Practical Aspects of PSC IRSE Phase II Course

Practical Aspects of PSC IRSE Phase II Course

What is Pre Stressing ? • It is intentional application of a predetermined force on a system for resisting the internal stresses due to external loads. P P

Thus PSC……. …is the special reinforced concrete which makes use of the intrinsic properties of steel and concrete i. e. using the properties they are good at CONCRETE in compression STEEL in tension

PSC is called Active concrete • Because steel is tensioned to compress the concrete so that there is no or hardly any tension in concrete under the service loads • This system needs high strength concrete (brittle) and high tensile steel (ductile) • Makes effective use of modern high strength materials

In-spite of the good designs… • There will be problems in the construction due to • Improper understanding or lack of understanding of the • basic principles • Right method of the application of the principles • Practical aspects of execution (because everything can’t be reduced to writing) • In fact the failures enlighten us to highlight the inconsistencies between assumptions on the paper and the understanding in the field

Practical problems and remedies • Specifications of works are based on theory and also to a large extent on observations from minor to major deformations, observed in already executed works. • There are many factors which are too difficult to be precisely laid down. In such cases the decisions are based on discretion or intuition of the Engineer-in. Charge or the field executive

Practical problems and remedies Broadly there are following four causes of failures. 1. Defective design. 2. Faulty methods or wrong sequence of construction. 3. Natural causes, such as, unanticipated floods, scouring and settlement of foundations, etc. 4. Sub-standard specifications. • Majority of the cases of failures are found to be on account of (2) and (3) above.

PRE-STRESSING STEEL • Uncoated Stress Relieved Strand As Per IS : 6006 • Uncoated Stress Relieved Low Relaxation Strands to IS : 14268 • Hard Drawn Plain Steel Wires ( Cold Drawn Stress Relieved Wires) to IS : 1785(part-I) 1983 • High Tensile Steel Bars to IS: 2090

PROPERTIES TWO WIRE STRAND THREE WIRE STRAND • THE TWO WIRE AND THREE WIRE STRANDS ARE DESIGNATED BY NUMBER OF ELEMENTAL WIRES AND DIA. OF ELEMENTAL WIRES. • Nomenclature A-B – A REPRESENTS NO. OF WIRES IN THE STRAND – B REPRESENTS DIA. OF INDIVIDUAL WIRE IN THE STRAND

PROPERTIES • SEVEN WIRE STRAND – Outer wires enclose inner wire in a helix with a uniform pitch of 12 to 16 times nominal diameter – Nomenclature - A-B – A REPRESENTS NO. OF WIRES IN THE STRAND – B REPRESENTS NOMINAL DIA. OF STRANDS

PROPERTIES • THE STRAND SHALL BE EITHER CLASS I OR CLASS II DEPENDING UPON THE BREKING STRENGTH OF STRAND. THE BREAKING STRENGTH OF CLASS II STRAND IS MORE. • THE TOTAL ELONGATION UNDER LOAD SHALL NOT BE LESS THAN 3. 5%.

Detailing of Reinforcement and Cables

Cable Layout • Cable layout means – Deciding about the location of cable at various section • Vertical profile • Horizontal profile – The locations between which the cable will be in straight and on curve • Working out the ordinates at every meter and at every change of curvature from curved to straight and vice versa in vertical as well horizontal plane

Importance of Cable Layout • Proper moment resisting couple so as to – Carry the dead and live load moments – Not to induce tension in the concrete under dead load as well as live load • Local Imperfections – Cause increase in the losses due to friction on account of the wobble effect

Loss due to Friction (Wobble) The permissible tolerance in the location of the prestressing tendons (sheathing duct) shall be ± 5 mm

Cable Profile and Ordinate details for half span

END BLOCK DEATAILS

How Proper Positioning of Cable is ensured • Cable tends to sag due to its self weight if not supported properly on reinforcement chairs and supports • Cable tends to float and move upwards due to buoyancy effect when concrete is poured (and is in liquid form), if not tied down properly • So cable has to be secured against downward as well as upward movement unlike reinforcement

What else is important • The angle of the cable at the end – To provide the proper force – Not to induce unintended forces causing tensions in the direction not catered to for in design • This can be ensured and checked only at the time of fabrication of shuttering for end block

What else is important? • Sequencing of the stressing operations in Posttensioned construction is important and that given in the drawing should be followed. • If not given in the drawing this should be asked for from the designer.

