Can consistent quality from mobile batch plants be achieved? 5

[mikeh]

We are currently looking at doing a substaintial amount (8000m^3)of concrete work in an underdeveloped country. As such we must bring with us everything that we we need to produce, transport and place concrete.<br><br>We are currently exploring two options.<br><br>1) Set up a traditional batch plant on site, using ready mix trucks for transport.<br><br>2) Use a self contained, truck mounted, mobile batch plant. i.e. this unit carries water, cement, aggregate and uses a converyor belt and auger sytem to move, mix and place concrete.<br><br>With option #1 we know what we are getting into with respect to producing consistent quality concrete. <br><br>I feel there a quality control issues with Option #2. Can anyone provide some insight into quality control issues using this type of &quot;mix-it-as-you need-it&quot; type mobile batch plant? What adjustments need to be made to our quality control/assurance program do we need to make if we use the truck mounted plant?

 

[Ron]

Mike,<br>Would agree that your QA job is going to be significantly more difficult with the Self Contained system.&nbsp;&nbsp;Unless you are placing only small quantities at a time, I would certainly opt for a traditional batch plant and transit mixers, both from a production standpoint and QA/QC.&nbsp;&nbsp;Keep in mind that many small batches will inevitably incur more variance than larger batches. <br><br>If you are looking at achieving something comparable to ACI recommendations for statistical quality control, the traditional setup would be better.

 

[ggIMG]

This may be late for the original question, but in case anyone else needs this info, our firm faces a similar issue for a series of upcoming projects, and I did some fairly extensive research on this topic.

First of all, there are a few key considerations for remote concrete production. First is the target placement rate in cubic yards per hour, assuming a normal workday with time out for breaks and cleaning the equipment. Choice of a system (e.g. how many units you need) is governed by this number. Second is: reliability matters a whole lot more than first cost. Third, ease of cleaning the equipment (if you use typical 8 to 10 yard transit mixers, how do you expect to keep the drums spotlessly clean inside so that buildup doesn't occur?). Fourth, ease of obtaining spare parts (assume you're already planning to hold spares locally for the most likely replacements).

Mobile batch plants, or Volumetric Mobile Mixers (VMMs) have traditionally been regarded with scepticism due to a) volumetric batching and b) relatively low contact time between materials and mixing blades. However, in the past five years, the reputable manufacturers of these machines have formed the Volumetric Mixer Manufacturers' Bureau to produce specifications and assure manufacturer compliance.

I've read the VMMB certification specs. The bottom line is, machines meeting these specs will produce concrete that is as good as anything produced with a conventional batch plant & transit mixers. However, I would personally use VMMs only for &quot;conventional concrete,&quot; not for high-spec mixes or heavily modified mixes (lots of admixtures etc.), *unless* I first arranged with the machine manufacturer to run some trial batches on one of their sales-demonstrator machines, and have the trial batches subjected to full lab testing. If you can come up with a really &quot;difficult&quot; mix design, and your preferred brand of VMM can handle it, chances are it can handle other less &quot;difficult&quot; special mixes as well.

Machines produced since 1999 or so should carry the VMMB certification plate. If a machine isn't VMMB certified, don't buy it; and if a concrete supplier using VMMs doesn't have certified machines, don't use that supplier.

From what I've been able to find, the most highly recommended VMMs are made by CemenTech and by Reimer. I don't work for either of those, so this isn't an advertisement! If our firm decides to use VMMs on an upcoming project, we'll probably go with CemenTech.

If you use VMMs, you absolutely need skilled and conscientious operators. VMM manufacturers will train, and even if they charge for training, it's worth the price. I would assume the hourly wage questions have been addressed by the relevant unions already, since these machines have been around a good long time.

There are two alternatives, that may turn out to be useful depending on your job situation.

The first is the &quot;self-loading transit mixer,&quot; commonplace in Europe (notably Italy) but barely known in North America. This is an offroad chassis with 4-wheel drive & 4-wheel steer, equipped with a mixer drum of from 1 to 4 cubic yards, mounted on a slewing (turntable) frame for 270-to-360-degree placement (front, rear, or side discharge), and equipped with a) a self-loading bucket similar to a front-end-loader shovel and b) a weigh-batching computer (optional accessory, highly worthwihle).

This machine is in effect a self-contained system with proper weigh-batching of all dry ingredients (water is batched by volume via a meter). The weigh-batch computers can handle 30 or so mix designs and up to 20 ingredients per mix design, and print batch tickets on demand. Unlike the VMM, it doesn't require a front-end loader or cement silo (you can use a silo or load cement by bag into the scoop). These machines will also climb steep grades fully loaded, go through deep water, and I suppose some day they might even learn to fly! Personally they are my first choice for remote concrete production.

Some manufacturers' names to put into a search engine: Carmix, Dieci, Merlo, Terex/Mariner, Italmacchine, Utranazz. There are a few more I can't recall offhand, but try putting &quot;mixer&quot; and &quot;dumper&quot; into the search engine, as the manufacturers of these often also make the Euro type of offroad site dumper. Price at the factory gate for a 3 to 4 yard machine should be in the neighborhood of US $75,000, which is amazingly good.

As these machines are virtually unknown in the USA market, the hourly rates for operators haven't been determined yet. What I would do is sit down with the head of the local Operating Engineers' union and discuss the machine in some detail. You want the most skilled operators you can get, and you're already saving money by using this machine (doesn't need ancillary supporting equipment & labour), so it's fair & reasonable to settle on what might otherwise look like a higher than normal hourly wage.

