I'm No Civil Engineer But....I Don't Think They Are Either
byu/ReesesNightmare inWTF
This looks really weird, but there's a perfectly reasonable explanation. Are they putting soccer balls between rebar to save on concrete? Yes, yes they are. This is common in concrete construction and it's called a biaxial voided slab. Notice that the balls are not near the walls or columns, but in the emptier areas. This not only saves on the amount of concrete used, but the ultimate weight of the floor. It also increases the floor's insulating qualities.
However, this particular project is more colorful than others. Builders more commonly use plastic boxes or styrofoam to made concrete voids. These are most likely not regulation soccer balls, but much cheaper plastic orbs. Still gets the job done. (via reddit)