The Table of Contents for this section is available.
This section covers the materials for and the production of precast, prestressed concrete members produced in accordance with the plans and these specifications.
Prestressing shall be of the pretensioning type in which steel reinforcing strands are initially stressed and anchored; the concrete is then placed, vibrated, and cured; and when the concrete has reached the required strength, the load is transferred from the anchorages to the concrete.
All materials shall meet the requirements of Division 10 shown below:
Coarse aggregate Article 1014-2
Fine aggregate Article 1014-1
Portland cement Article 1024-1
Type IP blended cement Article 1024-1
Fly ash Article 1024-5
Water Article 1024-4
Air entraining agent Article 1024-3
Chemical admixtures Article 1024-3
Reinforcing steel Section 1070
Prestressing strand Article 1070-5
Structural steel Section 1072
Miscellaneous metals Section 1074
Type IS blended cement Article 1024-1
Ground granulated blast furnace slag Article 1024-6
Changes in the source of aggregates, cements, or admixtures shall not be made during the casting of members in any one span or substructure unit unless the change is approved by the Engineer.
The Department reserves the right to place a duly authorized inspector in the plant at any or all times work related to the production of members for the Department is being performed. The Contractor shall notify the Engineer at least 7 days in advance when such work is scheduled to take place. An office area with an approximate floor space of 100 square feet, a desk or drafting table, 2 chairs, telephone, facilities for proper heating and cooling, and adequate lighting shall be provided at the plant for the exclusive use of the inspector. The inspector shall have the authority to reject any or all members not manufactured in accordance with these specifications. Approval of any member by the inspector at the plant, however, shall in no way be final, and further inspection will be made at the structure site both before and after the member has been placed in the final position. Any member found to be defective in any manner at any time shall be rejected and replaced by an acceptable member or repaired in a manner approved by the Engineer.
No member shall be transported from the plant to the job site prior to approval of that member by the plant inspector. Access to all surfaces of the member shall be provided so that the plant inspector has the opportunity to properly inspect the member prior to approval. This approval will be stamped on the member by the plant inspector.
1078-4 PORTLAND CEMENT CONCRETE.
(A) Composition and Design:
Portland cement concrete shall be composed of portland cement, coarse aggregate, fine aggregate, water, and an air entraining agent. Other cementitious materials and/or chemical admixtures may be added if approved by the Engineer. When admixtures are used, they shall be used in the proper proportions to obtain the optimum effect. Set accelerating admixtures, calcium chloride, or admixtures containing calcium chloride shall not be used.
All concrete shall develop a minimum compressive strength of 5,000 psi, at the age of 28 days, unless otherwise required by the plans or special provisions. Concrete shall be air entrained to provide an air content of 5 percent, plus or minus 2 percent. Concrete shall have a maximum slump of 3 1/2 inches. Concrete containing superplasticizer shall have a maximum slump of 5 inches or as approved by the Engineer. The cement content shall be not less than 564 lbs. nor more than 752 lbs. per cubic yard. The cement content of the mix design may be reduced by up to 20 percent and replaced with fly ash at a minimum rate of 1.2 pounds of fly ash for each pound of cement replaced. The maximum water-cement ratio shall be 0.450. All coarse aggregate used in prestressed concrete shall pass a 1 inch sieve.
The Contractor shall submit to the Engineer proposed concrete mix designs for each strength of concrete to be used in the work. Quantities of fine and coarse aggregates necessary to provide concrete meeting the requirements of the specifications shall be determined by the method described in ACI211, "Recommended Practice for Selecting Proportions for Normal Weight Concrete", using the absolute volume basis.
Mix designs, stated in terms of saturated surface dry weights, shall be submitted on M&T Form 312 at least 35 days prior to using the proposed mix. Batch proportions shall be adjusted to compensate for surface moisture contained in the aggregates at the time of batching. Changes in the saturated dry mix proportions will not be permitted unless revised mix designs have been submitted to the Engineer and have been determined to be acceptable for use.
Form 312 shall be accompanied by a listing of laboratory test results of aggregate gradation, air content, slump, and compressive strength. Compressive strength of at least three 6"x12" or 4"x8" cylinders shall be listed. The age of the cylinders at the time of testing and a detailed description of the curing procedure shall be shown. Laboratory tests shall be performed in accordance with the following test procedures:
Aggregate Gradation -- AASHTO T27
Air Content -- AASHTO T152
Slump -- AASHTO T119
Compressive Strength -- AASHTO T23 and T22
When the combination of materials is such that the required strength and/or a workable slump cannot be obtained at the minimum specified cement content with the maximum allowable water-cement ratio, the cement content shall be increased at no cost to the Department by whatever amount is required to produce the required strength and/or slump without exceeding the allowable water-cement ratio.
The Engineer will review the mix design only to ascertain general compliance with specification requirements. The Engineer will notify the Contractor, in writing, that the mix design is either acceptable or unacceptable. A mix shall not be used until the Contractor has been notified by the Engineer that the mix design is acceptable. Acceptance of the mix design shall not relieve the Contractor of his responsibility to furnish an end product meeting specification requirements. Upon request from the Contractor, a mix design accepted and used satisfactorily on any Department of Transportation project may be accepted for use on other projects.
