The following other characteristics of clay masonry units are not addressed by ASTM specifications: 1. Color: Metallic oxides give different units their characteristic color. Colors are commonly judged by eye from sample panels. Systems for color tolerance exist, but are not widely used.
2. Tensile strength: Parallel to the grain (in the direction of extrusion), this is typically 20 to 30 percent of the corresponding compressive strength. Perpendicular to the grain, it is typically 10 to 20 percent of the corresponding compressive strength.
3. Initial rate of absorption (IRA): This is defined as the number of grams of water absorbed in 1 min per 30 in.2 of bed area. An ideal range is 10 to 30. Many clay masonry units used in Texas have IRAs exceeding 30. Units with an IRA greater than 30 should be wetted briefly before laying. A simple field test for IRA is as follows: Place 20 drops of water in a quarter-sized area. If it takes longer than 1.5 min for the water to be absorbed, the units do not need to be wetted before laying.
4. Tensile bond strength (strength between mortar and units): This is typically about 100 psi when cement-lime mortar or mortar-cement mortar is used, and about 50 psi or less when masonry-cement mortar is used. Tensile bond strength is increased by compatibility between mortar and units: units with high IRA should be used with mortar having high water retention (high lime content); low IRA units should be used with low-retentivity mortar.
5. Modulus of elasticity: 1.4 − 5 × 106 psi. 6. Freeze-thaw expansion: Clay units exposed to cycles of freezing and thawing undergo permanent expansion (mean, standard deviation, and 97-percentile value of 118, 96, and 300 με, respectively).
7. Moisture expansion: Clay units exposed to moisture undergo permanent expansion caused by adsorption of water into unvitrified clay molecules (mean, standard deviation, and 97-percentile values of 200, 190, and 540 με, respectively).
8. Coefficient of thermal expansion: 3 − 4 με/°F.
