Pre-mixed cement or make your own?

[BearTricks]

Hi,

I'm making some concrete planters and dishes from moulds to put various plants in. I've made some moulds, but I mixed some concrete from cements and sand, let it cure a few days, took it out the moulds and it's just cracked and crumbled.

I checked my cement and it's way past its sell by date. My brain must be going.

Anyway, I need some more. I already have sharp sand but I'm wondering if it's worth just buying pre-mixed cement. It's just for concrete plant pots and containers and the like. I saw some at the garden centre a while ago that were a nice, modern mix of wood and concrete but they cost £20 - £30 for a little thing that I could knock together in an afternoon, not including waiting for the concrete to cure.

Is the pre-mixed stuff from the likes of Wickes much better for a small, non-weight bearing project like this, or am I just better off mixing to my own ratios? If so, what kind of ratio would work best and do I need to add anything else like plasticisers or hydralime? I have no idea about concrete or what any of these things are, and funnily enough there's not much information online because all the bloggers doing stuff like this seem to use an American product called Quickcrete. I have no idea what a UK equivalent might be.

As usual, any help would be appreciated. I'm also using the concrete to make some coasters because the set we have is nowhere near big enough for the builders mugs we drink our coffee out of.

Edit: I should say that it's Mastercrete cement that's in a waterproof bag, but it has been under shelter but outside possibly for the winter so I don't know if the waterproof bag will prevent it going off. There are a couple of the usual holes in the bag that water could have got in through, but in terms of consistency it doesn't seem any different from any of the cement I've used before.

 

[AJB Temple]

I am not a cement expert. Cement has a short shelf life, but I think that is to do with the anti irritants that it contains. It's hydroscopic and so will absorb water unless it is perfectly dry, so I would not use a bag that is much past its sell by date and certainly not older than 6 months, for anything important.

I make a lot of concrete as I am doing a fair amount of building work. Over the years the biggest mistakes I have seen people make are: 1) using old cement 2) not getting the quantities (usually printed on the back of the bag) anywhere near right and 3) not mixing properly.

Not mixing properly is my number one bugbear when I am helping people to do concreting jobs. It takes much longer than people think to get a good mix - including in a proper mixer. I never use ready mix stuff, but I always have sharp and soft sand and some aggregates to hand, as I buy in bulk bags. I have my own mixer and this is far better than mixing by hand. Cement I try to buy fresh for each job. Older stuff might go into dry mix for paths or get used up in small amounts with fresh cement when making concrete.

 

[finneyb]

I would have thought the commercial planters have surfactant in the water - use a dash of washing up liquid in the water. It will allow a lower water to cement ratio to get the same workability and make a stronger concrete.

They have external vibrators on the mould. Getting enough compaction of the wet concrete will be a big problem without it with such narrow sections - which may be your problem . You could place the wet concrete in the mould in layers and compact each layer thoroughly before adding the next. I would use round sand rather than sharp - easier to compact.

HTH

Brian

 

[ColeyS1]

My old place of work use to have a concrete department. I'd have to machine mouldings and cut stop ends for them to make pier caps and other fancy things.
The only time a surface was left smooth,straight from a trowel,was for aga and headstone bases.
Most things had a textured finish, the way to achieve it was to sponge it when it was still slightly wet, usually the following morning. On surfaces that couldn't be sponged fairly quickly due to it needing more time to dry, they'd brush on a retarder which meant the concrete that was up against the mould took longer to cure. A hose pipe a few days later washed off enough to make it look textured.
Sponging usually filled up any small holes from air pockets, might be worth a try.

Sent from my SM-G900F using Tapatalk

 

[AJB Temple]

The vibration thing from Brian is a good point. That could be achieved by putting the mould on a board and then vibrating the board with a compactor if you have one.

 

[Woody2Shoes]

A cheap way to vibrate a small concrete mould is to put a hammer drill on hammer only without a bit in the chuck.

If it were me, I'd prefer to mix it myself - but really just depends on quantities - main reasons being I get to choose the type/size of aggregate and how "strong" to make up the batch. I think that the pre-mixed bags often go off faster than the stuff you mix yourself - useful for setting gateposts, but perhaps not useful for other things!

Cheers, W2S

 

[BearTricks]

So I took everything out of its mould in preparation to throw it away. Most of the things just cracked and crumbled, apart from the one that was going to be my 'test' piece before I got carried away.

I basically half filled a yogurt pot with concrete and then pushed a smaller yogurt pot filled with sand for weight in to it. It was supposed to just look like a plantpot and would have done if the sides hadn't fallen apart. The base, however, was thicker than I thought it would be because I didn't push the inside down far enough. Whereas everything else crumbled like dirt, the base of this is rock hard, and more grey than the other attempts. Right at the very bottom in fact it's quite dark grey, smooth as glass and hard as anything. I wonder if I used too much sand in everything else, but under-mixed the concrete in the bottom of this pot, getting the right consistency by accident. It was just a tester to see if I was on the right track with mixtures though.

