Yes, the rhubarb crumb cake freezes very well. First, freeze the cooled pieces on the tray or a large plate. Once solid, transfer them to freezer bags and freeze them for up to 3 months. Defrost in the refrigerator or on the counter.  You can also place the frozen pieces on a baking tray and bake them until heated through and slightly crunchy again, about 10-15 minutes at 180°C/ 350°F, but check.