Note: The following answers are correct to the best of our knowledge, but may not apply in specific applications or situations. Most of them are designed to provide general information, rather than information for a particular project or application.
There are two methods for producing cellular concrete. The first is the batch production method in which externally generated foam is injected into the drum of a mixer for a calculated amount of time. The second is the continuous production method in which foam is injected in-line, on the discharge side of a pump. Richway offers equipment for both production methods.
As density is decreased, the compressive strength also decreases. See the strength tables and charts for more details, but as an example, 60 pcf density will have a strength ranging from 600 to 1000 psi.
Usually it will be easier, but if there is cement paste without any foam incorporated into it coating the drum, it may be more difficult.
The set time for cellular concrete is typically a little longer compared to “normal” concrete, due to the surfactants used in producing the foam. However, like anything produced with Portland cement, there is a finite time for production and placement. Generally, we recommend that the working time be limited to about four hours once the Portland is mixed with water, or about three hours after the foam is added. After this much time, the material should be left alone to continue the set process. Continuing to pump or move the material can result in collapsing and failure of the material. However, the set time can vary depending on the application, jobsite conditions, and the use of either retarders or accelerators.
We think “externally generated” is much more clear terminology and doesn’t suggest a rigid petroleum-based foam or something that is made a long time before it’s used. The foam has the consistency of rich stiff shampoo lather and is generated “on-the-go” as it is mixed or injected into the mixer. It is externally generated, rather than internally generated by the action of the mixer itself, as is the case with an air-entraining agent.
If a neat cement slurry is used with a .50 w/c ratio, the base slurry per yard will have approximately 2060 lbs of cement and 1030 lbs of water, with a density of 115 PCF. If foam is then added until the density is 30 PCF, we’d have 3.65 yards of 30 PCF material, with approximately 565 lbs of cement per yard. We have a mix design calculator available on our website that calculates mix design batch weights, foam dose times, and cost scenario analysis.
Yes. Just as in standard density concrete, the final material attributes will be affected, generally in much the same way alternative pozzolans will affect “normal” concrete. Regarding fly ash, it should be noted that high carbon content ashes may tend to break down the foam, and therefore should be avoided.
Yes, water reducers can be used and will help with dispersion and wetting of the cement powder before adding the foam. Most other admixtures can also be used, but in all cases tests should be run before mix design is finalized. Some super plasticizers may break down the foam, so thorough testing is essential. Air entrainment admixtures are usually not used in production of the slurry for making cellular concrete because the foam is the air that is added to the mix.
Cellular concrete is easily pumped. With high water content and low density, it may be virtually self-leveling, but will always be more easily moved than standard density concrete. It is usually easy to finish, but at some densities, it is sticky and hard to trowel. Generally no finishing is needed for geotechnical applications.
Cellular concrete pumps and flows extremely well.