Strength problems in Finnish concrete infrastructure - Robust Air Project

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Observed problem: In Summer 2016, surprising problems with concrete structures were observed during the post-tensioning work of the Kallanvaara railway bridge in Kemijärvi, Finland, when the tensioning anchors sunk into the concrete. It was soon realized that the compressive strength of concrete was clearly below the requirements. It was also revealed that the reason for the strength defects was the elevated air content of concrete. The Kallanvaara bridge was demolished and rebuilt. Similar cases were observed elsewhere in Finland, although the defects were smaller than in case of Kallanvaara Bridge. A ‘concrete crises’ broke out. The crisis received a lot of publicity and caused worries about the condition and safety of concrete infrastructures in Finland, especially bridges.
Frost resistance concrete: When a concrete structure is exposed to freeze-thaw loading, an air-entrained concrete is needed. A special admixture put into concrete during the mixing procedure generates this additional air. Air bubbles with a size of 0.03-0.8 mm are created and stabilized into the concrete mix. In addition to the air-entraining admixtures, water-reducing admixtures are also used in concrete. These admixtures were developed in recent years. In concrete production, the air content targeted is about 6%. However in some cases, the air content of concrete was increased to over 10%, causing a significant reduction of the strength properties of the concrete. The reason for the increased air content was unknown, but the assumption was that it was connected to the reactions initiated by new types of admixtures.
Robust Air project: The Finnish concrete sectors reacted to the crisis and quickly wanted to clarify the reasons for the elevated air contents in concrete. They wanted to solve this problem with a research project from the Department of Civil Engineering, expecting us to reveal the main causes for the problems and to propose corrective actions. They expected results in 6 months, a very short period. The department prepared a proposal for the project and received financing from the Finnish Transportation Agency, the Confederation of Finnish Construction Industries, seven concrete admixtures companies, and three ready-mix concrete producers, all of which participated in the project. The consortium covered all actors of the concrete branch in Finland.
Because of the strict time schedule, the project was mainly limited to concrete tests in the laboratory. We carried out large test series, in which the effects of concrete properties and different combinations of admixtures were investigated. We used new technologies to measure the air content of concrete directly from the concrete mixer. The concrete producers made also some tests in ready-mix concrete mixing stations.
The test series revealed that the main reason affecting the elevated air content of concrete was the efficiency of the mixing process. If the mixing process is too short or too ineffective, the air content of concrete may still increase when the concrete is mixed in the truck during the transport or at the construction site. Our recommendation was that the concrete industry should secure that the primary mixing process is effective enough.
The project fulfilled its targets. The results of the project explained to the concrete industry the mechanism behind the problematic elevated air contents and helped the industry to carry out corrective actions. The mixing time of air-entrained concrete was increased, and the control of the concrete mix design and the manufacturing process were improved. The elevated air content of concrete was a new phenomenon for the industry, but also for the research community. However, because of the long-term research activities in concrete technology and our existing laboratory equipment, we managed to carry out the project and successfully meet its objectives despite the strict timetable.
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