Water Control ?? at Concrete Batch Plants

[Rx4slump]

I am an inspector in Southern California. I have had an issue with the way "almost" all the batch plants have been adding the water to the load of concrete. Regularly, the batchman will batch very dry, then each driver will adjust the load at the wash rack. I have had to begin working at the wash rack just to account for the water added (which is a source that is NOT calibrated). The drier the loads, the more inconsistent our slumps seem to be at the jobsite. Concrete mixer drivers and their varying degrees of experience seem to be able to adjust the load properly when a reasonable amount of water is required (5 to 10 gallons), but when they add 20 to 30 gallons it is a mess. I don't even think this practice is allowed by ASTM C94. On large pours, I always encourage the batchman to add more water after it is apparent that each load is requiring 20+ gallons. Am I being unreasonable? They often have told me I am. I believe that all the moisture control devices are to allow the batchman to get close to achieving the proper slump without repeated adjustments. Is this practice universal or just in southern California? I have been working on a batch plant inspection method which scores the batchmans performance, including water initially batched. I could use your comments for my research. Thank You.

 

[genomty]

Rx4slump :

I believe that certain amounts of water can be added to concrete in order to achieve a specific workability (slump) in field, however, some restrictions should be observed. Guidelines outlined in ACI 304 mention that w/c ratio should not be exceeded. ASTM C 94 allows to add water at jobsite to adjust the slump only once. How much water can be added? it will depend on how much water was not added at the batching plant.

In my personal experience where a slump control has been required for large pours a pre-construction meeting has been awfully helpful. In those meetings the CMT guys from batching plant and from the field laboratory usually have agreed a solution that has demonstrated to be adequate:
Requiere to the plant manager not to send any truck with more water in the tanks than the water missing to the mix. Of course it has some restrictions, for instance, if the batching plant is too far how the driver will wash the drum?.

I'm not very familiar with the procedures at So. California, here in Monterrey Mexico, most of the batching plants always add to the mix an fluidizer admixture to control workability and setting time.

 

[Qshake]

My experience in this type of inspection goes back many years. I hope that some, or perhaps all, of it will be useful.

I worked for a government agency at the time and inspectors were provided at batch plant to oversee the quality of the concrete produced. Further it was our duty to inform the inspector at the job site how much water he could add on site without compromising the product. This, of course, was simply one of many tasks we performed.

At the beginning of the day we took samples of the coarse and fine aggregate. The samples were used to determine the moisture content and yield for a batch. Some of the plants we inspected were computerized and had moisture gauges in the aggregate hoppers while others did not. Most times often than not the mix water was adjusted at the hands of the operating engineer based on what information was taken from the samples. That is to say that we worked with one another never really believing that his gauges were perfect nor were my samples.

Typically, the driver (who is responsible for the cleanliness of the truck) will wash the small unmixed cement and aggregate from the truck's hopper and back fins. Typically this doesn't amount to much at all. Or shouldn't. If it did, I always spoke with the operator to make corrections. One thing that must be realized is that you must not try to overcompensate for each load. This will drive everyone crazy! However, since we alway wrote on the delivery ticket how much water could be added on site we didn't worry to much. If more water was added in the yard, the amount on the ticket was reduced.

Naturally, additional samples were taken throughout the day especially if it was a large pour. The moisture content will change of course throughtout the day. Since the water filtrates down during the night, the most difficult batches are the very first ones. The computer says the material is really wet and yet the water has dripped out the chute. This leaves a somewhat drier material actually in the bin. And I've had situations where we've actually tested the material's moisture content on the conveyor belts in order to determine the actual moisture.

Well, I won't go into the whole thing, but I think the best was reporting, on the ticket, what water can be added at the site. This depends, of course, on having someone in that role.

Keep your chin up (it doesn't appear that your being overzealous) and Good Luck.


Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Rx4slump]

More specifically the batchman are batching so dry that the drivers are adding an incredible amounts of water at the wash rack before the truck is leaving the batch plant. Then making another adjustment at the site. This practice is so widespread that I thought I must be missing something. ASTM C94 & ACI304R 3.4 certainly don't allow for multiple adjustments, especially from a source thats not calibrated. Could this sloppy water control account for strength specimens wide variances. They consistantly break above minimum values but they are very inconsistant. ?? Thanks

 

[Ron]

Rx4Slump...yes, this practice leads to significant strength variation and variation in shrinkage of the finished concrete. This can lead to durability and other long term issues with the concrete that don't present themselves in the review of the concrete strength alone.

This practice should be stopped and appropriate calibrated controls instituted. The process Qshake has outlined works and now with automation and computerization is even easier to implement.

 

[illeniumsd]

Rx4slump - I feel your pain. As an inspector in Southern California I deal with that problem every single day! If it's not the drivers adding too much water at the rack, it's the pumpers adding all the allowable without even looking at the load or consulting with me. I think every single one of us who Inspects loads from Hanson, Vulcan, Robertsons, etc... deals with this same problem.
Personally I would rather the batchman batch the load on the dry side that way the slump can be adjusted on site. However, if I know the batchman is good at what he does and understands the w/c ratio then i'll let him batch it right on. As far as drivers adding water at the rack sometimes you have to go out there and play "Mr. Inspector" which really blows. I don't like playing hardass and flexing the proverbial rejection muscle but if they keep adding 20 - 30 gallons i'll reject loads on the spot or let the Tech or Inspector on the site know and have em slump the trucks and reject on the site. When in doubt always remember to refer to the codes. Best of luck!!!

 

[dicksewerrat]

you have to be tough on the slump. If you lived downstream from the dam they are building and the concrete was 25% weaker than the design? What about the bridge over the freeway?after you send 5-10 loads to the dump, the drivers and batchmen will get the idea. never reject in the plant though, make them work for the right to throw away 9+ yards of concrete.