Civil Engineering Reference
In-Depth Information
from the top slab to eliminate such hard points (
9.6
). However, at the abutment the
slab must be well supported so that the roadway expansion joint does not suffer from
relative defl ections between the slab and the abutment. The discussion of the dynamic
effects on cantilever slabs close to expansion joints applies equally to top slabs.
9.3.8 Heat of hydration cracking
Whenever a bridge deck is cast in-situ with transverse construction joints, there is a
risk that as the more recent concrete cools from its peak setting temperature, it will be
cracked due to restraint offered by the hardened concrete of the previous pour (3.6).
A typical case is when bridge decks are built by the cast-in-situ balanced cantilever
method. The bridge segments are usually 3.5 m to 5 m long and the top slabs between
10 m and 20 m wide (including the side cantilevers). Although slabs are usually less than
300 mm thick, limiting the setting temperature, they frequently contain longitudinal
prestressing ducts, which create weak planes encouraging the formation of cracks,
Figure 9.16. As such deck slabs are always substantially reinforced in the transverse
direction, any cracks which form are likely to be within the specifi ed limits, usually less
than 0.25 mm. However, as the cracks penetrate through the thickness of the slab, rain
water collecting on the deck will pass through, drawing attention to their presence.
Sometimes they are only noticed when the tendons are grouted, and water appears at
the concrete surface.
If the deck is to be protected by a waterproof membrane, such cracks may be safely
ignored. However the designer needs to be forewarned of the possibility of this type of
cracking, and he would be wise to alert the client and the contractor in advance.
Figure 9.16
Restraint cracking in the top slab of cast-in-situ segments
