Constructing a Ferrocement Tank

Every long-lived and hearty culture has its monoliths. This last month, I took a stab at this ancient tradition by constructing a ferrocement tank on the most solid piece of ground I could find out here at Aprovecho.  We set out to manufacture a tank that would catch 10,000 gallons of water from our community building for the irrigation of perennial plantings around the perimeter of our garden.  What follows is an account of the project.

The foundation

Indeed the ground had to be solid; the sixteen foot wide and eight foot tall cylinder weighs 120,000 pounds.  That’s about 500 pounds per square foot (psf) at the floor.  Lucky for me, one of the only relatively flat spots on the property besides the handicapped parking slab is just up hill from our community meeting hall, the building whose 3,000 square feet of roof surface would dump rainwater into the tank over the course of the rainy season.  One doesn’t have to dig far at Aprovecho to encounter bedrock.  For this reason, it is almost impossible to dig a level bench sixteen feet into a slope without hitting a layer of rock impenetrable save for pneumatic drills and dynamite.  In most cases it would be fine to build a pad out from the slope with loose fill, packing it down as you go and build up from there.  It isn’t that easy if you are concerned about the slightest amount of settling irreparably cracking the foundation under a 500 psf water tank.  When doing the earthwork for a project like this, one wants to be certain to excavate a level pad down at hard compacted subsoil.  So Rick Valley and I rented a backhoe and dug a pad at the relatively flat spot uphill from the community hall.

We only dug a foot down before finding bedrock and judging by the mixed nature of the subsoil, it turned out that we would be building a tank on old fill dirt after all.  But the fill was at least fifty years old judging by the age of the tree roots we ripped out with the backhoe.  Probably an old logging road left from the cuts they did here in the late fifties.  Well, better a fifty-year-old pad than a fresh one, we mused.  After giving the site a good watering a day in advance in order to aid compaction, we proceeded to pour loads of ¾” minus gravel on the 16-foot diameter circle we had scratched in the ground representing the tank footprint.  We compacted the gravel every time we spread and moistened two inches of gravel.  For the compaction we used a droid-like gas-powered compacter.  We made sure in the end that the tank would be resting on a level gravel pad.  In all honesty, this ended the most concerning portion of this entire project.  I had been concerned from the start that the weight of the tank would force settlement of the soil it rested upon.  It was only after an obsessive and liberal application of compacted gravel that I was satisfied the tank would not shift the soil below it.

Consult the Masters

At this point, I was finally able to pay Nolan Scheid a visit.  He lives in the neighboring valley in a ferrocement castle he built out of rebar and chicken wire.  He sprayed the cement plaster on with mortar sprayers he and his family manufacture in his shop and sell around the world.  I had been hearing rumors of a genius who lived in a castle somewhere southwest of Eugene for a while now.  The helpful folks at the Masonry Supply Company spoke of him like a legend in his own time but could not recall his name.  I finally found his mention in online references to a summit held in Lorane a few years ago by the international ferrocement association.  He invited me out to his castle and poured over the blueprint I had drawn up at the behest of Tammie Stark, Eugene’s expert on water catchment systems.   In a domed cement library he advised me amid the noise of falling water from his freshwater lobster aquaculture tanks.  He was heating their water with a thermo-siphon from a black pipe he ran through his wall and into a rock pile at the south side of the castle.  He loaded me up with books on the subject of ferrocement construction and put me in touch with two other masters of ferrocement: author Garrett Connelly and Peter Epperson, a tank builder in Hawaii.  Before I left, he gave me my pick of pneumatic hog ring guns from a pile of at least twenty he spilled onto his shop floor.

The next day, I lay the newly revised blueprint in front of the newly arrived group of high school seniors and graduates I would be working with through the course of the project.  I set next to it the two books we would be referring to throughout the building process: Water Storage by Art Ludwig and Ferrocement Tank Construction by Garrett Connelly and laid out the project plan.


The Key and Water Main Box

Next we walked over to the freshly dug tank site and considered our considerables.  I was still not completely satisfied that the tank would not shift under its weight enough to risk cracking.  My friend, Tim Bailey, a tank builder in California, suggested we build out a block of concrete when we poured our slab on the downhill side of the tank as a “key” against slumping.  It would also help to prevent the chance of the tank pivoting on the section of bedrock at the opposite side.  We decided to incorporate the water outlet and overflow into the key (above). One could access the valves by opening up a cedar lid.  In this way, the plumbing joints and valves would be free from tampering and frost.

