That’s right! Butt naked! That is – the concrete will be butt naked – because two days after the pour it had cured enough for us to remove the formwork, thus undressing the concrete.
We released the Jahn “A” Brackets, removed the wales and peeled back the plywood.
Spraying the formwork interior with a release agent paid off. Separating the plywood from the concrete was relatively painless.
Pouring the concrete in two lifts and carefully compacting (or vibrating) each lift paid off too. The wall finish turned out nice and smooth with hardly any air bubbles.
We handled the formwork lumber, two by fours and plywood, rather carefully, as I would like to reuse it for part of the upcoming porch enclosure.
Our last job was to take care of the snap ties that were still sticking out of the wall. I pulled out the plastic cones with a pair of needle nosed pliers and broke them off, pushed a short piece of ¾ inch iron pipe over the tie, wiggled it up and down a couple of times, and – as advertised – it snapped right off. The small holes that were left behind by the cones were filled in with a sand-cement mix, as you can see in the image above.
Another two and a half cubic yards of concrete had arrived. I was prepared and very determined to make this a less hectic and painful experience compared to the footing pour. I did not run out of time and we already had dug up the gate post to have enough room for the truck to make it into the yard.
I also had built a small chute or funnel that I could set on top of the formwork. The wall is only eight inches wide. Hitting those eight inches with the large truck chute isn’t easy. My small chute, which we moved along the wall during the pour, prevented messy and expensive spills.
Back from my apprenticeship days, I remembered a basic rule when it comes vertical concrete walls. You don’t pour it all at once, but rather in several lifts. This eases the pressure on the formwork.
The first lift filled the formwork about half way. To make sure the concrete filled all nooks and to remove air pockets, I used an electric concrete vibrator. The added benefit of the vibrator is that it makes the concrete flow nicely within the formwork.
The second lift filled the formwork almost to the top. The third lift (if you want to call it a lift) was a top-up followed by troweling.
And – the formwork held up just fine, despite my concerns.
Any green in the gray?
Well, not much. Concrete unfortunately has a very high carbon footprint.
Out of the main ingredients, rock, sand, water and portland cement, the latter has the largest environmental impact. The manufacturing of portland cement requires a mixture of limestone, clays and silicas baked at 2,642 Fahrenheit. The associated carbon dioxide (CO²) release is four times greater than that of the global air traffic, according to one source, or 6% of the annual global CO² emission based on another source.
To reduce the carbon footprint of concrete, supplementary cementitious materials, such as fly ash, blast furnace slag and silica fume can be partial substitutes for portland cement. Their use can reduce the embodied CO² by 15 to 40%, according to the National Ready Mixed Concrete Association.
Recent research and development may decrease the concrete carbon footprint even further. The Karlsruher Institut für Technologie developed a portland cement substitute called Celitement, which is based on hydraulic calcium hydrosilicates. Celitement can be manufactured at temperatures below 572 Fahrenheit and thus should cut CO² in half compared to portland cement. An advertised side benefit is that concrete with Celitement appears to be more durable than conventional concrete.
The Romans again…
Talking about durability – the Romans figured that one out long ago. Their concrete has lasted more than 2,000 years, and that even in harsh maritime environments.
Recent research by the Lawrence Berkeley National Laboratory examined the composition of Roman concrete. Whereas our common concrete used calcium, silicates and hydrates, the Roman concrete relied on calcium-aluminum-silicate-hydrate, which made for an exceptionally stable binder. The Romans used less lime (less than 10% by weight) and baked at only 1,652 Fahrenheit, thus leaving a significantly smaller concrete carbon footprint.
The use of fly ash or volcanic ash as partial substitutes for portland cement also produces calcium-aluminum-silicate-hydrate. And with the Roman concrete precedent, we can get an idea about the potential durability and long term performance of these mixes.
Too good to be true?
Could concrete become carbon-neutral or even carbon-negative? It may, according to research at the University of Arizona. David Stone developed a product through his research that is a portland cement substitute. He used steel dust, a waste product of steel mills, and silica, which can be sourced from recycled glass bottles. The resulting material is called Ferrock.
