Judging by the abstracts submitted for this year's fib Symposium (November 2016), international research on concrete materials and concrete structural technology is primarily concerned with high-performance solutions for concrete mix design, reinforcing materials and structural systems. As concrete materials and structural systems become more complex, more ”modern“, so their behaviour also becomes increasingly difficult to predict with conventional materials models or analytical methods. The new edition of the fib Model Code for Concrete Structures 2010 makes allowance for this as it explicitly includes performance evaluation philosophy as well as conventional prescriptive design methods and established analytical models. For the research and development of concrete materials and structural systems, performance assessment through experimental investigations represents a well-accepted and frequently used approach. However, in practice, many design engineers prefer to rely on established analytical models and tend to avoid approaches that involve experimental investigations. While this might be acceptable and practical for standard design situations, experimental investigations become necessary for the design and conformity assessment of new materials and systems. One of the most critical foundations for the application of performance approaches in practice is the development of reliable test methods that deliver reproducible results and relate to the behaviour of the as-built concrete structure. The testing of mechanical material properties is generally based on well-established methods and linked to accepted analytical models. In contrast, considerable work is still needed in order to develop reliable test methods and interpretation criteria for concrete durability properties. For example, despite the fact that reinforcement corrosion is probably the most significant threat to the durability and structural performance of the built infrastructure, associated test methods and service life models still contain an abundance of uncertainties. The prediction of reinforcement corrosion is based on modelling chloride ingress and carbonation in concrete. Considering chloride ingress, the expected material performance is typically based on diffusion models and the determination of diffusion coefficients by testing. However, diffusion is just one of several transport mechanisms responsible for chloride ingress into concrete and may not describe the performance of the material adequately. For application in practice, the associated service life models rely on ambiguous assumptions for chloride surface concentrations and environmental exposure conditions. In addition, they do not take sufficient account of other parameters that influence reinforcement corrosion, such as the concrete's electrical resistivity, cover depth, cracking characteristics, moisture and oxygen availability, temperature, etc. This has resulted in practising engineers and concrete producers frequently voicing criticism of existing approaches to durability design. In turn, researchers often respond by developing ever more complicated models and applying increasingly sophisticated statistical approaches (which, however, do not necessarily improve the accuracy of the models). As research progresses, so it becomes increasingly clear just how complex the task of durability modelling really is. We will probably soon conclude that it is time to simplify our approaches, considering that the sophisticated models have not really improved the reliability of durability predictions. However, safe simplifications can only be made once the fundamental material behaviour has been understood. The simplified modelling of complex material behaviour represents one of the most powerful tools for engineering design in practice. This is well demonstrated by the oldest and most widely used performance-based approach for the specification and conformity assessment of concrete material properties – the compressive strength test. This test was developed in the first half of the previous century and initially widely criticized for not representing the material property of the as-built structure. It was argued that a small cubic or cylindrical concrete specimen subjected to short-term loading between two steel platens cannot represent the complex loading regimes and stress distributions of structural members subjected to real loads. However, despite its obvious limitations, this test has been successfully used worldwide for decades. The underlying aim of this test is to contribute to the design and construction of structurally safe structures, which has so far been a great success. Similarly, the ultimate aim of studies of concrete deterioration and service life modelling is to increase the durability of our built environment. And in this respect, major improvements have been made in recent years. Associate Professor Hans Beushausen University of Cape Town, Department of Civil Engineering Organizing Committee fib Symposium 2016, Cape Town, South Africa Hans Beushausen
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Apr 17, 2018
Moritz Volland + 6
Open Access