2-in. stock, and for floor panels and beam and girder sides, 1-in. stock may be used; while footings may take either 1-in. or 2-in. stock, depending upon local conditions. The use of uprights of 3 X 4 in. or 4 X 4 in., spaced from 3 to 6 ft. apart, is the usual practice.

396. Finish of Forms.-All interior surfaces of forms should be dressed true to give good surfaces to the casting, free from joint and other marks. On exposed work, all sharp corners should be bevelled by inserting small triangular strips in the forms. After forms have been erected, ice and snow should not be allowed to collect therein, and, if so, such accumulations should be removed before concrete is deposited. If forms are allowed to stand for any considerable period before filling, shrinkage cracks will have to be taken up and all supporting timbers and braces gone over and tightened before pouring is resumed

To give specially smooth surfaces and to prevent the production of grain in the concrete surface, as well as to permit easy removal of forms, the interior surfaces may be coated with soft soap, or with form oil, which is crude, or other heavy mineral oil.

39c. Removal of Forms.-The elapsed time within which forms may be removed with safety, is not entirely a matter of individual judgment. The normal process through which setting concrete passes is such that, with the standard proportions and mixes now universally used, a minimum period is definitely established, below which the forms can not be disturbed without permanent impairment of the work. This minimum, under most favorable weather conditions, can be assumed at 5 days for vertical compression members, such as columns and walls, and from 7 to 10 days for such other members as are subject to bending stresses. From this up to the maximum period, the elapsed time should increase in widening ratio, depending upon the departure from normal of the three following factors: (1) the consistency of the original mix; (2) the temperature encountered in the first 7 days; and (3) the atmospheric conditions in the same period, with particular reference as to its humidity. Short of freezing weather, the maximum period should not exceed 21 days.

Before striking any set of centers, inspection of the work should be made, and its soundness determined by tapping with some heavy object, the resultant ring indicating the degree of hardness then obtaining. Care should be taken not to mistake frozen concrete for that which has set; and for this purpose the system of daily reports heretofore mentioned should be referred to.

The successful removal of forms is an operation requiring considerable skill, and unless carefully watched will undoubtedly be responsible for many unsuspected drains upon the payroll, to say nothing of the lumber pile. The foreman or superintendent who will personally supervise the striking of a set of difficult forms will never thereafter be caught with a poor design, or permit careless and faulty work in the erection.

Forms for circular columns and for flaring column heads, are quite generally of steel. Steel forms for floor and wall forms are also coming into extended use. They are made sectionally and in extensionable units to provide easy handling and adaptability to varying constructions. Steel forms give a full, smooth finish to the concrete surface, and have a high salvage value.

40. Bending and Placing Reinforcement.Steel should be checked, assorted, and stored as soon as it is delivered at the site. It should be blocked up free from the ground, and should be stored in such a manner that those rods needed first may be easily reached.

In localities where trade custom permits, the bending of reinforcement can quite often be done more economically at the mill than at the building site. Where this is not the case, suitable equipment, in the way of bending machines, many varieties of which are now on the market, should be provided, and will be found a good investment, particularly if the tonnage is heavy and the bends are complicated. For light tonnage and simple bends, a very satisfactory makeshift can be devised on a home-made bench with the aid of dowels or pins adjustable in position to suit the varying templates, and a piece of pipe to act as lever in applying the necessary bending force.

All ordinary bending is done cold. Only when working in sizes 14 in. and over will heat be required, and then only to a low degree. The bending force should be applied gradually, as a jerk is liable to snap or break the bars, particularly in the high grade and rerolled steels.

It will be found advisable to so arrange that all rods of the same size and shape are bent at one time, so as to avoid the resetting of templates.

1 See also Sect. 6, Art. 13; for more complete treatment see Hool and Johnson's, "Concrete Engineers' Handbook," pp. 139 to 146.

