Will PostMix Set Under Water ?

[iDIY]

I have read that 'concrete' will set under water, for example where footings are dug deeper than the top of the water table.

Is this any concrete or must it be a specific type of concrete mix? I have a fence on a sloping garden and the last couple of post holes are down at the water table so the bottom half of the holes fill with water quite quickly.

Can I use normal postfix concrete in these holes or must it be some special type?

 

[JohnD]

How quick is "quite?"

you can use quick-setting mixes.

Sometimes called "prompt cement" or "ciment fondue"

OOI, OPC is specially treated to delay its setting time.

Excess water in the mix weakens concrete, so I'd consider pumping the hole out immediately before your pour (a wet and dry vacuum cleaner will do this) and maybe putting some sort of bag or membrane into the hole. As soon as you pour the mix it will displace the water and expand the bag to tightly fit the hole, because the mix is much denser than water. This will also prevent mud from mixing in and weakening it. It doesn't matter if the bag later rots away.

Another trick for damp ground is to shovel in a dry, lean mix first and tamp it well down, it will absorb moisture and give a clean surface for your main pour.

To prevent mud falling in, put formwork round the top few inches of your hole. This will also make the job look neater.

Wet ground will improve the long-term cure of the concrete, so after a year or so it will be harder and stronger than anything you have done above ground.

Use concrete posts, not wood. When you have dug out a few rotten posts you will understand why.

 

[Mike10]

Since the instructions for postcrete say to half fill the hole with water before pouring it in from the bag I'd say you were probably safe to use that.