POST TENSIONING IN BUILDING CONSTRUCTION Nitish Gaurav1
Research Scholar (Structural Engineering), Department of Civil Engineering,, School of Engineering and Technology, KK University, Nalanda, Bihar
Mr. Sandeep Kumar2
Assistant Professor, Department of Civil Engineering,
School of Engineering and Technology, KK University, Nalanda, Bihar Mr. Deepak Kumar3
Assistant Professor, Department of Civil Engineering,
School of Engineering and Technology, KK University, Nalanda, Bihar
Abstract- One of the more significant advancements in the domains of structural engineering and construction has been the creation of prestressing technologies. Regarding post tensioning applications in particular, it is well acknowledged that these techniques allow concrete solutions to be more economical, structurally sound, and aesthetically pleasing. As of late, post tensioning has proven to be more cost-effective than RCC construction. The use of post tensioning in construction slabs and beams is covered in the project. The construction of theatres, multistory buildings, and retail malls all frequently adopts this technique. Despite the ease of its fundamental ideas and its well-known advantages, post-tensioning solutions' application scope cannot be regarded as uniform throughout all domains and structural applications. In fact, it appears that the potential of prestressing is not being fully utilised, particularly in the field of constructing structures, for a variety of reasons. After all, it often happens that a more conventional, non- prestressed solution is chosen in many situations when post tensioning would offer a superior answer. For the building of industrial, commercial, and residential floor slabs, post-tensioned slabs are the favoured option. Due to its benefit and ease of application to a wide range of structure geometry and design solutions, this technology is being used more and more frequently. In high-rise building, the usage of post-tensioned floor slabs and reinforced concrete core walls has grown in popularity. Discussions of the post-tensioning slab system's economics cover relative material contents, construction speed, and post- tensioning cost-affecting variables. The flexibility of post-tensioned building structures is finally discussed in relation to potential future applications, new floor penetrations, and demolition.
Keywords: Domains of Structural Engineering, RCC Construction, Prestressing Technologies, Post-Tensioning Solutions, Application and Scope.
1 INTRODUCTION
When Eugene Freyssinet developed and patented the technology of prestressing concrete In 1928 he knew little about the applications for which he would be invented in future years. The use of prestressed concrete increased exponentially after second World War With materials used to repair and rebuild bridges in Europe. It is now an accepted civil Engineering construction material. The A.C.I Committee on Prestressed Concrete gives one of the most appropriate descriptions of post tensioned concrete. Prestressed concrete is concrete in which the internal forces of Such magnitude and distribution that the force produced by a given external load is retribution to a desirable degree'.
After stress we get many different benefits:-
(a) Designers have the opportunity to apply forces internally to the concrete structure the counterbalance and balance loads created by the structure thereby enabling design optimization.
(b) Designers can make use of the advantage of compressive strength of concrete while bypassing your inherent weakness under stress.
(c) Post-tensioned concrete combines and adapts to today's very high strength concrete and steel to result in a practical and efficient structural system.
In the 1950s the first post-tensioned buildings in the United States were built using unbonded post tensioning. Some post-tensioned structures were built long ago in Europe but the actual development
took place in Australia and the United States. joint efforts of the pressing companies, researchers and design engineers in these early stages resulted in standards and recommendations which helped promote the widespread use of this form of construction in Australia, in the United States and throughout the Asian region. Recent extensive research in these countries as well as in Europe has led to much expanded the knowledge available on such structures and now forms the basis of standards and codes of practice in these countries. Current architecture in India continues to place emphasis on the necessity of providing large uninterrupted floor space, flexibility of internal layout, versatility of use and freedom of movement.
All of these are facilitated by the use of post-tensioning in the construction of concrete floor slabs, giving large clear spans, fewer columns and supports, and reduced floor thickness. Post-tensioning in buildings can be loosely divided into two categories. The first application is for specialized structural elements such as raft foundations, transfer plates, transfer beams, tie beams and the like. For large multi-strand tendons used in these elements, 15.2 mm diameter seven wire strands are preferred.
2 BENEFITS OF POST-TENSIONED FLOORS
The Primary advantages of post-tensioned flooring over traditional reinforced Concrete in-situ flooring can be summarized as follows:
(a) (a) Longer Spans-Long spans can be used to reducing the number of columns. This results in bigger Column free floor areas which greatly increase the flexibility of use for the structure and May result in higher rental returns.
