Department of Civil Engineering, N-W.F.P UET, Peshawar

Prestressed Concrete

But the concrete experiences a shrinkage strain εsh with the passage of time and, in addition, if held under compression will suffer a creep strain εcu. The total length change in the member ∆lc = (εsh + εcu)lc……………………..(a) may be such that it exceeds the stretch in the steel that produced the initial stress, and complete loss of prestress force will result. The importance of shrinkage and creep strain can be minimized by using a very high initial strain and a high initial stress in the steel. The reduction in steel stress from these causes depends only on the unit strains in the concrete associated with shrinkage and creep, and the elastic modulus Es of the steel: ∆fs = (εsh + εcu)Es……………………(b) It is independent of the initial steel stress. It is informative to study the results of calculations of representative values of the various parameters. Suppose first that the member is prestressed using ordinary reinforcing steel at an initial stress fst of 30 ksi. The modulus of elasticity Es for all steels is about the same and will be taken here 29,000 ksi. The initial strain in the steel is: εsi = fsi/Es = 30/29000 = 1.03 x 10-3 And the steel elongation is: ∆s = εsls = 1.03 × 10-3ls………………………(c) But a conservative estimate of the sum of shrinkage and creep strain in the concrete is about 0.90 × 10-3, and the corresponding length change is: (εsh + εcu)lc = 0.90 × 10-3lc…………………...(d) Since ls and lc are nearly the same, it is clear by comparing (c) and (d) that the combined effects of shrinkage and creep of the concrete is almost a complete loss of the stress in steel. The effective steel stress remaining after time-dependent effects would be fse = (1.03 – 0.90) × 10-3 × 29 × 103 = 4 ksi Alternatively, suppose that the prestress were applied using high strength steel at an initial stress of 150 ksi. In this case the initial strain would be: εsi = 150/29000 = 5.17 × 10-3………………..(e) εsls = 5.17 × 10-3ls……………………………(f) Prof Dr. Qaisar Ali (http://www.eec.edu.pk)

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lecture-07-prestressed-concrete

Page 1 of 19 Prof Dr. Qaisar Ali (http://www.eec.edu.pk) Department of Civil Engineering, N-W.F.P UET, Peshawar Prestressed Concrete Prof Dr...

lecture-07-prestressed-concrete

Page 1 of 19 Prof Dr. Qaisar Ali (http://www.eec.edu.pk) Department of Civil Engineering, N-W.F.P UET, Peshawar Prestressed Concrete Prof Dr...