(B) Air Entrainment:
Air entrainment shall be obtained by adding an approved air entraining agent at the time of mixing. The air content of the freshly mixed concrete when discharged into the forms shall be 5 percent plus or minus 2 percent.
(C) Testing:
All testing required by this subarticle shall be performed by a certified concrete technician employed by the Contractor in the presence of the plant inspector unless otherwise approved by the Engineer. Certification of technicians will be awarded upon satisfactory completion of examinations prepared and administered by the Division of Highways or other agency approved by the Engineer.
Before allowing placement of the first load in a bed, the air content shall be determined by a calibrated Chace indicator, AASHTO T199. during the placement of the first load, the air content shall be determined by AASHTO T152, T196, or T121. The air content in each subsequent 10 cubic yards shall be determined by the Chace indicator, AASHTO T199, prior to allowing placement. The air content shall be determined by AASHTO T152, T196, or T121 from all loads from which cylinders are made. If the air content as determined by the Chace indicator, AASHTO T199, fails to meet the specification requirements, a second test will be run on material from the same load and the results of the 2 tests averaged. If the average does not meet the specification requirements, a test on the same load will be conducted using AASHTO T152, T196, or T121. Acceptance or rejection of the load will be based on the results of this test.
Slump shall be determined in accordance with AASHTO T119.
For the purpose of testing for the required 28 day compressive strength and also for the required compressive strength for the transfer of load, the Contractor shall furnish, at no cost to the Department, a minimum of 6 concrete cylinders made from a sample of concrete placed near the live end of the bed and a minimum of 6 concrete cylinders made from a sample of concrete placed near the dead end of the bed. Cylinders shall be made in accordance with AASHTO T23, except that the cylinders shall be cured in the same manner as the members represented until the strands have been released. Cylinders shall be placed in clusters at random points along the casting bed. After the strands have been released, cylinders shall be air cured in an approved common area near the testing apparatus for the remainder of the 28 day curing period. Cylinders shall be tested in accordance with AASHTO T22. Approved apparatus for testing the transfer strength of the cylinders shall be provided by the Contractor. This apparatus shall be maintained to within 1.0 percent accuracy and shall be calibrated at intervals not to exceed 12 months by an approved testing company at no cost to the Department. The Engineer reserves the right to require verification immediately after a testing machine is relocated and whenever there is reason to doubt the accuracy of the indicated load, regardless of the time interval since the last verification.
If the average of 2 cylinders tested to determine compressive strength at the age of 28 days fails to indicate a compressive strength of at least 5,000 psi, or such compressive strength as is required by the plans or special provisions, such failure will be cause for rejection of the members represented.
(D) Temperature Requirements:
The concrete temperature at the time of placing in the forms shall be not less than 50F nor more than 95F.
No concrete shall be placed when the air temperature, measured at the location of the concreting operation in the shade away from artificial heat, is below 35F.
(E) Elapsed Time for Placing Concrete:
The elapsed time for placing concrete shall be in accordance with Subarticle 10004(E). The provisions of Subarticle 10004(E) pertaining to Class AA concrete shall also apply to prestressed concrete.
(F) Use of Set Retarding Admixtures:
By permission of the Engineer, the Contractor may use an approved set retarding admixture should he desire to take advantage of the extended time interval between adding mixing water and placing the concrete.
The quantity of set retarding admixture per 100 lbs. of cement shall be within the range recommended on the current list of approved set retarding admixtures issued by the Materials and Tests Unit.
(G) Use of Water Reducing Admixtures:
Water reducing admixtures shall be used in accordance with Subarticle 10004(G).
(H) Measuring Materials:
Materials shall be measured in accordance with Article 10008.
(I) Mixers and Agitators:
Mixers and agitators shall meet the requirements of Article 100010.
(J) Mixing and Delivery:
(1) General:
Concrete shall be mixed and delivered to the site of the work by means of one of the following methods, except where other methods are approved by the Engineer. The Engineer may approve the mixing of concrete by methods other than those listed below provided the proposed method is capable of satisfying job requirements and there is adequate evidence that the proposed method will produce concrete complying with specification requirements. The Contractor shall be responsible for controlling the materials and operations in such a manner as to produce uniform concrete meeting specification requirements.
1. Central Mixed Concrete:
Mixed completely in a stationary mixer and the mixed concrete transported to the point of delivery in a truck agitator or in a truck mixer operating at agitating speed or in non-agitating equipment approved by the Engineer. Mixing shall be done within the capacity and at the mixing speeds recommended by the equipment manufacturer.
2. Transit Mixed Concrete:
Mixed completely in a truck mixer while at the batching plant, in transit, or at the work site.
