Concrete pour

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.

concrete-pour-01

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.

concrete-pour-02

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.

Celitement

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.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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