Signs of sea salt could spell the end of our Spanish dream
load-bearing and structural element within the property.
Here in the UK it is not unusual for there to be some slight expression of salts that usually prove to be quite harmless.
Typically a sulphate resistant concrete mix would be specified if there are active chemicals in the soil, or where the concrete is to be exposed to salt water. However, this is achieved through the introduction of an additive during the mixing stage and not by any retrospective treatment. Whilst it is common to use sea sand in the construction industry it is always washed thoroughly beforehand. Unfortunately regulations in other parts of Europe are not always adhered to and I suspect that none of this happened with your prospective house when it was built 15 years ago.
Unquestionably salt will weaken the concrete and attack any steel reinforcement within it unless precautions are taken during construction. It is worth having a detailed look to see if there is any staining or bulging cracks in the concrete.
This is caused by the reinforcement bars corroding and, as it does so, expanding. It literally breaks up the concrete from within. If this has started to happen then there is very little that can be done and the building is essentially doomed. I know of no post-construction treatment and suspect the estate agent may at best be ill-informed. Whilst I haven’t seen the property, from your description I suggest you think very carefully before proceeding with the purchase.
Generally, where public sewers are concerned water companies refuse “ build-over” agreements. In fact, my experience is that they tend to say “no” as a matter of course. Then, when pressed, they consider the matter, and may occasionally come down on the side of pragmatism, agreeing to either to building over or a diversion, at your expense of course.
Should they accept a buildover, then the foundations on your extension will have to be designed to avoid any bearing on the drains. This can be achieved by allowing the pipe to pass beneath lintels.
The manhole cover would also have to be changed to a sealed type and a legal agreement signed allowing them access for maintenance if and when needed. This is far from ideal.
Assuming there is sufficient fall and the water board gives approval it would be better to divert the sewer around the extension.
This is done by putting in a new section of sewer with manholes at each junction or change of direction, leaving the obsolete sewer running through the new construction.