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A couple quick points here:
In the U.S., the term 'Concrete' is used to define a mixture of portland cement and one or more aggregates. So Roverhi is correct in using the term if he is in the States, but as Wheels points out it is not the your normal concrete. Ferrocement boats generally use a very low slump 'high-early' portland and with a very carefully sized sand aggregate.
I think that Roverhi's point is quite valid though. It is extremely difficult to get a secondary bond with cement. The diffiulty in getting a secondary bond between even 'green ' cured cement and frresh cement is prime reason that a properly built ferrocement boat must be laid up all at one time.
In one of the earlier discussions on this Forum someone had posted a link to a website that had a technical paper on the strength of secondary bonds in ferrocement. There was a very significant decrease in strength across the cold joint. To the point that the author discouraged repairs of ferrocement with portland cement. The recommendation was to use a reinforced epoxy/fiberglass to make the repair with a larger contact area (taper) than would be normal in a fiberglass repair.
Respectfully,
Jeff
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