Losses in prestressed concrete

losses in prestressed concrete. All losses explained.

1. Losses in Prestressed concrete Shubham Singh 16CM22F National institute of technology Karnataka, surathkal

2. Introduction Prestressed concrete is a concrete construction material which is placed under compression prior to it supporting any applied loads.

3. Types of Prestressed concrete Pre-tension Post-tension Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast. Minimum grade of concrete used is M40. Concrete element is manufactured remotely from the final structure location and transported to site once cured. Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast. Minimum grade of concrete used is M30. Concrete element can be manufactured at the site hence it can reduce transportation cost.

4. Losses in Prestressed concrete Pre-tension Post-tension Short term Elastic shortening loss No frictional loss No anchorage slip Long term Creep loss Shrinkage loss Relaxation loss Short term No elastic shortening if all bars are tensioned at same time Frictional loss Anchorage slip Long term Creep loss Shrinkage loss Relaxation loss

5. High strength tendon bars have to use. As initial prestress is around 1500-2000 N/mm 2 . Total number of loss is more in Post-tensioned concrete compare to Pre-tensioned concrete. But total loss of prestress is more in Pre-tensioned concrete compare to Post-tensioned concrete. Total loss of prestress is around 15-20%. Transportation of prestressed concrete is also a big challenge. Heavy equipment and precise design.

6. Elastic Shortening loss In pre-tensioned concrete, when the prestress is transferred to concrete, the member shortens and the prestressing steel also shortens in it. Hence there is a loss of prestress. In case of post-tensioning, if all the cables are tensioned simultaneously there is no loss since the applied stress is recorded after the elastic shortening has completely occurred. If the cables are tensioned sequentially, there is loss in a tendon during subsequent stretching of other tendons. Loss due to elastic shortening is quantified by drop in prestress ( Δf p ) in a tendon due to change in strain in tendon ( Δε p ). The change in strain in tendon is equal to the strain in concrete ( ε c ) at the level of tendon due to prestressing force.

7. Strain compatibility Loss due to elastic shortening is quantified by the drop in prestress (∆ f p ) in a tendon due to change in strain in tendon (∆ ε p ). Change in strain in tendon is equal to strain in concrete ( ε c ) at the level of tendon due to prestressing force, which is called strain compatibility between concrete and steel. Strain in concrete at the level of tendon is calculated from the stress in concrete (f c ) at the same level due to the prestressing force. A linear elastic relationship is used to calculate the strain from the stress. Δf p = E p Δε p = E p ε c =E p (f c / E c ) Δf p = mf c

8. Anchorage slip loss In most Post-tensioning systems when the tendon force is transferred from the jack to the anchoring ends, the friction wedges slip over a small distance. Anchorage block also moves before it settles on concrete. Loss of prestress is due to the consequent reduction in the length of the tendon. Certain quantity of prestress is released due to this slip of wire through the anchorages. Percentage loss is higher for shorter members. Due to setting of anchorage block, as the tendon shortens, there develops a reverse friction.

9. Frictional loss Post-tensioned Members Friction is generated due to curvature of tendon, and vertical component of the prestressing force. The magnitude of prestressing force, P x at any distance, x from the tensioning end follows an exponential function of the type. A typical continuous post-tensioned member

10. Relaxation loss Relaxation is the reduction in stress with time at constant strain. decrease in the stress is due to the fact that some of the initial elastic strain is transformed in to inelastic strain under constant strain. stress decreases according to the remaining elastic strain. Factors effecting Relaxation : Time Initial stress Temperature and Type of steel. Relaxation loss can be calculated according to the IS 1343-1980 code.

11. Creep and Shrinkage loss Time-dependent increase of deformation under sustained load. Due to creep, the prestress in tendons decreases with time. For stress in concrete less than one-third of the characteristic strength, the ultimate creep strain ( ε cr , ult ) is found to be proportional to the elastic strain ( ε el ). The ratio of the ultimate creep strain to the elastic strain is defined as the ultimate creep coefficient or simply creep coefficient, θ. ε cr , ult = θε el IS: 1343 considers only the age of loading of the prestressed concrete structure in calculating the ultimate creep strain. Creep is due to sustained (permanent) loads only. Temporary loads are not considered in calculation of creep.

12. Since the prestress may vary along the length of the member, an average value of the prestress is considered. Prestress changes due to creep, which is related to the instantaneous prestress. To consider this interaction, the calculation of creep can be iterated over small time steps. The approximate value of shrinkage strain for design shall be assumed as follows (IS 1383): For pre-tensioning = 0.0003 For post-tensioning = Where t = age of concrete at transfer in days.