In the placing of reinforcement, the importance of a thorough supervision and inspection cannot be too greatly enlarged upon. Whether it be in the shape of rods, spirals, stirrups, fabric, or lath, the accurate placement and adjustment to position, so that it may not be displaced during subsequent operations, are details deserving of the closest attention. Not only is the safety of the work involved, but through the application of systematic principles, improved costs will likewise be found to result. For the proper spacing and support of reinforcement in the forms, many ingenious devices, such as chairs, clips, and spacers have been perfected and their use can often be recommended.

Steel should be thoroughly cleaned before being placed in the forms, in order to obtain a positive adhesion of the concrete to the steel. A slight film of rod rust is not objectionable, but no rod should be set in place on which rust scales have formed.

The fabrication of beam and girder reinforcement into units before placing, similar to that followed for columns, will, in many instances, work out to advantage both in time and money.

After reinforcement has been placed, and just before pouring, a thorough inspection should be made to determine whether all metal required by the plans has been supplied, that the forms are clean, and that all inserts, hangers, pipes, and other auxiliaries are in their proper position. On combination arches, filler tile, if broken, should be replaced, and the tile which have been disturbed again brought into alignment. In extremely dry and hot weather, a drenching of forms with water just before the concrete is deposited, will prevent excessive absorption and consequent robbing of water from the concrete mix.

41. Handling and Storage of Concrete Materials.—Satisfactory construction in concrete cannot be obtained if slipshod methods are allowed to dominate the field practice. While the quality and selection of materials entering into the work are generally determined by the purchasing or contract departments, the burden of final responsibility always rests with the field organization, who by the acceptance of inferior materials, may entirely defeat the promises of the most carefully prepared designs. Not only is the quality of the various materials to be watched, but the method of job handling and storage should be so worked out that no opportunity for their deterioration is permitted after delivery and before actual use.

In this respect, different operations require different treatment. On some, it may be quite sufficient to rely on daily deliveries of the stone, sand, and cement required. On others, where delivery facilities cannot be so well depended upon, a stock reserve will have to be provided. This may be either ground storage with planks beneath to prevent contamination from earth and other foreign matter, or it may take the shape of bins, should the storage involved be sufficient to warrant their construction. On large operations, where concreting is proceeding more or less continuously, the use of hoppers or bins becomes almost mandatory, and this is particularly true where a direct rail delivery of materials is had to the job. For filling such bins, use is made of either the continuous bucket conveyor, skip buckets operated by a hoist cable, or the clam-shell type of grab bucket, the latter being the more flexible arrangement if the equipment is available and if also there is space within which to operate. If storage bins can be set up so as to feed direct to the mixer, a second handling and considerable labor will be saved.

Remembering that the useful performance of cement is brought about through its chemical union with water during the mixing with other materials, it follows that storage of cement, whether in bags or bulk, should be in dry, air-tight storage rooms. Cement is avid for water at all times, and exposure to dampness in any degree before use proportionately lessens its ability when called upon to give useful service. Cement may be delivered in bags, barrels, or bulk. The latter method has many advocates, and when its use is permitted, the greatly lessened labor cost of handling will usually compensate for any risks taken in shipment. For bagged cement, gravity unloaders leading from the car or truck to the warehouse floor can be used to advantage. Individual shipments of cement should not be allowed to collect and remain for extended periods in job storage. Even at the expense of extra labor, if that be necessary, the older cement should be identified and worked up at frequent intervals.

The important relation which exists between the value of the empty cement container and that commodity itself is quite often underestimated by even those who should be the most interested. With cloth sacks carrying a reclaim value of 15c. each, it can be seen that from 20 to 25% of the initial cost per barrel is made up in this one item, and, as a consequence, any unaccounted for or damaged bags can produce a very marked effect on the cost units. One prolific source of loss comes through the original acceptance from the dealer of damaged or unsound bags, and a rigid inspection at time of delivery should be instituted to guard against such practice. Empty cement bags should be collected daily from each mixer, and suitably protected from the weather and against pilfering until they can be counted and bundled for return to the manufacturer.

