I suspect the slipping is on account of algae. If you use a watering can occasionally with a fine rose and a solution of bleach it will remove the algae and grip should return. Wear old trousers and wellies each time as very little holes will appear in clothes. Another solution would be to pressure wash the concrete. This tends to roughen up the surface. Or you could lay slabs on top if you wished. Pebbles won’t adhere well and are not ideal to walk on.