3. Shrink Mixed Concrete:
Mixed partially in a stationary mixer at a central mixing plant and completed as transit mixed concrete. All ingredients for a batch shall be in the stationary mixer and partially mixed before any concrete is discharged to the truck mixer, and the size of the batch shall not exceed the rated capacity of the equipment. The mixing time at the stationary mixer may be reduced to the minimum necessary to intermingle the ingredients, and the mixing completed in the truck mixer. The mixing revolutions in the truck mixer shall be as specified for transit mixed concrete or may be reduced as indicated by mixer performance tests of concrete as discharged from the truck mixer.
The Contractor shall have present during all batching operations a certified concrete technician employed by the Contractor, prestressed concrete producer, or concrete supplier while concrete is being batched and delivered to the site of the work. The sole duty of this employee shall be to have charge of and exercise close supervision of the production and control of the concrete. He shall perform moisture tests, adjust mix proportions of aggregates for free moisture, complete batch tickets (M & T Form 903) or approved delivery tickets, sign batch tickets or approved delivery tickets, and assure quality control of the batching operations. Delivery tickets will be permitted in lieu of batch tickets (M & T Form 903) provided they have been reviewed and approved by the Materials and Tests Unit. Certification of technicians will be awarded upon satisfactory completion of examinations prepared and administered by the Division of Highways or other agency approved by the Engineer.
(2) Mixing Time for Central Mixed Concrete:
The mixing time is defined as starting when all the solid materials are in the mixing compartment and ending when any part of the concrete begins to discharge. The ingredients shall be so charged into the mixer that some of the water will enter in advance of cement and aggregate, and substantially all the water shall be in the drum before one-third of the specified mixing time has elapsed. Transfer time in multiple drum mixers shall be counted as part of the mixing time.
The minimum mixing time may be established by:
1. Mixer performance tests as described in this subarticle,
2. The manufacturer of the equipment,
3. The requirement of 1 minute for mixers of 1 cubic yard capacity or less with an increase of 15 seconds for each cubic yard or fraction thereof in increased capacity.
The Engineer reserves the right to require a mixer performance test at any time. The minimum mixing time as determined by the mixer performance test will be that which will produce concrete meeting the requirements of Table 10781.
Sampling and testing for mixer performance tests shall be done as provided below. The mixer shall be charged to its rated capacity with the materials and proportions to be used in the work and mixed at the recommended mixing speed to the target time. Mixing shall then be stopped and discharging shall begin. Two samples of sufficient size to make the required tests will be taken after discharge of approximately 15 and 85 percent of the load by an appropriate method of sampling which will provide representative samples of the concrete.
Each of the 2 samples of concrete shall be separately tested for the properties listed in Table 10781. Tests shall be conducted in accordance with the standard methods shown in Table 10781 or procedures established by the Materials and Tests Unit.
The mixer performance test described above shall be performed on a minimum of 2 batches of concrete. For the performance test to be acceptable, all tests in each batch shall meet the requirements listed above.
The Engineer may recheck mixer performance at any time when in his judgment acceptable mixing is not being accomplished.
Where acceptable mixing cannot be accomplished in the established mixing time, the Engineer may increase the mixing time or require that the mixer be repaired or replaced before any further mixing can be done.
(3) Truck Mixers and Truck Agitators:
Truck mixers and truck agitators shall meet the requirements of Subarticle 100011(C).
(4) Delivery:
For central mixed concrete delivered in truck agitators or truck mixers, transit mixed concrete, or shrink mixed concrete, the Contractor shall use a ticket system for recording the transportation of batches from the proportioning plant to the site of the work. The tickets (M & T Form 903) or approved delivery tickets shall be filled out in accordance with the instructions issued by the Engineer. The tickets shall be issued to the truck operator at the proportioning plant for each load and shall be signed by the certified concrete technician, which will signify that the concrete in the truck has been inspected prior to departure. Each ticket shall show the time batching was completed and if transit mixed, the number of revolutions at mixing speed, if any, at the plant. The tickets shall be delivered to the inspector at the site of the work. For central mixed concrete delivered in non-agitating equipment, alternate methods of documenting batch proportions will be considered by the Engineer. Loads which do not arrive in satisfactory condition within the time limits specified shall not be used in the work.
(K) Read Mixed Concrete Plant:
Ready mixed concrete plants shall have been inspected and approved by the Department before they are used to produce concrete for the project. Plants shall meet all applicable requirements of the specifications. Plants approved by the Department will be placed on a list of approved plants which will be made available to the Contractor. All plants shall be subject to reinspection at intervals selected by the Engineer. Reapproval after each inspection shall be contingent on continuing compliance with the specifications.
| |
| Air content, percent by volume
of concrete (AASHTO T152) | |
| Slump, inches (AASHTO T119) | |
| Coarse aggregate content, portion
by weight of each sample retained on the No. 4 sieve, percent | |
| Weight per cubic foot (AASHTO T121) | |
| Average compressive strength at 7 days,
percent of average (AASHTO T22 and T23) |
* Tentative approval may be granted pending 7 day compressive strength tests.
