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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
Cost Effectiveness of HDPE Sheathing For Post-Tensioned
Prestressed Concrete Structures Over Galvanised Metallic Ducts –
A Study
S.G. Bapat, Chief Engineer (Civil-Designs) Retd; Mr. Arvind Shrivastava, Addl. Chief Engineer
(Civil) Nuclear Power Corporation of India Ltd. Mumbai, and Mr. Umesh K. Rajeshirke,
Spectrum Consulting Engineers, Navi Mumbai.
Introduction
Prestressing duct is one of the vital elements of total prestressing system.
The main functions of the duct are:
To create cavity / void in the concrete, along the profile of the prestressing cable so that
the cable can be threaded into it after the concrete gets hardened.
To provide the protection against the corrosion to the prestressing steel.
Different types of ducts are available and they can be classified depending upon their surface
profile, such as plain or corrugated, and depending on the material from which these ducts are
manufactured, such as metallic or non-metallic.
Metallic ducts made from steel strips with corrugations are being used over long period for
bonded post tensioning tendons. Plastic ducts have been used for many years in the industry,
but mostly in the form of smooth pipes for applications involving unbounded cables like external
prestressing, ground anchors etc. Only recently, the corrugated thick-walled HDPE (High
Density Polyethylene) ducts have become popular internationally for other applications of posttensioned tendons like bridges etc. They offer excellent features over metallic sheathing such as
improved corrosion protection of the tendons, reduced friction losses during stressing of the
tendons, increased fretting fatigue resistance of the tendon, their feasibility for corrosion
monitoring and the durability of the ducts themselves. Many times it is found that, when there is
large time gap between the concreting of the structure and prestressing operations, the metallic
duct gets corroded and then leads to a major problem of leakage of grout from one duct to the
adjacent ducts. This problem can be completely avoided if the HDPE ducts are used.
The cost of corrugated plastic ducts for bonded post-tensioning was strongly influenced by the
production method. These ducts were typically produced by extrusion & then spiral winding
process requiring significant investment. Since they were produced in relatively small quantities,
corrugated plastic ducts for bonded post tensioning system were typically more expensive, by
15–20% higher when compared with corrugated thin walled steel ducts till late 2000. But with
era of enormous use of PE ducts in large quantities coupled with modern speedy processing
methods, made PE ducts very competitive since then. The price gap further became not only
narrow but also made metallic ducts costly due to skyrocketing prices of metallic strips since
2002. The PE ducts since then are becoming very competitive & cost effective than metallic
ducts. Ducts represent about 5% of the total post-tensioning cost, which is about 10% of total
construction cost of a bridge structure.
In addition, significant improvement of durability and quality of the main reinforcement of a
structure can be achieved. Long tendons with significant friction losses due to tendon deviations
will benefit from the reduced friction coefficient of PE ducts and show an economy due to a
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
more effective use and therefore, a reduction of the required amount of prestressing steel
leading to an overall economy of the structure. The value of prestressing force available at a
particular section of the structure (e.g. superstructure of the bridge) to resist the tensile stresses
depends on the loss in the cable force due to friction between the cable and duct during
stressing operation. If the coefficient of friction is small, more is the force available and less is
the prestressing steel required. This results in saving in cost, especially for the structures like
long span bridges, continuous and curved girders, prestressed concrete silos, water tanks, pipe
lines, etc.
This report aims at providing a quantitative assessment of the cost saving in prestressing
system for some of the typical prestressed concrete structures.
The theoretical background on loss in cable force due to friction between cable and duct is
presented followed by the losses due to wobble of duct. The case studies are presented, which
are simply supported I girders for various spans, box girders, continuous box girder, simply
supported curved box girders and continuous curved box girders.
Theoretical Background
The physical phenomenon of frictional loss of a cable around a curve is well known [1]. For a
ready reference, a derivation for the same is given below.
