1、Section 10 PRESTRESSING 10.1 GENERAL 10.1.1 Description This work shall consist of prestressing precast or cast- in-place concrete by furnishing, placing, and tensioning of prestressing steel in accordance with details shown on the plans, and as specified in these specifications and the spe- cial pr
2、ovisions. It includes prestressing by either the pre- tensioning or post-tensioning methods or by a combina- tion of these methods. This work shall include the furnishing and installation of any appurtenant items necessary for the particular pre- stressing system to be used, including but not limite
3、d to ducts, anchorage assemblies and grout used for pressure grouting ducts. For cast-in-place prestressed concrete, the term “mem- ber” as used in this section shall be considered to mean the concrete which is to be prestressed. When members are to be constructed with part of the reinforcement pret
4、ensioned and part post-tensioned, the applicable requirement of this Specification shall apply to each method. 10.1.2 Details of Design When the design for the prestressing work is not fully detailed on the plans, the Contractor shall determine the details or type of prestressing system for use and
5、select materials and details conforming to these Specifications as needed to satisfy the prestressing requirements speci- fied. The system selected shall provide the magnitude and distribution of prestressing force and ultimate strength re- quired by the plans without exceeding allowable tempo- rary
6、 stresses. Unless otherwise shown on the plans, all de- sign procedures, coefficients and allowable stresses, friction and prestress losses as well as tendon spacing and clearances shall be in accordance with the Division I, De- sign, of the AASHTO Standard Specifications for High- way Bridges. The
7、prestressing may be performed by either preten- sioning or post-tensioning methods unless the plans show only pretensioning details. If the plans show only preten- sioning details, the use of a post-tensioning system will be allowed only if complete details of any necessary modifi- cations are appro
8、ved by the Engineer. When the effective or working force or stress is shown on the plans, it shall be considered to be the force or stress remaining in the prestressing steel after all losses, including creep and shrinkage of concrete, elastic short- ening of concrete, relaxation of steel, friction
9、and take up or seating of anchorages, and all other losses peculiar to the method or system of prestressing have taken place or have been provided for. When the jacking force is shown on the plans, it shall be considered to be the force applied to the tendon prior to anchorage and the occurrence of
10、any losses, including the anchor set loss. 10.2 SUPPLEMENTARY DRAWINGS 10.2.1 Working Drawings Whenever the plans do not include complete details for a prestressing system and its method of installation, or when complete details are provided in the plans and the Contractor wishes to propose any chan
11、ge, the Contractor shall prepare and submit to the Engineer working draw- ings of the prestressing system proposed for use. Fabrica- tion or installation of prestressing material shall not begin until the Engineer has approved the drawings. The working drawings of the prestressing system shall show
12、complete details and substantiating calculations of the method, materials and equipment the Contractor pro- poses to use in the prestressing operations, including any additions or rearrangement of reinforcing steel and any re- vision in concrete dimensions from that shown on the plans. Such details
13、shall outline the method and sequence of stressing and shall include complete specifications and details of the prestressing steel and anchoring devices, working stresses, anchoring stresses, tendon elongations, type of ducts, and all other data pertaining to the pre- stressing operation, including
14、the proposed arrangement of the prestressing steel in the members. Working drawings shall be submitted sufficiently in advance of the start of the affected work to allow time for 553 554 HIGHWAY BRIDGES 10.2.1 review by the Engineer and correction by the Contractor of the drawings without delaying t
15、he work. 10.2.2 Composite Placing Drawings When required by the special provisions, in addition to all required working drawings, the Contractor shall prepare composite placing drawings to scale and in suf- ficient detail to show the relative positions of all items that are to be embeddedin the conc
16、rete, and their em- bedment depth, for the portions of the structure that are to be prestressed. Such embedded items include the pre- stressing ducts, vents, anchorage reinforcement and hard- ware, reinforcing steel, anchor bolts, earthquake restrain- ers, deck joint seal assemblies, drainage system
17、s, utility conduits and other such items. Such drawings shall be ad- equate to ensure that there will be no conflict between the planned positions of any embedded items and that con- crete cover will be adequate. If during the preparation of such drawings conflicts are discovered, the Contractor sha
