Different mixes in same placement

[Hemifun]

We have a large concrete placement planned and the contractor wants to use 2 different batch plants. The issue is each plant has a different proposed mix. Each mix by itself would be acceptable but I don't know if it would be a good idea to mingle the 2 mixes. One mix proposes the use of Type 2 cement while the other is proposing Type 1 with GGBS. In addition, they are using different AE and WR admixtures. Any thoughts would be appreciated. Thanks.

 

[flight7]

I don't have any definitive information to provide, but I would expect that your result would be patchy differences in the appearance of the cured concrete. If this is a slab pour you would probably have differences in the finishing characteristics of the two mixes, resulting in a patchy and very inconsistent finish. I have no idea how the different admixtures would react with each other when they come into contact. If you call ACI or PCA they might be able to give you more specific information.

 

[woodman88]

Have the contractor get the two plants together and have the plants decide on a mix they can both do?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BigH]

Just wondering what the concrete placement is for? Large gravity section or is aethestics important? You need to be careful on admixtures because there are two types and they can interact negatively. One thing is to get the admixture manufacturers to confirm that should the two admixtures intermingle in the interface zone that they will not prove to be harmful. Would the mixes be in different locations within the placement - i.e., one starting on the left and the other on the right? If so, this probably wouldn't create much of a problem so long as the admixture issue is negated.

 

[Hemifun]

The concrete will be used for a base slab of an oxidation ditch at a wastewater plant. The contractor will be getting trucks from both plants during the slab placement. It will be a large pour and 1 plant will not be able to keep up with it hence his proposal to use 2 plants. We have told him that we do not want 2 different mixes. Thanks for the input. Anyone else ever run into this situation?

 

[dcarr82775]

I know large pours are done using multiple plants. Is it possible to use two different plants by the same producer? That way at least you should have conformity in the admixtures.

 

[concretemasonry]

A lot will depend on the concrete producer's facility - number of cement silos, aggregate silos and capacities and the mixing capability.

I assume the hourly plant capacity of a single plant is not enough. Trucks are not a problem, since most trucks can be used at any plant that is a central mix facility.

If this job is that big, there should be a way to work out a way to upset the normal regular customer demands. Admixtures could be a problem since often the admixture equipment is installed and maintained by the admix supplier that has a long history of testing and documentation. Concrete suppliers often have a loyalty to different suppliers (aggregate, cement, cement replacements and admixtures) and a single job must be big enough to upset the apple-cart. The selection of the date and time of the pour can usually be shifted to early or late hours or a different day of the week to

As a concrete inspector, I was involved in many large pours where I had to be there the day before of materials sampling and often be there at 1:00 AM to 5:00 AM to do the preliminaries for a 500 to 3000 yard pour. There is not enough margin in a single bid project to upset the normal customers. Very often the pour schedule can be adjusted to make the most out of the situation to get a better time frame for traffic.

It is usually a team effort between contractors and suppliers to create a compatible situation. There are always possible problems with large pours that cannot be predicted.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[jike]

I would require the same mix design for both plants. This also means using the same cement and admixtures.

 

[msquared48]

If this is CDF, there may not be a problem.

Mike McCann
MMC Engineering

 

[BAretired]

Why, Mike?

BA

 

[msquared48]

If it is just fill material, CDF, what could be the problem if they are spec'd to the same density and f'c?

Perhaps I am missing something here...

Mike McCann
MMC Engineering

 

[BAretired]

I suppose a problem could develop if the admixtures are incompatible, but I don't know enough about it to offer any useful advice. In the absence of more definitive information, I would be inclined to specify that admixtures be identical irrespective of the supplier.

BA

 

[msquared48]

OK. I'll have to agree that a chemical comparison would be warranted.

Mike McCann
MMC Engineering

 

[BUGGAR]

Just tell the finishers on the job that they will have to finish two different mixes in the same pour and watch what happens.

In New York City, that would get them to walk off the job.

In Southern California, they would just add so much water you couldn't tell the mixes were different anyway.

 

[JLNJ]

God forbid that you have a problem, but, if you do, how will the courts (or arbitrator) determine responsibility and liability? It seems that the water would be very muddy.

Maybe the owner needs to be apprised of the risks and be allowed to have some input.