Segregation in concrete is generally due to excessive water in the matrix but can also be caused by over compaction (vibration), dropping the concrete from a height or simply through inappropriate mix design.
If the mix looses its cohesion, excess water will naturally attempt to escape at the path of least resistance, upwards at the form face. Once a path of escape is achieved more excess water will find this path and a sand run will be created. Sand runs are also exacerbated by minimal form face absorption. If the formwork face has the capacity to absorb some moisture then this will reduce the strength or path of a sand run.
Sand runs are difficult repairs to deal with. Primarily because they flow in a vertical direction and tend to be very well defined. Fair-faced concrete when compacted tends to have natural variations in tone that run horizontally as the concrete is placed and compacted. We generally accept these horizontal variations more easily so a sharply defined vertical repair will need a lot of colour matching horizontally to force the eye to stop following the defect.
Sand runs can be vast and really long. I have seen some 6+ metres in length covering large areas of boards. This leads to large-scale intervention and the requirement to colour match over a big area. This means that inevitably lots of good concrete will need to be treated as well.
Sand runs are not excessively challenging in terms of filling flush but if they are extensive, this will take a lot of time. Also, some are very thin in depth and material may have to be removed beforehand.
However, the challenging aspect of segregation is that Architectural Concrete Repair should be a ‘less is more’ exercise of minimal intervention. Sand runs simply do not allow this philosophy to prevail. If a project displays sand runs over a few pours then I would suggest the approach should be changed quickly so as to avoid a lot of potential hassle and cost later.
The image here shows a section of segregation (top half) that has been repaired.