cement mix pier base?

[iwols]

hi guys weather permitting im going  to start the base for my shed/obsy today and the base for my pier just wondered what mix is best for the pier base,was going to make it  about 18" square with some rods knocked into for support cheers

[spillage]

I'm sure I used 3:1 for mine. I also used a petrol auger to drill out the bottom of all four corners and a central one (for the pier to sit in). This meant I did not have to go too deep and kept down the amount of concrete I used.

[coatesg]

I used 3:1 too. How deep it is going to be will depend on what it's going into (sub soil) and how much is going on top. My pier base is about 15" square and 3' deep with reinforcing rods.

Make sure the pier bolts miss the rods if you are drilling them in after (using expanding rawlbolts or similar)!!

[iwols]

thanks its going into clay with a pier and ed80 heq5 and camera filters ect,did you put in any of that small aggregate or just sand(grit) and cement thankks,maybe i could get away with  something smaller than 18" square?

[Davey-T]

12 minutes ago, iwols said:

thanks its going into clay with a pier and ed80 heq5 and camera filters ect,did you put in any of that small aggregate or just sand(grit) and cement thankks,maybe i could get away with  something smaller than 18" square?

You need concrete with aggregate in not sand / cement, bags of what's called ballast ,wouldn't go smaller than 18"

Dave

[iwols]

1 hour ago, Davey-T said:

You need concrete with aggregate in not sand / cement, bags of what's called ballast ,wouldn't go smaller than 18"

Dave

was thinking 3 sand ,2 ballast and 2 cement?

[spillage]

If you are going into clay I would go at least 1m deep as the deeper you go the more stable it becomes. I think above 900 clay is susceptible to movement due to changes in moisture.

My ground is a bit peaty so needed to go deeper. Thats why I went down about 500mm then an extra 1000m with the corners using the auger.

 

unless you have sand and ballast lying around already I would just buy some bags/dumpy bag of ballast.

[Davey-T]

Mines in clay in 18" square of concrete but I also have a 2 foot square base on the pier with bolts at each corner for extra stability, most piers only come with a small base.

Dave

[pete_l]

My HEQ5 + ED80 imaging rig stands on a 1m² base about 10 cm deep. Under that is a butyl sheet to act as a damp barrier, then earth. The mix was roughly 4:1 small aggregate to cement.

There is no need for anything more, it's rock solid

[coatesg]

2 hours ago, pete_l said:

My HEQ5 + ED80 imaging rig stands on a 1m² base about 10 cm deep. Under that is a butyl sheet to act as a damp barrier, then earth. The mix was roughly 4:1 small aggregate to cement.

There is no need for anything more, it's rock solid

If the clay is anything like that at my parents, a 10cm thick concrete base plonked on top will move around quite happily with changing seasons/moisture - the amount of heave is quite impressive!

[pete_l]

23 minutes ago, coatesg said:

If the clay is anything like that at my parents, a 10cm thick concrete base plonked on top will move around quite happily with changing seasons/moisture - the amount of heave is quite impressive!

I lived for 25 years in the S.E. (England) with a clay subsoil garden. I never had any problems with heave. And that was with a 20m² patio made simply from pavers on top of a "wacked" sand base.

[spillage]

My thoughts are that I would rather over do it then find out my pier suffers heave and I have to break up all that concrete and start again.

[coatesg]

Yep - it going to depend a lot on how the soil behaves, whether there are trees nearby, etc. Their patio, slabs concreted on 3" base has moved around 2-3" up and down. 

I'd prefer to overdo it than do it twice!

 

[25585]

Postcrete?

[tooth_dr]

23 minutes ago, 25585 said:

Postcrete?

I used postcrete to top up my concrete pier. It sets too fast for pouring a base you would have no working time at all. 

[bottletopburly]

Mine was 600 sq, 900 deep with five lengths of rebar 1,4m long driven down into the clay 

 

[laser_jock99]

1 hour ago, bottletopburly said:

Mine was 600 sq, 900 deep with five lengths of rebar 1,4m long driven down into the clay 

 

Nice hole- I struggled to make one at all......

I belive my foundations and pier are pretty stable though!

[Swoop1]

On 15/01/2018 at 19:19, bottletopburly said:

Mine was 600 sq, 900 deep with five lengths of rebar 1,4m long driven down into the clay 

 

I don't know what is more impressive- the neatness of the hole, the symetry of the rebar placing or the slippers?

 

[bottletopburly]

4 hours ago, Swoop1 said:

I don't know what is more impressive- the neatness of the hole, the symetry of the rebar placing or the slippers?

 

?  they are m&s slippers , It wouldn’t have mattered the hole finished round in the end  

https://flic.kr/p/ZaZLyw

[martin_h]

On 15/01/2018 at 19:02, tooth_dr said:

I used postcrete to top up my concrete pier. It sets too fast for pouring a base you would have no working time at all. 

I used postcrete for all of my block, dug the hole and chucked in a bucket full of water then 3 or 4 bags of postcrete then another bucket of water then 3 or 4 bags of postcrete etc....helps if there are two of you doing it.

Been holding up my mount and scope for a good few years now.

[Miguel1983]

Hi,

I'm a contractor here in Belgium, and i see some doubt about the concrete mix.

So, if we make concrete on site we use the 3-2-1 rule : 3 parts gravel/balast (7-16 mm (you may go higher on the second number, but start where the sand ends, so 7mm - ... ) - 2 parts zand (0-7mm) - 1 part cement (CEM2 42,5R or 52,5 (the higher the number the faster it has its final strength in N/mm2) )

You need more gravel than sand, and end up with a mix that has about 350-400 kg cement/m3.

About the rebar, its beter not to have contact with the soil, rust wil get in to the foundation and make it crack over time.

It's beter to make a few U-shaped bars and shape 4 or more of them into a basket, poor 10-15 cm of concrete in the hole, put the basket in and poor the rest in.

[timwetherell]

I used a 6" auger to drill 1m deep piles into clay which seems to have done a god job. The polar alignment hasn't drifted at all in two years. Reading Miguel's post above, If I was doing it again i'd probably protect the ends of the rebar in some way rather than just poking them into the clay!

[Miguel1983]

Don't worry, i said "over time", maybe more of a problem for a later generation