nor ASTM C140 contain a standardized method of accounting for the reduction in

available equivalent web thickness – or its impact on the strength of an assembly constructed of

such units.

In cases where the configuration of a concrete masonry unit or its webs do not comply with the

generic requirements of ASTM C90, or the testing procedures of ASTM C140, the recommended

practices listed in Note 5 shall be used to evaluate the impact of the reduced web configuration.

Note 5 was introduced in the 1990 edition of ASTM C90 to provide the necessary flexibility of

using concrete masonry units that incorporated unconventional web configurations – recognizing

the need for such applications – but provides only vague guidance on how to evaluate units with

alternative web configurations and how they can or should be used.

Prior to 1990, this note read somewhat differently, but still addressed the same fundamental issue

of transferring loads and stresses between the face shells of a unit. The equivalent note taken

from the 1964 edition of ASTM C90 read as follows:

Note 2 – Special unit designs involving corrosion-resistant metal ties between

face-shells may be approved provided tests show such ties are structurally

equivalent to the minimum specified concrete webs in stiffness, strength and

anchorage to the face-shells.

The purpose of the minimum equivalent web thickness requirements in ASTM C90 is to

ensure that sufficient material is provided between the two face shells of a unit to transfer

stresses from one face shell to the other so that the unit acts as a composite structural

assembly. A minimum equivalent web thickness criterion was probably a more critical

aspect of ASTM C90 when the standard was first introduced as many of the concrete

masonry assemblies constructed in the early part of the 20th

century were ungrouted and

unreinforced. More contemporary concrete masonry construction tends to have larger

amounts of reinforcement and grout – lessening, but not eliminating, the need for a

minimum equivalent web thickness, specifically in cases where a concrete masonry

assembly is ungrouted.

Note that Footnote E of Table 1 from ASTM C90 provides an exemption to units that do not

conform to the minimum equivalent web thickness, but are solidly grouted. By allowing grout to


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fulfill the structural role of connecting two face shells, ASTM C90 effectively permits webs to be

reduced, or removed, to any degree.

While not explicitly stated within ASTM C90, this standard does in effect define a de facto

minimum connection strength between two face shells by:

1) defining a minimum equivalent web thickness; and

2) defining a minimum unit compressive strength.

By defining the minimum equivalent web thickness and compressive strength for a concrete

masonry unit, the strength of the ‘connection’ between the two face shells is inherently defined as

well. ASTM C90 simply defines this value in a prescriptive manner. By determining this

inherent minimum ‘connection’ strength, one could then extrapolate this prescriptive requirement

to a more generic application through:

1) requiring that concrete masonry units with alternative web configurations that do not

meet the prescriptive minimum equivalent web thickness requirements of ASTM C90

provide an equal or greater ‘connection’ strength to that inherent to these prescriptive

minimums; or

2) in cases where the web configuration of a concrete masonry unit provides less

‘connection’ strength between the face shells, explicitly taking this limitation into

consideration when designing a concrete masonry assembly. In this case, the presence

and volume of grout present in an assembly constructed of such units, the support

conditions of the assembly, and the loads applied to the assembly are all unique variables

that would need to be taken into consideration when designing such an assembly and a

supplemental design check introduced to ensure that the assembly would not fail or be

compromised due to the reduced web configuration.

For additional information on assessing the performance attributes of concrete masonry unit

manufactured with varying web thickness, refer to NCMA’s publication Guide for

Ascertaining Performance Requirements for ASTM C90.

5.4 Solid Units:

5.4.1 The net cross-sectional area of solid units in every plane parallel to the bearing surface shall be not

less than 75% of the gross cross-sectional area measured in the same plane.


One of the more confusing definitions for masonry units within contemporary ASTM standards is

the definition of a solid masonry unit. Contrary to expectations, a solid concrete masonry unit

need not be 100% solid. Instead, up to 25% of the cross section of the unit is permitted to contain

cells or cores. Where a completely solid concrete masonry unit is desired, the specification

should stipulate 100% solid concrete masonry units.

5.5 End Flanges:

5.5.1 For units having end flanges, the thickness of each flange shall not be less than the minimum face

shell thickness.


Where concrete masonry unit are produced with end flanges, commonly referred to as “stretcher

units”, the minimum thickness of the end flanges is not permitted to be less than the face shell

thickness. This requirement ensures that the ends of the units have the same or greater robustness

and strength as the remainder of the exposed face of the unit.

NOTE 6—Flanges beveled at the ends for mortarless head joint applications that will be filled with grout

are exempt from this requirement. Flanges which are specially shaped for mortarless head joint


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applications which have been shown by testing or field experience to provide equivalent performance are

exempt from this requirement.


Note 6 applies to a unique unit configuration that is intended to be installed in direct contact with

adjacent units without mortar in the head joints. The ends of these units have a bevel on the

inside of the end flanges that permits grout to flow into the resulting key to provide mechanical

bond between adjacent units similar to what would be otherwise provided with conventionally

mortared head joints.

