die lah, concrete and the reinforced steel bars should be bonded together so not to let air bubbles between them which could rust the bars and cause concrete cancer.
Repair to concrete cancer are expensive and for high rise may lead to pull down the whole building if the rust penetrated and spread to all the reinforced steel bars. There should a concrete cover of 30-50mm and look at the picture die lah where is the concrete cover.
Expose this part to moistures for long time the rust will slowly creep into the bars and spread further to other bars cause structural defects to the column, beams or support, structural or non-structural.
Always suspect HDB poor quality construction and this has proven to me it is real.
Chip out the concrete and get it tested for concrete strength. depending on which part of the structures should be 25-30Mpa concrete strength. Get it tested to see HBD got tau jiak the concrete dilute with water or not.
My guess is the contractor did not use concrete vibrator when pouring this wall. Concrete vibrator tube is dipped into the wet concrete as it pour and the vibration will mixed the concrete well making sure the concrete covers all gaps and holes evenly and no tau jiak leow (makan money) from the contractor.
Can sue HBD or the contractor now for poor workmanship tau jiak leow on labour amd materials. Definitely conctractor tau jiak workmanship without using the concrete vibrator to settle the concrete evenly.
https://en.wikipedia.org/wiki/Vibrator_(mechanical)
http://www.alleneng.com/concrete-equipment/placing/concrete-vibrator