Figure 1: Frictional loss along length dx
Curvature Effect: Consider an infinitesimal length dx of a prestressing tendon which follows the
arc of a circle of radius R, (Ref. Fig. 1). Then the change in angle of the tendon along its length
dx is
dα = dx / R
For this infinitesimal length dx, the force in the tendon may be considered constant and equal to
F; then the normal component of pressure N produced by the force F bending around an angle
dα is given by
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
N = Fdα = Fdx / R
The amount of frictional loss dF around the length dx is given by the pressure times a coefficient
of friction µ, thus,
dF = -µN
dF = -µFdx / R = -µFdα
Transposing, we get
dF / F = -µdα
Integrating this on both sides, we have
logeF = -µα -----------(1)
Using the limits F1 and F2, we have the conventional friction formula
F2 = F1e-µα = F1e-µL/R
Since α = L/R for a section of constant R.
For tendons with a succession of curves of varying radii, it is necessary to apply this formula to
the different section in order to obtain the total loss.
Wobble effect: Wobble coefficient, b depends physically on the rigidity of the duct which mainly
depends on the duct diameter and the intermediate supports provided while laying the duct.
Typically, the value varies from 0.5o / m for smaller ducts to 0.3o / m for big stiff ducts. The
wobble effect can also be directly expressed in term of length using parameter K, which is K =
bµ. To compute frictional loss due to wobble or length effect, KL can be substituted for ìá in
formula 1, and then we have,
LogeF = -KL F2 = F1e-KL
If the length and curvature effects are combined, we get
LogeF = -µα -KL
For limits F1 and F2,
F2 = F1e-µα-KL
Or, in terms of unit stresses,
ƒ2 = ƒ1e-µα-KL
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
Figure 2: Approximate frictional loss along circular curve
The friction loss is obtained from this expression. Loss of prestress is given as FR = F1 – F2.
The stress in cable at the jacking end is ƒ1, and length to the point is L where the stress is ƒ2,
(Ref. Fig 2) Then,
FR = ƒ1 – ƒ2 = ƒ1 - ƒ1e-µα-KL = ƒ1 (1 - e-µα-KL) --------------(2)
Case Studies
Table showing values of μ & k
Equation (2) shows the influence of µ on the net
μ
k
effective prestressing force available for resisting a) As per IRC
0.17
0.002
b) As per fib
0.10 – 0.14
Not specified
the externally applied forces. Lower the value of
c) As per Experiment*
0.068
*0.00116
the µ, higher is the prestressing force available
*The experiment was carried out by IIT for NPCIL for Kaiga
and less is the prestressing steel required. To
Atomic Power Project pre-qualification exercise.
assess the extent of this benefit, four case studies are carried out, the summary of which is
presented in following sections.
The cost comparison is made between the conventional bright metal sheathing and the HDPE
sheathing. In case of HDPE, two sets of values for µ & k are used, first is based on the values
specified in IRC-18 [2] and other is based on average value specified in fib bulletin No. 7 [2].
Girder Bridges
For this study the standardized design of the girders of various spans viz. 30, 35, & 40 ms
published by MoST [4] are used as the base design, i.e. the same cable profile, number of
cables, type of cables are used for the comparison.
Span
Saving in prestressing steel
In MoST design, the number of cables and their
IRC Coefficients fib Coefficients
profile is arrived at using µ & K values of bright
30 m with footpath
2.78%
2.78%
30 m without footpath
2.38%
3.57%
metal sheathing which are 0.25 and 0.0046
35 m without footpath
5.21%
6.25%
respectively. In this study, it is tried to find out that 40 m with footpath
6.25%
7.29%
what could have been the reduction in
cable/strands if the HDPF sheathing would have been used instead of white metal sheathing.
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
Brief calculations are presented in Table No. 1 to 3 for girders with span of 30, 35, & 40 m
respectively and summary of the results are tabulated below:
It can be seen that the saving in prestressing steel is of the order of 6.25 to 7.3% for span of 40
m by just changing the white metal sheathing to HDPE sheathing.
Box Girder (Simply Supported)
Similar study has been carried out for box girders
with different spans. The results are presented in
table No. 4 through 8 for 30, 35, 40, 45 & 50 m
spans respectively. The table below gives the
summary of the results:
Span
30
35
40
45
50
m
m
m
m
m
Saving in prestressing steel
IRC Coefficients
fib Coefficients
2.57%
2.69%
4.02%
4.10%
5.32%
5.69%
5.93%
7.01%
6.50%
7.86%
Box Girder (Continuous Over 3 Spans)
Three span continuous box girder with span arrangement of 40m + 40m + 40m has been
considered for this case. For the comparison, longest cable in the girder which is running from
one end to the other is selected. The profile of this cable is shown in Fig. 3. The loss in
prestressing force is worked out at midpoint of cable i.e. center of the middle span. (The cables
are to be stressed from both the ends.)
