Causes of Ghosting
Question:
There are dark lines in my countertop where the reinforcing seems to show through. How can I fix this?
Answer:
This is called ghosting and is characterized by visible lines that outline the pattern of the reinforcing on the concrete surface. It is most often caused by changes in the concrete matrix due to aggregate segregation, changes in the water-cement ratio, and other changes in the cement paste. Any changes in the cement paste affect how it cures, and this affects the appearance of the finished product.
The primary cause of ghosting is pushing reinforcing material into the concrete after the forms are filled, which causes a pumping action directly under the reinforcing. This pumping action disrupts and changes the cement paste, which in turn results in ghosting.
Another cause of ghosting is to hang the reinforcing in the forms but pour the concrete through it. If the stream of concrete is split by the reinforcing but does not get a chance to remix below it, the aggregate gets pushed aside but the paste and cream flow back together. This causes a paste/mortar matrix change directly below the reinforcing that will cure slightly differently, resulting in ghosting.
A third way ghosting occurs is when reinforcing already buried in fresh concrete is moved or vibrated. The movement causes localized segregation, which in turn causes ghosting.
Other surface marks that resemble ghosting can be caused by leaky forms, excessive form release agent pooled or puddled in the forms, and items left on the curing concrete surface (like sponges, soda cans, polishing discs and hand pads). Anything that traps moisture in or on the concrete in some areas while other areas of the concrete remain uncovered and allowed to dry will cure differently. Concrete that is moist longer will cure more fully and the color will be more intense in those areas.
Ghosting is NOT caused by the reinforcing being too close to the visible surface. The fact that the reinforcing is close to the visible surface does not cause any changes in the curing of the cement paste.
Unfortunately, once these surface marks occur, there is no way to remove them. Prevention is the only way to ensure that ghosting doesn’t happen
A severe case of ghosting. This was caused by pushing the reinforcing
through a stiff, all-sand concrete mix.
There are dark lines in my countertop where the reinforcing seems to show through. How can I fix this?
Answer:
This is called ghosting and is characterized by visible lines that outline the pattern of the reinforcing on the concrete surface. It is most often caused by changes in the concrete matrix due to aggregate segregation, changes in the water-cement ratio, and other changes in the cement paste. Any changes in the cement paste affect how it cures, and this affects the appearance of the finished product.
The primary cause of ghosting is pushing reinforcing material into the concrete after the forms are filled, which causes a pumping action directly under the reinforcing. This pumping action disrupts and changes the cement paste, which in turn results in ghosting.
Another cause of ghosting is to hang the reinforcing in the forms but pour the concrete through it. If the stream of concrete is split by the reinforcing but does not get a chance to remix below it, the aggregate gets pushed aside but the paste and cream flow back together. This causes a paste/mortar matrix change directly below the reinforcing that will cure slightly differently, resulting in ghosting.
A third way ghosting occurs is when reinforcing already buried in fresh concrete is moved or vibrated. The movement causes localized segregation, which in turn causes ghosting.
Other surface marks that resemble ghosting can be caused by leaky forms, excessive form release agent pooled or puddled in the forms, and items left on the curing concrete surface (like sponges, soda cans, polishing discs and hand pads). Anything that traps moisture in or on the concrete in some areas while other areas of the concrete remain uncovered and allowed to dry will cure differently. Concrete that is moist longer will cure more fully and the color will be more intense in those areas.
Ghosting is NOT caused by the reinforcing being too close to the visible surface. The fact that the reinforcing is close to the visible surface does not cause any changes in the curing of the cement paste.
Unfortunately, once these surface marks occur, there is no way to remove them. Prevention is the only way to ensure that ghosting doesn’t happen
A severe case of ghosting. This was caused by pushing the reinforcing
through a stiff, all-sand concrete mix.
posted by Jeff Girard
at
1:09 PM
1 Comments:
Your advice on "ghosting" comes just in time! I am about to lay-up a concrete bathtub - male mold, comproco stuctural skin, fiberglass reinforcing.
My test section shows the ghost of my reinforcing, telling me that I had worked it to hard in my attempt to avoid tiny ratholes.
Thanks,
Clarke Olsen
Spencertown, NY
By
Clarke Olsen, at 8:57 AM
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