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Satisfactory performance of post-tensioned bridges depends upon the appropriate selection, design, specification and fabrication of various materials and components that make up the post-tensioning system. This chapter offers general guidance and information for materials and components. Some of the information in this chapter is taken from various industry specifications and information from manufacturers and suppliers. The most current versions of this information should be consulted when developing specific project data.
Strand for post-tensioning is made of high tensile strength steel wire. A strand is comprised of 7 individual wires, with six wires helically wound to a long pitch around a center wire. All strand should be Grade 1860 MPa (270 ksi) low relaxation, seven-wire strand conforming to the requirements of ASTM A 416 "Standard Specification for Steel Strand, Uncoated Seven Wire Strand for Prestressed Concrete". ASTM A 416 provides minimum requirements for mechanical properties (yield, breaking strength, elongation) and maximum allowable dimensional tolerances. Strand from different sources may meet ASTM A 416 but is not necessarily identical in all respects.
Strand is mostly available in two nominal sizes, 13mm (0.5in) and 15mm (0.6in) diameter, with nominal cross sectional areas of 99mm 2 and 140mm 2 (0.153 and 0.217 square inches), respectively. The majority of post-tensioning hardware and stressing equipment is based on these sizes. Strand size tolerances may result in strands being manufactured consistently smaller than or larger than nominal values. Recognizing this, industry ("Acceptance Standards for Post-Tensioning Systems", Post-Tensioning Institute, 1998 refers to the "Minimum Ultimate Tensile Strength" (MUTS) which is the minimum specified breaking force for a strand. Strand size tolerance may also affect strand-wedge action leading to possible wedge slip if the wedges and strands are at opposite ends of the size tolerance range.
Strand conforming to ASTM A 416 is relatively resistant to stress corrosion and hydrogen embrittlement, due to the cold drawing process. However, since susceptibility to corrosion increases with increasing tensile strength, caution is necessary if strand is exposed to corrosive conditions such as marine environments and solutions containing chloride or sulfate, phosphate, nitrate ions or similar. Consequently, ASTM A 416 requires proper protection of strand throughout manufacture, shipping and handling. Protection during the project, before and after installation, should be specified in project specifications, details, drawings and documents.
In recent years, various innovations have been developed in order to provide additional corrosion protection. Some of these measures include:
Plastic coated strand for unbonded tendons has been widely used in buildings, but not generally in bridges in the United States. However, greased and sheathed mono-strands are now available for cable-stays or external tendon applications for new structures and the repair of old ones.
Epoxy coated strand meeting the same requirements as ASTM A 416 is available and should also conform to ASTM A 882 "Standard Specification for Epoxy-Coated Seven Wire Strand". Epoxy coated strand is available as an outer coating only, or as a coating that also fully fills the interstices between wires. The latter is preferred for post-tensioning or cable stay applications. Special wedges are required that bite through the thickness of the coating and engage the strand; power seating of the wedges is usually required.
Strand made from fiber material (such as carbon or aramid fibers) has limited application as post-tensioning to date. These composite materials offer advantages for enhanced corrosion resistance, but lack the benefit of a high modulus of elasticity that is routinely provided by steel and which is crucial to good load-deflection behavior of a prestressed structure without excessive cracking under service loads.
Few manufacturers supply galvanized strand. Heating during galvanizing reduces the tensile strength to about 1660MPa (240 ksi). This strand is not used in bridges.
Tendons in prestressed concrete structures do not experience stress cycling significant enough to induce fatigue problems. Fatigue is a concern only in certain applications such as cable-stays in cable-stayed bridges where traffic loads significantly affect stresses.
Bars should be Grade 1035 MPa (150 ksi), high strength, thread bar meeting the requirements of ASTM A 722, "Standard Specification for Uncoated High-Strength Steel Bar for Prestressing Concrete", Type II bar. Coarse thread bars are used for most permanent and temporary applications. Fine thread bars are available if necessary for special applications. It is good practice to limit the stress level and number of re-uses for temporary applications, according to recommendations of the Manufacturer. In the absence of such information, it is suggested that for new bars, the stress should not exceed 50% MUTS and the number of re-uses be less than ten for applications such as temporary stressing or lifting.
Post-tensioning bars are available in various sizes from 16mm (5/8in) to over 50mm (2in) diameter. However, for convenience in handling, installation, and removal and re-use in normal applications for post-tensioned bridges, 32mm (1-1/4in) or 35mm (1-3/8in) diameter bars are typically used.
Bars are not as easily damaged by corrosion as strands because of their lower strength, large diameter and smaller ratio of exposed surface to cross section area. Hot rolled bars also acquire a natural surface oxidation from the rolling process that enhances their protection. Nevertheless, bars need to be protected during extended periods of exposure especially in aggressive environments. Hot-dip galvanizing and epoxy coating are available for corrosion protection if necessary.