The informal use of bags by workmen for protective coverings, foot wear, aprons, tool bags, etc., should at all times be prohibited.

Before bundling, each bag should be shaken in order to reclaim as much loose cement as possible. It has been estimated that the average waste per bag is 34 lb. from this source alone. Mechanical bag shakers have been devised for this purpose, and are being successfully used on many operations.

42. Measurement of Materials. In the proportioning and measurement of the several ingredients entering into concrete, long and continued custom has developed certain general methods which, while they do not by any means approach laboratory practice, at least, when handled with judgment, provide a certain amount of consistency of practice against the faults of which the designing engineer can and does guard by the selection of proper safety factors.

It is therefore essential that when 1-2-4 or 1-3-5 mix is specified, that the workmen and those in charge understand and can place the proper trade interpretation on the requirements. Ordinarily, a barrow of stone or sand used for charging purposes, is taken at 2 cu. ft., while the commercial bag of cement is considered as having a content of 1 cu. ft.

43. Mixing Concrete.-Mechanical mixers are now procurable in such sizes that any of the standard mixes can be run through at full batch capacity without resorting to split bags of cement. Water should be introduced only in measured quantities. Clean water only should be used, and having once found the quantity per batch required for the desired consistency, that amount only should be used for all subsequent batches that are formed from the same aggregate.

A daily count of all batches turned out by any one mixer should be kept by the workman, against which can be checked its total consumption of cement. If followed up consistently this will aid in detecting any departure from the uniform mix. Mechanical mixers are now built in so many different sizes that one will be found suitable for even the smallest of operations. As a result, hand-mixed concrete in these days has almost entirely disappeared from building construction practice.

44. Transporting Concrete.-The safe and economical transportation of concrete from the mixer to the forms is a subject always to be accorded the most careful consideration-not that the proven methods are innumerable, or in themselves greatly at variance, but so that the particular concrete operation under study is viewed in the proper relation which it bears to the structure as a whole. If it dominates, then the mixing plant and transportation system will be given the first consideration and all other facilities are made subordinate thereto. On the other hand, if this particular placement of concrete is not the controlling trade, then that arrangement will be chosen which best lends itself to the general scheme of material transportation, all trades being considered.

Some elementary principles upon which the ideal transportation system should be based, have been summed up briefly by one eminent authority as follows: (1) that the time interval elapsed between reception of concrete and its delivery to forms will not cause it to dry, or to take initial set; (2) that the system shall be tight, so that the more fluid portions may not be lost in transit; (3) that the mode of transit shall not permit a separation of ingredients; (4) that the delivery shall be approximately continuous, so that mixtures of varying composition may not be caused by stoppage and settling; (5) that it shall be efficient, rapid, and economical. In this summary, the order of importance is such as to emphasize quality of product delivered, as well as cheapness. With the foregoing in mind, the proper selection of the system, be it hand propulsion, vertical hoist, gravity, or a combination of the three, must be left to that individual best conversant with the situation, and responsible for results.

45. Placing of Concrete.-Hardly less important than the transportation of concrete is its placement in the forms. The placement should be such as not to permit the separation of the concrete into its component parts, as would occur if it were deposited continuously in one spot. Beams and slabs should be filled to the top in one continuous operation. When interruption to the work occurs, such as the end of the day's shift, stoppage should be made only at predetermined points where the security and safety of the work will not be affected. The general practice on floor slabs and beams is to make this at the center of the span with the stopping plane perpendicular and at right angles to same. Columns should be poured complete to the bottom of beams or girders in one operation, and then allowed to set before proceeding with the floor construction. When new concrete is deposited on old, special precaution must be taken to secure proper union.

