Feasibility of surface sampling in automated inspection of concrete aggregates during bulk transport on a conveyor

Abstract

Automated optic inspection of concrete aggregates for pollutants (e.g. wood, plastics, gypsum and brick) is required to establish the suitability for reuse in new concrete products. Inspection is more efficient when directly sampling the materials on the conveyor belt instead of feeding them in a sampled stream to the sensor system. However, this does not reveal the true volume contents if the concrete and pollutants have segregated. Moreover, even if the concrete was homogenous, the relation between the number of visible pollutants and those in the volume varies depending on the size of the pollutants relative to the size of the concrete. A few fundamental issues must be investigated and technically resolved before this improved way of automated inspection becomes possible. First we investigated if it is possible to feed concrete at a high rate in a homogenous volume to a conveyor belt using batches of several tonnes. Second we investigated the surface-volume numbers for specified pollutants. Progress in the first part was a promising method of feeding that appears to produce not a true homogenous volume, but at least a volume with a mild and most of all reproducible degree of segregation for the different pollutants. This suggests the segregation could be corrected for in the data using a model. As a follow-up, a theory was developed that predicted accurately how thin plastic flakes of different sizes correlate with the number of visible flakes on the surface of a granular matrix. This theory is now extended and tested successfully in the laboratory using batches of 2-8 mm dry and moist river gravel with added 1% plastic flakes.