The document discusses prestressed concrete, highlighting its definition, procedures, and the advantages and disadvantages of using this technique. Advantages include reduced size and maintenance, crack-free performance, and increased resistance to loads, while disadvantages involve higher costs and complexity in manufacturing. It also covers the materials used, stressing the importance of high-strength steel and concrete in achieving the desired performance in prestressed concrete structures.

1. PRESTRESSED CONCRETE
ENGR. SUBHAN
M.SC. STRUCTURE INU PESHAWAR

2. INTRODUCTION
• PRESTRESSING CAN BE DEFINED AS THE IMPOSITION OF INTERNAL STRESSES INTO A
STRUCTURE THAT ARE OF OPPOSITE CHARACTER TO THOSE THAT WILL BE CAUSED BY THE
SERVICE OR WORKING LOADS.

3. INTRODUCTION

4. INTRODUCTION

5. PROCEDURE OF PRESTRESSING
• STEEL STRANDS (REPRESENTED BY THE DASHED LINES) WERE PLACED IN THE LOWER PART OF THE
BEAM
• FORM.
• THE STRANDS WERE TENSIONED TO A VERY HIGH STRESS.
• THE CONCRETE WAS PLACED IN THE FORM AND ALLOWED TO GAIN SUFFICIENT STRENGTH FOR THE
• PRESTRESSED STRANDS TO BE CUT.
• THE STRANDS WERE CUT.

6. ADVANTAGES & DISADVANTAGES OF PRESTRESS
CONCRETE
ADVANTAGES
• SMALLER MEMBERS CAN BE USED TO SUPPORT SAME LOADS AS
SUPPORTED BY LARGER R. C. C MEMBERS OR SAME-SIZE MEMBERS CAN BE
USED TO SUPPORT LONGER SPANS.
• PRESTRESSED MEMBERS ARE CRACK-FREE UNDER WORKING LOADS AND,
AS A RESULT, LOOK BETTER AND ARE MORE WATERTIGHT, PROVIDING
BETTER CORROSION PROTECTION FOR THE STEEL. FURTHERMORE, CRACK
FREE PRESTRESSED MEMBERS REQUIRE LESS MAINTENANCE AND LAST
LONGER THAN CRACKED REINFORCED CONCRETE MEMBERS.
• REDUCED DEFLECTION, REDUCED STRESSES IN DIAGONAL TENSION,
GREATER STIFFNESS UNDER WORKING LOADS, INCREASED FATIGUE AND
IMPACT LOADS RESISTANCE UNDER WORKING LOADS.
DISADVANTAGES
• REQUIRES HIGHER STRENGTH OF CONCRETE AND STEEL, COMPLEX FORMWORK AND
HIGHER LABOR COST.
• CLOSER QUALITY CONTROL REQUIRED IN MANUFACTURE
• LOSSES IN THE INITIAL PRESTRESSING FORCES. WHEN THE COMPRESSIVE FORCES FROM
PRESTRESSING ARE APPLIED TO THE CONCRETE, IT WILL SHORTEN SOMEWHAT,
PARTIALLY RELAXING THE CABLES. THE RESULT IS SOME REDUCTION IN CABLE TENSION
WITH A RESULTING LOSS IN PRESTRESSING FORCES. SHRINKAGE AND CREEP OF THE
CONCRETE ADD TO THIS EFFECT.
• ADDITIONAL STRESS CONDITIONS MUST BE CHECKED IN DESIGN, SUCH AS THE
STRESSES OCCURRING WHEN PRESTRESS FORCES ARE FIRST APPLIED AND THEN AFTER
PRESTRESS LOSSES HAVE TAKEN PLACE, AS WELL AS THE STRESSES OCCURRING FOR
DIFFERENT LOADING CONDITIONS
• COST OF END ANCHORAGE DEVICES AND END-BEAM PLATES THAT MAY BE REQUIRED
.

