Optimizing the appropriate analytical area for Damage Rating Index (DRI) determination through applied statistics

The Damage Rating Index (DRI) method is increasingly used to assess the damage degree of concrete affected by alkali-silica reaction, although it has been criticized for its time-consuming character. This paper presents an optimized approach on how the method can be performed to provide a reliable estimate of the DRI number. A thorough sampling and statistical analysis was performed at the scale of 1 cm 2 squares on 56 cores extracted from various structures, with a wide range of concrete mixtures and deterioration levels. The inherent variability between cores extracted in a “restricted” area of ± 1 m 2 was also investigated in seven locations. The average 90% confidence prediction DRI number range between cores extracted in a “restricted” area reached 263, but such variability was location dependent. The best sampling approach for DRI determination over the whole core is to divide the specimen in the form of lines , which are test units composed of squares that are oriented parallel to one another in the direction of the core’s longitudinal axis. It was found that there is no minimum examination surface area for DRI determination, because there is usually internal damage variations within the investigated components and the inherent precision level is influenced by the damage degree. A representative number of lines should be targeted instead. An empirical model was developed to predict conservative estimates of the required number of lines to achieve a margin of error of ± 150, 200, 250 and 300, based on the core’s length and DRI number.