Metal forms shall be used except where other materials have been approved by the Engineer. The forms shall be of adequate thickness, braced, stiffened, anchored, and aligned adequately to consistently produce members within the limits of dimensional tolerances. The forms shall be designed and aligned so they will not restrict longitudinal movement of the casting when the prestressing force is transferred. Corners and angles shall be chamfered or rounded. All joints in forms shall be smooth and tight enough to prevent leakage of mortar. Holes and slots in forms, pallets and bulkheads shall be plugged neatly to prevent leakage of mortar. The inside surfaces of forms shall be accessible for cleaning. After each use, the beds and forms shall be thoroughly cleaned. Prior to casting, the inside surfaces of the forms shall be free from rust, grease, or other foreign matter. Coatings used for release of members shall not be allowed to build up. Forms which do not present a smooth surface shall not be used.
Form release agents may be applied to the forms before or after stringing of strands. If applied prior to stringing, the release agent shall be of a type that dries to a degree so that it cannot contaminate any strand that comes in contact with it. If the release agent is applied after stringing, the greatest care shall be exercised and a sheet metal or similar type shield shall be provided for protection of the strands.
Devices for tensioning shall be adequate to produce and maintain the required tension in all strands until the concrete has reached the required transfer strength. All jacks shall be equipped with accurate and calibrated gages for registering jacking loads. Gages shall be calibrated with the jacks with which they are used. All jacks and gages shall be calibrated by an approved testing company at no cost to the Department at intervals not to exceed 12 months. During progress of the work, if gage readings and elongations indicate materially differing loads, recalibration may be required. Gages shall have a full load capacity of 1 1/2 to 2 times their normal working load. The loads to be gaged shall be not less than one-fourth or more than three-fourths of the total graduated capacity unless calibration data clearly establishes consistent accuracy over a wider range. Gages shall have indicating dials at least 6 inches in diameter and the gage pointers shall not fluctuate, preventing an accurate reading, but shall remain steady until the jacking load is released. All gages shall have an accuracy of reading within 2 percent. Means shall be provided for measuring the elongation of strands within 1/4 inch.
1078-7 PLACING STRANDS, TIES, AND REINFORCING STEEL.
Strands, ties, supports, reinforcing bars of the sizes shown on the plans and bearing plates shall be positioned in accordance with the detailed dimensions shown on the plans and effectively secured against displacement from their correct positions. The steel reinforcing shall be placed in final position after tensioning of the strands. All tie wires shall be bent to the inside of the member so that the ends are farther from the edge than the material being tied. Bottom strands shall be supported at spacings not to exceed 20 feet by supports meeting the requirements of Article 10704 or by other means approved by the Engineer.
Strands with kinks, bends, nicks, scale, excessive rust, or other defects will not be permitted. No more than one broken wire per casting bed will be permitted. Slight rusting will not be cause for rejection, provided it is not sufficient to cause visible pits. Precautions shall be taken to prevent contamination of strands and reinforcing steel. The strands and reinforcing steel shall be cleaned to an acceptable condition before concrete is poured. Concrete shall not be placed in the forms until the strand and reinforcement condition and arrangement has been inspected by the plant inspector.
Strand splices will only be permitted at the end of a reel and when using a single strand jack. The strand lengths to be spliced together shall have the same lay of wire to avoid unraveling and the splice shall be positioned so that it does not fall within a member. The ends of the strand lengths to be spliced shall not be torch cut. They shall be cut by shears, abrasive grinders, or other means approved by the Engineer. No more than one strand splice will be permitted on an individual strand and the reuse of strands will not be permitted.
Where bonding of strands is to be prevented, it shall be accomplished by encasing the strand in a tubular conduit capable of resisting the pressure exerted by the concrete. Slit conduit shall not be used. The conduit used shall be of high density polyethylene or polypropylene with a minimum wall thickness of 0.025 inch. The inside diameter of the conduit shall be of sufficient size to allow free movement of the encased strand but it shall not be greater than the diameter of the strand plus 1/8 inch. The conduit shall be secured so that longitudinal movement along the strand will be prevented, and bonding of the strand will be prevented at the location shown on the plans plus or minus 1 inch. Concrete shall be prevented from entering the conduit by taping. The tape shall be manufactured from a non-corrosive material compatible with the concrete, conduit, and steel.
Each strand shall be tensioned to the load shown on the plans prior to placing the concrete.
The load induced in the prestressing strand shall be measured both by jacking gages and strand elongations on at least the first 5 strands and every third strand thereafter on each pour. Loads on all other strands shall be measured by either jacking gages or strand elongations. When both methods of measurement are used, if a discrepancy between gage and elongation of more than 5 percent is apparent, the entire operation shall be carefully checked and the source of error determined before proceeding further. The computed elongation and jacking loads shall make appropriate allowances for load losses due to friction and all possible slippage or relaxation of the anchorage. References shall be established periodically at each strand anchorage to indicate any yielding or slippage that may occur between the time of initial tensioning and final release of the strands.
