Once again I am a bit surprised to find how long it takes to cover a topic in some semblance of detail. Even with something as relatively mundane, perhaps, as concrete, there is much more to it than meets the eye.
In part one of this series, I walked down to one end of the proverbial see-saw, where I mentioned a visit to a local contractor’s residence, to see a passive-solar oriented, earth-bermed ICF (Insulated Concrete Form) house. There the friendly contractor, smitten with the ICF system apparently, regaled me for a couple of hours with glowing accounts of the numerous advantages to building structures from polystyrene foam forms and concrete. I related many of those pluses to you all, and mentioned that given a choice, based on what I know, between an ICF house and either a conventional stick-framed wooden house, or a trimberframe with SIP cloak, I would choose the ICF house. Reading that first post, it seemed perhaps that I was completely sold on the ICF way of building.
In the second post, I walked down to the other end of the see-saw, where I looked at the materials comprising the ICF system, namely the concrete and the polystyrene foam, in more detail, and concluded that they weren’t especially ‘green’ after all. I did omit a few things, for the sake of brevity – I didn’t consider every type of form used in ICF systems – some are made from concrete-polystyrene mixes, and so forth, however these account for a small slice of the market, and I would say are inferior in terms of insulative value. I also didn’t mention the plastic clips typical in ICF systems, clips which fix the foam panels to one another and allow for the attachment of other materials to the formed wall afterwards. These clips are either plastic, usually recycled plastic, so are relatively benign items, or they are made of steel, which are a somewhat high-embedded energy material and a little less benign. Still, given that the clips are a relatively minor component, they could be set aside in the big-picture consideration.
I also left out the topic of steel rebar in concrete, as that is an area I wanted to look at in a bit more detail today as I come to what I hope is a balanced view of ICF construction and concrete in general. Yes, I will see if I might be able to balance in the middle of that see-saw for a spell, a difficult task for me as I tend to be an ‘all-or-nothing’ sort of person most times.
One of the comments the contractor made upon my visit, after I asked him about durability of ICF construction, was that they “should last for millions of years” and that “the ancient Romans built with concrete, and that has lasted some 2000 years” – so apparently that’s all he needed to know. That wasn’t quite ringing true for me however, as I knew that Portland cement is hardly a product that stretches back into antiquity. Portland cement and Roman cement had to be different somehow.
In fact, the history of Portland cement goes back only to 1824, when a British stone mason named Joseph Aspdin invented it in his kitchen by heating a mixture of finely ground limestone and clay to create a hydraulic lime cement – that is, a cement that hardens with the addition of water. So it seems doubtful that the ancient Romans were using the same product, and if they were they were certainly remiss in their moving forward with the patent application :^). What were the ancient Romans using then?
To find out more on this topic means referring to a fellow named David Moore, a professional engineer who in his retirement devoted 10 years to the study of ancient Roman buildings and the concrete they were made of – buildings like this one for example, the Roman Pantheon:
Looking at the above structure, which I think quite beautiful, consider for a moment that it is made with un-reinforced concrete (i.e., no re-bar or other internal ties) , and that it stood as the largest dome ever constructed for over 1000 years, only being matched, size-wise, in Medieval Italy by Brunelleschi’s dome in Florence (which was actually not circular, but octagonal, and had iron chains for reinforcement). In truth, considering that modern concrete domes, like, say, the Tacoma Dome, are reinforced, the Roman Pantheon has not been equalled by modern man. It’s an amazing structure, and it is concrete.
Roman concrete, like the modern stuff, was composed of sand and aggregates, mixed with a cementitious paste which hardens. The Roman cement was based on limestone, which was heated in a kiln. Limestone contains calcium, carbon, and oxygen. The heating causes the limestone to undergo a chemical reaction in which the carbon and some of the oxygen are driven off, leaving behind a highly reactive product known as quicklime. A scientific term for quicklime is ‘calcium oxide’. Putting the quicklime into water causes another chemical reaction, much bubbling and heat, which yields a white paste known as slaked lime, aka ‘hydrated lime’. Mix the slaked lime with clean sand and you have a great mortar.
Unlike modern Portland cement, in which clinker is reground with a bit of gypsum to produce a powder (as detailed in the previous post) that can be conveniently stored in bags and activated with the simple addition of water, slaked lime has a somewhat shorter shelf life.
