Concrete Masonry Units

Materials and Manufacturing of Concrete Masonry Units

Concrete masonry units are formed from zero-slump concrete, sometimes using lightweight aggregate. The concrete mixture is usually vibrated under pressure in multiple-block molds. After stripping the molds, the units are usually cured under atmospheric conditions in a chamber that is maintained at warm and humid conditions by the presence of the curing units. Atmospheric steam or high-pressure steam (autoclaving) can also be used for curing. Concrete units normally have a much higher void ratio than clay units, making determination of (c/b) ratios unnecessary.

Visual and Serviceability Characteristics of Concrete Masonry Units

The following visual and serviceability characteristics are addressed by ASTM C90 (Standard Specification for Hollow Load-Bearing Concrete Masonry Units):
1. Dimensional tolerances: ASTM C90 prescribes maximum dimensional tolerances of ± 1/8 in. Thicknesses of face shells and webs are specified.
2. Chippage: According to ASTM C90, up to 5 percent of a shipment may contain units with chips up to 1 in. in size.
Other visual and serviceability characteristics, such as color, are not addressed by ASTM C90. Color is gray or white, unless metallic oxide pigments are used.

Mechanical Characteristics of Concrete Masonry Units The following mechanical characteristics are covered by ASTM C90, ASTM C140, and ASTM C426:

1. Compressive strength is typically 1500 to 3000 psi on the net area (actual area of concrete). ASTM C90 requires a minimum compressive strength (average of 3 units) of 1900 psi, measured on the net area.

2. Absorption (used to measure void volume) is evaluated in the following manner. The unit is immersed in cold water for 24 h. It is weighed immersed (weight F), and weighed in air while still wet (weight E). It is then dried for at least 24 h at a temperature of 212 to 239°F, and again weighed (weight C). Absorption in lb/ft3 is calculated as [(E − C)/(E − F)] × 62.4. Maximum permissible absorption is 18 lb/ft3 for light-weight units (less than 105 lb/ft3 oven-dried weight), 15 lb/ft3 for medium-weight units (105 − 125 lb/ft3), and 13 lb/ft3 for normalweight units (more than 125 lb/ft3).

3. Shrinkage of concrete masonry units due to drying and carbonation is 300−600 με. In general, shrinkage is controlled by controlling the concrete mix used to make the units, and by limiting the moisture content of the units between the time of production and when they are placed in the wall.

The concrete masonry industry formerly produced Type I (moisturecontrolled) units, which due to a combination of inherent characteristics and packaging were designed to shrink less, and Type II units (nonmoisture-controlled). This distinction was not as successful as originally hoped, because it was difficult to control the condition of Type I units in the field. As a result, ASTM C90 now does not refer to Type I and Type II units. All C90 units must demonstrate a potential drying shrinkage of less than 0.065 percent (650 με), which was the shrinkage requirement that formerly applied to Type II units.

Other Characteristics of Concrete Masonry Units

The following characteristic are not covered by ASTM specifications:

1. Surface texture can be smooth, slump block, split-face block, ribbed block, various patterns, or polished face.

2. Tensile strength is about 10 percent of compressive strength.

3. Tensile bond strength (strength between mortar and CMU) is typically about 40 to 75 psi when portland cement-lime mortar is used, and about 35 psi or less when masonry-cement mortar is used.

4. Initial rate of absorption (IRA) is typically 40 to 160 g/min per 30 in.2 of bed area. It is much less than this in units with integral water-repellent admixtures. In contrast to clay units, the tensile bond strength of concrete masonry units is not sensitive to initial rate of absorption. For this reason, specifications for concrete masonry units do not require determination of IRA. 5. Modulus of elasticity is typically 1 − 3 × 106 psi. 6. Coefficient of thermal expansion is typically: 4 − 5 με/°F.

Scroll to Top