In determining the applied load by measuring the elongation of the strand, a modulus of elasticity taken from the typical stress-strain curve for the brand, size, and type of strand being tensioned shall be used. The Contractor shall submit stress-strain curve data for the actual heats of material used in the strands to the plant inspector before the strands are used. Each reel or strand shall be identified by tagging in accordance with AASHTO M203. A standard color marking every 100 ft. on the strands will be required. The standard color markings shall be white for 270K stress relieved strand, green for low relaxation strand, and a double marking of green and red for special low relaxation strand.
Strands may be tensioned in a group or individually. Prior to being given its full tensioning, each strand shall be brought to an initial tension of 2,000 lbs. for all beds under 150 ft. in length, 3,000 lbs. for all beds 150 to 300 ft. in length, and 4,000 lbs. for all beds longer than 300 ft. in length. This initial tension shall be measured by a calibrated gauge or other approved means, and the elongation due to initial tensioning shall then be computed. The difference between the required final tension and the initial tension shall be used to compute the expected additional elongation.
After initial tensioning, the strands shall be tensioned until the required elongation and jacking load are attained and reconciled within the limits specified above. A permanent record shall be kept of the initial jacking load, the final jacking load, and the elongation produced thereby.
In single strand tensioning, rotation of the jacking ram will not be allowed.
When draped strands are to be used, the Contractor shall submit to the Engineer for his approval six (6) sets of the bed layout showing the method of draping and tensioning the draped strands and also calculations determining the loads required for tensioning the draped strands. The strands for all members to be cast in any one tensioning operation shall be draped before any beam is cast. End templates or bulkheads at ends of beams shall remain vertical or as otherwise shown on the plans. Draping for all members shall be done either simultaneously or in single or incremental lifts beginning at the center of the bed and working outward toward each end of the bed. Complete tensioning in the fully draped position will not be allowed unless approved in writing by the Engineer.
All devices used for deflecting the draped strands shall be round steel rollers of a type and dimensions approved by the Engineer. The part in contact with the strand shall be rounded to a diameter of not less than 3/4 of an inch. Support and hold-down devices shall be of sufficient rigidity and have adequate support so that the final position of the strands will be as shown on the plans.
With strands tensioned in accordance with the above requirements and with other reinforcement in place, the concrete members shall be cast so as to achieve the required lengths. Strand load shall be maintained between anchorages until the concrete has reached the required compressive strength for transfer of load from the anchorages to the members.
For personnel engaged in the tensioning operation, protection shall be provided by means of effective shields adequate to stop a flying strand. These shields shall be provided at both ends of the bed and shall be of steel, reinforced concrete, heavy timbers, and other material approved by the Engineer.
The procedures and equipment for handling, placing, and consolidating the concrete shall be such that a uniformly dense and high grade concrete is obtained in all parts of the member under all working and weather conditions. Concrete shall not be mixed, handled, delivered, placed, or finished using devices made of aluminum or containing aluminum.
Concrete for girders 54" or less in height, and concrete for all cored slabs, shall be placed in 2 or more equal horizontal layers. Concrete for girders over 54" height shall be placed in 3 horizontal layers. When placing concrete in 3 layers, the top of the first layer shall be located approximately at the top of the bottom flange and the top of the second layer shall be located approximately at the top of the web. Each layer shall be placed and compacted before the preceding layer has taken initial set so that there is no surface of separation between layers. Should shrinkage or settlement cracks occur, the Engineer reserves the right to require additional layers and/or vibration.
Internal or a combination of internal and external vibration will be required as is necessary to produce uniformly dense concrete without honeycomb.
Placing concrete in cold weather shall be in accordance with the provisions of Article 4209.
Concrete shall be placed in daylight unless an adequate lighting system meeting the approval of the Engineer is provided.
The temperature of the freshly mixed concrete when placed in the forms shall not exceed 95F.
The tops of girders shall be rough floated. The top surface of cored slabs shall be given a broom finish. Prestressed concrete members which are to be made composite with subsequently placed concrete shall be finished with a roughened surface for bonding. No laitance shall remain on the surfaces to be bonded.
(A) General:
Concrete shall be cured by steam curing, radiant heat curing or water curing, as set forth below. In addition concrete for prestressed piles may be cured with membrane curing compound as set forth below. The method or methods used shall prevent the concrete from losing moisture at any time before curing is completed. The methods used shall not deface or injure the concrete. Curing procedures shall be such as to prevent cracks occurring in the members.
The curing period shall continue at least until the concrete has reached sufficient strength to permit transfer of load from the anchorage to the members.
(B) Curing at Elevated Temperatures:
Radiant heat curing shall be done under a suitable enclosure which will contain the heat and prevent moisture loss. Moisture may be applied by a cover of moist burlap, cotton matting, or similar approved material. Moisture may be retained by covering the member with an approved waterproof sheeting in combination with an insulating cover. The cover shall be supported at a sufficient distance above the member being cured to allow circulation of the heat.
Steam curing enclosures shall be essentially free of steam leakage in order to minimize moisture and heat losses. The enclosure shall not be in contact with the members or forms for the members. Steam jets shall not be directed on the forms so as to cause localized high temperatures.
