Never Pour Fresh Concrete Directly Over Old Concrete!

The glue gets the credit. The prep does the work. Grind or shot-blast the old top and soak it the night before so it doesn't drink the water out of the new mix. Then cut your joints right over the old ones, or every old crack shows back up. How thick was this pour?

If this was the standard, we'd have buildings collapsing everyday It only fully takes effect when you're casting a large area of fresh concrete on a large area of set concrete

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I mostly agree, but I’d say “never without prep.” Fresh concrete over old can work if the surface is clean, roughened and properly bonded. Skip that and it delaminates, and water finds its way in between the layers.