We know prestressed concrete need relatively high strength concrete. Now our question is-how much strength is economical? Generally it is found to be economical to produce 4 ksi to 5 ksi concrete for prestressed concrete. Though concrete strength should be unique for each job and should be considered individually, the mixes falls within this range are proved economical; i.e. there have some established reasons.
Strength ranging from 4 ksi to 6 ksi can be produced without excessive cement or labor. As an example, we can include that only 15% (Average) more cost is required to achieve a mix of 6 ksi strength relative to production of 3 ksi concrete; notice that at the cost of 15% more cost we can have 100% more strength which is very crucial for prestressed concrete and often of serious requirement in this type of construction.
When strength requirement exceeds 6 ksi, the cost involvement becomes excessive, this type of mix have to be designed carefully and extensive care in control of mixing and subsequent placing and curing are required which are never easy to introduce in field.
For 6 ksi concrete, the plant operations are required where quality of concrete production can be monitored and maintained with great accuracy. 6 ksi to 8 ksi strength are sometimes specified for both precast and prestressed concrete beams. More strength may sometimes be required but not a usual practice.
When strength of more than 5 ksi is the target, the water-cement ratio should not be more than 0.45 (by weight). To have ease of placement, 2 to 4 inch slump should be provided, otherwise specialized vibration, other than ordinary vibration is required.
For a concrete mix of water-cement ratio 0.45, to achieve 76mm slump, about eight bags cement/yd3 of concrete is required. Where utmost care is taken during vibration, half inch or no slump concrete can be designed for only 7 bags cement/yd3 of concrete.
We know excessive cement content in concrete is always associated with increased shrinkage. So a lower cement content is always desirable. In one word, proper vibration is essential and where possible appropriate admixtures should be employed to increase workability of mix.