Is this in a area that freezes hard, if so that is when this crack will
get worse fast from the outside. An idea, drain it down grind in a
bigger gap1/4 - 3/4 or so deep, first use bleach to kill plant
growth-mold, any growth or dirt will never allow a repair to hold, Acid
etch the cracks , flush and wire brush them clean then use a mortar base
product or a modern sealant forced in. Inside coat with a sealant.
Outside consider a metal strap.