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Concrete Engineering

Is curing compound suitable for all concrete?

For concrete structures with low water-cement ratio (i.e. less than 0.4), it may not be suitable to use curing compound for curing. When hydration takes place, the relative humidity of interior concrete drops which leads to self-desiccation and drying-out.

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What is the purpose of uniform rate of application of water (i.e. 2m depth in 24 hours) in watertightness test of water retaining structures?

In watertightness test of water retaining structures, it normally requires the filling of water at a uniform rate or letting the water pool stand alone for some time before actual measurement is carried out. The reason for such provision in watertightness test is to allow sufficient time for water absorption to take place in concrete.

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Curing time in summer is less than that in winter. Why?

While concrete sets, it gains hardness and strength as the process of hydration slowly spreads the entire body of material. Curing should be allowed to continue for several days before subjecting the new concrete to significant stress.

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What are the potential problems of excessive application of form oils?

The problems of excessive application of form oils are:

(i) It stains the surface of hardened concrete.

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How can release agent help to separate formwork from concrete?

There are generally two main types of form releaser: barrier type or chemically active type.

For barrier type (e.g. form oil), it creates a barrier between the form and the fresh concrete. However, the quick evaporation of diesel oils affects clean air.

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How does rain affect the freshly placed concrete?

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.

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What is the difference between release agent, form oils, form releaser, demoulding agent?

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.

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What is the purpose of adding gypsum in 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.

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Some concrete specification requires the testing of compressive strength for both 7 days and 28 days. Why?

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.

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What is the main purpose of chamfers in concrete structures?

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.

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