All prestressing steel should be protected against physical damage and corrosion at all times from manufacture to final installation and grouting. It should be packed in containers for shipping handling and storage. A rust-preventing corrosion inhibitor should be placed in the package or be incorporated in the carrier type packaging material. Corrosion inhibitor should have no deleterious effect on the steel or grout or on the bond strength of steel to grout. Inhibitor carrier type packaging should conform to Federal Specification MIL-P-3420. Damaged packaging should be replaced or restored to its original condition.
Shipping containers should be clearly marked with a statement that it contains high-strength prestressing steel, the type of care needed for handling, the type and amount of corrosion inhibitor used and the date it was placed, and any other safety precautions and instructions. Strand should be clearly identified that it is low-relaxation (stabilized) strand per the requirements of ASTM A 416 and the corresponding LOT number for which quality control test samples have been taken. Strands not so designated should be rejected.
Reels of strand should be examined by the Contractor and inspected by the CEI when first received on site and periodically while in storage. During use, any reel that is found to contain broken wires or corrosion should be carefully examined. Lengths of strand containing broken wires or corrosion should be removed and discarded. Prestressing steel should also be protected during installation in the structure.
Post-tensioning bars for both temporary and permanent applications should be identified in a similar manner and inspected for damage or excessive corrosion. At any time during construction, the inspector (CEI) should have the authority to reject any prestressing steel that has sustained physical or corrosion damage.
To ensure that correct materials are supplied and used, specific quality control procedures for material acceptance should be in place. Procedures may differ from State to State or from Owner to Owner. In some cases, an Owner may require that only post-tensioning systems be used that have been approved and pre-qualified under the Owners qualification program. Pre-qualification in this manner involves prior submission and approval of test reports and certifications.
Samples for testing should be furnished at the job site for each manufacturer of prestressing steel and bar. Each sample furnished for testing should be accompanied by certification stating the manufacturers "Guaranteed Ultimate Tensile Strength (GUTS)", "Minimum Ultimate Tensile Strength" (MUTS) or "Actual Ultimate Tensile Strength", (AUTS).
An example of typical frequencies of sampling and LOT designations are, as follows:
For strand: three randomly selected samples, 1.5M (5ft) long, per manufacturer, per size of strand, per shipment, with a minimum of one sample per ten delivered reels.
For bar: three randomly selected samples, 1.5 M (5ft) long, per size of bar, per heat of steel with a minimum of one sample per shipment.
One of each of the sample(s) furnished to represent a LOT should be tested in accordance with appropriate ASTM standard, and the remaining samples properly identified and tagged should be stored for future testing. In the event of a loss or failure the stored sample(s) should be used to evaluate the strength. For acceptance of the LOT represented, test results must demonstrate 100% of the guaranteed ultimate tensile strength.
All bars of each size from each mill heat of steel and all strand from each manufactured reel to be shipped to the site should be assigned an individual LOT number and be tagged in a manner that each such LOT can be accurately identified at the site. All unidentified prestressing steel (strand or bar) or loss of positive proof of identification is sufficient reason for rejection.
Following initial acceptance, the user of the prestressing steel (Contractor) should maintain good control over storage and identification, maintain records and supply copies of certifications and test results to the inspector (CEI). The latter should regularly and periodically check stored components, records and results.
Approval of any prestressing materials by the Engineer (CEI) should not preclude subsequent rejection if material is damaged in transit or later found to be defective for any reason. Costs of acceptance and quality control tests are typically included in the project bid items for post-tensioning work and no separate payment is made. Testing should conform to the applicable ASTM Specifications. The location where the post-tensioning is to be installed is considered the "site" and may be the project site or a casting yard.
Cement grout is chemically basic and provides a passive environment around the post-tensioning bars or strands. In addition, grout serves to bond internal tendons to the structure. In the free lengths of external tendons the principal role of the grout is to provide an alkaline environment inside the polyethylene duct. Nevertheless, complete filling of the duct with grout is essential for proper protection.
The primary constituent of grout is ordinary Portland cement (Type I or II). Other cementitious material may be added to enhance certain qualities of the final product. For example, fly ash improves corrosion resistance in aggressive environments. The addition of dry silica fume (micro-silica) also improves resistance to chloride penetration because the particles help fill the interstices between hydrated cementitious grains thus reducing the permeability.
The water-cementitious material ratio should be limited to a maximum of 0.45 to avoid excessive water retention and bleed and to optimize the hydration process. Any temptation to add water to improve fluidity on-site must be resisted at all times. Fluidity may be enhanced by adding a high range water-reducer, HRWR, (Type F or G) - see 2.2.5.
Grouts made of cementitious materials, water and admixtures batched on site do not always have uniform properties. This arises from variations in materials, day to day mixing differences, crew changes, weather conditions and so forth. Grouts made of only cement and water often exhibit segregation and voids due to excessive bleed water. In an endeavor to eliminate problems related to grout variations and voids, several State DOT's have obtained greater quality control by requiring "pre-bagged" grouts. In a pre-bagged grout, all the constituent (cementitious) materials have been thoroughly mixed and blended at the factory in the dry condition. This ensures proper blending and requires only that a measured amount of water be added for mixing on site.