To this point, it is hard to see what the Romans had that we don’t; in fact, all in all, Portland cement seems far more convenient. Well, there’s more to it of course. While one type of mortar the Romans used is indeed slaked lime mixed with river sand, three parts sand to one part cement, they also had another type of cement, in which the slaked lime was mixed with pozzolan. Pozzolan takes its name from where it was first found, in the Pozzouli region of Naples. In the classic work by Vitruvius, “Ten Books on Architecture”, in Book II he devotes all of Chapter 6 to “Pozzolana for Concrete Masonry”. I’ll quote him directly:
“There is also a type of powder that brings about marvelous things naturally. It occurs in the region of Baiae and in the countryside that belongs to the towns around Mt. Vesuvius. Mixed with lime and rubble, it lends strength to all the other sorts of construction, but in addition, when moles (employing this powder) are built into the sea, they solidify underwater. Evidently this is why it happens: under these mountains are boiling earths and plentiful springs. These would not exist unless deep beneath there were huge fires, blazing with sulphur or alum or pitch. Therefore these interior fires and the vapor of their flames seep through veins in the ground and make this earth light, and the tufa created there has risen up without any component of moisture. Hence, when these three ingredients [lime, fired rubble, and pozzolana], forged in a similar fashion by fire’s intensity, meet in a single mixture, when this mixture is put into contact with water the ingredients cling together as one and, stiffened by water, quickly solidify. Neither waves nor the force of water can dissolve them.“
The ‘powder’ Vitruvius was referring to was volcanic ash, in case the reader hasn’t guessed. Pozzolan, in the the modern technical definition, is: A siliceous or aluminous material which in itself possesses no cementitious value, but will, in a finely divided form, and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
So, the Romans used the volcanic ash, the pozzolan, in place of the river sand, and like the heating process used for Portland cement in which lime is chemically bonded with clay to form the clinker, with the pozzolan, the heating work had already been done – by the volcano of course! The volcano calcined the pozzolan (that is, the heat drove off the carbon dioxide and water), which enabled it to chemically react with the calcium hydroxide in the slaked lime. A further advantage of the pozzolan over the sand is that the sand crystals, the silica I mean, are so condensed that at the atomic level it is difficult for the calcium hydroxide in the lime to chemically attach and react with it – thus a hardened mix of slaked lime and sand is essentially a material in which the sand particles are merely suspended in the hardened lime. With the pozzolan, on the other hand, there is an amorphous silica atomic structure with many holes in the molecular network, and thus the molecules of calcium hydroxide have many places to attach themselves and subsequently chemically react with the pozzolan. Thus a hardened slaked lime-pozzolan mix is a permanently bonded, chemically altered material, similar to Portland cement.
So, it would seem then that, unless one considers volcanoes to be somehow more environmentally friendly than modern-day concrete plants – both after all use a lot of energy and raw material and produce a lot of greenhouse gases, dust, and so forth, there isn’t much to pick between them in terms how the Romans producing concrete cement and how we do it today- or is there? That Roman concrete still lasted an awfully long time, and judging by the volume of repair work I’m observing to various concrete highway underpasses in Massachusetts these days, with spalled concrete very much apparent, there must have been something the Romans were doing which were are not – what could that be?
According to David Moore, what set the Roman concrete apart were three things:
1) rigid quality control
2) low water to cement ratio
3) expert placement and compaction
It must be said that while cement is produced in plants with extremely high quality control these days, the same cannot be said for how it is mixed into concrete or placed however. The Romans were very picky about their lime quality- the whiter the better (which meant it was free from impurities). Even the stone that would be mixed into the concrete was finely graded and carefully chosen. As Vitruvius writes in Chapter 7 of Book II,
“When it is time to begin building, let the stone be extracted two years earlier, not in winter, but in summer, and lie about in an open place. Whatever stones have been touched and damaged by bad weather in the two years shall be thrown into the foundation courses. All the rest that have not been damaged, once they have passed the test of Nature, will be capable of enduring in construction above ground. These provisions shall be observed not only for squared stone but also in rubble structures“
It seems they weren’t in such a hurry as we seem to be these days. The low ratio of water to cement was an important key to the success of Roman concrete. The Roman product was akin to a zero-slump concrete, a mixture that is so stiff that it will not flow in between the rocks in the aggregate. The placed concrete was then carefully tamped down, the compaction driving out the air bubbles which would otherwise weaken the finished product. In short, unlike modern Portland Cement-based concrete, a wet liquid which pours readily and sets up to near full strength in some 28 days, the Roman concrete was a very dry mix, packed and tamped layer by layer. This type of concrete would not be the equal of Portland cement-based concrete after 28 days, however one or two years on it would have had strength far in excess of the modern stuff.
