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Dark lane below the top of sidewalk core

Dark lane below the top of sidewalk core

Dark lane below the top of sidewalk core

(OP)
I am investigating the scaling of the sidewalk after 2 winters after placing. The dark lane locates about 1 mm below the surface. I am a bit puzzled with a fact that this lane marked the gravel aggregates as well. It's hard to explain the creation of this lane just by improper finishing. Yes, finishing, but again why the lane is going through aggregate particles? Sidewalk was placed 4-5 weeks before snow. A lot of chlorides in. Air is OK. Any thoughts?


https://res.cloudinary.com/engineering-com/image/upload/v1562174175/tips/line_through_aggregate_c-19-424_qaj4ka.bmp


https://res.cloudinary.com/engineering-com/image/upload/v1562174066/tips/wide_view_of_line_c-19-424_nf3o9g.bmp


https://res.cloudinary.com/engineering-com/image/upload/v1562174091/tips/line_through_sand_and_paste_c-19-424_zucqb9.bmp

RE: Dark lane below the top of sidewalk core

This is clearly a finishing issue. The dark zone that drifts upward into the coarse aggregate was caused by bleed water trying to reach the surface but the surface was dried and finished before that could happen. The distinct line just below the surface results from overworking the drying surface with a weakened cement paste....that is why it separates from the coarse aggregate.

The darkening below the coarse aggregate is also an indication of retempering, that doesn't mix well in the paste.

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