A manufacturer of a pre-bagged grout may already have had the material pre-qualified by a State DOT or other agency. In this case, it is appropriate to accept it on the basis of a written certification; providing that the manufacturer has on-going quality control tests that can be confirmed by submitting test reports to the Engineer. The certification should show the mixed grout will meet the pre-qualified standard. On site, daily grout production must be monitored by various field tests in order to maintain quality control and performance.
A thixotropic grout is one that begins to gel and stiffen in a relatively short time while at rest after mixing, yet when mechanically agitated, returns to a fluid state with much lower viscosity. Most grouts made with cementitious materials, admixtures and water are non-thixotropic. Thixotropy may be exhibited by some, but not necessarily all, pre-bagged grouts.
A critical feature of a grout is that it should remain pump-able for the anticipated time to fully inject the tendon. This may be significant for long tendons or where a group of several tendons is to be injected in one continuous operation. Some thixotropic grouts can have very low viscosity after agitation, becoming easy to pump.
Like concrete, admixtures may be used to improve workability and reduce the water required, reduce bleed, improve pumping properties or entrain air. Care must be exercised to use the correct quantities in the proper way according to manufacturer's instructions and to remain within the mix properties established by qualifying laboratory tests.
Calcium nitrite may help improve corrosion resistance in some situations by bonding to the steel to form a passive layer and prevent attack by chloride ions.
High range water reducer (HRWR) improves short term fluidity. However, a grout with HRWR may lose fluidity later when being injected through hoses and ducts. Unlike a concrete mix, it is not possible to re-dose a grout especially when it is in the, pump, hoses and ducts. Also, HRWR tends to cause bleed in grouts. On-site grout mixing with HRWR is not recommended.
Other admixtures include:
Shrinkage compensating agents
The addition of these should be strictly in accordance with manufacturer's recommendations. Furthermore, the mix should be qualified by appropriate laboratory testing. On site, daily grout production must be monitored by various field tests in order to maintain quality control and performance.
Acceptance of a grout is usually based upon the results of laboratory tests. Laboratory tests on trial batches of the proposed grout using the same materials and equipment to be used on site are used to qualify a grout. Trial grout should be prepared by personnel experienced in preparing and testing grout mixes. This should be done at an approved material testing laboratory. All tests should be performed at temperature and humidity conditions expected on site. Trials should precede construction by at least eight weeks in order to allow time for testing and resolution of any concerns.
Laboratory tests are normally performed for the properties listed in the following sections. Details of the tests to be preformed are provided in summary fashion. This is a summary of the key aspects only. For further details refer to the "Specification for Grouting of Post-Tensioned Structures", latest edition, by the Post-Tensioning Institute and/or the specific project contract documents.
Grout set time is tested in accordance with ASTM C 953 "Standard Test Method for Setting Time of Grouts." The setting time should be more than 3 but less than 12 hours. The tested setting time does not relate to the placement or working life of the mix.
Grout cube specimens, 50mm (2 in), are prepared and tested according to ASTM C 942 "Standard Test Method for Compressive Strength of Grouts". The strength should be 21MPa (3000 psi) at seven days and 35MPa (5,000 psi) at 28 days.
Grout permeability should be tested in accordance with ASTM C1202 "Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration". A value less than 2500 Coulombs after 6 hours is generally acceptable when subjected to a potential of 30 volts.
Volume change should be tested in accordance with ASTM C1090 "Standard Test Method for Measuring Changes in Height of Cylindrical Specimen from Hydraulic Cement Grout". A value of 0.0% to less than 0.1% at 24 hours and no more than +0.2% at 28 days is acceptable.
For non-thixotropic grouts, when tested according to ASTM C939 "Standard Test Method for Flow of Grout" the efflux time should be between 11 and 30 seconds immediately after mixing (Figure 2.1). After allowing the grout to stand for 30 minutes without further agitation, the efflux time should be less than 30 seconds. The initial lower limit of 11 seconds is intended to indicate that the mix contains the necessary amount of cementitious material. The upper limits are intended to indicate satisfactory fluidity for pumping.
For thixotropic grouts, the flow cone is filled to the top, i.e. above the standard level, and the time to fill a one-liter container is measured. The efflux time should be between 5 and 30 seconds immediately after mixing. After allowing the grout to stand for 30 minutes without agitation and then remixing for 30 seconds, the efflux time should be less than 30 seconds. It is recommended that some of the laboratory qualification tests be run at the ends of this spectrum. There are some commercial pre-bagged, thixotropic grouts that meet all other requirements yet show very low viscosity (high fluidity) after agitation, resulting in the 5 second lower limit (ref. recent 2003, revision to PTI "Specification for Grouting of Post-Tensioned Structures").
Figure 2.1 - Standard and Modified ASTM C939 Flow Cone Test
This is not a standard test. However, it was developed by the Florida Department of Transportation to ensure that a mix remains sufficiently workable for pumping under simulated site conditions after re-circulating for a one hour period. The following procedure, taken from FDOT Standard Specification, Section 938, may be used for guidance:
Perform the test in a temperature conditioned laboratory. Condition the room, grout, water, duct, pump, mixer and all other equipment to be used to a temperature of 32.5 °C (90°F) for a minimum of 12 hours prior to the test.
