DIY Concrete basin, problems with crumbling

[Frogstar]

Hi All

I have made myself a DIY concrete basin/sink. I used a simple cement premix. I have a problem in that after the curing for 48 hours the surface of the basin is weak and is crumbling in areas exposing the aggregate. Some of the areas are fine.

Im trying to figure out what I did wrong, perhaps the mix was too wet? or not mixed properly? Would like to give it another try

An advice will be appreciated. Picture attached.

Thanks
Ryan

 

[Retired]

Hi,

Welcome to the forum Ryan.

What did you use for the former? The former needs to be as smooth as possible and waterproof also it will need treating with some kind of release like diesel.

You say you used a premix; was this premix mortar or concrete. Adding too much water will cause problems resulting in weakness.

https://www.everything-about-concre... slabs, footings, steps, and foundation walls.

If you try again which I suggest you do; use your own mix based on the information at the link included then you've got full control of the mix and take care to get the ratio right. More pictures would be useful as to how you are doing the job and what you're using for the former etc. In order to expel air pockets tamp the mix down to compress it and it pays to tap the former with a hammer; hard enough to expel air pockets but not so hard as to damage the former; I've used polythene sheeting to ensure no sticking to formers simply stapling the sheeting to something like plywood; it gives a decent smooth finish but if you're new to this you'll improve with a bit of practice.

Just one example shown above; I hope it helps and good luck. Don't ever give up.

Kind regards, Colin.

 

[Frogstar]

HI Colin

Thanks so much for the reply. Appreciate it. My form is made of melamine board that i coated with epoxy, I did treat it with a petroleum jelly/turpentine mixture recipe i found online. It seamed to release well.

The pre mix was a dry concrete/aggregate mix. I will try your suggestion to make my own mix for sure. Although looking at the link and the video you posted mine was far too wet. Parts of my basin are solid and smooth but the parts as pictured the surface is crumbling, so too wet or poor quality mix or I didn't mix properly to start with?

 

[Retired]

Hi,

You're most welcome. I'm not an expert on concreting but have done a bit over the years so I'm happy to pass on the bit of knowledge I have.

If you mix by hand using a shovel mix the sand; aggregate and cement well before adding water then only add enough water making the mix useable; too much water makes the mix run and will result in weak concrete also be careful in winter to avoid it freezing. It's surprising how only a cupful of water changes the mix characteristics.

You could experiment with smaller items first then you won't be wasting materials and energy. Your former/mould sounds to be constructed OK and if it released OK then you're making a lot of progress. Mix it well with enough water and tamp it down; you will get better with a bit more practice.

Kind regards, Colin.

 

[Silentrunning]

In addition: most of the problems causing weak concrete are due to letting the concrete dry instead of cure. I make concrete pots and slabs for my bonsai. Once they have set up I put them in garbage bags with some water to keep them very moist. If they are very delicate I will submerge them completely in a tub for a couple of weeks. The slower concrete dries the stronger it will cure.

 

[Frogstar]

Hi,

You're most welcome. I'm not an expert on concreting but have done a bit over the years so I'm happy to pass on the bit of knowledge I have.

If you mix by hand using a shovel mix the sand; aggregate and cement well before adding water then only add enough water making the mix useable; too much water makes the mix run and will result in weak concrete also be careful in winter to avoid it freezing. It's surprising how only a cupful of water changes the mix characteristics.

You could experiment with smaller items first then you won't be wasting materials and energy. Your former/mould sounds to be constructed OK and if it released OK then you're making a lot of progress. Mix it well with enough water and tamp it down; you will get better with a bit more practice.

Kind regards, Colin.

Thank you Colin Appreciate the adivce

 

[Frogstar]

In addition: most of the problems causing weak concrete are due to letting the concrete dry instead of cure. I make concrete pots and slabs for my bonsai. Once they have set up I put them in garbage bags with some water to keep them very moist. If they are very delicate I will submerge them completely in a tub for a couple of weeks. The slower concrete dries the stronger it will cure.

Thanks very much for the tip. I will definitely add this to the list of to-dos.

 

[piglet11]

Might be a mixing problem even if it is premixed.
As Doug says, slow curing is the best way. When they built the Hoover Dam, they had to run cooled water in pipes through the concrete to slow the cure.
Also, getting air out is important. That's why you see vibrating pokers being used on large concrete pours.
Some bridge structures are impressively smooth when formwork is removed. On others, the fill is so good, you can see the wood formwork grains, used as a decorative feature.
Repairing is very difficult. Things like kerbstones are often repaired with a concrete resin if a corner gets knocked off.
Yes Colin, that extra bit of water can sure mess things up. Applies equally to Bisto gravy.

 

[CACJL]

Hi All

I have made myself a DIY concrete basin/sink. I used a simple cement premix. I have a problem in that after the curing for 48 hours the surface of the basin is weak and is crumbling in areas exposing the aggregate. Some of the areas are fine.

Im trying to figure out what I did wrong, perhaps the mix was too wet? or not mixed properly? Would like to give it another try

An advice will be appreciated. Picture attached.

Thanks
Ryan

It sounds like the mix is to week and the wrong type of sand, you need to use a three parts sharp sand to one part cement with the addition of waterproof additive, which is available from builders merchants.