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Formation of thermal contraction cracks

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

where:

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BS 8110 has been withdrawn and superceded by Eurocodes.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, CIRIA Report C660 Early thermal crack control in concrete.

 

 
Acknowledgement: The Concrete Society
 

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