Mike Reynolds [0] of "Garbage Warrior" and "Earthship" fame has been reusing empty plastic waste bottles in a similar fashion for decades. The cement forms a matrix around vacancies created by the bottles which become a permanent part of the structure.

He's also used aluminum cans and glass bottles in a similar fashion. Colored glass bottles can produce visual results resembling stained glass, it's interesting.

But these are hand laid like brick laying, so not quite what you're thinking with the ping-pong balls.

[0] https://en.wikipedia.org/wiki/Mike_Reynolds_(architect)

They already do, with mixed results. This[1] is a coverage of a collaps of a parking structure in the city of Eindhoven, the Netherlands. Although it appears the plastic-ball filled concrete was not the cause, but they way they incorporated it into the building was. 1. https://www.newcivilengineer.com/archive/technical-cause-of-...

Two issues: 1) unequal dispersion of the balls could create super weak spots that are mostly empty space. I'm not sure how engineers could use a material where the structural integrity isn't consistent. 2) If water ever penetrated the swiss-cheesyness of the concrete in freezing weather, you'd be in big trouble.

Yeah, this is already a nightmare with rebar in reinforced concrete. Just imagine actual ping pong ball sized voids filling with moisture. Swiss cheese, indeed.

But it wouldn't be holes, it would be water-tight ping-pong balls.

OK, what if you carefully place the balls in a regular pattern?

Imagine placing ping pong balls in a regular pattern in the bottom of an empty swimming pool, then filling it up with water.

You'd have to secure them in place. Perhaps with a metal grid.

Sure it's doable, it just makes the task that much more difficult. Plus you need to ensure there are no unintentional voids from the balls blocking the pour. Code requires that they can't be within 1-2 inches of any steel or the surface of the concrete.

Also, most designs assume a uniform cross section, and changing that greatly complicates the design method.

And remember that the laborers doing this don't always understand (or care) about these sort of restrictions.

It's usually cheaper and less risky to just use a lightweight mix, with injected air, if needed.

It is rather common for slabs to be made lighter by embedding in the concrete pipes (usually either plastic or metal/tin) or actual "pockets" (think of large - usually plastic - largish and squarish containers). Here is a photo of a commercial solution:

https://www.geoplastglobal.com/en/products/slabs/new-nautilu...

There is also an as well already available commercial solution using "balls":

https://www.cobiax.com/intl/produkte/cobiax-el/

similar to the "bubbledeck" technology used in Eindhoven:

https://en.wikipedia.org/wiki/Voided_biaxial_slab

Anyway these are basically an evolution (involution?) of "waffle slabs":

https://en.wikipedia.org/wiki/Waffle_slab

Thinking bit more generically, you might be really thinking about concrete with air pockets (though ensuring they're spherical might make for excellent load distribution through the side walls). There are already concrete variants called Aerated Concrete and Foam Concrete where bubbles are formed inside the concrete, resulting in a lighter product, though with different structural properties.

https://en.wikipedia.org/wiki/Autoclaved_aerated_concrete https://en.wikipedia.org/wiki/Foam_concrete