6. Permissible Variations in Dimensions

6.1 Standard Units—For standard units, no overall dimension (width, height, and length) shall differ by

more than +/-1/8 in. (3.2 mm) from the specified dimensions.


Overall unit dimensions (width, height, and length) are determined in accordance with ASTM

C140. When architectural finishes are added to a unit, exemptions to these dimensional

tolerances may be waived as discussed in Section 6.2.

6.2 Particular Feature Units—For particular feature units, dimensions shall be in accordance with the

following:


Particular feature units include scored block, split surfaces, slump block, ground or burnished

block, and units with similar architectural features either molded or mechanically introduced into

the unit during production.

6.2.1 For molded face units, no overall dimension (width, height, and length) shall differ by more than +/-1/8 in. (3.2 mm) from the specified standard dimension. Dimensions of molded features shall be within +/-

1/16 in. (1.6 mm) of the specified standard dimensions and shall be within +/-

1/16 in. (1.6 mm) of the

specified placement of the molded feature.


Section 6.2.1 addresses the placement and location tolerances of molded features, such as those

covered by Note 7.

NOTE 7—Molded features include, but are not limited to: ribs, scores, hex-shapes, and patterns.


No Commentary.

6.2.2 For split-faced units, all non-split overall dimensions shall differ by not more than +/-1/8 in. (3.2

mm) from the specified standard dimensions.


Because split-faced units are intentionally manufactured to provide an irregular finished surface,

such surfaces are exempt from complying with the overall dimensional tolerance requirement of

+/- 1/8 inch. Overall unit dimensions that are not affected by a split surface must still comply

with this tolerance. For the majority of split-face concrete masonry units, this exemption from

dimension tolerance applies to the width.

6.2.3 For slump units, no overall height dimension shall differ by more than +/-1/8 in. (3.2 mm) from the

specified standard dimension.


Like split-faced units, slump units are intentionally manufactured to provide an irregular finished

surface and are therefore exempt from complying with the overall dimensional tolerance

requirement of +/- 1/8 in. for length and width.


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NOTE 8—On faces that are split or slumped, overall dimensions will vary. Consult with suppliers to

determine achievable dimensional tolerances for products including these features.


No Commentary.

7. Finish and Appearance

7.1 All units shall be sound and free of cracks or other defects that interfere with the proper placement of

the unit or significantly impair the strength or permanence of the construction. Minor cracks, incidental to

the usual method of manufacture or minor chipping resulting from customary methods of handling in

shipment and delivery, are not grounds for rejection.


As a general note, the finish and appearance requirements of ASTM C90 are often the most

controversial, and most commonly misinterpreted, requirements of the standard. This stems in

part from the fact that finish and appearance requirements are by their very nature subjective –

interpreted differently depending upon the individual performing the assessment. The

requirements are also vague, in part because it is not easy to standardize such requirements – but

also because they are intended to be flexible to apply to a broad set of conditions. Refer to the

commentary provided for Section 7.2.1 for additional discussion on minor chips and cracks.

7.2 Where units are to be used in exposed wall construction, the face or faces that are to be exposed shall

not show chips or cracks, not otherwise permitted, or other imperfections when viewed from a distance of

not less than 20 ft (6.1 m) under diffused lighting.


ASTM C90 is a manufacturing standard – addressing the minimum requirements for block. It

does not cover design, application, workmanship, etc. – each of which is necessary for the

successful application of a C90-compliant concrete masonry unit. It is often interpreted that the

20-foot criterion above applies to units that are installed. While this requirement could be applied

as such, its true intent is for assessing units prior to their installation – again, because this is a

product standard. When applying these limits to units that have already been installed, one

should note that units may become damaged between the time they were delivered and the time

they were installed. This should be taken into consideration. One reason that these requirements

are applied to a finished assembly is because there are not standardized means of assessing

workmanship or appearance of completed masonry assemblies.

7.2.1 Five percent of a shipment containing chips, not larger than 1 in. (25.4 mm) in any dimension, or

cracks not wider than 0.02 in. (0.5 mm) and not longer than 25% of the nominal height of the unit, is

permitted.


Minor chipping and cracking incidental to customary production and handling methods are not by

themselves grounds for rejecting a given shipment of concrete masonry units, provided that no

more than 5% of that shipment contains such chips or cracks. Broken units are considered to be

units with a chip or crack larger than that permitted by Section 7.2.1.

7.3 The color and texture of units shall be specified by the purchaser. The finished surfaces that will be

exposed in place shall conform to an approved sample, consisting of not less than four units, representing

the range of texture and color permitted.


At a minimum, samples should consist of at least 4 individual units. Concrete masonry units,

because they are manufactured with natural materials, will vary some in final color and


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appearance. Where workmanship and aesthetics are a key component of the project, sample

panels may also be specified.