Use 122M (400 ft, 3M (10 ft) of duct (tube) for the test. Use a duct with an inside diameter of 25mm (1 inch).
Mix the grout to the specified water content. Pump the grout through the duct until the grout discharges from the outlet end of the duct and is returned to the pump.
Start the one hour test period after the duct is completely filled with grout. Record the time to circulate the grout through the duct. Constantly pump and re-circulate the grout into the commercial grout mixer storage tank.
Pump and re-circulate the grout for a minimum of one hour.
Record at 15 minute intervals throughout the test period, the pumping pressure at the inlet, grout temperature, and fluidity at the discharge outlet.
The result is satisfactory if the flow-cone efflux time (standard or modified ASTM C 939) after one hour of recirculation is not greater than 30 seconds.
The "Wick Induced Bleed Test" involves completely immersing a 0.5M (20 in) length of strand in a cylinder of carefully prepared grout and following a modified version of ASTM C940 to record the bleed water above the grout. A bleed of 0.0% after 3 hours at normal room temperature (70° F) is acceptable (Figure 2.2).
Figure 2.2 - Wick Induced Bleed Test
The "Schupack Pressure Bleed Test" uses a Gelman Filter to retain grout particles and records the bleed water expelled under air pressure applied up to 0.34MPa (50 psi) (Figure 2.3). Table 2.1 shows permissible maximum bleed water percentages at specific pressure values that should indicate the grout will have little or no bleed for the given vertical rise.
|Vertical Rise||Pressure MPa (psi)||Max% Bleed|
|0 to 0.6M (0 to 2 feet)||0.14 (20)||4|
|0.6M to 1.8M (2 to 6 feet)||0.21 (30)||2|
|1.8 to 30.5M
(6 to 100 feet)
Figure 2.3 - Bleed Under Pressure Test (Gelman Filtration Funnel)
An Accelerated Corrosion Test (ACT) may be used to quantify the expected level of corrosion for a specific grout. The test is based on research made under FHWA-RD-91-092 which indicates that a mean time to corrosion of 1,000 hours when tested at 0.2V is suitable. This test is not yet standardized. However, it is particularly useful in determining combinations of admixtures that may adversely affect the corrosion protection performance of a grout.
A wet density value for grout can be established in the laboratory using ASTM C185 "Standard Test Method for Air Content of Hydraulic Cement Mortar". Once established, it can be monitored in the field using an American Petroleum Institute Mud Balance (API Recommended Practice 13B-1: "Standard Procedures for Field Testing Water-Based Drilling Fluids").
Cement and other materials may be delivered in bags but should be stored in a weatherproof building. Storage in the open may be allowed providing that materials are on a raised, dry platform with adequate weatherproof covering. Additives should be stored in a warm environment. Dissolvable packaging materials should not be allowed for any components as they can break down to pulp and cause equipment or duct blockage.
It is essential that the user (Contractor) maintain a record of all delivered materials. A copy of the manufacturer's quality control data sheet should accompany each LOT of grout components shipped to the site. A LOT is that parcel of material from the same production run shipped to the site. Each shipment should be clearly identified with the corresponding LOT number so that it can be tracked to the manufacturer's quality control records. Copies of shipment records and quality control test reports should be maintained by the Contractor and copies provided to the Inspector (CEI).
Prior to use, all materials in storage should be checked to make sure they have not exceeded the manufacturer's shelf life or have not absorbed moisture and begun to clump or hydrate. It is recommended that cementitious materials and pre-bagged grouts not be stored on site for more than one month before they are used.
Dry silica fume is available in bags. Special care is essential when mixing dry silica fume with cement and additives in order to produce a job-site grout mix, as it can lead to clumping and a poor result. Pre-bagged grouts containing silica fume have been dry blended and do not exhibit this problem.
Any material with a total time from manufacture to use in excess of six months should be retested, or recertified by the supplier before use or else be rejected and replaced. Approval of any grout or grout materials by the Inspector (CEI) should not preclude subsequent rejection if material is damaged in transit or later found to be defective for any reason.
A proposed grout is normally accepted on the basis of the laboratory tests listed in Section 2.2.6 performed before construction, or on the basis of certification from the manufacturer that the (pre-bagged) grout materials meet the pre-qualification requirements of the Owner or project specifications. The manufacturer should have a continuing quality control program to ensure that production continues to meet the specified requirements. Copies of certificates should be checked and a record kept by the Contractor and the Inspector (CEI). Use of a particular grout on site may continue providing that certification and documentation is kept up to date, that materials in storage remain usable and that daily grout mix production tests meet specified limits. Approval to use a grout should be withdrawn if these quality control standards are not maintained.
When specified in the Contract Documents, field mock-up tests may be used to demonstrate that materials, components such as inlets and outlets, mixer, pumping and grout injection methods will result in complete filling of a duct. Mock-ups are appropriate for new means and methods, new types of components or grout materials. Production tendons should not be used for the mock-up test.
