The code of practice for prestressed concrete is IS:1343-1980. In this post some of the very important specifications which one should keep in mind before going for design of prestressed concrete is discussed.
Minimum cover to prestressed steel
- Pre-tensioned members - 20 mm
- Post-tensioned members - 30 mm or dia of cable whichever is bigger.
- For an aggressive environment, the above values should be inc. by 10 mm.
Spacing of cables or bars
- In the case of single wires for pretension system
- < 3 times dia. of wire
- < 1.5 times the max. size of aggregate
- In the case of cables or large bars
- < 40 mm
- < max. size of cable or bar
- < 5 mm plus the max. size of aggregate
Breaking of wires
- During tensioning it should not exceed 2.5 % of total wires.
- After tensioning it should not be condoned without special investigations.
Grouting
- Cement paste should be used for grouting with a water-cement ratio of 0.5.
- For large ducts, cement mortar with fine sand passing 150 micro m, IS sieve can be used.
- Compressive strength of 100 mm cubes of the grout shall not be less than 17 MPa at 7 days.
- The grouting pressure should not be less than 0.7 MPs.
Minimum reinforcement
- Longitudinal reinforcement - 0.2 % Ag for mild steel and 0.15 % Ag for HYSD steel.
- Members with dynamic loadings - 0.3 % Ag not greater than 0.2 % Ag.
- Without dynamic loadings - Not greater than 0.1 % Ag.
Side face reinforcement
When the depth of the web exceeds 500 mm, longitudinal reinforcement 0.1 % area of the web should be provided and distributed equally on each face of the web with spacing not exceed 200 mm.
Deflection
Total deflections should not exceed span/250.
- Deflections after erection of partitions not exceed span / 350 or 20 mm.
- If finishes are present , the upward deflection should not exceed span/300.
Check this post to know more about prestress concrete.
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