I have heard the washing up liquid thing before. I might have another go with this cement tomorrow before I get a new bag.

 

[n0legs]

Don't put washing up liquid in concrete or mortar mixes it's bad for it. Use plasticiser.

 

[skipdiver]

If you are just using sand and cement, then it is not concrete. You need an aggregate as well. Concrete is made from mixed ballast and cement and does not need plasticiser in my experience. I think modern cements contain plasticisers anyway. A general mix for concrete is 6 parts mixed ballast to one part cement, mixed with enough water to bind it and as stated earlier, it needs to be mixed thoroughly, something which is not easy in large quantities, but should be do-able in smaller batches. It also takes quite a while to cure, so don't rush it. Vibration to remove air pockets is also a good idea, which is why concrete floors for example are mechanically done with vibrating rods, or more simply by just tamping with a large batten. If you are concreting in moulds, then placing an orbital sander minus the paper on the mould for a few minutes will do the job. You may also want to think about putting some reinforcing bars in larger pieces.

Going back to the earlier point about aggregates. This is usually flint chippings if you buy it from a merchants but you would probably be better mixing your own using sharp sand and adding smaller pea gravel if it is for stuff in moulds.

 

[skelph]

+1 for what n0legs said

Don't put washing up liquid in concrete or mortar mixes it's bad for it. Use plasticiser.

Many years ago a bunch of cowboys built a new bungalow in our area and cleared the local shop of washing up liquid for using in mortar and render mixes. A year later my family business was asked to sort out the problem of crumbling mortar and render. We could remove pointing/mortar by running a finger tip along the joints and leave "claw marks" in the rendering by simply pulling fingers across the surface. The rendering was easily stripped off using rotary wire brushes and we then had to seal the blockwork before re-coating the walls. It cost the house owner a fortune as the cowboys had disappeared leaving him to foot the bill himself.

Back then we were able to buy sachets of plasticiser (I think it was Febplast) so that our guys were able to use the correct amount of plasticiser for each batch of mortar the made up - 1 sachet per bag of cement.

 

[finneyb]

+1 for what n0legs said

Don't put washing up liquid in concrete or mortar mixes it's bad for it. Use plasticiser.

Many years ago a bunch of cowboys built a new bungalow in our area and cleared the local shop of washing up liquid for using in mortar and render mixes. A year later my family business was asked to sort out the problem of crumbling mortar and render. We could remove pointing/mortar by running a finger tip along the joints and leave "claw marks" in the rendering by simply pulling fingers across the surface. The rendering was easily stripped off using rotary wire brushes and we then had to seal the blockwork before re-coating the walls. It cost the house owner a fortune as the cowboys had disappeared leaving him to foot the bill himself.

Back then we were able to buy sachets of plasticiser (I think it was Febplast) so that our guys were able to use the correct amount of plasticiser for each batch of mortar the made up - 1 sachet per bag of cement.

That sounds like too little cement - cement being the most expensive part of the mix.
A dash of washing up liquid in the water will be fine for garden planters - if it was bridge beams maybe not washing up liquid.

Brian

 

[BearTricks]

+1 for what n0legs said

Don't put washing up liquid in concrete or mortar mixes it's bad for it. Use plasticiser.

Many years ago a bunch of cowboys built a new bungalow in our area and cleared the local shop of washing up liquid for using in mortar and render mixes. A year later my family business was asked to sort out the problem of crumbling mortar and render. We could remove pointing/mortar by running a finger tip along the joints and leave "claw marks" in the rendering by simply pulling fingers across the surface. The rendering was easily stripped off using rotary wire brushes and we then had to seal the blockwork before re-coating the walls. It cost the house owner a fortune as the cowboys had disappeared leaving him to foot the bill himself.

Back then we were able to buy sachets of plasticiser (I think it was Febplast) so that our guys were able to use the correct amount of plasticiser for each batch of mortar the made up - 1 sachet per bag of cement.

That sounds like too little cement - cement being the most expensive part of the mix.
A dash of washing up liquid in the water will be fine for garden planters - if it was bridge beams maybe not washing up liquid.

Brian

Judging by the fact that the one pot I made was rock hard and dark grey at the base, and crumblier and sandier the further up it got, I'd wager that I mixed too much sand in and mixed it badly so the cement at the bottom wasn't properly incorporated.

I've tried again and given it a thorough mix. If that doesn't work I'll chalk it up to bad cement and get some new stuff. I've also used washing up liquid because they're mostly for pots to put cacti in. I'm not losing anything by giving it a go. I also vibrated it with a hammer drill, which seems to have worked a treat. The other option is that we're having a massive clothes clear out and I'm washing everything before we sell it or give it away so I was considering putting them on top of the washing machine when it's rattling about on full whack.

 

[skipdiver]

The chemicals in washing up liquid, especially the biological kind, weakens cement and should not be used ever. Use a proper placticiser, like Febmix.

Having said that, i can't see the point of putting it in concrete; i never have. It's more suited to mortar or render, and makes it more pliable and easier to work with.