Cement recipe for bridge footings

[dornfield]

Hi. What ratio of Portland Cement/ sharp sand/ aggregate would you recommend for building up existing concrete footings of a small bridge? 3m span. The river water has 'undermined' the exposed edges of the footings. Not so much 'undermined' as 'side scoured'. So some of the concrete will be below water level, and need to cure underwater, whereas some will be above water level.
Is there a 'one size fits all recipe that I should use?
Thanks.
Brian

 

[Scarlet Pimpernel]

Concrete will cure underwater, but it takes much longer. Maybe up to a month.
You need to protect it from any scouring action while it cures otherwise it'll simply wash away while wet.

 

[dornfield]

Yes, I though of that. I think I will try to build a wooden frame to contain the concrete, with the possible inclusion of galvanised butterfly ties to allow me to then build and tie in a more attractive 'natural stone facade. It is quite a fast flowing stream, and when it rains very heavily, it can become frighteningly swollen. So 1, 2, 4 cement sand aggregate?

 

[JohnD]

Concrete will cure underwater, but it takes much longer.

it will cure underwater, and does not take longer.

You might be able to protect it from being washed away by enclosing it in shuttering; or you can use "prompt cement" (quick setting cement, ciment fondu) which can be used in wet conditions because it sets almost immediately.

Because concrete mix is heavier than water, you can shovel it into shuttering through the open top and it will displace the water, but the mix will contain excessive water, which weakens it. It will tend to rise to the top so you can skim off the slurry or "laitance". After skimming it off you can add more of the proper mix. Fill the shuttering to the maximum possible depth, and above water level, to prevent water ingress through joints. The heavier concrete mix will try to push out through any cracks.

You might be able to use a pump or wet-and-dry vac to get the water out of your shuttering before you start to pour.

You can use wooden, or even chipboard, shuttering, which will eventually rot away underwater, but the concrete will be cured long before.