Concrete is a mix of cement and aggregate, which is sand or gravel of an appropriate type for the concrete's intended use. The aggregate usually comes from a source near where the concrete is made. In this instance, that might well be close to the scene of the crime. So Patenik concentrated on the aggregate. It was sand, and the sand grains were all nearly the same size.
Beach sand has that characteristic, because wave action tends to sort the grains by size.
Such marks develop when waves bash the sharp edge of one sand grain into the smooth face of another. Taken together, those features indicated that the concrete came from a coast, but picking which coast demanded other sorts of research. First, chemical analyses showed that each sand grain had a particular mix of heavy minerals. Then came a review of oil company records, particularly those from the 1930s and 1940s. During those decades the companies funded many studies of the heavy-mineral content of rocks and sands, believing these minerals might hint at underlying petroleum.