A large concrete pour appears to be happening now, although its not the secondary mat. Looks to be very large area just to the east of it.
http://www.vistatowerchicago.com/webcam/
Also, are those concrete pumps arranged in a daisy-chain fashion to send it a very long distance? Looks like the reach of a single machine is insufficient, and some sort of temporary pumping tower is in place where a vehicle can not reach.