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Rain may affect the water cement ratio at top portion of freshly placed concrete provided that the concrete is not properly protected from rain. To substantially change the water-cement ratio of the concrete at the surface of the slab, external energy must be supplied to the system such as troweling passes with excess water on the concrete surface.
Release agent, form oils, form releaser and demoulding agent are materials for separating formwork from hardened concrete. Though they generally refer to the same meaning, there are slight differences among
these terms.
Release agent: Materials that contain ingredients which are chemically combined with cement.
Gypsum is a mineral and is hydrated calcium sulfate in chemical form. Gypsum plays a very important role in controlling the rate of hardening of the cement. During the cement manufacturing process, upon the cooling of clinker, a small amount of gypsum is introduced during the final grinding process.
7-day compressive strength test results are usually not used for acceptance purpose but for information only. Instead, 28-day compressive strength test results are commonly adopted for acceptance purpose.
Some would suggest aesthetics as the answer of the question. Others may consider safety on the contrary because chamfered edge is less liable to damage to the concrete structure and any objects hitting on it. However, the main reason of the provision of chamfer is to make formworks easier to pull out after concreting operation.
Traditionally, chilled water and ice have been employed to reduce the temperature of concrete mix in hot weather condition. Chilled water has a limitation in its cooling potential. For instance, even if all mixing water has been converted into chilled water, the temperature reduction achieved in concrete mix is only about 2.7oC.
In balanced cantilever method, the moment is balanced along the length of the piers. However, along the extended cantilevers only a part of negative bending moment is balanced by prestressing bending moment arising from normal force induced by prestressing.
Balanced cantilever construction simply cantilevers segments from a pier in a balanced manner on each side until the mid-span is reached and a closure is made with a previous half span cantilever from the preceding
pier.
For single-span coupled cable, the length of cable is one span and they are coupled at the construction joint which is located at 0.25 of span. The use of single-span coupled cable in span-by-span construction suffers the following drawbacks:
(i) Stressing all tendons in one span is time consuming. Moreover, the construction team has to wait until the concrete has gained enough strength before all tendons in the span to be stressed.
The null point is the position of zero movement in the bridge deck. When the bridge deck is pinned at a single pier, it provides the location of null point with no deck movement. However, when the bridge deck is pinned to more piers, the position of null point has to be calculated. The determination of null point is important because it serves to estimate the forces on the piers by deck length changes and to calculate the sliding movement of sliding and free bearings.
