The Path Between the Seas

by David McCullough

To build the Great Pyramid or the Wall of China or the cathedrals of France, blocks of stone were set one on top of the other in the age-old fashion. But the walls of the Panama locks were poured from overhead, bucket by bucket, into gigantic forms. And within those forms there had to be still other forms to create the different culverts and tunnels, the special chambers and passageways, required inside the walls. Everything had to be created first in the negative, in order to achieve the positive structure wanted. Moreover, the creation of the building material itself was a “science” requiring specific, controlled measurements and a streamlined system of delivery from mixing plant to construction site. Timing was vital. Concrete—a combination of sand, gravel, and portland cement (itself a mixture of limestone and clay)—had been known since the time of the Romans, but was used very little as a building material until the late nineteenth century and then mainly for subbasements and floors. Dry docks and breakwaters were built of reinforced (or ferro) concrete—concrete in which metal rods are added—and in the early 1900’s several major buildings were built of the same material in Europe and the United States, as well as silos, some small bridges, and a Montgomery Ward warehouse in Chicago. George Morison drew up plans for his concrete bridge over Rock Creek Park in Washington, and by 1912 a tremendous concrete railroad bridge, the Tunkhannock Viaduct, was under way near Scra...