Many of these thoughts have been gleaned from international sites, with some editing and additions by BHE but we believe that the information is equally applicable to Australasian operations within concrete and block manufacturing plants.
Globally, wet batch mixing concrete plants are the preferred standard for ready-mix. On-site batching is indicating a distinct shift towards this (especially in South Africa), where both high-strength and high quality concrete is required in minimal time and plant owners are trending towards the wet plants.
A wet batch plant is fitted with either a planetary or a twin-shaft mixer, depending on the capacity of the plant and the final quality required. The aggregates are batched automatically from bulk storage bins onto a weighing conveyor, from where it is mostly elevated via skip-hoist to a surge hopper located above the mixer.
The weighed aggregates are then discharged into the mixer where cement, water and optional admixtures are automatically added. The constituents are then mixed by forced action of the mixing paddles or arms before being discharge fully mixed into the transit truck (and occasionally directly into a pump as may be required with on-site batching). These plants have an average 90 second cycle time between mixes.
Wet mix plants create fully-mixed concrete which ultimately reduces costs, as it eliminates the need for mixing at speed in the truck prior to dispatch. It also reduces wear and tear on the trucks which is a ready-mix plant's highest maintenance cost.
There is also no need for manual checking of the mix prior to to dispatch, as the mix will be completely homogenous by the time it is discharged into the truck.
A dry batch plant has aggregates loaded into a weigh hopper by a front-end loader until the correct weight is achieved, although large throughput plants with multi bin storage may also have the products batched out of the appropriate storage hoppers. The batched products are then sent via a conveyor up into a collector chute where they are discharged into a truck-mounted agitator drum, where water, cement and any other additives are also dispensed, after which, the mixture is free-fall mixed in the agitator.
Once sufficient time has elapsed with the drum rotating at speed, the plant "Batcher" typically inspects the consistency of the mix, adds more water if required, decides when the mixture is homogeneous then gives approval for the truck to leave.
On arrival at the site, the concreter foreman may request more water or more mixing at speed before the concrete is discharged.
The Italian Institute for Concrete, in research conducted on the quality of concrete produced in dry versus wet batch plants found that wet mix plants produce concrete of a higher quality than the dry process.
Various reasons were cited for this, including:
Lower permeability of the concrete produced
Repeatability, which reduces the standard deviation
A reduction in errors made by possibly inexperienced or inattentive personnel.
When using the same water/cement ratio, concrete produced in a wet mix plant had a higher workability, which ultimately resulted in a reduction of the water/cement ratio for a higher strength concrete with the same cement content.
Finally, the research showed that the mixing effect of wet concrete mixers facilitates total cement hydration, as confirmed by mix designs with a higher strength class that nevertheless use the same quantity of cement.