Introduction and methods of Prestressed Concrete
- 6. Beam under four-point bending (Beam fails immediately once tensile strength capacity reaches in concrete)
- 7. How to make concrete strong in TENSION?
- 11. Concept of balanced section, under reinforced section and over reinforced section
- 13. What is Pre-Stressed Concrete ? • Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from external loads are counteracted to a desired degree. • In reinforced concrete members, the prestress is commonly introduced by tensioning the steel reinforcement.
- 22. • Steel Wires – wire is a single unit made of steel. • Strands – Group of wire not more than 7 wounded to form a prestressing strand. • Tendon – A number of strands or wires are packaged together to form a prestressing tendon. • Cable – When a number of tendons are grouped together and form a single wire then it is called a Cable. Based upon the concrete-steel interface it can be classified into two-part: • Unbounded tendon – This tendon is one that is not bonded or in which no concrete grouting is done after prestressing. • Bonded tendon – This tendon is bounded by injecting concrete grout. Tendons In Prestressed Concrete
- 26. Prestressed Concrete (PSC) Reinforced Cement Concrete (RCC) In PSC members steel plays active role. The stress in steel prevails whether external load is there or not. In R.C.C members 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. In PSC, the stresses in steel is almost constant In R.C.C the stress in steel is variable with the lever arm. It has more shear resistance It has less shear resistance In PSC members, deflections are less because the eccentric prestressing force will induce couple which will cause upward deflections. In R.C.C., deflections are more. In PSC fatigue resistance is more compare to R.C.C. because in PSC stress in steel is load independent In RCC fatigue resistance is less compare to PSC because in RCC stress in steel is load dependent
- 27. Prestressed Concrete (PSC) Reinforced Cement Concrete (RCC) PSC is more durable as high grade of concrete is used which are more dense in nature. R.C.C. is less durable. In PSC dimensions are less because external stresses are counterbalance by the internal stress induced by prestress. In RCC dimensions are large compared to PSC 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.
- 34. Pretensioning Post tensioning Prestressing is done before pouring of concrete. Prestressing is done after concrete attains sufficient strength. Pretensioning is developed due to bonding between steel and concrete. Post tensioning is developed due to bearing. Preferred for small structural element and easy to transport. Preferred for large structural element and difficult to transport. Similar structural members are casted. Members are casted according to market requirements. Casted in moulds. Cables are used in place of wires and jacks for stretching. Greater certainty about the prestressing force. More economical to use a few cables or bars with large forces in than many small ones. Suitable for bulk production. Suited for medium to long-span in situ work where the tensioning cost is only a small proportion of the cost of the whole job .
- 36. Advantages of PSC Increase in durability. Less deformations (improved serviceability). Increase in shear capacity. Lower construction cost. Thinner slabs, which are especially important in high rise buildings where floor thickness savings can translate into additional floors for the same or lower cost. More aesthetic appeal due to slender sections. More economical sections ( Mass production – it is economical). Long span structures are possible.