790. Character Of Materials For Mortar And Concrete

It is unnecessary to repeat here the requisites for proper materials for mortar and concrete. The cement required for concrete blocks is first-class Portland, but no special properties are necessary. The early hardening rotary-kiln cements now on the market are well adapted to block making. As it is not usually possible for the block manufacturer to make elaborate series of tests, he should confine his choice to brands of established reputation. The factory tests, checked occasionally by a reliable independent laboratory, will indicate uniformity.

Having selected a brand, it is well to continue its use as long as it is satisfactory. It is almost useless to attempt to explain away the effects of a bad run of blocks by saying the cement was poor; the reputation gained by months of honest endeavor and good results receives a serious set-back.

The first essential of a cement is soundness; strength for a given Cost comes next, and the great majority of tests of cement are made to determine these two properties. Ease of working and color are also important in cement block manufacture. Having become acquainted with the manipulation necessary to obtain the best results with one brand, some experimentation will be necessary to become as familiar with a new brand. The color of the cement is of more moment here than in other uses. The color of the resulting product is influenced much more by the sand and aggregate than by the cement, yet in working with the same aggregate a change in the cement may affect the color of the block.

The requisites for good sand are given elsewhere in this volume. Impurities in sand may be objectionable in concrete block making, although the strength of the mixture may not be affected thereby. Dirty sand used in mortar appearing on the face of a block may give a " flat " appearance, lacking in character and tone. On the other hand, some impurities give to the product a most desirable color without any serious effect upon the strength and durability. Where a choice of sands is possible, the block manufacturer, if not himself capable of judging between them, should seek competent advice. The grading of the fineness of the sand is another important question in its relation to strength and texture. (See Chapter XI).

Aggregate for concrete blocks must in general be quite fine. Whether it be broken stone or gravel, it is seldom wise to use particles too coarse to pass a one-inch square mesh, and usually three-quarter inch pieces are better. The effect of size and character of stone, and the desirability of decreasing the voids by careful grading of size, have been fully covered elsewhere. (See Chapter XIII.) The character of surface and color of the aggregate are of great importance also in their effect on the appearance of the product. If several kinds of aggregate are available, the maker should experiment with them to determine what variations he can make in the surface appearance without too much expense. One who is destined to success will not be content until he has learned the possibilities- of the available materials.

791. Proportions Of Materials

The considerations which must fix the proportions to be used in concrete block manufacture are impermeability, strength, especially at early periods, and appearance. While strength is of great importance, it is not this property that will usually fix the amount of cement necessary in a given quantity of sand or aggregate. If made with sound cement it is not at all likely that a block which stands transportation to the building will fail from lack of strength when properly laid in an ordinary wall. This is evident if we recall that the ultimate compressive strength of mortar containing six parts sand to one Portland cement is about 1,000 lbs. per square inch, and that a prism of mortar one inch square and a foot high weighs only one pound. In order that a mortar or concrete shall be impervious, however, the voids in the sand must be filled with cement, and the voids in the broken stone or gravel must be filled with the mortar; and as impermeability is one of the most important qualities of a concrete block, the aggregate should be so graded that the voids shall be as low as possible.

Since, as a general rule, concrete properly proportioned is not only as strong, but nearly as impervious as the mortar alone, it follows that concrete is much more economical than mortar for use in making blocks. Thus, with a well-graded sand containing both coarse and fine particles, a compact mortar may be made with one part cement to three parts sand, while to the same mortar we may add at least five or six parts of broken stone, and the resulting concrete will be nearly as impervious as the mortar alone. With ordinary sand it will be found necessary to use one part cement to not more than three parts sand to assure impermeability, but of well-graded gravel as much as five parts to one cement may be employed. With artificial crushed products, that is, broken stone, one cement to two and a half or three parts sand by weight makes a good mortar. The quantity of mortar to be used should somewhat exceed the voids in the broken stone. If no broken stone is employed, the proportion of cement required to secure imperviousness is so large as to make the blocks expensive. This Cost may be reduced by using silica cement (§ 33). The use of hydrated, or slaked, lime has also been recommended to assist in filling the voids; the possibility of efflorescence from such use is sufficient to dictate caution, however, until it is proved to be unobjectionable. (Articles 34, 40, and 61 should be read in this connection).



