Factors which affect workability of concrete are:

• Cement content of concrete
• Water content of concrete
• Mix proportions of concrete
• Size of aggregates
• Shape of aggregates
• Grading of aggregates
• Surface texture of aggregates
• Use of admixtures in concrete
• Use of supplementary cementitious materials

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Effect of Aggregate Cement Ratio on Workability of Concrete

Aggregate cement ratio is the ratio of weights of aggregate to the weight of cement. If this ratio is more, that implies aggregates are more and cement is less and if this ratio is less, that implies weight of aggregate is less and weight of cement is more (relatively).

When the weight of cement is more, i.e. aggregate cement ratio is less, the concrete has more cement paste required to coat aggregates and fill the voids between them. This more cement paste makes the concrete relatively easy to  mix, place and compact as it reduces the friction between aggregates and allows its smooth movement. Thus the workability of concrete increases.

When the weight of cement is less, i.e. aggregate cement ratio is more, then there will be very less cement paste to coat aggregate surfaces and fill the voids, thus, mixing, placing and compacting of concrete will be more harder than previous case. This implies, the workability of concrete will be less than previous case.

Reference: Factors Affecting Workability of Concrete