This is another argument to add LWA to FC besides it making it less brittle. Some people in the FC industry claim that FC only improves over time. This could be because of the internal curing that may have been going on. Why this happened could be an interesting study. There are housing developments build 20 years ago with FC and still in very good condition according to my sources.
The website listed promotes LWA made from expanded clay, and is added to assist the internal curing. They made no mention how difficult it is to control the rheology as (I am assuming) the wet LWA releases water as soon as it is put in the mix. This could be problematic for use in FC. Someone may be working on a LWA that releases the water slowly or only after 24 hours or more.
Alternatively a Super Absorbent Polymer (SAP) can be used to store the water.
I pasted a preview of the website below
Using lightweight aggregate to cure concrete slabs yields big results.
By John Ries
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?
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 pre-wetted 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.
- 7 cubic feet x 55 pounds per cubic foot = 259 pounds of prewetted fine LW aggregate per cubic yard of concrete.
Lightweight concrete has long been used for suspended floors in order to reduce the weight on the structure and to increase the fire rating. And since a lightweight floor is typically thinner (5.25 inches versus 6.5 inches with normal weight for the equivalent fire rating), a lightweight floor will be 40% to 45% lighter than a normal-weight floor. So while lightweight concrete does cost a little more, with the reduction in weight, the building frame costs less. But lightweight concrete has other advantages when designed to take advantage of internal curing.
Concrete slabs must be surface-cured to end up with strong, crack-free faces. But most curing of slabs only affects a very thin layer at the surface — which is certainly important since that’s the wearing surface, but what’s happening farther down in the slab is also important and surface curing has little effect in the interior. Using lightweight aggregate to cure the concrete inside the slab helps resolve some issues for relatively little cost.
The American Concrete Institute defines internal curing as “a process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water.” This is accomplished by replacing some of the fine or intermediate aggregate with pre-wetted lightweight aggregate. We call it curing from the inside out.
There are many advantages to using internal curing for floors, including:
- Improved hydration…more efficient use of cement and SCM (limestone, fly ash, slag, silica fume)
- Reduced number of shrinkage cracks
- Reduced width of cracks
- Reduced curling and warping
- Increased strength of the concrete
- Reduced permeability
- Improved durability
- Helps offset poor surface curing and improves good curing