Introduction to Pre-stressed and Precast Concrete Technology

This the first lecture of prestressed and precast technology. In this lecture overview of prestressed concrete is presented in such a way, that it will be very helpful for civil engineering professionals and students new to the field of prestressed concrete technology.
this lecture covers the following topics
Definition of prestressed and precast concrete
Difference between prestressed and normal reinforced concrete
Terminologies related to prestressed like tendons, Anchorage, Pre-tensioning, post-tensioning, etc
Brief History of prestressed concrete
Development of building materials for prestressed concrete
Advantages and disadvantages of prestressed concrete
Difference between pre-tension and post-tension prestressing.
and Difference between prestressed and precast concrete.
#CivilEngineering #CivilEngineer #Prestressed #Concrete #precast
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1. Lecture# 01 Introduction PRESTRESSED AND PRECAST CONCRETE TECHNOLOGY 7/1/2020 1 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq Engr Shah Farooq Lecturer GCT Swat

2. Introduction: Definition of Pre-Stress: Pre-Stress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete. The pre-compression applied (may be axial or eccentric) will induce the compressive stress below the neutral axis or as a whole of the beam c/s. Resulting either no tension or compression. 7/1/2020 2 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

3. Basic Concept Pre-Stressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree. 7/1/2020 3 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

4. A pre-stressed concrete structure is different from a conventional reinforced concrete structure due to the application of an initial load on the structure prior to its use. The initial load or ‘pre-stress’ is applied to enable the structure to counteract the stresses arising during its service period. The pre-stressing of a structure is not the only instance of pre-stressing. The concept of pre-stressing existed before the applications in concrete. Two examples of pre-stressing before the development of pre-stressed concrete are provided. 7/1/2020 4 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

5. Force-fitting of metal bands on wooden barrels: The metal bands induce a state of initial hoop compression, to counteract the hoop tension caused by filling of liquid in the barrels. 7/1/2020 5 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

6. Pre-tensioning the spokes in a bicycle wheel: The pre-tension of a spoke in a bicycle wheel is applied to such an extent that there will always be a residual tension in the spoke. 7/1/2020 6 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

7. Terminology 1. Tendon: A stretched element used in a concrete member of structure to impart pre-stress to the concrete. 2. Anchorage: A device generally used to enable the tendon to impart and maintain pre-stress in concrete. 3. Pre-Tensioning: A method of pre-stressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the concrete is introduced by bond between steel & concrete. 4. Post-Tensioning: A method of pre-stressing concrete by tensioning the tendons against hardened concrete. In this method, the pre-stress is imparted to concrete by bearing. 7/1/2020 7 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

8. Brief History: Before the development of pre-stressed concrete, two significant developments of reinforced concrete are the invention of Portland cement and introduction of steel in concrete. These are also mentioned as the part of the history. The key developments are mentioned next to the corresponding year. 1824 Aspdin , J., (England) Obtained a patent for the manufacture of Portland cement. 1857 Monier , J., (France) Introduced steel wires in concrete to make flower pots, pipes, arches and slabs. 7/1/2020 8 By: Engr Shah Farooq

9. The following events were significant in the development of pre-stressed concrete. 1886 Jackson, P. H., (USA) Introduced the concept of tightening steel tie rods in artificial stone and concrete arches. 7/1/2020 9 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

10. 1888 Doehring , C. E. W., (Germany): Manufactured concrete slabs and small beams with embedded tensioned steel. 1908 Stainer , C. R., (USA) Recognised losses due to shrinkage and creep, and suggested retightening the rods to recover lost prestress . 1923 Emperger , F., (Austria) Developed a method of winding and pre- tensioning high tensile steel wires around concrete pipes. 1924 Hewett, W. H., (USA) Introduced hoop-stressed horizontal reinforcement around walls of concrete tanks through the use of turnbuckles. 7/1/2020 10 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

11. 1925 Dill, R. H., (USA) Used high strength un-bonded steel rods. The rods were tensioned and anchored after hardening of the concrete. 1926 Eugene Freyssinet (France) Used high tensile steel wires, with ultimate strength as high as 1725 MPa and yield stress over 1240 MPa . In 1939, he developed conical wedges for end anchorages for post-tensioning and developed double-acting jacks. He is often referred to as the Father of Pres- tressed concrete. 7/1/2020 11 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

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13. 7/1/2020 By: Engr Shah Farooq 13 1938 Hoyer, E., (Germany) Developed ‘long line’ pre-tensioning method. 1940 Magnel , G., (Belgium) Developed an anchoring system for post-tensioning, using flat wedges. During the Second World War, applications of pre-stressed and precast concrete increased rapidly. The names of a few persons involved in developing pre-stressed concrete are mentioned. Guyon , Y., (France) built numerous pre-stressed concrete bridges in western and central Europe. Abeles, P. W., (England) introduced the concept of partial pre-stressing. Leonhardt , F., (Germany), Mikhailor , V., (Russia) and Lin, T. Y., (USA) are famous in the field of pre-stressed concrete. The International Federation for Pre-stressing (FIP), a professional organization in Europe was established in 1952. The Precast/Pre-stressed Concrete Institute (PCI) was established in USA in 1954. Pre-stressed concrete was started to be used in building frames, parking structures, stadiums, railway sleepers, transmission line poles and other types of structures and elements.

