Aggregate was the stuff inside concrete that was not cement. The sand. The gravel. The crushed stone. The aggregate was seventy percent of the concrete by volume. The cement was the glue but the aggregate was the body. The cement held the aggregate together and the aggregate gave the cement something to hold. Without aggregate the cement was paste. Without cement the aggregate was a pile of rocks. Together they were concrete and concrete was the material that built the modern world.

The aggregate came in two sizes. Fine aggregate was sand. Coarse aggregate was stone. The sand filled the spaces between the stones and the cement filled the spaces between the grains of sand. The gradation was the key. The right mix of large stones and small stones and sand and cement left no voids. No voids meant no weak spots. No weak spots meant the concrete held. The engineer specified the gradation and the batch plant measured the proportions and the truck mixed it on the way to the job.

The old way was mixing on site. The laborer shoveled gravel and sand onto a board and added cement from the bag and turned the pile with a shovel until the color was even. Gray and even meant mixed. Streaks meant unmixed. The laborer added water from a hose and turned the pile again and the pile became mud and the mud became concrete when it hardened. The laborer was the batch plant. The shovel was the mixer. The board was the drum. The whole operation fit in a wheelbarrow and the wheelbarrow went wherever the building needed concrete.

You could read the aggregate in old concrete. The broken edge of a sidewalk showed the stones inside. Round stones from the river. Angular stones from the quarry. The aggregate told you where the concrete came from. Local aggregate. The builder used what was close because aggregate was heavy and hauling was expensive. The sidewalk on your block was made from the hillside up the road. The building on your corner was made from the quarry across the river. The city was built from itself. The ground became the building and the building stood on the ground it came from.

See also: Gravel, Screed