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Structural Concerns with Lightweight Concrete

Fernando Martinez Lewels has a M.S.C.E degree from the University of Texas at El Paso. He is now working with the Agartif company in Chihuahua, Mexico (about 170 miles from El Paso Texas). This Company has developed a type of lightweight aggregate that provides material for all types of construction needs, at reasonable cost and with good thermal insulation values. They manufacture the equipment required to do this according to the needs of their customers; the feed stock are common construction materials that should be available in most locations. Their philosophy in developing this type of aggregate is to be able to use this everywhere, without depending on a lightweight aggregate quarry, so you can have access to this material in any part of the world. In Mexico we have a saying that "we build our homes so we have to go outside in the summer to be fresh, and in the winter we go outside to catch the rays of sun to be warm". Lightweight concrete can help this situation by making available materials for more comfortable homes.

Questions and Answers

Q: I am interested in your recommendations on using lightweight concrete for a super-insulated house in a wet, cold Pacific Northwest climate (76 inches of precipitation; 30 days of snowfall per year; lots of freeze-thaw cycles; nice warm summers (75-85). What is your general advice. I am particularly undecided about the use of a vapor barrier. I anticipate using additional insulation on the outside of the exterior walls (hard rock wool?) and then another thin wall with a stucco finish. Should I be concerned about condensation in the rock wool? How would that affect the rock wool? Should I put a vapor barrier between the wall and the insulation, or between the inside finish and the wall?

A: (Bruce Schundler)Water can lower the thermal insulation values of any kind of product. As a result, you should be concerned about water penetration, vapor barriers, etc. Most lightweight concretes are permeable (e.g. water can pass through them) and they have to be treated with water proofing sprays (silicone for instance) when used in exterior applications. There are a number of exterior wrapping materials that would be good, and polystyrene board would probably be better than hard rock wool (rock wool can hold the moisture and eventually develop problems like mold if it is never allowed to dry.) These house wrapping materials allow moisture to leave the house while preventing water from getting in. The problem with absolute vapor barriers like plastic sheeting, for instance, is that they will not let any water in or out, and this wouldn't be good.

Q: I have completed the the frame of my house(columns & beams) with steel I- section...I,m now trying to make a big panel from lightweight concrete using perlite, lime & cement for the walls (which have no loads on them) what is your advice? Can I use the panels (with reinforcement) in the roof? Do I need to plaster the walls and paint them to keep rain from them?

A: (Kelly) Perlite can be used as a lightweight concrete aggregate but there are many variables involved. Test mixes should be done and the thickness of the wall is important.

A: (Bruce Schundler) Perlite concrete could be used in tilt up construction---but it should have some reinforcement (both fibers and metal rods--depending on the spans). The roof panels could also be manufactured from perlite concrete, but these would need to have reinforcement too. Years ago several companies made "pre-stressed" roofing panels, and others have made less engineered panels. BUT, these do have to be supported or engineered so as to minimize the spans between beams. Perlite concrete does have to be waterproofed (as does all concrete.)

Q: I am designing a custom design build house in Los angeles west side area and the main structure of the house is an 2 stores high steel frame and wood stud in between. My questions are: 1) Is light weight concrete appropriate for this type of structure? 2) Can I just design the light weight concrete as a pre-cast concrete panel, or I can apply the concrete on the surface of a plywood on the exterior wall and make it look like an concrete finish building? Which way is better and cheaper, if I just want the finish to look like concrete finish?

A: (Bruce Schundler) Essentially you are asking both structural questions and some exterior design questions. On the structural side, you really need to check with an engineer or builder. To be sure, how much a building can or cannot hold is based on the kind of steel used, the thickness and style, etc. Stucco and stucco cements have been applied on the surface of buildings for years. It can be applied over masonry units or over wood. Usually the stucco cement would be applied over expanded metal nailed the the exterior surface. There also are "systems" like the Dryvit system which uses a stucco veneer. To be sure, there are many options. For more definitive answers four your area, however, you should contact both stucco plasterers, local masons or contractors, and perlite suppliers like the Redco company in N. Hollywood (www.perlite.net.)

