Recipe for Concrete using postcrete and 20mm gravel please

[escapizm]

Im a simple subsea engineer and have googled the above and got mixed info.

Can anyone please advise the ratio of water, postcrete and gravel (ive loads of gravel) for a simple foundation I’m preparing for a green house. I will dig a foundation pit 8 inches deep for 3 inches of hardcore (using the gravel alone) then my concrete mix of 3 inches then 2" slabs on top next weekend when it drys?

I was thinking postcrete for ease but if non runner i can always get sand/cement!

A DIY forum question really but as a longstanding here I will give it a shot.

 

[escapizm]

Found this

A basic mixture of mortar can be made using the volume proportions of 1 water : 2 cement : 3 sand. Most of the student activities can be conducted using this basic mixture. Another "old rule of thumb" for mixing concrete is 1 cement : 2 sand : 3 gravel by volume. Mix the dry ingredients and slowly add water until the concrete is workable. This mixture may need to be modified depending on the aggregate used to provide a concrete of the right workability. The mix should not be too stiff or too sloppy. It is difficult to form good test specimens if it is too stiff. If it is too sloppy, water may separate (bleed) from the mixture.

 

[Ron]

I would not add gravel to a pre-bagged mix of any sort, simply because those pre-bagged mixes are formulated with the aggregate ratio they contain and you might not have sufficient cement paste to adequately cover the additional surface area of added aggregate, resulting in a lower strength, more porous concrete. Not good for a foundation.

Considering your terminology "Postcrete", I will assume you are in the UK. If so, Blue Circle Cement/LaFarge/Tarmac have pre-bagged, multi-purpose concrete mixes for the purpose you intend. These are formulated as concrete mixes and will perform in your application better than an adulterated Postcrete mix. Check with the major home improvement stores in the UK for these materials. Stick strictly with the manufacturer's mixing instructions, particularly the amount of water to be added. The primary control of concrete strength in simple mixes is the ratio of water to cement (Abram's Law).

Pre-bagged mixes are not cost effective if you need as much as a cubic meter of concrete. The are fine for small quantities. If you want to mix your own concrete, this formulation will yield a simple mix having a strength of about 25 MPa:

75 kg of nominal 20mm coarse aggregate
45 kg of concrete sand
25 kg of standard portland cement
13 kg of clean water

This will yield enough material (about 1/10 of a cubic meter) to charge a typical small rental mixer.

These proportions are approximate and should yield reasonable strength. If you drop the nominal size of the coarse aggregate down to 10mm, you will need to increase the sand to 50 kg, the cement to 30 kg and the water to 15 kg.

 

[cvg]

I would recommend that you place the concrete in one lift, not two thin ones. if this is a slab than you may want to have ready mix delivered as this will be a lot of concrete to mix by hand. if these are just foundations for the walls or corners, than you may want to go a little deeper than 8 inches.

 

[bimr]

If you are using concrete, you must be planning a larger greenhouse. In addition to what Ron posted, you probably should us a thicker foundation wall. A thicker foundation will keep the greenhouse walls square and level. The linked article recommends 6 inches of concrete or 8-Inches of wood.

An episode of the this old house show featured an interesting tunnel greenhouse suitable for milder climates.

http://www.thisoldhouse.com/toh/tv/ask-toh/video/0,,20790479,00.html

Ask TOH |Greenhouse, Pendant Light
Original Air Date: February 27, 2014
Season 12: Ep. 17

 

[escapizm]

Thanks folks great replies, it's a small 6ft x 4ft lean to greenhouse for my daughter against a garage wall so half the found exists in the pathway around the garage, I'm adding 4 x paving slabs 60cm sq ea, I'm doing founds really as the adjacent soil is a boggy clay, probably over kill but I enjoy working in the garden.

Using the 1,2,3 ration 40 kg cement, 80 kg sand 120 kg gravel so only need 240kg for 75mm DP x 0.6 x 2.4. I'll add 75mm of gravel first and place slabs on top of it all.

Tks again.