【正文】 the industry had bee well established not only in England, but also in Germany and Belgium. Shipments to the United States were started in 1868 and reached a peak about 1895, at which time production was well under way in the United States. Meanwhile the United States production of natural cement had been started early in the 19th century as a result of the demand for cement for construction of the Erie Canal and related works. Subsequent development of the rotary kiln led to large scale production of cement throughout the world. The use of concrete was expanded by the construction of railroads, bridges ,buildings and street pavements. Research in reinforcing concrete with steel rods had been started in France, and the year 1875 saw first use of reinforced concrete in the United States. Much39。of the concrete at this time contained barely enough water to enable the concrete to be rammed into place by the application of much hand labor. There then ensued a period of wet concrete in which the concrete was flowed into place. Many users of concrete, however, realized the folly of wet mixes, and about 1920 Duff Abrams revealed the results of his research and observations. He stated that the quality of concrete was directly affected by the amount of water in relation to the amount of cement 。 it usually makes up about 75 percent of a given mass of concrete, by volume, although a poor aggregate can reduce the strength of a batch of concrete considerably, good aggregate adds only slightly to the strength of the cement. The two principal advantages of concrete as a construction material are its relative cheapness and the ease with which it can be handled and placed while it is in the plastic state. The principal structural advantages of concrete are its great pressive strength and its durability , Concrete can withstand very high pressive loads. This is what makes concrete so suitable for the foundations, walls, and columns of buildings, and for driveways and walks as well. The principal structural disadvantage of concrete is its poor tensile strength. That is, it cannot withstand pulling or bending loads without cracking or breaking. For this reason, steel rods, or reinforcement steel, are often embedded in concrete, the reinforcement steel providing the tensile strength the concrete lacks. Concrete with reinforcement steel embedded in it is reinforced concrete. In addition to its poor tensile strength, concrete, like most construction materials, expands in hot weather and when wet and contracts in cold weather and as it dries out. Unless these movements are allowed for during construction, the concrete will crack. And, contrary to mon belief, solid concrete is not impervious to water. Some moisture will migrate into the bestmade concrete. But if the concrete should be excessively porous ,which can happen if too much water has been used in mixing it, moisture can easily enter the concrete after it has cured. If this moisture should be present within the concrete when cold weather es, the moisture may freeze, which may result in serious frost damage to the structure. Despite these limitations, concrete is an inherently strong and durable construction material. If the proportions of water, cement, and aggregate are carefully calculated and if the concrete is placed and allowed to cure according to simple but definite rules, it is possible to obtain from the concrete all the strength and durability that is inherent in it. The ratio of water to cement in a batch of concrete is the principal determinant of the concrete39。 and the less the amount