Can these be used outdoors? For example, a playground underneath rubber mulch?

Since our 1/2" Eco-Soft + tiles are made from EVA foam, they are not the best option to be used outdoors, but they can be used outdoors. Since they are non-porous, they may not be the best option under rubber mulch, as water would not be able to drain properly. We would recommend using one of our Lawn Pads or Play Pads under the mulch to allow water to drain properly.