Inlet, Outlet, Drain, Overflow, and Vent

Every tank has these five plumbing elements.  Water goes in at the inlet.  The inlet is six inches above the overflow.  The vent goes at the top and lets out air as the tank fills with water.  The drain is at the absolute bottom of the tank and allows for the extraction of all the water in an emergency.  The outlet is where the usable water issues from.  It is usually about a foot above the drain.  This way a foot of space is allowed at the bottom of the tank for sediment and anaerobic conditions before water is drawn off for use.  Most commonly the drain and outlet poke through the bottom of the wall.  We decided to run them together under the tank and through its center.  This would save us from compromising the wall at its most critical point (the bottom is where the weight is) by poking pipes through it.  Before we went any further we would have to address the outlet and drain.  First, we trenched out a sloped ditch rising from the side of the tank footprint into its center.  Into this ditch, we set a three-inch schedule 40 pvc pipe with a ninety bend rising up into the tank’s center six inches above the level of the gravel.  With a six-inch slab for the floor, this pipe would come flush to the bottom of the floor and serve as a perfect drain.  The pipe flush with the floor had a female thread to it.  We attached a threaded male pipe one foot long to it.  This left us with a removable pipe standing one foot above floor level (see photo upper right).  We had a dual drain and outlet.  When we wished to clean the whole tank we could thread out the final foot of pipe, drain the last foot of water and scrub the floor down.  Outside the tanks perimeter we tee’d off of the main three-inch pipe for our outlet to the garden and attached a fire hose fitting to the drain end.  This would allow a fire truck to pump out of the tank if needed.  At this point the tank site resembled a pac-man clamping down on a cob pipe.  We mixed a few bags of concrete in a wheelbarrow and poured it into the trench around the pipe.  This would keep it set in there as we began to build up.  The inlet, and overflow would go through the top of the wall and be affixed with bulkhead fittings. The overflow will eventually direct excess water to a swale system in the gardens below the strawbale dormitory.  The inlet would also be plumbed to catch water off of the roof of the tank itself from a gutter along the roofline.

The Floor Metal

Stretching a string from the vertical pipe that now poked up at the center of the tank, we scratched a line representing the perimeter of our circle.  Our tank would be 16 feet wide and the circumference would be 50.8 feet.  As we were putting the finishing touches on the gravel pad, the truck arrived with our steel.  As the evening shade finally crept across our site, we staged the materials: 60 pieces of 20 foot 3/8 inch rebar, 15 pieces of 20-foot ½-inch rebar, 40 pieces of 27” by 8-foot lath, two 200-foot roles of 6×6-inch welded wire mesh, and two 100-foot roles of ½-inch hardware cloth.  To hold this all together, we would require eight 5,000-count roles of metal twist ties.  The first step was to develop the floor of the tank.  This would involve making a steel framework, capable of holding the entire weight of the tank once it was filled with concrete.  In order to create this, we sandwiched ½-inch rebar bound together in a 1×1-foot grid between 6×6-inch wire mesh. The top mesh was set off from the bottom mesh by three inches in a diagonal dimension so that the end result was a grid of 3-inch wire mesh encompassing a diagonal grid of rebar. This last layer of mesh went in after the wall went on so that it could bend up onto and attach to the wall metal.  We had many pieces of rebar passing out and beyond the perimeter of the circle. These we bent up at a ninety-degree angle so as to rise vertically along the future wall of the tank.

