How to repair thin concrete deck cover
You need a flexible seal Quickcrete will only crack again. A urethane calking would be your best bet to seal the cracks, urethane will flex and maintain a seal (you have 2 forms of flexing one from weight and walking on the deck and the 2nd from expansion heating and cooling) urethane sealant or caulking is the best product to use in this case to seal the cracks and there are colored types that may be a better match for your surface.