18、ll revise his or her working drawing for one or more of the embedded items or propose changes in the dimen- sions of the work as necessary to eliminate the conflicts or provide proper cover. Any such revisions shall be ap- proved by the Engineer before work on any affected item is started. All costs
19、 involved with the preparation of such drawings and with making the necessary modifications to the work resulting therefrom shall be borne by the Contractor. 10.3 MATERIALS 10.3.1 Prestressing Steel and Anchorages Prestressing reinforcement shall be high-strength seven-wire strand, high-strength ste
20、el wire, or high- strength alloy bars of the grade and type called for on the plans or in the special provisions and shall conform to the requirements of the following specifications. 10.3.1.1 Strand Uncoated seven-wire strand shall conform to the requirements of AASHTO M 203 (ASTM A 416). Supplemen
21、t S 1 (Low-Relaxation) shall apply when specified. 10.3.1.2 Wire Uncoated stress-relieved steel wire shall conform to the requirements of AASHTO M 204 (ASTM A 421). 10.3.1.3 Bars Uncoated high-strength bars shall conform to the re- quirements of AASHTO M 275 (ASTM A 722). Bars with greater minimum u
22、ltimate strength, but otherwise pro- duced and tested in accordance with AASHTO M 275 (ASTM A 722), may be used provided they have no prop- erties that make them less satisfactory than the specified material. 10.3.2 Post-Tensioning Anchorages and Couplers All anchorages and couplers shall develop at
23、 least 95% of the actual ultimate strength of the prestressing steel, when tested in an unbonded state, without exceeding an- ticipated set. The coupling of tendons shall not reduce the elongation at rupture below the requirements of the ten- don itself. Couplers and/or coupler components shall be e
24、nclosed in housings long enough to permit the necessary movements. Couplers for tendons shall be used only at lo- cations specifically indicated and/or approved by the En- gineer. Couplers shall not be used at points of sharp ten- don curvature. 10.3.2.1 Bonded Systems Bond transfer lengths between
25、anchorages and the zone where full prestressing force is required under ser- vice and ultimate loads shall normally be sufficient to de- velop the minimum specified ultimate strength of the pre- stressing steel. When anchorages or couplers are located at critical sections under ultimate load, the ul
26、timate strength required of the bonded tendons shall not exceed the ultimate capacity of the tendon assembly, including the anchorage or coupler, tested in an unbonded state. Housings shall be designed so that complete grouting of all of the coupler components will be accomplished during grouting of
27、 tendons. 10.3.2.2 Unbonded Systems For unbonded tendons, a dynamic test shall be per- formed on a representative anchorage and coupler speci- men and the tendon shall withstand, without failure, 500,000 cycles from 60% to 66% of its minimum specified ultimate strength, and also 50 cycles from 40% t
28、o 80% of its minimum specified ultimate strength. The period of each cycle involves the change from the lower stress level to the upper stress level and back to the lower. The speci- men used for the second dynamic test need not be the same used for the first dynamic test. Systems utilizing multiple
29、 strands, wires, or bars may be tested utilizing a test tendon of smaller capacity than the full-sized tendon. The test ten- 10.3.2.2 DIVISION II-CONSTRUCTION 555 don shall duplicate the behavior of the full-sized tendon and generally shall not have less than 10% of the capacity of the full-sized te
30、ndon. Dynamic tests are not required on bonded tendons, unless the anchorage is located or used in such manner that repeated load applications can be ex- pected on the anchorage. Anchorages for unbonded tendons shall not cause a re- duction in the total elongation under ultimate load of the tendon t
31、o less than 2% measured in a minimum gauge length of 10 feet. All the coupling components shall be completely pro- tected with a coating material prior to final encasement in concrete. 10.3.2.3 Special Anchorage Device Acceptance Test 10.3.2.3.1 The test block shall be a rectangular prism. It shall
32、contain those anchorage components which will also be embedded in the structure?s concrete. Their arrangement has to comply with the practical application and the suppliers specifications. The test block shall con- tain an empty duct of size appropriate for the maximum tendon size which can be accom
33、modated by the anchor- age device. 10.3.2.3.2 The dimensions of the test block perpen- dicular to the tendon in each direction shall be the smaller of the minimum edge distance or the minimum spacing specified by the anchorage device supplier, with the stip- ulation that the cover over any confining
