Occasionally, whether precast or in-situ formed concrete, the apparent position of the reinforcement is visible on the formed surface, typically as darker lines. Reinforcement shadow, or perhaps ghosting, is caused by changes in the concrete matrix due to aggregate segregation, localised variation in the water-cement ratio, and in the paste/mortar. Any changes in the cement paste affect how it cures and this affects the light and dark tones of the formed surface.
Three typical scenarios.
1. Consider the casting of a precast element horizontally that, when hardened, will then be turned over to reveal the formed surface. If the reinforcement is pushed into the fresh concrete after the form is filled it can cause a pumping action directly under the reinforcement. This pumping action disrupts and changes the cement paste/mortar and may result in a shadow affect.
2. If the concrete is sieved by the reinforcement as it is poured and then does not fully remix around the bar, the paste and mortar will flow back together in preference to the aggregate. This causes a change in the paste/mortar matrix directly below the reinforcing that will cure slightly differently and may result in a shadow affect.
3. When reinforcement is moved or is subjected to vibration within the concrete matrix, e.g. the poker vibrator is constantly in contact with the reinforcement cage, this movement causes localized segregation of the concrete resulting in the shadow affect.
A related stripy effect can occur where the formwork walers (or bracing) create stiff regions in comparison to the formwork panel between the walers. The less stiff regions move more than the stiffened regions as the concrete is vibrated resulting in slight variations in matrix and therefore shadowing.