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What Is Cellular Concrete?

The key construction material in this project is cellular concrete. This is a cement, sand, and water concrete to which a roughly equal amount of air is entrained by the introduction of a detergent foam into the wet mix before placement. (Smaller amounts of normal concrete will be used in a surface layer to add strength, wear resistance, and a substrate for tie-downs and other fittings.)

This material is often used to line trenches and tunnels, to cover or fill large areas, and for roofs and upper floors. The advantages are:

The main disadvantage is that strength is reduced in proportion to the weight reduction, both due to the replacement of cement by air. It cannot be used for major load-bearing elements. Compressive strengths of cellular concrete ranges from 500-1200 psi, as compared to 3000-4000 psi for concretes commonly used in construction. It has good freeze-thaw resistance, low permeability, excellent insulation properties, and can be worked with wood tools.

Normal concretes have a typical density of about 2.4 times that of (fresh) water. When composed with cement, sand, and foam, densities between .8 and 1.6 are produced. If the sand is left out, densities as low as .3 are possible. Below about .8 the result is ornamentally but not structurally useful.The COE of aerated concretes is comparable to that of normal concretes.

In all concretes, during curing, the water in the mix is taken up by the formation of crystalline calcium-silica-hydrates (Tobormorite). Some of the water content may be lost to evaporation, leading to shrinkage and cracking as unbound moisture migrates to the surface of the block. Many factors affect this process, mostly temperature, cement type, water to cement ratio in the mix, and whether covers or sprays are used to retard evaporation. Normal concrete made with general purpose type I cement will set in 2-4 hours, achieve half of it’s final strength in 3 days, and most of it in 28 days. A list of resources for information on concrete, cellular concrete, and where to get related materials and equipment is here.

Like normal concrete, care must be taken to prevent evaporative loss and cracking in curing. This is even more important with cellular concrete. A sealant layer of some kind is applied to the surface, and wire mesh reinforcement is used to distribute shrinkage forces. We will use 10ga square wire remesh for reinforcement, and shaping the honeycomb cells, and a surface layer of normal concrete. I have not yet seen any info on interfacing the two, except that their COEs are very close. The risk will be at cure time. The cellular concrete may shrink a similar amount, but more rapidly. In a small scale test in a 5 gal bucket mold, the 2 layers, poured at the same time, bonded well without cracking.

The surface will be covered and kept wet through the early part of curing. It will be covered permanently with a layer of normal concrete.

It would be convenient if beach sand could be used to produce cellular concrete. One source specified sand of at least 80-90% Silica. Another source said beach sand can only be used if all the salt is washed out. For the same reason, fresh water is required, sea water will not work.

The normal concrete used in the surface layer will be entrained with 5% air to improve freeze/thaw cycle resistance. This is commonly done.