Concrete evidence: Analysis of aggregate and cement in a homicide investigation

Abstract

The unusual body deposition site described comprised three elements of concealment: i) a covert stream-based ravine some 60 m from the suspect’s home; ii) partial grave dug into the ravine bank and iii) final concealment using concrete slabs. Disaggregation and sieving of concrete samples from the site, suspect residence(s) and control samples was carried out. These allowed informative exclusion of all but one control sample and provided a range of possible comparisons that may reflect the sequence of concrete slab selection, transport and use in covering the victim. The textures/colours of disaggregated, dried sediment size fractions also proved useful in conveying principles of exclusion to the court and jury at a subsequent murder trial. This work flows from basic (visual) observation of dry, cut blocks, through regular laboratory procedures of thin section work to disaggregation and size separation of aggregate-cement fractions. Graphical presentation of each analysis provided effective communication of geological science during the trial at court, concluding with a verdict of guilty by aggravated murder.