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”

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  1. How do I find the mix ratio of masonry foundation

  2. Your water/cement ratio is critical.Any high strength concrete design should include the “slump” allowance. This is the amount that concrete placed in a cylinder will sag or slump when the cylinder is removed. A layman should not be thinking of designing 60 MPa concrete without the advice of a civil engineer or the Cement & Concrete Institute who are always most helpful in my experience (011) 315 0300.

  3. Hi

    I have to design a high strength mix of 60 Mpa for a school project and im almost clueless on where to start. All we have been instructed is to use 42.5 N Cement. Has anybody designed something similar and what material were used to achieve the high strength?
    in my mind im already thinking something Fly ash would be necessary to help increase strength but at the same time help mitigate the low water cement ratio necessary to achieve high strength. Crushed limestone with aggregate size > 8 mm.
    Am I on the right track, any help would be appreciated.

    • Fly ash is used as an inexpensive cement extender – it is used by cement manufacturers. It is not going to increase the strength of your concrete – in fact it will reduce the strength though it will improve workability. You might add FA to your concrete if the concrete was going to be placed in an “aggressive” environment.
      The materials you will use are cement, sand and crushed stone (available sizes 9.5 mm, 13,2 mm, 19,5 mm or 26,5 mm). I do have tables for trial concrete mixes but they only go up to 40 MPa. According to Fulton’s Concrete Technology published by the CCI, which has a chapter on concrete mix design (maybe there’s a copy in your school library?), high-performance concrete with a 28-day compressive strength of 68,9 MPa = high strength concrete. The chapter on high-performance concrete says that the best cements to use for (high-performance concrete) “for South African conditions,” are CEM1 42,5R, 52,5N, 52,5R. From a table in another chapter, I see that the early compressive strength (MPa) of 42,5N at two days is about 10 MPa and 42,R about 20 MPa. Both reach between 42,5 and 62,5 after 28 days.
      In the chapter on high-performance concrete, it states that substituting about 8% of the cement with condensed silica fume “could enhance strength”.
      It also says that tests done with South African aggregates have shown that the “optimum nominal size for coarse aggregate” i.e. crushed stone – is 19-26.5 mm.
      As far as sand is concerned, “a fineness modulus of 2,7 to 3,0 should be suitable.” Crusher sand is preferred in SA.
      It also states that a “good quality superplasticizer is an essential ingredient of high-strength concrete.” It explains how to work out the quantity required – tests need to be done in a lab.
      NB In this chapter it is also stated that “there is little experience in South Africa of the manufacture of high-strength concrete.”
      So they have given you a very difficult task.
      Maybe these trial concrete mix quantities might help:
      MPa at 28 days Cement Sand Stone (19 or 26,5 mm)
      10 50 kg 230 kg 196 kg
      20 50 kg 130 kg 164 kg
      30 50 kg 90 kg 114 kg
      40 50 kg 64 kg 98 kg
      Notice how much less sand and stone you need the higher the MPa needs to be.

  4. Slump tests also determines if the amount of water added is ok, it is used to determine the consistence of the concrete mix.

  5. I find this guide very problematical and thus misleading and therefore dangerous as follows:
    1. Your pictures clearly show the ratio of sand, stone and cement THEY are so clear that it is more than likely that the ratios in the pictures will be used without consulting the ‘fine’ print.
    2. The fine print says that these ratios are for wet sand therefore your pictures should also say wet sand!
    3. You also need to say that the maximum water content of the mix, including the water in the sand is xxx litres, on each picture.

    You really need to correct this.

    It would be really good.

    • Hi Ross, Thanks for the comment. The water part of mixing concrete is NOT an exact sience I’m afraid to say. It is impossible to say how much water is in a damp sand that gets delivered to any one site. We do say “You will also need to add more water to compensate for the lack of water in the sand.”, so the saying “The mixture must be not too dry and not too wet” applies.

  6. how do you control the amount of water for the above mix, 15mpa, 25mpa & 30mpa ?

    • When you mix concrete you add water gradually until it has the right consistency. It shouldn’t be runny, but must be pliable enough to work with.

  7. Hi, I can’t make complete sense of the difference of mixing ratios between’ Large Batches of Concrete Mixes’ ( 25 Mpa =7 bags cement(0.231m3) , 0.7 m3 sand , 0.7 m3 stone gives a ratio of 1: 3: 3) and ‘Small Batches of Concrete Mixes’ which states that the 25 Mpa ratio is 1: 2.5: 2.5) that almost 17 % less sand & stone ?

    • Hi Glynn, You can’t just convert quantities for small batches to a ratio and then compare the ratios to large batches. Mostly small batches will use a smaller stone and will be hand mixed and large batches will be mixed using a larger stone and a concrete mixer or delivered by a concrete pre-mix company. This is a confusing thing with concrete and mostly this is to do with the size stone being used, commonly 19mm, 13.2mm or 9.5mm. The larger the stone the more empty volume/space between the stones that has to be filled with sand and cement and conversly the smaller the stone size then, by proportion, there will be less sand and stone. This will also vary with the concrete strength High (30 Mpa), Medium (25 Mpa) or Low (15 Mpa).

  8. Good day
    I’m having my foundation lifted on the side of my house and under pinned. I’ve been told to order different quantities from quote to quote regarding the mix ratio. One is what I see here 50/50 mix medium strength. The other is a 1:2:3 cement, sand, stone. Is the later weaker, and should i reject the quote or is it still acceptible?

    • Hi Ally, Sorry for the delay, we do the replies on a voluntary basis when we can. This can only be answered by a “competent person” who should be overseeing the build and will specify the ratios needed for your soil conditions etc. This should have been specified when plans were submitted to council.

  9. Hello. Let’s say I order 6m3 of ready mix concrete, how do I check, in the truck, if the quantity of concrete is what I ordered?

    • You tell me! Use a reputable company that won’t rip you off.

    • The trucks normally take only 6m3 so if you look in the truck should be full, bear in mind a little space for the mixing so its not overflowing. I’d say about 85% full. Also calculate the place where you pour the concrete if its filled up if you worked it out to be 6m3.

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