While filling forms with concrete, too much attention cannot be given to the spading required if good workmanship is to be obtained. This action not only carries the heavier materials away from the face of the forms and so produces a denser and smoother surface, but it also eliminates to a large degree the air bubbles which otherwise form and if not removed account in part for porous work. The tapping of forms on the outside while concrete is being deposited is likewise beneficial in producing stronger work.

Concrete drippings from the runways, as well as any other material that has be mixed with the good, but should be rejected and removed from the building.

taken on the initial set, must not At the close of a day's work, all

mixers, buckets, spouts, buggies, runways, etc., should be thoroughly cleaned and flushed out with clean water. Particular attention should be given to mixers, and all accumulations of drippings removed while yet in a plastic state.

45a. Bonding New Concrete to Old.-True union between new and old concrete is difficult to secure. This difficulty increases as the water content of concrete is increased, both by tending to produce a film or layer of fluffy laitance at the top of the concrete first poured and also by diluting the cementing solutions in the new concrete, which solutions furnish the attaching or bonding means by contacting with and entering into the old or set material.

Customary recommendations are: (1) To roughen the old surface, (2) to thoroughly wet it, and (3) to coat it with a paste of neat cement, well rubbed in, before pouring the new concrete. These have a sound basis, except: (1) that the depth of roughening or removing old material should be so specified as to give assurance that clean, sound material, without dirt or laitance will be exposed; and (2) that the surface should be dry, not wet at the time the cement wash is applied, as wetting fills the pores in the set material with a fluid unchanged by cementing products and makes weak the cementing solutions directly at the plane of contact where they should be strongest for the result desired.

46. Finishing Concrete Surfaces.—Exposed surfaces of concrete require different treatments according: (1) to the excellence of their materials and workmanship, and (2) to their exposure and the character of their service.

The first of these is, as always, of chief importance. Thoroughly first-grade concrete, well placed and true to line, needs little or no finishing after it has set, except where architectural reasons require some special finish, such as, a matt, bush hammered, re-cut, or exposed aggregate finish. Inferior or careless work, on the other hand, necessitates long and arduous labor to make it presentable and useful.

46a. Removing Form Marks.-Careless building or bracing of forms always leaves a multitude of fins, edges, and blemishes. Fins may be readily removed by hand labor with hammer and chisel. Edges may be removed in like manner, but only with patient and careful labor. Blemishes, such as wood-graining, etc., can either be ground off, or can be effectively concealed by brushing with a wash of neat cement in water. Blemishes left by cutting off tie wire may be concealed by painting carefully with cement mortar before the cement wash is applied.

466. Repairing Surface Honeycomb.-Honeycomb is commonly viewed with lenience, but it is one of the surest evidences of poor workmanship in placing concrete. According to its location and the character of materials used in the concrete, it may or may not indicate a serious weakness. If strength is not impaired by its extent and presence, it may be concealed more or less permanently by painting or plastering over with cement-sand mortar, with a final cement wash to blend it into adjacent material. Such methods are, however, apologies at best. Honeycomb should properly be cut out and replaced with sound concrete.

46c. Grinding Concrete Surfaces. When concrete is moist and green, it is treated more easily than when dry and hard. Grinding of green surfaces may be readily done either by hand labor, using carborundum bricks, or by electric machines marketed for these purposes. Grinding makes a very effective finish where the cost is not prohibitive.

46d. Grinding Concrete Floor Surfaces.-Where "dusting," or other surface defects are found in concrete floors, grinding off from 16 tog in. is the surest and most effective remedial means. The dusting of concrete floors is usually caused by the use of excess water in the making of the concrete, or else by excessive ramming or floating. Removal of the loosetextured surface in such cases is necessary for lasting relief. Grinding machines, usually electrically driven, perform the work quite efficiently, though the process is tedious at best. Temporary relief may be had from chemical "hardeners" marketed under various trade names.

