The Brew Cabin

Our concrete block driveway is horizontal at the top alongside the house, then slopes downwards to the road. It's a normal herringbone pattern. Where the slope angle changes, the gaps at the top of the blocks are obviously larger (I don't know if they could/should have been chamfered to a tight fit, but they are not) and rain washes the sand out. The gaps are maybe 3mm - 5mm or so at the top of each block, for a row or two.
Is there any reason not to refill the gaps with weak sand and cement dry mix so it sets hard and doesn't wash out?

A 3-5mm gap is nothing! Don't be making a mess of the blocks by using a cement-bound jointing material. It won't last more than a season!

Just use normal jointing sand and, within 3 months, it will have crusted over with natural 'crud' and will seal itself. If you have any problem with 'scour', then use Keybond or similar to stabilise the sand in the short term, but, I can more or less guarantee that,m by this time next year, you'll not even notice the slightly wider joints.

Incidentally, just how severe is this change of direction to create joints that are noticeably wider than normal? Most 'rollovers' only widen the joint by 1-2mm.

It's been down for five years! I fill in the gaps with kiln-dried sand once or twice a year but it washes out. Maybe I underestimated the gap size; I'll take some photos and post a pointer; it's quite a steep angle change which couldn't be smoothed out much because of a sewer pipe.

If it's 5 years old, then this is an ongoing problem, and you should treat it with a joint stabilising compound, as mentioned in my last message, or look to seal the entire pavement with a decent urethane-based sealant.

I'd still like to see the pics, though. :)

Thanks! Pics are:
www.elessar.org.uk/blocks_1.jpg
www.elessar.org.uk/blocks_2.jpg
www.elessar.org.uk/blocks_3.jpg
- and the gaps are wider than I first estimated - 10mm or more. It also can't help that the car is turning on full lock as it goes over this area. I'll try Keybond.

Nice measure! :)

Yes, try Keybond or WJC from Resiblock. That should eliminate the problem of scour. It's probably worth cleaning the pavement first and topping up the joints with fresh, genuine jointing sand before applying one of these stabilising compounds.

Let me know how you get on.