Rules Specifications

"Work in Cold Weather. Don't use frozen mate rials for concrete work. By cutting off the crust of stone or sand, you will get unfrozen material. All hard lumps should be thrown out. When concreting below the freezing point, the stone and sand should be heated, preferably with a coil of steam pipe or with a steam jet inserted at the points where the material is being re moved. In this connection, however, it is well to remem ber that if the material so steamed is not used at once it will give you more water and therefore more ice to contend with in your pile. With a little care you should be able to place your concrete in the work at a tempera ture of 60° F., regardless of the temperature of the surrounding atmosphere.

"In the winter time it is well to cut down the area of your concrete work by use of bulkheads, so as to decrease the time of exposure of a single layer, which, of course, will result in running your work up to a greater height for a day's work, thereby necessitating stronger forms. There is no trouble at all in executing heavy masonry work, particularly below ground, in the coldest weather, if your materials are heated. However, in thin work (walls under 2 ft. in thickness) it is somewhat dangerous below 25° F.

"Where you have a spaded or mortar finish, extreme care should be taken to protect the work, preferably by covering the outside of the studs with a layer of tar paper. This makes an ice-box sort of arrangement, leaving an air-space of the thickness of your studs be tween the paper and your lagging. Care should be taken to tack this paper on securely, as it is not infrequent to see paper used in this manner blown off before your concrete has had time to set. Remember that cold weather retards the time of setting of concrete. I have known a number of cases where the time of setting was two or three times as long as in hot weather. This, of course, does not apply to mass concrete.

"On thin work I would suggest limiting the time of concreting to 28° on a rising thermometer, and 32° on a falling thermometer. On particular work (thin work) I have found the forecast of the Weather Bureau of great assistance in determining whether or not it would be advisable to concrete. During three months

of the winter when I was carrying on a very careful piece of work, I found the minimum temperatures pre dicted by the Weather Bureau very accurate indeed.

"Where I have used tar paper on the outside of trimming, I have found the temperature of the air be tween the paper and the lagging from 8° to 10° warmer than the outside air at its minimum temperature; and it is well to bear in mind also that the side from which the wind blows is more liable to be frozen than the sides that are protected from the wind. In my own work I take special pains to protect the north and west side of the structure.

"When concreting at low temperature, particularly with a mortar facing, I would suggest to the inexperi enced engineer cutting out small blocks of the lagging, say, 4x12 in., at different points of the most exposed sides of the work, in order to note whether or not the mortar facing is being affected by frost. One morning, when the temperature at 8 a. m. was 10° above zero, with a stiff wind blowing, I found the temperature of the air between the paper and the lagging to be 20°; and, upon cutting out the several blocks of lagging, I found a few ice crystals on the inside of the lagging, but the mortar facing was warm and was not affected at all. The temperature of the concrete at placing was about 60°. The temperature of the air at the time of concreting was probably a little below 30° F. This same morning I took off from the top surface two layers of cement bags which had been used to protect the top surface of the concrete, and found the bags frozen, and ice crystals on the under side of the bags. At two or three spots where the bags had not been properly lapped the concrete was frozen to a depth of 1 in. The balance of the concrete was not affected by frost.

The specifications given below are an exam ple of an attempt made by a municipality to standardize reinforced concrete bridge construc tion.