Although flyash mixes are often stated as to the percent of flyash of the total of flyash plus cement, this is misleading unless it is specifically stated that the flyash was used to replace cement (see flyash discussion in materials section)

Our usage is as follows:

According to statistics from the National Association of Homebuilders (1998 data), the average house built that year is 2085 sq ft and uses 14 tons of concrete (or about 7.5 yards), which is dramatically lower than what we used (reference: EBN V8#1, Jan 1999).  However, when we compute how much concrete we expect a house to use, we get a much different number.  Although we didn't find the concrete use statistic on the NAHB website, we did find characteristics of homes from that year:  average home size was 2190 sq ft (median was 2000), 48% of which were one story homes.  As for foundations, 37% had a full or partial basement, 43% were on a slab and 78% had at least a two car garage.

Although there are a variety of configurations,  the fact that 80% had a basement or were on a slab, and 78% had a two car garage, it would seem reasonable to assume that the average home had at least a 1000 sq ft of concrete floor.  Since a typical slab floor is at least 4" thick, we can calculate the concrete use as 1000/3=333 cu ft, or 12.3 yards.  Then, all the load bearing sections have to be thickened to have footings, which at about 120 linear feet footing that uses at least .75 cu ft of concrete/linear foot (ie the footing is 8" deep and 12" wide), that adds another 3.3 yards, bring us to a total of 15.6 yards, all using what would seem like very conservative guesses, and still resulting in double the use reported.

If anyone has better numbers, please contact us.

Analyzing our Usage

Even ignoring the concrete in the rainwater tank, we used a lot of concrete.  There are two classes of reasons of why we used so much concrete: choices we could have made, and issues with structural engineering and building codes. 

We could have chosen not to build a basement at all (see design for why we did), and we also could have used ICFs for the foundation (we didn't because they cost a lot more).  But that leads to the question of why an ICF walls with much less concrete is deemed sufficient, but a six inch thick foundation, which still uses more concrete than an ICF wall, isn't (we had to use 8" thick walls).

Structurally, both the footings and the slab consumed what seemed like excessive amounts of concrete.  The footings are 18" wide and 9" deep and filled with re-bar, although their only task is to spread the load of the foundation over a bigger area to prevent it from sinking into the ground.  It seems like there ought to be a way to use little to no concrete by substituting rock, broken concrete, crushed stone etc.  A significant amount of the concrete use in the slab is for areas where it is thickened to provide an integral footing.  Without these footings, the slab would have used only about 13 yards of concrete.  Again, the thickened slab footings seem like huge overkill, and that there ought to be a better way.

In general, it would appear that the world really is in need of a green structural engineer to come up with creative solutions to using less concrete.

The amount of flyash that can be used depends both on the kind of flyash that's available and the ability of the concrete finisher to deal with its unique finishing properties.  Walls cure in the forms, so don't present much problem, but slabs require a different technique for any significant amount of flyash.  The concrete finishers didn't like working with our 20% flyash mixture because of this, but in spite of their complaining, our untrained eye thought the finished slab came out as smooth as anyone would want.

Embodied Energy Usage

The energy to produce 5 sack concrete is usually given as around 1.5mBTU/yd.  In  EBN V2#2 (mar/apr 1993), the energy use is given as 1.7mBTU/yd, but the assumptions behind this number includes transportation the materials to the plant, and transportation of the cement to the job.  Since 94% of this embodied energy is in the Portland cement, reducing the amount should save energy, but the flyash also must be transported so the cost isn't zero.  Using numbers from EBN V8#6, train transport is about 310kBTU/ton or 15.5kBTU/100 lbs.  Compared to the total energy use this is so insignificant that it can be ignored (these numbers have so many assumptions built into them, they aren't that accurate anyhow).  So if five sacks of Portland cement uses 1574kBTU then 1 sack uses 315kBTU.

(final embodied energy calc goes here)

Foundation Drainage

The biggest issue with building below ground is keeping ground water out of the building, and this is complicated by the fact that concrete absorbs water and will wick it upward for a long way.  We use two main strategies to keep water out:

• Surround the concrete with rapidly draining material, such as gravel.
• Put a water barrier on the concrete itself.

Polyethylene capillary break under the footing.

While some aspects of these strategies are normally used, standard practice does not create a complete barrier to ground water entry into the foundation, and we wanted to do that: mostly because we think its a good idea, but also because the forthcoming IEQ standard being created by the EPA requires that these extra precautions be taken.  Given the frequency of mold problems in house, and the ubiquitous nature of water in basements, it seems likely that standard practice is going to include additional strategies for preventing groundwater infiltration, although they may not be the ones we've used.

Our first new technique is to install a capillary break so that water doesn't wick up the footing and into the foundation.  Since our footings are sitting on nearly pure clay which holds water but doesn't drain it well, that this was a reasonable precaution and after some consultation, it was decided that a polyethylene sheet under the footing would do the trick.  Traditional construction relies on using gravel both inside and outside the footings to drain water away from the footing, but this does nothing to prevent water from coming up through the footing via capillary action.  We would expect this technique, or something like it to become standard since it is low cost and easy to do.

Delta-drain material attached to foundation and footing drain in place (left).  The footing drain will get covered in gravel and a filter fabric before backfilling.  Gravel under the slab (right).  The dirt area in the center is a footing under the central load bearing walls.  This is obviously a weak spot for water, since there is no gravel under it. Cross sectional diagram of foundation wall (middle).

The only other significant difference is in using a drain fabric on the outside of the foundation.  We used Delta-drain, which is a heavy weight (but still somewhat flexible) plastic material that is composed of a sheet of plastic molded like hundreds of little egg cartons that faced the concrete and a plastic filter fabric on the dirt side (see photo, above.  The black material came off a different spool, but has the same egg carton plastic under it.)  The idea is that the delta-drain creates air gaps that won't hold water or promote capillary action (just like the gravel).  This keeps ground water away from the foundation.  It is fine if water gets behind the delta drain, since the wall is sealed with Thoroseal, and any water that gets back there will quickly drain down in the footing drain.

The main issue is keeping dirt from keep between the concrete and the delta-drain & in between the layers of the delta drain.  Since the layers are bonded together, dirt will stay out as long as the filter fabric isn't punctured by either shovels or plant roots.  The area between the concrete and the drain material is more problematic, since the drain material is attached by nails.  While it is probably possible to get the drain material tight to the wall, it isn't easy and dirt will likely fall down through any small crack that is left.   The instruction say to install is "at grade", but our experience is that "grade" can easily change over the lifetime of a house due to the addition/removal of topsoil and mulch.  It would seem like a better idea to embed a strip of some sort into the concrete (ie before the pour), and then snap the drain material into the attachment strip, thereby creating a continuous dirt barrier.

Our sales rep suggested we use the delta drain over the footing as well, but this proved to be very frustrating because the delta drain does not bend all that well, and wouldn't conform well.  In addition to this, it was difficult to install the material around corners. Overall it seems like a great idea that could use a little work.

Although the basement is designed to be unheated, unfinished space, we installed perimeter insulation under the slab in case someone wanted to use the space in the future.

	Vapor barrier & perimeter insulation on top of gravel (left).  Pouring  the slab (right).