715 - PRESTRESSED CONCRETE MEMBERS
700-79 Notify the Engineer a minimum of 2 business days for in-state production and 5 business days for out-of-
state production, in advance of the date when casting is to begin to afford an opportunity for inspection of the casting beds and forms, the equipment for placement and tensioning of the strands, the equipment for proportioning, mixing, placing and consolidating the concrete, and the equipment for handling the units.
b.Equipment.
1.Condition. Repair or replace unsafe or inadequately operating equipment.
2.Concrete Mixers. Mix concrete in truck mixers or in a central mixing plant that comply with SECTION 154 . When concrete is mixed in a central mixing plant and can not be adequately mixed according to SECTION 401 , conduct tests, as required by the Engineer, to determine the minimum mixing time to achieve uniformity of the concrete mixture. For air-entrained concrete, the maxi mum mixing time may not exceed the mixing time established from uniformity tests by more than 60 seconds.
3.Casting Beds. Portable casting beds are prohibited. The supporting foundations for casting beds must be such that no settlement will occur during the casting and curing of the units.
4.Forms. Use forms that are true to line, mortar tight and provide access for placement of the reinforcement and concrete.
5.Stressing Jacks. For stress application, use jacks equipped with hydraulic gauges or other approved stress measuring devices as a check against the applied load as measured by elongation. Recalibrate gauges when directed by the Engineer.
6.Curing Enclosures. Use stea m curing enclosures reasonably fr ee from steam leakage and providing adequate circulation of steam. Arrange steam jets so th at the steam shall not play directly on the forms or the concrete as it enters the enclosure. If non-waterproof tarpaulins are used for the enclosure, use a minimum of 2 layers. Use only waterproof tarpaulins or plastic sheeting for enclosures when prestressed concrete units are cured by radiant heat. (7) Instruments. When accelerat ed curing is used, install instruments during the curing period for measuring and recording temperature and humidity inside curing enclosures and for measuring and recording temperature in the concrete for each 200 feet of casting be d. Provide a minimum of 2 instrument installations of each type for each enclosure. The locatio n of each instrument must be approve d by the Engineer. Humidity level indicators may be waived by the Engineer.
c.Manufacturing Requirements.
1.General. Except as modified by the Contract Documents or approved by the Engineer, follow the latest edition of the Prestressed Concrete Institute’s, "Manual fo r Quality Control for Plants and Production of Precast and Prestressed Concrete Products".
2.Reinforcement. Install reinforcing bars as shown in the Contract Documents, and rigidly secure them to prevent movement during placement of the concrete. Welding of reinforcing steel bars is prohibited. Substituting deformed welded steel wire fabric in prestressed beams for reinforcing bars is acceptable, provided the spacing of the wires is less than or equal to the spacing shown in the plans, and the area of steel per foot is equivalent or greater than the reinforcing bars s hown in the plans. Use the ta ble shown in the plans for area of steel equivalences. Higher yield strength welded st eel wire fabric is allowed, but will not change the equivalences. Post production welding of wire fabric is prohibited. Accurately position steel strand within the tolerances sp ecified in the Contract Do cuments. Rigidly secure the strand so it shall be retained in the specified locations. The minimum horizontal spacing, center to center of strands at the ends, is 2 inches. Install supports to pr event dead load sag. The roller size on the holddown device must match the strand size used. Provide strand supports as shown in the Co ntract Documents that consist of a device with freely turning rollers a minimum of ⅞ inch in diameter at each defl ection point. A yoke type device may be used for top depressing of strands when approved by the Engineer. The prestress force and center of gravity must be as shown in the Contract Documents. Perform tensioning and elongation according to the Contract Documents. No tensioning of strands or placement of concrete will be permitted when the ambient air temperature is below 20ºF. At the option of the Engineer, strand shall be brought to within 25 F of the concrete at placement in lie u of corrections in elongation due to temperature. The Engineer may use suitable equipment to determine if the strand tension is proper throughout the entire bed length. Make corrections as required. 715 - PRESTRESSED CONCRETE MEMBERS
700-80 (3) Concrete. Handle and place the concrete by met hods that shall produce a dense, uniform product, free
from sand streaks and honeycomb areas. The presence of any deleterious substance such as “bag paper” is cause for rejection. Deliver concrete to the producer’s site of the work and comp lete placement within the time limits specified in SECTION 401 . Deliver and place successive batches at a c onstant rate and before the preceding batch has been perceptibly hardened or dried, or is no longer pliable, and in no case may the interval between successive batches in a unit exceed 20 minutes, or if the concrete mix or environmental conditions require otherwise, a period established by the Engineer. Do not add water to temper deposited concrete. Place concrete during co ld weather according to SECTION 401 . In addition, the form temperature shall be a minimum of 40ºF before the concrete may be placed. When necessary, continue heating the forms during the placement and finishing of the concrete. Provide concrete units meeting the tolerances specified in TABLE 715-1 , unless shown otherwise in the Contract Documents.
