post tensioning slabs

design and analyze post tensioning slabs

1. Project on Design and construction of post tensioning slab PRESENTING BY:- CH .Gopichand (10631A0110) D.Nagender (10631A0120) J.Paramesh (10631A0126) N.Prudhviraj (10631A0129) S.Sathyanarayana(10631A0147) CIVIL ENGINEERING 4 th YEAR SRI VENKATESWARA ENGINEERING COLLEGE INTERNAL GUIDE: Mrs.P.Jhansi

2. Objectives The objective of the present report is to summarize the experience available today in the field of post-tensioning in building construction and in particular to discuss the design and construction of post tensioned slab structures, especially post tensioned flat slabs* A detailed explanation will be given of the checks to be carried out, the aspects to be considered in the design and the construction procedures and sequences of a post-tensioned slab. The execution of the design will be explained.

3. Prestressed concrete PRINCIPLE – Using high tensile strength steel alloys producing permanent pre-compression in areas subjected to Tension. A portion of tensile stress is counteracted thereby reducing the cross-sectional area of the steel reinforcement . METHODS :- a) Pretensioning b)Post-tensioning PRETENSIONING :- Placing of concrete around reinforcing tendons that have been stressed to the desired degree. POST-TENSIONING :- Reinforcing tendons are stretched by jacks whilst keeping them in serted in voids left pre-hand during curing of concrete. These spaces are then pumped full of grout to bond steel tightly to the concrete. STEEL BARS BEING STRETCHED BY JACKS

4. Introduction Methods of Pre-stressing Pre-tensioning Post-tensioning

5. Introduction Pre-tensioning Steel tendons are stressed before the concrete is placed at a precast plant remote from the construction site.

6. Introduction Post-tensioning Steel tendon are stressed after the concrete has been placed and gained sufficient strength at the construction site.

7. Introduction Post-tensioning Systems Un-bonded Post-tensioning System Bonded Post-tensioning Systems Single strand Multi strands flat duct Multi strands round duct Single strand

8. Design of PT Slabs Flat Plate with Drop Panels Common geometries* Two-way system Suitable span: 12.2 m Limiting criterion: Deflection Rebar**: 2.94 kg/m 2 PT: 3.87 kg/m 2 * for typical office/residential buildings using ACI/UBC requirements ** quantity assume no bottom reinforcement

9. Design process

10. Materiel properties CONCRETE: Fc^28 → Compressive strength of concrete 28 days. Fcd → Design value for compressive strength on concrete. → 0.6 × fc^28 = 21 N/MM^2 PRE STRESSING STEEL: Ap → cross sectional area of pj steel 146 mm^2 Fpy →yield strength of PT steel 1570 N/MM^2 Fpu → characteristic strength of PT steel 1770 N/MM^2 PRE-TENSIONING STEEL: Ep → modulus of elasticity of pre stressing steel 1.95 × 10^5 N/MM^2 (very low relaxation (3%) Admissible stressing 0.75 fpu Reinforcing steel: Fsy →yield strength of reinforcing steel is 460 N/MM^2 Long-term losses (assumed to be 10%)

11. Details of building Type of structure: commercial building - Loadings: Live load p = 2.5 kN /m2 Floor finishes gB = 1.OkN/m2 Walls g w = 1.5 kN /m2 q = 5.0 kN /m2

12. Plan showing dimensions

13. Design Determination of slab thickness: Assumption l/h = 35 Self wt of slab g = yc × h L → length of span 8.4 h → 0.24 mt h → thickness of slab. Yc → volumetric wt.of concrete →2.5 KN/M^3 → g=6KN/M^3 → q =5 KN/M^3 → ( g+q )/g) = 6+5/6 = 1.83 (( g+q ) – service load g→ self wt )

14. (l/h as a function of ( g+q /g)) → For a value of 1.83 on y- axis l/h is coming to 36 → 0.233 which is approximately (0.24)

15. Determination of prestress µ → it is transfer component from pre stressing / unit length ( g+q /g) → 1.83 based on previous caluculation

16. Pre stress in longitudinal direction → for 1.83 the u/g value in is 1.39 → u = 8.34 KN/m^2 K → woober ’ s coefficient =(0.24×10^3)/(8.4^2×25) = 0.136 →h = 0.24 →length of slab = 8.4 → yc =25 £ c =concrete tensile stress=1000 Pre tensioning force → P = 4×l^2/8×hp →sag of tendon parabola Hp →0.178mt (p=8.34× 8.4^2/8×0.178) P =413 KN/M P =7.8 × 413 for a width of 78 mt P = 3221 KN/strand Pl → pre tensioning force per strand Pl → Ap × fpu × 0.7 ×10^-3 Ap =416 mm^2 Fpu = 1770 N/mm^2 Pl = 181 KN

