Q: Would you be kind enough to recommend a lightweight formula for our tabletops. Size: 48x48x1.5". These tops are tiled with marble mosaics. We can embed steel if necessary. Aesthetics are not important as they will be fully encased. However they must be strong enough to perform as tabletops without worry of breakage.

A: (Bruce Schundler) If it won't be exposed, we would recommend a one to four mix (one part Portland to four parts by volume of perlite or vermiculite) Add some fiber reinforcement (available at mason stores). The problem with the concrete if it is exposed is that it isn't very hard---and the surface would have to be hardened with some special coatings. Of course, I am presuming this concrete will have some kind of structural support under it. If not, lightweight concrete wouldn't work. All concretes are very bad in terms of tensile strengths--or their ability to support weight across spans. They are good for compressive strength, but not tensile strength. Often corrugated metal or metal forms have to be used if the concrete is spanning an area between two beams.

Q: You did not recommend the use of concrete on an exposed deck. We are planning to enclose our deck with a three seasons application. Our current deck boards are 5/4 treated lumber. We want to install radiant heat in the floor using pex tubing incorporated into a suspended slab of 1 1/2 " of concrete that we would lay tile on. Can you recommend a mix for this purpose?

A: (Bruce) I would recommend contacting a local redimix concrete supplier for his advice on what is available locally. Appropriate lightweight aggregates are different depending on what part of the country one is in.

Q: Can you tell me an easy way to aerate concrete with or without chemicals, as here in Indonesia chemicals & pumice etc. are hard to obtain!! I have made a large high speed mixer and am using an emulsifier (detergent) and have had mixed results. Problems in curing! Tried adding more lime etc. Can you advise?

A: (Bruce) Normally lightweight concretes need air entrainment--and normally the air entrainment can be purchased in the concrete by ordering Type 1A portland instead of regular Type 1. Or, in a pinch, people have used non-sudsing detergent (e.g. the type used in washing machines or dish washing machines--not the type used in kitchen sinks. But......different cements and different ingredients and different products can all affect what is used and how and why.

Q: I live in Eugene, OR, and every year this city has what can only be described as a "leaf problem". In other words leaves clog up the streets, drains, etc... Has anyone ever looked into turning leaves into paper products or building materials?

A: (Kelly)This website might interest you, since Phillip has been including various plant materials in his "hybrid adobe": hybridadobe.com

Q: Have you heard of this new building material yet? Its called hempcrete a mixture of concrete and hemp. The Oglala Sioux tribe who have been fighting for years to grow industrial hemp as a crop on their reservation want to also make hempcrete to help with the cost of housebuilding there. I was wondering what you think?

A: (Kelly) No, I have not heard of hempcrete, but I know that folks have been experimenting with adding straw to concrete to help with reinforcement, and hemp could be used for a similar purpose. The idea of making more insulating concrete with the addition of hemp could yield some improvement, but I would not expect much in this regard; any time you try to make a basically dense thermal mass material (such as concrete) serve as an insulation material, the compromise is rarely worth the trouble. If the hemp provided a significant portion of the volume, it might approach a good insulation material, similar to papercrete. I would suggest quite a bit of experimentation to ascertain how well it might insulate and what the properties of the material might be regarding durability, susceptibility to rot or deterioration from moisture, etc.

Q: I am having difficulty locating a source for a test quantity of cellular concrete foaming agent. Seems that that are not many suppliers in USA.....it has not 'caught-on' here yet. I have sent a few e-mail inquiries and seem to be to 'too little' to be of interest. I want to make homes that are non-traditional/contemporary......ie...no brick....no sheetrock....no asphalt shingles.....etc.

A: (Bruce Schundler) Especially in Texas, there is a lot of cellular concrete. Many new flat commercial roof decks in the south are "poured" with cellular concrete. And it's been used for over ten years now. Probably redi-mix companies and commercial concrete/masonry supply stores would know more about it. The problem is that it's not usually the kind of concrete that is used or purchased by DIY or small contractors, and usually is manufactured on the job by the roof deck applicators. Another possibility would be the easiest---go to www.google.com and type in cellular concrete and you will find the manufacturers , information, etc, etc.