(b) Overall Structural Cost- The total cost of materials, labour and formwork required to build the floor is reduced For spans over 7 meters thereby providing better economy.
(c) (c) Lowered to Floor-to-Floor Height- For the same applied load, thinner slabs can be used. Allow less section depth resultant savings in facade cost with minimum building height. Alternatively, for tall buildings may allow for the construction of more
storeys within the original building envelope.
(d) Deflection Free Slab-Unwanted deflection under service load can be virtually eliminated.
(e) Waterproof Slab- Post-tensioned slabs can be designed to be crack free and therefore waterproof slabs are possible. The realization of this objective depends on careful design, detailing and Construction. The Choice of Concrete Mixing and Curing Methods with quality workmanship also plays an important role.
(f) Preliminary formwork stripping-The earlier stripping of formwork and lower back propping requirements enable faster Construction cycle and quick reuse of formwork. This is an increase in the speed of construction.
Economics is explained in detail in the next section.
(g) (g) Material Management-The low material quantity in concrete and reinforcement is of great benefit to the site The strength of the post- tensioning strand is about 4 times that of traditional reinforcement.
Therefore the total weight of the reinforcing material is greatly reduced.
(h) Column and Footing Design- Lower floor dead loads can be used in a more economical design of the reinforced concrete columns and footings. In multi-storey buildings, reduced column sizes can be increase floor net let table area. These benefits can result in significant savings in overall costs. There are some Situations where building height is limited, with low floor height has permission for construction of additional storeys within the building envelope.
3 BONDED OR UNBONDED TENDON SYSTEMS
Post-tensioned floors can be constructed using bonded or unbonded tendons. The relative merits of the two techniques are subject to debate. The following points may be made in favor of each.
(a) Bonded System -The post-tensioned strands are inserted in galvanised steel or plastic ducts that are cast into the concrete section at the desired profile and create a voided channel through which the strands
may be installed in a bonded system. The ducts can be circular or oval in design, and the size of each duct can vary to accommodate a different number of steel strands. A combined anchorage casting is given at the duct's ends, which anchors all of the strands. The stressing jack's force is transferred to the concrete via the anchorage. After the strands have been stressed, a cementitious grout is used to fill the void around them, completely bonding the strands to the concrete. A tendon is the name given to the duct and the strands that go through it.
Figure1 Bonded Systems (b) Unbonded System-Individual steel strands are encased in a polyurethane sheath and spaces between the sheath and the strand are filled with rust-inhibiting grease in an unbonded system. Under factory circumstances, the sheath and grease are applied, and the completed tendon is electronically tested to ensure that the process was completed successfully. Anchorage castings are used to secure the separate tendons at either end. Once the concrete has reached the required strength, the tendons are jacked to impart the required prestress force.
Figure 2 Unbonded Tendon Systems 4 MATERIALS
All of the elements used in a reinforced concrete floorformwork, rod reinforcement, and concrete are used in post-tensioned
floors, as well as high tensile steel strand and post-tensioning hardware. Rod reinforcement in post-tensioned floors is identical to that used in reinforced concrete in every way. The yield stress of typical high tensile steel used in rod reinforcing is 460 N/mm2 and the modulus of elasticity is 200 KN/mm2. It has a poisons ratio of 0.3 and a thermal expansion coefficient of 12.5 x 10-6 degrees centigrade. Temperature has an effect on high tensioned steel strength, which drops from 100 percent at 300 degrees Celsius to only 5% at 800 degrees Celsius. Concrete production, compaction, and curing technology is well understood, hence it will not be detailed here. Only the concrete properties that are important for postensioning are taken into account. In post-tensioning, normal dense concrete with a density of 2400 kg/m3 is more frequent. However, in the correct circumstances, light weight concrete has some advantages. Both are addressed in their own parts. The qualities of two concretes are highly different, therefore mixing them together is not a smart idea;
differential movement, as well as differences in their elasticity, shrinkage, and creep modules, can cause issues.
(a) Formwork- The vertical edge boards of the form work must have holes drilled in them for the tendons to pass through at live anchorages in a post-tensioned floor. Due to the axial component of the prestress, the concrete undergoes a little length reduction during stressing. Though formwork trapping between any downstands is not a severe issue, formwork designers should be aware of the possibility. The post-tensioned slab lifts off the formwork during stressing, causing the weight to be redistributed.