17. strands No.of strands = p/ pl =413/pl =17.8 =͠ 18 18 strands of dia 15mm on 78 mt width. For 7.4 mt width =7.4/7.8 ×17.8 =16.88 17 mono strands of dia 15 mm of 7.4 mt width. On 6.6 mt width = 6.6/7.8 ×17.8 =15.1 16 mono strands of dia 15 mm of 6.6 mt width. For 2.4 mt width =2.4/7.8 ×17.8 =5.5 6 mono strand of dia of 15mm on 2.4 mt width Transverse direction: g+q /g = 1.83 k = 0.24 × 1000/ 7.8 m^2×25 k = 0.158 on design chart 2 for a k value of 0.158 & ( g+q /g) value of 1.83 the value of u/g is found be 1.41 → u= 8.46 kn /m^2 P = (u×l^2/8×hp ) →8.46 ×7.8^2/( 8× 0.167) P = 3.85 kn /m On 8.4 mt width p=8.4×385 P =3234 kn Pc 181 kn No. Of strands Np = p/pl =3234/181 = 17.9 18 mono strands of dia 15mm on 8.4 mt width On 7.2 mt width np = 7.2/8.4× 7.9 = 15.3 16 mono strands of dia 15mm on 7.2 mt width.

18. Execution Materials & Equipment Anchorage Markings Laying of Tendon Concrete pouring Pre stressing grouting

19. MATERIALS AND EQUIPMENT FORMWORK CONCRETE STRANDS TENDONS DUCTS ANCHORAGES WEDGES

20. Formwork

21. strands

22. Wedges

23. POST –TENSIONING METHOD

24. Anchorage marking

25. Laying of tendons

26. Concrete pourig Mix design of M35 Grade of Concrete : M35 Characteristic Strength ( Fck ) : 35 Mpa Standard Deviation : 1.91 Mpa * Target Mean Strength : T.M.S.= Fck +1.65 x S.D. (from I.S 456-2000) = 35+ 1.65×1.91 = 38.15 Mpa Test Data For Material: Aggregate Type : Crushed Specific Gravity Cement : 3.15 Coarse Aggregate : 2.67 Fine Aggregate : 2.62 Water Absorption Coarse Aggregate : 0.5% Fine Aggregate : 1.0 %

27. Concrete pouring

28. Mix Design: Take Sand content as percentage of total aggregates = 36% Select Water Cement Ratio = 0.43 for concrete grade M35 Select Water Content = 172 Kg (From IS: 10262 for 20 mm nominal size of aggregates Maximum Water Content = 186 Kg/m 3 ) Hence, Cement Content= 172 / 0.43 = 400 Kg /m 3 Formula for Mix Proportion of Fine and Coarse Aggregate: 1000(1-a 0 )= {(Cement Content / Sp. Gr. Of Cement) + Water Content +(F a / Sp. Gr.* P f )} 1000(1-a 0 )= {(Cement Content / Sp. Gr. Of Cement) + Water Content +C a / Sp. Gr.* Pc )} Where C a = Coarse Aggregate Content F a = Fine Aggregate Content P f = Sand Content as percentage of total Aggregates = 0.36 P c = Coarse Aggregate Content as percentage of total Aggregates. = 0.64 a 0 = Percentage air content in concrete (As per IS :10262 for 20 mm nominal size of aggregates air content is 2 %) = 0.02 Hence, 1000(1-0.02) = {(400 /3.15) + 172 +(F a / 2.62 x 0.36)} Fa = 642 Kg/ Cum As the sand is of Zone II no adjustment is required for sand. Sand Content = 642 Kg/ Cum 1000(1-0.02)= {(400 /3.15) + 172 +(C a / 2.67 x 0.64)} Hence, Ca = 1165 Kg/ Cum

29. Prestressing jack For prestressing, mono strand stressing jack is used and pressure is applied in a controlled way with the help of prestressing power pack. Initially, a gradual pressure of about 5 kg/cm 2 is applied. anchorage bursting when prestressing is applied and also to check anchorage slip. The perimeter of the rod is then marked with paint and then once the anchorage is known to be stable, the pressure is increasing up to 430 kg/cm 2 .

30. Jacks Prestressing powerpack Mono strandstressing jack

31. Grouting Grouting is done with the help of grout pump. The mixture of cement, water and admixtures must be done under a strict mixing time and velocity control and must not contain lumps nor any air bubbles during injection into the ducts

32. EQUIPMENTS :- T6Z-08 Air Powered Grout Pump Pumps cement grout only, no sand. 32 Gallon Mixing Tank. Mixes up to 2 sacks of material at once and allows for grout to be pumped during mixing or mixed without pumping. Approximate size 50" long 30.5" high 52" wide Weight 560 lbs. Production Rate 8 gallons per minute at 150 psi

33. ADVANTAGES OF POST-TENSIONING Longer clear spans Thinner slabs Lesser floor-to-floor heights Shorter building height Lesser weight Improved seismic performance Faster construction cycle

34. Conclusions Prestressed concrete offers great technical advantages in comparison with other forms of construction such as reinforced concrete and steel. They possess improved resistance to shearing forces, due to the effect of compressive prestress , which reduces the principles tensile stress Prestressing of concrete helps in improving the ability of the material for energy absorption under impact loads. The economy of prestressed concrete is well established for long span structures. Standardized precast bridge beams between 10m and 30 m long and precast prestressed piles have proved to be more economical than steel and reinforced concrete. Due to utilization of concrete in the tension zone, an extra saving of 15 to 30% in concrete is possible in comparison with reinforced concrete

35. Thank You!