After placing and vibrating, the concrete shall attain its initial set prior to the application of heat or steam. The concrete shall be considered to have obtained its initial set when it has a penetration resistance of at least 500 psi when tested in accordance with AASHTO T197. The sample of concrete to be tested for penetration resistance shall be taken from the last load to be cast in the bed. The sample of concrete shall be stored with the precast member and maintained in the same condition and environment as the member except for the periods of time necessary to prepare the test specimen and to perform the penetration resistance test. The penetration resistance test shall be performed by the Contractor.
The Contractor may submit data indicating that an approved concrete mix attains its initial set after some particular time period. Different periods may be established for different weather conditions. If such data is submitted, consideration will be given to permitting heat or steam to be introduced after the time indicated by such data in lieu of having to perform the penetration resistance test. Consideration will also be given to determining the time of initial set by methods other than AASHTO T197 provided the Contractor has submitted data supporting such other methods.
When the ambient air temperature is below 50°F, the forms shall be covered after the placement of concrete and sufficient heat shall be applied to maintain the temperature of the air surrounding the unit between 50° and 70°F.
When the ambient air temperature is above 70°F, a water cure as set forth below or other approved method shall be initiated as soon as the concrete is able to receive the water without physical damage to its surface. The cure may be discontinued upon introduction of steam, provided that a relative humidity of 100 percent is maintained.
Curing at elevated temperatures shall be done at a temperature of not more than 160 degrees F. Steam curing shall be done at a temperature of at least 130 degrees F.
The rate of increase of the ambient temperature within the curing enclosure shall be relatively uniform at a rate of approximately 40 degrees F per hour, not to exceed 15 degrees F per 15 minutes. The temperature increase shall be relatively uniform throughout the length and on both sides and top of the concrete unit. Recording thermometers shall be placed within 50 feet of each end of the bed and at points not to exceed 100 feet between the end thermometers. There shall be a minimum of 2 thermometers for bed lengths of 100 feet or less. Recording thermometers shall be calibrated at intervals not to exceed 6 months. The temperature differential within the curing enclosure shall not be more than 15 degrees F. Complete temperature records for all cures shall be submitted to the Engineer before final approval of the members.
Steam curing shall continue until the concrete has reached the required transfer strength.
Immediately after the steam curing or heat curing has been discontinued, the forms shall be released and the members shall be detensioned.
(C) Water Curing
The concrete shall be kept continuously wet by the application of water as soon as it can be applied without damage to the concrete surface, and before the concrete obtains an initial set of 500 psi. The water shall be applied using soaker hoses and wet burlap or other approved means for the full length of each member. Water shall be applied evenly along the entire length of the bed.
When the ambient air temperature is below 50 degrees F the forms shall be covered after the placement of the concrete and sufficient heat shall be applied in an approved manner to maintain the temperature of the air surrounding the member between 50 degrees F and 70 degrees F. After the concrete has obtained an initial set of 500 psi, the air temperature surrounding the member may be increased to 100 degrees F while continually maintaining moisture on the surface of the concrete. Whenever heat is applied to the member, temperature recording clocks shall be placed on the bed as required when curing at elevated temperatures, and the requirements for rate of temperature increase shall also apply.
The application of heat (if used) and water shall be maintained until the concrete has obtained release strength. The forms shall be released and the members shall be detensioned immediately after the curing has been discontinued.
(D) Curing with Membrane Curing Compound.
Prestressed concrete piles may be cured with a membrane curing compound. The entire surface of the concrete shall be sprayed uniformly with a wax-free, resin-base curing compound conforming to the requirements of Article 10262. Clear curing compound to which a fugitive dye has been added for color contrast shall be used.
The membrane curing compound shall be applied after the surface finishing has been completed, and immediately after the free surface moisture has disappeared. In the event the application of curing compound is delayed, another curing method shall be started immediately and shall be continued until the application of the curing compound is started or resumed or until the concrete has reached the required detensioning strength.
The surface shall be sealed with a single uniform coating of the specified type of curing compound applied at the rate of coverage recommended by the manufacturer or as directed by the Engineer, but not less than 1 gallon per 150 square feet of area.
At the time of use, the compound shall be in a thoroughly mixed condition with the pigment uniformly dispersed throughout the vehicle. If the application of the compound does not result in satisfactory coverage, the method shall be stopped and water curing, as set out above, shall be applied until the cause of the defective work is corrected.
At locations where the coating shows discontinuities, pinholes, or other defects, or if rain falls on the newly coated surface before the film has dried sufficiently to resist damage, an additional coat of the compound shall be applied immediately after the rain has stopped at the same rate specified herein.
When the ambient air temperature is below 50 degrees F, the forms shall be covered after the application of the curing compound and sufficient heat shall be applied in an approved manner to maintain the temperature of the air surrounding the member between 50 degrees F and 70 degrees F. Whenever heat is applied to the members, recording thermometers shall be placed on the bed as required when curing at elevated temperatures. The requirements for rate of temperature increase shall also apply.
Any curing compound adhering to a surface to which new concrete is to be bonded shall be completely removed by sandblasting, steel wirebrushes, bush hammers, or other approved means.