Parts of the formwork may be subjected to greater loads as a result of this than those imposed by the weight of wet concrete.
Figure 3 Formwork
(b) Cement-With the previous consent of the engineer-in-charge, any of the following cements may be used:
Ordinary Portland cement, as defined by IS : 269-1976*;
Portland slag cement, as defined by IS : 455-1976† but with no more than 50% slag content;
Rapid-hardening Portland cement, as defined by IS : 8041-1978‡; and
High strength ordinary Portland cement, as defined by IS : 8112- 1976§.
(c) Aggregates -All aggregates shall comply with the requirements of IS: 383-1970. The nominal maximum size of coarse aggregate is as large as possible, subject to the following conditions:
No more than one-fourth the minimum thickness of the member, provided that the concrete can be put easily to surround all prestressing tendons and reinforcements, as well as fill the form's corners.
It shall be 5 mm less than the spacing between the cables, strand sheathing are provided.
A maximum nominal size of 20 mm or less is normally considered satisfactory; aggregates with a maximum nominal size of 40 mm or less are generally considered satisfactory.
(d) Water -The requirements of water used for mixing and curing shall conform to the requirements given in IS: 456-1978*.
However use of sea water is prohibited.
(e) Admixtures -Admixtures may be used with the approval of the engineer-in-charge.
However use of any admixture containing chlorides in any form is prohibited. The admixtures shall conform to IS: 9103-1979.
(f) Prestressing Steel -The prestressing steel shall be any one of the following:
Plain hard-drawn steel wire conforming to IS: 1785 (Part I)-1966‡
and IS: 1785
Cold-drawn indented wire conforming to IS: 6003-1970
High tensile steel bar conforming to IS: 2090-1962, and
Uncoated stress relieved strand conforming to IS: 6006-1970.
5 EQUIPMENT
The specialist equipment required for post- tensioning consists of the following items, not all of which would be required on site.
Stressing jack
Swaging jack
Strand threading machine
Strand cutters or shears
Grout mixer and pump
Only bonded tendons require the use of a strand threading machine and grouting equipment. Strand can be threaded into the sheathing before, during, or after it has been installed, but before or after concreting. Of course, if the tendon lengths are short enough to be delivered threaded, the pushing machine would be unnecessary on site. Jacks are made to grab the strand(s) from either the front or the back.
Figure 4 Prestressing Jack
The jack is mechanically easier in the latter scenario, but it requires an extra length of strand (of the order of one metre, but verify for the specific jack to be used) that is cut off after stressing. After stressing, previous jacks had the option of manually pushing the wedge cone forward; now, practically all jacks automatically drive the wedges forward into the barrel to lock the strand as part of the automated stressing operation.
Jacks have a provision for measuring strand extension and are calibrated with their hydraulic pumps so that a direct reading of jacking force can be obtained.
Figure 5 Monojack
Figure 6 Grout mixer and pump
Figure 7 Strand threading machine Apart from flexibility in terms of handling equipment and available space at a given location, the vital aspects of a short stroke jack may have to be emphasised in stages.
The anchoring cones must grip and release the strand multiple times during the procedure, which may weaken the serrations. Varied systems have different sizes and weights of equipment, but those for monostrand use are quite similar in shape and size. These, as well as their pumps, are light enough to be handled by hand. The monostrand jack is commonly used to stress flat tendons. Multistrand jacks come in a variety of shapes, sizes, and weights. Some have a similar shape to a standard monostrand jack but are larger, while others have a much larger diameter and are shorter (up to 400 mm/16inch). A multistrand jack is significantly heavier than a monostrand jack, weighing up to 300kg. As a result, crane time is required throughout the stressing process. No specifics are offered due to the large range of equipment available from various vendors. It is suggested that specifics be sought from experts in the field.
6 ASSEMBLY OF PRESTRESSING AND REINFORCING STEEL
PRESTRESSING STEEL
(a) Straightening- When uncoiled, the wire should preferably be self-straightening. If this is not the case, the wire may need to be
straightened mechanically before use. In this case, care must be given to avoid changing the wire's qualities during the straightening process, and a test should preferably be performed on a sample of the wire after straightening. Any straightening (or bending, if the design allows for curved bars) of high tensile alloy steel bars must be done with a bar-bending machine. When the temperature of the bars is less than 10°C, they must not be bent. Heat should never be used to aid in the straightening or bending of prestressing steel.