46e. Special Surface Finishes.-Tool-cut surfaces may be obtained in the usual way where cost is not prohibitive. Stippled surfaces, or exposed-aggregate surfaces may be obtained by wire-brushing the concrete when green, or by acid washes, or by both. The effects obtained can be greatly varied by the selection of various colors of aggregates and the means of treatment adopted.1

1 See "Concrete Engineers' Handbook" by Hool and Johnson for detailed treatment.

47. Concreting in Hot and Cold Weather.-During hot weather, excessive evaporation is liable to dry out the concrete before the cement has had a chance to set. The usual procedure under such conditions is to keep the work moistened and protected against the direct rays of the sun by a covering of some sort, such as sawdust or canvas.

In cold weather, even before the actual freezing point has been reached, it is customary to take special precautions. The day's work should not be started until there are indications that there will not be a sudden or decided drop in temperature, and the work should be suspended sufficiently early in the day to permit of proper protection from the cold nights and possible frost. This can be obtained by spreading out a coating of straw, canvas, or even sand and cinders, if they are being used on the work. When work has to be carried on below the freezing point, and 20 deg. is about the limit, artificial heat will have to be supplied by means of coke or coal fires in salamanders, the heat thus produced being confined within the building by a liberal use of canvas, or other protection around the exterior at the level at which the work is being carried on.

When working under extreme conditions of this nature, the preheating of water and aggregate will be found necessary in order that the concrete shall not freeze before it reaches the forms. Steam coils or live steam is the usual method of providing the heated water. Sand and stone if received at the job in a frozen condition, must be thawed out before being placed in the mixer. A wood fire within a metal drum or old section of smoke stack, and around which are piled the frozen lumps, is perhaps the most satisfactory method. Steam jets are helpful in working out frozen car loads or piles of sand and stone.

48. Floor Arch Systems. Most floor arch systems are called upon to provide concealment for horizontal pipe runs, conduits, etc. In concrete construction these pipes are often embedded in the arch itself. In tile construction, they must be run in a fill, generally of cinder concrete laid on top of the arch for that purpose. If the former system is being used, then the electrician, plumber, and any other pipe trade, whose lines may be involved, should be on the building with materials and full information, so as to install his work directly the centers have been erected. If the second method is being used, then the lines will not be required and should not be laid until means are provided to protect them with the concrete fill. On concrete arches where the upper surface provides also the floor finish, it is customary to complete the trowel or float finish at the time the arches are poured. When the cinder fill intervenes, the finish being cement or hard wood or strips, the arch only is installed at this time, leaving the balance of the work to be followed up by succeeding trades and at a later date.

Whether this floor fill should be placed before or after the ceilings are plastered, is dependent upon individual conditions. If the work is being done in winter weather, it is preferable to lay the floor fill first, thus obviating any possible damage to plaster ceilings below through dampness and freezing. In summer work this element is not present and if due precaution is taken not to permit the tamping of the fill into place to break the plaster bond below, then the plastering should be completed first.

Another consideration which sometimes holds the determining value, if viewed from the standpoint of economy, lies in the size of the various rooms in which the floor is to be divided. If the spaces are small and badly cut up, then it will be found much cheaper to lay the fill and finish as a loft floor holding back the dividing partitions and plastering for the next operation.

In the construction of floor systems each level should be completed in itself as far as possible before proceeding to the next, only such open spaces being left behind as are absolutely unavoidable. If the lower stories are to be devoted to construction driveways and to temporary storage of materials, the arches should be first installed and then planked for protection. It is not advisable to plank first and depend upon getting the arches in at a later and more convenient date. That time will never come and will only result in throwing the entire building out of sequence and step. The same holds true as well for upper levels.

Floor finishes of wood, cork, rubber, linoleum, etc., should never be laid until all danger of dampness from plaster or floor fill has been eliminated. A small piece of roofing felt or building paper laid upon the floor fill and sealed against the action of the air with a non-absorbent material, will upon inspection a few days later disclose the amount of dampness then prevalent in the building. This is a simple test and should be made use of before proceeding to install expensive floor finishes.