This 1925 cover image depicts a concrete mixing plant (one of three) for a vital project in downtown Chicago, in which South Water Street was transformed into Wacker Drive, a major multilevel street running along the Chicago River in the Loop.

The project entailed 300,000 cu yd of excavation and 120,000 cu yd of concrete placement. To achieve the desired quality of concrete the uncommon practice of sand inundation was employed.

This eliminated the uncertainty of using sand with unknown and varying moisture content, speeded up the hydration of the cement and also gave a mixture that flowed more freely with a smaller amount of water.

The mechanical inundator was a steel drum of fixed volume suspended under the sand bin. The drum was partly filled with water.

Then sand was gated into the drum until it was filled to the top, with any surplus water overflowing. The drumful of saturated sand was then rotated and dumped into the mixer hopper.

Additionally, proportioning by fineness modulus and water-cement ratio helped ensure concrete quality. Three crews of 19 men operated each outfit. Eleven men handled the cement from stock, loading the conveyor and emptying the 94-lb bags into the cement charging hopper.

One man operated the inundator, one the stone batcher, one the mixer, three worked the concrete conveyor, one chuted the concrete into the carts and there was one engineer.

Distribution was done by hand carts operating on runways.