Figure 3: Span arrangement of cable layout for continuous box girder
The change in angle from the stressing end to the mid point of cable = 96.93° and cumulative
length upto this point is 60.87 m. Hence the available prestressing force at the mid point and
saving in prestressing steel considering various values of µ & k is given below,
Available Prestressing
Of course not all the cables in the girder will be
Saving in
Type of Duct
force w.r.t force at
prestressing Steel
having similar profile and length. Some of the
stressing end
Bright metal
49.5 %
0.0 % (Reference)
cables run locally over short distance having
HDPE (IRC)
67.6 %
36.6%
HDPE (fib)
74.5 %
50.5%
lesser change in angle having different values of
prestressing force available. The overall saving in prestressing steel for the girder is found to be
about 14 to 16 % i.e. about 4.8 t, which in terms of cost is about Rs.3,60,000/- (Assuming
Rs.75,000.0 / t, which includes the cost of installation, stressing, grouting, etc. complete)
Continuous Box Girder With Curvature in Plan
Similar study has been carried out for continuous curved box girder having similar span
arrangement i.e. 40m +40m +40 m and the same cable profile in elevation. The radius of
curvature, in plan, is 60 m. Again for the comparison, longest cable in the girder which is running
from one end to the other is selected. The losses in prestressing force is worked out at midpoint
of cable i.e. center of the middle span & the cable is to be stressed from both the ends.
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
Available Prestressing
The change in angle from the stressing end to
Type of Duct
force w.r.t force at
the mid point of cable = 136.4° and cumulative
stressing end
Bright metal
41.6 %
length, up to this point is 60.9 m. the available
HDPE (IRC)
59.0 %
HDPE (fib)
68.6 %
prestressing force at the mid point and saving in
prestressing steel, considering for various values of m & k, is given below,
Saving in
prestressing Steel
0.0 % (Reference)
41.7%
64.62 %
Again not all the cables in the girder will be having similar profile and length. Some of the cable
may run locally over short distance having lesser change in angle and having different values of
effective prestressing force available. The overall saving in prestressing steel for the girder is
found to be about 16 to 18 % i.e. about 6.25 t, which in term of cost is Rs. 4,68,750/Table 1.0 A : I Girder 30m span (without foot path)
Span
30m without FP
30m without FP
30m without FP
Type of Sheathing
Bright Metal
HDPE
HDPE
Coeff. as per IRC Coeff. as per IRC
Coeff. as per fib
Wobble and Friction coeff.
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
No of Cables
6
6
6
Frictional Losses
Cable No.
Strands % loss Strands % loss Strands % loss
Cable no 1
12
9.67%
12
6.87%
12
6.57%
Cable no 2
12
9.67%
12
6.87%
12
6.57%
Cable no 3
12
10.14%
12
6.85%
12
6.56%
Cable no 4
12
10.14%
12
6.85%
12
6.56%
Cable no 5
12
9.67%
12
5.99%
12
5.81%
Cable no 6
12
10.35%
12
6.04%
12
5.90%
Total
72
9.94%
72
6.58%
72
6.33%
Jack End Force
1031.4 t
1031.4 t
1031.4 t
Loss of Force
102.5 t
67.9 t
65.3 t
Saving in force
0.0 t
34.7 t
37.3 t
Saving in strands in each Girder
0
2
2
Percentage Saving
2.78%
2.78%
Table 1.0 B : I Girder 30m span (without foot path)
Span
30m with FP
30m with FP
30m with FP
Type of Sheathing
Bright Metal
HDPE
HDPE
Coeff. as per IRC
Coeff. as per IRC
Coeff. as per fib
Wobble and Friction coeff.
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
No of Cables
7
7
7
Frictional Losses
Cable No.
