Civil Engineering Reference
In-Depth Information
Figure 2.17
Photomicrograph showing a shrinkage fine crack developed in a sprayed
is patchy carbonation either side of the crack shown by the very thin bright areas
adjacent to the crack. Note the crack passes through the limestone aggregate particles
as well as through the paste. Limestone aggregate particles occur on the left and right
sides of the field of view.
cements will help control cracking of this type but may not entirely eliminate
shrinkage cracking.
Drying shrinkage cracking is normally very shallow and the cracks may
only be a few micrometres wide. However, where reinforcement is present
within the repair it is not uncommon for shrinkage cracks to continue
for several centimetres and to intersect the surface of the reinforcement.
Shrinkage microcracks that are very shallow may self-anneal with time
in damp conditions. Shrinkage cracking that intersects the surface of
reinforcement may be detrimental to the long-term durability of the repair
if they allow chlorides, moisture or carbonation to reach the surface of
the reinforcement. Such cracking is readily detected using petrographic
techniques; the example in
Figure 2.17
shows the localised penetration
of carbonation along a vertical shrinkage microcrack which intersects the
surface of the, as yet, uncorroded reinforcement.
Plastic shrinkage can develop in the surfaces of concrete repairs where the
repair is exposed to warm windy conditions before it has cured. It occurs
where the rate of evaporation exceeds the rate of bleeding. Very thin repairs
may be prone to cracking of this type, particularly where there is potential
for moisture from the repair material to be absorbed by the substrate. Plastic
shrinkage cracks are generally restricted to the cement paste, tend to be non-
parallel sided and commonly decrease in width rapidly with depth.
2.5.2 The effects of on-going deterioration in the concrete
substrate
In some cases expansion of the concrete substrate due to on-going
deterioration can lead to cracking along the interface between the repair
