Assessment of fatigue resistance and strength in existing concrete structures

Abstract: During the last few decades, it has become more and more important to assess, maintain and strengthen structures like bridges, dams and buildings. This is mainly due to the fact that: (a) many structures are getting old and many have started to deteriorate, (b) there is sometimes a need to increase the load carrying capacity of an existing structure due to e.g. a demand for higher loads or (c) the cost to build new infrastructure is often higher than to repair/strengthen existing structures. Therefore it is of great interest to find methods to evaluate existing concrete structures in an efficient way. In this thesis parameters influencing the evaluation process have been investigated and analysed and the results are presented in the appended papers. Below, findings from the main areas are presented. The development and variation of compressive and tensile strength of concrete are presented for old concrete bridges in Sweden. The mean increase in concrete compressive strength was about 70% for twenty bridges built during 1931-1962 (a rather high dispersion must be taken into consideration). The increase is related to the original 28-day concrete compressive strength which varied between 18 and 51 MPa. The compressive strength within a typical reinforced railway concrete trough bridge was approximately 15% higher in the longitudinal beams than in the bottom slab (measured on drilled cores). A pullout test method, the Capo-test, has been examined as an alternative to drilled cores to determine the in-place concrete compressive strength. A strength relationship is proposed between the compressive strength of a drilled core with the diameter and the height of about 100 mm, fcore, and the pullout force, F, from the Capo-test. A probabilistic approach has been proposed for the evaluation of the shear force fatigue capacity of a concrete bridge slab. In the reliability analysis three different combinations of shear and fatigue models have been compared. The models have been used to determine the safety index ¦Â (and the probability of failure) after another 5 or 25 years of traffic with higher axle loads (300 kN) than the bridge already has been exposed to. The most interesting combination seems to be the shear model of Hedman & Losberg (1975)/BBK04 (2004) and the fatigue model of Tepfers (1979). Results and analyses are presented from cyclic uniaxial tensile tests performed on new and old concrete. The results from the tests indicate that the deformation criterion proposed by Bal¨¢zs (1991) for bond slip may also be applied to plain concrete exposed to cyclic tensile load. A method is proposed for how the deformation criterion may be used also for assessment of existing structures. The load carrying capacity of damaged prestressed concrete railway sleepers has been investigated. The sleepers had an age of five to ten years and the damage, in form of more or less severe cracking, is believed to be caused by delayed ettringite formation. The following tests have been performed: (a) bending capacity of the midsection and the rail section, (b) horizontal load capacity of the fastener, (c) control of the concrete properties and (d) fatigue capacity in bending of the rail section. A visual inspection and classification of the damages are also presented. The test results show that railway sleepers are quite robust. Small cracks do not seem to influence the load carrying capacity and it is first when the cracking is very severe that the load carrying capacity is reduced significantly.