Concrete Construction: Methods and Costs

by Halbert Powers Gillette; Charles Shattuck Hill

CHAPTER XIX.

METHODS AND COST OF REINFORCED CONCRETE BUILDING CONSTRUCTION.

If we set aside concrete block construction, virtually all concrete used in building construction is reinforced; plain monolithic or mass concrete now, as in the past, is one of the secondary building materials. It is reinforced concrete building construction that is discussed in this chapter. In no class of concrete work is the contractor's responsibility for the successful outcome of the work greater than in reinforced concrete building construction. No degree of excellence in design can make up for incompetent, careless or dishonest work in construction. This is true not merely in the general way that it is true of all engineering construction—it is true in a special way peculiar to the material. Except for the reinforcing steel, the contractor for concrete building work has no guarantee of the quality of any element of his work except his own faithful care in performing every task that combines to produce that element. The quality of his concrete depends upon the care with which he has chosen his cement, sand and stone, and on the perfection with which he has incorporated them into a homogeneous mixture. The quality of his beam or column, then, depends upon the care with which the concrete is placed in position with the reinforcement and with which the supporting forms are maintained until the member is amply strong to do without support. There is no certainty of any detail except the certainty that is had by performing every part of the work as experience has taught that it should be performed if perfect results are to be attained. We have dwelt thus emphatically on the responsibility in concrete building work of the contractor for the reason that in the past it has been upon the contractor that the burden of failure has been generally shifted.

The construction work of buildings is divided into (1) construction, erection and removal of forms; (2) fabrication and placing of reinforcement; (3) mixing, transporting and placing concrete.

CONSTRUCTION, ERECTION AND REMOVAL OF FORMS.

The stereotyped text-book statement that forms must be true to dimensions and shape and rigid enough in construction to maintain this condition under all loads that they have to sustain mentions only one of the factors that the constructing engineer or the contractor has to keep in mind in designing such forms. His design must be made true and rigid at the least possible cost for first construction of lumber and carpenter work; it must be made with the plan in mind of using either the same forms as a whole or the same form material several times in one structure; it must be made with a view to convenience in taking down, carrying and re-erecting the forms the second or third time; and it must be made with the object in sight of securing the greatest salvage value either in forms fit for use again or in form lumber that can be sold or worked up for other purposes.

The general conditions governing the computation and design of economic form work are discussed in Chapter IX.

COLUMN FORMS.—Concrete columns are usually square or rectangular in section, with, commonly, chamfered or beveled corners. The popularity of these sections is due very largely to the simplicity of the forms required. When hooped reinforcement is used, the column section is always circular or polygonal. Hollow sections, T-section and channel sections are rarely employed and then only for wall columns.

Column forms should be made in units which can readily be assembled, taken apart and re-assembled. The number, arrangement and size of the units are determined by the shape and size of the column and the means adopted for handling the forms. For square or rectangular columns there will be usually four units of lagging, one for each side, plus the number of clamps or yokes used to bind the sides together. Yokes or clamps will seldom be spaced over 3 ft. apart unless very heavy lagging is used; 2 ft. spacing for yokes is common. For circular columns two units of lagging are necessary and this is the number commonly used; the yokes or hoops are spaced about as for rectangular columns. Metal forms can be used to good advantage for cylindrical columns. Forms for polygonal columns are difficult to construct in convenient units. Forms built complete a full story high and concreted from the top are essential where wet and sloppy concretes are used. In Europe, where comparatively dry concretes are employed and where the reinforcement is commonly placed a piece at a time as concreting progresses, three sides of a rectangular form are erected full height and the fourth side is built up as the concrete and metal are placed. This construction is now less common, even abroad, than it was, since wetter mixtures are coming to be approved by European engineers to a greater extent now than formerly. It is a time consuming method and with wet mixtures it has nothing to recommend it. For lagging 1¼ and 2-in. plank are commonly used; with yokes spaced 2 ft. apart the lighter plank is amply strong and reduces the weight of the units to be handled as well as the amount of form lumber required.