When the cows are standing/eating, they will not be standing on top of the sand..... their feet will be just above the concrete, sunk into the sand.    Height will not be affected.  

I've seen it done several ways.  PVC pipe is brittle, and will break, but you can by HDPE pipe in several sizes.... it gets sold as natural gas pipe.  Can be purchased by the stick... about 20 foot, or on a coil.  

Personally, I've seen wood 4x4's and 6x6's used.  In tie stalls, I've only seen 4x4's and 6x6's in free stalls.  Obviously, you could run the 4x4's through the table saw and angle back the top edge toward the cow.  Wood is much easier to anchor down.  Also, I would strongly consider adding a spacer, say 1/2" or 3/4" pieces of treated plywood under the 4x4 at every anchor, say every 24" to give a bit of drainage.  

A local guy just built a new 100 cow tie stall barn with sand stalls.  I've not visited him yet, but we've been selling him our excess fresh 2 year olds for a while.  If by chance you get the Dairy Star newspaper.... they have visited with him several times.  I do not know what he used as a rear curb, but it may be concrete, and there too, precast parking stops would probably work well, and have drainage slots beneath.