How to repair concrete letterbox?

[emckatz]

 hello team! How would I fix the chip in the top right corner of my cement pebble letterbox? I would also love to press pebbles into what ever product I use to fix the letterbox! Any idea would be much appreciated! Thanks!

[EricL]

Hello @emckatz 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your concrete letterbox.

I suggest using Selleys 415g Brick And Concrete Sealant to re-attach the broken piece back onto your letterbox. I'm not sure if you can press pebbles into the gap where the crack has occurred, but you can certainly give it a try with Tuscan Path 2kg 1-2mm Decorative Mixed Pebbles in combination with Selleys 250g Liquid Nails Clear Construction Adhesive. My other suggestion is to spread fine light-coloured sand on the surface of the join to disguise the adhesive line that will occur once the letterbox is re-assembled.

Let me call on our experienced members @Dave-1, @Nailbag and @Noyade for their recommendations.

If you need further assistance, please let us know.

Eric

[emckatz]

Thanks Eric, but I should have added that I no longer have the broken piece, I want to replace the broken off part! Any suggestions on how to do that? 

[EricL]

Hello @emckatz 

This does present a challenge! Theoretically, in order to recreate the broken section, you'll need to build form work made of plywood around the top of the letterbox. Formwork is a temporary mould or structure used to hold wet concrete in place until it sets and gains enough strength to support itself. It shapes the concrete into the desired form such as slabs, beams, or columns and is usually made from timber, plywood, steel, or plastic. Once the concrete hardens, the formwork is removed.

The formwork can be joined together using screws and clamps, it will involve a fair bit of trial and error as you try to get the right shape to put the mortar in. Your cuts must be precise as you don't want any of the mortar dripping onto the finished pebbled surface. 

Before putting the mortar on the surface of the formwork it must be sprayed with WD40 to serve as a release agent. This will prevent the mortar from sticking to the timber formwork. Once the mortar has been put in, it would be a good opportunity to put the pebbles on top. Once the mortar cures you can then remove the formwork carefully and expose the repaired section.

I recommend using Dingo Fast Set Mortar - 3kg for the repairs. The remaining pebbles can be stuck on the front section of the repaired piece using the Selleys clear adhesive. My best advice is to mix the mortar thick to prevent it from turning it into slurry and dripping out of the form work.

Let me call on our experienced members @Dave-1, @Nailbag and @Noyade for their recommendations. 

If you need further assistance, please let us know.

Eric

[Nailbag]

Hi @emckatz 

Reading through @EricL last post, I think thats the way I would tackle it too. Its all about taking your time to make the min-formwork which you may find you need to build in stages.

Nailbag

[Dave-1]

Good Evening @emckatz 

I am in agreement with @Nailbag  with @EricL's suggested way to go forward. I like using melamine (the shiney white board) when I have been making formwork up for concrete repairs as I found it does pop off easily. Forming a concrete proof seal might be a bit hard but you have the existing smooth surfaces to form up around the new piece you will be making. 

Depending on how fragile the existing surface is, maybe drill some small 1/8" nholes into the cracked surface and then before you fill the area insert some thin bullet head nails to act as reo?

Id go very easy on the drilling and judge as you go. That concrete doesnt look strong. So iffy iffy if Id go with the steel pin suggestion.

Dave

[Noyade]

How is the 'roof' attached?

I was thinking concrete sheeting.

[emckatz]

Thank you so much for all your input!! I shall report back on how I go!!

[emckatz]

It looks like a concrete sheet attached with a glue

 

[EricL]

Helo @emckatz 

Thank you for sharing that extra picture of your letterbox. It does appear that you'll need to re-build that piece of concrete onto the existing piece. It's a shame you no longer have the broken piece as it would have expedited the repair process.

Eric