north, with a tunnel under the city, avoiding a change of direction for the through trains via the eastern ring rail line. However, budgets could not be found for such tremendous works during the 1970s to 1990s. However, thanks to the extension of the European high- speed railway network through Belgium, this is now becoming reality – the construction work for the North-South link for through trains to the Netherlands are now in progress. The work began in 1998, and are foreseen to last for a period of eight years. The construction of the North-South link can be divided into four large distinct zones, as shown in Figure 2: q Zone 1: the construction of the southern access slope to levels -1 and -2 between the Berchem station and the ‘Antwerp Central’ station; q Zone 2: the renovation works in and under the Central Station, including a tunnel on level -2 under the station for the through traffic; q Zone 3: the construction of two bored tunnels (approximately 1200 m) from the station to the north (Dam Place) and the incline to level -2 as a cut-and-cover tunnel; q Zone 4: the works beyond the Dam place, to the north, connecting to the existing line, and the new high speed line, both in the direction of the Netherlands. This paper is concerned with measurements carried out within the framework of the constructions between Berchem and the Central Station, which must be built taking into account some very specific conditions: q The Central Station has to remain in service during the works and 70% of the train traffic must be permanently able to reach the station, which has to employ at least three platforms at all times; NDT IN CIVIL ENGINEERING A case study on strain gauge measurements on large post-tensioned concrete beams of a railway support structure H De Backer, W De Corte and P Van Bogaert Non-destructive testing by means of strain gauges may be a straightforward operation under laboratory conditions. When applied on a construction site it becomes a wholly different matter. This paper is concerned with the use of strain gauges as a control mechanism on the effectiveness of the post-tensioning of large concrete beams, used as part of the roof of a cut-and-cover tunnel for the high-speed train link passing under the Antwerp Central Station. The emphasis of this paper is placed on the practical execution of the measurements and the specific problems encountered when working on site. To validate the method used, a comparison is made between calculated and measured values of concrete stresses caused by the tensioning operations, proving that the measuring method is more than adequate. This case study shows the usefulness of strain gauges applied to the surface of concrete beams, as a practical, fast and accurate way of performing non-destructive strain measurements on major construction sites for civil engineering projects. Hans De Backer received his civil engineering degree in 2002 from Ghent University, where he is now working as a researcher and PhD student with the Civil Engineering Department. Wouter De Corte received his civil engineering degree in 1998 from Ghent University, where he is now working as a research assistant and PhD student with the Civil Engineering Department. Philippe Van Bogaert received his civil engineering degree from Ghent University in 1974, obtained his PhD in 1988 and is professor with the Civil Engineering Department of Ghent University. During the past few years, the Civil Engineering Department of Ghent University has performed a substantial number of measurements on important construction works, for example the bridges for the high speed train link in Halle and Leuven, the support structure under the bridge of the Plantin-Moretuslei in Antwerp and on the precast bridge girders with pre- tensioned carbon fibre wires of the Kortenberg road bridge. Contact address: Ghent University - Department of Civil Engineering, Technologiepark Zwijnaarde 904, B-9052 Gent, Belgium. Tel: +32 9 264 54 34; E-mail: Hans.DeBacker@UGent.be, Wouter.DeCorte@UGent.be, Philippe.Vanbogaert@UGent.be 1. Introduction At the turn of the 19th century, Antwerp celebrated: from that time on, the railway lines coming from Brussels and Ghent entered the ‘Metropolis’ on elevated rails. In 1905, a new and monumental station building, ‘the new Antwerp Railway Cathedral’, shown in Figure 1, became a reality. From then on, the urban road traffic was no longer disrupted by trains: all level crossings were eliminated from the city. The necessary structures, the station itself and the iron and masonry bridges (across the Plantin and Moretuslei and the Belgiëlei) are monumental constructions. For the last 30 years, railway engineers have been planning a link between the dead-end station of ‘Antwerp Central’ and the Figure 1. Antwerp Central Station 822 Insight Vol 45 No 12 December 2003