1. 2E8 Materials (Civil Engineering Component) Civil Engineering Materials Department of Civil, Structural and Environmental Engineering Trinity College Dublin Dr. Roger P. West and Mr Peter Flynn

2. 2E8 Materials (Civil Engineering Component) Section A:Concrete A1Basic Materials: A2Fresh Concrete Properties: A3Hardened Concrete Properties: A4Concrete Mix Design: A5Reinforced Concrete: A6Pre-stressed Concrete:

3. 2E8 Materials (Civil Engineering Component) 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? Pre-stressed Concrete:

4. 2E8 Materials (Civil Engineering Component) 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. 2E8 Materials (Civil Engineering Component) 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. 2E8 Materials (Civil Engineering Component) 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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Addition of pre-compression can also help to overcome shear stresses, which result in diagonal tensile stresses

8. 2E8 Materials (Civil Engineering Component) 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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Materials & 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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-tensioning

15. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Pre-tensioning

18. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Post-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Post-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Post-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Post-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Post-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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Materials & 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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Materials & Equipment Pre-tensioning Equipment: –Temporary grips to hold wires or strand during and after tensioning –Consist of barrel and wedge

25. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Materials & 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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Materials & Equipment Post-tensioning Equipment: –Multi –strand system:

27. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Materials & 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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Materials & Equipment Post-tensioning Equipment: Flat Duct System

29. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Advantages 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. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete Disadvantages 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

31. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete

32. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete

33. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete

34. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete

35. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete

36. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete

37. 2E8 Materials (Civil Engineering Component) Pre-stressed Concrete

38. 2E8 Materials (Civil Engineering Component) Section A:Concrete A1Basic Materials: A2Fresh Concrete Properties: A3Hardened Concrete Properties: A4Concrete Mix Design: A5Reinforced Concrete: A6Pre-stressed Concrete: