" 'Each batch of concrete shall be spread in place in horizontal layers not exceeding 6 in. in thickness, and shall be at once thoroughly compacted by ramming with rammers weighing about 25 lbs. each. Sufficient water must be used in making the concrete to enable the proper results to be obtained, but not enough to cause the concrete to quake under the rammer.' "In regard to the second paragraph quoted, I wish to state that on a great deal of work where a reasonably rich concrete is in use it is neither impracticable, in my judgment, nor undesirable, to bring the concrete up in 2-ft. layers if wet concrete is used. This, of course, is impossible when you get above ground—that is, on fin ished work—as such a course would be too deep for the spaded face work. There is a tendency, I believe, in the profession, to increase the layers on mass work; and I think possibly it results in getting better concrete. These large layers, of course, were impossible under the old system of using dry concrete. As to the rammer referred to, I might say that most work at present is spaded, the rammers only being used occasionally when a dry batch is brought on the work, more often by accident than design.
"The use of fine sand for concrete is deprecated where the concrete will be highly stressed.
"Good crushed rock makes the very best sand; but in passing upon crushed stone in lieu of sand, look out for rotten dust. Rotten rock going through the crusher is, as a rule, crushed to dust.
"Do not specify ornate details with spaded work, nor fine arrises. If these are desired, use a mortar face, or, what is better, use broken stone throughout from or down, depending upon the size of details desired. Tests indicate that a concrete made of the small aggregate is practically as strong as concrete of a large aggregate for the same percentage of cement.
"Plastering the surfaces of concrete before the next day's work is begun is unnecessary. Concrete should be kept wet as long as practicable, provided efflorescence is not more of a factor than 8 or 10 per cent additional strength.
"Concrete can be laid in cold weather in large masses in water and below the surface of the ground. One of the best ways to protect it below ground, is to permit the water to rise up over night, and then pump it out the following morning. The laitance and silt which accumulate under such procedure, of course, should be carefully cleaned off.
"Forms. The engineer should carefully study the forming of the work to be done, prior to the award of contract, and should specify in a general way the class of work which will be required of the contractor.
"A great deal of work is seriously marred by the lack of rigidity in the forms. Designers use the same
size wales and studding, whether or not the work is to be carried up 2 ft. or 10 ft. The height of a day's work should be always considered in the design of form, and the maximum height of a day's work used. Extreme care should be taken to reinforce corners of the framing. In designing forms for my own work I assume that con crete is a liquid of half its own weight; that is, 75 lbs. per cu. ft. This rule has resulted very satisfactorily.
"Forms should be designed so that they can be used in duplicate. It requires a great deal of study frequently, to design the forms so that they can be so used; but it means a great deal of money in the contractor's pocket.
"Bevel all small projections on forms for rustic joints, etc., so that these forms can be taken down without tearing off the edges of finished work. Bevel at least h-in. to the foot. The designer of forms should always consider the practicability of the economical removal of forms without injury to the concrete.
"Forms built up in section lengths should have these section lengths not longer than 6 or 8 ft. for convenience in handling and removal without injury to the complete work. For beams, camber the bottom of the forms slightly, so that any sag in the form will not give the appearance of sag to the beam.
"The contractor should be required to stop up all cracks in the forms with mortar or putty, so that the mortar will not give way. Tongued-and-grooved dressed flooring or similar lagging should be used on all face work, even if this work is to be dressed afterwards, because, if these forms are not practically water-tight, the mortar facing will get away, necessitating unsightly patches on the completed work.
"There is no practical difference in the appearance or strength of concrete, whether wood, sand, galvanized iron, cast iron, or plaster paris is used in the face-work forms.
"Arch Centers. It is thought that better results can be obtained, and also a lower price secured for the work, if the engineer designs the centering before bids are asked. It is certain that it gives the average contractor an opportunity to make a more intelligent bid. If the engineer is not experienced in this class of work, I would suggest that he leave the design entirely in the hands of the contractor, not attempting to cover the descrip tion of the center except in a very general way. I would suggest the following specifications for an inexperienced engineer : '"Complete drawings of the centers with a complete description of the various materials to be used shall be submitted by the contractor to the engineer.