Prestressed Concrete

Why is prestressing useful in concrete members?
-Concrete is strong in compression and weak in tension – Flexural cracks occur at relatively low loads
– A compressive prestressing force can be applied concentrically or eccentrically in the longitudinal direction of the member. This prevents cracks at critical midspan and supports at service load.
-Sections can behave elastically.
-Nearly the full capacity of the concrete in compression can be used over entire depth under full loading.
-Bending, shear and torsional capacities are increased at these sections.

Differences between Reinforced Concrete (RC) and Prestressed Concrete (PC)
– Tensile Strength
• RC: Tensile strength of concrete is assumed to be negligible.
• PC: Permanent stresses are created prior to full DL and LL application to reduce net tensile stresses.

– Cracking and Deflection
• RC:
RC cracks — only 1/2-1/3 of concrete crosssection contributes to effectiveness.
– Cracking and deflection are typically irrecoverable after reaching these limit states.
– Limited by deflections because cracking reduces effective moment of inertia.
– Service deflections are typically large.
• PC:
– Little, if any, cracking. Entire cross-section contributes to effectiveness.
– A higher level of recovery of cracking and deflection due to prestressing force.
– Service deflections are small (camber reduces ? and full section is stiffer).
• Both RC and PC deflect appreciably at ultimate loads.

– Stiffness
• RC: Stiffness is difficult to control economically.
• PC: Stiffness can be flexible or rigid by controlling amount of prestress for a given strength.
• ** PC acts like RC when applied moment exceeds flexural tensile strength.

– Shear
• RC: No direct assistance from longitudinal steel.
• PC: Some assistance from longitudinal steel.

– Serviceability
• RC: Heavy
• PC:
– Lighter weight
– Can accommodate longer spans
– More adaptable to precasting.

– Safety
• RC: Safe
• PC: Partial testing of steel and concrete by tensioning loads.

– Economy
• RC: More material.
• PC: Less material, but higher cost materials used.
Tend to save on other parts of structure because PC is lighter.

Disadvantages of Prestressed Concrete
– Higher material costs
– Prestressing is an added cost
– Formwork is more complex than for RC (flanged sections, thin webs) – thus, precast not as ductile as RC

Advantages of Prestressed Concrete
– High strength steel & concrete
• PC: 20-35% reinforcement in RC
– Lighter section, Longer spans
• PC depth ~ 65-80% of RC depth
• Lighter foundations possible
– Entire section is active (more efficient)
– Crackless
• Better corrosion resistance, better durability
– Deflection control
• Critical for spans greater than 70-90 ft. (excessive DL)
– Better shear resistance
– Inherent safety: highest prestress force is applied when concrete is weakest (during construction); but, costs can be brought closer to RC for large quantities
– Indirect long-term savings is large (less maintenance, longer life possible – quality materials)
• Best solution: probably a combination of PC and RC = partial prestressing.

Definitions
– Linear Prestressing
• Prestressing applied longitudinal to the axis of the member

– Circular Prestressing
• Circumferential hoop (“hugging”) stress on cylindrical or spherical structure

Prestressing Methods
– Pretensioning
• Do not self-react against member.
• Load is transferred by bond of wire with concrete.
• Disadvantage:
– Need for large abutments.
– Only practical for prefabricators in casting yards.

– Post-tensioning (Bonded, Unbonded)
• React against beam itself.
• Jack and then fix tendon with one end anchored.
• Bonded conduits – pump grout into conduit to fix location of tendons in conduit.
• Unbonded conduits – no grout. Slabs: Unbonded tendons save time & $ (use greased tendons or tendons wrapped in greased paper).
• Most useful in cast-in-place construction.