Strands % loss Strands % loss Strands % loss
Cable no 1
12
9.67%
12
6.87%
12
6.57%
Cable no 2
12
9.67%
12
6.87%
12
6.57%
Cable no 3
12
10.14%
12
6.85%
12
6.56%
Cable no 4
12
10.14%
12
6.85%
12
6.56%
Cable no 5
12
9.02%
12
5.81%
12
5.72%
Cable no 6
12
9.67%
12
5.99%
12
5.81%
Cable no 7
12
10.35%
12
6.04%
12
5.90%
Total
84
9.81%
84
6.47%
84
6.24%
Jack End Force
1203.3 t
1203.3 t
1203.3 t
Loss of Force
118.0 t
77.8 t
75.1 t
Saving in force
0.0 t
40.2 t
42.9 t
Saving in strands in each Girder
0
2
3
Percentage Saving
2.38%
3.57%
Table 2.0 : I Girder 35 m span
35m with FP
35m with FP
35m with FP
Bright Metal
HDPE
HDPE
Coeff. as per IRC Coeff. as per IRC
Coeff. as per fib
Wobble and Friction coeff.
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
No of Cables
8
8
8
Frictional Losses
Cable No.
Strands % loss Strands % loss Strands
% loss
Cable no 1
12
11.96%
12
6.80%
12
6.36%
Cable no 2
12
11.96%
12
6.80%
12
6.36%
Cable no 3
12
11.17%
12
6.20%
12
5.88%
Span
Type of Sheathing
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
Cable no 4
Cable no 5
Cable no 6
Cable no 7
Cable no 8
Total
Jack End Force
Loss of Force
Saving in force
Saving in strands in each Girder
Percentage Saving
12
12
12
12
12
96
11.17%
12.65%
13.83%
13.17%
12.31%
12.28%
1375.2 t
168.8 t
0.0 t
0
12
12
12
12
12
96
6.20%
6.77%
6.30%
6.87%
6.32%
6.53%
1375.2 t
89.8 t
79.0 t
5
5.21%
12
12
12
12
12
96
5.88%
6.33%
6.74%
6.02%
5.29%
6.11%
1375.2 t
84.0 t
84.8 t
6
6.25%
Table 3.0 : I Girder 40m span
40m without FP
40m without FP
40m without FP
Bright Metal
HDPE
HDPE
Coeff. as per IRC
Coeff. as per IRC
Coeff. as per fib
Wobble and Friction coeff.
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
No of Cables
8
8
8
Frictional Losses
Cable No.
Strands % loss Strands % loss Strands % loss
Cable no 1
12
12.97%
12
6.57%
12
5.86%
Cable no 2
12
12.97%
12
6.57%
12
5.86%
Cable no 3
12
12.18%
12
6.06%
12
5.34%
Cable no 4
12
12.18%
12
6.06%
12
5.34%
Cable no 5
12
13.65%
12
7.01%
12
5.87%
Cable no 6
12
14.82%
12
7.76%
12
6.30%
Cable no 7
12
14.16%
12
7.33%
12
5.72%
Cable no 8
12
13.13%
12
6.67%
12
5.18%
Total
96
13.26%
96
6.75%
96
5.68%
Jack End Force
1375.2 t
1375.2 t
1375.2 t
Loss of Force
182.3 t
92.9 t
78.1 t
Saving in force
0.0 t
89.4 t
104.2 t
Saving in strands in each Girder
0
6
7
Percentage Saving
6.25%
7.29%
Span
Type of Sheathing
Span
Type of Sheathing
Wobble and Friction coeff.
No of Cables
Cable No.
Cable no 1
Cable no 2
Cable no 3
Cable no 4
Cable no 5
Cable no 6
Cable no 7
Cable no 8
Cable no 9
Cable no 10
Cable no 11
Total
Jack End Force
Loss of Force
Saving in force
Saving in strands
Percentage Saving
Span
Type of Sheathing
Wobble and Friction coeff.
No of Cables
Cable No.
