【導(dǎo)讀】腐蝕機制引起預(yù)應(yīng)力鋼筋的惡化和斷裂，進而對橋梁的可靠性產(chǎn)生影響，這比對預(yù)應(yīng)力混凝土橋的其他部分的影響更加嚴重，也更加迅速。在這些結(jié)構(gòu)中，有粘結(jié)預(yù)應(yīng)力鋼筋的惡化也不必要通過混凝土外部糟糕的狀況表現(xiàn)出來。這些退化機制包括：。電化學(xué)(小區(qū))和孔蝕。應(yīng)力腐蝕開裂和鋼的脆斷。最近，世界各地的預(yù)應(yīng)力、后張法預(yù)應(yīng)力混凝土結(jié)構(gòu)已經(jīng)被認為受到某些早期性能破壞的威脅，尤其是已經(jīng)存在的缺陷。這些問題可能繼續(xù)危害結(jié)構(gòu)。此，我將對預(yù)防腐蝕的方法和施工與維護期間的檢測進行介紹。對于遭受早期破壞的現(xiàn)有結(jié)構(gòu)，作者討論案例研究是基于探測技術(shù)和有效的修復(fù)手段。美國的第一個預(yù)應(yīng)力混凝土橋建于1950年；接受這種施工技術(shù)的步伐迅速加快;在這個時尚潮流中建造了當(dāng)今大多數(shù)的混凝土橋梁上部結(jié)構(gòu)。于新型結(jié)構(gòu)和延長預(yù)應(yīng)力和后張法預(yù)應(yīng)力混凝土橋梁的使用壽命需要專注于這種特殊施工技術(shù)的顯著特點和固有特征。

　　

【正文】 c protection systems which produce excessive electrical currents. Rupture is sudden and brittle. Fretting Corrosion. Fretting (surface wear) at surfaces in contact between adjacent tensioned steel elements can cause cyclic relative motion to induce surface deformations. For example, tightly bundled strand in tendons at deviation points can be susceptible, When interelement contact forces between these tension elements are high (such as those at post tensioning tendon deviators and cable saddles), fretting leads to localized transverse cracking of the high strength steel. The cracking locations can then bee susceptible to accelerated strength deterioration through tensile fatigue or corrosion. Corrosion and Fatigue. The fatigue resistance of high strength steels used for prestressing is reduced wherever corrosion is present. Under these circumstances, fatigue stresses will cause a shorter time to failure than a static stress level of the same magnitude. This mechanism will manifest itself more frequently in unbonded (external) tendons than bonded systems. Corrosion Arrest, Monitoring and Control When a post tensioned structure has deteriorated as a result of corrosion processes, or if defects permitting corrosion to initiate and progress are discovered, three different, general approaches have been considered for mitigating future damage. These include: ? Providing or restoring conditions which prevent corrosion of susceptible steel surfaces (passivation) ? Prevent ingress of aggressive solutions with barriers (sheathing, coatings, sealants and sealers) ? Controlling the electrochemical balance within the reinforced concrete environment to halt metal loss. Cathodic protection functions in this manner. Accurate and reliable definition of deterioration mechanisms and extent is required in all circumstances. Experience of the last 5 years has contributed significantly to advancement of practical methods, largely due to bridge rehabilitation efforts initiated as a result of discovery of tendon voids and other defects in Florida and other states. When voids and defects are discovered in grouted post tensioning ducts, regrouting is a practical solution for assuring that design life of the structure will not be promised, as long as the damage accumulated to the point of tendon defect discovery is managable. Recent innovations in nondestructive test technicues and vacuum grouting permit accurate identification and repair of voids. These methods can be used in 8 Copyright ASCE 2020 Structures 2020 Structures Congress 2020Downloadedfromascelibrary.orgbyTHREEGORGESUNIVERSITYon01/07/1opyrightASCorpersonaluseonly。allrightsreserved. conjunction with techniques of the second approach which prevent infiltration of aggressive agents from the structures environment. Considerations related to installation of cathodic protection systems are also presented in following sections. NDT Methods. The bination of the two methods is presently used for the location of voids in ducts for grouted posttensioned strands and bars. The nondestructive Impulse Radar and ImpactEcho (IE) methods are the most promising for the detection and quantification of voids in grouted internally post tensioned tendons. These methods are described in the American Concrete Institute Report ACI , “Nondestructive Test Methods for Evaluation of Concrete in Structures.” Also, Sansalone and Streett (ImpactEcho: nondestructive evaluation of concrete and masonry, Bulbrier Press, Ithaca, NY, 1997) describe the use of ImpactEcho testing for the detection of grouted or ungrouted tendon ducts. Impulse Radar is used to locate the longitudinal alignment and depth to the ducts of interest, before IE testing. It is essential that the ducts be located accurately before any stress wave testing is performed, even in precast units. In addition, post tensioned decks and piers usually contain a considerable quantity of standard reinforcing steel, which has to be avoided during IE testing. Ferro magic devices such as covermeters are not adequate for this purpose, because of the concentration of different types of steel reinforcement and the depth of duct cover usually encountered. The location of all ducts in the deck or pier should be clearly marked on the concrete surface to facilitate subsequent IE testing. The IE pression wave velocity of the concrete can be tested, for example, at drainage openings in the outer deck flange where the flange thickness can be confirmed, or at the edges of piers. Three IE tests should be made at each location, and the individual results stored on puter. In addition, an average test result is pounded from the three tests, and processed according to the method outlined in Abraham, M., C. L233。onard, P. C244。te and B. Piwakowski (Time Frequency Analysis of ImpactEcho Signals: Numerical Modeling and Experimental Validation, ACI Materials Journal, V. 97, No. 6, . 2020, pp. 645657). This process eliminates the influence of the Rayleigh wave on the result, and also minimizes any effect from background noise. Florida DOT’s Central Structures Offices offers a detailed report on its website documenting the selection and parative evaluation of various NDT systems for prestressed construction ( Manuals/). The study was initiated as a definitive parison between various available technologies and systems resulting from Florida DOT’s need to evaluate existing post tensioned bridges. Vacuum Grouting. Vacuum grouting techniques are known to have been refined to the point that prehensive restoration of grout’s contribution to strength and durability of post tensioned construction can be assured. Reliable techniques have been developed by post tensioning contractors for the process. The volume of