we have recently completed an extension on to our house and with the periods of heavy rain in Sydney we end up with water stains on the concrete. The slab was put in place around two existing brick pillars that are off the house that are supporting an upstairs balcony. When we get a day or two of rain the concrete floor around the bottom of the pillars goes dark grey with a water stain. The explanation that we have had from our builder is that the moisture in the pillars (which are part of the existing building) is higher than that of the new slab and as such the water stains on the new slab are from moisture being taken from the pillars. When things dry up the water stains go away.
To me I do not expect to see any water stains on a concrete floor that is visible on the inside of the room. I am reluctant to have the flooring put down whilst there are still stains showing up when it rains. I am not sure if this is expected or whether we have a problem where water is leaking inside of the extension and on to the floor somehow.
If anyone could shed some light on whether this is a common thing or whether we need to have it looked at further it would be a great help.