The sand and stone will naturally vary slightly from time to time, but should not differ materially from the requirements of the specifications or the standards adopted by the engineer. Stone or gravel that contains any considerable quantity, say more than 5%, of foreign matter, such as soil or refuse, should be rejected. Stone containing an excessive quantity of fine fragments or screenings bunched together in the pile, should be mixed with the mass of larger stone before it is used, so as to produce a mass of fairly even composition. Stone that is coated with mud, or that is not sound and hard should be thrown out.
Sand that is moderately coarse and does not contain more than 7½% of clay or soil, and is free from vegetable refuse, may be used.
21. Storing Materials.—Cement stored on the street must be stacked up on dry lumber floors at least four inches from the ground and effectually covered to protect it from rain or snow. Packages of cement which, when turned out for use contain hard lumps should be rejected.
Sand and stone must be stored on tight lumber floors to prevent their admixture, when being shoveled up, with street soil or other impurities. This requirement must be enforced.
22. Proportioning Concrete.—The proportions of cement, sand and stone or gravel named in the specifications must be strictly adhered to.
A barrel of Portland cement will be considered as four cubic feet and a standard bag of cement as one cubic foot.
It is usual for the contractor to measure the sand and stone by wheelbarrow loads. In that case you will measure and determine the capacity, in cubic feet, of the convenient or average wheelbarrow load, and determine the size and number of loads each of sand and stone required for one batch of concrete (if made by hand) and thereafter see that the quantities thus determined are used, and the wheelbarrow loading uniform. To measure the contents of a wheelbarrow load, use a square box holding one cubic foot. It cannot be accurately measured on the barrow.
As a check upon the proportioning of materials, you should occasionally keep a record of the quantities of cement, sand and stone used for, say, one or two hours, and mark and measure up the quantity of concrete made with them, and from this data compute the ratios of the materials used. (In ordinary street concrete, proportioned 1:3:6, the volume of rammed concrete made will just about equal the volume of the stone used.)
Where the concrete is mixed by a machine with automatic feeding devices, reliance must not be placed upon the machine for proper proportioning. Frequent tests of the materials used and the volume of concrete turned out should be made as described above, and any necessary corrections made in the feeding devices of the machine. This test should be made daily if practicable. The eye will usually detect any material change in the composition of the concrete, and whenever such changes are observed the composition ought to be at once checked up by measurement.
23. Mixing Concrete.—See that the specifications are followed. Use sufficient water to make what is commonly called a wet concrete but not so wet that free water will drain from the mixed batch before ramming. The test for proper mixing will be that all the fragments of stone are completely covered with mortar.