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10.2.3.3 Calculation of crack width

/3 = a coefficient relating the average crack width to the design value

= 1.3 for restraint cracking in members with a minimum dimension of 300 mm or less.

Therefore

Eqn 4.80

wk = 1.3 x 263 x 8.4 x 10"4 = 0.29 < 0.3 mm (limit)

10.2.3.4 Concluding remark

The Code suggests a minimum value of 3 N/mm2 be taken when the time of cracking cannot be confidently predicted as being less than 28 days.

Whilst the values given for f seem high, it is difficult at the design stage to assess accurately the as placed concrete strength because this often exceeds the class specified. Consequently, unless strict site control is exercised, it would be prudent to adopt the apparently conservative figures given in EC2 Table 3.1.

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