To some, concrete may seem like just a mixture of water, stone, sand and cement which sets solid and is useful for building driveways and disposing of patsies. To many others it's probably even less than that. However, when one requires concrete, to build a patio, for example, simply telephoning the local 'ready-mixed' supplier and asking for 'some concrete' just isn't going to wash.
For starters, you need to know how much concrete you need, ie, you will need to provide a volume, calculated in cubic metres1. For the mathematically adept, this is not usually a troublesome issue. And anyway, on construction projects, a specially-trained quantity surveyor may be employed to perform the necessary calculation. However computation of such a volume may induce pre-mensural tension in the numerically-challenged, in which case, don't panic and don't guess, but get some advice. The supplier may even be willing to come and do the maths for you.
Recipe For Success
You'll also need to know what kind of concrete you want. The properties of the concrete you'll need to build a canoe are (probably) not the same as the concrete you'll need to make flagstones or pave paradise and put up a parking lot. Strength of the hardened product and workability of the freshly-mixed stuff are the key attributes to be specified for run-of-the-mill concrete, but other special requirements may need to be considered; such as special finish, resistance (to saline or nuclear environments), whether the concrete is to be pumped, nominal aggregate size... and so on. If you are ordering the concrete yourself, it's your decision to make, so you should be sure to know what kind of concrete you require. Again, in most instances, a qualified professional civil engineer, structural engineer or architect should be able to advise. Ultimately, you have two alternatives:
Prescribed-Mix where the purchaser provides the exact recipe for the concrete that the supplier should provide, and as such relieves the supplier of many of his obligations and responsibilities for the concrete properties. This would normally be used only for 'special applications'.
Specified-Strength where the purchaser provides his strength requirements, and the supplier is duty-bound to produce a concrete which meets those requirements. The supplier may also ask for some additional information, such as 'slump'.
For laying a concrete driveway, which may experience freezing or salty conditions, a request for a 32MPa mix with an 80mm maximum slump and 6% entrained air should suffice.
The 'MPa' used in this context is a measurement of the characteristic strength of the concrete, and 32MPa means that a 100mm x 100mm x 100mm cube of concrete when made and tested in accordance with the requirements of British Standard 18812 should resist a crushing strength of at least 32 Megapascals or 32 N/mm2.
'Slump' (as described in UK by British Standard, 1881) is an indication (but not a measure) of the 'workability' ('flowability' or 'runniness') of the fresh concrete mix, and is usually taken at the point of delivery, ie, at the site. The higher the slump, the more workable the concrete and, hence, the easier it is to work with. The test is performed by filling with concrete (in a prescribed manner) a truncated cone, and then lifting the cone. The distance by which the concrete 'slumps' is its slump. Typically, automatic road-paver laid concretes can be very stiff, (say 25mm slump) while beams and columns could be 'very wet' (above 100mm slump), especially if there is very congested reinforcement. Notably, the strength of concrete is more or less proportional to the cement/water ratio (sometimes confusingly called the water/cement ratio). Adding water to improve the workability of a mix will decrease its structural strength. It is an absolute no-no and any respectable concrete delivery truck driver will not condone it.
'Entrained air' comprises pin-head sized air-bubbles purposely introduced into the concrete, and which, by acting as pressure relief-valves held to protect concrete against weather-induced freeze-thaw cycles.
Coming, Ready or Not
Finally, logistics. You need to have an idea where the concrete is coming from and where it is to be delivered. The process from mixing to pouring really should take no more than two hours, so it is pointless ordering from a supplier in Cardiff if you live in Glasgow3. Perhaps discuss the route the trucks will take with your supplier, and if possible furnish him with a map showing where you want the concrete to be delivered. Also, you need to be more than just confident when you order the concrete that you'll be ready to receive it when it arrives. For example, will the truck fit through your garden gate? These are issues people have been known to ignore, and a rufty-tufty concrete truck driver won't be kept waiting. If you are not ready, internal poker-vibrator in hand, when he arrives, be prepared to have 8-10 cubic metres of rapidly hardening concrete deposited in your goldfish pond, gnomes or not.
Health and Safety Issues
Concrete is a known irritant, and people working in a 'cementitious' environment may be well-advised to wear goggles and a mask, as well as full overalls and gloves. Use of vibration equipment should also be limited to avoid any repetitive stress injuries. Lifting heavy loads should also be regulated. These precautions, and others, may be mandatory depending on local/national regulations so anyone dealing with concrete is advised to consult their local Health and Safety body for advice.