ITEM 406 – PRESTRESSED CONCRETE STRUCTURES
406.1 Description
This Item shall consist of prestressed concrete structures and the
prestressed concrete portions of composite structures, constructed in
reasonably close conformity with the lines, grades and dimensions shown on
the Plans or established by the Engineer and in accordance with this Specification. It shall also include the furnishing and installing of any
appurtenant items necessary for the particular prestressing system to be
used, including but not limited to ducts, anchorage assemblies and grouts
used for pressure grouting ducts.
406.2 Material Requirements
406.2.1 Concrete and Grout
The materials for concrete and grout shall conform to Item 405, Structural
Concrete. The concrete shall be Class P as shown in Table 405.2, unless
otherwise shown on the Plans or specified in the Special Provisions. The
proportions of the grout will be as set out in Subsection 406.3.11, Bonding
Steel.
406.2.2 Prestressing Reinforcing Steel
It shall conform to Item 710, Reinforcing Steel and Wire Rope.
406.2.3 Prestressing Steel
It shall conform to the requirements of Item 710, Reinforcing Steel and
Wire Rope.
All prestressing steel shall be protected against physical damage and rust
or other results of corrosion at all times from manufacture to grouting.
Prestressing steel that has sustained physical damage at any time shall be
rejected.
406.2.4 Packaging, Storing and Shipping
Prestressing steel shall be packaged in containers or other shipping forms
for the protection of the steel against physical damage and corrosion
during shipping and storage. A corrosion inhibitor which prevents rust or
other results of corrosion shall be placed in the package or form, or when
permitted by the Engineer, may be applied directly to the steel. The
corrosion inhibitor shall have no deleterious effect on the steel or
concrete or bond strength of concrete to steel. Packaging or forms damaged
from any cause shall be immediately replaced or restored to original
condition.
This shipping package or form shall be clearly marked with a statement that
the package contains high-strength prestressing steel and the care to be
used in handling, and the type, kind and amount of corrosion inhibitor
used, including the date when placed, safety orders and instructions for
use.
406.2.5 Elastomeric Bearing Pads
This shall conform to Item 412, Elastomeric Bearing Pads.
406.2.6 Water
It shall conform to the requirements of Subsection 311.2.4, Water.
406.2.7 Enclosures
Duct enclosures for prestressing steel shall be galvanized ferrous metal or
of a type approved by the Engineer, mortar tight and accurately placed at
the locations shown on the Plans or approved by the Engineer. Transition
couplings connecting said ducts to anchoring devices need not be
galvanized.
406.2.8 Sampling and Testing
All wire, strand, anchorage assemblies or bars to be shipped to the site
shall be assigned a lot number and tagged for identification purposes.
All samples submitted shall be representative of the lot to be furnished
and in the case of wire or strand, shall be taken from the same master
roll.
All of the materials specified for testing shall be furnished free of cost
and shall be delivered in time for tests to be made well in advance of
anticipated time of use.
The Contractor shall furnish for testing the following samples selected
from each lot, if ordered by the Engineer. The selection of samples will be
made at the manufacturer’s plant by the Engineer or his representative.
1. For pretensioning work-samples at least 2 m long shall be furnished of
each size of wire or strand proposed
.
2. For post-tensioning work-samples of the following lengths shall be
furnished of each size of wire proposed.
a. For wire requiring heading, 2 m.
b. For wires not requiring heading, sufficient length to make up one
parallel-lay cable of 1.50 m long consisting of the same number of wires as
the cable to be furnished.
c. For strand to be furnished with fittings, 1.50 m between near ends of
fittings.
d. For bars to be furnished with thread ends and nuts, 1.5 m between
threads at ends.
3. Anchorage assemblies – If anchorage assemblies are not attached to
reinforcement samples, two (2) anchorage assemblies shall be furnished,
completed with distribution plates of each size and type to be used.