The concrete surfaces to which the compound has been applied shall be protected from abrasion or other damage which results in perforation of the membrane film until the concrete has achieved design strength and the members are detensioned. The forms shall not be released until immediately before the members are detensioned.
Transfer of load from the anchorages to the members shall be done when the concrete has reached the required compressive strength shown on the plans.
For any particular group of members cast in the same bed, the load shall not be transferred to any concrete until the test cylinder breaks indicate that the concrete in all these members has reached the required strength. Two cylinders, one from each end of the bed, shall be tested for the purpose of determining whether the concrete has reached the required strength for transfer of load. The average of the strength tests on the two cylinders shall meet or exceed the required strength and the lowest cylinder shall not be more than 200 psi less than the required strength. If both these conditions are not met, transfer of the prestressing load to the concrete shall be delayed until tests of additional cylinders show that the required strength has been reached.
When curing has been done at elevated temperatures, the procedures for transferring prestressing load shall begin immediately after curing has been discontinued and the forms have been released, and while the concrete is still hot in order to prevent cooling shrinkage and cracking. If so directed by the Engineer, members shall be covered or otherwise protected so as to cool the concrete slowly after release in order to prevent thermal shock and the evaporation of moisture in the members.
Transfer of load procedures shall be such as to not cause cracks in members. Transfer of load shall be by gradual release of the strands as a group, by gradual release of part of the group, or by burning of the fully tensioned strands at the ends of the members. If strands are to be released by a method other than gradual release of the entire group, six (6) copies of the proposed method and pattern of release, if not so shown on the plans, shall be submitted by the Contractor to the Engineer for his approval. The approved method and pattern of release shall be rigidly followed. When the fully tensioned strands are being burned, each strand or group of strands shall be burned simultaneously at each end of the bed in its indicated order in the pattern and at each end of each member before proceeding to the strands in the next group in the pattern at any point.
When detensioning all girders, cored slabs, and piles, strands shall not be burned quickly but shall be heated with a low oxygen flame played along the strand for a minimum of 5 inches until the metal gradually loses its strength. Heat shall be applied at such a rate that failure of the first wire in each strand shall not occur until at least 5 seconds after heat is first applied. When detensioning other members, the above procedure shall be followed unless an alternate procedure has been approved by the Engineer. Detensioning by arc welder will not be allowed.
The method for single strand detensioning of members having draped strands shall incorporate the following:
1. The pair of straight strands located in the uppermost position in the lower flange shall be released first.
2. Then the tension in the draped strands at the ends and uplift points shall be released in accordance with an approved pattern.
3. All hold-down devices for draped strands shall be disengaged and the hold-downs released.
4. Then the pair of straight strands located in the upper flange shall be released.
5. The remaining straight strands of the pattern shall be released in accordance with an approved sequence.
All strand releases shall be in a manner as to cause a minimum shock and lateral eccentricity of loading and shall meet the Engineer's approval.
Failure to follow the above procedures for transfer of load will be grounds for rejection of the members involved.
1078-12 PRESTRESSED CONCRETE GIRDER WEB SPLITTING
Upon detensioning of certain girders with draped strands, cracks may occur in the webs at the ends of the girders. If such cracks occur, the contractor shall employ a method to remedy this condition on all subsequent girders of the same type and strand pattern. If debonding of strands is used, the following criteria shall be satisfied:
1. Alternate rows of draped strands shall be debonded 2 feet from each end of the girder.
2. The two straight strands in the top of the girder shall not be debonded. One half of the straight strands, as nearly as possible, in the bottom flange shall be debonded. As nearly as possible one quarter of the straight strands in bottom of girder shall be debonded 4 feet from each end of the girder and one quarter of the straight strands shall be debonded 2 feet from each end of the girder.
3. The debonding pattern shall be symmetrical about the vertical axis of the girder for both draped and straight strands.
4. Strands shall be debonded so that the centers of gravity of the draped strands and the straight strands in the bottom of the girder will remain within 1 inch of their original location at the end of the girder.
5. Strands shall be debonded by encasing the strand in a conduit. The conduit shall meet the approval of the Engineer.
No separate payment will be made for debonding strands. Payment shall be included in the contract unit price bid for prestressed concrete girders.
1078-13 HANDLING, TRANSPORTING, AND STORING.
Members damaged while being handled or transported will be rejected or shall be repaired in a manner approved by the Engineer. All members may be handled immediately after transfer of load from the anchorages to the members has been completed.
All prestressed members shall be stored on solid, unyielding, storage blocks in a manner to prevent torsion or objectionable bending. In handling girders and cored slabs, they shall be maintained in an upright position at all times and shall be picked up within 3 feet of the points of bearing and transported and stored so as to be supported only within 3 feet of points of bearing.
In handling, transporting, and storing prestressed members, the number and location of supports shall be in accordance with the plan requirements for the sizes, lengths, and types of members involved, or as approved by the Engineer.
Members shall not be transported away from the casting yard until the concrete has reached the minimum required 28 day compressive strength and a period of at least 5 days has elapsed since casting, unless otherwise permitted by the Engineer.