Q: Our Computer Aided Drafting class is looking for a lightweight concrete formula for making concrete canoes. We need a recipe that is less than 40lbs per cubic foot.

A: (Kelly) There are a number of mix designs on our web site at www.schundler.com/const.htm in the concrete section. Both perlite and vermiculite, mixed with other products, have been used in the concrete canoe "races".

Q: I am looking for a solution to a muddy fenced in dog yard (with lots of digging). Found cellular cement grassy pavers which are used in Europe for grassy parking areas. Thought that might prevent dogs from digging while still allowing for grass to grow. Have my own plastic forming machine (for signs) and thought I would create 2' x 2' x 2" molds to form cellular blocks for molding purpose. Wondering if lightweight cement might work (easier for me to handle). If so, what would be the suggested mix?

A: (Bruce Schundler) Lightweight perlite concrete could be used, but like cellular concrete, it does not have a lot of flexural or tensile strength. It probably should be mixed with fibers to give it more strength, and the mix could vary depending on a balancing of strength and weight. I wouldn't use a mix weaker than a four to one mix, however. (see our charts---especially those for special mix designs.)

Q: Would it work to use rice hulls as a concrete aggregate? I would think with their high insulation value, low moisture-rating capabilities, and economical value, they would be a good choice for green builders. Using fly ash as well as rice hulls would be only better, I would think. I would appreciate your thoughts on this.

A: (Kelly) I have not heard of anyone using rice hulls in cement. They might work, but I question the advantage of using them. Being encased in cement would severely limit their insulative qualities, and the product would not likely be as durable as mineral aggregate materials. I suggest that you run some experiments to test the results...and let me know how they turn out. I prefer to find ways to avoid using cement at all, since its manufacture is polluting. Rice hulls work well in earthbags.

Q: We are a company in the Dominican Republic who are looking for a lightweight concrete to build a number of affordable or low-cost housing. We have found a supplier in Romania of a product that allows us to build a house in 7 hours but it is very expensive. Do you have any suggestions for us to make a good project in affordable housing.

A: (Kelly) Lightweight concrete does not necessarily require the use of expensive or unusual additives. It is basically concrete that has replaced the standard heavy gravel aggregate with a lightweight substitute, such as crushed volcanic stone, perlite or vermiculite. There are special entrainment methods that introduce air pockets into the mix, and these do require additives.

Q: I have large supply of natural shell grit ranging in size from 3 to 15mm and a few 25 to 40mm shells. The type of shells closer to the 15mm end are generally broken bivalves while the smaller end are snail type shell and are generally complete. The grit weighs in at 700kg per cubic meter. Would this be suitable for lightweight concrete and what mixing ratio should I use. I have also much beach sand which is fairly fine also. Should I use this or not?

I have read that lime is used by Steve Kornher...for what reason? I would need to plaster the inside and outside on completion to seal it, and as the concrete would be porous, would this still have a good insulation value?

A: I would think that the smaller snail-type shells that contain a fair amount of trapped air would provide a reasonable aggregate for lightweight concrete; the larger broken bivalves are probably less useful in this regard, since they are more solid or hefty.

As for the beach sand, it may not be ideal for concrete, since beach sand is often rounded and not particularly sharp (which is the ideal)...but less than ideal sand does work for concrete in any case. I would not plan to use much of either type of sand with a lightweight concrete mix, as it will lessen the insulative value.

Lime is used for several reasons in concrete work:

1) It is cheaper than Portland cement.
2) It is lighter than Portland cement.
3) It provides a creamier, more adhesive mortar.
4) It combines well with Portland cement.

Interior and exterior plaster over lightweight concrete will not affect its insulative value. My suggestion is that you do some experiments with various mixes of these materials and see what works best.