As is said concrete dont help batteries, reduces core temperature which is bad.
Ctek and similar smart chargers are worth their weight in gold, BUT dont pay the silly money for them. Aldi and Lidl do an exact copy now and then for around £13, and the only difference between the 60 quid jobbies and the "cheaper" ones is if the power to the charger goes you reset the cheap one, Ctek will resume from its point of charge.
I deal with and teach lessons to do with batteries and chargers and the Aldi/Lidl ones are as good as anything out there, the only charger you will ever need.