No member shall be transported from the plant to the job site prior to approval of that member by the plant inspector. This approval will be stamped on the member by the plant inspector.
The diagonal face of the bottom flange of prestressed concrete girders and the outside face of exterior cored slabs shall have all voids filled with a sand-cement or other approved grout. All voids in piles greater than 1/2 inch in diameter or depth shall be filled as above. The resulting surface finish shall have essentially the same color and surface finish as the surrounding concrete. Voids greater than 1/4 inch in diameter or depth in other faces of these and other members except piles shall be repaired in a like manner. Where an excessive number of smaller voids exist in any member, the Engineer may require a similar repair.
Honeycomb, excessively large fins, and other projections shall be repaired as directed by the Engineer. Members with honeycomb, cracks, or spalls shall not be repaired until inspected by the Engineer. Any appreciable impairment of structural adequacy shall be cause for rejection.
Holes caused by strand hold downs shall be cleaned and filled upon removal from the casting bed. All patches shall be of materials approved by the Engineer and shall have developed strength at least equal to the minimum 28 day strength requirement for the concrete prior to approval of the member. Members shall be clean and surfaces shall have a uniform appearance.
Reinforcing bars exposed on the tops of girders and exterior cored slabs shall be free of mortar build up and excessive rust.
1078-15 ALIGNMENT AND DIMENSIONAL TOLERANCES.
(A) Cored Slabs:
Cored slabs shall be manufactured within the tolerances indicated in Table 10782.
| Depth (A) | |
| Width (B) | |
| Length (C) | |
| Position of voids - Vertical (D) | |
| Position of voids - Horizontal (E) | |
| Position of void Ends - Longitudinal | |
| Square ends - Deviation from square (horizontal)
or vertical) or designated skew (F) | |
| Horizontal alignment - Deviation from a straight
line parallel to the centerline of member (G) |
|
| Camber - Differential between adjacent units (H) | |
| Camber - Differential between high and low
members of same span (H) | |
| Position of dowel holes - Deviation from plan
position (I) | |
| Width - Any one span | |
| Width - Differential of adjacent spans in the
same structure | |
| Bearing area - Deviation from plane surface | |
| Local smoothness (J) | |
Position of holes for transverse strands |
|
| Position of strands (M) |
NOTE: Dimensions followed by an alphabetical suffix are shown in Figure 1078-1
(B) Girders:
Girders shall be manufactured within the tolerances indicated in Table 10783.
(C) Piles:
Piles shall be manufactured within the tolerances indicated in Table 10784.
(D) Sheet Piles:
Sheet piles shall be manufactured within tolerances indicated in Table 1078-5.
1078-16 IDENTIFICATION OF MEMBERS.
Each prestressed member shall be permanently identified by the Contractor by number and date of manufacture, and this information shall also be painted, or otherwise marked as approved by the Engineer, on at least one end of the member as soon as practical after manufacture. In the case of girders or cored slabs, other identification as to station, span, and position within the span shall also be painted on at least one end of the member.
A daily quality control record form shall be maintained by the Contractor. This form shall have been approved by the Engineer and shall include pertinent information concerning tensioning, concrete quality and placement, curing, and detensioning. This form shall be signed and dated by a certified concrete technician. A copy of the completed or up-to-date form shall be furnished to the Materials and Tests Unit upon request and also before any members are approved. A sample form, indicating the minimum required information, may be obtained from the Materials and Tests Unit.
| Length (A) |
|
| Width - Flanges(B) | |
| Width - Web (C) | |
| Depth - Overall (D) | |
| Depth - Flanges (E) | |
| Horizontal alignment (top or bottom flange) -
Deviation from a straight line parallel to the centerline of beam (F) |
|
| Bearing plate - Deviation from plane surface | |
| Beam ends - Deviation from square or
designated skew (G and H) |
|
| Position of stirrups - Projection
above top of beam (I) | |
| Local smoothness of any surface (J) | |
| Position of holes for tie rods (K) | |
| Position of strands (L) |
NOTE: Dimensions followed by an alphabetical suffix are shown in Figure 1078-2
| Width (A) | |
| Length (B) | |
| Horizontal alignment - Deviation from a straight
line parallel to the centerline of pile (C) | |
| Squareness of ends (D) |
|
| Local smoothness (E) | |
| Position of strands (F) |
NOTE: Dimensions followed by an alphabetical suffix are shown in Figure 1078-3
| Length(A) | |
| Width(B) | |
| Depth (C) | |
| Position of Tendons (D) | |
| Position of Voids (E) | |
| Longitudinal | |
| Transverse | |
| Vertical | |
| Wall Thickness (F) | |
| Longitudinal Spacing of Sirrups (G) | |
| Position of Handling Devices (H) | |
| Longitudinal | |
| Transverse | |
| Vertical | |
| Variation from Specified End Squareness
or Skew (I) |
|
| Sweep (J) | |
| Position of Blockouts (K) | |
| Local Smoothness (L) |