I've got a utility room with a suspended floor. Despite stiffening the joists, the washing machine thumps around on it and (apart from being noisy) the vibration seems likely to eventually destroy the flooring. What I'd like to do is build a solid plinth under the section of floor supporting the washing machine - level with the present joists so that the floorboards can be replaced. This would need a block about 70x60cm area, and 25cm deep (down to the concrete floor underneath). My plan is to build a temporary wooden former around the area then pour concrete up to the required level. A few questions: (1) Is this a sensible plan? If not, then any other ideas for stopping the vibration? (2) What mix of sand/cement would be good for this? How much shrinkage is likely on drying, and how long would it be before the washing machine could be put back? (3) Foundation floor is concrete screed with a bitumin layer. How should I key the new concrete block onto it?