Specifying resistance against carbonation-induced corrosion by performance

The determination of performance levels for carbonation resistance from modelling or from benchmarking current national provisions does not give consistent results. Current modelling tends to inaccurately determine values for concretes containing cements with less than 55% Portland cement clinker. This is probably due to the input from standard carbonation tests, where the relative humidity of 57% to 65% significantly reduces further hydration. Data from sheltered natural exposure sites show that concrete made from slowly reacting cements perform much better than predicted by modelling. Testing the carbonation resistance of concretes conforming to the recommendations in BS 8500-1 is not normally necessary, but in the author's opinion there are a few situations, identified in the paper, where carbonation testing is advisable. As it is almost essential that performance results are obtained in a reasonable timescale, carbonation-resistance requirements should be specified in terms of accelerated test criteria. Specifying the performance requirements given in this paper for the accelerated test procedure (prEN 12390-12) will be safe but very conservative for slowly reacting cements. Where carbonation resistance is specified by performance, it is prudent to specify, in addition to the performance requirements, limiting values, until sufficient experience is obtained from specifying durability by performance.