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Masonry 101: How to Repair A Concrete (Cinder) Block Wall

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As part of my parents' boathouse makeover, they had to tear down a bunch of cinder block walls in order to remove an unpermitted addition.   You can see it was no small feat:

 The demo involved breaking up the blocks and resulted in rough unfinished edges on the blocks they did want to leave standing:

For aesthetic reasons we decided to cap the blocks to give it a smooth finished edge.  The easiest/fastest/cheapest solution to repairing the cinder block was using poured concrete and then painting the entire wall to mask the patch.

To do this, you can't just spread new concrete onto the old blocks like cake frosting.  You won't be able to get a nice flat finish, and it won't adhere very well. 

To attach the new concrete to the end of the wall, you need to use wall ties.  The purpose of these metal straps is to provide a connection between your new concrete and the original wall. The straps help transfer the load and allows for lateral movement (so the new concrete doesn't peel away from the wall with settling or impact.)
Here is an example of wall ties on a wall being constructed from scratch. You can see how they keep the brick layer attached to the cinder block layer of this wall:
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In our case, we needed to add the strapping after the fact, so we nailed them into the cinder blocks using masonry nails:
Next you have to build a concrete form (we used scrap lumber) to give the finished edge of the wall patch its shape:
We attached the form to the damaged end of the wall using a bunch of clamps:
You can see how much of a void there was between the form and the blocks.  This will need to be completely filled with concrete to give you a nice square finished end.
Next up: The concrete mix. For this project we used Ready-to-Use Concrete Mix.
Concrete mix is a mix of sand/gravel/stone with cement.  You just add water and it is ready to go!  It is the ratios of the ingredients that change its strength and drying time. 
Concrete mix is similar to mortar mix (the stuff I used to pour the floor of my shower) but it is made up of sand and cement only. Concrete is is a lot coarser and includes large rocks and gravel. You can use mortar mix to repair masonry, but it is about 4-5x as expensive, and the finish isn't that much nicer. 
The correct ratio of concrete mix to water depends on your conditions, but it is always better to add it little by little, since it is easier to moisten dry mix then try to absorb excess water.  We used a large shallow mixing pan and a garden hoe to mix the concrete:
 The finished consistency should be a lot like cottage cheese. 

For the best adhesion, you should wet down the surfaces which will come into contact with the new cement.  You want it damp not wet.  A hand held mister bottle works great.  Using a trowel we filled the hole with the wet concrete, making sure the top was smooth:

After about 24 hours (a little more since we had high humidity) we removed the form and had a beautiful finished edge to our wall:

If you do have any areas which didn't get concrete, you can go back and patch them with a trowel and a little more concrete mix, one again making sure to wet down your surface before adding the new concrete:
And there you have it.  The under $5 solution for fixing up a cinder block wall!

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