When prestressing system has been previously tested and approved for
similar projects by an agency acceptable to the Engineer, complete tendon
samples need not be furnished, provided there is no change whatsoever in
the materials, design or details previously approved.
406.3 Construction Requirements
406.3.1 General
Prestressed concrete structural members shall be constructed in accordance
with the requirements of Item 405, Structural Concrete and Reinforcing
Steel shall be placed in accordance with the requirements of Item404,
Reinforcing Steel, subject to the modifications and amendments contained
herein.
406.3.2 Prestressing Method
The method of prestressing to be used shall be optional with the Contractor
subject to all requirements hereinafter specified.
The Contractor, prior to casting any members to be prestressed, shall
submit to the Engineer for approval complete details of the methods,
materials and equipment he proposes to use in the prestressing operations.
Such details shall outline the method and sequence of stressing, complete
specifications and details of the prestressing, steel and anchoring devices
proposed for use, anchoring stresses, type of enclosures and all other data
pertaining to the prestressing operations, including the proposed
arrangement of the prestressing units in the members, pressure grouting
materials and equipment.
406.3.3 Prestressing Equipment
Hydraulic jacks used to stress tendons shall be equipped with either a
pressure gauge or a load cell for determining the jacking stress. The
pressure gauge, if used, shall have an accurate reading dial at least 154
mm (6 inches) in diameter and each jack and its gauge shall be calibrated
as a unit with the cylinder extension in the approximate position that it
will be at final jacking force, and shall be accompanied by a certified
calibration chart. The load cell, if used, shall be calibrated and shall be
provided with an indicator by means of which the prestressing force in the
tendon may be determined. The range of the load cell shall be such that the
lower ten (10) percent of the manufacturer’s rated capacity will not be
used in determining the jacking stress.
Safety measures shall be taken by the Contractor to prevent accidents due
to possible breaking of the prestressing steel or the slipping of the grips
during the prestressing process.
406.3.4 Casting Yard
The precasting of prestressed concrete structural members may be done a ta
location selected by the Contractor, subject to the approval of the
Engineer.
406.3.5 Placing Enclosures
Enclosures for prestressed reinforcement shall be accurately placed at
locations shown on the Plans or approved by the Engineer.
406.3.6 Placing Steel
Steel units shall be accurately placed at the position shown on the Plans
and firmly held during the placing and setting of the concrete.
Ducts may be fabricated with either welded or interlocked seams.
Galvanizing of the welded steel will not be required. Ducts shall have
sufficient strength to maintain their correct alignment and shape during
placing of concrete. Joints between sections of ducts shall be positive
metallic connections which do not result in angle changes at the joints.
Waterproof tape shall be used at the connections.
All ducts or anchorage assemblies shall be provided with pipes or other
suitable connections for the injection of grout after prestressing.
Ducts for prestressing steel shall be securely fastened in place to prevent
movement.
After installation in the forms, the end of ducts shall at all times be
covered as necessary to prevent the entry of water of debris.
All ducts for continuous structures shall be vented over each intermediate
support, and at additional locations as shown on the Plans. Vents shall be
12.7 mm minimum diameter standard pipe.
Connections to ducts shall be made
with metallic structural fasteners. The vents shall be mortar tight, taped
as necessary, and shall provide means for injection of grout through the
vents and for sealing the vents. Ends of vents shall be removed 25.4 mm (1
inch) below the roadway surface after grouting has been completed.
Distances from the forms shall be maintained by stays, blocks, ties,
hangers or other approved supports. Blocks for holding units from contact
with the forms shall be precast mortar blocks of approved shape and
dimensions. Layers of units shall be separated by mortar blocks or other
equally suitable devices. Wooden blocks shall not be left in the concrete.
When acceptable prestressing steel for post-tensioning is installed in
ducts after completion of concrete curing, and if stressing and grouting
are completed within ten (10) calendar days after the installation of the
prestressing steel, rust which may form during said ten (10) days will not
be caused for rejection of the steel. Prestressing steel, installed,
tensioned and grouted in this manner, all within ten (10) calendar days,
shall be subject to all the requirements in this Item pertaining to
corrosion protection and rejection because of rust.
