**Concrete Mixes & Batch Ratios**

**For Different Applications**

The basic techniques that will enable you to cast a foundation or slab are relatively straightforward. The secret to success lies largely in the correct quantities being mixed together in the concrete mixes you use. Although the principles of mixing concrete and mortar are simple, this can be backbreaking work, especially if you decide to mix the concrete by hand yourself. If there is a reasonably large quantity of concrete to be used, it is advisable to hire a concrete mixer. Whether you are mixing by hand or in a concrete mixer, you will have to measure materials accurately to ensure you achieve concrete of consistent quality.

**Batching** Generally, materials for smaller jobs are batched by volume. Recommended ratios will enable you to mix concrete to match the function for which it is intended, or the strength of the concrete required.

One 50 kg sack of cement has a volume of 33 litres (0,033 cu m). A builders’ wheelbarrow, filled level to the top, has a volume of 65 litres (0.065 cu m, which is almost double the volume of a sack). When batching by volume, it is safe to assume that one wheelbarrow-load is equivalent to two sacks of cement. Since sand bulks in volume when it is damp, the mix ratio table (below) is based on the use of damp bulked sand. If you are measuring dry sand, reduce the quantity of each batch by 20–25 percent. You will also need to add more water to compensate for the lack of water in the sand. The recommended ratios are based on the use of either 19 mm or 13,2 mm commercial crushed concrete stone. Stone does not bulk in volume when it gets wet and so no correction is necessary.

## Large Batches of Concrete Mixes

**15 Mpa** This is a low-strength concrete mix and is suitable for house foundations that are not reinforced, and for boundary walls and freestanding retaining walls.

**25 Mpa** This is a medium strength concrete and is suitable for reinforced foundations, light-duty house floors, patio slabs, footpaths, steps, driveways and garage floors.

**30 Mpa** This is a high strength concrete and is suitable for suspended structural beams, pre-cast beams and flagstones, heavy-duty workshop floors and suspended reinforced floors.

## Small Batches of Concrete Mixes

You can use containers such as buckets, drums or tins. It is **importan**t that the same size container is used for all materials in a batch.

**Moving and placing the concrete**

**Time limits**

**Moving the concrete**

**Retempering**

## Concrete Mixer

## Ready-Mixed Concrete

If you have large quantities of concrete to place, it is much more convenient to order it ready mixed. It is then mixed in a factory environment, according to your specifications. You must just ensure that workers are on site to place the concrete as soon as it is poured from the truck.

what is the implication of concrete mix of ratio 1:4:6 of elephant cement : wet(river Sand): Rice (granite) in a suspended beam& reinforced floors.

Hi Koko,

That mix sounds dangerously weak for a suspended beam and reinforced floor. I would contact PPC (the elephant cement people) or a qualified engineer to give you some advice before you have a major disaster on your hands.

what will be the outcome strength of 1:3 mix?? in PSI

What do you mean 1:3 mix? Are you asking about a mortar mix or a concrete mix that needs a stone component? It also depends on the class of cement that you are using.

Hi, I am looking for the ratio by weight of cement to a premix of sand and stone (already combined) to produce 40MPa concrete.

Thanx

Hi Peter,

You will have to figure out what the ratio of sand to stone is in your premix and also gauge/guess what size the stone is.

Here are the trial mixes for 40MPa from the Cement and Concrete Institute handbook:

Stone size 13,2mm Mass/bag———–Cement= 50Kg—-Sand= 68Kg—-Stone= 68Kg

Mass/Cubic Meter—Cement= 575Kg—Sand= 780Kg—Stone= 770Kg

Stone size 19,0mm Mass/bag—–Cement= 50Kg—Sand= 64Kg—Stone= 98Kg

Mass/Cubic Meter—Cement= 520Kg—Sand= 650Kg—Stone= 1020Kg

These are based on certain assumptions such as the cement type is class 32,5. If you use a cement that is class 42,5 and higher it will give you a stronger concrete but will be less economical. The sand and stone is also given with an average moisture content. Please be cautious as to what you are using the concrete for and for structural uses it would be best to consult a professional.

Hi. Where can I find the minimum required strength for a building block? Thank you for an excellent website.

Hannes, this link will take you to our downloads page. Scroll down and you will find a link to How To Make Concrete Bricks and Blocks. This is a very useful little publication that was produced by the now defunct Cement & Concrete Institute. The Concrete Masonry Manual – link on this same page – also has specifications for building blocks.

Thank you Penny it helps a lot.

Has another body took over C&C institude? Plse email me hannesrichard@gmail.com

Hannes, from their old website I see that a new non-profit organization has been formed.”The new company will be funded by AfriSam, Lafarge, and Sephaku and will begin operations on Thursday 2 May 2013.”

From what I read at the time, they closed because their main sponsor, AfriSam withdrew their funding. Looks to me like it’s got to do with BEE and suchlike! But I really don’t know.

Hi Hannes,

They are now called “The Concrete Institute”. Here is a link: http://www.cnci.org.za/EN/Content/Home

Why fc’ is Only 15, 25 & 30 Mpa???..

How if I want to know the fc’ of 1Cement:2Sand:3Gravel ?

How can I calculate the fc’ ?..

Or vice versa , I want to produce fc’ =21Mpa, how can i calculate the concrete mix?

thanks..

Surely you have a text book. We do not do students homework for them.

Fulton’s concrete technologyis an excellent resource.Thank you for the clear information tou put inthis article. It’s helpful.

GOOD ARTICLE, IT CAME TIMELY. I HAVE SEARCHING FOR ANSWERS ON CORRECT RATIO FOR CONCRETE MIXES, YOUR ARTICLE GAVE ME THE EXACT ANSWER I NEEDED.