5-173 Measure and record the concrete cover at each void location aft er void hold-downs have been removed. The QAI will indicate the number and location of the measurements.
535.15 Acceptance and Quality Control Testing of Concrete Acce ptance of structural
precast/prestressed units, for each day’s production, will be d etermined by the Department, based on compliance with this specification and satisfactory co ncrete testing results. At least once per week, the QAI will make 2 concrete cylinders (6 cylinders when the Contract includes permeability requirements) for use by the Department; cylinders shall be standard cured in accordance with AASHTO T23 (ASTM C31). The QAI will p erform entrained air content and slump flow testing, determine water-cement ratio an d determine temperature of the sampled concrete at the time of cylinder casting. All test ing equipment required by the QAI to perform this testing shall be provided in accordance wit h Standard Specification Section 502.041, Testing Equipment. In addition, the Contracto r shall provide a slump cone meeting the requirements of AASHTO T 119. Providing and mainta ining testing and curing equipment shall be considered incidental to the work and no add itional payment will be made. Quality Control concrete test cylinders shall be made for each day’s cast and each form bed used. Cylinders tested to determine strand release strengt h and design strength shall be field cured in accordance with AASHTO T23 (ASTM C31). 28 day c ylinders shall be standard cured. Record unit identification, entrained air cont ent, water-cement ratio, slump flow and temperature of the sampled concrete at the time of cyl inder casting If the Contractor fails to make enough cylinders to demonstrate that the product meets the Contract requirements, the product will be considered nonco nforming work. The compressive strength of the concrete will be determined by averaging the compressive strength of two test cylinders made from the same s ample. For the purpose of detensioning prestressed products, neither of the test cylinder s shall have a compressive strength less than the minimum required transfer strength. For the purpose of determining design strength, the average of two cylinders shall meet or exc eed the design strength, and the difference in strength between the two shall be no more tha n 10 percent of the higher strength cylinder. Perform compressive strength testing to determine transfer and design strength in the presence of the QAI. Cylinder tests not witnessed by the QAI w ill not be acceptable.
535.16 Curing by Moisture Retention Cure the concrete in accor dance with MNL 116,
Section 4.20. Moist cure the concrete until it has reached des ign strength. Do not use membrane- forming curing compounds without the appro val of the Fabrication Engineer. If membrane-forming curing compounds are authorized, follow the requirements of MNL 116 and the curing compound manufacturer’s published recommendations. 5-174
535.17 Accelerated Curing of Concrete (Optional) Cure the conc rete in accordance with
MNL 116, Section 4.19, except as modified herein. After initial set, the temperature gain of the concrete shall n ot exceed 40 F per hour. Initial set shall be determined in accordance with ASTM C403, Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resista nce; a strength gain of 500 psi indicates initial set. The maximum allowable concrete tempe rature shall be 160F. Concrete temperature shall be measured near each end of the casting bed and at intervals not to exceed 100 feet. In order to qualify for accel erated cure, the concrete temperature shall attain a minimum temperature of 120 F; that temperature shall be maintained for a minimum of 8 hours and the concrete shall achi eve a minimum of 80 percent of design strength. Detension precast/prestressed products immediately after the co mpletion of the curing cycle while the products are warm and moist. The accelerated c uring cycle shall be considered complete when the method of supplying heat is stoppe d and/or the concrete temperature drops below 120°F. Two cylinders shall be tested i mmediately upon completion of the accelerated cure cycle. Products that have n ot achieved all of the above criteria shall be moist cured until the concrete has achieved d esign strength. If the precast/prestressed concrete products have achieved 80 p ercent of design strength during the accelerated curing c ycle, no further curing will be required.
535.18 Prestressing Tension the strands in accordance with MNL 116 and this
Specification. Provide stressing calculations t o the QAI prior to tensioning s trands. Do not tension the strands until the QAI has reviewed the calculations. Apply ini tial force and final force to the strands in the presence of the QAI. The QCI shall be present t o monitor and document the application of initial force, final force and the elongation of the strands. Tension strands in an orderly sequence to avoid snags and entan glements. When strands from two, or more, coils are used, identify the locations of th e different strand lots. Calculate the elongation and adjusted gauge pressure readings f or each modulus of elasticity and cross sectional area of the strands. Prior to tensioning, cycle the hydraulic jacking devices until the hydraulic fluid reaches normal operating temperature. After initial tensioning, establish a permanent and clearly vis ible reference mark on the strand to determine strand elongation after final tensioning. Measure the strand elongation to the nearest 1/16 inch. If the strands have been tensioned for more than 48 hours witho ut concrete being placed in the forms, test a minimum of 10 percent of the strands, but not less than two strands in each row and not less than two strands in draped strand arrays, by applying the theoretical