The steel dust in the Ferrock reacts with CO² in the concrete curing process to form iron carbonate, and as such reduces the carbon footprint to the point where it may be carbon-neutral or carbon-negative. And similar to the other portland cement alternatives listed above, Ferrock is said to produce a concrete with a higher compressive strength and better deflection properties.
These developments, whether Celitement, calcium-aluminum-silicate-hydrate or Ferrock, appear very encouraging. But it may be another few years before they are market ready – before they truly begin to disrupt the current portland cement market.
The rebar was in place and we could indulge in playing with plywood sheets for the foundation wall formwork.
I had taken great care to assure the footing, the footing plates and plywood strips were all level. While we were installing the plywood sheets for the formwork, everything turned out plumb and square. Our previous due diligence paid off and I had a huge weight lifted off my shoulders!
But that took only care of half of my worries. How to keep the formwork from buckling under the pressure of the freshly poured concrete was the next big thing on my mind.
I had purchased a box of snap ties that would help us tying the formwork together. My plan was to space them 24 inches horizontally and 16 inches vertically across the formwork. That meant that I had to pre-drill the plywood sheets in the 24 by 16 inch pattern while setting up the formwork. It was critical to keep the holes on the outside lined up with those on the inside to make sure the snap ties fit across the formwork.
Although I haven’t even ordered the concrete yet, it was already time to think about the finished wall and how to remove the formwork once the concrete had cured. To prevent the concrete from stubbornly sticking to the plywood, I sprayed the formwork interior with an oil based release agent, which makes it relatively easy to remove the formwork.
All the plywood sheets were up, and the job turned counter-intuitive. We had to take the interior formwork back down and lay it flat so that we could install the snap ties through the outside formwork. To keep the ties horizontal and in place, we installed the brackets and wales and tightened them up.
We knew that if we did a good job on the pre-drilling and lined up all the holes, we should be able to lift the inner formwork back up and in the process slip the snap ties though the pre-drillled holes. Bingo! Everything lined up again and we finished the job by installing the brackets and wales on the inside, and braced everything at critical points along the inside and outside.
Everything on this formwork is straight, with the exception of the existing limestone foundation wall to which I have to connect. If I don’t want the concrete to pour all over the place but rather keep it in the formwork, I need to produce a tight fit to that rather fancy limestone wall face.
The old scrib method came in handy again. I ran a simple compass along the limestone face and thus translated that profile onto an adjacent plywood sheet. To cut the profile, I used a jigsaw, and after a little bit of fine tuning here and there, I got the tight fit I needed.
My helpers were rather impressed by the simplicity and ease of this method.
I had given some thought to what utilities may need to pass through the concrete foundation wall. There is for instance the sump discharge line that would connect to a future cistern. But there are also other items such as electrical lines that may lead from the house to the back porch and into the yard.
Rather than pouring the foundation wall and core-drilling through the concrete later to accommodate those utilities, I decided to preemptively install sleeves into the wall through which I can feed those utilities and avoid the whole core-drilling issue.
Just one of those little details that may pay off one day…
There is a silver lining in almost everything! My sprained ankle gave me time to think about how to put the formwork for the concrete foundation wall together. I learned about concrete form work during my apprenticeship in Germany, but that was over 25 years ago. It was time for a refresher!
What makes concrete work so interesting is that you get one shot. If something goes wrong with the formwork during the pour, there typically is no fixing it. And starting over is time consuming and very expensive.
We needed an eight inch wide and 54 inch tall foundation wall. The outward pressure from the concrete onto the formwork is considerable – 675 pounds per square foot (psf) at the bottom and 150 psf towards the top. (The pressure was calculated following Lateral Pressure Design Formula in Guide for Formwork for Concrete (ACI 347) by the American Concrete Institute).
I was on a mission to find a reliable formwork construction method that would keep the eight inches wall thickness from top to bottom, despite all the concrete pushing on it. What I settled on is a system with snap ties, brackets and horizontal wales.
The snap ties are the spacers between the plywood sheets. The Jahn “A” Brackets slip over the snap tie head and hold a two by four – the horizontal wale. The brackets are tightened up by pushing/hammering the eccentric over the tie head which holds the formwork in place. The snap ties have a working pressure of 3,350 pounds.
