【正文】 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。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。allrightsreserved. quality, porous concretes and grouts. When the depth of concrete’s carbonation reaches the reinforcement, passivity is lost and metal’s corrosion can proceed unimpeded in the presence of moisture and oxygen. Concrete Cracking as Initiator. Cracks in the concrete or grout surrounding prestressing steel shorten the time to initiation of corrosion by acting as a path for introduction of aggressive environment, which can locally affect the steel surface. Locations where these cracks intersect reinforcement can also create localized corrosion and pitting. Attention to detailing of reinforcement for crack control is required in pretensioned structures. In post tensioned structures, cracking of tendon grout can bee problematical when it provides access points to reinforcement for electrolytic solutions in the event that other corrosion barriers (ducts and sheathing) are breached. Electrochemical (Macrocell) and Pitting Corrosion. Because the surface of steel reinforcement embedded in concrete is not always uniform electrochemically, separate corrosion cells can be created in regions of local differences in an otherwise benign, alkaline environment. Steel tendons in nonuniformly grouted ducts, for example are exposed to zones where different moisture and oxygen conditions occur. Similarly, strand in pretensioned ponents can be exposed to fluctuating depths of carbonation or concentrations of chlorides creating localized conditions favoring corrosion. In both these instances, macrocells are set up between the small anode area and a large cathodic region, when an electrolytic solution is present. In these cases, the macrocell effect can cause considerable corrosion in small regions, and high strength prestressing steels will exhibit severe pitting in these local areas at boundaries between different environmental conditions, ., grouted/ungrouted, wet/dry, carbonated/uncarbonated, etc. While total weight loss of the metal resulting from pitting may be small, the consequences for prestressing reinforcement can be severe, since corrosion rates at pits are relatively higher and will lead to fracture sooner than general loss of sectioncorrosion. Stress Corrosion Cracking and Embrittlement. This deterioration is a highly localized corrosion that produces cracking in the simultaneous presence of corrosion conditions and externally applied or residual tensile stresses. High strength, colddrawn reinforcing and prestressing steels are susceptible. Stress corrosion cracking induces sudden, brittle, unexpected failure as SCCinitiated cracks propagate across the ponent’s loadcarrying section. Most significantly, the process progresses at a far faster rate than the more monly re