Mock-up tests should be conducted sufficiently in advance of production grouting (at least 4 weeks) to allow time to resolve any problems. As far as possible, a mock-up should simulate the type and size of tendon, duct, anchorages and proposed attachments and be arranged to a similar, representative, geometric duct profile. Acceptance requirements should include provisions for bleed, settlement, shrinkage or expansion, flow of grout, completeness of filling and the absence of bleed pockets.
The following field mock-up tests are suggested for guidance:
For continuously draped tendons in spliced girders or cast-in-place construction: one tendon mock-up of the longest tendon from anchor to anchor, including all proposed intermediate duct couplings and grout inlets and vents. The profile should simulate the tendon with the maximum accumulated curvature from anchor to anchor.
For cantilever or continuity internal and external tendons in precast or cast-in-place segmental construction: one tendon mock-up of the longest tendon from anchor to anchor, including all proposed intermediate duct couplings and grout inlets and vents. The profile should simulate the tendon with the maximum accumulated curvature from anchor to anchor.
For vertical tendons: one tendon mock-up of the longest tendon from anchor to anchor, including all proposed intermediate duct couplings and grout inlets and vents. The profile should simulate the tendon with the maximum accumulated curvature from anchor to anchor.
The following tests should be conducted and satisfied during the field mock-up trials:
Pumpability and Fluidity (Flow Cone) (18.104.22.168)
Simulated Field High Temperature Fluidity Test (22.214.171.124 - Optional)
Wick Induced Bleed Test (126.96.36.199)
Wet Density (188.8.131.52)
The Schupack Pressure Bleed Test (184.108.40.206) should be satisfied for projects there longitudinal tendons have a maximum difference in height at any point over 6 feet or vertical tendons are over 20 feet high.
Corrosion performance should be tested separately and at a much earlier time before imminent use in construction. Refer to the PTI "Specification for the Grouting of Post-Tensioned Structures" for further information. Mock-up tests may be waived at the discretion of the Engineer, given satisfactory results of earlier tests or use of the same materials, equipment and methods by the same personnel.
For field (on-site) tests of daily production grout, refer to Chapter 4.
Ducts are available in different materials for different applications and types of tendons. Originally, duct was considered primarily as a means of forming a void through the concrete for the tendon and little attention was paid to the possible role of the duct as a barrier to corrosive agents. Largely as a consequence of finding voids in grouted tendons, more emphasis is now placed on the quality, integrity and continuity of the duct as a corrosion barrier in itself. This has resulted in a move toward the use of high density plastic ducts in some states. Nevertheless, previous duct materials are still available and their use continues in other regions. Consequently, the following recommendations should be adapted as appropriate to meet local needs and conditions.
The nominal internal cross sectional area of circular duct should be at least 2.25 times the net area of the post-tensioning strands or 2.50 times for tendons installed by the pull through method. In case of space limitations, the minimum duct area may be only 2.00 times the strand area for relatively short tendons up to approximately 30M (100 ft) long.
Oval "flat" ducts are commonly used for transverse tendons comprising up to 4 strands of 0.6in diameter in deck slabs of box girders. The internal clear dimensions of oval duct should be a minimum of 25mm (1 in) vertically and 75mm (3 in)horizontally.
For tendons containing a single post-tensioning bar the internal duct diameter should be at least 6mm (¼ in) greater than the maximum outside dimension of the bar. A greater clearance may be preferred or be necessary for some applications. Examples of this use would be to provide greater tolerance for temporary bars or to accommodate bridges with slightly curved alignments.
Ducts are spirally wound to the necessary diameter from strip steel with a minimum wall thickness of 0.45mm (26-gauge) for ducts less than 66mm (2-5/8 in) diameter or 0.6mm (24-gauge) for ducts of greater diameter. The strip steel should be galvanized to ASTM A653 with a coating weight of G90. Ducts should be manufactured with welded or interlocking seams with sufficient rigidity to maintain the correct profile between supports during concrete placement (Figure 2.4 (a)). Ducts should also be able to flex without crimping or flattening. Joints between sections of duct and between ducts and anchor components should be made with positive, metallic connections that provide a smooth interior alignment with no lips or abrupt angle changes.
Figure 2.4 - Spiral Wound Steel Duct and Rigid Steel Pipe
Smooth steel pipes should conform to ASTM A53 "Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc Coated, Welded and Seamless", Grade B Schedule 40. When required for curved tendon alignments (e.g. deviation saddles and similar) the pipe should be pre-fabricated to the required radius (Figure 2.4 (b)).
Corrugated plastic duct (Figure 2.5) to be completely embedded in concrete should be constructed from either polyethylene or polypropylene. The minimum acceptable radius of curvature should be established by the duct supplier according to standard test methods. Polyethylene duct should be fabricated from resins meeting or exceeding the requirements of ASTM D3350 with a cell classification of 344434C. Polypropylene duct should be fabricated from resins meeting or exceeding the requirements of ASTM D4101 with a cell classification range of PP0340B44544 to PP0340B65884. The duct should have a minimum material thickness of 2.0 mm + 0.25 mm (0.079 in + 0.010 in). Ducts should have a white coating on the outside or should be of white material with ultraviolet stabilizers added.
