Masonry Structural design

Visual and Serviceability Characteristics of Clay Masonry Units

The following characteristics are covered by ASTM C62 (Standard Specification for Building Brick) or by ASTM C216 (Standard Specification for Facing Brick):
• Dimensional tolerances
• Durability
• Freeze-thaw resistance
• Appearance

  Different ASTM requirements for clay masonry units are summarized in a table at the end of this section, and are described in more detail below:1. Dimensional tolerances for building brick (ASTM C62) vary with nominal dimensions, but are typically ± 1/4 in. For facing brick (ASTM C216), corresponding typical required tolerances are ± 8/32 in. for Type FBS (face brick, standard) and ± 5/32 in. for Type FBX (face brick, extra). Tolerances are also specified for distortion.2. Durability is controlled indirectly in terms of boiling-water absorption. Units are dried at 230 to 239°F, placed in boiling water for 5 h, then reweighed. Boiling water absorption equals weight gain divided by original dry weight.
Boiling-water absorption is taken as a general index of durability. Building brick (ASTM C62) must have a boilingwater absorption (average of 5 units) of at most 17 percent for Grade SW (severe weathering), and at most 22 percent for Grade MW (moderate weathering). No limit is imposed for Grade NW (negligible weathering). Facing brick (ASTM C216) must have corresponding boiling-water absorptions of 17 percent (Grade SW) and 22 percent (Grade MW). Grade NW does not exist under ASTM C216.

3. Freeze-thaw resistance is controlled indirectly in terms of a “saturation coefficient,” defined as follows:
a. Cold-water absorption (24 h): Units are dried at 230 to 239°F, placed in cold water for 24 h, then reweighed. Cold-water absorption equals weight gain divided by original dry weight.
b. Boiling-water absorption (5 h): After the cold-water absorption test described above, units are placed in boiling water for an additional 5 h, and again weighed. Boiling-water absorption equals weight gain (cold plus boiling-water absorption) divided by original dry weight.
c. Saturation coefficient (c/b ratio): Cold-water absorption (24 h) divided by boiling water absorption (5 h).
The saturation coefficient is a measure of the additional void space available in the units after saturation by cold water. A saturation coefficient of 1.0 indicates no additional void space.
The lower the saturation coefficient, the more additional void space is available. This is taken as a rough index of resistance to freeze-thaw degradation (additional void space is available for the freezing water).

Building brick (ASTM C62) must have a saturation coefficient (average of 5 units) of at most 0.78 for Grade SW (severe weathering), 0.88 for Grade MW (moderate weathering), and at most 1.0 (no limit) for Grade NW (negligible weathering).
Facing brick (ASTM C216) must have corresponding saturation coefficients of 0.78 (Grade SW) and 0.88 (Grade MW). Grade NW does not exist under ASTM C216.
4. Appearance is not addressed by ASTM C62 (Building brick).
ASTM C216 (Facing brick) addresses chippage and efflorescence potential.
a. Chippage: Under ASTM C216, up to 10 percent of units complying with Grade FBS can have chips up to 1 in. in size; for Grade FBX, the corresponding percentage is 5 percent.
b. Efflorescence: This is a white or colored chemical residue on the surface of the masonry. It is produced by water-soluble compounds within or in contact with the masonry. If water gains access into the masonry in sufficient amounts, and comes in contact with the water-soluble compounds for a sufficient time, it dissolves those compounds into positive and negative ions. The water containing the dissolved compounds (in ionic form) moves through the masonry and evaporates; the dissolved ions combine to form deposits. If these deposits form on the surface of the masonry, they are called “efflorescence;” if they form within the pores of the masonry near the surface, they are called “cryptoflorescence.” The positive ions are usually potassium, sodium, or calcium. The negative ions are usually sulfates, chlorides, or hydroxides. In general, because the positive and negative ions are present in all masonry, efflorescence is reduced by limiting the amount of water in contact with the masonry, and by limiting the passage of water to the surface of the masonry.
When required to be tested, Facing Brick (ASTM C216) are required to show “no efflorescence.” Efflorescence Testing (in accordance with ASTM C67), uses distilled water and is not a complete check for efflorescence.


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