Rule 3. Under average conditions, forms should remain in place for the following periods:

Walls in mass work from 1 to 3 days.

Thin walls and columns from 2 to 5 days, according to weather conditions as noted above.

Slabs up to 6 feet span, from 1 to 2 weeks.

Beams and girders, from 2 to 4 weeks.

Small arches, from 1 to 3 weeks; large arches from 1 to 2 months. Rule 4. Forms should be removed gently without chipping or jarring the concrete. Prying with bars or striking with a sledge should be prohibited.

Rule 5. Column forms should be so constructed as to permit of their removal without disturbing the beam or slab forms.

Rule 6. Beam forms should be so constructed as to permit of the removal of the sides before the bottom is disturbed in order that the condition of the concrete can be examined.

Rule 7. Beams should be supported by shores for a considerable time after the forms have been removed, or until the concrete has become thoroughly cured.

Rule 8. Arch centres must be removed without shock or jar to the arch ring. Centres should be lowered evenly and gradually, so that the ring can settle uniformly.

Rules for Surface Finish.-Surface finishes are of two kinds: (a) Those in which the moulded surface is treated after the forms are removed.

(b) Those in which the moulding is so done that the finish is a part of the moulding process.

The following rules should be observed for class (a):

Rule 1. If the surface is to be grouted all holes and joint marks must be filled or smoothed down before the grout is applied.

Rule 2. When the surface is to be tooled, from 30 to 60 days must elapse before the concrete is hard enough to give a good, clean tool cut.

Rule 3. When scrubbed, the scrubbing should be continued just long enough to remove the surface cement and to partially expose the aggregate without loosening it.

Rule 4. When etched with acid, the acid must not be allowed to remain too long, and all excess acid must be removed by washing.

Spaded or mortar finishes are used for class (b). The following rules should be observed:

Rule 1. Spading is best done with a special flat-bladed spade, having the blade perforated with holes or slots, which will screen back the stones and allow the mortar to pass.

Rule 2. In a mortar finish, the facing mortar and concrete backing are placed at the same time and are tamped together. The tamping should not be so hard as to force pieces of stone through the facing, but hard enough to bond thoroughly the facing mortar and backing.

Rule 3. The preferable method of construction is to use a facing form between the lagging and the backing. Fill between the facing form and the lagging with mortar, then fill behind the facing form with the backing, and finally withdraw the facing form and tamp backing and facing together.

Moulded Blocks, Piles, Ornamental Castings, Etc.-Three general processes are employed for moulding cast concrete work:

(a) A dry mixture is heavily tamped into a mould and the block is immediately released and set aside for curing.

(b) A liquid mixture is poured into moulds where the blocks remain until hard.

(c) A medium wet mixture is compressed into moulds by hydraulic presses or other means of securing great pressure. The following rules should be observed for Dry Mixture Blocks.Rule 1. For dry mixtures the mixing and tamping must be thorough and the water uniformly distributed. Tamping should begin with the first shovelful and should be continued until the mould is filled.

Rule 2. Dry mixtures should have a consistency such that the block will part from the mould without sticking, sloughing, sagging, or loss of form. Dryness in excess of these requirements should not be allowed.

Rule 3. Moulds must be rigid and adequately clamped. The construction should be such that the green blocks are not injured when removed.

Rule 4. After removal, the dry mixture blocks must be stacked in a horizontal position on immovable supports and freely sprinkled with water. A dry mixture block does not have enough mixing

water to enable the cement to set and harden perfectly, and this deficiency must be supplied by sprinkling. The sprinkling should begin within an hour after moulding and should continue for at least ten days. While the block is soft, the sprinkling should consist of a gentle spray, that will not wash the concrete.

Rule 5. Blocks should be cured for at least 30 days before they are removed from the storage yards for use in construction.

The following rules apply to wet mixtures: Rule 1. The mixture must be thoroughly stirred and churned to eliminate air voids, prevent arching and fill corners and edges of moulds.

Rule 2. The mould must not be removed until the concrete has thoroughly set and is hard enough to do without its support.

