At the site of the proposed plant the sand and gravel must be mixed with the cement and carried to place. It has been found by experience when the mixing is done thoroughly and by hand, and when the resulting concrete can be shovelled directly behind the forms, that the cost of the mixing and placing is about one dollar per cubic yard of concrete. If the concrete has to be wheeled into place this cost will be added to.
In order to make a cubic yard of concrete, it is necessary to have nearly one cubic yard of the coarse material, whether this be rounded stones from a gravel bank or angular stones from a stone-crusher. Seven-eighths of a cubic yard of stone may be safely considered as necessary for a cubic yard of concrete. To this must be added three-eighths of a cubic yard of sand for a one to two and a half to five mixture. When this amount of stone and sand have been thoroughly mixed together, four and a half bags of cement should be added. Inasmuch as the variation in sizes of the individual particles of rounded gravel is such that a dense concrete results naturally, it is quite reasonable both to increase the amount of stone and decrease the amount of cement if that variation in size seems to be one which will produce a dense mixture. Thus one cubic yard of stone, one-third cubic yard of sand, and four bags of cement may be used and will, under favorable conditions, result in a good concrete. In order to determine whether this latter combination is permissible on any particular piece of work, a test may be made by thoroughly mixing the materials together in the proportions named and testing the volume of this mixture (B) in a box of measured dimensions. Then the same volumes mixed together in the former proportions (mixture A), and tested in the same box will show the relative value by occupying either more or less space than the other mixture (B). If less, mixture A is a better one, and should be used; if more, then the latter mixture, B, is the better one.
The amount of water required for mixing concrete depends upon the temperature of the outside air as well as upon the personal ideas of the person in charge of the mixing. Some builders like wet concrete and some like dry concrete. It should be noticed, however, that wet concrete is cheaper because it requires little or no tamping. Wet concrete, however, should be spaded, that is, a spade forced down into the mixture, particularly against the forms, so that particles of air caught between the stones may escape, and so that there may be no pockets between the stones into which the liquid cement mixture does not penetrate. It is generally considered that about fifteen per cent of the volume of concrete is the necessary volume of water for the mixture. This amounts to thirty gallons, or a barrel of water, to a yard of concrete, although the sizes of barrels vary, and a cement barrel would not be large enough, and a road-oil barrel would be too large.
The cost of forms depends, again, on the cost of material and on the cost of labor. Rough lumber varies in price from twenty to forty dollars a thousand feet, board measure, delivered on the grounds, and the cost of framing and placing it varies from eight to twenty dollars per thousand, depending on the skill of the carpenters and on their daily wages. In order to estimate the cost of the lumber required for building false work, it is best to determine exactly the amount of lumber required, and get the price from a lumber yard on that quantity. Ordinarily, it is safe to say that a carpenter in building forms will be able to saw and nail in place 250 board feet per day, so that, knowing the amount of lumber to be used and the wages of the carpenter, it will be easy to determine the cost of the forms as first set up. They may be taken down and removed for the purpose of re-assembling in another place for about half the cost of placing originally, and by carefully arranging to build the forms in panels or sections, they may be removed by a carpenter at the rate of 4,000 or 5,000 board feet per day.
Valves.—In connection with a sewage-disposal plant, valves are essential at many points. At the entrance to the several parts of the settling tank, flap valves are suitable to admit or keep out sewage from the several compartments. Gate valves are used on the by-pass lines and on connecting lines between the tank and the filter beds in order to be of service when it is occasionally necessary to clean the beds. More simple valves may be used in manholes where a diversion of the flow is required and where perfect and complete water-tightness is not essential. These valves may be made of plank, sliding up and down in grooves left in the concrete walls for that purpose. Sludge valves may be made to fit in the bottom of the tank, and depend for their water-tightness on the weight of the valve itself with the aid of a rubber packing which is placed between the valve and its setting. The cost of these various kinds of valves cannot be given exactly, since their cost depends upon freight and profit of the various commission men through whom the valves are ordered, but, generally speaking, they will be found to differ but little from the costs given in the following table:
| TABLE | |
|---|---|
| Flap valve as shown in Fig. [16] | $5.00 |
| Gate valve (iron bearings) for 6–inch pipe (Fig. [13]) | 20.00 |
| Gate valve (bronze bearings) for 6–inch pipe | 30.00 |
| Sludge valve as shown in Fig. [10] | 4.00 |
| Iron slide valve as shown in Fig. [11] | 15.00 |
Dosing Devices.—Dosing devices referred to in Chapter III are usually purchased directly from the manufacturer, and while their cost varies a little, depending upon the cost of freight, an ordinary single automatic siphon may be estimated at $15, the difference in price varying a little with the different makes of siphon. If an alternate discharge is required, then two siphons must be installed, by means of which alternate intermittency is secured, the variation, however, being only from one to the other and back again to the first. If a plural alternate discharge is to be used, the cost may be estimated roughly for a 6–inch siphon at from $50 to $75 for each unit, this price including the necessary piping but not the cost of setting.
Filling Material.—Artificial sand filters require a sand of uniform size and one free from dirt. These two requirements add very materially to the cost of sand, since it is almost impossible to find a natural sand which fulfils the necessary requirements. A few sections of the country are fortunate in having sand in the vicinity suitable for filtration purposes without any washing or screening. Such parts of the country, however, are limited to those where sand has been deposited by glacial action, and is essentially silicious in character. It is hopeless to expect to find suitable sand in the centre of New York State, for example, and even with washing and screening, sand in this locality is far from being desirable. It will be found, further, that after this undesirable sand is washed and screened the cost of the final product is so great that it is usually cheaper to use broken stone either as a filter or as a contact bed.
Washing sand in small quantities is done by throwing the sand into a channel through which water is passing, the sand being retained by a series of low partitions in the channel. If the water enters the box or channel through a pipe at the bottom, frequent entrance holes being provided along the sides and bottom of this pipe, the sand is kept in a state of suspension, the dirt more readily washed out, and a much smaller amount of water used. The cost of shovelling the sand into the washer and again out of the washer, about thirty cents per cubic yard, must be added to the original cost of the sand. The cost of water, if pumped by hand or by steam, will be about ten cents per cubic yard of sand cleaned, making the total cost about forty cents per cubic yard. If only three-fourths of the unwashed sand is available for use, then the cost of the final product is a little less than fifty cents per cubic yard. The sand before being brought to the washer will have been sifted at an additional cost of perhaps thirty cents per cubic yard. Hauling from the bank to the washer, or from the washer to the site of the disposal works, or both, if the water supply requires the washer to be placed at some distance from the sand bank, will add from fifty cents to one dollar a yard to the costs already indicated. It may generally be assumed that it will be impossible to put sand into an artificial filter for less than $1.50 a cubic yard, and it may easily cost $2.50 a yard if the sand bank is at considerable distance from the site of the works.
Broken stone in most parts of the country can now be bought from a stone-crushing plant. If road construction has been in progress in the vicinity, the contractor for the work has been obliged to open a quarry and set up a crushing-and-screening plant, and it will generally be possible to buy broken stone from such a contractor at about fifty cents per cubic yard. The cost of hauling and the cost of shovelling into the beds must be added to determine the cost of the stone in place. Sometimes it is cheaper to bring the stone from a distance by rail, such stone costing about $1.25 at the railroad station. Then the cost of hauling and shovelling must be added. It will be noticed that the cost of stone does not differ materially from the cost of sand, and since the amount of stone needed is only about one-quarter of the sand needed, it is generally cheaper to build a stone bed. The purification, it will be remembered, however, is decidedly inferior.