Surprise-surprise: when you get down to it, modern Portland cement, in terms of the type usually employed in construction, is a product meant to satisfy convenience and economy over any other concern. To quote David Moore,
“We have to use a higher water content because, for economic reasons, we use automated machinery to mix our mortar and aggregate off-site to make concrete and then bring it to the site and pour it into forms that contain steel reinforcing. The mixture must be fluid enough to be worked by the machinery and flow around all the re-bar and forms with minimal manual interaction.“
Ah, yes, the good old ‘minimal manual interaction’ ploy. The modern concrete hardens quickly, and this brings with it a propensity to shrink and crack. Thus the need for re-bar. The cracks form regardless, especially if the mix was over-hydrated, and while the re-bar does hold things together over the short term, eventually, given its exposure to the air or water, the metal begins to corrode and eventually spalls the concrete. And so we spend millions repairing our concrete structures, and in fact are locked into a cycle of having to regularly do such things, with costs escalating with each round of repair. How wise, eh? Makes perfect sense if you are in the concrete business I suppose, but it is hardly a means of creating really durable architecture. And given the cost we so willingly pay in the production of Portland cement, in terms of the greenhouse gases that go along with that, it seems a shame we haven’t the wisdom to justify that profligacy by creating structures that have the potential to last for millennia. That would be a reasonable trade, but no, we’re evidently too efficient (greedy?) for such things.
It ought to be said as well that tamping concrete by hand is hardly easy afternoon work, and that the ancient Romans had thousands of slaves for such tasks – today this sort of process would certainly not be economical on a large scale or for large concrete structures.
The ancient Romans had no need for re-bar or other means of reinforcement because the low amount of water in their mix meant far less shrinkage, and fewer cracks as a result.
Some people are exploring options in terms of creating more durable cement, using the conventional infrastructure to place and pour liquid concrete mixes. A few modern materials exist which can take the place of the pozzolan – the volcanic ash – that the Romans used: fly ash and rice hull ash are two prominent choices. Fly ash is a by-product of coal power plants that otherwise gets trucked to landfills. Rice hull ash is the by-product of burning rice hulls for power generation. Either waste product is an excellent option to replace some of the cement in the concrete mix. Such ash by-products, as they serve to not only strengthen the concrete but to put material to use that would otherwise clog up the landfill, are definitely worthy of consideration. I came across a good read on the topic of using fly ash in concrete, in hope of producing a 1000 year foundation (please click on the .pdf link found on that page).
When employing conventional concrete, even with the addition of fly ash, ensuring that the contractor does not use any more water than necessary and uses sound practices for placement and finishing is very important. For instance, a frequent problem with modern concrete is that the contractor, frankly, does not know what they are doing, and they “overwork” the concrete when finishing the surface, which causes excess water to flow to the surface. When the concrete cures, the surface of the concrete is very weak, and will pop off in a few years, leaving a very rough surface. This is not a fault of the material, but it is the fault of the contractor.
Concrete is a big deal, especially when the lens is set to a wide view. Globally, some 1.6 billion tons of Portland Cement is produced, a figure expected to reach 2 billion tons by next year – the 7% of greenhouse gases mentioned the the previous post. Since the cement is but 12~15% of the content of a typical concrete mix, we are also staring at the fact that the aggregates from that mix amount to some 10 to 11 billion tons of material, and all the mining, trucking and processing that goes along with that. Then there is the water employed in mixing concrete too, some 1 trillion liters per year, potable water no less, and this is not counting the water used for cleaning and washing concrete mixing equipment and in slowing the cure of poured concrete. Add to that the environmental impacts of producing the steel rebar used for reinforcement of that concrete. It’s quite a horrific picture really, and when you think that virtually all that concrete is produced with specifications demanding a curing speed which means that it will crack and likely have an ultimate durability of perhaps 100 years, well, one has to consider the sanity of the whole process. In fact, I am being generous – according to P. Kumar Mehta, PhD (of UC Berkeley, now retired), one of the world’s leading experts on concrete, the designed service life of concrete is but 50 years, and experience has shown that in urban and coastal environments many concrete structures begin to deteriorate in 20 to 30 years. As a side note, the American Society of Civil Engineers gave the US infrastructure an average grade of ‘D’ and estimated that it would take $1.3 trillion to fix the problems. With the recent Stimulus spending, some of that work is now underway across the nation, but really, what’s the wisdom in how we are using this material, given the energy expended to make it, place it and then, gosh darn it, do it all over again?
