Concrete Repair Category No.6. Deflected Construction Joints
When a construction joint is required on a wall on wall or column on column pour the joint line can cause problems with deflection, grout loss and achieving a true straight edge to the joint.
Deflection is the biggest issue and should not be ‘feathered in’ to the concrete plane of the pour below. It may appear to be a solution but the concrete will always look worse if this is attempted. It will always appear to be ill considered and look funny to the eye. The only solution is to continue the plane of the concrete and form a true straight joint line. The joint line, even though it is not flush, will help the repair to be more accepted by the eye (see image).
Grout loss will also be present but this can be resolved but protect the pour below!
If the wall from the previous pour is not ‘struck off’ to a straight line with the formwork then this will also require the re-instatement of a new joint line as a wobbly line will prevail. Finishing the top face of a pour to flatness is important to give the joint line a considered straight line for the eye to zip across.
Another way is to implement design into a joint line. This may be to incorporate a joint detail such as a ‘birds beak’. This is a Toblerone shaped section than is fixed to the inside of the formwork creating a recessed triangular void in the concrete face at the joint intersection. The concrete is finished to the apex of the triangle (deep within the concrete form) and the joint will be hidden from view.
Whether you are using this type of detail or not deflection can still be an issue and the way to resolve it is to ‘nip’ or tie in the formwork for the new pour as close as possible to the top of the old pour. If the new formwork starts too far down it will exaggerate any deflection that the old pour has. A lot of formwork utilises tie holes lower down on the first pour to fix the new formwork to. This will increase the amount of deflection further up.
The solution is to design in some sacrificial tie holes close to the top of the first pour, clamp the formwork with the ties sandwiching a grout check material between. Later fill and repair the holes for a flush grout-loss-free-non-deflected wall or column pour. The result is much better concrete and much less repair.