Cable no 1
Table 4 : Box Girder 30 m span
30m
30m
30m
Bright Metal
HDPE
HDPE
Coeff. as per IRC
Coeff. as per IRC
Coeff. as per fib
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
11
11
11
Frictional Losses ( at midspan of the girder )
Strands % loss Strands % loss Strands % loss
19
7.95%
19
6.04%
19
5.90%
19
7.95%
19
6.04%
19
5.90%
19
7.95%
19
6.04%
19
5.90%
19
7.95%
19
6.04%
19
5.90%
19
7.95%
19
6.04%
19
5.90%
19
7.95%
19
6.04%
19
5.90%
6
7.95%
6
6.04%
6
5.90%
19
9.33%
19
5.87%
19
5.76%
19
9.33%
19
5.87%
19
5.76%
19
9.67%
19
5.93%
19
5.81%
19
9.67%
19
5.93%
19
5.81%
196
8.55%
196
5.98%
196
5.86%
2807.8 t
2807.8 t
2807.8 t
240.1 t
167.9 t
164.5 t
0.0 t
72.1 t
75.6 t
0
5
5
2.55%
2.55%
Table 5: Box Girder 35m span
35m
35m
35m
Bright Metal
HDPE
HDPE
Coeff. as per IRC
Coeff. as per IRC
Coeff. as per fib
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
13
13
13
Frictional Losses ( at midspan of the girder )
Strands % loss
Strands % loss Strands % loss
19
8.51%
19
4.98%
19
4.91%
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Cable no 2
Cable no 3
Cable no 4
Cable no 5
Cable no 6
Cable no 7
Cable no 8
Cable no 9
Cable no 10
Cable no 11
Cable no 12
Cable no 13
Total
Jack End Force
Loss of Force
Saving in force
Saving in strands
Percentage Saving
Span
Type of Sheathing
Wobble and Friction coeff.
No of Cables
Cable No.
Cable no 1
Cable no 2
Cable no 3
Cable no 4
Cable no 5
Cable no 6
Cable no 7
Cable no 8
Cable no 9
Cable no 10
Cable no 11
Cable no 12
Cable no 13
Cable no 14
Cable no 15
Cable no 16
Total
Jack End Force( Total)
Loss of Force
Saving in force
Saving in strands
Percentage Saving
19
19
19
19
19
19
19
13
19
19
19
19
241
Wobble and Friction coeff.
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
Cable
No.
no 1
no 2
no 3
no 4
no 5
no 6
no 7
no 8
no 9
no 10
no 11
no 12
no 13
no 14
19
19
19
19
19
19
19
13
19
19
19
19
241
4.98%
4.98%
4.98%
4.98%
4.98%
4.98%
4.98%
4.98%
5.07%
5.07%
5.12%
5.12%
5.02%
3452.4 t
173.2 t
138.7 t
10
4.15%
19
19
19
19
19
19
19
13
19
19
19
19
241
4.91%
4.91%
4.91%
4.91%
4.91%
4.91%
4.91%
4.91%
4.97%
4.97%
5.01%
5.01%
4.93%
3452.4 t
170.3 t
141.5 t
10
4.15%
Table 6 : Box Girder 40 m span
40m
40m
40m
Bright Metal
HDPE
HDPE
Coeff. as per IRC
Coeff. as per IRC
Coeff. as per fib
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
16
16
16
Frictional Losses ( at midspan of the girder )
Strands
% loss
Strands
% loss
Strands
% loss
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
19
9.55%
19
4.43%
19
4.35%
15
9.55%
15
4.43%
15
4.35%
15
9.55%
15
4.43%
15
4.35%
19
11.44%
19
5.59%
19
4.47%
19
11.44%
19
5.59%
19
4.47%
19
11.69%
19
5.75%
19
4.51%
19
11.69%
19
5.75%
19
4.51%
296
10.07%
296
4.75%
296
4.38%
4240.3 t
4240.3 t
4240.3 t
427.0 t
201.3 t
185.9 t
0.0 t
225.8 t
241.1 t
0
16
17
5.41%
5.74%
Span
Type of Sheathing
No of Cables
8.51%
8.51%
8.51%
8.51%
8.51%
8.51%
8.51%
8.51%
10.03%
10.03%
10.32%
10.32%
9.03%
3452.4 t
311.9 t
0.0 t
0
Table 7: Box Girder 45m span
45m
45m
45m
Bright Metal
HDPE
HDPE
Coeff. as per IRC
Coeff. as per IRC
Coeff. as per fib
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
17
17
17
Frictional Losses ( at midspan of the girder )
Strands
% loss
Strands
% loss
Strands
% loss
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
19
10.59%
19
4.86%
19
3.92%
10
10.59%
10
4.86%
10
3.92%
19
12.94%
19
6.37%
19
4.91%
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
Cable no 15
Cable no 16
Cable no 17
Total
Jack End Force( Total)
Loss of Force
Saving in force
Saving in strands
Percentage Saving
Span
Type of Sheathing
Wobble and Friction coeff.