4.Surface Finish. Make all surfaces of the units reason ably straight and true to lines and grades, and free from fins or other proj ections. Form joint marks will be permitted. Gi ve top surfaces of beams a wire brush or stiff broom finish applied transverse to the length of the beam. Rake the top surf aces of the panels, perpendicular to the prestressing strand, making depressions of approximately ¼ inch. Do not pull out the co arse aggregate when raking.
5.Cracks. Produce concrete units free from cracks of sufficient width to impair the unit’s strength and durability. (6) Curing. Curing may be accomplished by either th e moist curing method or accelerated curing with low pressure steam or radiant heat.
a.Moist Curing Method (Normal Curing Temperatures). As soon as possible after the units have been cast, cover them with a minimum of 2 layers of burlap and keep wet until the side forms are removed. After the side forms are removed, protect the units with wet burlap or a vapor proof cover until they have attained the strength requirement for release shown in the Contract Documents. If repairs to the concrete surface are required or to give the units a su rface finish, remove the protective covering and complete the surface work, but keep the surfaces of the unit moist during the entire time that the protective covering is removed.
b.Accelerated Curing with Low Pressure Steam or Radiant Heat. Perform low pressure steam curing or radiant heat curing under an enclosure to contain the live steam or the heat. Allow the initial set of the concrete to take place by delaying the initial application of steam or heat from 2 to 4 hours after the final placement of concrete. If retarders are used, increase the waiting period before application of the steam or radiant heat to 4 to 6 hours. If the time of initial set is determined by the standard method of test for "Time of Setting of Concrete Mixtures by Penetration Resistance", ASTM C 403, the time limits described above may be waived. Do not apply live steam directly on the concre te forms causing locali zed high temperatures. Radiant heat may be applied by means of pipes circulating steam, hot oil or hot water, or by electric heating elements. Perfor m radiant heat curing under a suitable waterproof enclosure to contain both heat and moisture. Minimize moisture loss by covering all exposed concrete surfaces with 2 layers of wet burlap. While waiting for the initial set, the minimum temperature within the curing chamber shall be 50ºF. During this time, live steam or radiant heat may only be used to maintain the curing chamber at the minimum temperature. During the initial application of live steam or radiant heat, increase the ambient air temperature within the curing enclosure at a maximum average rate of 40ºF per hour. The maximum curing temperature within the enclosure is 160ºF, while the maximum internal concrete temperature shall be limited to 180 F. Hold the maximum temperature until the concrete has reached the desired strength. Immediately after the steam or heat curing has been discontinued, accomplish release. Additional curing is not required after release. Do not allow the temperature of the concrete to drop below 50ºF at any time. Provide recording thermometers showing the time-temperature relationship throughout the curing period from placing concrete to transfer of prestress. All temperature records will be retained by the Engineer as part of the curing records. Missing or incomplete time-temperature records shall be cause for rejection of the corresponding prestressed units. Temperature limits and use of recording thermo meters are the same wh en curing with steam or radiant heat. Due to the slow rise of ambient temperatures with radiant heat, app lication of heat
Source: Kansas Standard Specifications for State Road and Bridge Construction, 2015 Edition. Pages 410–411 of 978.