The other option is a large site-mixer with an automatic loading hopper with a scale for weigh-batching. In the USA we used to have a wide choice of manufacturers of machines up to 16-S (16 cubic foot mixed batch) but alas they are no more, so you have to import. Check out

http://www.winget.co.uk

for some good examples of these; Winget has been around for about a century. Other manufacturers include Utranazz (UK), Altrad/Lescha (Germany), IMER (Italy; their USA branch doesn't advertise these but the USA reps may be able to handle the importation issues), and most of the European makers of contractors' grade concrete mixers. Aside from Winget and Utranazz in the UK, it would appear that Germany and Italy are the places to look for these.

These machines are made in two types. One is a large version of the common &quot;portable mixer,&quot; with a 200-litre drum (equivalent to a 6-S in the old AGC-USA ratings). The other has a horizontal drum that reverses rotation to discharge (like the old horizontal transit mixers). In both cases you have a hydraulic loading hopper on one side, with a scale; and a calibrated water tank. Optionally you can order a cable-controlled drag-scraper that allows one person to load the aggregates into the hopper without fatigue (highly recommended). These machines range from 200 litres to 500 litres in mixed batch output. Price range is from about $18,000 US to about $45,000 US depending on size and options.

By the way, the cool thing about the British manufacturers of these (e.g. Winget, Utranazz) is that the quoted capacity of the machine (in accord with BSS specs) is the actual mixed batch output, not the dry materials input.

For labour, you need: One person in good physical condition to operate the aggregate drag-scraper (low to medium skill level). One person to batch the cement (from sacks), admixtures if any, and monitor the water system, and operate the mixer controls. This person needs both strength and smarts. Or you could have two people switching off on aggs and cement, and have a mixer operator who is responsible for water & admixes; in which case the mixer operator needs the smarts but not necessarily the strength. The people loading aggs & cement are probably at Labourers' wage rates. The mixer operator might be something to discuss with the local union representatives; again, be willing to go for a higher hourly rate in exchange for a higher standard of training & skill.

One thing these machines don't have is a drum revolution counter or batch timer. However it's a simple matter to add a rev counter, you can build it yourself from parts obtainable at any decent electronics store (e.g. Radio Snack or better). I've asked a couple of the manufacturers of these machines to figure out if a weigh-batching computer could be added (similar to the one on the self-loading transit mixer), and so far, one manufacturer said yes and another is working on it. If you can't get the computer option, you won't have weigh-batch tickets for each batch, and in that case a conscientious operator is a major asset.

For any new concrete production system, it's probably worthwhile to do some rehearsal-runs using only sand or coarse aggs, just to get the pace of the work figured out and all the workers choreographed like a dance performance before an actual project comes up.

Also, regardless of what the manufacturers quote as maximum number of batches per hour, you want to re-calculate the timing yourself based on your own job conditions. Your jobsite conditions will probably be less favourable than the manufacturer's test conditions, so your production rate will be affected accordingly. It's better to know in advance so you can buy an extra machine ahead of time to increase your production capacity (rather than ending up doing &quot;concrete marathons&quot; or getting unplanned cold joints if you have to stop overnight in the middle of a pour).

I could go on about all of this for another two pages but I think I'll stop now (!).

If anyone finds this useful, post to this thread so at least I know I haven't wasted my time writing it!

 

[mikeh]

Thank you for reply (if somewhat belatedly) The info you provided will be helpful should this situation arise again.

Just so you know, we ended up setting up a traditional batch plant.

 

[BigH]

Perhaps I have been in the bush too much lately, but on a China expressway project, we did all of our concrete work using the portable 0.125 m3 mixers. Unless you have a massive pour, these, working in tandem or in triplet are quite find, in my opinion. We also did the same in Laos where it would have been riduculous to have set up any kind of stationary plant.

We found that the concrete we produced was quite consistent in properties - our standard deviations (in Laos) was in the order of 3.5% or so on some 200 samples. The nice thing about having the portable mixer on site is that the concrete goes into the structure almost immediately. You can get away with a lower initial slump - slump will decrease with time after mixing. We never did our work using volumes as a measure of proportions - except for some lined ditches. We always had a scale on site that wheelbarrows could be pushed onto to obtain the &quot;right&quot; weight.

I was quite happy with it - sometimes better so than with a ready mix plant.

I do caution that sometimes we, if we are from developed countries, try to &quot;out tech&quot; the normal ways of doing things in developing countries. I tried, for instance to introduce wheelbarrows on a job here in India - then roll the barrows over to the place of pour - but the concreting people didn't even know how to set up their ramps! - they overlapped the wrong way. Instead, they used little pans that they carried on their heads (the hard hats here come with a little cylinder on top of the hat onto which the pan fits nicely for stability. Then they carry it to the place of pour. Like pouring concrete one and half litres at a time. Instead of worrying, perhaps, about the mixing of the concrete, it is more so about how you are going to place it expediently once the concrete arrives at the jobsite - or, in the case of portable mixers already there, at the point of pour.

Anyway, that's my two cents worth (oops, 4 rupees!).

 

[nisko]

Techinical designer and producer of mobile concrete batching plants in Germany Euromix 100 & Euromix75

http://www.nisamidis.de