【Haichuan Chemical Construction Technology】What causes color differences and sand lines on the concrete surface? 90% of people don’t understand it!

First point: The formwork wasn’t cleaned properly, so contaminants were left on the wall. After the formwork was used multiple times, hardened cement paste from previous pouring, an oxide layer from the release agent, and rust spots remained on its surface. If you don’t clean it or apply a sealant, those contaminants will stick to the surface of the new concrete once the concrete is poured in. After the formwork is removed, the contaminated areas appear with varying shades of color; this is not caused by defects in the concrete, but rather by dirt on the formwork that has been transferred to it. There are even more concealed situations: carbon-containing particles and light oily substances in the admixtures float on the surface of the concrete and adhere to the formwork. These things are not very noticeable, but after hardening, the surface color becomes darker and uneven. Simply put, template cleaning isn’t just a quick rinse; it involves brushing off all the rust, old glue residue, and oil films. Second point: when the slump is too high, segregation and bleeding cause the paste to be carried away. Once the slump exceeds the appropriate range, the excess water in the concrete rises to the surface, carrying with it the finest particles in the cement. These fine particles accumulate on the surface of the formwork and on the back side of the rebar; once they harden, they turn into dark patches. When the water is gone, the sand and aggregates become visible; the surface turns white, and this is what is known as a sand line. Water flows along the cracks in the formwork, beneath the rebar, and through the gaps in the aggregates; wherever it flows, sand lines are left behind. It’s not caused by vibration; rather, the water carries away the paste, leaving only the bones exposed in the sand. A higher slump is not necessarily better; it should be kept within the acceptable range for construction. For the separated concrete, the surface first takes on an unsightly appearance, which I’ll show you. Third: The sand ratio is too high and the sand is too coarse, preventing the cement paste from covering it properly. When the sand ratio is high, there is an excess of fine aggregates, and the cement paste is not sufficient to cover them all. Sand was filled between the stones, but there wasn’t enough mortar to cover the sand as well. After hardening, the surface has no luster and appears white or gray. The sand itself is coarse, with a low specific surface area, resulting in less coating. The coarse sand has large gaps, allowing the slurry to seep through; only a layer of fine sand grains remains on the surface, which falls off easily when touched with a finger. This is also a type of sand line; it’s not caused by water flow, but rather by the fact that the mix ratio itself does not provide sufficient support. Fourth: The workability is poor; the paste is not soft, and there is excessive vibration in certain areas. The workability of the concrete is not good – it becomes very rigid as soon as it comes out of the tank. It seems that the paste and the aggregates are on top of each other, but in reality they are not properly combined. Once inside the formwork, the vibrator can’t help but vibrate more. In the areas affected by over-vibration, the slurry rises upward while the aggregates sink downward; the thin layer of slurry on the surface turns black and shiny after drying. In the areas adjacent where vibration is lacking, there is no slurry coverage, so the sand surface turns white. On a board, the areas with excessive vibration appear black, while those with insufficient vibration appear white; this is how color differences are created. It’s not that the batches of material are different; rather, it’s that shaking one container of material in your hands results in two different colors. Fifth point: The admixtures were not properly adjusted; as a result, the concrete lacked cohesion, and excessive water was used. The incorrect dosage of admixtures led to poor cohesiveness in the concrete, with insufficient bonding between water and aggregates. With too much water, there is more free water, which increases the risk of exudation; as a result, color differences and sand lines appear. Admixtures cannot be simply added; their optimal dosage must be determined to ensure that the slurry possesses both viscosity and thixotropy. The prepared concrete has a full and smooth surface, without any widespread whitening or sanding.

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