Workability and Quality Control of Concrete
The word workability is a term that refers to properties of fresh concrete, that is, of the concrete before it has set and hardened, and it is legitimate to ask why any attention should be given to these properties at all.
The performance of concrete will in practice be assessed in terms of whether the hardened material performs in the way intended and continues to do so: it will be judged in terms of shape and finish, strength, deflection, dimensional changes, permeability and durability.
So why should the properties of the fresh concrete be considered to be important, and why should they be the concern of the practising engineer?
The answer to the first of these questions lies in the fact that the properties of any finished material are affected by the properties at an earlier stage and by the processes applied to it, while the answer to the second one is that all, or a major part of, the processing of concrete is actually carried out on site.
The first stage is, of course, the making of a homogeneous mix and then, assuming this has been done properly, the material is subjected to other processes as follows.
The concrete must be capable of giving a good finish direct from the formwork, withouth oneycombing or an excessive number of blowholes or other surface defects.
If there is a free surface, it must also be capable of giving a good finish in response to an operation such as floating or trowelling.
A workable concrete is one that satisfies these requirements without difficulty and, ingeneral, the more workable it is, that is, the higher its workability, the more easily it can be placed, compacted and finished.
Workability can be increased by simply increasing the water content of the mix but, if that method is used, a point will be reached at which segregation and/or bleeding become unacceptable so that the concrete is no longer homogeneous and, before that, the water/cement ratio may have reached a level such that the hardened concrete will not attain the required strength.
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