792. Special Forms

In the construction of a building many special forms are required, such as sills, lintels, columns, steps, etc. Block machines are now made to mold many of these forms when of moderate length. Special molds of steel with hinged sides are also made into which the concrete is tamped. When the size of block required is beyond the capacity of the machine in use, wooden molds may be readily made. Unless for some form which is not to be duplicated, such wooden molds should be so made that they may be used repeatedly. To make a convenient form, fasten one plank on edge along one side of the bottom piece or pallet, thus making an L; the remaining side piece is not fastened, but is held in place by a strong iron clamp of U form. Between the movable side piece and the clamps place wedges to bring the mold to the proper width. The end pieces are held by cleats tasked to the side pieces and pallets, which are made longer than the proposed block to allow for this. For steps or sills requiring rounded corners, a strip of molding is tacked along one corner of the mold. Lumber for this purpose should be one and three-quarter inches thick, dressed. To give a fine finish and to prevent the absorption of water by the mold, sandpaper the interior and apply two or three coats of shellac dissolved in alcohol.

Sand molds are also employed for casting concrete, much the same as for cast iron, and beautiful effects have been obtained by this method. This process,1 which is patented by the Stevens Cast Stone Company of Chicago, consists in preparing a sand mold from a pattern, and pouring the mold with a wet mixture of mortar or of concrete made with fine aggregate. While this process is employed for making concrete blocks of the ordinary hollow form, it is especially suited to molding lintels, columns, treads, and all ornamental pieces. A bracket cast by this process is shown in the cut, and the character exhibited in the finish is especially noteworthy.

793. Facing

The application of a special facing to concrete blocks is for one of two purposes to make the block more nearly waterproof, or give some special surface appearance. The need of such a facing is most apparent in the tamped block made of mortar alone. To make such a block waterproof throughout requires a large percentage of cement, and by using a waterproof facing and leaner mortar for the body of the block it is sought to decrease the Cost without sacrificing the impervious quality. To insure this result the mortar for the facing should not contain more than two or two and a half parts sand to one cement, and should form a layer about a half inch in thickness. If the surface can be troweled, a smooth face is formed to repel the moisture.

1 For detailed description, see article by Mr. Charles D. Watson in Cement Age, May, 1906.

When applied to improve the appearance of a block, the mortar may be colored, or special sand or stone screenings may be employed to give the desired finish. Since the quantity of such facing mortar is small, the Cost of special materials will be correspondingly less important than would be the case if the entire block were made homogeneous.

The method of applying such a facing differs with the method of manufacture and the form of machine used. Some of the machines for tamped mortar blocks mold the block face down in order to place the facing mortar first. If molded in the ordinary manner a separating plate must be placed just back of the face plate to keep the two mortars apart until the mold is full. With the pressed block the facing mortar is placed last and the pressure is applied directly upon it. The poured block is usually molded face up and the facing mortar spread last. It may then be troweled or screenings may be sprinkled upon it to give the desired special finish. With the poured block the facing is more frequently applied for the sake of appearance, since by this method the body of the block may be made waterproof at a reasonable Cost. Whatever method of manufacture is employed, the face and body of the block should be made together, or the facing applied immediately after the block is completed, in order that the two may be thoroughly bonded together.

The difficulty in applying a facing mortar is greatest in that method of manufacture in which such facing is most needed. It is frequently a question whether the increased Cost and trouble of applying such a facing does not exceed the saving resulting from the use of a leaner mixture in the body of the block. A block made of an impervious mixture throughout is undoubtedly better than one of which only the face is waterproof, and the facing should only be adopted when it is found on trial to be appreciably less expensive.

794. Waterproofing

The precautions necessary to make waterproof mortar and concrete and the application of surface washes have already been discussed in Art. 61, and these remarks are entirely applicable to concrete blocks. As already stated, the cement paste should be in excess of the voids in the sand, and the mortar in excess of the voids in the broken stone or gravel. That sufficient water be used in mixing is also one of the first requisites, as there is much greater danger of permeability being due to a deficiency of water than to an excess, especially in a comparatively small mass like a concrete block. The washes mentioned in Art. 61 may be applied to blocks as to other concrete surfaces, and they are sometimes used even after the block is in the wall. In addition to the washes mentioned above, there are certain trade preparations used in a similar way and to mix with the water used in gauging the mortar. There is also a powder, the formula for which is a trade secret, to mix with the cement for making waterproof concrete. This has been on the market but a short time, but has given good results.