Mar 132013
 

Concrete Mixes & Batch Ratios

For Different Applications

workers mixing concrete
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.

15mpa mix graphic

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

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.

30Mpa mix

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 batches with buckets graphic

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 mixer

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 truck delivery

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

  110 Responses to “Concrete Mixes”

Comments (109) Pingbacks (1)
  1. Hi Janek

    I am in Pinetown (KZN) and i would like to find out if you know any store where i can get your book?

    • Which book Pule? Our only printed book that is still in print is Owner Building in South Africa and that is available at all good bookshops including Exclusive Books. Our ebooks are available from howtobooks.co.za

  2. thanks for the lecture, I love it

  3. Very very educative. Thanks a million.

  4. Hi

    Is there a huge difference financially between ready mix and “self mix”. I received a quote for ready mix 25mpa @R1444.00 per m3

    • Hi Leticia, With self mix you must calculate the cost of sand:stone:cement:water as well as the cost of hiring a concrete mixing machine for the time it will take to mix the quantity that you need for your build. Then you must calculate the cost of your labour to get your required amount of concrete where you need it. The m3 costs will vary from city to city and town to town in SA.

    • It all depends in the area you are situated like durban price is not the same as Richards bay.The distance between where you want concrete and the batching plant is a big factor when pricing

  5. really interesting topics for CE. missed this site from a long. keep the good works going on.

  6. hello,I am a new student in civil engineering field and I want you to reply me my question’s answer.
    1.what are the mix ratio that can be use for this following concrete:slab,bathroom floors,kitchen floors,car pack,garden,masonry concrete slab,columns,foundations.
    2.is it necessary that the mix ratio for columns should be different from the one for the foundation.

  7. Hi.my head is very quick and intelligent in the construction field but I always find my self sidelined due to luck of funds for studies.am not sure if have any advise for me.thanks.

  8. Hi there, is this book available in stores, beside online? If it does where can buy it?

  9. hello i want to find quantities per cube for each strength of the following materials cement, stone, sand, water. i would like to see the steps in calculating it. Please help i so stuck i feel like my head is about to explode.

  10. Hi how do i download this e book do i need a lot of data and how do i pay the money in thanks karl

    • Hi Karl, I have just checked the links from the books and they are all working fine. For the Concrete and Mortar Handbook and the Energy Efficient EBooks you will be directed to my book site howtobooks.co.za with a secure PayFast payment gateway. The Owner Building book is sold for me by Loot. Thanks for the support. Janek

  11. How much concrete mix and how many bags cement

  12. How do I measure how much concrete mix is needed to lay are 300mm thick foundation 15m long

    • Julian you also need to know how wide the foundation is. You then multiply the length by the width and then by the depth/thickness and get your cubic meters then work out the quantities you need for that many cubic meters.

  13. Hi
    We want to make our own concrete kitchen countertops, but are battling to find info on the ratio of the mix we should use for this.
    Websites from other countries indicate that one can buy special cement for this purpose. Is this available in SA as well.
    Also do you have any advise on the best stain to use for this kind of project.

  14. I want to throw a white cement and fine crusher mix on a bathroom floor, which I want to polish. What mix do I need

  15. Hi. I am building a diy pizza oven.
    I want to build a small rectangular slab which will be suspended, but supported on all 4 sides. The slab total size is 1200 x 1200. I want to sit the slab on top of blocks on all 4 sides. The overlap will be approx 170mm on each side.
    This is to be the cooking floor of the oven. I need to know what ratio is best and how thick the slab should be. I have mesh to put inside the slab as extra support.
    i purchased bags of concrete mix from the local hardware store. When I opened the bag, the ratio of cement looks very low to the sand. I have since bought seperate bags of sand and grey stone. I will need to purchase more cement.
    The medium/strong ratio as above looks good. But how much of each will I need for the slab size needed?
    The pockets of stone and sand come in 40kg bags. Not sure of the cement.

    thanks

    • For what you are wanting to do and to make it a safe structure I would advise asking a structural engineer to assist you, if the structure were to collapse and someone was hurt then you would be liable and you would have no insurance cover either.

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