I've moved into a recent conversion (completed 1 year ago) and I suspect the contractors have skimped in a number of inportant areas.
Example- they've concreted the yard outside the back door and inserted a new man-hole sewage cover and the surface is becoming increasingly pitted with holes 1-2inches across in places and up to 1inch deep. The edging has broken down around the man-hole cover which is now loose and a tripping hazzard. If I put a stick into the developing holes and twiddle it around I get sand and bit's of gravel coming out and the area never properly dries out (the yard never get's direct sun) and it's proving difficult to maintain. Whenever I attempt to sweep up I get a load of sand and grit. It's looking like a Moonscape.
Any suggestions as to why the concrete is degrading like this? I think they didn't use the right mixture in an attempt to save on the concrete and/or they simply didn't know what they were doing. You should see the rest of the place!
I think that you have answered your own question. Sand is cheaper than cement, and I suspect that the builders have simply used a weak mix to save money. Concrete gets it's strength from having disimilar sized stones and chippings. Without theses stones is is simply cement mortar, which will wear away if used as a path.
Adding too much water can also weaken the strength of concrete, but I have seen it very runny and still set to a very hard finish.
The other possibility is that the concrete was laid when it was frosty. If the water in the concrete freezes before the mix sets, then even the strongest of mixes will crumble.
Unfortunately it sounds like it will need to be taken up (not too difficult if it is as weak as you say), and relaid.
If your home is only one year old, this may be covered by the NHBC quarantee, but they probably have a clause that exempts paying out for this.
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