Precast Slab - Contamination On Mould Face

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  #1  
Old 01-11-18, 06:48 AM
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Question 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). This is an uncommon defect and does not affect all units cast.

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 for compaction.

We've had some of the darker material sent off for testing, only to find 'trace' elements of mineral oil. My gut feeling is that this is not a material issue, but some sort of water-cement differential locally at these points. Could this be aggregate shadowing, from too much (or too little) compaction?

We've experienced a similar issue 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!
 
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  #2  
Old 01-11-18, 08:31 AM
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I would assume it's related to the release agent. Try making a panel and mark/mask off strips of the mold. Spray one strip very heavily with release agent and another strip with no release agent and do the remainder of the mold with a normal amount of release agent so you can get it out of the mold. If you have more dark spots with the heavy release agent and few or no spots where there was no release agent you'll know that it is probably the release agent.
 
  #3  
Old 02-05-18, 01:26 AM
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Thanks for your input!

We've performed many trials on release agents and I'm confident that isn't the issue in this case.

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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Last edited by JoeLOR; 02-05-18 at 01:45 AM. Reason: Included photos
  #4  
Old 02-05-18, 04:47 AM
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I was curious what you found out. Thank you for posting back.
 
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