Ocmer North America

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??"

Some 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 FitzpatrickMark 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.

As 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 and equipment.

If I had to produce 650yd to 1000yds a day I'd go with the wet plant 10 times out of 10."

Robin ShepherdsonRobin 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?"

Mark FitzpatrickMark 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: 

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 mixing speed. 
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 site. 
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 advantages include: 

Faster production capability than a transit-mix plant 
Improved concrete quality control and consistency and 
Reduced wear on the truck mixer drums. 
There are several types of plant mixers, including: 

Tilt drum mixer 
Horizontal shaft paddle mixer 
Dual shaft paddle mixer 
Pan mixer 
Slurry 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.