34、 reinforcing steel or supplementary skin reinforcement be appropriate for the particular application and environment. The length of the block along the axis of the tendon shall be at least two times the larger of the cross-section dimensions. 10.3.2.3.3 The confining reinforcing steel in the local z
35、one shall be the same as that specified by the anchorage device supplier for the particular system. 10.3.2.3.4 In addition to the anchorage device and its specified confining reinforcement steel, supplementary skin reinforcement may be provided throughout the spec- imen. This supplementary skin rein
36、forcement shall be specified by the anchorage device supplier but shall not exceed a volumetric ratio of 0.01. 10.3.2.3.5 The concrete strength at the time of stress- ing shall be greater than the concrete strength of the test specimen at time of testing. 10.3.2.3.6 Either of three test procedures i
37、s ac- ceptable: cyclic loading described in Article 10.3.2.3.7, sustained loading described in Article 10.3.2.3.8, or monotonic loading described in Article 10.3.2.3.9. The loads specified for the tests are given in fractions of the ultimate load F, of the largest tendon that the anchorage device is
38、 designed to accommodate. The specimen shall be loaded in accordance with normal usage of the device in post-tensioning applications except that load can be applied directly to the wedge plate or equivalent area. 10.3.2.3.7 Cyclic Loading Test 10.3.2.3.7.1 In a cyclic loading test, the load shall be
39、 increased to 0.8F,. The load shall then be cycled between O.lF, and 0.8F, until crack widths stabilize, but for not less than 10 cycles. Crack widths are considered stabilized if they do not change by more than 0.001 inch over the last three readings. Upon completion of the cyclic loading the speci
40、men shall be preferably loaded to failure or, if lim- ited by the capacity of the loading equipment, to at least 1 . 1 Fpu. 10.3.2.3.7.2 Crack widths and crack patterns shall be recorded at the initial load of 0.8Fp, at least at the last three consecutive peak loadings before termination of the cycl
41、ic loading, and at 0.9Fp,. The maximum load shall also be reported. 10.3.2.3.8 Sustained Loading Test 10.3.2.3.8.1 In a sustained loading test, the load shall be increased to 0.8F, and held constant until crack widths stabilize but for not less than 48 hours. Crack widths are considered stabilized i
42、f they do not change by more than 0.001 inch over the last three readings. After sustained loading is completed, the specimen shall be preferably loaded to failure or, if limited by the capacity of the load- ing equipment, to at least l.lFpu. 10.3.2.3.8.2 Crack widths and crack patterns shall be rec
43、orded at the initial load of 0.8F, at least three times at intervals of not less than 4 hours during the last 12 hours before termination of the sustained loading, and during loading to failure at 0.9F,. The maximum load shall also be reported. 10.3.2.3.9 Monotonic Loading Test 10.3.2.3.9.1 In a mon
44、otonic loading test, the load shall be increased to 0.9Fp, and held constant for 1 hour. The specimen shall then be preferably loaded to failure or, 556 HIGHWAY BRIDGES 10.3.2.3.9.1 if limited by the capacity of the loading equipment, to at least 1 .2F,. 10.3.2.3.9.2 Crack widths and crack patterns
45、shall be recorded at 0.9FP, after the 1-hour period, and at l.OF,. The maximum load shall also be reported. 10.3.2.3.10 The strength of the anchorage zone must exceed: Specimens tested under cyclic or sustained loading . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . lF, Speci
46、mens tested under monotonic loading . . . . . . 1 .2F, The maximum crack width criteria specified below must be met for moderately aggressive environments. For higher aggressivity environments the crack width criteria shall be reduced by at least 50%. (1) No cracks greater than 0.010 inch at 0.8FP,
47、after completion of the cyclic or sustained loading, or at 0.9FP, after the 1-hour period for monotonic loading. (2) No cracks greater than 0.016 inch at O.9Fp, for cyclic or sustained loading, or at 1 .OF, for monotonic loading. 10.3.2.3.11 A test series shall consist of three test specimens. Each
48、one of the tested specimens must meet the acceptance criteria. If one of the three specimens fails to pass the test, a supplementary test of three additional specimens is allowed. The three additional test specimen results must meet all acceptance criteria of Article 10.3.2.3.10. For a series of sim
49、ilar special anchorage devices, tests are only required for representative samples unless tests for each capacity of the anchorages in the series are re- quired by the engineer-of-record. 10.3.2.3.12 Records of the anchorage device accep- tance test shall include: (1 j Dimensions of the test specimen. (2) Drawings and dimensions of the anchorage device, including all confining reinforcing steel. (3) Amount and arrangement of supplementary skin reinforcement. (4) Type and yield strength of reinforcing steel. (5) Type and compressive strength at time of testing of concrete. (6) Ty
