523 © 2016 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Structural Concrete (2016), No. 4 Technical Paper DOI: 10.1002/suco.201600067 Stefanie von Greve-Dierfeld* Christoph Gehlen Performance-based durability design, carbonation part 2 – Classification of concrete At present, prescriptive regulations with regard to concrete cover and composition are applied to provide sufficient durability of re- inforced concrete members under exposure conditions with dif- ferent degrees of severity. In view of current knowledge on dete- rioration mechanisms and their modelling, it is planned to change from these deemed-to-satisfy specifications to a performance- based design approach in future standards. In such specifica- tions, concrete durability design is based on the statistically characterized performance of concrete, determined in standard- ized tests with respect to defined classes of concretes with simi- lar performance. This paper presents the results of a study in which concrete mix- es were tested and analysed with respect to their carbonation re- sistance. Compositions with similar performance are grouped into carbonation resistance classes. These classes are described statistically and requirements for performance testing are given. In addition, composition requirements are introduced in order to determine concrete performance depending on mix composition prescriptively. Finally, an example is given for the assessment of concrete performance with regard to carbonation. This work was carried out at the request of JWG under CEN TC 250/SC2 and CEN TC 104/SC1 as an input and starting point for the ongoing committee work to implement the methodology from the fib Model Code for Concrete Structures 2010 in the next genera- tion (2021) of European concrete standards. Keywords: concrete durability, material resistance, performance class, performance-based specifcation, carbonation, durability conformity testing 1 Introduction Under moderate humidity conditions, gaseous carbon di- oxide difuses into partially saturated concrete and dis- solves in the concrete pore solution to form mainly calcite with calcium supplied by the dissolution of Portlandite and, depending on the pH of the pore solution, C-S-H phases. If the pH of the pore solution falls below a certain value at the surface of the reinforcement, the steel surface becomes depassivated and is no longer protected against corrosion. The rate at which the carbonation front penetrates the concrete depends mainly on difusion because the re- action kinetics is much faster, i.e. the carbonation of con- crete is controlled by porosity (and thus by concrete com- position) and the moisture conditions of the concrete. If concrete is exposed to standardized (i.e. reference) labora- tory test conditions (65 % RH, 20 °C, natural CO 2 concen- tration), the increase in the depth of carbonation may be described by a simple root-time law, [1], [2]: (1) where: x c (t exp ) carbonation depth at time t exp [mm] t exp time of exposure [year] k NAC a factor refecting the basic resistance of the cho- sen concrete mix (w/b ratio, cement type or bind- ing capacity, etc.) to natural carbonation. (The specimens are produced and cured in a standard manner and stored under reference conditions.) [mm/year 0.5 ] In order to determine concrete performance, a reliable short-term test method is required. The test should be able to diferentiate concrete performance with sufcient pre- cision. 2 Performance testing 2.1 Test specification In the case of tests performed at increased CO 2 concentra- tions (accelerated carbonation, ACC), it is known that the high concentration changes the location of calcite precipi- tation, which can reduce pore space and thus slow down the carbonation process. Furthermore, accelerated car- bonation leads to an increase in moisture content, which also results in a slower carbonation process. For this rea- son, this study considers the classifcation of performance with regard to carbonation resistance under natural con- ditions (NAC). Several test methods are currently used for testing natural carbonation; they are described in detail in [1]. The basis for the test specifcations is the widely accepted prEN 12390-10 “Testing hardened concrete – Part 10: De- ( ) exp exp xt k t c NAC = ⋅ * Corresponding author: stefanie.vongreve@tfb.ch Submitted for review: 08 April 2016; revision: 16 June 2016; accepted for publication: 21 June 2016. Discussion on this paper must be submitted within two months of the print publication. The discussion will then be published in print, along with the authors’ closure, if any, approximately nine months after the print publication.