During the drilling of the rebars the following points must be kept in mind;
1. The surface should be thoroughly inspected whether it has reinforcement underneath or not. If it had reinforcement within the range of proposed depth of drilling than straightaway it must be stopped as these rebars might get damaged during drilling.
2. Demarcation should be done so as to avoid wrong drilling.
3. The drilling machine we used was of hydraulic type with circular action of drilling with diameter 42 mm. Actually the available drilling machine was of two types one is of larger dia which can drill upto 1 m but that of smaller dia can only drill upto around 500 mm. As for #6 rebar the development length is 910 mm for Class C30 grade concrete, thus we opt the bigger one.
4. The drilling plunger must be kept straight and the hole must be complete vertical as we need to place vertical bar not inclined one. For this reason some hanging weight can be used.
5. After drilling the surface must be properly cleaned by air pressure and must be dry prior to epoxy or grouting.
6. The grouting must be of proper mix design and of approved brand as suggested by the engineer.
Note this is not the only solution for any damaged or cut rebar other options are also available depending on the site situation and local practice. If the rebar projecting is cut or damaged from such a height that overlap can be provided than overlap is also an option. Moreover even if overlap length is not available the damaged rebar can be rectified by welding as per ASW standards.
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How To Rectify Damaged Broken Reinforcement?