A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
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.
When the temperature drops, the concrete at the outside drops to
surrounding temperature while the concrete at the central region continues
to cool down. The contraction associated with inner concrete induces
tensile strains and forms cracks tangential to the circular radius.
13. Is the material of formwork (timber or steel) helps to reduce
thermal cracks in concreting operation? (C1)
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.
If the section under consideration is thick and internal restraint is the likely
cause to thermal cracking, then timber formwork should be used. On the
other hand, if external restraint is the main concern for thermal cracking,
then steel formwork should be used instead.
14. 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.
Is the crack width induced by concrete hydration and flexure should
be considered individually to satisfy the above maximum crack width
requirements? (C1)
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