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 prewetted lightweight aggregate. We call it curing from the inside out.
As the cementitious material (portland cement and supplementary cementitious materials, or SCMs) in the concrete slab react, it consumes water. Concrete with a lower water-cementitious materials ratio (below about 0.48) consumes more water than is available in the mix. When that happens, the concrete dries out, can no longer gain strength, and begins to develop shrinkage cracks.
Using internal curing, though, as the cementitious material hydrates, tiny dry pores develop in the paste. Since these pores are smaller than the pores in the lightweight aggregate, moisture is pulled out of the aggregate. This allows the cementitious paste to continue to gain strength and prevents chemical shrinkage.
Chemical shrinkage is a result of the fact that the volume of cement paste is less than the volume of the cementitious materials plus the water. In fact, 1 cubic centimeter of cement plus 1 cubic centimeter of water yields only 1.8 cubic centimeters of dry cement paste. But if water is continuously provided, the chemical shrinkage can be prevented, yielding a more robust concrete.
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
These advantages have proven true in a variety of settings. Research performed at the University of Illinois by Professor Jeff Roesler and Armen Amirkhanian showed up to a 75% reduction in curling when using internal curing. And a large pavement built in 2005 at the Union Pacific Intermodal Hub in Hutchins, Texas, consumed 250,000 cubic yards of internally cured concrete. A visual inspection after six months found only one crack; after about eight years there was still only minuscule plastic or drying shrinkage cracks.
All this raises the question of whether a slab with lightweight aggregate will dry out enough that impermeable flooring can be installed according to the construction schedule. Research performed by Concrete Construction magazine in 2000 (“Long Wait for Lightweight”) indicated that it takes two to four times longer for lightweight concrete to dry than normal weight concrete. The Expanded Shale, Clay and Slate Institute (ESCSI) had experience that seemed to indicate this wasn’t true so they teamed up with Peter Craig for some research. The team performed a 13-month study comparing normal weight concrete to lightweight in an unconditioned space in Dalton, Ga., with both burnished and unburnished floor finishes. The result was that the lightweight concrete did take longer to dry but much less than previously reported. The LW concrete lagged the normal-weight concrete in reaching an acceptable level of drying but only by about two weeks.
Considering all these advantages for lightweight concrete, shouldn’t you consider using it on your next project?
John Ries is the president and technical director of the Expanded Shale, Clay and Slate Institute.