DIY Pier related questions

[sadia]

Hello everyone, I am considering to build a Pier in my backyard. Reading through the threads I understand that at least 25mpa concreate is recommended for pier footing. I have one initial query:

Where do i source the premixed 25 mpa concrete because bunnings one seems to be 20mpa premix.

Regards
Sad

[terrynz]

Hi Sad,

Standard portland cement mixed 7-8 per cubic metre will make 35~40Mpa concrete. Remember to keep it cool, avoid and frosts whilst it sets; normal strength in 7 days with full strength after 28-30 days.

hope this helps
Terry

[Louwai]

Sad,
Pretty much all pre-mix will be 20mpa or less. The higher mpa is reached by more cement powder being added but with no additional water.
The mixture should be wet right through, but stiff, not runny. Too much water & the finished concrete will be brittle & very dusty.
When 'Finishing' the surface, try to get it to the required smoothness as quickly as possible. The more you trowel the top, the more water will be drawn up to the surface. Too much water on the surface will have the powdering effect as noted above after the concrete dries.


Also, a higher strength is acquired by the mix curing slowly. If the cement dries too quickly it will crack.
Eg. If you mix & lay the concrete in the morning at say 8am, as soon as you finish trowelling the surface you should cover the concrete with a hession bag or similar & keep it moist.
This will;
1) keep the sun off to stop it drying too quickly
2) The wet bag will help to keep the concrete damp & dry very slowly.
Hose the bag lightly every hr or so for a couple of days.

[marki]

20 mpa concrete will be fine for a pier. Its more about mass then strength as telescopes do not exert any real force on the concrete. 25 and 32 mpa mixs are used in suspended slabs and are really way over the top for a simple telescope pier. If you got 20 mpa with 20mm aggregate it will be more than enough. Chuck a bit of mesh in and Bob's your uncle. In Perth we have concrete taxi's where the guy rocks up in a little truck and mixs it for you a wheel barrow at a time. Clean, reasonably priced and effiecent as there is no waste. Do you have something similar over your way?

Mark

[sadia]

Thank you for all the advice guys. Mark I heard about concrete taxi type of service in melbourne, I will get few quotes to weight my options.

Thanks again :)

Regards
Sad