cement and allow it to set protected from sun and wind. At the end of 20 minutes it should be soft and pliable, damp and not warm on exterior surface. At 10 hours it should be dry, firm, and hard enough to resist pressure of thumb nail. If it hardens or heats in less than 20 minutes the cement should be rejected, as it will set before the concrete is put into place. A cement which will not set in 10 hours will cause difficulty in placing the concrete and a satisfactory cement should set within these limits.

The heating referred to is due to free lime and, if in excess, the cement should not be used. Storage will convert the free lime into a hydrated condition in time, and it can then possibly be used.

Both the pat and ball tests are also serviceable in determining the presence of free lime. This free lime causes heating when mixed with water and also expands in volume, causing cracking of the pat or ball. When used in the work, over-limed cement may cause disintegration.

The presence of free lime can better be determined by subjecting the pat or ball to a hot steam bath for an hour or two, after which it should still remain sound and free from cracks. The presence of lime may also be determined by treating the cement with muriatic acid 3 parts, water I part, cement 1/3 part. A good cement will effervesce about two seconds. If it effervesces continually it contains too much limestone or natural cement and should not be used.

Cement in the laboratory is also subjected to what is known as the "Specific Gravity" test made by special apparatus and not available to the cement user on the work. The usual specific gravity of a good Portland is about 3.2. A much greater value shows overburning and a lower quantity indicates underburning or adulteration.

The test most frequently quoted is that for tensile strength which, like the previous one, cannot usually be made by the cement user, as time and apparatus are required, while uniform and reliable results depend upon the skill and experience of the tester. The tests are made by moulding briquettes into shape like a figure 8 having a cross-section in the centre of exactly 1 square inch. These briquettes are allowed to set either 1, 7, or 28 days and sometimes for even longer periods running into many years. They are then broken in testing machines. The briquettes are made both of pure

cement and of cement mortar mixed with varying proportions of sand. These tests are of value because long years of experience have fixed certain values which a good cement should obtain and although pure cement is little used in practice, a standard is thus fixed which serves as a basis of comparison for different cements.

Tests are also made on large works by moulding beams, slabs, blocks, and columns of various mixtures of concrete, which are later subjected to special machines, and broken by bending, shearing, or compression, and the actual strength determined.

It is proper to say here that to the credit of American cement manufacturers, the consumer need have but little fear of the quality of the cement he uses. The great bulk of cement of any of the standard brands will pass the ordinary requirements. Moreover the cement work in most structures is never subjected to anything like the stresses that the strength tests show it is able to withstand. It is only in work where very high unit stresses are employed, such as in reinforced concrete structures that the actual strength of the material is really approached. It is due largely to the uniformly good quality of cement turned out that the greatest confidence has been established in the mind of the consumer as to its use without testing, and it is due largely to such confidence that the cement industry owes its rapid growth, for without it the present phenomenal expansion would have been impossible.

The ordinary cement user should be particularly careful about two things in a newly received shipment of cement. In times of great building activity when the cement mills are run up to full capacity, there is danger of having the cement too fresh, and in such cases he should order it a month or so ahead of time so as to improve it by storage as already referred to. The second thing is to see that the cement has not been injured in transit or storage, for if dampness has reached the cement it will be lumpy and partially set and its usefulness be largely destroyed.

REQUIREMENTS FOR CEMENTS

The following are the requirements for natural and Portland. cement prepared by the National Association of Cement Users, after an exhaustive study of the subject.

STANDARD REQUIREMENTS FOR NATURAL CEMENT

Definition. This term shall be applied to the finely pulverized product resulting from the calcination of an argillaceous limestone at a temperature only sufficient to drive off the carbonic acid gas.

Fineness. It shall leave by weight a residue of not more than 10 per cent on the No. 100, and 30 per cent on the No. 200 sieve. Time of Setting.-It shall not develop initial set in less than ten minutes, and shall not hard set in less than thirty minutes, or in more than three hours.

