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Concrete Masonry Products
1) History Of Concrete Block
2) Manufacturing Process
3) CMU Today
4) Properties & Characteristics
5) Types Of Concrete Masonry Units
6) CMU Shapes
3. CMU Today
The dominant unit manufactured today is the 8x8x16 inch nominal module. It is currently manufactured as a two-core unit for reasons of providing for vertical reinforcing, lighter weight and balance in pick-up by the mason. Filled cell insulation can also be added in the block cores. This unit has gained popularity for the following reasons:
a) An optimum relationship of weight and size.
b) This unit bonds easily at corners as the width is half of the length allowing for running bond.
The materials used in the manufacture of concrete block in the N.W., include portland cement, sand, gravel, and lightweight aggregate. The specific lightweight aggregate available in this area is pumice.
Block are manufactured as Lightweight units (less than 105 lbs/cu ft.) or medium weight (105-1 25 lbs/cu ft.). The density requirements are included in ASTM C-90.
Lightweight block utilize either all pumice aggregate or a combination (usually 50/50) of pumice and sand and gravel. Full pumice block provide superior fire resistance and energy saving properties, but are lower in compressive strength.
Another lightweight block manufactured is the 50/50 unit which is the dominant type block in the NW. This block provides overall construction economy because of its strength and weight. Normal Weight block are manufactured with all sand and gravel aggregate. These units have slightly less shrinkage and are denser than light weight block. They do weigh more which increases the laid in place cost somewhat
Freshly molded units are transferred into
the low pressure steam and drying kilns
4. Properties and Characteristics of CMU
The infinite number of uses of concrete masonry are limited only to the imagination of the building designer. Few other materials have its fire-resistive qualities; few can match its structural strength. In a new era of energy saving consciousness it is finding much appreciation for its thermal resistive properties, not to overlook its effective ability to act as a barrier to the transmission of sound. Architecturally, it has embellished the finest of modern structures. With inflation leaping out of sight late in the twentieth century, concrete masonry's freedom from maintenance, and its fire resistance are other important features of this building material.
Color. Concrete block can be manufactured with integral color. Mineral oxide pigments are used to accomplish this. The normal colors possible are buff, red, brown, and yellow.
4.1 Texture: Texture in block is dependent upon the aggregate types and blend used in a particular mix as well as the water content during the molding process. Smooth units may have a nearly closed face texture (wipe) or very porous and open appearance. Split Face and Striated (Scratch) Face units have face textures created mechanically by a fracturing or combing process.
4.2. Hollow Load-Bearing Block. This section covers the standard hollow core concrete block. Such units are less than 75% solid. ASTM C-90 is the most widely used hollow block specification.
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 net concrete 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 wall design 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 and wrong 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, Non Moisture-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 Total Absorption (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 to 0.045 | 40 | 35 | 30 |
| 0.045 to 0.065, max | 35 | 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.
| Grade | Compressive Strength min, psi (MPa) |
Water Ap sorption, max lb/ft3 (kg/m3) (Average of 3 units) with Oven-dry Weight of Concrete, lb/ft3 (kg/m3) |
||||
| Average Gross Area | Weight Classification | |||||
| Average of 3 Units | Individual Unit | Light weight | Medium weight Less than 125 to 105 (2002 to 1682) |
Normal weight 125 (2002) or more | ||
| Less than 85 (1362) | Less than 105 (1682) | |||||
| N-1 N-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) of Units, 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)c |
1 1/8 (29) | 2 1/2 (209) |
| 12 (305) | 1 1/2 (38) 1 1/4 (32)c |
1 1/8 (29) | 2 1/2 (209) |
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 by the length of the unit.
cThis face-shell thickness (FST) is applicable where allowable design load is reduced in proportion to the reduction in thickness from basic face-shell thickness shown.
Moisture content requirements are included to control the amount of shrinkage that can occur 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 moisture content 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 gives three 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 moisture content because the loss of water will not create excessive shrinkage in the block. Units 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 much moisture 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 normal position 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 when desired.
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 humidity conditions 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 more than 1/8 inch from the specified standard dimensions.
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". 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.
4.3.3. Compression. Block shall develop not less than 1200 p.s.i. for grade S units, and 1800 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.
Gross and net CMU area
Concrete Masonry Unit interior wall
The NW Masonry Guide Table of Contents
Masonry Institute of Washington
Washington State Conference of Mason Contractors
