Curing From the Inside Out: How much LWA do I need?

Saturated lightweight aggregate is typically added as fine aggregate, since that is more dispersed throughout the concrete matrix. But how much is needed? Let’s figure it out:

• Through extensive research by Dale Bentz at the National Institute of Standards and Technology, Jason Weiss at Purdue, and others it’s been proven that we need 7 pounds of internal curing water per 100 pounds of cementitious material.
• If we assume a mix with 600 pounds of cementitious material, we therefore need 600/100 x 7 = 42 pounds of internal curing water.
• We will assume that the lightweight aggregate has 18% absorption in the field (this is about average) and that 90% of the water will desorb (give the water back to the concrete mix).
• We will further assume that the prewetted fine lightweight aggregate weighs 55 pounds per cubic foot.
• So 55 pounds x 18% (0.18) = 9.9 pounds/cubic foot x 0.9 (desorption) = 8.9 pounds of internal curing water available per cubic foot of fine LW aggregate.
• We need 42 pounds of internal curing water divided by 8.9 pounds of internal curing water available per cubic foot = 4.7 cubic feet of fine LWA per cubic yard of concrete.
• 4.7 cubic feet x 55 pounds per cubic foot = 259 pounds of prewetted fine LW aggregate per cubic yard of concrete.