TABLE NO. 30. Proportions: cement 1, sand 1, volume; cement 21 ounces; sand 23 ounces; water 6 ounces. Figures are tensile strength per square inch. 90 Days. 6 Months. 9 Months. 12 Months 18 Months 2 Years. Immersed in test-room when removed from moulds....... Exposed to air and frozen three days, then immersed in test-room four days...... Immersed in test-room when removed from moulds... 327 357 375 392 429 402 415 388 402 316 378 411 374 415 405 392 383 409 336 422 421 399 394 384 390 356 387 Exposed to air and frozen six days, then exposed to air in test-room at 70° one day. 169 198 167 217 215 208 221 221 239 Set No. 2 was kept on damp. boards in a closed tank for the whole period, and never allowed to dry out. Set No. 3 was allowed to set in the laboratory, and then exposed to the severe frost and left in open air for the whole period. Set No. 4 was exposed in from 8 to 10 minutes to the severe frost and left there for the whole period. The important deductions from the Portland tests are: 1. That mortar immersed in water is stronger than when used in air; 2. That mortar exposed to temperature below freezing and kept there till set is stronger than when allowed to set in air and then exposed to frost; 3. That mortar kept in damp air was the weakest of all the different conditions experimented on. It will be noticed from the results of the tests of the natural cement: 1. That, contrary to the Portlands, these cements should not be used if the mortar must be exposed at once to frosts; 2. That from the neat tests no time deductions can be made of a sand mixture, as in every case when mixed with fresh water the 1-to-1 compound was considerably stronger than the neat; 3. That No. 2 in every case but one was the strongest, while with the Portland it was the weakest; 4. That the addition of salt to the mixing water added very materially to the strength of the briquettes when exposed to the frost. Table No. 33 gives the results of some experiments made by Mr. A. C. Hobart and published in The Technograph, No. 12, 1897-98. In all cases the briquettes were frozen six days after having been allowed to set, as shown in the table. They were thawed from 18 to 20 hours and then broken. The upper line of figures for each mortar is the strength in pounds per square inch of the unfrozen briquettes, and the lower is the percentage of the strength frozen to the strength unfrozen. Table No. 34 gives the result of some tests made on 12-inch concrete cubes by Mr. W. A. Rogers, Assistant Engineer of the Chicago, Milwaukee, and St. Paul Railway at Chicago. "Atlas" Portland and Louisville natural cements were used. The proportions were: Atlas, 1 cement, 3 gravel, and 4 broken stone; and Louisville, 1 cement, 2 gravel, and 4 broken stone. Eight cubes were made of each cement, two being mixed with water to which Age in Hours when Frozen. 1 2 3 6 12 24 481 72 168 336 321 337 341 372 374 400 352 379 327 672 68 72 73 79 80 80 67 69 52 100 186 187 193 296 331 133 177 172 172 184 58 77 75 75 33 43 62 58 26 34 48 45 00 00 10 14 8 18 00 00 285 238 234 268 284 248 240 300 590 671 60 50 50 59 50 45 56 110 110 105 144 176 173 175 179 255 260 271 303 391 47 57 56 57 58 79 79 82 68 87 61 81 96 102 129 143 138 146 161 226 55 73 86 92 116 129 120 108 98 128 18 26 15 33 48 54 69 74 87 124 32 46 27 59 86 96 113 114 121 125 120 116 127 152 163 143 135 140 154 199 one pint of salt to ten quarts of water had been added, and the others with fresh water. Capacity of machine 185,000 pounds. a showed signs of failure, b showed no signs of failure. The cubes kept out of doors were subjected at once to a temperature considerably below zero. During this exposure the weather was the coldest experienced in Chicago for twenty years, but subsequently grew warmer, so the cubes froze during the night and thawed during the day. The deductions the author of the paper makes for the mixture is: "Freezing before setting does not seem to injure the Portland-cement concrete even if, after having frozen hard, the concrete is exposed to freezing and thawing weather. Exposing green Portland cement concrete to a freezing temperature seems to affect its rate of hardening, making it slower, but eventually the concrete will be just as good as if it had not been exposed to the cold. The use of salt seems largely to counteract the effect of cold in causing slow hardening." He also makes the same deductions for Louisville cement, except that he thinks the use of salt seems to have little if any effect on the strength of the cubes exposed to the cold. Mr. Noble describes the construction of an anchor-block of concrete. This was built during freezing weather, a portion of the time below zero, with about one-half of the mass below water. The mixture was 1 part Milwaukee cement, 2 parts sand, and 4 to 5 parts broken stone. The material and water were heated, a double handful of salt being added to each part of water. Ice formed over the top of the concrete every night until the mass was above the water-level. No attempt was made to protect the concrete from frost, and six months after it was laid it was found to be thoroughly set. These experiments cover quite a period of time and were made by different people under very different conditions. As a rule the same general deductions can be made from them. That is, that with proper precautions good results can be obtained by the use of cement mortar in cold weather; that a freezing temperature 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. Cement fresh from barrel, average of five repulverized after 3 days' set, average of six.... . . .. 236 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 |