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 > e_ult

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
e_ult = 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BS 8110 has been withdrawn and superceded by Eurocodes.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. Also see BS EN 1992-1-1:2023 Design of concrete structures. General rules and rules for buildings bridges and civil engineering structures which will superceed part 3.

Alternatively, Report C766 published by CIRIA (2019), which replaced C660 (2007), can be used.

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