post tensioning slabs

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^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%)

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

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^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

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.

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

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 %

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

post tensioning slabs

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