Discoloured blotchy concrete

[Jonathan Martin]

Hi,

I was hoping somebody might be able to shed some light on how the concrete in the attached photos has ended up blotchy (see attached photos) and how we might avoid it in the future... There was no issue during the pour of this sample panel but the discolouration has got us all stumped.

Thanks in advance, Jonny

 

[darthsoilsguy2]

maybe those areas have a higher paste content? maybe due to consolidation issues and the mix design? is it old? if not, it might look better soon

 

[Jonathan Martin]

The attached picture better shows the location/extent of the blotches - they seem to be concentrated around the dywidag bars and spacers which in this pour were of the "wagon wheel" type. Perhaps the vibration in this area was increased as these items also acted as vibrator when the poker came into contact with the Dywidags and reinforcement and this led to increased compaction/overworking in these areas....

The pour is fresh - these photos were taken within a day or 2 of the formwork being removed. It is something that I haven't come across before and I am concerned that we will have the same issue when it comes to pouring the permanent works.

 

[darthsoilsguy2]

considering it is 2 days old, i think my hypothesis makes sense. the vibration takes more paste and water into the areas where aggregate has a harder time getting too. i suspect the mix design could be improved upon on future pieces for this application so that consolidation occurs with less vibration with aggregate sizes which aren't dimensionally constrained. If you still have these things in September, post followup photos when they are a month old.

 

[Ingenuity]

When I looked at your first photo I initially thought uneven coating of form release compound, but the second photo does not support that.

Are the forms steel-faced?

 

[Ron]

Actually agree with both DSG2 and Ingenuity. There is a vibration problem. If you look at the bottom of the slab, you see underconsolidated concrete and at features that intersect the forms you see laitance. This is indicative of mix variations and variations in the bleed water accumulation. Most of the blotches are oval or elliptical with a vertical orientation, indicating an issue with bleed water rising.

There is also a likely contribution from the inconsistent application of form release. This can either trap or release bleed water at the surface interface, depending on the texture of the form surface, thus presumably, the reason for Ingenuity's question about the form material.