Mar 132013
 

Concrete Mixes & Batch Ratios

For Different Applications

Mixing concrete472 Concrete Mixes
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.

Concrete Big Batches Barrows15Mpa s Concrete Mixes

To make 1 cubic metre of 15 Mpa concrete you will need to mix 5 1/2 bags of cement with 0,75 cubic metres of sand and 0,75 cubic metres of stone.

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.

Concrete Big Batches Barrows25mpa s Concrete Mixes

To make 1 cubic metre of 25 Mpa concrete you will need to mix 7 bags of cement with 0,70 cubic metres of sand and 0,70 cubic metres of stone.

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.

Concrete Big Batches Barrows30mpa s Concrete Mixes

To make 1 cubic metre of 30 Mpa concrete you will need to mix 10 bags of cement with 0,65 cubic metres of sand and 0,65 cubic metres of stone.

Small Batches of Concrete Mixes

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

Concrete Small Batches Buckets s Concrete Mixes

Moving and placing the concrete

Time limits
The time that elapses between the start of mixing a batch and when that batch is placed and compacted should ideally not exceed 45 minutes. If concrete is not placed immediately after batching, cover it with plastic sheets or wet sacking so that it does not dry out in the sun or wind. Concrete not placed and compacted within this time, or which has stiffened to a degree that its workability (consistency) cannot be restored fully by turning it over a couple of times with spades, should be discarded. This is because the hydration process would be in an advanced stage and retempering of the concrete will weaken it.
Moving the concrete
The concrete can be moved in buckets or wheelbarrows. If it is jolted too much, the stone will settle at the bottom. If this happens, remix the concrete before placing it. Do not let the concrete stand so long that it stiffens before it is placed.
Retempering
The concrete mix should be used within a maximum of two hours of being mixed and must never be retempered by mixing in additional water, as this reduces the resultant strength of the mix.

Concrete Mixer

Concrete Hand Pour487 Concrete Mixes

A concrete mixer has been used to mix the concrete, and wheelbarrows are used to place it.
Note the reinforcing and the plastic waterproofing underlay.

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.

Concrete Pour520 Concrete Mixes

Here a batch of concrete is mixed off-site and delivered and placed into the pre-dug foundation trenches.

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  55 Responses to “Concrete Mixes”

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  1. 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

  2. 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!

  3. 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.

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

  5. 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.

  6. 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 :-D

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

  8. 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-weight If 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.

  9. 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 Estimating shrinkage. Because of the number and complexity of factors , estimating is, at best imperfect.” So with this in mind rather add a bit more cement to the mix and make it stronger rather than weaker.

  10. hi, may I know the correct mixed ratio for concrete grade “7″

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