Concrete Basics – Mixing On Site.

Mixing concrete on site sometimes is the only way to go, even though it is less convenient and usually costs more than ready mixed.

  • The location is just too far away from the batching plant.
  • Once the materials are combined the setting of the concrete begins, the mix will inexorably start to set.  It will not remain plastic after a certain time.  So the farther away from the plant, the less time you have to do the job.
  • I have done one job at a remote location, where we added the water and cement on site, to the previously batched aggregates in a transit mixer.   Expensive but it worked well for a one off job.
  • The dread of all concrete truck drivers is to have the concrete set in the bowl. Tell me about it!  I once had a truck with 6cuM on board bogged at the end of an airport runway. Happy days.
  • Some jobs are very hard to do in the large quantities that a truck brings in. You may need just more time to do the job.
  • Mixing on site allows you to set your own pace and make adjustments as you go along. You can add your own colouring, strength or waterproofing admixtures, feature aggregates etc.
  • Quite large paving, kerbing or pre casting jobs can be done by just one man, working at his own speed.
  • Buy or hire a concrete mixer to do the actual mixing. They come in various sizes.
  • The smaller ones have electric motors and are very convenient.
  • The larger ones need the extra power that a petrol or diesel engine will deliver.
  • A mixer should be loaded up just right.  By this I mean that if you buy a 3 cu ft. mixer, then mix 3 cu ft.  Don’t try to fit in 4 cu ft.  It might fit in, but it will take ages to mix, or not do the correct job.

I won’t go into the specific details of mixes here, because there are so many combinations of what you may be doing and the materials available to do it. By far the best way is to get the proportions from your own material supplier.

  • Our local supplier will deliver what we call pre-mix. That is the aggregates and sand have been mixed at his depot, so there is no need for separate piles of stone and sand.
  • The mixing ratios are batched using the volume method.  Ready mix companies measure the mix ingredients by weight.
  • It is so common, to just measure shovels full, and throw the materials into the mixer that way, but this is very inaccurate.  Try always to measure the volumes, except for trivial jobs.
  • I have done jobs where we made up ply volume boxes, but an easier way is to use metal buckets. clean empty 10 litre paint tins or similar.
  • At the least, use a bucket to measure the cement and water.  The water cement ratio is critical.
  • Try to keep your material clean and your bags of cement dry.
  • The mix may be 2:5:7 with enough water to give a slump of 100mm
  • That is 2 parts cement, 5 parts sand and 7 parts aggregate.

When mixing by hand, the amount of water used is usually, initially done by eye, then when a mix that gives the correct slump is made, the same amount of water is used every time after that to achieve consistent results.(Your sand and aggregate could be bone dry, or sopping wet, so that effects the amount of water to be used).