Concrete work is usually undertaken in changing conditions. External factors such as the weather plays a critical part in carrying out the work.
Furthermore, ground conditions vary and is not constant from area to area. A concrete foundation needs to be adjusted in order to take into account whether the ground contains rock, buried rubbish or tree roots. The consistency of the ground also needs to be taken into account, e.g. whether the ground consists mainly of sand or clay. Ground conditions will have a direct impact on the quoted price as it will be extremely difficult to quote a set price without taking the subsequent conditions into consideration.
Concrete might also show signs of cracking over an extended period of time. Little to nothing can be done to avoid this, except for ongoing maintenance and remedial work. Due to the nature of concrete this is likely to end up in a debate between contracting parties due to the high cost of the remedial or maintenance steps on the one hand and the aesthetics of the surface on the other hand.