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instruction in the use of the prestressing equipment and installation of materials as may be necessary to obtain required results.
601A.04 Concrete and Grout. The materials for concrete and grout shall conform to section 601. The concrete shall be class D as shown in table 601-1 unless otherwise shown on the plans or specified in the special provisions. The proportions of the grout will be as set out in subsection 601A.17, Bonding Steel.
601A.05 Reinforcing Steel. Reinforcing steel shall conform to subsection 709.01.
601A.06 Prestressing Reinforcing Steel. Prestressing reinforcing steel wire shall conform to the requirements of subsection 709.03.
601A.07 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 vendor 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 Inspector:
(a) Pretensioning method.-Samples at least 7 feet long shall be furnished of each strand size. A sample shall be taken from each end of every coil. (b) Post-tensioning method.-Samples of the following lengths shall be furnished:
For wires requiring heading, 17 feet.
For wires not requiring heading, sufficient length to make up one parallel-lay cable 5 feet long consisting of the same number of wires as the cable to be furnished.
For strand to be furnished with fittings, 5 feet between near ends of fittings.
For bars to be furnished with threaded ends and nuts, 5 feet between threads at ends.
Anchorage assemblies.—Two anchorage assemblies shall be furnished, complete with distribution plates of each size and type to be used, if anchorage assemblies are not attached to reinforcement samples.
Construction Requirements 601A.08 General. Prestressed concrete structural members shall be constructed in accordance with the requirements of section 601 and reinforcement steel shall be placed in accordance with the requirements of section 602, subject to the modifications an amendments contained herein.
601A.09 Prestressing Equipment. Hydraulic jacks shall be equipped with accurate pressure gages. The Contractor may elect to substitute screw jacks or other types for hydraulic jacks. In that case, proving rings or other approved devices shall be used in connection with the jacks. All devices, whether hydraulic jack gages or otherwise, shall be calibrated and, if necessary, recalibrated so as to permit the stress in the prestressing steel to be computed at all times. A certified calibration curve shall accompany each device.
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.
601A.10 Casting Yard. The precasting of prestressed concrete structural members may be done at any location selected by the Contractor, subject to the approval of the Engineer.
Before any site on Government property is approved for use as a casting yard, the Contractor shall submit to the Engineer a plan of operation showing any anticipated leveling or altering of the selected area. Upon completion of the work, the site shall be cleared of equipment and rubbish and restored as nearly as possible to its original condition.
601A.11 Enclosures. Enclosures for prestressed reinforcement shall be accurately placed at locations shown on the plans or approved by the Engineer.
All enclosures shall be metallic and shall be mortar-tight, with the exception that the Contractor, at his option, may form the enclosures by means of cores or ducts composed of rubber or other suitable material which can be removed prior to installing the prestressing reinforcement. Enclosures shall be strong enough to maintain their shapes under such forces as will come upon them. They shall be 14 inch larger in internal diameter than the bar, cable, strand, or group of wires which they enclose. Where pressure grouting is specified, cores or ducts shall be provided with pipes or other suitable connections for the injection of grout after the prestressing operations have been completed.
601A.12 Placing Steel. All steel units shall be accurately placed in the position shown on the plans, and firmly held during the placing and setting of the concrete.
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.
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.
601A.13 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 4000 psi unless otherwise indicated. The elements shall be cut or released in such an order that lateral eccentricity of prestress will be a minimum.
601A.14 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 internally or externally or both as ordered by the Engineer. The vibrating shall be done with care and in such a manner as to avoid displacement of reinforcement, conduits, or wires.
601A.15 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 4 hours after placing concrete and after the concrete has undergone initial set, the first application of steam shall be made, unless retarders are used, in which case the waiting period before application of the steam shall be increased to from 4 to 6 hours. Water curing methods shall be used from the time the concrete is placed until steam is first applied.
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, the ambient air temperature shall increase at a rate not to exceed 40° F. per hour until a maximum temperature of from 140° F. to 160° F. is reached. The maximum temperature shall be held until the concrete has reached the desired strength. In discontinuing the steam application, the ambient air temperature shall not decrease a rate to exceed 40° F. per hour until a temperature has been reached 20° F. above the temperature of the air to which the concrete will be exposed. The concrete shall not be exposed to temperatures below freezing until at least 6 days after casting.
After transfer or post-tensioning, the prestressed members shall be protected by suitable coverings or moist curing against extreme ambient temperatures or humidities prior to movement from the casting plant.
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.
601A.16 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 4000 psi unless otherwise indicated.
After the 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.
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 at the jack and the minimum tension, shall be determined in accordance with article 1.6.8 of AASHO “Standard Specifications for Highway Bridges."
A record shall be kept of gage pressures and elongation at all times and submitted to the Engineer for his approval.
601A.17 Bonding Steel. Post-tensioned steel shall be bonded to the concrete except when such bonding is not practicable.
All prestressing reinforcement to be bonded to the concrete shall be free of dirt, loose rust, grease, or other deleterious substances.
The annular space between the perimeter of the enclosure and the steel shall be pressure grouted after the prestressing process has been completed. The grout shall be made to the
consistency of thick paint and shall be mixed in the proportions by volume of one part portland cement to 0.75 part (max.) of sand passing a No. 30 sieve and 0.75 part (max.) of water. Within the limits specified, the proportions of sand and water shall be varied to produce the required strength and fluidity. To that end it may be necessary to eliminate the sand from the mix and use neat cement grout.
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 or 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 mortar 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 1 part powder to 50 parts pumicite (or other inert powder) by weight. The blend shall be thoroughly mixed with the cement and sand before water is added to the batch to prevent the blend from floating in the water. The amount of the blend used should vary from 412 ounces per sack of cement for concrete having a temperature of 70° F. to 7 ounces for a temperature of 40° F. After all ingredients are added, the batch shall be mixed for 3 minutes. Batches of grout shall be placed within 45 minutes after mixing.
The final pressure placed on the grout shall be 50 to 100 psi.
601A.18 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 or other waterproofing material, in addition to any galvanizing which may be specified in addition to the requirements of ASTM A 416 and ASTM A 421.
601A.19 Handling. Extreme care shall be exercised in handling and moving precast prestressed concrete 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 member is in its 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.
Care shall be taken during storage, hoisting, and handling of the precast units to prevent cracking or damage. Units damaged by improper storage or handling shall be replaced by the Contractor at his expense.