Fly ash layer in concrete

Now a days fly ash is being used with cement as a binding material & as well as for other benefits where economic is one of them, some times due to bad concreting a flyash layer is observed after de-shuttering of the casted member.

Fly ash layer is generally found when bleeding occurs in a concrete mix. The slurry along with the carbon layer rise to the top surface. During successive castings, a layer is formed in between good concrete where weak layer of fly ash is found.

Repairing methodology for Fly ash layer:

·         First, we have to determine the extent of fly ash layer in a particular structural member or portion.

·         Secondly, using chisel/steel wire brush we have to get rid of the weak fly ash layer till we find good concrete.

·         If the depth of the chipped out part is less than 5 mm then the void shall be filled with finish materials such as gypsum/putty as per client's instruction.

·         If the depth is more than 5 mm but less than 10 mm, then the void shall be cleaned properly followed by application of concrete bonding agent such as Fosroc Nitobond EP. After which modified cement mortar such as Sika Microcrete 4 or Sika 122 top HS shall be used to fill up the void.

·         If the depth is more than 10 mm but less than the cover provided, then the void shall be cleaned properly followed by application of concrete bonding agent such as Fosroc Nitobond EP. After which modified cement mortar such as Sika Microcrete 4 mixed with 10 mm coarse aggregate in the ratio (1:4) shall be used to fill up the void.

However, in the last case & cases where the depth is more than cover the engineer in-charge shall be notified and suggestion shall be taken for carrying out repairing work. Approval from the structural engineer / consultant shall also be taken before commencing.