SELECTING A CONCRETE MIXER - what is the best type for you?
The best concrete mixer for your
application can be selected by following a few simple steps and rules:
Size by batch or
production per hour
- Filling a container,
such as a bucket, truck or mould, where you need to fill it with a
particular volume of mix each time? This makes it a
FIXED BATCH SIZE system.
- Container needs several
batches to fill it, batch sizes are fixed and you need to size the mixer to
deliver this required amount of concrete. Example: a 2 yd bucket requires a
mixer with 2 yd output. For multiple-batch containers,
- Feeding a continuous
process? examples: pipe, block, paver machines with holding hoppers,
concrete pumps and conveyor belts feeding large volume applications (dams,
backfill). These cases do not require a particular batch size but do
require a particular
THROUGHPUT PER HOUR.
Once the batch size is settled, how many batches do you need per hour?
crews, each using their own 2 yd bucket, return their bucket every 5
- 2 yd batch required every 2.5
minutes, 4 x 12 = 48 yd per hour.
- 2.5 minute cycle time is
- Three crews with 2 yd buckets
every 5 minutes.
- 2 yd batch every 1.66 minutes
or 6 x 12 = 72 yd per hour.
- Depending on the product
being produced, at 2 yd per batch, even the fastest mixer may not be able to
keep up. A good twin-shaft or planetary mixer can almost produce a batch of
high-slump concrete in this time, but it's pushing it. If the aggregate and
cement discharge times are no more than 5 seconds each, and water is added
in 15 seconds or less, it can be done. But if the product is SCC, dry cast
or block mix, even the best mixer cannot mix a batch properly in less than 2
minutes. You need to use a bigger bucket, re-think your production schedule
to stagger the crews' needs, or plan for a second mixer.
Two rules appear here:
Compare production requirements
with mixer throughput. If too great, increase batch and container size or
stagger production over a greater time.
If still too much for a reasonable
mixing cycle time, you need a second mixer.
First, you need to calculate the amount of concrete required per hour, then you
need to determine the mixing cycle time. To determine the hourly
throughput, consider the process the batch plant is feeding.
- Block plant producing 1
pallet of 4 blocks every 6 seconds. Each block weighs 30 lb.
- 4 x 30 = 120 lb every 6
seconds, and 120 x 60/6 = 1200 lb per minute, x 60 = 72,000 lb per hour.
- Assuming the yield per yd is
4050 lb, you need 72,000 / 4050 = 17.8 yd of dry cast mix per hour.
- Mixing cycle time for block
mix, even in a high-speed planetary mixer, may be 3 minutes due to the small
percentage of cement. Mixer output, therefore, is 17.8 x 3/60 = 0.9 yd.
- A mixer delivering 1 yd of
compacted concrete would do the job. If a spiral blade mixer with a mixing
cycle time of 5 minutes were used, the mixer would have to deliver 1.5 yd
compacted, raising its cost to perhaps more than a high-speed mixer.
- Its less intense mixing
action would force you to use more cement for the same strength product,
raising the cost even further.
- Two pipe machines, the first
casting 3 pipe with combined volume of 10 yd (for largest form combination)
every 15 minutes.
- Second with combined volume
15 yd every 20 minutes, of different mix design.
- Required output = 10 x 60/15
+ 15 x 60/20 = 85 yd of dry cast mix per hour, at peak production with
- Mixing cycle time for
dry-cast mix in a high-speed planetary mixer is typically 2 minutes but at
worst case may be up to 2.5 minutes when the moisture measurement system is
coping with changing aggregate moistures.
- Mixer output, therefore, is
85 x 2.5/60 = 3.5 yd compacted concrete.
- Note that some mixers need to
be de-rated when making dry-cast mixes and this will increase the size
required. Again, drum or spiral blade mixer would need 3 to 4 minutes to
make this mix and the mixer size would increase to 5 to 7.5 yd as a result,
increasing cost and again requiring up to 20% more cement, increasing
production costs for the life of the mixer.
The rule here is simple:
Taking the combined, maximum
throughput per hour, divided by the number of batches that can be made per hour,
gives the required mixer output.
Where the bucket, hopper, skip or
truck holds more than one batch from the mixer, first determine the time in
which the container must be filled. This will allow you to size the mixer
to fill it with your required number of batches.
- Trucks are 6 yd each and the
crews can use, at most, 90 yd per hour.
- 90/6 = 15 trucks to be filled
each hour, or 1 truck every 4 minutes.
- A twin-shaft mixer producing
SCC has a maximum mixing cycle time of 2 minutes, therefore two batches of 3
yd each are required for each truck; the mixer must be capable of 3 yd
- Note that, even with a small
increase in either number of trucks per hour or mixing cycle time, it would
be necessary to fill the trucks with single batches to avoid a back-up. You
could opt for under-filling the trucks (say, 4 yd in the 6 yd truck) and use
a 4 yd mixer, or go for the full 6 yd mixer and have extra capacity if
needed later on.
What mixer is best for my
- Planetary mixers give the
best quality product, with minimum cement usage, for all types of mix and
batches of up to 4 yd.
- Above 4 yd, consider multiple
batches from a planetary type, due to its high speed and superior mixing
- If this is not possible, the
twin-shaft is the concrete mixer of choice, especially for wet cast slump concrete
- If throughput is low and a
slower mixer, such as spiral blade, could be used, consider the saving in
cement over the life of the mixer if a planetary is used instead. Even if
cement saving is not a factor (as with products where the cement percentage
is specified beforehand) the cost of a planetary mixer might not be much
higher than a spiral blade type of the same size, making it still the type
- Note that lifetime cost
per batch of twin-shaft models is usually much lower than that of planetary
and pan mixers, due to their lower wear.
More mixer facts
The effects of mixer charging
order to get the fastest mixing cycle time from pan, planetary or twin-shaft
batch mixers, the mixer has to be filled quickly.
Often overlooked, the system for getting aggregates and cement into the mixer is
critical and can prevent a plant from ever living up to expectations if not
considered up-front. See
mixing cycles for details.
Aggregate charging – 10
Cement charging - 15 seconds
Dry mix time - 20 seconds
Water charging - 20 seconds
Wet mixing time - 20 seconds
Discharge - 10 seconds (twin shaft)
20-40 seconds (planetary)
If a mixer moisture correction
system is used, expect to add from 20 to 40 seconds onto the complete cycle.
Also, if high rate plasticizers or SCC admixtures are added after the water,
expect a further 20 to 40 seconds for full dispersal and action. Note also
that SCC will increase in slump as mixing continues; a compromise must be
reached between the amount of admixture and the acceptable mixing time.
Note that we consider only the
mixing cycle time, from completion of one batch discharge to the next. The
batching (weighing up) time does not come into the equation because after the
first batch of the day it always happens in parallel with the mixing - as mixing
proceeds, the next batch is being weighed up. Even the fastest mixer
cannot outpace a batch weighing system, and most batch plants can keep two
mixers supplied without missing a beat.
Skype or email us - see Contact Us on menu, for fast, free advice and a full evaluation of your plant and
mixer needs. We refurbish older plants, supply mixers, hoppers and scales,
fabricate on site and install the whole system if you wish.