Superplasticizers make it possible to make strong holds while making the concrete easily pourable. See the graph below for a comparison of concrete where the slumps are similar (rheoplastic and high slump with water) but the strengths are greatly varied.
See a .pdf document from Master Builders on High Performance Concrete.
This is the stuff that makes it easy to do. They work by repelling negative charges in the concrete, which in turn makes the concrete more 'slippery' and more easily pourable. I have been using RheoBuild 1000 from MasterBuilders, located here in Cleveland, OH.