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• When prestressing steel for post-tensioning is installed in the ducts after completion of concrete curing, and if <br /> stressing and grouting are completed within 10 days after the installation of the prestressing steel, rust which may form <br /> during those 10 days will not be cause for rejection of the steel. Prestressing steel installed, tensioned, and grouted in this <br /> manner, all within 10 days, will not require the use of a contusion inhibitor in the duct following installation of the <br /> prestressing steel. Prestressing steel installed as above but not grouted within 10 days shall be subject to all the requirements <br /> in this section pertaining to corrosion protection and rejection because of rust. The requirements in this section pertaining to <br /> tensioning and grouting within 10 days shall not apply to epoxy-coated prestressing steel strand. <br /> • Any time prestressing steel for pretensioning is placed in the stressing bed and is exposed to the elements for more <br /> than 36 hours prior to encasement in concrete, adequate measures shall be taken by the Contractor, as approved by the <br /> Engineer,to protect the steel from contamination or corrosion. <br /> • After final fabrication of the seven-wire prestressing steel strand,no electric welding of any form shall be performed <br /> on the prestressing steel. Whenever electric welding is performed on or near members containing prestressing steel, the <br /> welding ground shall be attached directly to the steel being welded. <br /> • Pretensioned prestressing steel shall be cut off flush with the end of the member. For epoxy-coated prestressing <br /> steel, only abrasive saws shall be used to cut the steel. The exposed ends of the prestressing steel and a 25-mm strip of <br /> adjoining concrete shall be cleaned and painted. Cleaning shall be by wire brushing or abrasive blast cleaning to remove all <br /> dirt and residue on the metal or concrete surfaces. Immediately after cleaning, the surfaces shall be covered with one <br /> application of unthinned zinc-rich primer(organic vehicle type) conforming to the provisions in Section 91, "Paint," except <br /> that 2 applications shall be applied to surfaces which will not be covered by concrete or mortar. Aerosol cans shall not be <br /> used. The paint shall be thoroughly mixed at the time of application and shall be worked into any voids in the prestressing <br /> tendons. <br /> The seventh paragraph in Section 50-1.07, "Ducts,"of the Standard Specifications is amended to read: <br /> • All ducts with a total length of 120 in or more shall be vented. Vents shall be placed at intervals of not more than <br /> 120 in and shall be located within 2 in of every high point in the duct profile. Vents shall be 12 mm minimum diameter <br /> standard pipe or suitable plastic pipe. Connections to ducts shall be made with metallic or plastic structural fasteners. Plastic <br /> components, if selected, shall not react with the concrete or enhance corrosion of the prestressing steel and shall be free of <br /> water soluble chlorides. The vents shall be mortar tight, taped as necessary, and shall provide means for injection of grout <br /> through the vents and for sealing the vents. Ends of vents shall be removed 25 mm below the roadway surface after grouting <br /> has been completed. <br /> The sixth paragraph of Section 50-1.08, "Prestressing,"of the Standard Specifications is amended to read: <br /> • The following formula and friction coefficients shall be used in calculating friction losses in tendons: <br /> To=Txe(µa+KL) <br /> Where: <br /> To=steel stress at jacking end <br /> Tx=steel stress at any point x <br /> e=base of Naperian logarithms <br /> g=friction curvature coefficient <br /> a=total angular change of prestressing steel profile in radians from jacking end to point x <br /> K=friction wobble coefficient(=0.00066/m) <br /> L=length of prestressing steel from jacking end to point x <br /> Contract No. 10-OM7104 <br /> 27 <br />