Chikara is not licensed for hard surfaces due to run off.
Guessing you don't have a ticket for spraying weedkiller in general it's the least of your problems
I have heard it works well and i have heard it is better if it rains shortly after application. As it does not work by contact with the foliage as does Glyphosate so it wont wash off .
Flazasulfuron ..the active ingredient in chikara works by "wrapping" itself around the seed preventing germination.
As it is illegal for you too use it i could not offer you advice as i might be aiding and abetting. I believe the criminals amongst us who do use it spray the job with glyphosate before washing (spot spraying the weeds only as blanket spraying is banned) wash,then after resanding spray Chikara.
They then "maintenance" spray Glysophate/chikara mix every 3/4 months to keep the area weed free.
As i say this is only what i have heard on the criminal grapevine and no way endorse it myself.