Sometimes lines appear in precast concrete countertops where the steel reinforcing seems to show through. It can also happen in GFRC with scrim or foam coring. This is called ghosting.
Ghosting 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 hanging the reinforcing in the forms and pouring 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.