The Wall Metal

There would be two cement jobs involved in the construction of the tank.  The first, soon to come, would be the floor.  The floor represents the foundation of the tank and has to be extremely strong and sit on a solid footing.  The second would be the plastering process, completed in one day, in which we cover the roof and walls with 2 ½-inches of cement, forcing the cement through the fine mesh of the walls and roof.  We would work from the roof down, winding our way down the cylinder of the tank, until we brought our plastering to the joint of the floor.  Before we went ahead and poured the floor of the tank, we would have to at least complete the bottom layer of the wall so as to have a continuity of wire mesh coming out of the concrete floor and continuing up the wall.  After we had the rebar sticking up vertically, we cut a strip of 6-inch mesh to a length two feet longer than the tank’s circumference and seven feet wide (representing the height of the tank).  This we wound into a circle with a circumference of 50.8 inches inside the vertical rebar. We overlapped it by four squares and tied it together every six inches with the wire ties.  The next order of business was to create the vertical rebar framework.  We bent 3/8-inch rebar to a ninety degree angle 18 inches in from its tip and cut it to a length of seven feet eighteen inches on the other end.  The 18-inch ends we slipped through the wire mesh forming our new wall and tied them to the rebar grid at the floor.  The long ends stretched a perfect 18 inches beyond the top of our wall.  These got bent over to another ninety-degree angle facing into the tanks center.  We tied the floor ends to the floor rebar at three points and tied the vertical seven foot portion of rebar to the wall mesh just once for each bar.  Next we lashed three twenty-foot ½ inch rebars together with a twenty-five inch overlap where they came together (you want to overlap the bars to 50 times their width).  Attaching this to the vertical rebar one foot up the height of the wall we wrapped it around assuring that it was level as we went.  Each rebar was made plumb then tied to the level horizontal band.  As we wrapped around the tank we came back to the starting point of the rebar and were happy to find we had leveled evenly across.  We overlapped the remaining rebar and had a level horizontal hoop tied to our now plumb vertical rebars.  Next we crawled inside the tank and adjusted our wire mesh to line up on its horizontal axis with the level hoop of rebar and tied it off every foot.  Now we had perfectly plumb vertical rebars and a level interior mesh.  Using the six-inch mesh as our guide we proceeded to wrap horizontal rebar hoops around the tank every foot going up.  We were assured to have level hoops as we climbed.  This attention to leveling proved priceless.  The sooner you can work this out the better.  We were now beyond the wankadoodle stage.  Our tank was assured an acceptable degree of symmetry as we built up (see photo below).

We had built our grid of wall rebar and attached the inside mesh.  Next we attached an outside wall of mesh and again offset it so as to create 3-inch squares.  We found we had to cut it into ten foot strips however in order to compensate for the increased circumference.  Ten feet was the farthest we could stretch without getting the overlap extremely off.

We next attached the metal lath to the inside of the tank.  The half-inch hardware cloth was attached to the entire exterior.  We were always making sure to line everything up with our level mesh and tying it tight enough that nothing budged under pressure from our palms.  This meant tying and hog-ringing the lath and hardware cloth about every six inches.

In order to access the inside of the tank, we cut a hole at waist level just large enough to crawl into and lined the sharp edges with poly pipe.  It would also make the cement application a lot easier.  We would close this up the day we plastered.

Pouring the Floor

Before we could pour, we had to set one more layer of mesh on top of the floor rebar.  As mentioned above, this was offset from the mesh below it in order to create three-inch squares.  We forced it into a bend at the wall and tied it to the wall mesh.  The pump company arrived at the same time as the cement truck and attached their hoses to the truck.  The hose went through the hole in the wall and began churning out cement at a cubic yard per minute.  We rushed to shift the cement down through all of the floor metal with a concrete vibrator and smooth out the floor in a mild slope to the center drain.  At the walls, we tapered up the concrete about a foot.  At the top of this lip we formed a trench along the wall metal and scored it with our trowels in an attempt to create texture for the wall cement to adhere to.  We also poured a diluted amount of muriatic acid along the top of the lip to further score the area that would make up the cold joint.  We then pulled out the hoses and poured the key that would embed the outlet and drain.

The Roof

The roof rebar is arranged in a radial pattern.  While a grid is the strongest arrangement for the floor, the tension on the roof is best contained by rebars meeting each other in the center like spokes of a wheel.  We wanted the roof to rise about a foot to form a dome so we stuck a pole in the center of the tank with a circle of plywood nailed onto it at the height all of the rebar would come together.  We then began to tie rebar to the 18 inches that were facing into the tank already.  Once all of the rays of rebar were in place, we tied them all to three concentric circles 16 inches apart (photo).  With all manner of difficulty, we next tied strips of six inch mesh to the underside of the roof followed by  the metal lath.  We then cut about 12 poles to size and stuck them all over the inside of the tank to support our weight on the roof.  This allowed us to climb up and tie wire mesh to the skyward side of the rebar.

The Gutter, Access Hatch, and Interior Ladder

No tank is complete without these three elements.  Although our artistry and skill in rebar manipulation was challenged as we manufactured these structures, we had all come along way from that first day laying out the mesh on our fresh gravel pad.  I am very happy with how these turned out  (photo).

The Plaster

We got the plaster done in two 13-hour days with an inexperienced crew of six to eight people.  There were two people mixing, four people applying the plaster (two inside and two outside of the tank), one person delivering the mix to the plasterers, and myself, scurrying around between everyone making sure everything was going ok.  All together we used 4 yards of sand, 40 bags of cement, and 4 bags of lime.  We mixed it together with a gas-powered mixer at a ratio of 1 part cement to 2.5 parts sand to 1/10th part lime.  In order to avoid the obscure occurrence of an alkali-sulfate reaction, the plaster damaging expansion caused by sulfur rich water reacting with lime, I opted not to use our sulfuric scented well water as our water source. Instead, I had to run a hose from our spring in the forest to the site.  We sprayed this into the mixer as we added our proportions.  Once we had it mixed it to a good looking consistency, we poured it out into wheelbarrows and let it sit for 10 minutes.  We then mixed it once more before wheeling it over to the plastering crew.  We knew it was ready when a thumbprint would very slowly lose its impression.