14. Development of Building Materials: 7/1/2020 By: Engr Shah Farooq 14 The development of pre-stressed concrete can be studied in the perspective of traditional building materials. In the ancient period, stones and bricks were extensively used. These materials are strong in compression, but weak in tension. For tension, bamboos and coir ropes were used in bridges. Subsequently iron and steel bars were used to resist tension. These members tend to buckle under compression. Wood and structural steel members were effective both in tension and compression. In reinforced concrete, concrete and steel are combined such that concrete resists compression and steel resists tension. This is a passive combination of the two materials. In pre-stressed concrete high strength concrete and high strength steel are combined such that the full section is effective in resisting tension and compression. This is an active combination of the two materials. The following sketch shows the use of the different materials with the progress of time. www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

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16. Advantage of Pre-Stressed Concrete 1. The use of high strength concrete and steel in pre-stressed members results in lighter and slender members than is possible with RC members. 2. In fully pre-stressed members the member is free from tensile stresses under working loads, thus whole of the section is effective. 3. In pre-stressed members, dead loads may be counter-balanced by eccentric pre-stressing. 4. Pre-stressed concrete member posses better resistance to shear forces due to effect of compressive stresses presence or eccentric cable profile. 5. Use of high strength concrete and freedom from cracks, contribute to improve durability under aggressive environmental conditions. 7/1/2020 16 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

17. Continued… 6. Long span structures are possible so that saving in weight is significant & thus it will be economic. 7. Factory products are possible. 8. Pre-stressed members are tested before use. 9. Pre-stressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before collapse. 10. Fatigue strength is better due to small variations in prestressing steel, recommended to dynamically loaded structures. 7/1/2020 17 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

18. Disadvantages of Pre-Stressed Concrete: 1. The availability of experienced builders is scanty. 2. Initial equipment cost is very high. 3. Availability of experienced engineers is scanty. 4. Pre-stressed sections are brittle 5. Pre-stressed concrete sections are less fire resistant. 7/1/2020 18 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

19. Classifications and Types Pre-stressed concrete structures can be classified in a number of ways depending upon the feature of designs and constructions. 1. Pre-tensioning: In which the tendons are tensioned before the concrete is placed, tendons are temporarily anchored and tensioned and the pre-stress is transferred to the concrete after it is hardened. 2. Post-tensioning: In which the tendon is tensioned after concrete has hardened. Tendons are placed in sheathing at suitable places in the member before casting and later after hardening of concrete. 7/1/2020 19 By: Engr Shah Farooq

20. Pre-Stressing System 1. Pre-Tensioning system: In the pre-tensioning systems, the tendons are first tensioned between rigid anchor-blocks cast on the ground or in a column or unit –mould types pre-tensioning bed, prior to the casting of concrete in the mould. The tendons comprising individual wires or strands are stretched with constant eccentricity or a variable eccentricity with tendon anchorage at one end and jacks at the other. With the forms in place, the concrete is cast around the stressed tendon. 7/1/2020 20 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

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23. 2. Post-tensioned system: In post-tensioning the concrete unit are first cast by incorporating ducts or grooves to house the tendons. When the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end of the face of the member and anchored by wedge or nuts. The forces are transmitted to the concrete by means of end anchorage and, when the cable is curved, through the radial pressure between the cable and the duct. The space between the tendons and the duct is generally grouted after the tensioning operation. 7/1/2020 23 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

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25. Difference of Pre-Stressed over Reinforced Cement Concrete 1. In pre-stress concrete member steel plays active role. The stress in steel prevails whether external load is there or not. But in R.C.C., steel plays a passive role. The stress in steel in R.C.C members depends upon the external loads. i.e., no external load, no stress in steel. 2. In pre-stress concrete the stresses in steel is almost constant where as in R.C.C the stress in steel is variable with the lever arm. 3. Pre-stress concrete has more shear resistance, where as shear resistance of R.C.C is less. 4. In pre-stress concrete members, deflections are less because the eccentric pre-stressing force will induce couple which will cause upward deflections, where as in R.C.C., deflections are more. 7/1/2020 25 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

26. Continued… 5. In pre-stress concrete fatigue resistance is more compare to R.C.C. because in R.C.C. stress in steel is external load dependent where as in P.S.C member it is load independent. 6. Pre-stress concrete is more durable as high grade of concrete is used which are more dense in nature. R.C.C. is less durable. 7. In pre-stress concrete dimensions are less because external stresses are counterbalance by the internal stress induced by pre-stress. Therefore reactions on column & footing are less as a whole the quantity of concrete is reduced by 30% and steel reduced by about 60 to 70%. R.C.C. is uneconomical for long span because in R.C.C. dimension of sections are large requiring more concrete & steel. Moreover as self-weight increases more reactions acted on columns & footings, which requires higher sizes. 7/1/2020 26 By: Engr Shah Farooq www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq

27. 7/1/2020 27 By: Engr Shah Farooq Thanks www.youtube.com/c/CivilEngineersite https://web.facebook.com/engrshahfarooq