If buildings and other structures were to be designed to have, say, 150 years of useful service life, that is to say if the life-cycle cost were actually factored in instead of being largely ignored in a rush to get stuff done and make money, then we would be truly maximizing the return on investment, both from a capital perspective but also in terms of natural resource use.
Now, how to reduce the environmental impact of concrete? Here’s a few ideas, gleaned and drawn from my readings thus far:
-reduce the amount of cement in the concrete mix by using pozzolanic by-products such as fly ash to amend the mix. This product will set more slowly than the regular mixes and thus the construction schedules ought to be modified in light of this. It is simply the question of maximizing resources over labor productivity. Less Portland cement means less greenhouse gas production.
-employ construction and demolition wastes (i.e., concrete and masonry rubble, a billion tons of which are produced worldwide each year) for a component of the coarse aggregate in concrete. Additionally, dredged sands and mining waste can serve as fine aggregate components. Some amount of recycled material in the aggregate is far better than the continued expansion of mining virgin aggregate sources.
-according to Professor Mehta, the 1 trillion liters of water used each year in the making of concrete could be cut in half by better aggregate grading and expanding the use of mineral admixtures and superplasticizers. Recycled water is to be preferred to potable water. There are large savings to be had in the use of water for curing concrete – saturated burlap covers with a plastic over sheet, for example, can be employed, as they were on the 1000 year foundation project linked to above.
-employ concrete mixes that use less water, and are enriched with fly ash and superplasticizers, (to 50~60% of the cement mix) to produce a concrete that hardens slowly. This produces a concrete more like the Romans employed, and eliminates all or most of the shrinkage – when you eliminate the cracks which lead to the premature degrade of the material you produce something with far greater durability. Such concrete would also largely preclude the use of rebar, saving money and material. If reinforcement is called for, fiber reinforcement is a viable option that is both quicker and cheaper than steel re-bar.
For those, like me, seeking to build really durable architecture, one has to think carefully about materials, and it is hard to get away from using concrete. While I greatly prefer clay-based walls to concrete walls, ICF type or otherwise, given the superior handling of moisture clay provides, its easy fixability and capacity of being altered down the line with ease, and the low embedded energy of such a material, one is still faced with the foundation and how to make it. The foundation is second only in importance to the roof. Dry laid, mortarless stone is very nice, and when carefully fitted can approach art, as with this classic Peruvian example:
That said, if I don’t have years to spare for such a laborious process, and want a watertight basement, then I am really needing to go with some form of concrete. ICF makes a lot of sense, given the incorporation of both form and insulation with the system. I wouldn’t use regular Portland cement however, but a mix with a good volume of fly ash, some recycled content in the aggregate, fiber reinforcement, superplasticizers, and with as low a slump (minimal water content) as I could get away with. I would specify a 90 day period for curing to usable strength, and use a burlap wrap to keep the temperature of the cure down low and the hardening chemical reaction moving along slowly. I realize I would need an engineer to cooperate with my plans in that regard as most building departments and their codes are not quite with such radical ideas quite yet. It does seem though, if one takes the steps, it is possible to have a really durable foundation, something upon which would be worth putting a truly durable building.
As I said, I’m not so keen on a full ICF house, just ICF for the foundation for me, except where my choice would be limited to choosing between ICF and stick framed, or ICF and SIP/timberframe. I consider those systems even worse for the environment than ICF, both in initial production and in long term recyclability, and poorer uses of a resource which I consider most precious: wood.
On balance, one has to weigh both the environmental impact of producing and obtaining a material, along with any benefits that material may convey over the long term, especially in regards to improved energy use, re-useability, convertability, and so forth. There are some definite negatives to the production of Portland Cement and polystyrene, and the conventional mixed product of concrete is not all that durable. With amendments and modifications to the mix, however, a durable insulated wall would offer convenient construction, excellent strength, along with better thermal performance which conserves fossil and other fuels over the long term. The incorporation of the foam as both form and insulation also means a relatively streamlined building process and less waste. Concrete walls, however, once cast are not so easy or convenient to modify (as will inevitably happen as ‘buildings learn’ – for more, see the book by Stewart Brand linked in the sidebar at the right of this page). The difficulty of structural modification is a major drawback in my mind.
As this blog rolls along, I plan to post more in the future about sustainable building and some of the systems I have developed, and while you can’t go wrong with clay and other low embedded-energy materials generally speaking, it’s nice to now to feel a little less anxious, personally, when it comes to using concrete, so long as it is the right concrete.
17 Replies to “I See ICFs III: Alternate Realities”
New reader here. Enjoy your blog, especially the trigonometric digressions and the sawhorses. I'd love to see some posts related to drafting and how you go about developing a drawing of, say, the japanese trestle. I've been thinking about the non-square legs ever since that post.
In that vein I was curious as to what books you might recommend regarding layout, and also what you might recommend as a good book to start for timber framing?
it's good to hear from you, especially given your enthusiasm for both trigonometry and sawhorses. Yay! In response to your first question, I will be bringing out more material on carpentry drawing soon enough, though I may not go into lots of detail in a blog format.
As for what books to recommend on carpentry drawing, that's a little tough to say. I already link a modern French layout book in the book list on right of this page. As for Japanese layout books, they are many, however their methods are different and there is a language barrier to try and cross just to access them to any extent, so its not such an easy course to take. In German, take a look for books by Manfred Euchner. In English, well, the selection is not so great I'm afraid. The best stuff I have seen is very hard to come by indeed: the works of James Monckton from the mid 1800's. His books can only be found through the major universities and libraries, or occasionally on the used book market for lots of $$$. The reprint of James Newland's “The Carpenter's Assistant”, also linked to at the side of the page, is very worthwhile.
As for basic texts on timber framing, that's also hard to say. It depends upon what sort of timber framing you wish to explore, and from what angle you are approaching it. A History? A how-to?
I think the last semi-comprehensive English/American book would have been Fred Hodgson's “Light and Heavy Timber Framing Made Easy” – another very scarce title last published in 1909 or so. For a detailed look at various English cathedral timber frames, (not a 'how-to' type of book) it's hard to beat the works of Cecil Hewett.
Also, I'll be publishing some layout material in the next few months, which will be available for purchase. This will be a graduated series of study materials.
I realized after I posted that I was a little vague. I'm interested more in a how-to… Monckton, Newlands and Hodgson sound like they will be good places to start.
I noticed that you didn't have any of the more recent timber framing books on your reading list, are they that bad?
well, I've looked at most of the more modern books on North American framing methods, some good, some not so good – depends upon where you're coming from and what you're looking to do of course. Perhaps it would be worth your while to post a message on the TF Guild forum to see what people over there might recommend.
Most of the newer books advocate for using SIPs and bent-style frame systems and all that, and I can't really get behind that stuff. Not my thing.
You might also want to consider taking some courses in timber framing – there are plenty of options in that regard.
I am enjoying your posts very much!! Please keep it up. I have been coming to the blog for a couple of months and I like the way you think. Thanks for the book links and all the information – I too am very interested in the splayed leg construction of saw horses and hopper boxes and will be making my own attempt as soon as I get past some of the major home renovations I am in the middle of. I am insulating insulating insulating with careful attention to vapour barrier. It's an old house. I have added 8 inches of ROXUL to the interior of the foundation and will be adding 3 1/2 inches of polyicocianate (no idea how to spell such a toxic sounding thing) to the exterior of the house in the spring and then “hardy board” siding. The garage foundation is ruined so it will be coming out in the spring and replaced and re built. Gotta have a shop and crooked shop won't do. Just last night I put the finishing touches on a cast iron wood burning insert for the fireplace upstairs….
Hey, stumbled across your blog a while ago and have been catching up through the archives. I'm a journeyman carpenter up here in Alberta, where we've got a really interesting combination of the best and the worst in building technologies being used – mostly the worst, unfortunately. A quick addendum to your take on rebar: it's rather glib to say it simply 'keeps the concrete from cracking' .. it serves important structural purposes as well, especially in the case of beams or large, complicated structures that can't be completed in a single pour, or structures that will be subject to any sort of tension; sharing those duties with pre-and-post-stressed steel cable. I do agree with you wholeheartedly regarding the composition of concrete (fly ash vs. portland), but there again you have issues with large pours and reinforcement. I would say the issue is as much, if not more related to the design of large structures, as to the materials used – many current designs, like a standard skyscraper formed with tables and slabs, would be almost impossible to form with such thick concrete that set so slowly. I'd be interested to hear your take on large structures being built with pre-cast components (a major industry here in Calgary) – I used to work for the largest one in town (Con-Force) about a decade ago when I was an apprentice. It was a horrible job, but educational. The quality of the components tends to be much higher, as the pours can take place in what is essentially a giant controlled environment/shop/lab, leaving you with much more durable concrete and components. Also, little to no form waste, as they re-use permanent custom steel forms in the shop. Also, on the topic of ICFs – I've only built a few, but I've stayed in contact with a couple of clients over the years – a great example I like to use is one we built on a farm about an hour north of Calgary. In combination with good design (passive solar gain from orientation, etc.), really good quality triple-glazed low-e windows, and a good sod roof, that house ran two entire winters without the furnace coming on even ONCE. This is a 2500 ft2 home, in Alberta winters (-20 to -50 degrees celsius), in the middle of exposed prairie. To my mind, the environmental cost of producing the foam/concrete is offset by not having to heat it, basically. Studies on exact numbers here that I've found are contradictory and biased, I'd love to see more independent research into the long-term maintenance costs of these structures as well. Anyway, keep up the good work.
please refer to paragraph one in the above piece, where I note that it is hard to cover the topic of concrete “in some semblance of detail” despite the output of three posts on the topic. It is impossible to cover every aspect of concrete design and technology in this format. other people are telling me I'm being too long-winded. How does the writer win in such a situation? My posts on this topic are a heck of a lot more detailed than 99% of the stuff I came across when reading up on concrete and ICF construction.
Now, obviously, rebar and other metal tensioning systems are commonly employed in concrete for other functions besides controlling cracking. As the example of the Roman Pantheon clearly shows, large structures can be also accomplished without rebar.
Having rebar, and other metal tensioning systems allows concrete structures to perform, in a sense, against the nature of the material – in tension rather than in compression. That's how technology often develops – one idea leads to another, or is borrowing in quite unexpected ways.
Who would have thought that a perfume spritzer would have become adopted for use as a carburetor for early automotive engines, for example?
Once a technology becomes exploited in new ways, people often come up with further innovations and systems undreamed of by the people who invented, say, concrete. So, sure, many current designs would be impossible with Roman methods, that goes without saying. And on the flip side, modern man with all his innovations and technological sophistication cannot duplicate (or even understand) quite a bit of stuff that pre-industrial societies created. The Pantheon is using concrete in a way completely harmonious with concrete's nature – it works well in compression, and a dome is a design (generally) that well suits that fact. Similarly, modern industrial glues allow wood's end grain to be glued directly, but I still prefer to follow long-established traditional practice and design in harmony with the wood's nature, not the glues'.
Alex, did you read the link in one of the prior posts in this series to the mega Temple project in Hawaii, designed to last 1000 years? Huge amounts of concrete, but no rebar – pour methods and the concrete ingredients were adjusted to suit. I think that addresses some of your comments in terms of the difficulties of doing large pours.
As far as my take on “large structure being built with pre-cast components” – well, you won't be getting one here. This blog is about carpentry, which generally limits the subject material to structures of three stories at most, and such topics as pre-cast concrete technology is beyond the scope I intend to address in this forum. The argument for pre-cast would seem to dovetail with the argument for pre-fab, factory built wooden housing, I can say that much. Few houses employ pre-cast concrete (though I'm sure there are industry advocates out there who would like to change that!)
I concur with you on the favorable environmental performance of ICF houses, and how that balances off very well with the impacts incurred in the production of such structures. I do believe I made the same point in the above series – at least I hope so!? The fellow down the street from me with the ICF house was also quite proud to show me in detail how good the thermal performance of the house was, and how low his energy use was in the winter. It was an impressive point to be sure, and changed my perspective on the technology to a degree.