No welds or grounds for welding equipment shall be made on the forms or on
the steel in the manner after the prestressing steel has been installed.
Wires, wire groups, parallel-lay cables and any other prestressing elements
shall be straightened to insure proper positioning in the enclosures.
Suitable horizontal and vertical spacers shall be provided, if required, to
hold the wires in place in true position in the enclosures.
406.3.7 Pretensioning
The prestressing elements shall be accurately held in position and stressed
by jacks. A record shall be kept of the jacking force and the elongations
produced thereby. Several units may be cast in one continuous line and
stressed at one time. Sufficient space shall be left between ends of units
to permit access for cutting after the concrete has attained the required
strength. No bond stress shall be transferred to the concrete, nor end
anchorages released until the concrete has attained a compressive strength,
as shown by cylinder tests, of at least 28MPa unless otherwise specified.
The elements shall be cut or released in such an order that lateral
eccentricity or prestress will be a minimum.
406.3.8 Placing Concrete
Concrete shall not be deposited in the forms until the Engineer has
inspected the placing of the reinforcement, enclosures, anchorages and
prestressing steel and given his approval thereof. The concrete shall be
vibrated with care and in such a manner as to avoid displacement of
reinforcement, conduits, or wires.
Prior to placing concrete, the Contractor shall demonstrate to the Engineer
that all ducts are unobstructed.
406.3.9 Curing
Steam curing process may be used as an alternative to water curing. The
casting bed for any unit cured with steam shall be completely enclosed by a
suitable type of housing, tightly constructed so as to prevent the escape
of steam and simultaneously exclude outside atmosphere. Two to four hours
after placing concrete and after the concrete has undergone initial set,
the first application of steam or radiant heat shall be made unless
retarders are used, in which case the waiting period before application of
the steam or radiant heat shall be increased to from four to six hours.
During the waiting period, the temperature within the curing chamber shall
not be less than 10.00C and live steam or radiant heat may be
used to maintain the curing chamber at the proper minimum temperature. The
steam shall be at 100 percent relative humidity to prevent loss of moisture
and to provide moisture for proper hydration of the cement. Application of
the steam shall not be directly on the concrete. During application of the
steam, or of radiant heat, the ambient air temperature shall increase at a
rate not to exceed 4.410C per hour until the curing temperature
is reached. The maximum curing temperature within the enclosure shall not
exceed 71.10C. The maximum temperature shall be held until the
concrete has reached the desired strength. Detensioning shall be
accomplished immediately after the steam curing or the heat curing has been
discontinued and additional curing is not required after detensioning.
1. Curing with Low Pressure Steam
Application of live steam shall not be directed on the concrete forms as to
cause localized high temperatures.
2. Curing with Radiant Heat
Radiant heat may be applied by means of pipes circulating steam, hot oil or
hot water, or by electric heating elements. Radiant heat curing shall be
done under a suitable enclosure to contain the heat and moisture loss shall
be minimized by covering all exposed concrete surfaces with plastic
sheeting or by applying an approved liquid membrane curing compound to all
exposed concrete surfaces. Top surface of concrete members to be used in
composite construction shall be clear of residue of the membrane curing
compound so as not to reduce bond below design limits.
If the Contractor proposes to cure by any other special method, the method
and its details shall be subject to the approval of the Engineer.
406.3.10 Post-tensioning
Tensioning of the prestressing reinforcement shall not be commenced until
tests on concrete cylinders, manufactured of the same concrete and cured
under the same conditions, indicate that the concrete of the particular
member to be prestressed has attained compressive strength of at least 28
MPa unless otherwise specified.
After all concrete has attained the required strength, the prestressing
reinforcement shall be stressed by means of jacks to the desired tension
and the stress transferred to the end anchorage.