Tensile Strength. The minimum requirements for tensile strength for briquettes one inch square in cross-section shall be within the following limits, and shall show no retrogression in strength within the periods specified:

Constancy of Volume.-Pats of neat cement about three inches in diameter, one-half inch thick at centre, tapering to a thin edge, shall be kept in moist air for a period of twenty-four hours.

(a) A pat is then kept in air at normal temperature.

(b) Another is kept in water maintained as near 70° F. as practicable.

These pats are observed at intervals for at least 28 days, and, to satisfactorily pass the tests, should remain firm and hard and show no signs of distortion, checking, cracking, or disintegrating.

STANDARD REQUIREMENTS FOR PORTLAND CEMENT

Definition. This term is applied to the finely pulverized product resulting from the calcination to incipient fusion of an

intimate mixture of properly proportioned argillaceous and calcareous materials, and to which no addition greater than 3 per cent has been made subsequent to calcination.

Specific Gravity.-The specific gravity of the cement ignited at a low red heat shall not be less than 3.10; and the cement shall not show a loss on ignition of more than 4 per cent.

Fineness. It shall leave by weight a residue of not more than 8 per cent on the No. 100, and not more than 25 per cent on the No. 200 sieve.

Time of Setting.-It shall not develop initial set in less than thirty minutes; and must develop hard set in not less than one hour, nor more than ten hours.

Tensile Strength. The minimum requirements for tensile strength for briquettes one inch square in section shall be within the following limits, and shall show no retrogression in strength within the periods specified:

ONE PART CEMENT, THREE PARTS SAND.

7 days (1 day in moist air, 6 days in water) 28 days (1 day in moist air, 27 days in water)

150-200

200-300

Constancy of Volume.-Pats of neat cement about three inches in diameter, one-half inch thick at the centre, and tapering to a thin edge, shall be kept in moist air for a period of twenty-four hours.

(a) A pat is then kept in air at normal temperature and observed at intervals for at least 28 days.

(b) Another is kept in water maintained as near 70° F. as practicable and observed at intervals of at least 28 days.

(c) A third pat is exposed in any convenient way in an atmosphere of steam, above boiling water, in a loosely closed vessel, for five hours. These pats, to satisfactorily pass the requirements, shall remain firm and hard and show no sign of checking, cracking, and disintegrating.

CHAPTER IV

CONCRETE AND ITS PROPERTIES

What Concrete Is.-Kinds of Concrete.-Function and Effect of the Cement, Aggregates, Water, Chemicals, Weather Conditions, Gases, Sewage, etc.—Laws of Strength and Permeability.

CONCRETE is an artificial rock, made by uniting sand, broken stone, gravel, etc., by means of lime or cement. Its principal ingredients are as follows:

1. The matrix or mortar; consisting of cement and sand mixed with water.

2. The coarse aggregate; broken stone, gravel, etc.

Concrete made with good Portland cement, in proper proportions, becomes so hard and strong that when pieces are broken, the line of fracture will often be found to pass through the particles of stone, showing that the adhesion of the cement to the stone is greater than the strength of the stone itself.

Kinds of Concrete.-While concrete is generally composed of cement, sand, and broken stone or gravel, the following special combinations are also used:

1. Rubble concrete, also called Cyclopean masonry.

2. Cinder concrete.

3. Asphalt concrete.

4. Reinforced concrete.

In constructing massive walls and dams, a reduction in cost may often be obtained by introducing large stones into the concrete. Concrete of this character is called Rubble Concrete or Cyclopean Masonry. The percentage of rubble stones employed varies from a few per cent to over half the volume. The saving effected comes partly from the reduction in the cement required per cubic yard of concrete and partly from the saving in crushing.

Cinder concrete is used where great strength is not required. Its most valuable properties are its light weight and the resistance which it offers to heat. It is therefore used for fireproofing and light floor construction.