Why Reinforcement is required in PSC • In the end block – To take the local transverse tension around the tendon behind the anchorage – To cater for the tension developed between two or more anchorages, which tends to split the member

Why Reinforcement is required in PSC • In the web for carrying shear • Shear is carried in PSC by – the vertical component of tendon – the concrete section – vertical reinforcement in the form of stirrups

Why Reinforcement is required in PSC • When the concrete section is sufficient to take the shear, theoretically no web reinforcement is required • This is seldom the case and shear reinforcement in the form of vertical stirrups is provided

Vertical Shear links

Why Reinforcement is required in PSC • At the junction of the web and the flange – As shear connectors for transferring the forces for enabling the member to carrying the moment – These are required between the bottom flange and the web – As well as between the top flange and the web

Shear Connectors

Why Reinforcement is required in PSC • Over the bearing area • This is required to distribute the stresses due to distribute the reaction to the larger section of the concrete

Material test data 4. Strand /wire coil no. = Tested UTS value = 5. Design Area of Cable (Ad ) = mm 2 Measured Area of Cable (Am ) = mm 2 6. Design Value of E (Ed) = kg/cm 2 Measured Value of E (Em) = kg/cm 2

7. Modified elongation : 8. Jack Area (Aj ) = cm 2 9. Design jack efficiency (nd) = 10. Measured jack efficiency ( nf ) = (as per certificate) 11. Pre-stressing design force (Pd ) = t x 103 kg 12. Modified pressure = Pd /Aj x nd /nf kg/cm 2

STRESSING OF CABLES

LOSS DURING ANCHORAGE • This loss – occurs when Pre-stressing force is transferred from tensioning equipment to anchorage. – It is particularly important in short members – It should be cross checked at site & compared with the values adopted by designer – ( it depends on type of anchorage and pre stressing system)

FUTURE CABLES • For easy installation at later date • Made in box girder to cater for increased pre-stress force • Provision of 15% (minimum) of design pre -stressing force.

EFFECT OF PRESTRESS

Other Important Issues • Proper Storage of the HTS – HTS coils should be stored in a closed go-down to protect it from the harmful effects of atmosphere and protect it from corrosion • Use of water soluble oil coating – Insist on the factory application of the water soluble oil coating on the HTS to prevent corrosion

Other Important Issues • HTS should be handled with great care like a baby so that it does not get a cut or even a minor nick. The handling should be done on raised supports avoiding dragging on ground. • Cable should be grouted after stressing without delay – and in no case it be allowed to remain un-grouted after 7 days of stressing.

Difference between pressure and elongation The difference between the elongation and the pressure should not be more than 5%

Other Important Issues • Grouting of the ducts – Non shrink grout or non shrink admixture to be used (but take care to use admixtures that do not cause corrosion like Aluminum salts • For longer Girders, it is preferable to provide Air Vents to release trapped air and ensure complete filling of the ducts with grout.

Other Important Issues • Cutting of HTS after prestressing – HTS should be cut using the abrasive disc cutters and in no case using the gas cutting • Ends of the HTS after cutting should be protected and should be buried in rich concrete ensuring covering of the end block in rich concrete

Windows in Forms • Windows/openings should be left in the formwork for vibration of the concrete in case of tall members like web. • Checking by wooden mallet should be done continuously during the concreting particularly at the difficult locations to ensure proper concreting

Practical problems and remedies • Problem: Cracks in pre-cast pre-stressed girders in stacking yard, girders were supported such that part of girder length was overhanging. • Solution: Stacking was improved. One of the randomly selected girders was tested to its ultimate load & found satisfactory. All girders were used.

Practical problems and remedies • Problem: Cracks in cast-in-situ pre-stressed deck as pre-stressing was started after 3 Calendar days as per drawing note. • Reason: It was winter & the concrete strength gain was not sufficient to take pre-stressing load. Due to winter the strength gain was slow. • • Solution: Since than pre-stressing was taken up after testing field cubes only. Cubes were cured with the parent girder/s only.