It seems that, as a general rule, the energy-saving benefits to be realized by added insulation, of whatever form, more than offset the environmental impacts of manufacture/resource mining/transport side of the equation.
Like you I would love to see some further independent research on ICF construction, and hopefully some will come along at some point, because most of what there is to read out there is industry propaganda, which I always take with a grain of salt.
Fair enough, fair enough. Though I will throw one last point back, which is that falsework/formwork is and has always been a part of carpentry, often shared with masons/bricklayers. I don't like doing it, and only did it for a couple of years myself. But even the pre-cast components are all formed and the pours are supervised by carpenters, though the forms are mostly steel, wooden ones are used as well, and you need a carpenter's knowledge of layout and structure to build them. I don't mean to try to divert the focus of this blog here, but you've done quite a few posts touching on large structures (such as that aforementioned temple in Hawaii) and the use of concrete – I thought you may have more to say about them, though most people probably don't find them terribly interesting compared to the fine joinery of japanese framing 🙂 And one last point about precast concrete houses – they are hugely popular in continental Europe, and I suspect they will make much further inroads here soon as well. They're ugly as sin, but very efficient to construct and maintain – so if previous experience in the market here is an indicator, people should just looooove them :\ Anyway wasn't trying to start an argument or anything, I find your opinions genuinely interesting and well-informed, hence my requests 🙂 So few carpenters are interested in discussing these things … like you say, it really is a 9-5 job for most it seems.
Your blog came up when I was doing a Google search for something else. But I think it's very interesting. I'm an engineer working with fiber-reinforced composites but I have become very interested in wood construction. I particularly liked the recent articles on the japanese screen. Particularly the joints I found interesting. I'm looking forward to perusing the archives. 🙂
But about concrete, the topic of these posts. I think that you make a very good point in using less water. But I do think you missed one of the main points in using rebar; tension strength. While concrete is very strong in compression, it fares rather poorly in tension. As I understand it, that is the main reason for using rebar; giving the concrete tensile strength.
thanks for your comment. Like you, I used to think also that the main purpose in using rebar was because of concrete's poor tensile strength, however my research led me to understand that rebar was added primarily to combat the shrinking and cracking problem. Consider how most concrete foundations are laced with rebar or mesh when the only loads on them are compressive. Consider the use of fiber supplement to concrete mixes for poured foundations, mentioned in the above series of posts on ICF's. Same function as rebar.
In writing the above piece, I considered the tensile issue secondary in regards to rebar use (especially given that the bulk of residential construction foundation work where ICF's are employed do not have tensile issues to deal with) and therefore choose to not bring the matter up.
Your assumption that the only loads on foundations are compressive is not generally correct. If you are talking about a concrete beam lying in the soil supporting a wall above it, you're correct.
But most foundations that I've seen consist of a concrete slab resting on the soil that supports walls and pillars. So if you look at the equilibrium of that slab you'll see point and line loads acting on the top surface, and a distributed load on the bottom surface. The sum of all these forces cancel each other out. Imagine the stresses in the slab where a pillar (or wall) rests on it. In your minds eye, cut out the piece of slab around the circumference of the pillar, and contemplate the stresses acting on it. The (concentrated) load from the pillar will generally not be cancelled out by the part of the distributed load acting on the underside of the slab on the same area. For the forces to balance out, there _must_ be a shear load acting on the imaginary cut surfaces in the slab. Shear stresses in the material imply bending (basic mechanics), so the underside of the slab is locally loaded in tension which requires rebar to keep it from failing.
thanks for pointing that out. I should not have said that the 'only' loads on foundations are compressive. And of course, given an earthquake, loads can rapidly change in direction.
I think though that in 95% of residential construction, there are no loads from 'pillars' as the building loads are distributed across many sticks and plywood sheeting tends to preclude the sort of point loads one gets with timber posts. also, locations in a slab are generally thickened considerably where the walls are located and where a point load is to be placed, no? And in the residential foundations I have seen, generally the walls are poured first, in a stem-wall fashion, and the slab is poured later, and is separated from the perimeter wall by expansion gaps so it floats. So, yes, generally I am talking about a “concrete beam laying in the soil supporting a wall above it”. I realize there are lots of other systems and ways of arranging structural parts.