Figure 2.5 - Corrugated Plastic Duct
HDPE smooth pipe is available in different diameters, wall thickness, physical and chemical properties. There is significant variability in commonly available materials. It is very important that it has satisfactory properties for handling, storage, installation and durability for the application. The color is normally black from a small amount of carbon in the material, to protect against degradation from ultraviolet light. The wall thickness, diameter and physical strength (Hydrostatic Design Basis) should be sufficient to initially withstand grouting pressures. In the long term it should not deteriorate or split. The requirements should be in accordance with AASHTO LRFD Bridge Construction Specifications.
All duct splices, joints and connections to anchorages should be made with couplings and connectors that produce a smooth interior duct alignment with no lips or kinks. Special duct connectors may be used in match-cast joints between precast segments and similar situations if necessary to create a continuous, air and water-tight seal. duct tape should not be used to join or repair ducts or make connections.
All fittings and connections between lengths of plastic duct and between ducts and steel components (e.g. anchors or steel pipe) should be made of materials compatible with corrugated plastic ducts. Plastic materials should contain antioxidant stabilizers and have an environmental stress cracking of not less than 192 hours as determined by ASTM D 1693 "Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics", Condition C.
Connections between sections of plastic pipe should be made using heat welding techniques or with mechanical couplers per the manufacturer's recommendations or as approved by the Engineer. Connections should have a minimum pressure rating of 1 MPa (145 psi) and produce a smooth interior alignment with no lips or kinks.
Connections between external HDPE pipe and steel pipe embedded in the concrete should be made using circular sleeve (boot) made of Ethylene Propylene Deine Monomer (EPDM) having a minimum (working) pressure rating of 1 MPa (145 psi). EPDM should have 100% quality retention as defined by ASTM D1171 "Standard Test Method for Rubber Deterioration-Surface Ozone Cracking Outdoors or Chamber (Triangular Specimens)" Ozone Chamber Exposure Method B. The minimum wall thickness should be 10mm (3/8 inch) reinforced with a minimum of four ply polyester reinforcement. Sleeves should be secured with 10mm (3/8 in) wide power seated, 316 stainless steel band clamps, using one on each end of the sleeve (boot) to seal against leaking grout. The power seating force should be between 356 and 534 N (80 and 120lbf). Alternatively, connections may be made using mechanical couplers with plastic components made of approved plastic resins meeting the same requirements as for external plastic pipes and metal components of grade 316 stainless-steel. Mechanical connections should meet the same pressure rating requirements (above) and have seals to prevent grout leaks.
Steel and plastic pipe may be connected directly when the outside diameters do not vary by more than + 2mm (0.08in). A reducer or spacer should be used when outside this tolerance. When installed correctly, a single band clamp around each end of the sleeve should be sufficient. Double banding may be necessary to fix an apparent leak of air, water or grout.
In some cases, external tendon connections may be enhanced by the use of shrink sleeve wrap overlaying the connection and portions of adjacent plastic and steel pipes. This may be used in aggressive environments where connections may be directly exposed to de-icing salts or other contaminants entering through expansion joints or other similar openings.
Shrink sleeves should consist of an irradiated and cross linked, high density polyethylene backing with an adhesive layer that will withstand 66 ° C (150 ° F). Sleeve materials should meet the following:
|Property||Test Method||Minimum Requirements|
|Internal Application||External Application||Units|
|Fully recovered Thickness*||2.3 (92)||2.8 (111)||mm (mils)|
|Peel Strength:||ASTM D 1000||5.0 (29)||8.0 (46)||KN/M (lb per inch )|
|Softening Point:||ASTM E 28||72 (162)||102 (216)||°C (°F)|
|Lap Shear:||DIN 30 672M||60 (87)||40 (58)||MPa (psi)|
|Tensile Strength:||ASTM D 638||20 (2,900)||24 (3,480)||MPa (psi)|
|Hardness:||ASTM D 2240||46||52||Shore D|
|Water Absorption:||ASTM D 570||< 0.05%||< 0.05%|
*The fully recovered thickness is the thickness after installation using heat.
Duct made from galvanized strip steel may be prefabricated or fabricated on site as necessary. Plastic duct may be shipped in coils or in bundles of straight lengths.
In order to avoid inadvertent introduction of contaminants or debris, it is recommended that the ends of duct coils or bundles be protected and covered during shipping and storage. Special temporary end caps may be used to seal the ends of individual ducts. Plastic ducts should be protected from sunlight, ultraviolet degradation, crushing and excessive bending until installed in the bridge. All ducts and pipes should be stored in a dry location, on a raised platform, protected from weather and contamination.