But before I get to play with the snap ties and brackets, I have myriad other tasks lined up, competing for my attention.
From the bottom up
The snap ties and brackets can’t be installed at the very bottom of the formwork. Instead, we installed footing plates, which are simple two by fours that get anchored into the concrete footing.
To get to the 54 inch wall height, we added a six inch plywood strip to the 48 inch plywood sheets. I decided to place the plywood strips at the bottom of the wall. This way any associated imperfections (like seams) will be hidden under the future concrete floor.
It also made for relatively simple 12 inch tall framing, with the footing plate at the bottom and the first wale 12 inches up. The 16 inches on center vertical two by fours kept the plywood strip and sheet lined up and prevented it from bowing outwards.
I also had to think about the steel reinforcement and how it fits into the installation sequence.
We started with the outside footing plate, plywood strip and framing. Once completed, I put up all of the #5 rebar, before installing the inside footing plate and framing.
At this point we had most of the tedious tasks behind us and could move on to installing the plywood sheets, snap ties and brackets. More about that in the next post.
The difficulties with architectural documents
This is a subject close to home, as I frequently produce construction documents myself in my capacity as a landscape architect. I personally don’t like to leave details unresolved or un-tested in my drawings. Items not properly resolved or detailed typically lead to pricey bids or change orders during construction because of field changes.
I had installed the footing rebar exactly as detailed in the architectural drawings, but was mindful enough to have Edgar, our porch contractor, stop by to check on everything.
He promptly pointed out that the rebar and the wall alignment on the short east and west footing was about four inches too far out. If I was to keep the alignment as per the drawings, the porch would end up wider than the building.
Edgar’s comment was: “I have to look at the drawing and figure out how to build it. Some of the details and dimensions don’t work as shown.”
Well, I got a taste of that and moved the rebar over by four inches, as you can see at the beginning of the time lapse.
Yes, I wish this post was about drinking beer… May be a beer would have helped!
This was the day of the porch footings concrete pour – a busy day. I started out by lining the form work with a 6 mil polyethylene sheet. It should act like a damp proof, minimizing the potential for soil moisture rising up into the footings and ultimately the foundation wall.
Next I put back the horizontal rebar I had laid out the day before. To tie the future foundation wall to the footing, I had a 48 L-shaped rebar section. I attached the 12 inch horizontal leg (the short leg) to the already placed rebar, with the 18 inch leg (the long leg) rising vertically from the footings.
These vertical legs need to lign up with the center of the future foundation wall. But keeping them in line and vertical proved to be rather challenging, and we ended up constantly adjusting them during and right after the concrete pour.
I had rented a concrete vibrator to make sure the concrete would flow nicely, fill all the nooks and crevices, and to remove air bubbles or air pockets. We screened the top of the footing to make sure it was level and finished the job by troweling a key along the vertical rebar legs. The key will help with the mechanical connection between the foundation wall and footing.
The process wasn’t nearly as smooth as it may appear in the above narrative or time lapse.
My problems started with the concrete truck arriving slightly early with me not being quite ready. Plus the truck was huge – or to be more precise – very long. We had an excellent driver and he made it into the narrow alley. But swivelling the truck into the back yard eluded us, despite trying for a good 30 minutes.
We finally had to dig out the fence post next to the gate, which gave us an opening large enough for the truck to make it into the yard and back up to the job site. We were ready to pour, when I twisted my ankle – and I twisted it good!
I had to sit down for a minute to contemplate my options, of which there was really only one: A $1,000 mass of concrete sitting in a truck that needed to be poured. So I got back up on my legs and started limping around as best as I could.
Our neighbor William saved the day by taking over the heavy lifting and basically running the show. I would have been in dire straights without him.
Thank God I didn’t untie my boot when I twisted my ankle! Once I did at the end of the job, there wasn’t even any limping around anymore. Cathy hauled me to the emergency room where I learned that it was a bad sprain and that I’d earned at least two weeks of forced vacation.
That footing will be tied to fond memories forever!