Anyone who works with concrete knows that it is heavy—about 150 pounds per cubic foot (pcf) for normal-weight mixes. In many applications, the weight is either advantageous or something we can accommodate easily. But in some applications lighter weight (actually lower density) concrete is a big advantage—either simply because of the reduced weight or for higher fire resistance, noise resistance, or thermal insulating value.
An advantage of lightweight aggregate that has been getting more attention recently is its ability to carry water into the concrete to achieve what is called “internal curing.” Using saturated fine lightweight aggregate, water can be released slowly into the concrete during hydration. This is especially advantageous for high-performance mixes, resulting in higher strength and lower permeability.
When even lower unit weight concrete is needed, or higher insulation value, cellular concrete is the solution. Cellular concrete is created by introducing foam into the concrete mix to result in a matrix of air bubbles. Cellular concrete can be produced at unit weights from 110 pcf to as low as 15 pcf. The newest innovation in cellular is pervious cellular concrete that allows the mix to retain water and to drain.
The notion that water-saturated lightweight aggregate can provide water for cement and pozzolan hydration is not a new concept—Paul Klieger first wrote about it in 1957. In the early 1990s, Bob Philleo coined the term “water-entrained concrete” to describe how water stored in lightweight aggregate can be pulled into the mix to augment normal curing.
Dale Bentz at the National Institute of Standards and Technology has developed a way to quantify how much fine (or intermediate-sized) lightweight aggregate is needed to fully hydrate high-performance concrete, which typically has a high cementitious content and a low water-cementitious materials ratio.
There are two ways saturated lightweight aggregate can help produce better concrete. First, with a typical mix, the transfer of moisture into and out of the lightweight aggregate particle prevents the buildup of a high w/c ratio area at the contact between the paste and the aggregate. Because moisture can move in and out of the aggregate, water lenses don't build up adjacent to impervious normal-weight aggregate particles and the bond between the paste and the aggregate is stronger. That weak area in normal concrete results in micro-cracks that allow the migration of water through the hardened concrete and reduced durability. When that weakness is eliminated, the concrete has lower permeability and higher durability.
The second way lightweight aggregate is used to improve concrete is in high-performance mixes (usually high strength), which tend to have low water content and high cementitious materials content. These are not lightweight concrete mixes but contain predominantly normal-weight aggregate and often have a high proportion of supplementary cementitious materials, such as fly ash, slag, and silica fume. A problem with this type of mix has been self-desiccation—where the hydration process consumes all the available water and the concrete dries out resulting in what's called autogenous shrinkage, which leaves the concrete with higher permeability and lower durability. In this situation, as the cementitious paste consumes all the water due to hydration, it pulls out additional water absorbed inside the lightweight aggregate, allowing the cementitious materials to hydrate more fully.
“The most recent uses of internal curing have been in pavement mixes in Texas and in bridge deck mixes in New York,” says Reid Castrodale, director of engineering for Stalite, a manufacturer of lightweight aggregate in Salisbury, N.C. “For bridge decks, you don't always get the best curing and moisture loss, which is an important issue in the deck's durability. The internal curing reduces the desiccation and autogenous shrinkage for these low-water, high-performance concrete mixes.”
This is where the entrained aggregate idea comes into play and is why the push with internal curing has focused on the use of fine lightweight aggregate in normal weight concrete. “Part of the idea is that with fines you are dispersing the entrained water throughout the mix better,” says Castrodale. “Although the effect would still be there using saturated coarse aggregate in lightweight concrete, the water is not as readily available.”
A twist on this, notes Castrodale, is that as the lightweight fines dry out, the pores actually can serve the same purpose as entrained air bubbles in protecting the concrete from freeze/thaw action. “With supplementary cementitious materials, there can be a problem in keeping your air content up,” he says. “But if you can have what amounts to hard air entrainment, you don't have to worry about admixtures and the possible loss of the bubbles.” This concept is so far unproven, but promising.
Internal curing does not, however, eliminate the need for good external curing to protect the concrete surface from drying out. It does, though, reduce the concrete's sensitivity to less than perfect wet-curing practices. “There may someday be a provision where you could go four or five days with your wet curing instead of seven,” says Castrodale, “but no one is proposing to eliminate curing. We want it to be a second line of defense. And anyway, with high-performance concrete, external curing is sort of wishful thinking. The point of high-performance concrete is to have very low permeability. So how is the water on the surface going to get in after a day or two when the permeability gets so low? Adding internal moisture makes up for that.”
An important question always is how much this will cost. Lightweight aggregate concrete is about $20 to $30 per cubic yard higher than a normal weight mix. For internal curing, you may only be replacing about a third of the aggregate with lightweight so it may only add an extra $10 per cubic yard.
A graphical explanation of internal curing is posted on our Web site under “News & Articles” and “Editors' Picks.”
