Is the crack width induced by concrete hydration and flexure should be considered individually to satisfy the above maximum crack width requirements?

For the purpose of defining the serviceability crack width limit state, the maximum design surface crack widths for the exposure conditions defined in BS8007 should be taken to be the following:

The maximum design surface crack widths for direct tension and flexure or restrained temperature and moisture effects are:
1) severe or very severe exposure: 0.2 mm;
2) critical aesthetic appearance: 0.1 mm.

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Is the material of formwork (timber or steel) helps to reduce thermal cracks in concreting operation?

To answer this question, one must fully understand the effect of formwork on the temperature of concreting structure. Without doubt, with better insulation of structure by timber formwork, the overall rise of temperature and hence the peak hydration temperature is also increased. However, for a well-insulated structure, the temperature gradient across concrete element is reduced. Therefore, the use of well-insulated formwork (like timber) increases the maximum temperature and reduces the temperature gradient across the structure at the same time. Hence, whether steel or timber formwork should be used to control thermal cracking is dependent on the restraints and the size of section.

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What is the crack pattern induced by hydration due to internal restraint?

Let’s take a circular column as an example to illustrate this.

When the temperature is rising, the inner concrete’s temperature is higher than outer concrete’s temperature and the inner concrete is expanding. This induces pressure to the outside and the induced compressive stress will result in formation of radial cracks near the surface of concrete.

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What is the purpose of critical steel ratio in concrete structures?

The purpose of critical steel ratio is to control the cracking pattern by having concrete failing in tension first. If steel reinforcement yields first before the limit of concrete tensile strength is reached, then wide and few cracks would be formed. In the calculation of critical steel ratio, the thickness of the whole concrete section is adopted for analysis.

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Is waterstop necessary in construction joints of water retaining structures?

Construction joints are introduced during construction and they are not intended for movement to take place. If the concrete panel on each side of the construction joint has been designed to have maximum crack width of 0.2mm, theoretically this joint would also behave in the same way as its adjoining material.

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