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According to the trade literature, the determinations of location, direction, and the cover thickness over a single steel bar in concrete are relatively easy and reliable from a magnetic measurement if the bar diameter is known. The estimation of the bar diameter is also possible if the cover thickness is known, although these results are less reliable. Only recent publications suggest double measurements from which both the cover thickness and the bar diameter can be estimated without previous knowledge of either of them. Unfortunately, the accuracy of diameter determination remains unimproved even with these methods. This paper attempts to present the further improvement of the magnetic determination of bar characteristics. The basic idea is to combine a magnetic device with a computer that calculates, without any previous knowledge about the bar: 1) the thickness of the concrete cover above the bar and, 2) the diameter of the bar. Preliminary data also indicate that distinction can be made whether the tested area is above a single bar or multiple bars, although this is not discussed in this paper. Experimental data obtained on laboratory specimens illustrate the new method.

Presents the results of radar tests conducted in a controlled laboratory environment to provide an independent assessment of the usefulness of the technique for investigating in situ concrete structures. Parameters studied include size and thickness of concrete elements, the presence of voids and steel reinforcement, and the influence of moisture. The use of a commercially available computerized signal enhancement system has also been examined to assess the potential benefits. Results are presented that include typical signal response traces and show the degree of precision that can reasonably be expected from radar measurements under a range of practical circumstances. Results are considered in the light of the basic theoretical factors and a number of important limitations are identified. It is concluded that the method offers considerable potential in a range of structural applications but is subject to a number of practical limitations, and guidance is offered concerning experimental technique and interpretation of results.

This paper describes repairs on two prestressed concrete pipelines (inverted siphons) that were found to have distressed areas. The distressed areas included disbonded mortar coating, and corroded and broken prestressing wires. The siphons were dewatered, during January and part of February 1991, for repairs and inside inspection. Repairs in areas of major corrosion and broken prestressed wires consisted of removing the earth cover, excavating a limited-length trench under a pipe unit, and completely removing the mortar coating and prestressing wire around the pipe for 360 deg. High-strength steel tendons were placed around the pipe and stressed to the initial prestressing force. Shotcrete was placed over the tendons and anchors, followed by backfilling. The two siphons were returned to service on Feb. 15, 1991. The repairs appear to be successful in restoring the concrete pipe to its initial design strength.

Injection methods for repairing concrete to maintain the integrity of structural members were studied. As a general rule, the lower is the viscosity of epoxy, the longer the setting time and the deeper the grout goes. Further, the deeper the placing, the better the result will be. In the actual case, splitting tension test of core cylinders from repaired sections show that the repair method that adopts a specially designed injector will do better work than that using a conventional one.

The Story Bridge is a densely trafficked, important corridor over the Brisbane River. Built in the 1940s, the bridge (particularly the southern approaches) are showing signs of distress. The main span of the bridge consists of two steel cantilever trusses with a drop-in steel span. The overall length of the main span is 446 m. The southern approach spans consists of steel trusses, concrete girders, and a grillage of beams with a concrete deck, for a combined length of 515 m. There have been longstanding concerns with respect to some elements of the bridge. These are expressed most visibly by extensive cracking and spalling of the concrete. In some locations primary reinforcement is completely rusted through, leaving doubts about the safety of local areas of the bridge. Some rocker bearings appear to be partially or totally seized. The investigation involved an historical review of the design and construction details used in the bridge. Design loads were compared with modern standards. The concrete quality was investigated and the level of corrosion determined. These are reported in the context of the original specifications and details, and repair procedures recommended.