9DBMC-2002 Paper 241 Page 1 Durability Predictions Using Early-Age Durability Index Testing JR Mackechnie & MG Alexander Summary: Durability of reinforced concrete structures is often dependent on the corrosion of reinforcement due to ingress of aggressive ions and fluids. Using the premise that the potential durability of concrete is determined by the protection provided by the cover concrete to the embedded steel, the resistance may be defined in terms of transport properties such as absorption, permeation and diffusion. A suite of durability index tests was developed to characterize the early- age resistance of concrete to transport of fluids and ions that affect corrosion of reinforcement. These tests were found to produce reasonable predictions of durability performance for reinforced concrete structures. Early-age laboratory characterization testing is discussed together with field data regarding carbonation rates and chloride ingress into concrete. Findings suggest that the approach may be usefully applied as performance specifications where durability of reinforced concrete structures must be guaranteed. Keywords. Carbonation, characterization, chlorides, durability, predictions 1 INTRODUCTION The bulk of durability problems concern the corrosion of reinforcing steel rather than deterioration of the concrete fabric itself. The adequacy of the concrete cover layer is therefore critically important in resisting aggressive agents from the surrounding environment. A plethora of durability tests has been developed to measure fluid transport rates by various mechanisms through concrete. Most methods require sophisticated equipment and complex monitoring and therefore have limited practical value for site concrete. The concept of durability index testing was proposed to provide practical means for characterizing the durability potential of concrete (Alexander 1997). Such index tests must be sensitive to important material, processing and environmental factors affecting concrete. The purpose of material indexing is to provide a reproducible engineering measure of microstructure and properties important to concrete durability at a relatively early age (e.g. 28 days). Thus it should be possible to produce concretes of similar durability by a number of different routes: additional curing, lower w/c ratio, different binder types, etc. In order to assess the effectiveness of these laboratory techniques, field studies of concrete in service are essential. These studies allow deterioration mechanisms to be accurately assessed under normal exposure conditions. The disadvantage of field exposure testing is that deterioration may take years to proceed to a measurable extent thereby requiring extrapolations to estimate long-term trends. These extrapolations may be misleading if the results are not independently validated with long- term data. 2 LABORATORY TESTING Transport of aggressive agents into concrete is primarily caused by absorption, permeation and diffusion mechanisms. The influence of each transport type is dependent on environmental conditions and material properties of concrete. Carbonation- induced corrosion of reinforcement is caused by gaseous diffusion of carbon dioxide through relatively dry concrete. Chloride-induced corrosion in contrast is caused by ionic diffusion through near- or fully saturated material. Results reported in this paper have been produced from several different South African studies over the last ten years. Concrete mixes were produced with local materials and are denoted: PC – 100% Portland cement, FA – 30% fly ash, SL – 50% blast-furnace slag and SF – 10% condensed silica fume. Grade of concrete refers to the nominal concrete cube strength of the material at 28 days. 2.1 Oxygen permeability test Permeability is defined as the capacity of a material to transfer fluids under the action of an externally applied pressure. The permeability of concrete is dependent on the concrete microstructure, the moisture condition of the material and the characteristics of the permeating fluid. Ballim developed a falling-head permeameter that allowed simple measurement of oven-dried concrete exposed to oxygen under pressure (1993). The test equipment and typical measurements are shown schematically in Fig. 1.
Durability Predictions Using Early-Age Durability Index Testing
Apr 29, 2023
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