Rule 3. The block must be true to shape and exact in dimensions, with faces true to plane, and edges true to line. Mouldings and other ornamentations must be perfect. A moulded block should be equal in perfection to cut stone in all particulars of shape and dimensions.

Rules for Concrete Piles.-Concrete piles are driven (a) by punching a hole in the ground by means of a metal mould and filling with concrete; (b) by casting the piles in moulds and driving by aid of a water jet.

The following rules should be observed for concrete piles in place:

Rule 1. In driving the shell for new piles, care must be taken that adjacent piles in which the concrete is still green are not jarred and injured.

Rule 2. In concreting piles in place, the concrete must be lowered in small buckets or in such a way that the cement is not separated from the stone.

Rule 3. The reinforcement must be set parallel to and concentric with the axis of the pile. The best practice is to assemble the reinforcement into a unit frame, and to place it as a unit.

The following should be observed for cast piles:

1. Cast piles should be straight, the metal points, when used, firmly attached and the pile should be without cracks or chipping. None of the reinforcing metal should be exposed. If cored for sinking by water jet, the cores must be open and unobstructed. If fluted on the sides to provide passages for the rise of

water used in jetting, the flutes or corrugations must not be obstructed.

2. Moulds should be straight and kept true to line and level.

3. The reinforcement must be kept parallel to and concentric with the axis of the mould.

4. The concrete should be poured at several points along the mould to prevent flowing and segregation.

5. The driving should be done in such a way that the pile is not fractured in the body. The head should be protected by a cushion cap to take the direct blow of the hammer. If the driving is done by a water jet, the pile should settle to a firm bearing.

6. Cast piles should not be dragged along the ground or otherwise roughly handled.

Ornamental Castings.-In ornamental castings, great care must be taken in the moulding, handling, and setting in place to preserve the true lines, flutings, and other ornamentations. When white cements, stainless mortars, or other special materials are required, the inspector should take particular pains to insure the use of the proper ingredients. The general rules for cast blocks and pile; apply with additional force to all ornamental work.

CHAPTER XXXIII

COST OF CONCRETE WORK

General Cost of Main Classes of Work.-Elements of Cost.-Cost of Materials.— Cost of Mixing.-Cost of Placing.--General Expenses.-Summary of Costs.-Cost of Mortar.-Actual Examples of Cost.-Building Blocks.—Paving.—Removing Efflorescence.-Stucco.-Forms.-Cost of Buildings in Terms of Cubical Contents. Cost of Residences.—Cost of Sewers.-Concrete Pipes.-Bridge Piers and Bridges.-Piles.-Trestles, Sidewalks, Curbs, and Gutters.-Fence Posts.-Poles. -Roofs.-Tunnel Lining.-Waterproofing.-Cost of Concrete Dams.

THE Cost of concrete construction is made up of the combined cost of materials and labor. The cost of materials for any given class of work is readily determined from the dimensions of the structure and the market prices of cement, sand, broken stone, timber, steel, etc.; the labor cost, however, is dependent not only upon the prevailing rate of wages, but also upon the efficiency of the men employed, the amount of form work, and the character of the construction.

The cheapest construction is obtained when the concrete is deposited in large masses and when the transportation, mixing, and depositing in place is performed by machinery. When laid in thin sections, as in tunnel linings, small arches, thin walls, etc., the use of forms and of hand labor per cubic yard of concrete is very largely increased, which greatly augments the unit cost of the construction.

General Cost of Main Classes of Work.-Where Portland cement can be obtained at $1.50 per barrel, sand at 80 cents per cubic yard, and broken stone at $1.50 per cubic yard delivered on the work; and where the cost of form timber does not exceed $25.00 per M; while the rate of wages for carpenters is $3.50, laborers, $1.75, and teams $3.75 per ten-hour day, the cost of concreting, including interest and depreciation on plant, but with no allowance for profits, will run about as follows:

Heavy mass constructions, as large dams, reservoir

walls, pavements, heavy foundations, abutments, rubble $3.50 to $5.00 per cu. yd. concrete, etc.