Cast-in-place concrete shall not be post-tensioned until at least ten (10)
days after the last concrete has been placed in the member to be the
post-tensioned and until the compressive strength of said placed concrete
has reached the strength specified for the concrete at the time of
stressing.
All side forms for girders shall be removed before post-tensioning. The
falsework under the bottom slab supporting the superstructure shall not be
released until a minimum of 48 hours have elapsed after grouting of the
post-tension tendons nor until all other conditions of the Specifications
have been met. The supporting falsework shall be constructed in such a
manner that the super restructure will be free to lift off the falsework
and shorten during post-tensioning. Formwork left inside box girders to
support the roadway slab shall be detailed in such a manner so as to offer
minimum resistance to girder shortening due to shrinkage and
post-tensioning.
The tensioning process shall be so conducted that the tension being applied
and the elongation of the prestressing elements may be measured at all
times. The friction loss in the element, i.e., the difference between the
tension of the jack and the minimum tension, shall be determined in
accordance with Article1.6.7 of AASHTO Standard Specifications for Highway
Bridges.
Suitable shims or other approved devices shall be used to insure that the
specified anchor set loss is attained.
Prestressing tendons in continuous post-tensioned members shall be
tensioned by jacking at each end of the tendon. Such jacking of both ends
need not be done simultaneously.
A record shall be kept of gage pressure and elongation at all times and
submitted to the Engineer for his approval.
406.3.11 Bonding Steel
Prestressing steel shall be bonded to the concrete by filling the void
space between the duct and the tendon with grout.
Grout shall consist of Portland Cement, water and an expansive admixture
approved by the Engineer.
Water shall be potable.
No admixture containing chlorides or nitrates shall be used.
Water shall first be added to the mixer followed by cement and admixture.
The grout shall be mixed in mechanical mixing equipment of a type that will
produce uniform and thoroughly mixed grout. The water content shall be not
more than 19 liters per sack of cement. Retempering of grout will not be
permitted. Grout shall be continuously agitated until it is pumped.
Grouting equipment shall be capable of grouting at a pressure of at
least0.6894 MPa.
Grouting equipment shall be furnished with a pressure gauge having a
full-scale reading of not more than 2.07 MPa.
Standby flushing equipment capable of developing a pumping pressure of1.72
MPa and of sufficient capacity to flush out any partially grouted ducts
shall be provided.
All ducts shall be clean and free of deleterious materials that would
impair bonding of the grout or interfere with grouting procedures. All
grout shall pass through a screen with a 2 mm (0.0787 inch) maximum clear
openings prior to being introduced into the grout pump.
Grout injection pipes shall be fitted with positive mechanical shutoff
valves. Vents and ejection pipes shall be fitted with valves, caps or other
devices capable of withstanding the pumping pressures. Valves and caps
shall not be removed or opened until the grout has set.
Post-tensioned steel shall be bonded to the concrete. All prestressing
steel to be bonded to the concrete shall be free of dirt, loose rust,
grease or other deleterious substances.
Immediately after completion of the concrete pour, the metal conduit shall
be blown out with compressed oil free air to the extent necessary to break
up and remove any mortar in the conduit before it hardens. Approximately 24
hours after the concrete pour, the metal conduits shall be flushed out with
water and then blown out with compressed oil free air.
Prior to placing forms for roadway slabs of box girder structures, the
Contractor shall demonstrate to the satisfaction of the Engineer that all
ducts are unobstructed and if the prestressing reinforcement has been
placed, that the steel is free and unbonded in the duct.
After the tendons have been stressed to the required tension, each conduit
encasing the prestressing steel shall be blown out with compressed oil free
air. The conduit shall then be completely filled from the low end with
grout under pressure. Grout shall be pumped through the duct and
continuously wasted at the outlet until no visible slugs of water or air
are ejected and the efflux time of ejected grout is not less than 11
seconds. All vents and openings shall then be closed and the grouting
pressure at the injection end shall be raised to a minimum of0.6894 MPa and
held for a minimum of 10 seconds.
