Expert: Dan Griffin - 10/7/2011

Question
Dan,
I need to pour a 18" x 10' x 42' pad for a machine. Total weight of machine is 44,000 lbs. What rebar and spacing do you recommend?
What strength concrete do you recommend?

Answer
Chris, I had two of these things to pour back when.  They were designed by the Japanese company that was providing the machines.  They were quite a bit larger than yours and they were 4' thick.  They had no reinforcement, the concrete was there to provide mass and stability.  They did call for dovetailed anchorages for the mounting points.  I was fortunate to run into another fella who suggested using upside down flower pots to create the shape.  I believe we used 3000# concrete.  The slab the next morning was so hot that you couldn't walk on it for more than few minutes.  This was inside a working manufacturing plant, so we could not flood cool the slab, but did keep it covered with wet burlap for days

I would expect that the machine company would have some minimum recommendations.  I would discuss the heat issue with our concrete provider.  If I were trying to give seat of the pants reinforcement advice I would put in one mat in the bottom 1/3 and another mat in the top 1/3.  I would use #6 bar at about 16" cneters.  If there is a way to discuss this with the machine people, ask there opinion.  I suspect more depth and more mass rather than reinforcement will be the intent.