The typical problems are those inherent in any mix that uses HRWR and VMA; slump retention and potential for segregation. As with all concrete mixes, many things can go wrong, but pretty much if the design properties are good, and it is placed in the field free-flowing without segregation, then the potential problems are no more than for conventional mixes. If the mix is not free flowing, that's a problem because it may no longer be self-consolidating, yet will probably segregate if vibration is applied. When this happens, it may be corrected by an additional dose of HRWR if proper attention is paid to time and temperature rise since the concrete was batched. If the loss of flowability is because of reduced effectiveness of admixtures over time, it can be corrected by an additional dose, or even by a few rapid drum rotations. If the loss of flow is because of cement hydration, the remaining concrete must be rejected. If the mix is segregating, that is a more serious mix problem that normally cannot be fixed in the field. Sometimes, if the mix is just barely at the point that it begins to segregate, stopping discharge and letting the truck sit a little while may allow the mix to stiffen enough to complete the placement. Better placement methods (like reducing the height of a drop) may also stop the segregation. If the segregation cannot be stopped, the load must be rejected in all cases and either the design is bad or, more likely, it was not batched per the design.
