High-strength concrete is used to cast the column all the way to the bottom of the beam. 2. Beams and slabs are made of lower strength concrete (including junctions). 3. The column above the slab is cast with high strength once more. This sequence leads in lesser strength concrete in the column's beam-column junction compared to the remainder of the column.
The purpose of this difference in strength is that you can repair or replace low-strength elements while leaving the high-strength ones intact. For example, if you find out that a slab has some cracks, you could simply patch them instead of replacing the whole thing. Same with the column next to it; you would only need to repair the low-strength concrete instead of replacing the whole thing.
There are also other factors that affect how strong a concrete structure is, such as the type of reinforcement used in the concrete. For example, steel rebar is often used to add strength to concrete structures. Other types of reinforcement include fiberglass rods and plastic tubes filled with cement. These materials are called "matrix composites" because they contain both rigid materials (the bars) and flexible ones (the fibers or tubes).
Finally, the strength of concrete depends on how much water it contains. Too much water makes the concrete weak, so it should be watered enough to keep it dry but not so little that air bubbles get trapped inside.
It is typical practice in multistory structures with slab-beam-column construction to employ greater strength concrete in columns rather than beams and slabs. The concrete is poured in phases as follows: 1. High-strength concrete is used to cast the column all the way to the bottom of the beam. 2. Standard-strength concrete is used to finish the top of the beam before moving on to the next phase.
The reason for this is that any open areas in the beam are filled with high-strength concrete, while the slab is always fully covered. This helps to protect the beam from corrosion when it remains exposed to the environment. It also helps to reduce the amount of reinforcement needed in the beam compared to if standard-strength concrete was used instead.
As long as the structure is not under load, there is no need to pour high-strength concrete into the beam. Once it is necessary to strengthen the beam, then the column should be poured first before standard-strength concrete is added to the top. This allows the high-strength concrete to cure properly before adding more concrete to the top.
The key here is to have a clear understanding of how the different components of the structure will be poured before starting work. If high-strength concrete is required in the beam but not the slab, then make sure you allow enough time for it to set before going on to the next stage or your beam will be weak at that point.
7.3 (A), the most frequent varieties of composite columns are concrete-encased composite columns and rectangular and circular concrete-filled steel tube columns, which have become more popular in a variety of modern constructions due to their strong structural performance. Concrete-encased composite columns consist of a hollow cylindrical shell of thin metal wrapped with thick layers of concrete. These columns can be used as column walls or as part of a floor/ceiling system. Rectangular concrete-filled steel tube columns are usually supplied in lengths of 20 feet (6 m) or 30 feet (9 m). Circular concrete-filled steel tube columns are usually supplied in diameters of 18 inches (45 cm), 20 inches (50 cm), and 24 inches (60 cm).
7.4 (B), other column types include: cast-in-place columns; precast concrete columns; steel pipe columns; and timber columns.
7.5 (C), composite columns are often defined by their construction method rather than by their material composition. For example, a concrete-wrapped steel tube column is not a composite column because it is made up of steel tubes embedded in concrete. On the other hand, a wood column that is wrapped in fiberglass is considered a composite column because it is made up of wood fibers mixed with resin and placed around a central pole.
Reinforced concrete beams are structural components intended to support transverse external loads. Depth of the beam (h)
| Beam span | Beam type | Span/depth ratio |
|---|---|---|
| Greater than 10m | Simply supported | 20*10/span |
| Cantilever | – | |
| Continuous | 26*10/span |
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