Civil Engineering Materials

1. Civil Engineering Materials Department of Civil, Structural and Environmental Engineering Trinity College Dublin Dr. Roger P. West and Mr Peter Flynn

2. Section A: Concrete • A1 Basic Materials: • A2 Fresh Concrete Properties: • A3 Hardened Concrete Properties: • A4 Concrete Mix Design: • A5 Reinforced Concrete: • A6 Pre-stressed Concrete:

3. Pre-stressed Concrete: • What is it? • How does it work? • Pre-tensioning • Post-tensioning • What are the materials and equipment used? • What are the advantages compared to RC?

4. Pre-stressed Concrete • What is Pre-stressed Concrete?: • Concrete structures tend to be heavy (due to their self-weight) • Inevitably cracks as steel takes up tensile load • However, if one adds a sufficiently large compressive (axial) load to a beam as well as bending => eliminate tension throughout

5. Pre-stressed Concrete • One way in which this is achieved is by stretching the steel before the concrete sets, then release it when concrete is hard • This adds compressive force to the concrete • Increased compression in concrete requires higher concrete compressive strength • No cracking should occur as tension no longer exists under bending • Deflections are smaller and more slender beams can be used

6. Pre-stressed Concrete • Example: • Consider picking up stack of bricks resting on each other vertically • It is possible to rotate them into a horizontal stack by applying pressure at each side with ones hands • Tensile strength of row of bricks is zero but as long as sufficient pressure is applied, the bricks can be moved together and are stable

7. Pre-stressed Concrete • Addition of pre-compression can also help to overcome shear stresses, which result in diagonal tensile stresses

8. Pre-stressed Concrete: Methods • There are two basic methods of applying pre-stress to a concrete member • Pre-tensioning – most often used in factory situations • Post-tensioning – site use

9. Pre-stressed ConcretePre-tensioning • Tendons usually straight • Steel stressed prior to concrete setting • When concrete has achieved correct strength, steel is released from tensioning device, putting beam in compression

10. Pre-stressed ConcretePre-tensioning • The release of the tendons transfers a compressive force into the concrete =>eliminates tensile stresses under working loads • Bond between the stressed steel and the concrete is very important – need to ensure that steel is kept clean • Some loss of pre-stress force is inevitable at transfer of load from tendons to concrete • Also get some longer term losses due to relaxation of tendons over time and creep in concrete

11. Pre-stressed ConcretePre-tensioning • Usually carried out in factory conditions for precast units • Permanent stressing beds constructed • Long-line production used – a number of similar units produced at the same time

12. Pre-stressed ConcreteMaterials & Equipment • Steel: • Usually in form of cold-drawn high tensile wires or alloy steel bars • Terms: • Bar – reinforcement of solid section – 20-60+ mm in diameter – may be ribbed or smooth. Tensile strength = 1030MPa • Wire – reinforcement of solid section, supplied in coils (diameter ranges from 3mm to 7mm). Tensile strength = 1570MPa • Strand – group of wires spun in helical form around common longitudinal axis (12.5-18mm diameter). Tensile strength = 1670-1860 MPa • Tendon – may be individual wires, bars or strands • Cable – group of tendons

13. Pre-stressed ConcretePre-tensioning • Steel tendons: wire for small units, steel strand for larger units. Tend to have flat profile • Tendons are anchored to a fixed anchor plate at one end of stressing bed and threaded through stop ends of each individual unit • Force is applied to tendons by a jack and the tendons are locked off using an anchor

14. Pre-stressed ConcretePre-tensioning

15. Pre-stressed ConcretePre-tensioning • Once stressing is complete, any required reinforcement is placed and mould is assembled for required concrete profile • Concrete is poured, compacted and cured • Very important to compact properly – voids adjacent to tendons reduce effective bond • Curing is usually accelerated (by choice of constituents and by applying heat in moist environment) to allow fast turn-around on units

16. Pre-stressed ConcretePre-tensioning • Once concrete has reached sufficient strength, tendons are cut • As tensioned steel tries to return to its original length, bond between steel and concrete prevents this and concrete is placed into compression • Tendons revert to original diameter at points where cut – provides beneficial wedge action

