greatly retards the setting of mortar, but does not seriously injure it if properly treated; that it is much safer to use Portland cement in cold weather, especially if the mortar is to be subjected to alternate freezing and thawing. The one exception to the latter conclusion is the experiments of Mr. Hobart. His results would show that the American cements are not only influenced less by freezing than the Portlands, but that their strength is actually increased. Mr. Hobart says that this is so different from all the former ideas on the subject that some of the tests were carefully duplicated with practically the same results. Specifications for work involving the use of cement mortar always provide that it shall be used within a certain time after it has been mixed, generally from half an hour to an hour and a half, according to the character of the work and the nature of the particular cement. This is because it is considered that cement mortar should be in its permanent place before it has begun to set, and that any disturbance after the first set reduces its ultimate strength. Not many experiments have been made to demonstrate this, and it can be readily understood that to be of value tests must be made of each individual cement. A slow-setting cement will of course permit more manipulation and disturbance than one that sets quickly, and just what the effect will be can only be known by experiment. Table No. 35 shows the result of some experiments. detailed by Gen. Gillmore. The sections used and the methods of constructing and breaking were the same as on page 111, except that the mortar was made of equal parts of natural cement and sand by volume, and the samples were kept in sea-water for 320 days. TABLE NO. 35. ...... Breaking Strength. 767 lbs. ... Cement fresh from barrel, average of five 66 66 Table No. 36 gives the results of Mr. Cooper as published in the paper previously referred to. The briquettes were made of Portland-cement mortar mixed 1:2 and broken at the end of one year. The figures represent tensile strength in pounds per square inch, and the different columns show the time of making the briquettes after the mixing of the mortar. The author of the paper concludes: "In practical working with most Portland cement, if it becomes necessary for the mortar to stand for one-half of a day even, no injury will result, provided the precaution is taken to keep the mortar wet." Another test to which cements are generally put is the one for maintaining its volume. This is sometimes done by placing the mortar in a cylinder of glass. If any expansion takes place in setting, the glass will be broken, and if any shrinkage, it can be easily detected. Mr. Clarke says in the Boston Main DrainageReport that in his tests the cylinders were invariably broken. Another method is the so-called "hot water" test. The Faija method is to mix a small pat of cement with as little water as possible, and place it on a glass plate in a covered vessel which contains water maintained at a temperature of about 112°. The pat is kept in the moist air for 6 or 8 hours, when it is immersed in water kept at a temperature of from 115° to 120° Fahrenheit for the remainder of 24 hours. If at the end of that time it remains intact with no signs of disintegration, it is ready for use. Manufacturers, however, can overcome the effect of the heat by adding sulphate of lime to the cement. In speaking of hot-water tests, Mr. Cummings in his work heretofore referred to says: "It is safe to assert that of the more than one hundred and fifty million barrels of American rock cements used in all of the great engineering works throughout the country during the past fifty years, and with no evidence of failure, not one per cent would have sustained the boiling test. A cement, whether natural or artificial, that will crystallize so rapidly as to sustain the boiling test ought to be looked upon with suspicion, as it is either naturally too quick-setting or too fresh and lacking in proper seasoning." Concrete. Concrete can be defined as masonry made up of broken stone, gravel, cinders, or other similar material, joined together by cement mortar. It has been in use for centuries. One of the oldest and most noted examples of concrete construction is that of the dome of the Pantheon at Rome. In early times it was used principally for foundations. But as its value has become recognized and cement has been produced better and more cheaply, its use has been extended until now it is put to practically as many uses as is stone itself. It is used as a monolith and also in blocks. It is particularly adapted to foundations of irregular form, as it is cheaply and easily shaped. It is used extensively in foundations for all classes. of work, bridge piers and abutments, sidewalks, curbing, sewer-pipe, fire-proof floors, and even as a monolith in arch bridges of quite extensive spans. Stone suitable for concrete is often found in localities where good building-stone is not obtainable, and thus the use of concrete allows masonry construction when the cost of natural stone would have been prohibitive. So it is not strange that it has become popular with engineers, as, when well made, its success has always been as great as its adaptability. One of the best examples of concrete construction of modern times is the Museum building of the Leland Stanford