In regards to the issue brought up in the above blog post, that of cracking in concrete as a result of rapid shrinkage and the addition of another material to hold things together, i.e., rebar, I defer again to P.K. Mehta, acknowledged as a leading expert on the topic. I reread a bit of his stuff again after reading your comments to be sure I hadn't mis-characterized his remarks, and realize I could have been a little more expansive in how I quoted, though, as with any blog entry, it is always a challenge to hold to a certain degree of brevity. Perhaps I am guilty of conflating rebar with 'reinforcement' in general?
In Mehta's work “Concrete: Microstructure, Properties, and Materials”, the acknowledged 'bible' of concrete work, he notes that Egyptians used straw fibers to reinforce mud bricks and that there was evidence that asbestos was used to reinforce clay posts about 5000 years ago. He notes that FIBERS are used to reduce shrinkage cracking in slabs and pavements that have large exposed surfaces more prone to high shrinkage cracking. Granite doesn't come with rebar, he argues, why should concrete? The use of concrete mixes with fly ash, plasticisers, etc., which do not reach ultimate strength for many months has shown very good results. Hopefully we'll catch up with the ancient Romans one day.
And while rebar does perform a useful purpose in terms of handling tensile loads, as you've pointed out, there is still the major drawback that rebar is one of the materials which tends to reduce the service life of concrete structures. In the “Handbook of Corrosion Engineering”, they cite Mehta in several places in regards to such points, i.e., “The soundness of concrete implies freedom from cracking”. Defects arise largely due to the fact that the high water mixes used produce a concrete with greater porosity, and that greater permeability leads to corrosion issues eventually in the rebar, through carbonation and chloride ion diffusion.
In short, the modern “traditional” concrete system does not produce a durable material – large scale environmental degradation, often prematurely, of the reinforced concrete infrastructure in many countries around the world is testament to that fact.
And finally, in terms of the first used of the 'modern' system in 1824 or so, how prevalent were designs incorporating tensile loads at that time? Were the initial concrete structures produced suddenly taking advantage of the tensile load capacity of rebar in concrete? I doubt it. I think that all they had to do was pour this liquid Portland cement a few times and note the great advantages, except for that darn cracking which shows up, especially across large exposed areas. How to solve? Well, not so different, conceptually, to add pre-placeable materials like rebar or metal mesh (and keep the apparent advantage of the liquid Portland mix) than it was for the Egyptians to think of adding straw. Later, others thought of ways that concrete could be made to perform in tension, and the rebar was configured for that, along with pre-stressed concrete and like technologies. I speculate a bit of course.
Your comments are much appreciated! Gave me some food for thought and reconsideration.
The inventor of reinforced concrete, Joseph Monier, started using steel mesh to be able to build stronger cement flowerpots initially. Later he started using it for other structures.
So I think anti-cracking properties of rebar are a side effect, not the primary goal.
Otherwise we'd be calling it “anti-cracking bar” instead of “reinforcement bar”.
again, many thanks for your comment. You may be right about the 'primary' vs 'secondary' goals of reinforcing concrete with rebar. I wasn't there to see what concrete mixes Monier used, whether he added the metal for tensile purposes or to hold the posts together to guard against cracking. According to U. Memphis's site (http://www.ce.memphis.edu/1101/notes/concrete/section_2_history.html) apparently Monier was not the first person to use reinforcement in concrete, nor did he quite understand what he was doing:
“It was subsequently shown that Monier never understood, as Wilkinson had, the need for the reinforcing to be near the tensile side of a beam.”
It's been a most interesting discussion with you, however at this point I feel I've devoted enough time/energy to it and the semantics of primary/secondary purposes of rebar is a topic for the concrete research crowd and is not the primary point of the blog post above, nor does it change the overall thrust of the argument in the posting.
Thanks for your contribution.
chris, have you ever researched ferrocement. I have been looking at getting a home from these guys eventually. it makes a lot of sense.
thanks for the comment, and I hadn't come across that use of Ferro Cement before, at least not in regards to buildings made from it. I do remember sailboats being made from Ferro Cement back in the 70's and 80's, and they were also touted as DIY. A lot of them looked bad aesthetically though, as without a mold to form the shape it is tough to get a fair surface on a boat hull with ferro cement. This may be less of a problem for making rectilinear buildings.