17. Pre-stressed ConcretePre-tensioning

18. Pre-stressed ConcretePost-tensioning • Ducts are placed in position prior to concrete pour to predetermined profile. Tendons lie inside ducts. • Concrete is cast then in mould/ formwork and allowed to harden • Bond needs to be prevented at this stage to allow post-tensioning • When concrete strong enough to take compressive loads, stress tendon and anchor off

19. Pre-stressed ConcretePost-tensioning • Tendons tend to be profiled within central portion of beam, especially for large sections where self-weight is significant • Effectiveness of pre-stressing force is a function of the force multiplied by the eccentricity • Can increase efficiency by increasing eccentricity or achieve same pre-stressing effect by applying a larger force at a given eccentricity • Curve in profile develops an upward camber in the concrete section when tendon is tensioned – cancels out some of the downward deflection due to the beam acting under full working load

20. Pre-stressed ConcretePost-tensioning • Preformed metal ducts are cast into concrete with specially made anchorages • Need to ensure no concrete grout enters duct; joints in duct need to be protected with tape • Ducts need to be accurately located to correct profile and securely anchored in position during concrete pour (would otherwise float) • Once concrete has reached sufficient strength, tendons are jacked against the face of the anchorage blocks • Need to check extension of tendons as will be unable to see movement of the tendon in the duct

21. Pre-stressed ConcretePost-tensioning • Ducts are often filled with grout after all tendons are stressed and locked off (bonded post-tensioning) • Grout ensures that tendons are not subject to corrosion but also provide a bond between the tendons and the concrete • Provides factor of safety against rupture of the system • Unbonded post-tensioning requires tendons to be in greased ducts to provide corrosion resistance • Unbonded tendons can be restressed or replaced • Create difficulties for demolition as tendons can “blow out” explosively

22. Pre-stressed ConcretePost-tensioning • At the end of post-tensioned sections, tendons apply a large force through an anchorage block of relatively small area • Similar to driving a wedge into a block of wood • Need to provide sufficient reinforcement to contain these bursting forces

23. Pre-stressed ConcreteMaterials & Equipment • Concrete: • Important to have reached correct strength at transfer • Accelerated hardening often used in pre-tensioning for example, using RHPC, accelerators or steam curing • Elastic deformation occurs under application of pre-compression • This shortens the unit and hence reduces the stress in the tendon – needs to be accounted for in calculations • Creep – inelastic deformation due to sustained stress; causes reduction in pre-stress due to application of sustained compressive stress

24. Pre-stressed ConcreteMaterials & Equipment • Pre-tensioning Equipment: • Temporary grips to hold wires or strand during and after tensioning • Consist of barrel and wedge

25. Pre-stressed ConcreteMaterials & Equipment • Post-tensioning Equipment: • Depends on type of system being used • Multi –strand system: • Large anchorage with many strands; usually all tensioned simultaneously using large jack (weighing > 1 tonne) • Tensions in strands locked off using wedges • Tendons located in large round spiral ducts • Suitable for large concrete sections (e.g. bridge beams, transfer structures)

26. Pre-stressed ConcreteMaterials & Equipment • Post-tensioning Equipment: • Multi –strand system:

27. Pre-stressed ConcreteMaterials & Equipment • Post-tensioning Equipment: • Flat duct system: • Smaller number of tendons stressed in rectangular flat ducts (70mm wide x 20mm deep) • Used in thinner concrete elements (e.g. flat slabs) • Stressed using mobile jack (weighs c. 20-30 kg)

28. Pre-stressed ConcreteMaterials & Equipment • Post-tensioning Equipment: Flat Duct System

29. Pre-stressed ConcreteAdvantages over RC: • Lighter structures possible • Savings in foundations, cladding etc. • Less materials and labour required • Longer spans • In buildings, may be possible to increase number of storeys for same overall building height • Useful for containment vessels due to lack of cracking • Useful for marine structures subject to cyclic loading

30. Pre-stressed ConcreteDisadvantages compared to RC: • Need higher quality materials • More complex technically • More expensive • Risk of sudden failure, especially if not lifted properly • Harder to re-cycle

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38. Section A: Concrete • A1 Basic Materials: • A2 Fresh Concrete Properties: • A3 Hardened Concrete Properties: • A4 Concrete Mix Design: • A5 Reinforced Concrete: • A6 Pre-stressed Concrete: