Civil Engineering Reference
In-Depth Information
60
50
40
30
20
10
0
0-10
11-20
21-30
31-40
41-50
51-60
61-70
71-100
Age when repaired (years)
Ages of the structures at the time of repair are shown in
Figure 14.4.
Most
(66 per cent) were in the range 10 to 40 years, 42 structures were over 50
years and the oldest when repaired was 100 years.
It is convenient to consider the initial problems under two groups:
Deterioration involving underlying processes of degradation, e.g. corrosion
and AAR, and Damage caused by mechanical or other actions.
14.3.1 Corrosion and AAR
Corrosion is by far the most common problem in concrete structures and
occurred in 137 of the case histories. The corrosion was almost exclusively in
the reinforcing steel but there were nine cases in prestressing steel. Corrosion
of prestressing steel is potentially more serious than reinforcement as it can,
and has on several occasions, led to collapse (Tilly, 2002b).
Corrosion invariably occurs as a result of a fundamental cause such as
inadequate thickness of cover or leaking expansion joints. It follows that
there are two aspects of the repair to be addressed: to take action to eliminate
the root cause (e.g. by waterproofing), and to halt the internal process of
corrosion (e.g. by removal of contaminated concrete).
AAR is less common and there were only 14 cases reported, five
accompanied by corrosion. There is considerably less experience of AAR and
the relative numbers of cases of corrosion and AAR are not unrepresentative
of the overall numbers in the field. AAR can be very damaging and, like
corrosion, has led to structures having to be demolished.
The processes of corrosion and AAR are difficult to stop and invariably
continue unless the repairs are very carefully (and expensively) carried out.
14.3.2 Mechanical and other damage
This group of problems includes freeze-thaw damage (due to cycles of
freezing and thawing), cracking caused by structural actions, wear and
leaching (of spillways on dams), impact damage (by vehicles or ships striking
the structure), vibrations and repeated loading causing fatigue damage,
