Repair concrete window sills

Beaker!

Hi, I have a concrete window sills which have some weathering damage on top. At one spot there is a circa 10cm by 6cm section where the concrete is crumbling away to depth of maybe 1cm in the middle of the area. Luckily it is a flat surface so filling it in shouldn't be too hard but any advice in what's best to use e.g. Would exterior polyfilla be ok?

jack of all

The best approach would be to use an epoxy repair mortar, sold especially for such repairs, any decent builder's providers should stock this in small tubs. Clean back the affected area, wire brushing to remove any loose material, if steel rebar is exposed, clean well and I would spot prime with red oxide primer before proceeding to repair with mortar. If the repair is very small you could repair it using exterior fillers that are cement based, clean as before and prime the area with PVA, allow to become tacky before trowelling on the filler, work with a filling knife or trowel to smooth the repair.

With large repairs it is recommended to fix dowels and steel screws in the repair area, wrap copper wire (to resist corrosion) around the screw heads in the repair area. The wire acts as reinforcement to retain the filler in the repair.

The success of all these types of repairs depends to a great degree on the quality of the existing concrete and thorough preparation. If corrosion of steel rebar is advanced it may be difficult to stem the spalling of the concrete as the rusting steel expands in volume as it corrodes. Your repair sounds straightforward enough, so good luck with it!

Beaker!

Thanks for your detailed reply!! Really appreciate it! One more question- why the need for PVA adhesive before filler option?

jack of all

PVA is often used in conjunction with cement based repairs, concrete and plasterwork. It primes and seals powdery areas and helps improve the bond. If not used the dry concrete may tend to draw the water out of the repair mortar leading to poor adhesion. It's good practice to use PVA in any case, a small tub will probably cost around €6 or thereabouts, you'll only need a small bit but you can use the remainder as a wood glue etc, handy to have around and will keep for a long time if kept from frost. Follow the directions on the tub and you won't go wrong.