Jr. University of California. The entire building is practically a monolith. In specifying proportions for concrete-mixing, it is customary to regulate them in units of cement. This is not the true way, and there is a growing tendency among engineers to change this and establish instead a certain quantity of mortar as standard unit. The province of the mortar is to bind the pieces of stone together, and when the voids of stone are positively filled, any excess is simply wasted. In deciding, then, upon the proportions to be used in the concrete, the amount of voids in the stone adopted must be first ascertained. This will vary with different kinds of stone and according to the uniformity with which it is broken. The actual size of the stone does not make so much difference. When the pieces are approximately cubical and of about the same size, the voids will be about 50 per cent of the stone. By grading the sizes, however, from the largest to a permissible minimum, the amount of voids can be materially reduced, thus accomplishing a saving of mortar and increasing the strength of the mixture. In order to insure the complete filling of the voids and making as solid a mass as possible, it is best to specify an amount of mortar, about ten per cent in excess of actual voids, as perfect work is very seldom attainable in practice. The exact composition of the mortar is important. The character of the work must determine the strength required for the concrete. Recognizing, then, that a concrete cannot be stronger than its mortar, the proportions of the concrete and sand can be decided upon. For a good concrete, stone should be hard, tough, and of such a texture as to permit of strong cohesion between the mortar and the different fragments. But it would not be allowable, or good engineering, to go to great expense to provide a stone that would be appreciably stronger than the mortar matrix. The ideal concrete would have its stone and mortar of equal strength, so that when broken the fracture will extend through mortar and stone alike. Clean gravel and gravel mixed with broken stone have been used with great success. In concrete for fire-proof floors, where weight is an important consideration, clean steam cinders are generally employed. This gives good results, and some of the tests of very flat arches made of this material show that its strength is surprisingly great. After having determined upon the amount and composition of the mortar required for any given amount of stone, the next step is its preparation. The sand and cement should first be thoroughly mixed dry. The importance of this cannot be overestimated. Without good mortar good concrete cannot be obtained. It is not sufficient that enough and good materials are provided, but they must also be properly applied. Water should next be added in such quantity as will assure the desired consistency, without drowning out the cement, and the entire mass mixed rapidly until every grain of sand is coated with cement, as this acts with the sand in precisely the same manner as the mortar acts with the stone. It is miniature concrete. As it is desirable to have as great cohesion as possible between the mortar and the stone, the latter should be thoroughly wet, so as to wash off all dust or other foreign. matter, and then added to the mortar. The resulting mass must then be turned over forward and backward until the mortar is scattered evenly among the interstices of the stone, so that each piece is completely covered and the concrete is finished. The material at all times must be kept on boards or platforms, so that it shall be kept free from all foreign matter. This operation of mixing should be done without delay and as expeditiously as possible, as the sooner the concrete is in place the more complete will be its final set. The place of mixing should be near its final location, preferably so that it can be shovelled to it from the boards; but this is seldom possible, and it must be carried in some conveyance and dumped. When used in any great mass it should be spread in layers from 9 to 12 inches in depth and at once thoroughly tamped till the mortar flushes to the surface, and then left undisturbed till completely set, or till another layer is ready to be placed upon it. In such work it is better to have one layer follow another before the first has entirely set, so that they can become thoroughly bonded together. Whenever fresh material is placed upon or against old that has become dry and hard, the latter should first be wet in order to aid in this bonding. The amount of materials of the different kinds necessary to produce a given quantity of concrete is important. Enough has already been said to show upon what this is conditioned. Whether it will be economy to mix gravel with the broken stone if that be used, or whether one or the other is to be adopted, depends upon the ease with which they can be obtained and their relative values. It is the business of the engineer to study this question till it can be correctly settled. Having then determined upon the aggregate, and the amount of voids it contains, the amount of mortar is at once decided upon. Ordinary sand contains loose about 37 per cent of voids. Some tests to determine this, made in the laboratory of the Department of Highways, Borough of Brooklyn, New York City, resulted as follows: |