Precast Slab - Contamination On Mould Face

 
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Old 01-11-2018, 05:56 AM   #1
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Precast Slab - Contamination On Mould Face


Hello,

I work for a precast concrete facility.

We are experiencing continued dark contamination to the mould-face of our lattice slab product (pictures attached).

Has anybody experienced anything similar?

The 'pattern' is seemingly random, sometimes large sometimes small, and never across the full face of the cast unit. The steel pallet that this face is cast upon is free of defects and misted with release agent prior to cast. After casting, the pallet and mould is also oscillated.

We've had some of the darker material sent off for testing, only to find 'trace' elements of mineral oil. My gut feeling wasn't a material issue, but some sort of water-cement differential locally at these points.

We've had a similar issue crop up recently, however in these cases there is evidence of efflorescence and actual damage to the face (with a very dusty texture) - we believe this issue is unrelated and is caused by unmixed material.

Any advice or recommendations would be appreciated.

Cheers!
Attached Thumbnails
Precast Slab - Contamination on Mould Face-2.jpeg   Precast Slab - Contamination on Mould Face-8.jpeg   Precast Slab - Contamination on Mould Face-55.jpeg   Precast Slab - Contamination on Mould Face-1.jpeg   Precast Slab - Contamination on Mould Face-3.jpeg  

Precast Slab - Contamination on Mould Face-44.jpeg  
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Old 02-04-2018, 10:14 AM   #2
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Re: Precast Slab - Contamination On Mould Face


Are you sure itís not curing out? How do you cure your precast ?
You can try consulting with a concrete tester.


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Old 02-05-2018, 03:25 AM   #3
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Re: Precast Slab - Contamination On Mould Face


Quote:
Originally Posted by Quality5280 View Post
Are you sure itís not curing out? How do you cure your precast ?
You can try consulting with a concrete tester.


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Thanks for your input!

We cure in a curing chamber, I believe this is at an ambient temperature of around 18 degrees Celsius.

We've recently installed a mesh above where our dry materials are batched. It seems due to the geometry of the drum that delivers sand dust etc, some material is becoming compacted and eventually released in the form of dry lumps (see photos below).

We believe these lumps are breaking down into smaller lumps when they reach the wet mix, and settling to the bottom face of the mould once compacted on the oscillation table. I imagine these dry lumps would become saturated with water and cure at a different rate to the surrounding cement, leaving us with the dark staining.

We haven't had an issue since installing this mesh, but are struggling to replicate the original issue through trials. Hopefully we'll be at a point soon where we can turn the problem on and off and confirm that the dry material was/is the problem.





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