1、1 CONCRETE AND MORTAR 1. Early History of Cement and Concrete Shelter from the very beginning of man/ existence, has demanded the application Of the best , available technology of the contemporary era. In the earliest ages, structures consisted of rammed earth, or stone blocks laid one on another wi
2、thout benefit of any bonding or cementing medium. Stability of the stone structures depended on the regular setting of the heavy stones , The earliest masonry probably consisted of sun-dried clay bricks, set in regular courses in thin layers of moist mud. When the moist mud dried, a solid clay wall
3、resulted. Construction: of this kind was common in the dry desert areas of the world. Burnt gypsum as a cementing material was developed early in the Egyptian period and was apparently used in construction of some of the pyramids. Later the Greeks and Romans discovered methods of burning lime
4、stone to produce quicklime which was subsequently slaked for use in making mortar. Both the Greeks and the Romans learned that certain fine soil or earth, when mixed with the lime and the sand, produced a superior cementing material. The Greek material, a volcanic tuff from the island of Santorin ,
5、is still used in that part Of the world. The best of the materials used by the Romans was a tuff or ash from the vicinity of Pozzuoli near Mt. Vesuvius, hence the name “pozzolan”used to identify a certain type of mineral admixture used in concrete today. The cement produced by the
6、Romans was a hydraulic cement, that is, it had the capability of hardening under water. Many of the Roman structures were constructed of a form of concrete, using these materials, and stone masonry was bonded with a mortar similarly composed. During the Middle Ages of history, the art of makin
7、g good mortar was nearly lost, the low point having been reached in about the llth century, when much inferior material was used. Quality of the lime started to improve at this time and in the 14th century or later the use of pozzolans was again practised. One of the most famous projects of th
8、e comparatively recent period was the 2 construction of the new Eddystone Lighthouse off the coast of England in 1757-59. John Smeaton, the engineer and designer of the structure, investigated many materials and methods of bonding the stones for the building. Engineering and scientific d
9、evelopment was beginning to move rapidly at this time, and many researchers in several countries were investigating cementing agents made from gypsum, limestone and other natural materials. One discovery was a method of making a cement by burning a naturally occurring mixture of lime and clay. prope
10、rties of the natural cement were very erratic because of variations in the proportions in the natural material, although use of this natural cement continued for many years. In 1824 Joseph Aspdin , a brickmason of Leeds, England, took out a patent on a material he called Portland cement
11、, so called because concrete made with it was supposed to resemble the limestone quarried near Portland, England. Aspdin is generally credited with inventing a method of proportioning limestone and clay, burning the mixture at high temperature to produce clinkers, then grinding the clinkers to
12、 produce a hydraulic cement. His small kiln, producing about 16 tons of clinker at a time, required several days/or each burn. Expansion and development of cement manufacturing was slow for a number of years. About 1850, however, the industry had become well established not only in England, but also
13、 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 r
14、esult 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 p
15、avements. 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. Much'of the concrete at this time contained barely enough water 3 to enable the concrete to be rammed into place by the applic
16、ation 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
17、 was directly affected by the amount of water in relation to the amount of cement ； within reasonable limits, the quality of the concrete decreases as the water-cement ratio goes up. This has become one of the basic laws of concrete technology. 2. Advantages and Disadvantages of Concrete and Its Wat
18、er-Cement Ratio Concrete is a mixture of Portland cement, water, sand, and crushed gravel or stone. The water and cement form a cement paste in which the sand and stone or gravel are mixed. The sand and stone or gravel together make up the aggregate of a concrete mixture. The aggregate serves
19、 no structural function. It is merely ,a filler that adds low-cost bulk to the cement paste； 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 st
20、rength 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 compressive strength and it
21、s durability , Concrete can withstand very high compressive loads. This is what makes concrete so suitable for the foundations, walls, and columns of buildings, and for driveways and walks as well. &nbs
22、p; 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