(b) Wire Arrangement and Positioning-All prestressing steel must be carefully and precisely placed in the exact locations indicated on the design drawings. The position of the prestressing tendon can have a tolerance of up to 5 mm. The designer's needed curves or bends in prestressing tendon must be moderate, and the prestressing tendon must not be forced around severe bends or created in any way that could cause unwanted secondary stresses.
Suitable methods, such as suitably robust and evenly distributed spacers, must be used to maintain the relative position of wires in a cable, whether curved or straight.
In post-tensioning operations, the spacing of wires in a cable must be sufficient to allow free flow of grout. The steel in the mould or formwork must be fixed and supported in such a way that it does not move during the pouring or compacting of the concrete, or during the tensioning of the steel. The sort of fittings utilised to position the steel must not cause friction that is more than that which is assumed in the design.
(c) Jointing-Other than hard-drawn wire, high-tensile wire can be linked together using proper methods as long as the strength of the joints is equal to or greater than the individual strengths of the wires being joined. The hard-drawn wire used in prestressed concrete work must be continuous throughout the tendon's length.
Couplings can be used to attach high- tensile steel bars together, as long as the coupling's strength is such that the bar fails before the coupling in a test to destruction.
Welding of wires or bars is strictly prohibited.
(d) Protection of Prestressing Steel and Anchorages — In all post-tensioned
projects where prestressing is first performed without bonding, the prestressing tendon must be given suitable corrosion protection at a later date, normally not later than one week following prestressing.
(e) Internal prestressing steel — The best protection for internal prestressing steel is a colloidal cement or cement-sand grout.
Segregation must be avoided at all costs, and only fine sand should be utilised for this reason. The grout should be applied under pressure, and the area between the duct and the prestressing tendon should be completely filled with grout. It is recommended that water be flushed via small ducts prior to grouting, with care made to ensure that all water is subsequently displaced by grout. In the event of butted assemblies, water flushing should be done only after the jointing material has set correctly. Injection should begin at one end and continue until the grout overflows from the other end, preferably from the lowest point of the curve in the case of curved ducts.
(f) External prestressing steel — The best way to safeguard external prestressing steel is to encase the tensioned wires, cables, or bars in dense concrete that is secured to the main concrete, for example, by wires left projecting from the latter. If a cement-sand mix is utilised, the cover and density provided should be sufficient to prevent corrosion. Alternatively, the steel might be encased in bitumen, or paint protection could be applied where the steel is accessible for inspection and maintenance.
Soon after the final stressing and grouting procedures, the anchoring must be appropriately safeguarded from damage or corrosion.
(g) Cover-The cover of concrete measured from the outside of the prestressing tendon must be at least 20 mm in pre-tensioned work. When cables and large-sized bars are used in post-tensioned work, the minimum clear cover from sheathing/duct must be at least 30 mm or the cable or bar's size, whichever is greater.
(h) Spacing-In the case of single wires used in pre-tension system, the minimum clear spacing shall not be less than greater of the following:
3 times diameter of wire, and
1 times the maximum size of aggregate.
In the case of cables or large bars, the minimum clear spacing (measured between sheathings/ducts, wherever used) shall not be less than greater of the following:
40 mm,
Maximum size of cable or bar, and
5 mm plus maximum size of aggregate.
(i) Grouped Cables -Cables or ducts may be grouped together in groups of not more than four as shown in Fig.7.
Figure 8 Spacing of cables
The minimum clear spacing between groups of cables or ducts of grouped cables shall be greater of the following:
40 mm, and
(b) 5 mm plus maximum size of aggregate.
The vertical distance between groups shall not be less than 50 mm.
7 RESULT
DESINGN OF RCC BEAM (INPUT)
8 CONCLUSION
In conclusion it is worthy to reinforce a few key points. There is a clear tendency towards long spans in buildings because there is increased emphasis on offering huge uninterrupted floor area, which can produce higher rental returns. A cost- effective method of getting these wider spans is post-tensioning. Post-tensioning will undoubtedly be economical for spans of 7.5 metres and beyond, and the savings grow as the spans do as well. The tendon length has the greatest impact on the price of slab system post-tensioning. Other variables scatter the findings, resulting in an upper and lower bound. Despite this, getting low pricing from a post-tensioning supplier is always a good idea.
