Civil Engineering Reference
In-Depth Information
reinforcement drawings are available, they will need to be validated with
checks on reinforcement size, location and cover. If they are not available, it
is essential that they are recreated from surveys. This is very expensive and
underlines the value to owners of maintaining full records of construction,
modifications and repairs.
The structural reinforcement cover surveys are complementary to and
should be integrated with the investigations to evaluate cover in considering
corrosion risk. However, missing bars and top reinforcement cover greater
than specified will go unremarked in a corrosion survey, but they may indicate
serious structural deficiencies even before deterioration is considered.
When damage or deterioration becomes obvious to the owner, the first
questions relate to overall safety and the usually more immediate risk to
the public from falling spalls. So, structural engineering input (Wood,
2006) to the investigation and subsequent repairs is essential. The amount
of deterioration concrete structures can suffer before remedial measures
are necessary is small. Codes for concrete design include no margin for
deterioration and modern design to minimise costs has reduced strength
reserves and robustness.
Focusing the investigation on the areas where there is structural risk from
vulnerable details and/or the worst deterioration usually enables a better and
more cost-effective diagnosis to be made than random sampling.
Figure 3.2
shows inadequate bearing seating of precast beams below a leaking car park
joint.
Careful evaluation of the causes of cracking is an essential procedure in
the appraisal of a concrete structure. Cracking arises from:
• Structural tensile strains from flexure and shear
• Early age 'non-structural' effects, thermal, shrinkage, etc
• Long-term 'non-structural' conditions corrosion (Concrete Society,
1992), AAR (BRE, 2007, IStructE, 1992, 2010), etc.
Relating crack widths to causes and monitoring changes in crack widths
are powerful diagnostic tools. Any area where tensile strains exceed about
150 microstrain will crack and the crack orientation indicates the principal
stress direction. Compressive stresses suppress cracking. Cracking relates to
the combination of all structural and 'non-structural' effects. For example,
the axial compression in a column suppresses map cracking from AAR so
only vertical cracking develops. Cracking provides a channel for the ingress
of chlorides and carbonation.
Sets of cores taken during an investigation which contain cut reinforcement
immediately raise alarm bells for the structural engineer. Not only is it
extremely difficult to structurally reconnect a cut reinforcing bar, but the
structural damage from cutting a bar may be far greater than that from the
deterioration being investigated. All coring proposals need to be checked
by a structural engineer and coring should be preceded by a cover survey of
