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.
IntroductionPrestressed 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 MaterialThree 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
|Table-1: Comparison of Wobble and Friction Coefficient Values|
|Type of sheath||Wobble coefficient||Friction coefficient|
|Lead coated Metal||0.0030||0.18|
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 StudyIn 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|
ResultsOn 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|
ConclusionIt 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.