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If early thermal contraction is restrained, cracking will occur if the restrained strain (or induced stress) exceeds the capacity of the concrete, i.e. if:
T1akR > eult
T1 = drop between peak temperature after casting and ambient temperature, °C
a = coefficient of thermal expansion, per °C
k = modification factor
R = restraint factor
eult = ultimate tensile strain capacity of concrete
BS 8110 takes a value of 0.8 for k and recommends values for R for various sequences of construction as given in the table below.
Two design approaches are possible, namely avoiding the formation of cracks or controlling them once they form.
Table: Values of restraint (taken from BS 8110)
|Pour configuration||Restraint factor R|
|Thin wall cast onto massive concrete base||0.6 to 0.8 at base: 0.1 to 0.2 at top|
|Massive pour cast onto blinding||0.1 to 0.2|
|Massive pour cast onto existing mass concrete||0.3 to 0.4 at base: 0.1 to 0.2 at top|
|Suspended slabs||0.2 to 0.4|
|Infill bays, i.e. rigid restraint||0.8 to 1.0|
BS 8110 has been withdrawn and superceded by Eurocodes. See BS EN 1992-3 Design on concrete structures, Part 3: liquid retaining and containg structures which as a similar process but may recommend different factors. Alternatively, Report C766 published by CIRIA (2019), which replaces C660 (2007) can be used.
Acknowledgement: The Concrete Society
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