The biggest concerns with the plaster are: that it fills all of the voids within the metal framework and does not leave air pockets between the inside and outside of the wall.  The pocket could seriously weaken the integrity of the wall and cause cracking.  The plasterers had to exert enough force on the outside of the tank with their trowels to force the cement all the way through the 2 ½ inches of mesh to the inside of the wall.  The cement that poured through to the inside was then covered by another layer of plaster dolloped out of the buckets of the plasterers on the interior of the tank and forced again through the framework to the outside.  The square shaped pattern that resulted from forcing the cement back through the wall to the outside made a nice plaster for the finishing coat.  Garrett Connelly said that working with inexperienced plasterers is preferable since their over-meticulous tendencies pay off with this kind of work.

After we had plastered the entirety of the wall save for the area that remained, a hole through which we were able to pass bucket and tools, we began work on the second coat of the inside.  We worked our way around the interior of the tank, laying a second smoother layer across the gritty already relatively solid first layer.  At the base of the tank, where the wall met the floor we brought the plaster out from the wall at a forty-five degree angle to meet the floor.  This covered up the key we had scored by muriatic acid with a good thick layer of cement and greatly increased the thickness of the wall at the most critical point.  By the time we reached the side of the tank with the hole, it was getting dark.  Under lantern light, we patched up the hole and plastered it over.  We had to finish all water holding portion of the tank in one day.  Otherwise we would leave a cold-joint in the wall where we plastered wet cement onto dry cement.  This would create a seam in the wall for water to travel through.  Once we were done with the wall around 9 pm, the whole crew went up to my house to an amazing feast prepared by Tao and a few interns.

The final day saw us gathered around the tank at 7 am.  5-gallon bucket by 5-gallon bucket, we hefted plaster to a crew on the roof.  They troweled it through the roof mesh to a crew below that attempted with a good degree of success to smear any cement that slipped through onto the bottom of the roof before it fell onto the floor or their heads.  The key to success here is getting the right consistency in your plaster.  Too wet and it drips through on the workers below.  We then put the finishing coat on the outside of the wall and the interior of the roof.  Another day well done, we made our way up the hill to another feast and toasted our new water tank.

Rainwater Downspouts

The downspouts come off at six points on the community hall.  The PVC is 3 inches until all of the points converge and flow in one pipe to the tank.  Here the PVC goes to 4 inches to accommodate the combined flow during peak rains.  At each downspout there is a first flush device that must fill before water flows to the tank.  This prevents dust, bird crap, and other detritus washing into the tank when the rains start.  They can be emptied by a simple turn of a spigot at each downspout.  In order to be able to run our pipe under a road and back up into the tank we made sure that all PVC at the downspouts was 18 inches above the tank inlet.  You can imagine a solid body of water in the pipes that come from each downspout, combine together, and go into the tank.  Since water always drains to the lowest point, whenever water runs off the roof and into the pipes, the water on the other end of the pipes overflows into the tank.

“OK you have the water… now what?”

In the immediate future, the water tank will supply water to the aquaculture tanks at the bottom of the garden as well as the forest garden that forms a perimeter around the vegetable gardens.  Ultimately the tank may act as a central point for all roof-water (it is placed to retrieve water by gravity from all buildings) on the campus.  With a pump powered by the winter creek flow, we could pump the water out of the tank into a larger storage further up the hill.  Every time the tank filled, the pump would automatically switch on and pump all 10,000 gallons.  This could happen all winter long in 10,000-gallon increments.  With all roofs combined we could capture 150,000 gallons, enough to keep our vegetable gardens irrigated year round!  The overflow will drain into a pipe that opens into a series of swales that will span across the contour of the garden.  Their downhill side will be planted to perennial food and medicine and they will feed water to our duck pond.

In Thanks

First of all, I want to thank Hazen Parsons, whose help was incalculable and temperament inspiring.  Thanks also to the experts: Nolan Scheid, Peter Epperson, Art Ludwig, Garrett Connelly, and Tim Baily.  Thanks to John, Tyler, Duva, Alex, Amber, David, and Josh.  Also thanks to Chris, Genevieve, Rick, Kate, and Dan for your consistent help.

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