A NEW APPROACH TO CRACK CONTROL FOR REINFORCED CONCRETE - An investigation of crack widhts close to the reinforcement and the correlation to service life
Abstract: One of the most important factors governing durability of concrete structures is reinforcement corrosion, and to prevent degradation associated with it, it is desirable to use large concrete covers. However, if the concrete cover is increased, the majority of design methods will predict that the crack widths at the concrete surface increase significantly. This can be contradictory, since the same design methods both specify smallest concrete cover and require crack width limitations to prevent reinforcement corrosion. To fulfil crack control demands current European structural codes imply that the engineer tend to use as small covers as possible, use small diameter reinforcement bars with narrow spacing and place excessive amounts of crack reinforcement in sections with high bending moments. All these measures are disadvantageous with respect to durability, which is best achieved by increasing the concrete cover and providing space for effective casting of the concrete. The structural codes focus on limitation of crack widths at the concrete surface, which is well defined and measurable. However, in view of durability the crack width close to the bars should be more decisive for the durability and service life of the structure. The width of the cracks close to the bar can be expected to control the total amount of free oxygen, moisture and other corrosive agents transported via the crack to the reinforcement. This thesis examines the crack width close to the bar surface and the correlation to parameters such as steel stress, concrete cover, steel diameter and concrete quality. The test results show that the crack widths on the concrete surface are of the order twice the crack width close to the bar surface and that the crack width near the reinforcement bar is much less affected by a change in concrete cover than the crack width at the concrete surface. The test results also showed that the crack width close to the bar is affected only to a limited extent by bar diameter and concrete quality and that steel stress is the dominating parameter. During the tests it was noticed that small inclined cracks developed at the bar interface and in the vicinity of the crack so that the concrete near the bar was separated from the rest of the concrete and more or less followed the bar. This mechanism has similarities with pull-out failures, but is limited by the continuous reinforcement. The formation of the small inclined cracks induced a cone shaped concrete piece, which is the main reason for the different development of crack widths close to the reinforcement and at the concrete surface. Non-linear FE-calculations have also been performed, showing similar relations between the crack width close to the reinforcement bar and the crack width at the concrete surface, as the experimental tests described above. The FE-analysis confirmed that the crack width close to the reinforcement bar was almost unaffected by the thickness of the concrete cover. In the results from the FE-calculations the formation of the cone shaped concrete piece during the tensile loading was described in more detail. vi To evaluate the total effect of crack widths and concrete cover on the corrosion risk, cracked beams exposed to chlorides have also been tested. The main conclusion from this test series was that large concrete covers protect the reinforcement better than small concrete covers from chloride induced corrosion for exposure from below, even if the crack widths are larger at the concrete surface. It was also shown that the corrosion risk increases for chloride exposure from above and for high steel stresses.
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