CENTRAL vs DRY MIX CONCRETE PLANTS - a discussion
From a LinkedIn
Concrete Producer Network discussion
The topic of the discussion was "Dry Batch or
Central Mix Plants: pros and cons??"
interesting views came up, and some very clear descriptions on
types of batch, based on the mixing given. The subject of
mobile mixers came up and I (Robin Shepherdson in this
discussion) added a comment on how you can make high slump
concrete, even though you are discharging via inclined belt.
Mark Fitzpatrick -
"Just to give you my point of view I, like you, setup and operate mobile batch
plant operations on project sites. So I think we're speaking the same
language which is not a commerial operations point of view.
pretty much everyone stated above wet mix plants are faster, with better
quality, and a more efficient use of your labor while have more wear
parts with a larger maintenance schedule.
As Mr. Beers stated dry
batch plants can be set up to produce 200cy's an hour, but don't get
confused with the load time of 2min 30seconds. That's just loading the
trucks. It doesn't account for the additional 5 to sometimes 10 minutes
it takes a driver to mix his load and wash down his truck before he
leaves the batch plant. With that in mind you need to ask yourself what
is the turnaround time for your trucks from the plant to the job back to
the plant. The major production issue with a dry batch plant other than
consistent quality is number of trucks needed to meet yards per hour
demand plus the amount of extra labor that is needed to staff that
equipment. That's really where you wet plant saves you money(and of
course the consistent quality).
My first question about the
twinshaft mixer is will it be a trailered mobile unit or fixed on a
stand so as to allow your delivery fleet to pull under the mixer to be
loaded? My questioning of this is because mobile units are typically not
capable of producing high slump concrete. The second item to consider is
that will you have a large enough mixer to single batch full size loads
into your mixer trucks? You can send smaller load sizes, but you will
again not be utilizing your delivery fleets full potential. You can
double batch if you have a smaller mixer, but this will again be a
little slower the running a full load each cycle. However, this still
should be faster than a dry batch while fully utilizing your labor force
If I had to produce 650yd to 1000yds a day I'd go
with the wet plant 10 times out of 10."
Shepherdson - "Mark, thanks for your excellent summary, from which we
can all learn. Many of us are on the outside of this and do not know how
the insiders manage their plants. I supply equipment and the trailer
mixer you describe is one of the items we supply (see
www.Ocmer North America/mobile-mixer.htm). Since these mixers have to
feed the truck from an inclined belt, there is a limit to the slump.
However, SCC has been successfully produced in this way so it is
possible. It is also possible to make a 2" slump mix and add more water
or your SCC admix at the truck.
I would also ask if I can have
your permission to print your comment on our web site?"
Fitzpatrick - "Robin, thank you very much for the compliment. I have
actually seen higher slump (excluding SCC) concrete produced out of
mobile twin shaft units and while it was sloppy it did work. You also
make an excellent point about adding the additional water or plasticizer
after loading the truck from the twin shaft unit. Adding HRWR after or
during the truck loading is probably the best method to accomplish high
slump concrete from these types of mixers from what I've seen. And I
definitely haven't seen everything. I don't mind you posting my comment
at all. I'll also check out your site today when i get a free moment.
Thanks again and take care.?
Thank you very much for the
compliment. I have actually seen higher slump (excluding SCC) concrete
produced out of mobile twin shaft units and while it was sloppy it did
work. You also make an excellent point about adding the additional water
or plasticizer after loading the truck from the twin shaft unit. Adding
HRWR after or during the truck loading is probably the best method to
accomplish high slump concrete from these types of mixers from what I've
seen. And I definitely haven't seen everything. I don't mind you posting
my comment at all. I'll also check out your site today when i get a free
moment. Thanks again and take care.
Butch Thomas - A. Transit
Mixed (or "truck-mixed") Concrete
In transit-mixed concrete, also called
truck mixed or dry-batched, all of the raw ingredients are charged
directly in the truck mixer. Most or all water is usually batched at the
plant. The mixer drum is turned at charging (fast) speed during the
loading of the materials. There are three options for truck mixed
Concrete mixed at the job site. While travelling
to the job site the drum is turned at agitating speed (slow speed).
After arriving at the job site, the concrete is completely mixed. The
drum is then turned for 70 to 100 revolutions, or about five minutes, at
Concrete mixed in the yard. The drum is turned at high
speed or 12-15 rpm for 50 revolutions. This allows a quick check of the
batch. The concrete is then agitated slowly while driving to the job
Concrete mixed in transit. The drum is turned at medium speed
or about 8 rpm for 70 revolutions while driving to the job site. The
drum is then slowed to agitating speed.
(More information on ready
mixed concrete trucks can be found in the Delivery section.)
B. Shrink Mixed Concrete
Concrete that is partially mixed in
a plant mixer and then discharged into the drum of the truck mixer for
completion of the mixing is called shrink mixed concrete.
Central mixing plants that include a stationary, plant-mounted mixer are
often actually used to shrink mix, or partially mix the concrete. The
amount of mixing that is needed in the truck mixer varies in these
applications and should be determined via mixer uniformity tests.
Generally, about thirty turns in the truck drum, or about two minutes at
mixing speed, is sufficient to completely mix shrink-mixed concrete.
C. Central Mixed Concrete
Central-mixing concrete batch
plants include a stationary, plant-mounted mixer that mixes the concrete
before it is discharged into a truck mixer. Central-mix plants are
sometimes referred to as wet batch or pre-mix plants. The truck mixer is
used primarily as an agitating haul unit at a central mix operation.
Dump trucks or other non-agitating units are sometimes be used for low
slump and mass concrete pours supplied by central mix plants. About 20%
of the concrete plants in the US use a central mixer. Principal
Faster production capability than a
Improved concrete quality control and consistency
Reduced wear on the truck mixer drums.
There are several
types of plant mixers, including:
Tilt drum mixer
shaft paddle mixer
Dual shaft paddle mixer
The tilting drum mixer is the most common American central
mixing unit. Many central-mix drums can accommodate up to 12 yd3 and can
mix in excess of 200 yd3 per hour. They are fast and efficient, but can
be maintenance-intensive since they include several moving parts that
are subjected to a heavy load.
Horizontal shaft mixers have a
stationary shell and rotating central shaft with blades or paddles. They
have either one or two mixing shafts that impart significantly higher
horsepower in mixing than the typical drum mixer. The intensity of the
mixing action is somewhat greater than that of the tilt drum mixer. This
high energy is reported to produce higher strength concrete via to
thoroughly blending the ingredients and more uniformly coating the
aggregate particles with cement paste. Because of the horsepower
required to mix and the short mixing cycle required to complete mixing,
many of these mixers are 4 or 5 yd3 units and two batches may be needed
to load a standard truck or agitator.
Pan mixers are generally
lower capacity mixers at about 4 to 5 yd3 and are used at precast
concrete plants. Central Mix hands down is the best.