All duct materials (metal or plastic) ducts should comply with the requirements of AASHTO LRFD Bridge Construction Specifications or the project specifications, as applicable
In general, post-tensioning duct will be acceptable if it meets the requirements of "Acceptance Standards for Post-Tensioning Systems", Section 5, "Sheathing", PTI, 1998 and for corrugated plastic ducts, FIB Bulletin #7,"Corrugated Plastic Ducts for Internal Bonded Post-Tensioning Systems", Article 4.2 "System Approval Testing" Stage 1 and Stage 2.
Key features for acceptance (according to PTI) for internal tendons are:
Duct cast into concrete should withstand at least 3.0M (10ft.) of concrete fluid pressure.
Duct shall not dent more than 3mm (1/8 inch) under a concentrated force of 0.45KN (100lbf) applied using a 13mm diameter [#4] reinforcing bar.
Where prestressing steel is pre-installed in the duct, the duct shall withstand at least 1.5 M (5 ft.) of concrete fluid pressure and resistance to denting is not required.
Duct with a diameter greater than 50mm (2 in.) shall not deflect more than 75mm (3 in.) when a 6M (20ft.) length is supported at its ends, although where tight radii are required, more flexible duct may be permitted.
Plastic duct should withstand the above at 38°C (100°F) except that longitudinal stiffness requirements may be reduced by 50% if the installation support spacing is reduced 50% from that for steel duct.
The above do not apply to ducts stiffened with bars, mandrels or inflatable tubes.
For acceptance, it is recommended that three successful and successive tests for each type of duct should comply with the above requirements.
FIB Bulletin #7 sets out procedures for the approval of a corrugated plastic duct system on the basis of a series of stages, using the same assembly of the system. The stages are:
Practicability of Assembly - the actual assembly of the ducts in a rebar cage
Water Tightness - by an air-pressure test of the same assembly prior to concreting
Stressing/Friction - by jacking and releasing at one end then the other
Grouting Test and Wear of Duct - no voids nor significant wear after grouting (autopsy)
Electrical Resistance Test - not less than 1 kilo-Ohm resistance between the reinforcing and post-tensioning steel (for electrically isolated systems - prior to autopsy).
FIB Bulletin #7 is suitable for qualifying new systems that have not been used before. The tests are not meant as project specific tests or production tests but as system approval tests and therefore, need to be carried out only once for certification or approval of the system.
Key acceptance requirements for external tendons:
Duct (HDPE pipe) including all welds, splices, grout fittings and connections should be vapor tight and capable of withstanding 1MPa (145psi) grout pressure.
For verification, an assembly containing plastic and steel pipe and connections may be pressure tested as follows: (1) Condition the assembly by sustaining an internal pressure of 1MPa (145psi) for 3 hours. (2) After conditioning, the assembly should retain an internal pressure of 1MPa (145psi) for five minutes with no more than 0.1MPa (15 psi) reduction.
It is recommended that a system supplier provide full documentation including:
Technical documents and drawings of general assembly of the system and details of components
Instructions and method statements for installation, stressing and grouting
A quality assurance plan covering production, shipping, handling, storage and installation of the system
Instructions for surveillance and maintenance of the system in service
For acceptance and approval of a post-tensioning system, all components tests and results of post-tensioning system approval tests should be carried out by an independent approved body or testing laboratory. This testing should be completed prior to submission of Shop Drawings and other related documents to the Engineer for approval.
Not all the above tests are standardized and are not formal requirements. The above may be used for guidance. Requirements for individual projects should be considered on a case-by-case basis. Proposals should be prepared by the Contractor for the approval of the Engineer.
On site, the Contractor should maintain a complete record of all documentation, test reports, shipping dockets and approvals. Copies should be provided to the Inspector (CEI) to ensure compliance. Also, it is recommended that for multi-strand tendons, internal tendon ducts be checked using a torpedo prior to installing tendons (see Chapter 3).
A basic bearing plate is a flat plate bearing directly against concrete. Covered by this definition are square, rectangular, or round plates, sheared or torch cut from readily available steel plate, normally ASTM A 36. Basic bearing plates are used in conjunction with galvanized sheet metal or plastic trumpets to transition from the strand spacing in the wedge plate to the duct (Figure 2.6).
Figure 2.6 - Basic Anchor Plate
For acceptance, a basic bearing plate should comply with the requirements of AASHTO LRFD Construction Specifications.
A special bearing plate or anchorage device is any anchorage hardware that transfers tendon force into the concrete but does not meet normal analytical design requirements for basic bearing plates. Covered by this definition are devices having single or multiple plane bearing surfaces, and devices combining bearing and wedge plate in once piece. These anchorages typically require confinement reinforcement and should be accepted on the basis of physical tests (Figure 2.7).
Figure 2.7 - Multi-plane Anchor
Use of a special bearing plate or anchorage device is acceptable if it complies with the testing requirements of AASHTO LRFD Construction Specification 10.3.2.3.
Wedge plates are part of the anchorage system and should comply with AASHTO LRFD, Section II - 10.3.2.3 for special anchorage devices. In the absence of any other specific contract requirements, in general, three successful qualification tests on wedge plates should meet the requirements of Section 4.1 "Wedge Plate Test Requirements" of "Acceptance Standards for Post-Tensioning Systems", PTI, 1998. These tests require that after loading to 95% MUTS and release, the deformation of the plates should not exceed 1/600 of the clear span and that the wedge plate sustain at least 120% MUTS without failure.
