Most of the construction of the Panama Canal was done in the years 1907 to 1914. In 1972 when removable bulkheads were installed in the lateral culverts located below the floors of the Miraflores Lock chambers of the Canal, the work required removal of some concrete. After about 58 years service this concrete was found to be hard and in excellent condition. Considering all the discoveries that have been made in the intervening years about concrete proportioning and mixing and about the sometime susceptibility of concrete to attack if not made properly for its specific environment, it may seem remarkable that concrete made without benefit of all this information had done so well. In looking at one sample of the concrete removed in 1972 from the floor of a Mireflores Lock chamber, what strikes the modern viewer immediately is the paucity of coarse aggregate. As seen in an accompanying photo (in the article) it is almost as though a few pieces of stone had been casually sprinkled though a mortar mix. Even the mortar itself seems to be finely graded, with little material that would be retained on the Number 4 or 8 sieves. This absence of concern for grading seems typical of those days when proportioning was done by volume and when water content was of so little concern that concrete was commonly "mixed rather wet". The proportions of cement:sand:stone in the mixes for Gatun Locks have been gives as 1:3:6 for the mass, 1:2:4 for the facing mixtures and 1:3:0 for finishing horizontal surfaces. Undoubtedly other proportions were used for other applications. It is also interesting that the concrete in the Canal is free of trouble form alkali-aggregate reaction. In 1940 disruptive expansion in several dams and other structures in the United States was found to be the result of this hitherto unrecognized destructive reaction between cement and certain kinds of siliceous aggregates. The discovery led the United State Panama Canal Company to make a field investigation of aggregates on the Isthmus of Panama. One purpose was to see if the aggregates that had been used in the Canal contained reactive minerals that could be dangerous to the life of the Canal. It was found that none of the aggregates contained these minerals in sufficient amounts.