" 'The center shall be built for the full width of the bridge, and the crowns of the centers shall be raised above the intended height of the finished arches to allow for settling when the centers are being loaded and when struck. The centers shall be provided with striking wedges or sand boxes so that the lowering of the center may be effected slowly. The lagging for the center may be in two or more thicknesses if the contractor so desires; but the top layer shall be tongued-and grooved lumber, and the lower layers shall be dressed to match the top surface when in place. Lagging shall have a deflection under the full load of the arch of not more than The settlement of the center under its full load shall not be greater than for each 30 ft. in vertical height.' "Under such specifications the centering should not be paid for directly, but payment for same should be included in the price of the arch concrete in place. On large work I would suggest that the contractor be re quired to furnish ample fire protection. On one piece of work under my supervision, requiring nearly 2,000,000 ft. of lumber, the contractor installed a 4-in. water pipe, with hose connection for each arch.
"Engineering Supervision. On small work it is neces sary frequently to get along with a single inspector, whose duty it is to see that the forms are erected on the lines given by the engineer ; that these forms are stiff; that the materials and workmanship are in accordance with the specifications and approved plans; and that the rein forcement is properly placed. In addition to this, he looks after the removal of the forms, to see that the concrete work is not injured in careless removal, and that all corners are properly protected by wooden strips upon the removal of the forms. To this is added, as a rule, the clerical work of keeping track of the extra work and also the general cost of the work in its several details.
"If you have to break in a green man for this work, I would suggest selecting a carpenter. It is more diffi cult to get a good man to look after the forming than any other part of the work, excepting that of getting a good face, and if you have not a man experienced on face work you had better camp out on the job when it comes to this part of the work. Contractors will help themselves a good deal by getting experienced foremen and mechanics for face work. Wood butchers and cheap labor will not pay a contractor who is trying to get a high grade of finished work. Contractors should bear in mind that finished concrete work requires skilled labor, just such labor as is required for first-class stone and brick work.
"On any work, say, over $20,000, to be executed in a working year, I would suggest placing an additional low-grade inspector on the mixer. It occasionally occurs
that the laborer pouring in the cement, through careless ness, does not put in the right amounts, and I recall two cases on my own work when the cement was entirely omitted. Of course, on work executed in daylight, this omission would probably be detected, owing to the varia tion in its general appearance and color ; but on night work it might get into the work and afterwards cause serious trouble. It is also quite common for a laborer to throw out one-third or more of hard, lumpy cement without compensating for it. This does not make so much difference in mass work; but in thin work it is, of course, vital.
"It has been my practice to prohibit finished concrete work at night, although, if the contractor would provide a sufficient number of arc lights, there is no reason why the finished work cannot be so executed. Another reason for having an additional inspector on the mixer is that, as a rule, material is more or less superficially inspected by the engineer or inspector by going over the top of a car or barge and digging up a few pockets here and there. Both sand and gravel, however, frequently con tain layers of mud near the bottom, which the additional inspector on the mixer would be able to see and condemn.
"Lines and grades must be given more frequently on finished concrete work than on other classes of masonry, as it is difficult to correct small errors without unsightly results. It may be said in general also that the cost of inspection under existing conditions is higher, in order to get a first-class finished concrete job, than it would be for the same amount of brick or stone. It is thought that from to per cent of the total cost of the work is a proper charge for inspection on first-class concrete work.
"Records of the actual cost of all work should be kept, where possible, in order to determine upon the proper price for the extra work. The various inspectors and foremen under the direction of the assistant engi neer should agree each day as to the actual cost of the extra work. Assistant engineers should go over the extra work at least twice a month with the contractor's super intendent, so that the detailed cost may be properly adjudicated while the several items are fresh in the minds of the entire force. Very complex bookkeeping details, of course, generally result in their untimely death before the completion of the work; or, what is worse, in the loss of efficiency in the engineering and inspection forces. Clerical work is a serious problem to most first class inspectors, and therefore their reports should be simplified for them as much as possible by the use of printed forms.