No of Cables
Cable No.
Cable no 1
Cable no 2
Cable no 3
Cable no 4
Cable no 5
Cable no 6
Cable no 7
Cable no 8
Cable no 9
Cable no 10
Cable no 11
Cable no 12
Cable no 13
Cable no 14
Cable no 15
Cable no 16
Cable no 17
Cable no 18
Total
Jack End Force( Total)
Loss of Force
Saving in force
Saving in strands
Percentage Saving
19
19
19
314
12.94%
13.16%
13.16%
11.18%
4498.1 t
503.0 t
0.0 t
0
19
19
19
314
6.37%
6.59%
6.59%
5.25%
4498.1 t
236.4 t
266.7 t
19
6.05%
19
19
19
314
4.91%
5.03%
5.03%
4.17%
4498.1 t
187.6 t
315.5 t
22
7.01%
Table 8: Box Girder 50m span
50m
50m
50m
Bright Metal
HDPE
HDPE
Coeff. as per IRC
Coeff. as per IRC
Coeff. as per fib
k
0.0046
k
0.0020
k
0.0015
μ
0.25
μ
0.17
μ
0.12
18
18
18
Frictional Losses ( at midspan of the girder )
Strands
% loss
Strands
% loss
Strands
% loss
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
19
11.61%
19
5.34%
19
4.05%
17
11.61%
17
5.34%
17
4.05%
17
11.61%
17
5.34%
17
4.05%
19
14.31%
19
7.08%
19
5.47%
19
14.31%
19
7.08%
19
5.47%
19
14.51%
19
7.21%
19
5.57%
19
14.51%
19
7.21%
19
5.57%
338
12.24%
338
5.74%
338
4.38%
4841.9 t
4841.9 t
4841.9 t
592.7 t
278.1 t
212.2 t
0.0 t
314.6 t
380.5 t
0
23
27
6.80%
7.99%
Conclusion
The main benefit of using HDPE ducts in prestressing system is the enhancement in the
durability of the prestressed concrete structure. It renders the excellent protection of the
prestressing steel, the most important element contributing to the safety of the structure against
the corrosion. The users may get this benefit at more of less the same cost of conventional
metal sheathings.
Due to the lesser value of friction & wobble co-coefficients, the HDPE sheathing offers overall
economy in prestressing system. This was an attempt made to quantify and bring out this
advantage with the help of few case studies. This would give rough idea to the user about the
possible saving in overall cost of the structure. The cost benefit increases with span of the
girder. In addition to cost benefit, other advantages are user & friendly (i.e. easy to handle and
joining), high durability etc. this, indeed, makes HDPE corrugated sheathing an ideal material in
prestressing industry.
A word of caution is necessary while describing and specifying the raw material to be used in
the manufacture of plastic ducts. Absolute care should be taken not to allow PVC (Polyvinyl
chloride) as the material. The reason being that PVC, when exposed to heat or fire, is likely to
give out chlorine gas and/or hydrochloric acid. In fact, the formation of these two hazardous
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HDPE Sheathing For Post-Tensioned Prestressed Concrete Structures
substances, namely chlorine gas and hydrochloric acid, occurs even at a temperature of about
70°C when exposed to weather. In case of high strength concrete, concrete temperatures are
reaching upto 70°C due to heat of hydration. Obviously, the presence of chlorine and/or
hydrochloric acid is extremely corrosive and detrimental to steel & concrete structures. Also
these two substances are environmentally most damaging. Thus care should be taken to
ascertain that the prestressing ducts are not made of PVC.
References
Prestressed Concrete Structure by T Y Lin
fib Bulletin No. 7: Technical Report on Corrugated plastic duct for internal
bounded post-tensioning, June 2000
IRC:18-2000: Design Criteria for Prestressed Concrete Road Bridges (Post-Tensioned
Concrete)
Standard Drawings for Road Bridges by MoST, 1990
NBMCW November 2010
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