Definition
Concrete refers to the waxy colored paste obtained after evaporation of the volatile solvent used to extract the aromatic molecules from a fragile plant material, most often a flower. It is a composite mix of aromatic compounds, natural plant waxes and pigments, and not a finished product for the bottle: the concrete is a process stage, which must be washed with alcohol to yield the absolute.
Origin and history
Volatile solvent extraction emerged in Grasse (France) in the second half of the 19th century. It progressively replaced cold enfleurage, which was judged too slow and too costly for the nascent industrial scale (source: Wikipedia).
The historical solvent was benzene, then light petroleum; today the industry favors food-grade hexane for its low boiling point and high volatility, which allow near-complete evaporation with residual rates capped by regulation (source: Societe Grassoise de Parfumerie).
Use in perfumery
Concrete is used almost exclusively as an intermediate material on the way to the absolute: it is dissolved in ethanol, the waxes are filtered out at low temperature, and the alcohol is then evaporated to retain only the concentrated aromatic fraction. It is also occasionally used as-is in solid perfumes and in some artisan work that values the depth of a waxy, colored material.
The yields are telling: roughly one ton of rose petals is needed to obtain one kilo of concrete, which then yields around 700 grams of absolute after alcohol washing (source: Delacourte). The most commonly produced concretes are rose, jasmine, tuberose, orange blossom, mimosa and violet leaf.