The various technical requirements for good lime and cement are very strict and detailed, and for the small house it is customary to cover their qualities in the briefest manner by referring to the standard specifications of the American Society for Testing Materials.
Slaked lime should be made from well-burned quicklime, free from ashes, clinker, and other foreign materials.
Dry hydrated lime should be the finely divided product resulting from mechanically slaking pure quicklime at the place of manufacture.
The specifications of the American Society for Testing Materials covering the quality of cement should be followed where large purchases are made. Where small quantities are to be used, the reliability of the dealer must be the basis of purchase.
As mortars and concretes made from these materials are as important as the cements or limes, it is essential to have definite standards for them.
Lime mortar should be made of 1 part by volume of slaked-lime putty or dry hydrated lime and not more than 4 parts by volume of sand. The use of hydrated lime is recommended, since the poor qualities which are apt to develop from careless slaking of quicklime are thus avoided. It also comes in smaller packages, and if the entire quantity is not used at once it may be stored without deterioration. It is only necessary to mix the hydrated lime with water until it becomes a paste, and then add the necessary sand. The purpose of adding sand is to increase the bulk and to reduce the shrinkage which pure lime paste will develop as it hardens. Pure lime paste, without sand, will shrink, crack, and develop very little strength. By introducing sand this contraction is reduced, but the addition of too much will decrease the strength slightly. However, this decrease of strength is very little. A mortar made of 1 part lime to 6 parts sand is nearly as strong as one made from 1 part lime and 3 parts sand. The maximum amount of sand to be used is generally governed by the ease of working, and not so much by the strength. A lime which is too sandy will not spread easily on the trowel.
Cement mortar is, of course, a stronger material and can be used in damp places where lime mortar would deteriorate. The theory of mixtures of both cement mortar and concrete is to proportion the materials so that they produce the most compact substance. For instance, in the cement mortar the cement should just fill the voids between the particles of sand, and in concrete this cement mortar should just fill the voids in between the larger aggregate, and this larger aggregate should be so graded in size that it makes the most compact body. It used to be thought that certain definite numerical proportions, as laid down by theory, of the various ingredients would hold true for all kinds of sands and aggregates. For instance, the proportion of 1 part of cement, 3 parts of sand, and 6 parts of aggregate was thought to be the best for ordinary use under all conditions. But extensive tests by the government have shown that the only real way to determine the correct proportions of mixtures is to experiment with the particular sand and gravel that will be used, and to test them to see what ratios give the most compact mass. It has also been found that round aggregates, like pebbles, produce the strongest concrete, since the particles flow into place better than the sharper aggregates, which formerly were considered necessary because of the supposed idea that they made a better mechanical bond with one another. The proportion of water is also important, a quaking mixture producing the best results.
It is customary in small work, however, where no experiments can be made on various mixtures to determine their proper proportions, to follow the old rules of thumb for amounts.
Cement mortar should be made of cement and sand in the proportions of 1 part of cement and not more than 3 parts of sand by volume.
If cement-lime mortar is to be used it should not have more than 15 per cent by volume of the cement replaced by an equal volume of dry hydrated lime. The addition of hydrated lime to cement mortar improves its working qualities, making it slide more readily on the trowel and also increasing its water-proofness. Its strength is not decreased within the limits prescribed.
Good. Very compact Bad. Not compact because of poor grading of aggregate, Good and bad concrete.
In concrete work it is as important to have good sand and aggregate as cement. Sand should be sharp, clean, coarse quartz. The sand used should not, when it is rubbed in the hand, leave the palm stained.
Gravel which is used as an aggregate should be free from clay or loam, except such as naturally adheres to the particles. If there is too much clay or loam, it should be washed with water. When bank gravel is used the best results will be obtained if it is screened from the sand and remixed in the proper proportions for fine and coarse aggregate. For ordinary mass concrete the size of aggregate should vary from 1/4 inch to 2 inches, and in reinforced work should not exceed 1 1/4 inches.
The best proportion of parts to use must vary according to the requirements, but for the small house good results will be obtained by using 1 part of cement, 2 parts of sand, and 4 parts of gravel or broken stone.
Stucco Work. - Stucco is really a Portland-cement plaster used on the exterior, and its success depends a great deal upon the quality of materials employed and workmanship. All stucco to a greater or less degree cracks, but the problem is to make the cracks as small as possible. The government is carrying on an extensive investigation of the problem of stucco through experiments on fifty-six exterior panels which have been under observation since 1915. Each one of these panels has been spread upon a different base or made with different proportions. So far only two panels have been found to be entirely free from cracks, although many are practically uninjured by the small cracks which have developed. It is therefore quite evident that as a rule it must be assumed that the stucco will crack to a certain extent, and in order to cover such defects a rough surface is the best. As to proportions of mixtures, there is a great variation of opinion. The commonest is 1 part of cement, 2 1/2 parts of sand, to which is added about 1/10 part of hydrated lime by weight of cement. For a more detailed account on stucco, send for the Progress Report issued by the Bureau of Standards on the Durability of Stucco and Plaster Construction.
Plastering. - The qualities of internal plaster depend upon the construction of the wall, the methods of application of the plaster, and the quality of the plastering material.
The walls and ceiling to which plaster is to be applied must be so constructed as to be practically rigid under the loads that they will carry. Since plaster is not elastic, any slight change in shape of the surface will cause it to crack. The common backings which are satisfactory for plastering are wood lath, metal lath, and masonry, such as concrete, terra-cotta tile, brick, plaster board, etc. Wood lath makes the least rigid back of all, and for this reason is not considered the best, although it is the cheapest. Unless the wood laths are wet before the plaster is applied, they will absorb the moisture from the plaster and swell, thus cracking the wall. Metal lath for this reason is superior. Masonry walls should be made rough to give the necessary key for the plaster to cling to. In brick walls the joints are raked out, in concrete walls the surface is picked, and the outside of terra-cotta tile is marked with grooves for this purpose.
The best results in plaster are secured with three coats. The first coat is called the scratch coat, and is intended to form a bond between the wall itself and the plaster. It should be pressed into the apertures between the lath to secure a good bonding key, and its surface should be scratched with a tool to give the required bond between it and the next coat, or brown coat. The brown coat forms the main body of the plaster and averages about 3/4 inch to 7/8 inch thick. The finished coat is then added on top of this and is intended to develop a plane surface with the desired color. Each coat should be allowed to dry out and then be wet before the next one is added. If wood lath is used, this drying and wetting will cause the lath to shrink and swell, so that cracks will be developed in the scratch and brown coats. These should be filled in before the finished coat is added.
The materials which should be used in the various coats depend upon the requirements which are necessary for each one.
Scratch coat is for bonding; brown coat for plasticity; finished coat for appearance.
As the most important characteristic of the scratch coat is strength, and that of the brown plasticity, and the final coat appearance, the materials must be proportioned accordingly.
1 part by volume of calcined gypsum. 3 parts " lime paste.