Progress > August 2007
How to Make a Concrete Superstructure
Now is the season of concrete at the Kroon Hall construction site. Having cleared the site down to the basement elevation, the construction team spent the month of August building up the footings and foundation walls. The first big pour of cement for the ground floor suspended slab is scheduled for mid-September, and preparation is well underway for this construction milestone.
Bird’s eye view, photograph by Peter Otis
Concrete in its most basic form is a simple recipe of stone, sand, cement and water, the modern use of which dates back to the middle of the 18th century in Britain. The aggregates – the sand and stone that make up the primary ingredients in concrete – for the Kroon Hall concrete are all being sourced locally and mixed in a plant in New Haven. The cement that holds these aggregates together is composed of a combination of Portland cement (a derivative of limestone) and Ground Granulated Blast Slag Cement (GGBSC). GGBSC is an industrial by-product of the blast furnaces in steel mills and using it as cement both increases the strength of concrete and keeps this industrial waste out of landfills.
Material for constructing concrete formwork, photograph by Peter Otis
Working with concrete involves an elaborate process of preparation, installation and inspection. In case you are thinking about building your own concrete superstructure, here is your guide to building with concrete in ten simple steps:
- The excavator prepares the site, bringing the rock surface to the proper elevation.
- The Geo-Technical Engineer inspects the rock for proper bearing capacity (these first two steps only apply to the footings – the foundation walls will then rest on these footings and the structure is built up from there).
- The concrete contractors place formwork – this is basically the frame that holds the concrete in place while it cures (the forms for Kroon are made of steel and timber prefabricated into modules which can be quickly assembled, disassembled and reused).
Formwork assembled, photograph by Peter Otis
- The concrete contractors place reinforcing steel bars (rebar) inside the forms.
- The rebar inspector inspects the rebar (as one would expect).
Rebar placed and inspected, photograph by Peter Otis
- The concrete is then placed – either directly off the chute from the concrete truck, pumped to the site, or poured from a concrete bucket.
- The wet concrete and forms are inspected to ensure proper placement methods.
Concrete poured, photograph by Peter Otis
- The formwork is left in place until the concrete has had enough time to cure – this takes at least a few days.
- The formwork is stripped away to reveal the smooth surface of set concrete.
- At the time of the pour, test cylinders of the concrete are also cast, allowed to cure, and then crushed to determine the strength of the concrete at any given location.
Concrete foundation walls taking shape, photograph by Peter Otis
The concrete is tested with every placement to verify the structural integrity. In an early stage of construction additional concrete tests were poured for architectural reasons – this was a trial and error process of refining the finish of exposed concrete. A reminder of these early tests is the small structure of columns on the edge of the building site that has been dubbed “mini-Kroon” by Peter Otis, the F&ES Career Development Director and official photographer of the Kroon construction process. As Kroon continues to take shape and emerges from the building site, the real building will soon rise above its “mini” emissary.
The view from “mini-Kroon,” photograph by Peter Otis