Zander's Aircrete Troubleshooting Guide
Problem A: Aircrete is weak/crumbly. This could be an actual problem, or the aircrete may simply need more time to cure. It is difficult to accurately judge the quality of aircrete until it has cured for 30 days. Aircrete is often extremely weak and crumbly for the first several days and gradually increases strength for up to one month. Weak and crumbly are very subjective attributes and certainly all aircrete is weak and crumbly if compared to hardwood, steel, or concrete. Roughly speaking, the average adult should be able to stand on a 12x12x3 inch block of aircrete that is well supported on the bottom without it breaking apart.
Problem B: Aircrete collapsing. It’s no fun to see aircrete collapse! Collapsed aircrete can still be useful, but it is not what you were hoping for.
If your aircrete is crumbly or collapsing, consider these points:
- Did you pour aircrete into a watertight form so that no water could leak out?
- Did you cover your aircrete immediately after pouring with a thin plastic sheet to trap in moisture?
- Did you pour your aircrete in a shaded location? (The first couple hours are most critical.)
- Was your cement thoroughly mixed with water before injecting the foam?
- Was your foam density close to 3.3 ounces per quart or 95 grams per liter? The closer to this density the better. If your foam density is off by more than 10% then this could be a serious issue.
- Did your Little Dragon air-pressure dial remain stable within a couple PSI while you were injecting foam into your aircrete batter?
- Did the foam get thoroughly mixed into the cement batter creating a fully homogenous batch?
- Have you tried a different foaming agent or a different ratio of water to foaming agent?
Is your cement getting thoroughly mixed?
Put your hand into the cement slurry before adding foam. Give special attention to feeling what is on the bottom, sides, and in any pockets or corners. Do you feel sludge, blobs, lumps or chunks? Break up those chunks and smooth the sludge with your hand, a whisker, or by scraping the sides with a shovel and re-mixing. When making concrete this is rarely an issue because the sand and gravel do an excellent job grinding the cement into a smooth slurry. However, when making aircrete, the cement is the hardest material in the mix. The addition of foam makes mixing cement even more difficult. The more cement that is not mixed completely before adding the foam, the more the aircrete is compromised.
Is your foam getting thoroughly mixed into the cement slurry?
After injecting the foam and turning off your mixer, feel around with shovel starting at the top and working your way down. Do you feel a different density between the bottom and top? If so, the foam injection process may be insufficient. Scrape the bottom of the barrel with the shovel and bring a sample up to the surface. Does the sample in your shovel look darker than what's at the top of the barrel? If the bottom feels heavy and dense compared to the top, and if your the bottom sample in your shovel looks dark compared to the surface then your foam has not been injected/mixed thoroughly. A common reason for this is that the mixer is not being held at the bottom of the barrel during the entire mix-injection process. A similar result occurs if the bottom of the injector tube is not close enough to the mixer blades - the injector tube should carry the foam to within 1/2" of the blades. Those with home-built injection-mixer systems should be especially aware of this issue. The underlying principle is that any foam that escapes the vortex and does not get sucked down into the vacuum of the spinning blades and mixed into the dense cement slurry at the very bottom of the barrel, instead floats up to the top of the barrel. The mixing paddle typically used on hand held mixers is not able to retrieve that foam and the result is a gradient mix of foam and cement slurry with mostly foam on the top and mostly cement on the bottom. Once this type of non-mixture has occurred, there is little that can be done with the mixer to resolve it.
What cement are you using?
At Domegaia workshops we try to use pure Portland Cement Type 1. There are many cement options that might work better or worse for your application and sometimes only certain types are available, but if you are just learning or having difficulty then please start with Portland Cement Type 1. If you are already using pure Portland Cement Type 1 and are having problems then consider the other issues in this troubleshooting guide before trying other types of cement.
Is your Little Dragon providing consistent foam density?
If the pressure dial on your Little Dragon moves more than a couple PSI during actual aircrete making operations, then your aircrete quality may suffer due to foam density fluctuations beyond the critical 90-100 grams per liter window, regardless of how well you tuned the foam density during testing.
The most common cause of fluctuating foam density is sharing electrical circuits with mixers, compressors, and the Little Dragon. The water pump on the Little Dragon is directly affected by slight changes in voltage which are easily caused by the electrical impact of an injection mixer or air-compressor running on the same circuit. It is highly recommended to provide the Little Dragon with a dedicated circuit, free from all other appliances.
The other primary cause of fluctuating foam density is fluctuating pressure of the air supply. While the Little Dragon is running, keep an eye on the output gauge of your air-compressor especially just before and after it's air pump kicks on. The output pressure dial on the compressor should remain steady (within one or two psi). If it is jumping around more than that during the cycle then some adjustment may need to be made on your air-compressor. If your compressor is rated for less than 2.5 CFM at 90 PSI it might not be able to keep up with the Little Dragon. Try setting the output pressure of the compressor lower than the kick-on pressure. The kick-on pressure switch may need to be adjusted to a higher minimum pressure setting. Perhaps the check valve needs to be cleaned and inspected. Improve the quantity and quality of electricity being supplied to your compressor by removing the extension cord, or reducing the length of extension cord and/or using a thicker gauge extension cord, and shut off or remove other appliances from the electrical circuit. If your compressor is still fluctuating in output pressure or unable to keep up with foam production, consult the unit’s manual or customer support.
I made some major edits to the original post today. Any comments?
good stuff Zander! There could also be the issue of cold damp environments for collapsing forms. If it's too cold and damp then the cement will have trouble "firing". Cement has a chemical reaction to water that heats itself to cure. You can put your hand on a curing form and actually feel it! If it's too cold and damp there will be little heat and evaporation for the curing to happen. Applying an accelerator to the mix could help solve this so it sets up faster. I have also heard of heating blankets used in freezing climates.