structural concrete mix

[BryanB]

I need to use C40 concrete for a reinforced beam. What is the required mix - and how does the usual cement/sand/aggregate ratio translate to a 3/4 to dust mixed aggregate. Bryan

[BigWallop]

Why not just buy a pre made beam, they're cheap enough. In fact, buying a pre made one might be cheaper than trying to make your own.

[dg]

Its not that easy to define a mix to meet the c40 strength, as the strength of the concrete is affected by how well you mix it and how well to place and compact (vibrate) the concrete.

You could mix a 1:2:3 mix, but if it is full of air pockets when dry, then it will be weaker than a 1:5:10 which has been well vibrated.

A 1:3:6 may do if properly mixed and vibrated.

The placement of the re-bars is important too.

Mixed sand/gravel depends on the ratios chosen by the supplier, and it is difficult to say what ratio they have used.

3/4 to dust is not a term used for concrete - it is used for grading stone for hardcore

dg

[BryanB]

similar hole I've discovered under where I need a wall for an extension. The beams have been designed by a structural engineer to meet the building inspctors requirements. They are 3500 x 450 x 375 - so don't think |I could move anything pre-cast.

[BigWallop]

Then, for your own safety, I'd definitely say, go for something bought in, rather than making your own, because the building inspector, and most insurance companies I'm sure, won't accept anything that hasn't been tested and certified as being able to take the stresses that have been designed.

Another idea you could put the structural engineer, is to have the hole sufficiently filled with high grade aggregate and soil or a heavy mix concrete pour. Of course, this totally depends on the size and depth of the hole, and what the sides of the hole are like in retaining the weight of being back filled.

The idea of a "T" shaped structure to span a hole must mean the hole is wider than it is deep, or of equal proportions, but I'd still assume that shuttering and pouring a block of concrete in the hole where your going to build, would be adequate to hold the weight of a new wall. If the hole is for a seasonal spring or burn that runs under the property, then a concrete pour into an arched shaped timber mould would, I'd have thought, also be able a better idea than a "T" made from pre-stressed concrete.

A solid arch shape would surely be able to take the weight of the wall on a new building and the arch would allow the burn to run under. Just have a look at the Roman bridges and via-ducts that are still standing after all these years, and they've allowed rivers and roads to run under and over them without the need to ballast them with "T" shaped beams.

I'm on the outside of this question and not really able to look in, so my thoughts might be well off mark, but maybe my comments could be used to make other judgements on the suitability of other means of spanning this gap.

Good luck with it.

[Ian Zejma]

Depending on the attitude of your structural engineer and building inspector, you may have difficulty in proving that your self mixed concrete is C40.

What C40 means is that the characteristic strength of 100mm cubes made, cured and testing in the standard way achieve a strength of 40 Newtons/mm^2.

Depending on the aggregates and cement used, to give you a rough idea, the required mix would probably need to be:-

Portland cement 375kg/m^3

20 - 10 mm aggregate 690kg/m^3 10 - 5 mm aggregate 345kg/m^3 Sand 800kg/m^3 Water 187 litres

Depending on where you are and if there are any sulfates in the ground, there may be other mix design restrictions that the building inspector may want satisfied.

Ian

Regards, Ian....

remove NOSPAM_ from address