Rolled concrete dams using gap-graded aggregate

Roller-compacted concrete dams, in which the concrete is placed and compacted in thin layers from abutment to abutment without interior formwork, have been found to be very economical. The writer suggests that potential problems of seepage along lift lines, temperature and drying shrinkage cracking, and segregation of aggregates may be minimized or eliminated by the use of gap-graded aggregate concrete. This type of concrete consists of a skeleton of single-size coarse aggregate, the voids of which are filled with a graded fine aggregate, cement, and water. The maximum size of the fine aggregate is one-eighth of the minimum size of the coarse aggregate and the very fine sand is eliminated to reduce the specific surface of the aggregates, thereby minimizing the water and cement requirements. The resulting material is a minimum slump, very dense concrete with low cement content. The proportioning method for the concrete mix is straightforward, depending only on the compaction characteristics of the aggregates, the specific gravities of the materials, and the water-cement ratio needed for design strength. The advantages of gap-graded aggregate for roller compacted concrete dams warrant the extra crushing or screening required.