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

4th YEAR

SRI VENKATESWARA ENGINEERING COLLEGE

INTERNAL GUIDE:

Mrs.P.Jhansi

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.

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

Introduction

Methods of Pre-stressing

Pre-tensioning

Post-tensioning

Introduction

Pre-tensioning

• Steel tendons are stressed before the concrete is placed

at a precast plant remote from the construction site.

Introduction

Post-tensioning

• Steel tendon are stressed after the concrete has been placed and gained sufficient strength at the construction site.

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

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/m2

• PT: 3.87 kg/m2

* for typical office/residential buildings using

ACI/UBC requirements

** quantity assume no bottom reinforcement

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^2PRE STRESSING STEEL:

Ap → cross sectional area of pj steel 146 mm^2Fpy →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%)

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

Plan showing dimensions

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 )

(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)

Determination of prestress

µ → it is transfer component from pre stressing / unit length

(g+q/g) → 1.83 based on previous caluculation

Pre stress in longitudinal direction

→ for 1.83 the u/g value in is 1.39

→ u = 8.34 KN/m^2K → 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 parabolaHp →0.178mt (p=8.34× 8.4^2/8×0.178)

P =413 KN/MP =7.8 × 413 for a width of 78 mt

P = 3221 KN/strand

Pl → pre tensioning force per strandPl → Ap × fpu × 0.7 ×10^-3

Ap =416 mm^2

Fpu = 1770 N/mm^2

Pl = 181 KN

strandsNo.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.

Execution

Materials & Equipment

Anchorage Markings

Laying of Tendon

Concrete pouring

Pre stressing

grouting

MATERIALS AND EQUIPMENT

a) FORMWORK

b) CONCRETE

c) STRANDS

d) TENDONS

e) DUCTS

f) ANCHORAGES

g) WEDGES

Formwork

strands

Wedges

POST –TENSIONING METHOD

Anchorage marking

Laying of tendons

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 %

Concrete pouring

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

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 .

Jacks

Prestressing powerpack

Mono strandstressing jack

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

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

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

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

Thank You!