527. A good finish may be obtained when the molds are smooth if the workmen will force the blade of a spade or shovel between the fresh concrete and the mold, and pull the handle away from the mold. This has the effect of forcing the large stone back from the face and allowing the mortar to flow in. A layer of mortar is thus left next the mold with no marked line of junction between mortar and concrete, as may be the case in using a mortar facing. A similar effect may be produced by throwing the concrete against the face of the mold with such force that the larger pieces of aggregate rebound. In very finely finished work this may mar the surface of the sheathing, but ordinarily this method is effective.
528. When a special layer of mortar is used for facing, there is more danger, perhaps, of making the layer too thick than too thin. As to the richness of the mortar, two parts sand by measure to one volume packed cement is usually sufficient though a more glossy finish may be made if desired, by using equal parts of cement and sand. It is better to avoid too great a variation between the richness of the mortar used for facing and that used in the body of the concrete.
One of the best ways of applying such a layer is to prepare a sheet of steel of width equal to the thickness of one layer of concrete, usually six to eight inches, with two handles on the upper edge to facilitate moving it. At the ends of the sheet, on the side next the mold, rivet short pieces of 1 1/2 in. by in. or 2 in. by 2 in. angle iron. This sheet of iron with the projecting legs of the angles against the face of the molds, forms, with the latter, a space one and one-half or two inches thick, which is to be entirely filled with the finishing mortar made rather moist and tamped lightly with edge rammers. The concrete is filled in behind the iron, after which the latter is withdrawn by means of the handles, and the whole mass is thoroughly rammed. The end sought is that the finishing mortar shall have some approximately definite thickness, and that the stones of the concrete shall be tamped into the finishing mortar, but not through it, and thus destroy any sharp line of demarcation between mortar and concrete, and ensure a perfect bonding of the two. It is evident that this can only be accomplished by placing the mortar and concrete at the same time.
529. One other cautionary remark concerning the use of finishing mortar. With the present state of our knowledge concerning the rates of expansion of mortars and concretes of different composition, it is not considered wise to use too many combinations in the same structure. To illustrate, a pavement or surfacing of a large concrete structure was once built in layers as follows: first, thick natural cement grout was placed on the concrete foundation; second, natural cement concrete; third, Portland cement concrete; fourth, a richer Portland cement concrete; fifth, Portland granolithic; sixth, rich Portland mortar; and seventh, floated with dry Portland cement and sand. We cannot be absolutely sure that this is bad practice, but it would seem that this structure might have served its purpose with fewer varieties of -material, and it is usually considered very doubtful whether Portland cement mixtures will always adhere well to mixtures of natural cement, although the author knows of instances where they have been used in juxtaposition apparently with good results.
530. Granolithic is a facing or surfacing mortar composed of crushed granite and cement. The granite is usually specified to contain no particles larger than J inch to one inch, and about one and one-half to two and one-half parts are used to one volume of cement. This is more frequently used for foot walks and other places where resistance to wear is required, but may also be used to surface walls, to line reservoirs, etc. It will be mentioned again in connection with cement sidewalk construction.
531. Exposed concrete surfaces frequently present a patchy appearance. This may be the result of lack of care in placing the concrete next the mold, or it may be due to variations in the purity of the sand or in the amount of water used in mixing. On mortar-faced work this lack of uniformity is less noticeable. The use of slag sand, or of a little fine pozzolanic material, may be advantageous, and a small amount of lampblack in the facing mortar also tends towards uniformity in appearance.
A very pleasing finish may be given by applying to the set concrete a thin wash of cement and plaster of Paris, though the permanence of such a wash may be open to question. The sheathing should be removed as early as it is perfectly safe to do so, and the concrete surface cleaned from any oil or grease that may have come from the mold planks. The wash, which should be very thin, may be applied with a whitewash brush. A mixture of equal parts Portland cement and plaster of Paris gives a very light gray finish, and one part plaster to three parts cement gives a trifle darker shade.
532. A rubbed finish of excellent appearance may be given by removing the sheathing before the concrete has set very hard, say after twenty-four to forty-eight hours, and rubbing the surface with white brick or with a wooden float. If there are small voids in the surface, it may be covered with a thin grout of equal parts of cement and sand and then rubbed hard with a circular motion. The grout should not leave a scale on the work, the object being only to fill surface imperfections.
If the mold boards are removed at just the proper time, a good finish may be given by rubbing with a wooden float, without the coating of thin grout. A somewhat similar effect is produced by brushing the surface with brooms or stiff brushes.
What is called a pebble-dash finish was used in the construction of a bridge in the National Park at Washington, D. C.1 Eighteen inches of the concrete next the face was made of one part cement, two parts sand, and five parts of gravel and rounded stone from one and one-half to two inches in their smallest diameter. After the removal of the forms the cement and sand were brushed from around the face of the gravel next the surface exposed to view. It was found by experiment that the brushing should be done when the concrete was about twenty-four hours old. At twelve hours the gravel was displaced by the brushing, and after thirty-six hours the mortar had become so hard as to be removed from the surface of the stones with difficulty. The forms were therefore designed so that sections of the lagging could be removed as desired. The Cost of the brushing was said to be about sixty cents per square yard.