Shriram Bapat, Retd.Chief Engineer Civil, NPCIL, Mumbai

Prestressed concrete slab (PT slab) construction has gained wide importance as a state of art technology, due to a number of engineering solutions and economy that it provides. Conventional usage of bright metal sheets as duct material have explicit problems of corrosion, duct opening leading to slurry intrusion and higher short-term losses due to the large values of friction and wobble coefficients. Further, one-end stressing operation may lead in to inefficiency of the prestressing system. The use of HDPE material, as prestressing duct, can eliminate the inherent problems associated with the metal ducts and reduce the prestress losses. Both end stressing using HDPE sheathing is the best combination for PT slabs.

Introduction

Prestressed concrete slabs (PT slabs) are the state of art construction technology and are provided these days for almost all commercial complexes. This concept is also being adopted for some large scale residential complexes. As the concrete quantities as well as reinforcement steel quantities get reduced when this concept is adopted, it is getting popular. Additionally, one gets clear headroom, as the floor beams are eliminated. This paper highlights the cost effectiveness of use of HDPE sheathing compared to bright metal sheathing which is conventionally used in the industry.

Sheathing Material

Three different types of sheathing materials are available in the market and are in general being used for housing the prestressing tendons.
  • Bright Metal Sheathing
  • Lead Coated Metal Sheathing
  • Galvanized Metal Sheathing
All the above sheathings are manufactured as round sheathing and then pressed to get a flat sheath, suitable for PT Slab. In the process of converting round sheath to flat sheath, there are chances of getting non-uniform profile of the flat duct as well as there are chances of opening of joints, which further leads to chances entry of slurry into the sheath while concreting. The flat HDPE duct is ideal in this situation, as the shape will be uniform and absolutely no possibility of leakage of slurry into the duct, while concreting.

Table-1: Comparison of Wobble and Friction Coefficient Values
Type of sheath Wobble coefficient Friction coefficient
Bright Metal 0.0046 0.25
Galvanized  Metal 0.0030 0.20
Lead coated Metal 0.0030 0.18
H.D.P.E. 0.0020 0.17

In addition to the advantages of the HDPE duct as mentioned above, HDPE ducts have lower values for co-efficient of friction and wobble co-efficient, leading to reduced short-term prestress losses during pre-stressing operation. Table-1 gives values of friction and wobble co-efficients for different types of sheathing ducts.

HDPE flat duct was not being manufactured in India till recently. Now, one manufacturer has successfully developed and came forward to manufacture and supply of HDPE flat duct.

Present Study

In view of lower value of friction and wobble coefficients for HDPE duct, when compared with those values for Bright Metal duct, which is commonly used in PT industry, a study was undertaken to work out economy in using HDPE duct. Different column grids, corresponding column sizes, slab thickness and drop slab thickness were considered. Table-2 gives the considered combinations.

Table-2: Combination Considered for the Case Study
Sr No. Grid Size(m) Column Size(mm) Slab Thickness(mm) Drop Size(mm)
1 8 x 8 700 X 700 200 3000 X 3000 X 375
2 9 x 9 700 X 700 200 3000 X 3000 X 375
3 10 x 10 800 X 800 225 3500 X 3500 X 400
4 11 X 11 900 X 900 250 4000 X 4000 X 450
5 12 X 12 900 X 900 265 4000 X 4000 X 475

Five panels have been considered in X-direction and three panels have been considered in Y-direction. Stressing has been done from alternate end in both the direction as one case and in other case, stressing is considered from alternate end in Y-direction and from both ends in X-direction. ADAPT FLOOR PRO 3D (Builder) software of Dr. ALAMI has been used for analytical work.

Table-3: Comparison of H.T. strand requirements using Bright Metal Duct and HDPE Duct
Column Slab Drop Panel COLUMN Both End Stressing Alternate End Stressing
BM HDPE BM HDPE
HTS HTS HTS HTS
Grid Thk. Size Size (Kg/sq.m.) (Kg/sq.m.) (Kg/sq.m.) (Kg/sq.m.)
8X8 200 3X3X0.375 700X700 3.4 3 3.4 3.1
9 200 3X3X0.375 700X700 4 3.64 4 3.81
10 225 3.5X3.5X0.400 800X800 4.4 3.9 4.5 4
11 250 4X4X0.450 900X900 4.6 4.24 4.7 4.3
12 250 4X4X.500 900X900 5.4 5.0 5.6 5.35

Results

On the basis of analytical results, the requirement of H.T. strand per square meter of slab has been worked out. Table-3 gives comparison of H.T. strand requirement using Bright Metal and HDPE flat duct and Table-4 gives percentage saving of H.T. strand when HDPE flat duct is used, in place of Bright Metal Duct.

Table-4: Percentage Saving in H.T. Strand
Column Grid % SAVING
Size Both End Stressing Alternate End Stressing
8X8 11.7 08.0
9X9 09.0 04.8
10X10 11.4 11.0
11X11 07.8 08.5
12X12 07.40 04.50

Conclusion

It can be seen from the Table-4 that there is substantial saving in requirement of H.T. strands when HDPE duct is used, in place of conventionally used Bright Metal duct. The range of saving is 4.50% to 8.5% when conventional alternate end stressing is resorted to. The saving goes up in the range of 7.4% to 11.7%, when both end stressing is resorted to. Additionally, there is marginal saving in requirement of duct length and number of anchorages.

When conventionally used Bright Metal Ducts are provided in PT Slab construction, they get corroded during construction stage itself (particularly in and around Mumbai coastal region). This increases friction and thus the frictional losses also go up. Use of HDPE flat duct will eliminate this problem.

On the basis of study following is recommended:
  • HDPE flat duct shall be used in PT Slab construction.
  • Both end stressing shall be resorted to, in place of conventional method of alternate end stressing.
  • 8 m x 8 m column grid seems to be most economical choice in selecting column grid at planning stage.

Acknowledgment

The author acknowledges the analytical help provided by Shri Umesh Bhujbalrao of M/s.VSIL, Bhopal. Preparation of this paper was possible due to constant pursuance by Shri C.B.Dandekar of M/s.REX Polyextrusion Ltd, Sangli, manufacturers of HDPE flat duct.
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