9 FUTURE SCOPE
Building models considered in this study are high building. The proposed results need to be validated by future case studies, taking more height of building. Stiffness can also be categorized based on seismic zone. In present study only variations due to heights of soft storey and variation due to beam and column section, while keeping are constant is considered. Same can be done for various zones along with height.
Another field of wide research could be to take into account the infill walls with door and window openings which have not been considered in this research work.
REFERENCES
1. Albercht, P., & Lenwari, A. (2008). Fatigue Strength of Repaired Prestressed Composite Beams. JOURNAL OF BRIDGE ENGINEERING, 13(4), 409-417.
2. Alkhairi, F., & Naaman, A. (1993). Analysis of beams prestressed with unbonded internal or external tendons. Journal of Structural Engineering, 119(9), 2680-2700.
3. ANSI/ASAE. (2003). Design Requirements and Bending Properties for Mechanically Laminated Columns.
4. ASCE. (2005). Report card for America’s infrastructure. Retrieved from Reston,VA, USA:
5. Ayyub, B. M., Sohn, Y. G., & Saadatmanesh, H.
(1992). Prestressed Composite Girders. II:
Analytical Study for Negative Moment. Journal of Structural Engineering, 118, 2763-2782.
6. Baxter, S. (2014). Low Grade Solid Timber Systems for Residential Construction.(Bachelor of Engineering Thesis), University of Tasmania.
7. Bernard, E. S. (2008). Dynamic serviceability in lightweight engineered timber floors. Journal of Structural Engineering, 134(2), 258-268.
8. Buchanan, A., & Fairweather, R. (1993).
Seismic design of glulam structures. Bulletin of the New Zealand national society for earthquake engineering,26(4), 415-436.
9. Buchanan, A., Palermo, A., Carradine, D., &
Pampanin, S. (2011). Post-Tensioned Timber Frame Buildings. The Structural Engineer, 89, 17.
10. CBS, & CBT. (2011a). D-Dalle. In C. B.
Technologie & C. B. Structure (Eds.).
11. CBS, & CBT. (2011b). La Dalle O'portune. In C.
B. Technologie & C. B. Structure (Eds.).
12. Chen, S. (2005). Experimental study of prestressed steel–concrete composite beams with external tendons for negative moments.
Journal of Constructional Steel Research, 61, 1613-1630.
13. Chen, S., & Gu, P. (2004). Load carrying capacity of composite beams prestressed with external tendons under positive moment.
Journal of Constructional Steel Research, 61, 515-530.
14. Colombi, G., & Poggi, C. (2006). An experimental, analytical and numerical study of the static behavior of steel beams reinforced by pultruded CFRPstrips. COMPOSITES PART B-
ENGINEERING, 37, 64-
73.doi:10.1016/j.compositesb.2005.03.002 15. Dall’Asta, A., & Zona, A. (2005). Finite element
model for externally prestressed composite beams with deformable connection. Journal of Structural Engineering, 131(5), 706-714.
16. Deng, J., & Lee, M. (2009). Adhesive bonding in steel beams strengthened with CFRP.
Proceedings of the Institution of Civil Engineers - Structures and Buildings, 162, 241-249.
doi:10.1680/stbu.2009.162.4.241
17. Dolan, J., Murray, T., Johnson, J., Runte, D., &
Shue, B. (1999). Preventing annoying wood floor vibrations. Journal of Structural Engineering,125(1), 19-24.
18. Durham, J. P., Lam, H. G. L., & He, M. (1999).
Seismic response of shear walls with oversized sheathing panels. Paper presented at the 8th Canadian Conference on Earthquake Engineering, Vancouver, Canada.
19. Ellingwood, B., & Tallin, A. (1984). Structural serviceability: floor vibrations. Journal of Structural Engineering, 110(2), 401-418.
20. Filiatrault, A., & Folz, B. (2002). Performance- Based Design of Wood Framed Buildings.
Journal of Structural Engineering, 128(1), 29- 47.
21. Foliente, G. C. (1995). Hysteresis Modeling of Wood Joints and Structural Systems. Journal of Structural Engineering, 121(6), 1013-1022.
22. Gilbert, R. I., & Mickleborough, N. C. (2019).
Design of Prestressed Concrete. London: Unwin Hyman.
23. Hamilton, J. (2020). Use of Low Grade Timber in Residential Flooring Systems. (Bachelor of Engineering with Honours Thesis), University of Tasmania.