7.4 A shipment shall not contain more than 5 % of units, including broken units, that do not meet the

requirements of 6.1, 7.1, 7.2, and 7.2.1.


Refer to the commentary provided for Section 7.2.1 for additional discussion on broken units.

8. Sampling and Testing

8.1 The purchaser or authorized representative shall be accorded proper facilities to inspect and sample

the units at the place of manufacture from the lots ready for delivery.


No Commentary.

8.2 Sample and test units in accordance with Test Methods C 140.


ASTM C140 covers such test as: compressive strength, dimensional tolerances, absorption, etc.

8.3 Total linear drying shrinkage shall be based on tests of concrete masonry units made with the same

materials, concrete mix design, manufacturing process, and curing method, conducted in accordance with

Test Method C 426 and not more than 24 months prior to delivery.


The linear drying shrinkage of a concrete masonry unit is influenced by the type of constituent

materials, their relative proportions, and the process by which the units are manufactured and

cured. As such, linear drying shrinkage is rarely performed on each lot of concrete masonry units

products, but instead when changes to the mix design or manufactured method are implemented.

ASTM C90, however, requires that linear drying shrinkage be performed at intervals not

exceeding 2 years.

9. Compliance

9.1 If a sample fails to conform to the specified requirements, the manufacturer shall be permitted to

remove units from the shipment. A new sample shall be selected by the purchaser from remaining units

from the shipment with a similar configuration and dimension and tested. If the second sample meets the

specified requirements, the remaining portion of the shipment represented by the sample meets the

specified requirements. If the second sample fails to meet the specified requirements, the remaining

portion of the shipment represented by the sample fails to meet the specified requirements.


This section includes the requirement that if the first sample of concrete masonry units tested

does not comply with ASTM C90 that a second can be sampled and retested. This provides an

opportunity for anomalous test results to be verified prior to rejection of shipment of units.

NOTE 9—Unless otherwise specified in the purchase order, the cost of tests is typically borne as follows:

(1) if the results of the tests show that the units do not conform to the requirements of this specification,

the cost is typically borne by the seller; (2) if the results of the tests show that the units conform to the

specification requirements, the cost is typically borne by the purchaser.


This note provides non-mandatory guidance on how the costs of testing are usually handled

between purchase and supplier.

10. Keywords


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10.1 absorption; concrete masonry units; equivalent web thickness; face shell; flange; lightweight; linear

shrinkage; loadbearing; medium weight; normal weight; webs


No commentary.

APPENDIXES

(Nonmandatory Information)

X1. WATER PENETRATION RESISTANCE

X1.1 Exterior walls are often subjected to moisture penetration from one or more sources. For example,

basement walls may be exposed to water from saturated soil. Above grade exterior walls are usually

exposed to wind-driven rain. To prevent water penetration, proper detailing, construction, flashing, and

drainage should be provided. Proper water penetration resistant treatments should be applied to the walls.

While it is not within the scope of Specification C 90 to include information on resistance to water

penetration, such information and guidelines are available from other organizations.

Appendix X1 Commentary Discussion

Multiple industry publications are available on the subject of water penetration resistance of

concrete masonry construction, including:

TEK 19-1, Water Repellents for Concrete Masonry Walls

TEK 19-2A, Design for Dry Single-Wythe Concrete Masonry Walls

TEK 19-3A, Preventing Water Penetration in Below-Grade Concrete Masonry Walls

TEK 19-4A, Flashing Strategies for Concrete Masonry Walls

TEK 19-5A, Flashing Details for Concrete Masonry Walls

TEK 19-6, Joint Sealants for Concrete Masonry Walls

TEK 19-7, Characteristics of Concrete Masonry Units with Integral Water Repellent

X2. CRACK CONTROL

X2.1 Restrained or differential movement in building elements and building materials can result in

cracking. Some common causes of movement are: loads created by wind, soil pressure, seismic forces, or

other external sources; settlement of foundations; or volume changes in materials. For example, volume

changes in concrete masonry units can be caused by moisture gain and loss, thermal expansion and

contraction, and carbonation. To limit and control cracking due to these and other causes, proper design,

detailing, construction, and materials are necessary. Specification C 90 provides a maximum limitation on

the total linear drying shrinkage potential of the units, but it is not within the scope of this specification to

address other design, detailing, construction, or material recommendations. This type of information and

related guidelines for crack control are available from other organizations.

Appendix X2 Commentary Discussions

Multiple industry publications are available on the subject of crack control in concrete masonry

construction, including:

TEK 10-1A, Crack Control in Concrete Masonry Walls

TEK 10-2C, Control Joints for Concrete Masonry Walls – Empirical Method

TEK 10-3, Control Joints for Concrete Masonry Walls – Alternative Engineered Method

TEK 10-4, Crack Control for Concrete Brick and Other Concrete Masonry Veneers