Pigments, Water Cement Ratio & Freeze thaw resistance

[fearghusq]

How do you normally account for pigments in concrete mix design?

Here is my problem. I am working on a project to increase the durability of products manufactured by a precast company. They are having major problems with freeze thaw durability of concrete flags.

To overcome this I am proposing changes to mix design to be in line with BS 8500 (Exposure XF3 - Horizontal, exposed to freeze thaw without deicing salts) which gives:

360 kg/m3 CEM

W/C 0.45

5.5% Target Entrained Air

I have been told by pigment manufacturers to treat the pigment as an aggregate. However they significantly increase the water demand of a mix, so is it acceptable to treat it as an aggregate with high water absorption? (ie 'soaks' up water contained in wet agg). Therefore allowing you to add additional water to mix and remain below the 0.45 W/C limit.

Or do you have to treat it as a binder (even though it will not contribute to strength development) thereby increasing your cement content allowing you to add additional water and maintain the 0.45 W/C ratio.

Any help any more experienced engineers or concrete technologists could give would be greatly experienced.

Thanks for reading!

 

[Ron]

Pigments are non-cementitious, so shouldn't be considered in the cement count. Treat as highly absorptive aggregate. Consider pre-soaking the pigment. Add a small amount of surfactant to improve wetting.

 

[concretemasonry]

Soaking can be very dangerous unless you really know how much water is added since for a production control in a plant it becomes when it comes to dosing/batching.

I assume you are using synthetic iron oxides (except for blues/greens). With these, the recommended maximum percentage is 10% and normally 2-3% is common in many products that are exposed since the heavy dosage is a little "gaudy"

In a plant you probably have some accelerated curing method.

Dick

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

 

[Ron]

Dick...using an aqueous colloid for the pigments is not really a big deal. The amount of water used for the soaking can easily be compensated in the mix proportions as can the amount of pigment...which as you note is usually relatively small. Creating an aqueous colloid just makes the dispersion a bit easier and more consistent.

 

[concretemasonry]

Suspending pigment is water is not new. Some plants use wet pigments for easy of batching. With these most systems use an agitator. Most concrete products are made using dry materials and not have to worry about freezing and transportation. - This for high volume production in modern automated plants.

Since this is obviously a wet concrete, water is only a factor in the mix that has a high water content. My background is high strength concrete products in high production automated plants where excess water is avoided for production purposes, even though it is the cheapest way to increase strength - A different world and technology. I always have to remind myself to shift gears.

Dick

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

 

[fearghusq]

Many thanks for your help and advice guys.

They are, as you pointed out, iron oxide based pigments (elementis manufactured).

Do either of you have an idea of typical moisture absorption values for these?

While ive done absorption testing for the other aggregates - i dont have all the neccessary kit here, have to travel to and use another lab 80 miles away!

Many thanks for all your help
Fearghus

 

[Ron]

If you make an aqueous suspension of the pigment, the absorption doesn't matter. Just put it in enough water to create the suspension, then subtract that amount of water from the amount of mix water you'll add for the concrete mix.

 

[concretemasonry]

True - The absorption is a non-factor, especially considering the small amount in a typical mix.

As an example, in zero slump concrete the absorption of the pigment is not enough to dry out a mix. Due to the large volume of aggregate used, a wet period can raise the moisture content of drained sand to a point where the absorption of a dry pigment will not dry out the mix to a level where an acceptable product could not be made, but the strength went up over the target, but the tolerances were lost and the color changed. This is based on millions of sf of products produced annually.

Elimentis may be the supplier, but when it comes to synthetic oxides, the different colors required include factory blending of various basic color oxides since the global processors are limited.

The major use of pigments in concrete world-wide is in zero slump concrete products where moisture control is important and the products are subjected to performance testing and not theoretical mix designs because the manufacturing systems are the controlling factor.

If you are using any color with a blue or green "tint", chrome oxides (pricey) may be used instead of iron oxides that are limited to yellows, reds to blacks with no cool tones. Avoid carbon based "pigments" for blacks and grays.

Dick

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