Especialidades JA/Albañilería/Respuestas
Nivel de destreza
3
Año
1937
Version
23.12.2024
Autoridad de aprobación
Asociación General
1
2
2a
2b
2c
2d
2e
2f
3
4
Water is added to either of these mixtures and worked in with a trowel or a hoe until it reaches the desired consistency. Both types can be purchased pre-mixed so the mason need only add the water. Mixing is done on a hard flat surface, often in a wheelbarrow or on a sheet of plywood, but more properly in a mortar box.
5
6
7
Floating
If a smoother surface is required than the one obtained by screeding, the surface should be worked sparingly with a wood or aluminum magnesium float. The aluminum float, which is used the same way as the wood float, gives the finished concrete a much smoother surface. To avoid cracking and dusting of the finished concrete, begin aluminum floating when the water sheen disappears from the freshly placed concrete surface. Do not use cement or water as an aid in finishing the surface. Floating has three purposes:
- to embed aggregate particles just beneath the surface;
- to remove slight imperfections (high and low spots); and,
- to compact the concrete at the surface in preparation for other finishing operations.
Begin floating immediately after screeding while the concrete is still plastic and workable. However, do not overwork the concrete while it is still plastic because you may bring an excess of water and paste to the surface. This fine material forms a thin, weak layer that will scale or quickly wear off under use. To remove a coarse texture as the final finish, you usually have to float the surface a second time after it partially hardens.
Edging
As the sheen of water begins to leave the surface, edging should begin. All edges of a slab that do not abut another structure should be finished with an edger. An edger dresses corners and rounds or bevels the concrete edges. Edging the slab helps prevent chipping at the corners and helps give the slab a finished appearance.
Troweling
If a dense, smooth finish is desired, floating must be followed by steel troweling . Troweling should begin after the moisture film or sheen disappears from the floated surface and when the concrete has hardened enough to prevent fine material and water from being worked to the surface. This step should be delayed as long as possible. Troweling too early tends to produce crazing and lack of durability. However, too long a delay in troweling results in a surface too hard to finish properly. The usual tendency is to start to trowel too soon. Troweling should leave the surface smooth, even, and free of marks and ripples. Spreading dry cement on a wet surface to take up excess water is not a good practice where a wear-resistant and durable surface is required. Wet spots must be avoided if possible. When they do occur, however, finishing operations should not be resumed until the water has been absorbed, has evaporated, or has been mopped up.
An unslippery, fine-textured surface can be obtained by troweling lightly over the surface with a circular motion immediately after the first regular troweling. In this process, the trowel is kept flat on the surface of the concrete. Where a hard steel-troweled finish is required, follow the first regular troweling by a second troweling. The second troweling should begin after the concrete has become hard enough so that no mortar adheres to the trowel, and a ringing sound is produced as the trowel passes over the surface. During this final troweling, the trowel should be tilted slightly and heavy pressure exerted to thoroughly compact the surface. Hairline cracks are usually due to a concentration of water and extremely fine aggregates at the surface. This results from overworking the concrete during finishing operations. Such cracking is aggravated by drying and cooling too rapidly. Checks that develop before troweling can usually be closed by pounding the concrete with a hand float.
8
Adding water to Portland cement to form the water-cement paste that holds concrete together starts a chemical reaction that makes the paste into a bonding agent. This reaction, called hydration, produces a stone-like substance — the hardened cement paste. Both the rate and degree of hydration, and the resulting strength of the final concrete, depend on the curing process that follows placing and consolidating the plastic concrete. Hydration continues indefinitely at a decreasing rate as long as the mixture contains water and the temperature conditions are favorable. Once the water is removed, hydration ceases and cannot be restarted. Curing is the period of time from consolidation to the point where the concrete reaches its design strength. During this period, you must take certain steps to keep the concrete moist and as near 73°F (23°C) as practical. The properties of concrete, such as freeze and thaw resistance, strength, watertightness, wear resistance, and volume stability, cure or improve with age as long as you maintain the moisture and temperature conditions favorable to continued hydration. The length of time that you must protect concrete against moisture loss depends on the type of cement used, mix proportions, required strength, size and shape of the concrete mass, weather, and future exposure conditions. The period can vary from a few days to a month or longer. For most structural use, the curing period for cast-in-place concrete is usually 3 days to 2 weeks. This period depends on such conditions as temperature, cement type, mix proportions, and so forth.
Historical Notes
Masonry was introduced in 1937, discontinued in 1956, and revised and reintroduced in 1986.
References
Much of the material for requirements 5 through 8 was taken verbatim from the U.S. Navy's training manual NAVEDTRA 14043. Since this manual is an original work of the U.S. Federal government, it is in the public domain, meaning it is legal (and ethical) to use it for any purpose (including providing the excellent instruction for this difficult requirement).
