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.

  45 Responses to “Concrete Mixes”

Comments (44) Pingbacks (1)
  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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 technology is an excellent resource.

  6. Thank you for the clear information tou put inthis article. It’s helpful.

  7. 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.

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