Mortar Joints
Great Importance
In examining the bond in a brick wall, the eye naturally is first attracted to the brick units as so many colored spots arranged in different order so as to produce a certain pattern effect; but this pattern effect in the brickwork depends very largely upon what at first may be overlooked or disregarded, the mortar joint. The mention of mortar suggests in the mind a very commonplace thing which the workman mixes and carries in a hod to the bricklayer; but it is concave one of the most important elements entering into the beauty, as well as the strength, of a brick wall.
Effect of Mortar Color
When you consider that all the joints in brickwork, both vertical or “head” joints, and horizontal or “bed” joints, are filled with mortar weathered of one color or another, amounting on an average to one-seventh of the wall, it is evident what a vital part they play in the appearance of the entire wall surface. An artist will tell you that this amount of color introduced into any surface will greatly modify, by contrast or analogy, the general effect, so that it is of the utmost importance, in selecting the sort of brick you wish for your wall surface, that you also select the mortar joint.
Three Elements Involved
Three elements must be carefully considered in dealing with the mortar joint: its color, its texture, and its size and kind. The color of the mortar joint may be such as entirely to destroy the beauty of the brick. On the other hand, if it is properly chosen, it will bring out the fine shades and tones of the brick in such a way as to enhance very greatly its natural beauty. Then, the mortar joint has a certain texture which is produced either by finishing it rough or smoothing it with the trowel or a tool made for that purpose. This mere treatment of the surface of the mortar joint has more to do with the appearance of the wall than one might at first suppose. In addition to that, the size of the mortar joint, running from a thin “buttered” joint up to an inch in width affects the color relation of the whole surface; and the kind of joint, whether cut flush, raked out, or tooled in various shapes, has a distinct bearing on the whole effect (Fig. 57). In a word, do not neglect the mortar joint, for it is one of the most important elements that go to make up the beautiful fabric of the brick wall, in the building of which there is deserved and required the exercise of a fine discriminating taste.
All in all, what with the convenient units of stretcher and header, each with its color and texture, you have a medium in the choice of bond, mortar joint, and pattern for weaving the most charming mosaic or tapestry effects into the wall surface, a possibility offered by no other material than face brick.
Mortar Colors
Colored mortar joints may be produced in two ways, first, by the use of a natural colored sand or ground granite or other stone, and secondly, by the use of artificial mortar colors. Pure white joints are obtained by using white sand or ground limestone or marble. As the color of the mortar greatly affects the appearance of the finished wall, much care should be given to the selection and proper use of these colors, whether natural or artificial. As the color of the finished mortar after it has set and dried is never the same as that of the fresh mortar, experience is the only guide for the proper preparation of the mortar. Someone has well said that the right way to make mortar is to mix brains with it.
When artificial color is used, it should first be thoroughly mixed with the dry sand. Never add this mixture to hot lime. But after the slaked lime has thoroughly cooled, mix it with the colored sand in batches in a separate box. If preferred, the color may be made into a thick paste with water and then added directly to the mortar. In either case, thorough mixing and uniform proportion of quantities are essential. The more thorough the mixing and uniform the proportions, the more permanent and uniform will be the color and the less color required. Use the best colors obtainable on the market, as cheap colors may ruin the appearance of the wall.
Quantities
As to quantity required, the exact amount of color varies greatly with the exact shade of the color selected, the width of the mortar joint, and the brand used. As a fair general estimate, it will take approximately 75 pounds of coloring matter for every 1000 bricks laid with 3/8 to 1/2-inch mortar joints. But it is always safest to follow the directions of the manufacturer as to the kind of color and the exact amount required.
Various Kinds of Mortar
Materials
Mortar, as the bonding material between brick, must be given careful attention as the strength of the wall depends on its quality. Mortar is composed of sand and either lime or cement, or both. In all cases the materials should be the best. The cement should pass the test of the American Society for Testing Materials. The lime should be either fresh and well-burned lump lime, slaked so as to produce a smooth, puttylike mass without lumps, or the prepared hydrated kind found on the market. The sand should be sharp, clean, and free from foreign matter, and screened through a mesh of the size to secure the proper fineness. For wide joints, the mortar needs, instead of sand, fine gravel of the size required to give it sufficient body.
Kinds of Mortar
In large cities the building ordinances require certain mixtures of mortar. For various mixtures see page 104. But where there are no legal requirements, the contractor’s own judgment and experience should guide him in the selection of the proper mix, as each one has certain qualities which recommend it. Portland cement mortar is stiffer and harder to work than lime mortar, but should always be used where permanence and strength are required, as in exposed or heavy bearing situations, or in cold weather when the mortar must set before it freezes. Lime mortar may do for the usual work, above the grade in dry warm climates. It should not, however, be used for basement walls, except possibly where the soil is extremely dry, nor for piers or points in the wall that carry heavy loads. It is smooth and works easily, so that more brick can be laid per day than with cement mortar, but its use should be limited to the proper situations. Cement-lime mortar has the combined qualities of both cement mortar and lime mortar. It has both strength and good working qualities, and costs less than cement mortar. It is preferred by many to cement mortar; we recommend its use in proper situations, throughout this Manual.
Mixing Cement Mortar
Cement mortar must be mixed and used immediately, as cement sets very quickly. On this account, it should not be made in large quantities and, once it has obtained a partial set, should never be re-tempered. The cement and sand for cement mortar should be thoroughly mixed while dry, by turning the material over three or four times with a shovel, when sufficient water to make a plastic mass should be added, turning again several times. It is always desirable to add a small quantity of lime, when tempering the cement mortar, in order to make it easier to work.
Mixing Lime Mortar
Lump lime should be slaked as soon as possible after delivery on the job. If it is allowed to stand for any length of time, it should be stored in an air-tight box; otherwise, it may become air slaked, reducing mostly to powder and soft, crumbly lumps. The fresh lime is always in hard lumps and should be free from cinders. Lime is slaked in a mortar box by adding water to the lump lime. On adding water the lime becomes very hot, gives off vapor, and finally burst into a powder which gradually reduces to a paste known as lime putty. The exact amount of water necessary varies with different limes, but in all cases, the right proportions used in slaking must be watched carefully. An excess of water will stop the slaking process. If not enough water is used the lime will “burn” and lose its strength. If left overnight, the mortar box should be covered with planks.
When slaking is completed, the lime is reduced to what is called “putty”, which should be of uniform consistency throughout, containing no unslaked lumps. The sand should now be added, the exact amount depending upon the quality of mortar desired, and thoroughly mixed with the putty, and then shoveled out on wooden planks where it should remain for at least a week before using.
Hydrated lime
Hydrated lime is in powdered form and needs no other preparation than the addition of water. Its quality is uniform and produces satisfactory results, even when experienced labor cannot be obtained, and may be used at once on the job. Its uniformity and the facility it gives in handling the mortar make it preferable to lump lime although it costs somewhat more.
The mixing of cement-lime mortar is very simple, as all that is needed is to add the proper amount of cement to the lime mortar, at the time it is tempered for use, and very thoroughly mix into a consistent mass.
Tempering Mortar
Working the mortar and adding water to bring it to proper consistency for actual use on the job is called tempering, a process that should be continued until the mortar slides easily off the trowel. All white spots, which are really small lumps of lime, should be made to disappear; otherwise, these lumps will swell and “pop” after the mortar is laid in the wall.