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Kamal Khodja, sales

Chris Shawcroft, service

Robin Shepherdson,
engineering

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, see below.
• 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.

Fixed batch size

Once the batch size is settled, how many batches do you need per hour? 

Example 1:

• Two crews, each using their own 2 yd bucket, return their bucket every 5 minutes.
• 2 yd batch required every 2.5 minutes, 4 x 12 = 48 yd per hour. 
• 2.5 minute cycle time is easily achieved

Example 2:

• 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.

Throughput per hour

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. 

Example 1: 

• 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.

Example 2: 

• 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 largest forms.
• 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.

Multiple-batch containers

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.

Example: 

• 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 compacted output.
• 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 application?

• 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 action. 
• If this is  not possible, the twin-shaft is the mixer of choice, especially for wet cast slump concrete and SCC. 
• 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 of choice. 
• 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 methods

In order to get the fastest mixing cycle time, 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 Planetary or Twin Shaft mixing cycles for details.

• Aggregate charging – 10 seconds

• 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.

Call us or use our "Ask the Experts" live chat box, on menu at left, 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.