When strengthening existing reinforced concrete structures, we can find many successful applications of post-tensioning. The method of substituted cable ducts is a very convenient especially in the case of strengthening existing reinforced concrete bridges whose load capacity can be increased up to 300 %. Strengthening of structures by post-tensioning is based on active influence of internal forces by prestressing force and radial forces that occur in places of bending the unbonded prestressing tendons. Thus, the forces which acts directly on structure in the anchorage areas and in the locations where the prestressing tendons change their trajectory (in saddles) are considered as the effects of prestressing. Saddles are one of the most important structural details of the additional prestressing system. For the purpose of closer examination of the action of monostrand in steel saddles with different radii of curvature, the methodology of experiment was developed and the concrete test panel in which holes for sensors and substituted cable ducts for monostrand are prepared was created. The concrete test panel enables change of the radius of curvature and was designed for analysis of monostrands in saddles with radii of 600, 1000 and 1500 mm. The aim of short-term experiments with monostrands is to determine suitable application of post-tensioning of existing structures with prestressing tendons in HDPE sheaths in saddles with small radii of curvature.

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Post-tensioning is a suitable, reliable and durable method for strengthening existing reinforced concrete bridge structures. The high efficiency of post-tensioning can be seen on many implemented applications for bridge reconstructions worldwide. There are still several thousands of beam and slab bridges the load capacity of which no longer meets the demanding transport conditions. The oldest reinforced concrete beam bridges, from 1905-1915, are designed according to the Austrian Ministry of Railways Bridge Standard of 1904 when the largest load to be considered was the 18-tonne road steamroller. These bridges are not dimensioned for the currently valid traffic load values. The paper deals with the strengthening of the parapet beam bridges from the period of 1905-1930. These bridges have two main beams pulled over the bridge deck which is supported by cross beams. The cross beams connect the two main beams, forming a half-frame in the transverse direction which provides spatial rigidity of the structure. The spans of these bridges are usually in the range of 15 to 25 m.

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