Page 129 states "Concrete is...beguilingly simple: stir together one bucket of cement with two buckets of sand or gravel, and enough water to make a thick "gloop". Not correct - cement plus sand make mortar, cement plus sand plus gravel make concrete. Assuming a loose density of 1.35 gm/cc for cement, and something like 1.80 gm/cc for sand or gravel, gives a sand or sand and gravel ratio to cement about 2.7:1, close to the accepted ratio of 3:1 for mortar, grossly over-cemented for concrete. As rule of thumb, for general construction purposes, the aggregate (sand and gravel)/cement ratio should be about 5:1, and the water/cement ratio about 0.5:1. A high cement content provokes heating, and the necessary high water content for workability provokes shrinkage - not a problem for mortar which is used in thin layers in brickwork, but definitely a problem in mass concrete.