Civil Engineering Reference
In-Depth Information
Roughening of the surface of the structure to be repaired by mechanical
means can leave behind a shattered zone in the substrate surface with
intense microcracking invisible to the naked eye. Cracking of this type is
readily detectable petrographically (illustrated in
Figure 2.4)
and can lead
to a weakness in the bond between the repair and the substrate. Such
microcracking can also act as a pathway for subsequent moisture ingress and
weakening due to moisture movement and leaching.
Water-jetting is much less commonly associated with shattering of the
concrete surface and commonly leaves a rough, cleanly broken surface.
The edges of concrete repairs may also have sawn margins or be feather-
edged. Feather-edged repairs can show a tendency to lift at their periphery
- especially if the concrete surface is smooth, allowing moisture and
carbonation penetration.
Figure 2.15
illustrates a feather-edged repair.
2.4.2 Common causes of post-repair weakening of the bond
between the repair and the substrate
Moisture ingress
It is not uncommon for there to be a concentration of voidage or
microcracking along the substrate surface. In damp conditions, these features
can act as a pathway for moisture ingress leading to localised recrystallisation
of the cement hydrates and porosity enhancement due to leaching of water-
soluble materials in the cement hydrates such as portlandite.
Figure 2.16
illustrates secondary ettringite formation in voids at the concrete surface due
to recrystallisation of the cement hydrates. The moisture may also contain
chloride ions contributing to the risk of the development of reinforcement
corrosion.
Carbonation in damp conditions can result from the presence of
carbonate ions in moisture that is able to penetrate into the repair. As well as
and fine sand. The hydrated cement paste is of high porosity and is deeply carbonated
and contains small quantities of unhydrated cement grains. The quartz aggregate
particles appear various shades of grey to white.
