C35 concrete mix... what is the right mix ?

Discussion in 'Builders' Talk' started by do it Alone, Sep 22, 2017.

  1. do it Alone

    do it Alone New Member

    Hi ,

    I have to mix a strong mix for foundations for a steel beam post. The initial foundation is going to be about 60x60 and at least 100 cm with some vertical bars. I have been advised to achieve C35 strength
    I have been online trying to find the correct mix for the cement, sharp sand and gravel, but it seems they all differ for C35.
    The water ratio seems to be 1 kg cement to 0.5 liter of water to get a good hydration but for the mix I have now got a few options, 1 - 2 - 2 or 1 - 2 - 4. or even 1 - 1.5 - 2.5 .
    I don't have a vibrating poker and was wondering what manual alternatives there are to get rid of the trapped air.
    Can anyone give me some direction on this,

    Thanks in advance
     
  2. stevie22

    stevie22 Screwfix Select

    Who has told you C35? All the concrete really needs to be in this situation is stronger than the soil it's sat on unless your post is doing more than just sitting on it.

    If you go with 1:2:2 you'll have plenty of strength but keep the water down: excess water is where strength is lost. Tamp it thoroughly in layers with a bit of 2x2
     
    KIAB likes this.
  3. KIAB

    KIAB Super Member

    4 to 1 using ballast & just enough water to allow enough movement tamp it down.
     
  4. do it Alone

    do it Alone New Member

    Thanks for the reply, the steel post is transferring the weight of the beam holding up the upper floor and a wall above, parallel to the beam, so there suppose to be a lot of weight which needs to be taken by this foundation. I have a span of almost 7 tmr, one post in the middle, then two either side
     
  5. do it Alone

    do it Alone New Member

    I'm not using ballast. Cheers
     
  6. KIAB

    KIAB Super Member

    2 to 2.5 gravel 1.5 to 2 sharp sand, all depends on size of gravel.
     
  7. Dr Bodgit

    Dr Bodgit Super Member

    Isn't strength more about how much water is used?
     
  8. KIAB

    KIAB Super Member

    Known as 'water to cement' ratio.

    Aggregates also play a part, large aggregates leave voids, as does too much water, it evaporates again leaving voids.
    And ratio of cement to aggregates, too much or too little, can also affect the mix.
    Peeps think wrongly adding more cement makes better mixes,etc.
     
    Last edited: Sep 22, 2017
  9. do it Alone

    do it Alone New Member

    Its 20 mm gravel
     
  10. stevie22

    stevie22 Screwfix Select

    All this is way over the top for the OP's requirement. All in ballast would be fine for this job. i would personally use 1:6 for this.

    Aggregate size is one of many factors in designing concrete. Surface texture, shape and uniformity are also factors. Large aggregate won't leave voids if the aggregate grading is right. Design is always finalized by trial mixes in industry as there so many variables.
     
  11. do it Alone

    do it Alone New Member

    Thanks, but I have the materials on site, it's sharp and 20 mm , no use of a vibrating poker, so just need a strong mix to be sure of the strenght
     
  12. PhilSo

    PhilSo Screwfix Select

    We used to slump test every batch on big pours.
    Good indicator of correct water content
    Regards
    PhilSo
     
  13. stevie22

    stevie22 Screwfix Select

    If you have the stuff then use it for sure. If you're worried about strength then go 1: 2 : 3 but keep the mix stiff and tamp well.
     
  14. Dr Bodgit

    Dr Bodgit Super Member

    Does the 1:2:3 ratio relate to volume or weight? Just curious...
     
  15. do it Alone

    do it Alone New Member

    That should be by volume
     
  16. Dr Bodgit

    Dr Bodgit Super Member

    I was hoping so, otherwise would need to convert to shovel loads!
     

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