The answer is quite simple. The rolled-up form is initially brought to the construction site. Imagine a larger, heavier version of one of those plug-in full-height air mattresses.The form is made of polyvinyl chloride, or PVC, which is one of the most commonly-used plastics in the world; it's found in everything from medical devices to utility pipes, flooring, and packaging.The form is inflated with an air pump after being laid out on a concrete slab or other foundation. The concrete is then pumped into the form by a ready-mix truck, which can either mix concrete on the way to the site or at the site itself.

https://interestingengineering.com/innovation/startup-builds-inflatable-houses-pumping-concrete-balloon-like-forms

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