Civil Engineering Reference
In-Depth Information
width under the quasi-permanent load combination is recommended to be
0.4 mm, with the following comment.
'For X0, XC1 exposure classes, crack width has no influence on
durability, and this limit is set to guarantee acceptable appearance. In
the absence of appearance conditions, this limit may be relaxed.'
Tighter control of crack width is normal in bridges, and is sometimes
needed in buildings: for example, in the humid environment of a laundry,
or in an open-air multi-storey car park. For these and most other environ-
ments, the recommended limiting crack width is 0.3 mm for reinforced
concrete, or 0.2 mm for some types of prestressed concrete. Prestressing
of composite members is rare, and is not considered further. The exposure
class also influences the specification for type of concrete and for minimum
cover to reinforcement.
The appearance of a concrete surface may be important where a
web-encased beam is visible from below, but the top surface of a slab is
usually concealed by the floor finish or roof covering. Where the finish is
flexible (e.g., a fitted carpet) there may be no need to specify a limit to the
width of cracks; but for brittle finishes or exposed concrete surfaces,
crack-width control is essential.
Limiting crack widths are normally specified as a characteristic value
w
k
, though EN 1992-1-1 refers also to a 'limiting calculated crack width,
w
max
', for which the same limiting values, 0.2 mm to 0.4 mm, are used.
The usual interpretation of a characteristic value is one with a 5% prob-
ability of exceedence. Crack width is a random variable, but the concept
of 'probability of exceedence' is difficult to apply in practice [42].
Design rules are given in EN 1994-1-1 for the following situations:
(1)
where 'the control of crack width is of no interest' and beams are
designed as simply-supported although the slab is continuous over
supports;
(2)
for the control of restraint-induced cracking, based on the tensile
strength of the concrete;
(3)
for load-induced cracking, with control of crack width to 0.2, 0.3 or
0.4 mm;
(4)
for the calculation of estimated crack width and maximum final
crack spacing.
For cases (1) to (3), simplified rules are given that do not involve the
calculation of crack widths. These are outlined below. For case (4), refer-
ence is made to provisions in EN 1992-1-1 for reinforced concrete mem-
bers. This situation rarely arises in buildings and is not considered further.
