Rain is not curing
Rain comes and goes. Between showers, a hot afternoon can dry the top few millimetres of a slab in an hour. Cement needs water continuously for the first week to keep reacting. Interrupted curing gives a weak, dusty skin even when the concrete below is fine.
The seven day routine
Cover the slab with wet sacks or polythene as soon as it can be walked on without marking. Keep the sacks wet morning and evening for seven days. On a hot day, add a midday soak. For 42.5R, the first three days matter most.
Cement needs water for a week, not for an afternoon.
Heavy rain on fresh concrete
If a storm is coming, do not pour. If it arrives mid pour, cover the finished section and stop at a construction joint your engineer approves. Rain on concrete less than two hours old washes out cement from the surface.
Columns and walls
Wrap columns in wet sacking or polythene once the formwork comes off. Vertical faces dry faster than you think.
Written for this demo. Always follow your engineer's instructions.