Concrete Block Industry

Curing.

This is a most important step in the process of manufacture, second only to the proportioning, mixing, and moulding, and, if not properly done, will result either in great in jury to or the complete ruin of the blocks. Blocks shall be kept moist by thorough and frequent sprinkling, or other suitable methods, under cover, protected from dry heat or wind currents for at least seven days. After removal from the curing shed, they shall be handled with extreme care, and at intervals of one or two days shall be thoroughly wet by hose sprinkling or other convenient methods. We recommend curing in an atmosphere thoroughly impregnated with steam. This method serves to supply needed moisture, prevents evaporation, and in some measure accelerates the hardening of the blocks.

We view with distrust, in the present knowl edge of the chemistry of cement, any artificial, patented, or mysterious methods of effecting the quick hardening of cement blocks or other cement products. If such method be proposed, it should be thoroughly investigated by compe tent authority before use.

Time of Curing.

This is also most important in its effect upon the industry, and is directly and vitally influenced by the following condi tions: 1. Quality, quantity, and setting properties of the cement used.

2. Quality, size, and quantity of the sand or fine aggregates used.

3. Amount and temperature of water used.

4. Degree of thoroughness with which the mixture is made.

5. Method of curing, weather conditions, and temperature.

6. Density of the block as affected by the method and thoroughness of tamping, or by the pressure applied.

Before fixing the minimum permissible time required in curing and aging blocks, it is well to consider the important effect of additions of sand upon the tensile strength of cement mortar.

The following tabulation has been interpo lated from the diagram of cement mortar tests prepared by Mr. W. Purves Taylor, of the Phila delphia Municipal Laboratory.

The results of the neat tests and the 1 to 3 mortar tests (that is, one part cement to 3 parts crushed quartz by weight) are averaged from over 100,000 tests, while the other results are based on from 300 to 500 tests.

It must also be kept in mind that these results are obtained under practically uniform and theo retically correct conditions, in the amount of water used, thoroughness of mixing, and mould ing and storage of samples until tested.

Comparing the results at 28 days, it is appar ent that the 1 to 5 mortar has only 71 per cent of the strength of the 1 to 4 mortar, and but 50 per cent of the strength of a 1 to 3 mortar. The 1 to 4 mortar has but 70 per cent of the strength of a 1 to 3 mortar, and 46 per cent of the strength of a 1 to 2 mortar.

The ratio of compressive strength to tensile strength is not quite constant for all periods of time, and for the several mixtures above given; but the compressive strength, or resistance to crushing per square inch, may be approximately obtained by multiplying the tensile strength given in the above table by the constant six (6). (Note—This would increase with the age of the mortar, and would be greater for good gravel or stone concrete than for the clear mortar of which a given concrete is made.) In fixing the minimum time required for cur ing and aging blocks before use, due regard should be given to the proportions used. It is manifestly wrong in principle to require as long a period for to 2 or al. to 3 block as might seem necessary for a 1 to 4 or a 1 to 5 block; and it is obviously unsafe to attempt to use a block of lean proportions in as short a time as a rich mixture would gain the necessary strength.

This might be supposed to be met by fixing the minimum resistance to crushing of blocks (of all compositions) ; but it must be kept in mind that a very small percentage of the blocks used are tested, by reason of the expense, incon venience, or lack of facilities.

The required minimum resistance to crush ing of first-class blocks used for exterior and bearing walls should not be imposed upon blocks for minor and less important uses.

Marking. All cement blocks should be stamped (in process of making), showing name of manufacturer, date (day, month, and year) made, and composition or proportions used. The place of manufacture, methods, and materials should also be open to inspection by representa tives of the Building Department, the architect, engineer, or individual buyer.