I know this is from Wikipedia, but hopefully it will give you an idea.

Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (surface water) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot be readily made more workable by addition of reasonable amounts of water.
http://en.wikipedia.org/wiki/Concrete

--------------------------------------------------
Note added at 37 mins (2012-05-07 09:51:02 GMT)
--------------------------------------------------

Batching is similar to that of normal concrete, generally from ready-mixed concrete outlets. Workability is commonly between 50 mm and 100 mm slump and the concrete can be easily placed by skip or chute. However, due to the low bulk density of the aggregate, separation from the mortar tends to occur especially at higher workabilities.

Pumped lightweight concrete requires a higher workability so that increased pump friction and blockage does not occur. It is usual to use a natural sand, and an admixture to provide plasticising and thickening properties to the mix. Pump mix workabilities should be measured using a flow table rather than by slump. Compaction for low workability concrete should be by immersed poker vibration (low amplitude and high frequency). Pump mixes tend to be self levelling and compacting and thus require minimal vibration.

Over-vibration will tend to cause segregation. It has been observed that good power float finishes to floor slabs can be difficult. This problem can be avoided with correct mix design.

Where a supplier is unfamiliar with lightweight concrete it may be advisable to construct a trial slab and assess the finish quality. Lightweight concrete should be ordered strictly in accordance with the Contract Specification. Suppliers of the material should be aware of the method of placing the finish requirements and consequently be able to supply a suitable mix. Responsibility for the mix design, batching and mixing lies with the supplier.
http://www.concrete.org.uk/services/fingertips_nuggets.asp?c...