7. PRE TENSIONING & POSTTENSIONING

8. MATERIAL USED FOR PRESTRESS CONCRETE
CONCRETE AND HIGH STRENGTH STEEL ARE USED FOR PRESTRESS CONCRETE.
THE STRENGTH OF MATERIAL IS HIGHER BECAUSE:
• THE MODULUS OF ELASTICITY OF SUCH CONCRETES IS HIGHER, WITH THE RESULT THAT THE ELASTIC STRAINS IN THE CONCRETE ARE SMALLER WHEN THE
TENDONS ARE CUT. THUS, THE RELAXATIONS OR LOSSES IN THE TENDON STRESSES ARE SMALLER.
• IN PRESTRESSED CONCRETE, THE ENTIRE MEMBERS ARE KEPT IN COMPRESSION, AND THUS ALL THE CONCRETE IS EFFECTIVE IN RESISTING FORCES. HENCE,
IT IS REASONABLE TO PAY FOR A MORE EXPENSIVE BUT STRONGER CONCRETE IF ALL OF IT IS GOING TO BE USED. (IN ORDINARY REINFORCED CONCRETE
BEAMS, MORE THAN HALF OF THE CROSS SECTIONS ARE IN TENSION AND, THUS, ASSUMED TO BE CRACKED. AS A RESULT, MORE THAN HALF OF A HIGHER-
STRENGTH CONCRETE USED THERE WOULD BE WASTED.)
• MOST PRESTRESSED WORK IN THE UNITED STATES IS OF THE PRECAST, PRETENSIONED TYPE DONE AT THE PRESTRESS YARD, WHERE THE WORK CAN BE
CAREFULLY CONTROLLED. CONSEQUENTLY, DEPENDABLE HIGHER-STRENGTH CONCRETE CAN READILY BE OBTAINED.
• FOR PRETENSIONED WORK, THE HIGHER-STRENGTH CONCRETES PERMIT THE USE OF HIGHER BOND STRESSES BETWEEN THE CABLES AND THE CONCRETE.

9. MATERIAL USED FOR PRESTRESS CONCRETE
HIGH-STRENGTH STEELS ARE NECESSARY TO PRODUCE AND KEEP SATISFACTORY PRESTRESS
FORCES IN MEMBERS. IT SHALL PROVIDE THE CAPACITY OF LOSS DUE TO
• SHORTENING DUE TO COMPRESSION
• SHORTENING DUE TO SHRINKAGE AND CREEP
AND SHALL PROVIDE ADDITIONAL CAPACITY FOR WORKING LOADS

10. MATERIAL USED FOR PRESTRESS CONCRETE
20000PSI PRESTRESSED ROD VS
• BUT STRAIN REQUIRED FOR LONG-TERM
CREEP & SHRINKAGE=0.008
• RESULT= THE STRESS WILL BE COMPLETELY
RELIVED IN THE STEEL.
150000PSI PRESTRESSED ROD
• STRESS REQUIRED FOR LONG-TERM CREEP
AND SHRINKAGE=
• REMAINING STRESS=
• %LOSS=15.47 OF THE STEEL STRESSES.

11. STRESS CALCULATIONS

12. EXAMPLE-01
• CALCULATE THE STRESSES IN THE TOP AND BOTTOM FIBERS AT THE CENTERLINE AND ENDS
OF THE BEAM SHOWN IN FIGURE.
• SOLUTION

13. EXAMPLE-01
• STRESSES AT BEAM CENTERLINE
• STRESSES AT BEAM ENDS

14. CLASS U, C AND T SECTIONS
• CLASS U: ARE THOSE SECTIONS WHICH ARE ASSUMED TO BE UNCRACKED BUT HAVE THE MAX.
TENSILE STRESSES
• CLASS C: ARE THOSE SECTIONS WHICH ARE ASSUMED TO BE CRACKED BUT HAVE THE MAX.
TENSILE STRESSES
• CLASS T: ARE THOSE SECTIONS WHICH ARE ASSUMED TO BE WITHIN TRANSITION BETWEEN
CRACKED AND UNCRACKED SECTIONS AND THE TENSILE STRESSES ARE ASSUMED TO BE
WITHING [,