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 715 - PRESTRESSED CONCRETE MEMBERS
700-81 cycles may be accelerated to meet climatic co nditions, however, do not increase the ambient air
temperature within the curing enclosure by more than 40ºF per hour. In all cases, the curing procedure to be used must be well established and carefully controlled.
7.Releasing Prestressed Units with D r a p e d S t r a n d s . U n i t s m a y b e d e - t e n s i o n e d a s s o o n a s t h e y h a v e attained the strength requirements shown in Contract Documents. If the units have been cured by accelerated curing methods, transfer the stressing force to th e concrete as soon as the release stre ngth of the concrete has been reached, and while the concrete is still warm.
d.Inspection and Testing.
1.General. Provide the Engineer free access to the manufacturing plant at all times for inspecting materials, plant facilities, manufacturing and curing proc edures. Inform the Engineer of the planned concrete placement and curing schedule in advanc e of the start of any work. The Engi neer will require time for testing of materials, inspection of equipment and re viewing of procedures that will be us ed in casting units, prior to beginning casting.
2.Testing Equipment.
a.Cylinder Molds. Provide an ample supply of cylinder molds for the casting of test cylinders. All molds are subject to approval of the Engineer. Use 6" x 12" cylinders. The Engineer may approve the use of 4" x 8" cylinders, provided reliable correction factors have been developed and submitted, along with supporting data, for review and acceptance.
b.Compression Machine. Provide a machine capa ble of measuring the compressive strengths of concrete cylinders cast during the manufacturing of the units. All testing machines must be calibrated and approved by the Engineer.
3.Test Cylinders.
a.Casting and Curing. All test cylinders are the responsibility of the Contractor. With the exception of 28 day cylinders, cure all cylinders under the same conditions (environment) as the concrete they represent. Initially store then cure 28 day cylinders as per KT-22. For the testing purposes described below, the “total volume of concrete placed” is defined as each 40 cubic yards of concrete or fraction thereof placed in each line, within each curing enclosure, between bulkheads, during a continuous working period. Make 1 group of 3 or more cylinders for each third of the tota l volume of concrete to be placed. Note the limits of the concrete in the beds represented by each of the groups of cylinders. Mark and identify all cylinders groups as 1, 2 and 3 with marked group 1 representing the first third of the total volume of concrete placed, marked group 2 representing the second third of the total concrete placed, and marked group 3 representin g the final third of the total concrete placed. To facilitate the testing of multiple sets of cylinders for release or shipping, each group may contain more than 3 cylinders. Mark cylinders within a group as xA, xB, xC, xD, xE where “x” is the group number (always 1, 2, or 3) and A through E are the unique cylinder identifying marks within the group. Letters D and E, etc., are optional additional cylinders which allow more than one set of test cylinders for release or shipping. Identify which cylinders will be used for 28 day testing (i.e. 1E, 2E, 3E). Store and cure these three cylinders as per KT-22. Follow the procedure of making sets of cylinders from early, middle and late placement, during normal production operations. When operations are interrupted or changed (i.e. equipment break-down, very small placements , etc.), adjust the cylinder fa brication schedule to match the production, and provide adequate cylinders for later release and shipping strength testing. Coordinate this revised schedule with the Engineer on the production site.
