My concrete seawall (on a freshwater lake) has a void in a bottom corner. It seems to have been created with the void -- it's not a result of trapped water freezing and pushing out a chunk. I'd like to fill the void to avoid future possible erosion. Multiple questions:

  • Should I fill it while it's warm and underwater (summer/fall), or dry but cold (winter)?
  • What should I fill it with?
  • what is a seawall? (It is a name that is only used in the USA?)
    – Walker
    Jul 22 '10 at 9:25
  • This is a seawall (construction methods vary): bbc.co.uk/essex/content/images/2007/08/06/…
    – tnorthcutt
    Aug 23 '10 at 3:37
  • You should add your solution as an answer, not an edit to the question. Future visitors might find this information useful, and it's easier to understand it as an answer, as opposed to a part of the question.
    – Tester101
    Dec 6 '13 at 16:59
  • Please accept your answer or one of the others so the question can be resolved.
    – isherwood
    Feb 9 '17 at 17:15

Almost all cementitious products cure better, stronger and harder with a steady presence of [fresh] water. As far as placing the product, whichever you choose, in the presence of water, that will depend on specifics that you have not listed. I will say that when large construction companies are pouring underwater footings, say for bridges, they will have the form in place and filled with a heavier than water gel that gets displaced by the concrete as it is poured in/injected. If the water is intermittent or tidal you can use a simple fast setting concrete, or a normal concrete with an accelerant additive. If it is saltwater you can use additives suited to saltwater. Your local concrete store will be able to direct you in the right direction as far as additives go. Be careful about what kind of reinforcement you use - many types of reinforcement are very susceptible to sulfates in the water and will cause spalling if placed too close to the outer edge of the concrete.

With regard to your initial description, I would say that most concrete pours end up with a void. As long as no water is penetrating through the void in your wall you likely have no concerns in re.: future erosion. Even if water were coming through this void the concrete has likely formed a crystalline structure that is resistant to "erosion," however if water is coming through then there are many cementitious products available for purchase that will solve this problem. I've used Xypex and at least one other for weeping foundation walls. Insofar as patching the void is concerned I've had good luck with a product called Thoropatch.

Best of luck in your endeavor, even if I'm not sure what said endeavor is...



I believe there are cement products that will cure in water, however I would wait until the winter. Cold is not a problem here, except for your fingers. And you can always find a day in the middle of winter when the temperature is quite moderate.

Find a high strength patch filler. Make sure that it attaches solidly to the existing hole. If possible, I would drill some holes into the sides of this hole, then put some non-rusting (brass, aluminum, etc.) bar stock into the hole so that it sticks out into the void area. The idea is to put a set of fingers into the cement patch area to prevent this patch from popping out.

  • Why should it be done in the winter? Sep 19 '12 at 17:20
  • 1
    @ChrisCudmore - because cement products usually work better when they are not asked to set underwater, especially if there is wave action involved. Yes, you can get products that will still set underwater, but they will be more expensive and may require some knowledge for their use.
    – user558
    Sep 19 '12 at 23:52

Note: What I actually did: Found a relatively warm winter day (the water line was below the void in wintertime), troweled in cement, built a form around the void (it was a corner so I was able to stake the form next to the void while leaving enough space to top off the cement and then push the form right up flush to the void). I went with a bonding cement because the space was deep but not so high and I thought that using stone would prevent me from packing it in tightly. I could have done this submerged, it just would have been a lot colder and wetter.

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