Unit Compressive Strength and Absorption/Saturation Testing
Unit strength and absorption
ASTM C140 provides procedures for sampling and testing concrete masonry units, including compressive strength, absorption, moisture content, and density. Compressive strength is determined on capped units or half units, and for hollow units the load is divided by the net area, not the gross area. Absorption testing involves oven-drying, immersion, and in many procedures boiling, followed by weighing in air and while suspended to determine absorption and saturation coefficient. The saturation coefficient is a ratio of absorptions that indicates the degree of pore saturation and is used in durability specifications for clay masonry.
Understanding the difference between absorption and saturation coefficient is essential. Absorption is the amount of water taken up under a specified condition, expressed as a percentage of dry mass. Saturation coefficient compares a milder soak to a more severe boil; a lower coefficient generally indicates better weathering resistance. For concrete masonry, C90 sets maximum absorption limits by unit type, while C216 sets requirements for facing brick.
Trap: dividing compressive load by the gross area of a hollow unit, testing a unit that has not been properly capped, or reporting absorption without stating the conditioning method.
Worked example: an oven-dry unit weighs 4000 g; after the specified 24-h soak it weighs 4400 g; after boiling it weighs 4500 g. Absorption from the soak = (4400 − 4000)/4000 × 100 = 10.0%. Saturation coefficient = 400/500 = 0.80. Report both the value and the method, because the numbers are meaningless without the conditioning basis.