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4.2.1. ASTM C-90 Northwest Modifications.

The typical concrete masonry unit manufactured in the Northwest is a Grade N block,

Grade N units require a minimum compressive strength of 1000 p.s.i., average (3 units)gross area. This equates to a netconcrete strength of over 2000 psi. since the block is

approximately 50 % void area.

Some confusion arises over the compressive strength portion of the specification. Many

specs call for the compressive strength of the concrete masonry unit to be 1350 p.s.i.rather than 1000 p.s.i. This is improper as 1350 p.s.i. is the walldesign strength allowable

with Grade N units. This value incorporates the strengths of the mortar and grout as well

as the block into one design value based on net area.

Another ASTM C-90 requirement that stirs up much controversy, misunderstanding andwrong application is that concerning the type classification of concrete masonry units.

C-90 states that there are two types of units for each grade: Type I,Moisture- Controlled

Units, which shall conform to the moisture content requirements of Table I, and Type II,NonMoisture-Controlled Units, which need not conform to the moisture content

requirement shown in Table 1.

ASTM C-90

Table 1 Moisture-Content Requirements for Type 1 Units

Linear

Shrinkage

Moisture Content, max, % of TotalAbsorption (Average of 3 Units)

Humidity Conditions at Job Site or Point of

Use

Humida Intermediateb Aridc

0.03 or less 45 40 35

From 0.03 to0.045

40 35 30

0.045 to 0.065,

max35 30 25

aAverage annual relative humidity above 75 %bAverage annual relative humidity 50 to 75 %cAverage annual relative humidity less than 50 %

Table 2 Strength and Absorption Requirements

Note -- To prevent water penetration protective coating should be applied on the exterior

face of basement walls and where required on the face of exterior walls above grade.

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Grade

Compressive

Strengthmin, psi (MPa)

Water Apsorption,

max lb/ft3 (kg/m3)

(Average of 3 units) with Oven-dry Weight of Concrete, lb/ft3 (kg/

m3)

Average GrossArea

Weight Classification

Averageof 3

Units

IndividualUnit

Light weight

Lessthan

85

(1362)

Lessthan

105

(1682)

Mediumweight

Less than

125 to

105

(2002 to1682)

Normal

weight125

(2002)

or more

N-1N-11

1000(6.9)

800 (5.5) -- 18(288)

15 (240) 13 (208)

S-1a

S-11a

700

(4.8)600 (4.1)

20

(320)-- -- --

aLimited to use above grade in exterior walls with weather-protective coatings and in

walls not exposed to the weather

Table 3 Minimum Thickness of Face-shell and Webs

Web Thickness (WT)

Nominal

Width (W) ofUnits, in. (mm)

Face-Shell

Thickness(FST) min, in.

(mm)a

Webs,a min,in. (mm)

Equivalent Web

Thickness, min.in./linear ftb

(mm/linear m)b

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)1 3/8 (35)

1 1/4 (32)c1 1/8 (29) 2 1/2 (209)

12 (305)1 1/2 (38)

1 1/4 (32)c1 1/8 (29) 2 1/2 (209)

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aAverage of measurements on 3 units taken at the thinnest point, when measured as

described in Methods C 140, Sections 15 and 17.2.

bSum of the measured thickness of all webs in the unit, multiplied by 12, and divided bythe length of the unit.

cThis face-shell thickness (FST) is applicable wher allowable design load is reduced in

proportion to the reduction in thickness from basic face-shell thickness shown.

Moisture content requirements are included to con trol the amount of shrinkage that canoccur in a concrete masonry unit. There are three facets to consider in evaluating

moisture content and shrinkage.

1. What are the humidity conditions at the jobsite or point of use? In an area that is

continually damp, rainy and very humid, masonry would not tend to dry out and shrink.Shrinkage takes place when excessive water leaves the unit through evaporation or

drying. Arid or desert area will cause the excessive moisture in the unit to leave rapidly

and thus shrink more than in a humid area. Accordingly, in humid areas, the moisturecontent of the block can be higher than the moisture content in arid areas.

The humidity conditions are defined as follows:

Humid, when the average humidity is above 75 %;

Intermediate, when the average annual humidity is between 50 % and 75 %;

Arid, when the average annual humidity is less than 50 %.

This average annual humidity is generally obtained from the local weather bureau, which

may or may not have the same condition as the jobsite.

2. What is the absorption of the masonry units? The heavier the unit, the less water it is

permitted to absorb.

3. What is the total shrinkage of the units from saturated wet to oven dry? Table 1 givesthree ranges of shrinkage:

1. 0.03 % or less

2. from 0.03 % to 0.045 %

3. from 0.045 % to 0.065 % max.

Masonry units that have very little shrinkage, 0.030 % or less, can have higher moisturecontent because the loss of water will not create excessive shrinkage in the block. Units

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with high shrinkage, 0.065 % may have excessive movement or shrinkage if the units are

very wet and they dry out in an arid climate.

The total linear shrinkage of the unit will be a governing factor in determining how muchmoisture may be in the block.

With the materials used in the manufacturing of block in the NW., it is recommended that

the shrinkage requirement be specified as 0.045 % - 0.065 % maximum. In Seattle for

example, the maximum moisture content would then be 35%, as taken from Table I

4.2.2. Basic Use. 1) Reinforced structural walls.

4.2.3. Grades. There are two grades of hollow concrete block: N and S. The grade N unit

is for general use in exterior walls above and below grade and for interior walls.

The Grade S unit is limited to use above grade in exterior walls with weather protective

coating and in walls not exposed to the weather.

4.2.4. Compression. This is the vertical load applied upon a block unit in its normalposition in accordance with ASTM specification. Block shall develop not less than 700

p.s.i. grade S, and 1000 p.s.i. for Grade N over the average gross area with an average of

three units, Higher strength block are available. Consult with local manufacturer whendesired.

4.2.5. Moisture Control. Two types of hollow concrete block are covered as follows:

TYPE I: Moisture controlled units shall conform to a maximum moisture content

requirement dependent upon the linear shrinkage properties of the block and the humidityconditions at the jobsite. Block with less shrinkage potential are permitted to have higher

moisture contents.

TYPE II: Non-moisture controlled units are not required to meet a moisture content

requirement.

4.2.6. Dimension.No overall dimension (width height, and length) shall differ by morethan 1/8 inch from the specified standard dimensions.

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Configurations and dimensions of a typical 8x8x16 unit

4.3. Solid Load-Bearing Units. This section covers solid concrete block (units with 75%

or more solid area) ASTM C-145 is the most widely used solid block specification.

4.3.1. Basic Use. 1) Veneer, 2) Cavity walls, 3) Multi-wythe reinforced walls.

4.3.2. Grades. There are two grades of solid load-bearing concrete block "N and S". Thegrade N unit is for general use in exterior walls above and below grade and for interior

walls.

The Grade S unit is limited to use above grade in exterior walls with weather protective

coating and in walls not exposed to the weather.

4.3.3. Compression. Block shall develop not less than 1200 p.s.i. for grade S units, and1800 p.s.i. for grade N over the average gross area with an average of three units.

4.3.4. Moisture Content. Two types of solid concrete block are covered as follows:

NOTE: Same info as 4.2.5.

4.3.5. Dimension. Two types of hollow concrete block are covered as follows: NOTE:Same info as 4.2.6.

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Gross and net CMU area

Concrete Masonry Unit interior wall

Next page

The NW Masonry Guide Table of Contents

Masonry Institute of Washington

Washington State Conference of Mason Contractors

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