**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.

Hi

I need an hand book Article or any Text book that will Educate me on different concrete mixes and morta mixes.

I need to learn more to be able to implement better

Hi David, go to our downloads page (ownerbuilding.co.za/free-downloads) there are a number of leaflets that will help you. You can also go to our other site for more info: buildingregulations.co.za

What is the good ratio for making quality bricks (Sand and Cement only) using Afrisam and how much water is needed in that particular ratio?

Hi Bernard, I have uploaded a file for you to download on our downloads page, just scroll down to “Concreting & Blockmaking”. This an AfriSam brochure that gives you all you need to know for your project. Basically they say that as a rough guide, using an aggregate cement ratio of 8:1 by loose volumes, three and a half bags of cement and a cubic metre of aggregate will be enough to make about 400 standard bricks. There are other factors that they talk about and you should read the whole paper first before starting. Good luck!

Hi there,

We are preparing to extend a house and according to the quotation, I was suppose to building sand 10 cubic meter and at cashbuild they gave me 1 cubic meter sand. What is the difference?

Hi Dumisani, If you look at the amount of sand delivered then times that by 10 then that will be what 10 cubic meters will be. BTW 10 cubic meters is a lot of sand.

which contractor that can build us concrete house with its required insulation

Nathi, unfortunately we cannot make recommendations. Make sure whoever you approach is registered with the NHBRC.

Hi, building a double storey house my ground floor is 210 sqm was advice to use 25mpa spec needed 19mm 70:30 mix 42.5mpa all in is 35cu just need to know how many ton (single truck) will a use and my first floor is 183sqm all in 36cu, please help just need to know how many trunks will i use?

Your best bet is to contact a ready-mix concrete company and ask them to do the estimates for you. They will be able to give you a pretty instant reply.

Good day. I’m a Quantity Surveying student who would like the estimated:

1) cement, sand, stone and water quantities for a 5000l trial mix of 30MPa concrete, with cement type CEM 1 42.5; and

2) cement, sand and water quantities for a 1l grout mixture to be used in CFA piles

I forgot: I look forward to hearing from you. Thank you for your gracious time.

Hi Lungelo, as much as we would like to help you it sounds like this is an assignment, and it is our policy not to do homework for students. You will have to do the calculations yourself. Good luck with your studies

hye.may i know the correct cement ratio with strength 30Mpa at 28 days? please help me.

Hi Sakinah, I have added a table at the end of this story with concrete mix proportions in ratios. I hope this helps.

hi , I am a little confuse , in relation of mixing of concrete , I need a 4500psi strength , what are the mix I should used

Hi Nanda,

I am also a little confused, because concrete strength is measured in Mpa (after 28 days). Have a look at the table on this page that gives the range mostly used in ordinary construction today:

concrete-mixes-by-weightIf you have a unique structure that requires a special concrete mix then you will have to consult a structural engineer who will draw up the specifications for you.I thought I had my foundation mix clear in my mind (house extension) but now my supplier has suggested pre-mixed sand and stone – they claim it is a 50 /50 mix by volume. This is great as it saves much-needed storage space but the question begs – how much cement /m3 of the mixed sand/stone for a 15Mpa mix?

I learnt on this page that ,75m3 of each + 5.5 bags of cement (plus water) only makes 1m3 of concrete, so it is obviously not a simple matter.

Thank you

Hi Steve,

I agree it looks confusing, if you have a look at the first illustration on his page it says below the wheelbarrows that for 1 cubic meter of 15Mpa concrete mix .75 cubic meters sand and .75 cubic meters stone with 5 1/2 bags cement. So for your premix you must take into account the volume shrinkage when the sand fills the cavities between the stone. So .75 plus .75 will not end up as 1.5 cubic meters of mix but closer to 1 cubic meter then you must add your cement which will further fill the smaller cavities between the sand and stone. The shrinkage will be greater with a bigger stone size and coarser sand as well. Water is not taken into account as this will evaporate and the mix will shrink to its final volume. The Cement and Concrete Institute do say under

“So with this in mind rather add a bit more cement to the mix and make it stronger rather than weaker.Estimating shrinkage.Because of the number and complexity of factors , estimating is, at best imperfect.”hi, may I know the correct mixed ratio for concrete grade “7”

Lameck I am not sure. Here is a link to a table that gives proportions for Grade 7 concrete – but I cannot guarantee its accuracy. Hope it helps.