Errors during casting 59
hardened cast. This problem is also illustrated in Figure  3.9. When new
concrete is placed on old concrete (already set or hardened), the old concrete
should be free of dirt (sand, oil), dust and laitance (see also Section 3.8 of
this chapter, bleeding). Failing to do so will result in a visible joint in the
finalized concrete surface. More importantly, it will also lead to a weak
connection between the old and new concrete.
During casting, unwanted inclusions can also be entrapped into the fresh
concrete. In many cases, these inclusions remain invisible and unknown,
although sometimes they become apparent on the final concrete surface, as
illustrated in Figure 3.10 where some timber leftovers have been entrapped.
Hardened concrete
1
st phase
2 nd phase concrete
Weak
connection
Laitance, dust, dirt
Figure 3.9 Casting joint.
Figure 3.10 Unwanted inclusions in concrete.