Concrete mix ratios

[McDaddy1815]

Hi all
Apologies if this has been answered already, I haven't found it if it has. Also apologies if this is in the wrong place, am new to the forum.

Im after some advice from the experts on good ratios to create an easily workable and most importantly smooth concrete mix. Background wise, I will be filling wooden forms for cast in place concrete pavers. Say around 1m2 each. Approx 50mm thick. Will be finishing the pavers with white colour hardener. Will be using ordinary cement, sharp sand, and I have a million tons of 10mm pea shingle, so that will be the course aggregate. Was also going to add some fibres to minimise shrinking.

Would like to ultimately achieve as smooth a finish as possible, otherwise would use a 1-2-3. I have been recommended a 2-1-1 mix (cement-sand-stone), how does this sound? They are literally just stepping stones, so will take no weight. Also, I am by no means a pro so needs to be fairly easy to work.

Any help would be appreciated, as I get a different answer whoever I ask!

[Brucieboy]

The generally accepted strongest mix ratio for concrete is 1:3 (1 cement : 3 combined aggregate). For your case, I would suggest a maximum 1:1.5:1.5 (1 of CEM I, not General Purpose CEM II: 1.5 of sharp sand: 1.5 of 10mm aggregate) all batched accurately by volume. Add as you plan the polypropylene fibres, but to provide better workability, also use a concrete superplasticiser (aka high range water reducing admixture) such as Sika Viscocrete. Adding a superplasticiser at the manufacturers recommended dosage rate will be far better than using lots of water to achieve the required workability thus resulting in improved durability of the finished slab. Depending on the sharpness of the sand and the finish you require, you may need to tweak the ratio of sand to 10mm aggregate slightly.

For a 1m2 slab, 50mm thick might be a bit on the low side, preferably go 75mm. It’s also essential to provide a smooth, well compacted base using Type 1 or similar to minimise any movement even though it’s for light use – presumably you want them to last!! It may also pay to cut some polythene sheeting and place on the compacted base prior to laying the concrete – this will act as a slip membrane and further minimise shrinkage by eliminating restraint. Finally, cure the slabs with polythene sheeting once finished – raise off slightly to prevent marring of the surface if necessary.

I always recommend that a trial mix is carried out first so you become familiar with the mix and its handling / finishing properties. If you feel the mix is too rich, i.e. too “sticky�, try a 1:2:2. Good luck.

[McDaddy1815]

Thanks so much for the detailed info

I had wondered about the depth, so will indeed increase to 75mm as you advise.

Last question - do I need to worry about an additive to frost proof the slabs? I haven't before, but this will be my first where finish is important

Thanks again

[Brucieboy]

At this cement content, an air entraining admixture (used to protect the hardened concrete from frost) is not normally required providing you use a superplasticiser (and fibres) as suggested above.

[McDaddy1815]

Great thanks so much for your help