In situations where very low density concrete is desirable and strength is not so important, an option is cellular concrete. Here again, this is not a new idea—cellular concrete was invented in Europe nearly a century ago. But there have been some recent advances that make it more appealing for certain applications.
Cellular concrete is concrete that has a very high percentage of what amounts to entrained air. Typically a cement and water or cement-sand-water slurry is made in a drum truck or stationary mixer. At the jobsite, a foam generator makes foam from surfactant that is added to the slurry in the mixer drum. This process is called preformed foam or externally generated foam since the bubbles are not created within the concrete through mixing action as they are in typical air-entrained concrete. “The foam doesn't have any impact on the concrete chemistry because it is roughly 97% air,” says Rich Borglum, Richway Industries, Janesville, Iowa. “And 97% of what isn't air is water so there is a very limited amount of active ingredients going into the concrete. Furthermore this is almost the same material as an air-entraining admixture or a retarder.”
The slurry for cellular concrete is mixed with a very high cement content—as much as 2300 pounds of cement in a cubic yard, which might result in concrete with a density of 120 pcf. When mixed with the foam, as the density goes down so does the cement content. Our 120 pcf concrete mixed with enough foam to become 2 cubic yards now has a density of 60 pcf with 1150 pounds of cement. Bring the density down to 40 pcf and you end up with 3 cubic yards of cellular concrete with only 770 pounds of cement in each cubic yard.
The compressive strength of cellular concrete is proportional to the density: as the density goes down so does the strength. For higher density concrete, more than 135 pcf, foam can serve as a flow enhancer. At low density, 20 to 50 pcf, compressive strength is between 30 and 900 psi, and the R-value is as high as 2 per inch of cellular concrete. Although compressive strengths of around 50 psi don't sound too impressive, in many applications that is adequate—the compressive strength of a good compressed soil is only around 25 psi.
That's a primary application of cellular concrete. “Typical uses are tunnel linings, filling mines, encasing pipes, or for road fill, or where soil isn't good,” says John Sedenquist, COO of Cellular Concrete, Allentown, Pa. “With poor soils, we'll excavate down as much as 15 feet and fill with cellular. Where soil weighs 110 pcf, we refill with 30 pcf material so there is less load on the underlying soil and no need to worry about consolidation.” Other primary applications are roof decks and floor decks, especially on metal decking. “Roofs are big in the southwest because we get a good wind uplift rating and we can eliminate insulation boards, which eliminate having to use fireproofing on the underside of the deck,” says Sedenquist.
Pervious cellular concrete is an innovation that has been introduced recently by Cellular Concrete. This product was named as one of the Most Innovative Products at the 2008 World of Concrete. Pervious cellular concrete retains high volumes of water and slowly allows water to percolate through. The highest profile use to date has been Citi Field in New York—the new home of the New York Mets baseball team—where 17,500 cubic yards was placed beneath the field. Built on unstable soil, traditional fill would have added enough weight to cause settlement, but 29 pcf pervious cellular concrete created no settlement and was much cheaper to install than a compacted gravel subbase.
In addition, cellular concrete can take the place of flowable fill (also called controlled low strength material)—a low-strength concrete mix. Borglum describes a recent project in an industrial pump house, where 4 feet of 40 pcf cellular (45 cubic yards) was installed in a single lift. “The engineer specified cellular for two reasons,” he says. “First, the low-density material wouldn't disturb the considerable plumbing and electrical conduits during placement and, second, should repairs or alternations be needed in the future, excavation would be easy. This required five ready-mix trucks, each bringing about 2½ yards of mix, consisting of sand, portland cement, and water with high-range water reducer added. About 7½ cubic yards of foam was added to each load.” In some cases, when used to backfill around utilities, color can be added to the cellular concrete to alert anyone excavating near pipes.
Cellular concrete has very high flowability—practically soup-like exiting the pump hose. In flat placements, it is virtually self-leveling. “At real low density, it doesn't flow that well,” says Borglum, “because there is no mass to move it—it's almost like shaving cream. But in the 30 to 60 pcf range it flows easily.” Around 60 to 80 pcf, a little troweling may be needed, although the material can be sticky and difficult to finish.
Most cellular concrete is placed by pumping. Cellular Concrete promotes a system they call “in-line mixing,” where the foam is injected and mixed as the slurry goes into the pump line. “Say it goes in at 110 pcf,” says Sedenquist, “then it will come out the other side at 30 pcf—even within a couple of feet. We are foaming in the pump line. A 10-cubic-yard load will become 38 cubic yards, so you've reduced the number of trucks on the road.”
Cellular concrete pumps easily, although with some types of foam air can be lost at higher pressures or in longer drops. “In Tampa, cellular concrete is being used to fill sinkholes,” says Sedenquist. “We inject it under 450 psi pump pressure into the sinkholes that are 40 or 50 feet under the ground. Our foam held up under these high pressures. We can even use it to raise roads or whatever has sunk. The bubbles are so stable that they elongate under pressure and then when the pressure is released, they return to spherical.”