Calculating material requirements for concrete

Discussion in 'Landscaping and Outdoors' started by Mike263, Jun 23, 2021.

  1. Mike263

    Mike263 New Member

    I have a question, I need to concrete small area, 0.95m x 0.90m x 0.2m - overall the volume is 0.17 m3 of concrete.

    The calculator here suggests I need 63 Kg of cement and 0.377 tonnes of ballast (which I assume is 377Kg) - this is just over 15 x 25Kg bags.

    When I look at the hole and lay some bags of ballast alongside it seems completely implausible that all those materials are going to fit inside the area to be concreted.

    A 'Major Bag' of Ballast from Tarmac measures 1m x 35cm by 5cm (the weight is unspecified except that the retailer states approx £0.10p per Kg and at £2.30 a bag that is 23Kg.

    That volume of one bag equates to 0.0175 m3. Multiply that by 16 and you've got 0.28 m3 - in ballast alone - this is way more than the 0.17 m3 of concrete I need.

    Is there some magic at play here, am I missing something?
    Last edited: Jun 23, 2021
  2. Severntrent

    Severntrent Screwfix Select

    Concrete weighs about 2400kg/m³ so your 0.17 m3 comes in at 410kg which is only 30kg less than your calc so you'll have 1/2 to 1 bag of concrete left over to dispose of at the end of mixing
  3. Mike263

    Mike263 New Member

    Thanks, I deliberately excluded the cement in the calc, so by the time I've added in 2.5 bags, I'll have made up the gap
  4. stevie22

    stevie22 Screwfix Select

    Don't get too hung up by the numbers. The 2400 is not fixed (depends on the aggregate size and specific gravity) and if your 200depth turns out at 210 you're 5% adrift or nearly 20kg.

    The apparent shrinkage is real enough and is simply down to small particles fitting into the spaces between the larger ones especially when lubricated by water. It's even more marked when you start with separate aggregates. In another life I was a C&G concrete technology lecturer and we had a pair of glass cylinders with the same amount of aggregates in each but one mixed and one left separate which proved the point very graphically..

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