Q: I am building a enclosed room with a flat roof patio/deck. Proposed construction of the roof is 3/4" ply with EPDM as water barrier and tiled top layer. I have been advised that I should have approx 2" of concrete between the EPDM and the tile layer, is lightweight concrete an option?.. does it have sufficient strength and how much weight (%) would I save?

A: (Bruce Schundler) We think perlite concrete could be used, but we aren't sure if it has to be used. The main thing would be for the floor (the 3/4 plywood) to be supported very well, and maybe two layers would be helpful. The problem with tile is that if there is any flexing or "give" in the floor, the tiles and the supporting concrete could be damaged. But no matter what, this is a question more for tile people than us. If perlite or vermiculite lightweight concrete is used, we would recommend a 1 to four mix with fibers or wire (for reinforcement.)

Q: About forty years ago my father built some apartments. For the second floor he used a lightweight concrete (type unknown). The floor is starting to crumble under the tile in the kitchen. Can you suggest a repair material? Can I use a standard floor repair such as Floorfix or similar product? Also, are there any special steps that I would need to follow to assure a proper repair procedure.

A: (Bruce) I think the answer is yes. As usual, the older material should be cleaned out carefully and completely.

Q: I have a concrete subfloor and bituminous/orangeburg piping for my plumbing. The Orangeburg pipes have deteriorated and I need to bust up the existing concrete slab, re-do the plumbing and then replace the concrete. My question is, could I use the light-weight concrete to replace the slab and if so, would this be cheaper and would it make the plumbing more accessible in the future if repairs were needed? I do not believe any load-bearing walls are involved, only a room-dividing wall (between kitchen and bathroom); the subfloor will have to support a bathtub.

A: (Kelly) I doubt that lightweight concrete would be any cheaper...most likely it would be more expensive, depending on what aggregate is used. Lightweight concrete would likely be easier to bust up in the future if this were necessary. If the concrete is a subfloor then, lightweight concrete might be a good option. As a final flooring material, it is not as hard and durable as ordinary concrete.

Q: I remodel older framed homes and was wondering if lightweight concrete would work to build an inner shell within a dry walled room anchoring the rebar into the concrete slab floor? In this way I could custom shape the vaults and incorporate a custom flowing wall unit with fireplace etc. Would this system blend to the floor and would the 4" concrete slab be able to support the structure? I could also tie into the existing framing to offset loads into the slab. Also can drywall be a backing using a mesh for structural support and what would be a minimum thickness I would have to use?

A: (Robert Alexander) I don't have any experience with that, but it is an interesting idea. I can tell you that pumice-crete weighs about the same as concrete during the pouring process and about 50 lbs./cu. ft. dry - which can take several weeks and you would have to vent the room to get rid of the moisture.

The pouring process is rather messy and sometimes excess water will flow out of the bottom of the forms, but, unless this is structural, perhaps you could go with a dryer mix. You can sculpt (carve) pumice-crete for a day or two after it is poured, but within a few days it hardens like concrete.

One concern I might have would be the different rates of thermal expansion - but inside you wouldn't expect much change. Expanded metal lath - or wire mesh - works well for tying the two together and you could make the pumice wall (for lack of a better term) as narrow as you wish or as wide as your foundation (slab) could support - that is just a matter of your form spacing.

A loop of heavy gauge wire (like concrete reinforcing mesh) embedded in the wall at the bottom would probably work better than rebar, but rebar stubs would probably keep the wall in place - and in reality neither may really be necessary due the the weight of the pumice wall - unless you ran a truck into it.

If you are pouring the wall on a fresh slab, it will bond to the slab since there is concrete in the mix. We didn't use any mechanical bonding (rebar./mesh) in any of the walls on the house I worked on. Those internal walls were 10" wide and the outer walls were 16" wide.

Q: My building recently had a pipe break and my lightweight concrete floors were soaked for 5 days. My question is, will the lightweight concrete be compromised in any way from the flooding?

A: (Kelly) I doubt that any real harm was done, as long as the concrete was able to dry out. There are a variety of light weight aggregates used, so it is hard to say for sure. Lightweight concrete is commonly used as an underlayment for roofs, in which case it would be essential for it to be fairly robust in damp situations.


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