If aluminum powder is used to expand the grout, it shall be added as
follows:
From 2 to 4 grams of the unpolished variety (about 1 to 2 teaspoons) shall
be added for each sack of cement used in the grout. The exact amount of
aluminum powder will be designated by the Engineer. The dosage per batch of
grout shall be carefully weighed. A number of weighings may be made in the
laboratory and doses placed in glass vials for convenient use in the mix.
The aluminum powder shall be blended with pumicite or other inert powder in
the proportion of one (1) part powder to fifty (50) parts pumicite (or
other inert powder) by weight. The blend shall be thoroughly mixed with the
cement. The amount of the blend used should vary from 120 g (4-1/2 ounces)
per sack of cement for concrete having a temperature of 21.10C
to 190 g (7 ounces) for a temperature of 4.440C after all
ingredients are added, the batch shall be mixed for 3 minutes. Batches of
grout shall be placed within 45 minutes after mixing.
406.3.12 Unbonded Steel
Where the steel is not to be bonded to the concrete, it shall be carefully
protected against corrosion by a coating of tar of other waterproofing
material, in addition to any galvanizing which may be specified in addition
to the requirements of ASTM A 416 (AASHTO M 203) and ASTM A 421 (AASHTO M
204.)
406.3.13 Handling
Extreme care shall be exercised in handling and moving precast prestressed
concrete mortar members. Precast girders and slabs shall be transported in
an upright position and the points of support and directions of the
reactions with respect to the member shall be approximately the same during
transportation and storage as when the members are in their final position.
If the Contractor deems it expedient to transport or store precast units in
other than this position, it shall be done at his own risk after notifying
the Engineer of his intention to do so.
Prestressed concrete girders shall not be shipped until tests on concrete
cylinders, manufactured of the same concrete and cured under the same
conditions as the girders, indicate that the concrete of the particular
girder has attained a compressive strength equal to the specified design
compressive strength of the concrete in the girder and has attained a
minimum age of 14 days.
406.3.14 Composite Slab Construction
The manufacturing tolerances for pre-cast members shall not exceed those
given for length, cross-section and straightness on the Plans, as specified
in the Contract or as approved by the Engineer. In addition, where beams
are laid side by side in a deck:
1. The difference on soffit level between adjacent units before the in-sit
concrete is placed shall not exceed 5 mm for units up to 10 mm for longer
units.
2. The width of the deck soffit shall be within ±25 mm.
3. In adjacent span, the continuity of the outside beams shall be
maintained.
4. The width of gap between individual beams shall not exceed twice the
nominal gap.
5. The alignment of transverse holes shall permit the reinforcement or
prestressing cables to be placed without distortion.
406.4 Method of Measurement406.4.1 Structural Members
The quantity to be measured for payment will be the actual number of
precast prestressed concrete structural members, except piling, of the
several types and sizes, installed in place, completed and accepted. Each
member will include the concrete, reinforcement and prestressing steel,
anchorages, plates, nuts, elastomeric bearing pads, and other such material
contained within or attached to the unit.
Piling will be measured as provided in Item 400.
406.4.2 Other Items
The quantities of other Contract items which enter into the completed and
accepted structure will be measured for payment in the manner prescribed
for the several items involved.
406.5 Basis of Payment
The accepted quantities, measured as prescribed in Section 406.4 shall be
paid for at the contract unit price for each of the particular item listed
below that is included in the Bill of Quantities, which price and payment
shall be full compensation for furnishing and placing all materials,
including all labor, equipment, tools and incidentals necessary to complete
the work prescribed in this Item.
Payment will be made under:
Pay Item Number
|
Description
|
Unit of Measurement
|
406 (1)
406 (2)
|
Prestressed Structural Concrete
Members (Identification) Prestressed Concrete |
Each
Lump Sum
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