Cement mix using additional pea gravel

[iwanttodiy]

Hi guys

I am planning to use 1:6 cement to ballast for a concrete sub base for a patio floor.

As I have lots of pea gravel left over, does anyone have any recommendations for throwing it into the mix? Maybe 1:1:6 (cement: pea gravel:ballast)

 

[SFK]

IwanttoDIY,
I am not going to advise on usage of the pea gravel (although as a DIYer I would use it), but wanted to highlight that your maths is wrong.

You need to keep the cement ratio the same. So if you are wanting to keep the 1:6 ratio when you use the pea gravel, the pea gravel should be replacing the Ballast, not weakening the cement mix.

So it should be a ratio 1:1:5 (cement: pea gravel : ballast).
SFK

 

[SFK]

Also, what you are calling "The Sub base layer" is normally compacted, hardcore and is cement free.

The 1:6 cement mix layer you are talking about is used in the "Concrete Layer" not what you call the "Sub Base Layer".

https://www.marshalls.co.uk/homeowners/inspire-me/articles/lay-paving-slabs-concrete-6022

SFK

 

[iwanttodiy]

Thanks that makes sense, I guess I just took the advice from the builders merchants when they said throw in a shovel load per mix

I guess as a worst case, if I still have loads of pea gravel left I can get few bags of sand and do a 1:2:4 mix (cement: sand : pea gravel). I guess my concern is I haven’t seen many mixes using pea gravel, and it is usually standard gravel.

I guess, stone is stone right?

 

[SFK]

Iwanttodiy,
I would be careful with: "Using pea gravel I can get few bags of sand and do a 1:2pea:4sand mix".

"Ballast" is designed so it has a continuous mix of gravel sizes from sand to small rocks so that when mixed with Cement every void is filled with rock. This makes for all the stones to lock together to make a strong mix.

Mixing only two gravel sixes together makes for a weaker mix.
And can leave sharp single stones that could protrude and break a paving stone.
So only Pea Gravel and Sand is a bad idea.

If you have a lot of pea gravel I would say start with a 1:2:4 (cement: pea gravel : ballast) mix, noting that I said ballast not sand, as this will go in the hole first. And when Pea Gravel all used up move to the 1:6 (cement: ballast) mix that will then go on top.

If you do use sand under patio, note that it is never "builders sand" (which is for mortar between bricks) but must always be "sharp sand" as this again locks together better.


Note that this instruction link shows the method I think you are using:
https://www.bradstone.com/advice/laying-guides/how-to-lay-a-patio/
Which is a Foundation bed of 1Cement:6Balast and then a mortar bed of 1Cement:6SharpSand.

SFK

 

[^woody^]

Using pea gravel may end up with a mushy slab.

 

[SFK]

Woody,
To improve my knowledge, why mushy?
Because Pea Gravel is smooth and regular shaped, or because it is changing the Ballast mix, or other?


Dooh...just got the joke.
SFK

 

[John D v2.0]

Personally i wouldn't use it if the mix strength was important, as sand for cement mixes is usually rough to promote interlocking, and the cement binds it together. Put some rounded aggregate in and it relies more on the cement therefore it is weaker.
However since you're basically just filling the space and spreading the load idI probably risk it. After all, even a wall is just concrete with bricks as large aggregate, laid individually

 

[Andehh]

Using pea gravel may end up with a mushy slab.

Woody,
To improve my knowledge, why mushy?
SFK

You will get the joke any second now....and will laugh to yourself!


Very good!

 

[SFK]

Andehh,
Thank you for the nudge - got it, sighed, laughed and then kicked myself for missing it.

SFK

 

[iwanttodiy]

Thanks, I like the idea of a 1:2:4 (cement : pea gravel : ballast) mix.

Iwanttodiy,
Note that this instruction link shows the method I think you are using:
https://www.bradstone.com/advice/laying-guides/how-to-lay-a-patio/
Which is a Foundation bed of 1Cement:6Balast and then a mortar bed of 1Cement:6SharpSand.

SFK

Thanks, yes this is the approach, it’s similar to what I found on the marshal site for laying marshal flags. Except marshal mentions to use a primer on the flags as well.

Sounds like a plan, saves wasting the pea gravel

 

[tomfe]

You'll be fine with pea gravel in the mix, it will move and level easier as the aggregate is smaller. It will be a weaker mix but as it's a base for a patio it'll be fine.

 

[iwanttodiy]

Did someone come by and give me a top up of pea gravel

Just found an extra 1/4 of a jumbo bag that suddenly appeared by magic ;P