Post tensioning is an useful method of reducing size or increasing load bearing capacities of concrete structural elements like bridge decks, girders and suchlike. Unlike pre-stressing, where stresses are applied to steel well before pouring concrete, in case of post-tensioning the same are applied only after concrete attains sufficient strength. Like pre-stressing, Post tensioning too is an extensive subject for studies. Some useful practical points on Post tensioning in a RCC bridge deck are briefly discussed herein.
The key materials involved in Post tensioning of a concrete bridge are:
a) Steel cables of required diameter. These are usually composed of several steel strands twisted together. These cables are located within concrete at suitable locations as per design requirements over and above normal steel reinforcement. Ordinary steel re-bars are installed prior to pouring, as usual, while the cables to be post-tensioned are introduced later and conduits for that purpose are left within concrete accordingly (as per construction drawings). As part of quality control measures, materials test certificates from the manufacturer of the cables are scrutinised. Besides, physical inspection and more importantly, laboratory tests, such as, tensile strength test, relaxation test etc. are conducted to ensure the quality of these all-important elements. For general idea, the ultimate tensile strength for these should be at least 1770 N/sq.mm.
b) Ducts & Couplers: These are the conduits for providing the steel cables inside concrete. The couplers join the ducts to render several pieces of ducts a single, leak-proof conduit of required length. The dia. of ducts depends upon the dia. of cables to be inserted and is specified in construction drawings. Quality control measures for these primarily include physical inspection & scrutiny of material test certificates from manufacturer.
c) Anchorage Assembly: This include trumplate, anchor blocks etc. and is essential in order to provide anchorage to the steel cables during application of tensile stress. The precise orientation of this assembly with relevant details is illustrated in construction drawings. Physical inspection & scrutiny of material test certificates from the manufacturers are important aspects of quality control measures. For general idea, the anchrage efficiency should not be less than 90%.
d) Grout: Grout is another important element of the process of post-tensioning of RCC bridges. The steel cables inside the conduits within concrete are fully grouted after application of tensile stress to the same is over. Stringent quality control measures are adopted for grouts & grouting of this kinds in order to ensure quality of high standard. Sufficient quality tests are conducted on grout, such as, fluidity test, bleeding test, volume change test, compressive strength test etc. in order to examine it,s quality. For more details on grouts & grouting, refer earlier posts on the topic.
Contd. in Part-II …..