The risk of cracking is usually expressed as the ratio (over time) between generated concrete tension stress and tensile strength.
If the value of total tensile stress in any point of the concrete wall exceeds the actual tensile strength of concrete in that point, the crack is formed, but cracks can also occur with a tensile stress below the tensile strength,
In practical case showed with crack index 0.75, the cracks occur during the first week after casting.
Regarding the climatic- and on-site (boundary) conditions there are always
some uncertainty and the relevant input must be assumed according to the most probable weather, for the season and the insulating properties for the formwork materials. The output from simulations is therefore associated with uncertainty, and what we get from simulations is therefore a probability for cracking (or not) within a certain level of confidence .
The level of confidence of simulations is impossible to verify accurately as it is more or less impossible to verify the actual properties and stresses in a real structure. However, through experience different “safety levels” have been proposed,Cracking occurs when stresses reach the tensile strength capacity of the concrete, hence the Crack
Index (Ci = the stress/strength-ratio) becomes 1.0. A crack risk criteria must take the aspects of variation into consideration. Hence, if it is of vital importance to obtain a high safety against cracking in a structure, the simulated Ci should be considerably lower than 1.0.
I agree.
Besides, time of cracking depends also on thickness and member type, e.g. ground slab (mainly bending restraint due considerable thermal interaction with the ground) and wall on foundation (mainly centric restraint due to uniformly distributet deformation impact but excentric restraint at wall bottom...)
A reliable crack assessment should take into account the type of cracking (micro-cracks when the stress in a single point reaches the tensile strength vs. macro-cracks when the restraint force or moment of a cross section reaches the cracking force or moment of this cross section. This is usually not considered by the cracking index...
If crack assessment concentrates only on cracking index, it is important to define the tensile strength (fctk0.05 vs fctm)
We propose for safe exlusion of cracking: Sigma (t) < 0,8 x fctk,0.05 = 0,8 x 0,7 x fctm
More details can be found here:
http://onlinelibrary.wiley.com/doi/10.1002/suco.201400058/pdf
You mentioned climatic factor/seasonal effect which is true. Here are other reasons why crack occurs during early age (< 1 week)
1. Excessive bleed water evaporates, leaving microcracks on the cement surface
2. Differential shrinkage: Cohesive mix sticks to side wall, causing internal cracks
3. Thermal effect (self-explanatory)
4. Differential settlement
5. Poor curing practices
At very early age, during the first week of hardening, the concrete experiences changes in the properties of concrete due to hydration process. the main properties affecting cracking of concrete are tensile strength, Elastic modulus, creep and shrinakge. Thus, it is not only the tensile strength that lead to cracking but the combined effects of these properties.
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