Concrete Slab and Wood Piles

Longitudinally, wood is generally dimensionally stable. Usually with wood piles, I use a concrete cap to support the slab.

Dik

I guess since the concrete slab will be poured around the columns, do I need something like an expansion joint around the columns?

Sounds like you are concerned about the radial expansion of the timber piles at the point where they penetrate the slab on grade. I suggest warping them with typical expansion joint material such as this

Even the thinest of theses materials (1/4" for the link above) should be sufficient. Often the concrete slabs are cast directly in contact with timber piling that pass thru the slab - with no problems. But a little "wiggle room" is better, IMHO.

I vote for pouring the concrete around the piles with no gap at all. This will give you a fixed connection at the base and really help in stiffening up the building. A pile embedded in the sand is not really a fixed base. Just make sure you have enough concrete and reinforcement around the piles.

The concrete is going to crack, so reinforce it to control them. I am not even sure if you would prevent any additional cracks anyway by providing a gap. I think the added stiffness you will give the building is more important anyway. A lot of beach houses I have been in will make you sea sick if there is a stiff wind.