Wedge performance is critical to the proper anchoring of strands. Different wedges have been developed for particular systems and applications such that there is no single standard wedge. However, all are similar. The length is at least 2.5 times the strand diameter, with a 5 to 7 degree wedge angle and serrated teeth for gripping the strand. They are of case-hardened low carbon or alloy steel. A wedge assembly typically has 2 or 3 part wedges with a spring wire retainer clip in a groove around the thick end.
Wedges are case hardened with a ductile core, in order to bite into the strand and conform to the irregularity between the strand and wedge hole. In so doing, the surface may crack. This is normally acceptable and does not affect performance so long as wedges do not break completely into separate pieces. Often, it is only the portion outside the retainer ring that cracks.
Performance requirements should be in accordance with Section 4.1.2, of "Acceptance Standards for Post-Tensioning Systems", PTI, 1998 which imposes quality control sampling and testing on manufactured lots of 3,000 wedges in order to certify compliance.
For acceptance of a post-tensioning system, the strand-wedge connection is part of the anchorage system and should comply with AASHTO LRFD, Bridge Construction Specifications article 10.3.2.3. In the absence of any other specific contract requirements, for guidance, strand-wedge connections should conform to Sections 4.1.3 and 6.1.6 of "Acceptance Standards for Post-Tensioning Systems", PTI, 1998. These tests require that for each strand, wedge and wedge hole, thirty consecutive static tests and four consecutive dynamic tests, for which half are on lubricated holes and half on non-lubricated holes be conducted. Static tests are required to sustain 95% of AUTS at a strand elongation of 2%. Dynamic tests comprise 500,000 cycles from 60% to 66% of AUTS followed by 50 cycles between 40% and 80% AUTS without failure.
For acceptance of a post-tensioning bar system, the bar nut and plate is part of the anchorage system (Figure 2.8) and should comply with AASHTO LRFD, Bridge Construction Specifications article 10.3.2.3. In the absence of any other specific contract requirements, for guidance, for permanent applications, three successful tests on each size, type and grade of bar nut connection and bar coupler connection are required for acceptance in accordance with Sections 4.2,and 6.1.7 of "Acceptance Standards for Post-Tensioning Systems", PTI, 1998".
This test requires that nuts carry the greater of 100% of bar MUTS or 95% AUTS, couplers carry the same with a central 1 inch of the coupler not engaged, and nuts permit 5° misalignment between the bar and bearing plate. Unbonded bar tendons should withstand 500,000 cycles from 60% to 66% MUTS and thereafter 50 cycles from 40% to 80% MUTS.
Figure 2.8 - PT-Bar Anchor Plate
Grout inlets, outlets, valves and plugs should be made of polypropylene or polyethylene meeting the requirements for plastic, corrugated ducts. Permanent threaded plugs should be made of stainless steel or any non-metallic material containing antioxidant stabilizers and having an environmental stress cracking of 192 hours as determined by ASTM D 1693, Condition C. Temporary items not included in the permanent features of the finished structure may be of any other suitable material.
Attachments for grout inlets and outlets (also referred to as "vents"), including seals between grout caps and anchors, should be capable of withstanding at least 1MPa (145 psi) internal pressure. For acceptance testing, see 2.3.4.
Tubes for inlets and outlets for strand tendons should have a minimum inside diameter of 20mm (3/4 in). For bar tendons and for tendons comprising up to 4 strands, tubes should be at least 10mm (3/8 in) internal diameter. Inlets and outlets should be closed with suitable valves or plugs. For grouting of long vertical tendons, dual mechanical shut-off valves are usually necessary to facilitate intermediate stages of grouting and venting.
Inlets and outlets should be arranged and attached to ducts, anchorages and grout caps in a manner that allows all air and water to escape in order to ensure that the system is completely filled with grout. (See Chapter 4 for examples of locations of inlets and outlets).
Permanent grout caps are recommended to provide an additional level of corrosion protection at an anchorage (Figure 2.9). Project specific documents should specify when caps are required.
Figure 2.9 - Permanent (Plastic) Grout Cap to Anchor
Permanent grout caps should be made from a fiber reinforced plastic containing an anti-oxidant additive to ensure an enduring, maintenance-free, life of 75 years with an environmental stress cracking endurance of 192 hours per ASTM D 1693. Caps should be sealed against the anchor bearing plate and have a grout vent on the top of the cap. Caps should be secured to the anchor plate using 316 stainless steel bolts. Caps should be rated for a minimum pressure of 1MPa (145 psi).
Project specific documents should state when a dynamic system qualification test for unbonded (external) tendons is necessary. This should comply with the most recent AASHTO LRFD Bridge Construction Specifications. In the absence of any other information, for guidance, reference may be made to "Acceptance Standards for Post-Tensioning Systems", PTI, 1998.