b.Testing. With the Engineer observing, te st cylinders to measure the release and shipping strength at the producer’s plant. (i) Release Strength. Test 3 cylinders, 1 cylinder from each of the 3 groups of cylinders (i.e. 1A, 2A, 3A) prior to strand release to determine if the specified (design) release strength shown in the Contract Documents has been reached. Release stre ngth is attained when the average compressive strength of the 3 tested cylinders equals or exceeds the design rel ease strength and no more than 1 cylinder in the tested set has a compressive strength which is below the design release strength by no more than 100 psi. If the above conditions are not met, a minimum of 1 hour must pass before a second cylinder from each of the same 3 groups is tested (i.e. 1B, 2B, 3B). (ii) Shipping Strength. Before reaching 28 days of age, th e Contractor, may test 3 cylinders, 1 cylinder from each of 3 groups of cylinders (i.e. 1C , 2C, 3C) to determine if the specified 28 day 715 - PRESTRESSED CONCRETE MEMBERS
700-82 strength shown in the Contract Documents has been reached. Shipping stre ngth is attained when
all 3 tested cylinders meet or exceed the specified minimum 28 day strength. If this requirement is met, the products represented by these cylinders are accepted for strength requirements and may be shipped 1 day (approximately 24 hours) after meeting the compressive strength requirement and 5 days (approximately 120 hours) after concrete placement, whichever is greater. If the above condition is not met, a minimum of 2 hours must pass before a se cond cylinder from each of the same 3 groups is tested (i.e. 1D, 2D, 3D). If a cylinder from each of the 3 groups is not available for testing, then early shipping will not be allo wed. Cylinders earmarked for 28 day testing cannot be tested for shipping. (iii) 28 Day Strength. A previously identified se t of 3 cylinders, 1 cylinder from each group of cylinders (i.e. 1E, 2E, 3E) shall be stored and moist cured as per KT-22 and then tested when the concrete has reached an actual age of 28 days. Tes ting will take place at a location chosen by the Engineer while being observed by the Engineer. When the early shipping requirements described above have not been met, the average strength of these 3 cylinders must meet or exceed the specified minimum 28 day strength. In addition, only 1 of the cylinders in the tested set may be below the minimum specified 28 day strength by no more than 5%. The average strength of 1 set of 3 cylinders may be less than the specified minimum 28 day strength by no more than 5% or 300 psi, whichever is less, provided that the previous 9 consecutive sets of 28 day cylinders manufactured for the same KDOT project and using the same mix design complied with the 28 day strength criteria described above. (iv) Coring. When either (but not both) of the following occur: the 28 day strength of an individual cylinder is less than the 5% criteria described above or a second of any 10 consecutive manufactured cy linder sets attains an average compressive strength below the specified minimum 28 day strength by no more than 5% or 300 psi, whichever is less. The Contractor may, with the approval of the Engineer, core the unit (or units) represented by such cylinder (or cylinders) and have them tested. The location of the cores must be approved by the Engineer. Follow AASHTO T 24 when obtaining, preparing, testing and calculating the strength of drilled cores. If the adjusted compressive strengths of any of the cores are below the specified minimum 28 day compressive strength, the represented units will be rejected. Coring is not allowed on product represented by more than 1 out of any 10 consecutively manufactured cylinder sets, regardless of reason, and can only take place after the concrete has reached an age of 28 days.
e.Handling, Storage and Transportation.
1.Handling. Do not lift or strain the units in any way before the stress application strength has developed. While lifting and handling, support the units only at points designated in the Contract Documents.
2.Storage. When units are stacked for storag e, support each unit at designated bearing points.
3.Transportation. The units may be shipped 1 day (approximately 24 hours) after test cylinders have reached the specified 28 day compre ssive strength, and the units have attained a minimum age of 5 days (approximately 120 hours), whichever is greater. Support beams in an upright position. The required points of support and direction of reactions with respect to the beam are approximately the same during transportation as when the beam is in its final position in the structure. Transport piling and panels with the points of support approximately below the lifting points designated in the Contract Documents. If during transportation, units are supported at points so that a portion of the unit is cantilevered past the points designated above, the unit must be adequately reinforced or the overhanging portion adequately supported to prevent damage.
4.Damage. Units damaged in shipment or pl acement may be accepted provided the damage does not impair the structural qualities of the unit, and such damage can be repaired at the work s ite to the satisfaction of the Engineer.
f.Field Construction. Do not place concrete on prestressed bri dge beam units until they have reached a minimum age of 28 days, or as noted in the Contract Documents. Install bridge beams as shown in the Contract Documents. 715 - PRESTRESSED CONCRETE MEMBERS
700-83 g. Piling . Do not place piling before the specified 28 day strength has been attained. See SECTION 704
for placing piling. 715.4 MEASUREMENT AND PAYMENT The Engineer will measure the length of pr estressed concrete beams by the linear foot. The Engineer will measure the area of prestressed concrete panels by the square foot. Payment for "Prestressed Concrete Beams" and "Prestre ssed Concrete Panels" at the contract unit prices is full compensation for the specified work.
Source: Kansas Standard Specifications for State Road and Bridge Construction, 2015 Edition. Pages 411–414 of 978.