Practical problems and remedies • Problem: Cracks appeared from deck slab towards end cross diaphragm window opening. • Reason: The concrete sections from soffit slab & webs were pre-stressed while deck was not. • Solution: Provided closely spaced mild steel surface reinforcement (6 mm diameter at 75 mm c/c both ways) and the cracks reduced to acceptable / vanished.

Cracks in deck slab

Practical problems and remedies • Problem: Cracks appeared from deck slab towards soffit through webs near end / intermediate supports. • Reason: The cable layout over these sections was dipping heavily to take benefit of vertical shear resisting component without checking stresses in concrete at respective stage. • • Solution: (1) Cable profiles at ends / intermediate supports were checked with respect to their vertical component resisting external shear and it was found that the relief was exceeding the permissible shear stress. (2) The top & bottom most fiber stresses were also worked out with respective BM and it was found that the tensile stress at top was exceeding the permissible limit. The cable profiles were flattened & the cracking problem has vanished.

Profile of prestressed cables

Practical problems and remedies • Problem: While pre-stressing from both ends, it is advised through drawing/s that pre-stressing be carried out simultaneously from both ends. Hydraulic pressure levels v/s extensions are monitored with the least possible difference but most of the time not satisfying the requirements as stipulated. • • Solution: (1) Pre-stress cables from both ends with minimum difference in hydraulic pressure. (2) Increase hydraulic pressure in multiples of say 15% to 20%. Unless the lagging end picks up the same pressure, do not proceed ahead with the leading end. (3) Best method of both end pre-stressing is, carry out pre-stressing from one end first & then from the other end. The results at the end of both end pre-stressing are the same as expected through design calculations

Practical problems and remedies • Problem: Duration and cost of consumables forms fixing & removal was not fitting into the duration and budget. • • Solution: Side forms were fabricated in 5 m long x 3. 250 m tall panels and assembled on trolleys in one piece side shutters to cast 40 m long x 3. 25 m tall pre-tensioned precast girders. It resulted in time & consumable saving and improved quality. Ladders, walkway platforms, toe boards & hand railings helped to save on concrete wastage, improve in supervision & efficiency of workmen.

One piece side forms (3. 25 m tall x 40 m long) with working platforms & hand railings.

Completed rebar cage being shifted

Rebar cage being lowered on pre-cast bed within one piece side forms for 40 m long span.

One piece side shutters with working platforms, toe boards, ladders & hand railings

One piece side shutter in position

Arial view of pre-cast bed for 40 m long pretensioned girders (2 lines each with 3 girders)

Practical problems and remedies • Problem: Concreting end block was taking almost 7 calendar days per end, not fitting in completion schedule. • • Solution: The end block reinforcement cage pre-tied against jig, anchorages & bearing sleeves and then launched in its position with the help of crane and could complete end block within 4 calendar days.

Shortcomings through permanent structure design Shortcoming Remedial Measure Edge distance from concrete surface Edge distances should be strictly as per the recommendations of the manufacturers of the strands In particular it is observed that along edges and corners bursting of concrete is observed due to reduced clear cover. The designer giving priority to HT steel without consideration to reinforcement clear cover.

Shortcomings through permanent structure design Shortcoming Remedial Measure Inadequate information & precautions provided on drawings, like shortening and hogging up of girder after release of pre-stress. Shortening and hogging up of girder after release of pre-stress be made available in drawings. Negative moment at the centre of Provision of additional reinforcement to span and at the support over the pier accommodate negative moment cap due to non availability of the reinforcement steel in these zones, results in cracks to the girder

Shortcomings through Execution Shortcomings Remedial Measure Honeycombing in girder bottom flange. Bottom bulb top slope should be steeper (1 H : 3 V), to allow entrapped air to get escaped. Even if it leads to additional cost one should go for this steeper bottom bulb top slope Form work for in-situ deck slab • Intermediate gaps between psc girders be provided with sacrificial rcc planks. • Cantilever deck slab be avoided by matching edge girders with deck slab edge. • For cantilever deck slab above is not possible and a traveling form set be used.

One piece cantilever deck slab forms on launching girder (2. 25 m wide x 40 m long)

Cantilever deck slab forms on launching girder (2. 25 m wide x 40 m long) to aligned for 2 spans at a time

Cantilever deck slab forms on launching girder (2. 25 m wide x 40 m long) completed cantilever deck slabs