New Zealand Engineering 1998 June
Are You
Managing Your Concrete Assets Effectively?
Sheldon Bruce
is a concrete technologist for Central Laboratories, Opus International
Consultants
| New Zealand has a long history of
construction in concrete, from the earliest unreinforced examples in the
19th century to the extensive use of reinforced concrete throughout
the 20th century to construct the bridges, dams, ports, pipelines
and buildings which contribute to New Zealand's infrastructure.
The durability of these reinforced concrete structures is becoming an
increasingly important issue as they age. On the state highway network,
for example, in excess of 30 percent of the reinforced concrete bridges
are more than 50 years old. Careful management of these assets is required
to ensure that durability is maintained and the design life is achieved
or enhanced.
Concrete
durability
Reinforcement corrosion as a result of external chloride contamination
from the sea is the most common cause of concrete deterioration in New
Zealand's reinforced concrete structures. Significant chloride contamination
will result in corrosion of the reinforcing steel and subsequent spalling
of the cover concrete. Structures in the coastal zone are at greater risk
of reinforcement corrosion. This is acknowledged in the Concrete Structures
Standard (NZS 3101 : 1995) which specifies minimum requirements for design
concrete strength and depth of cover concrete. |
Non-destructive measurement
of
the depth of cover concrete
over
the reinforcing steel
|
Reinforcing steel corrosion may also result from carbonation of the concrete,
a reaction which occurs between components in the hardened cement paste
and carbon dioxide in the atmosphere. This reaction lowers the pH of the
pore solutions in the concrete and can destroy the protective oxide film
formed on the surface of the reinforcing steel. In New Zealand, carbonation
is more likely to occur in drier inland areas but is a less common durability
problem than chloride contamination.
The risk of reinforcement corrosion is further increased when poor detailing
and inferior quality construction practices further reduce the natural
protection provided by the concrete to the reinforcing steel. Examples
include lack of concrete cover over reinforcing steel, poor compaction,
inadequate drainage, and the use of concrete of insufficient quality to
meet the design life requirements of the structure. In many cases the construction
practices may simply reflect the materials, methods and knowledge available
at the time of construction. The importance of design and detailing is
also recognised in NZS 3101 : 1995.
Testing in progress on
a concrete arch
bridge
Extensive deterioration
of a bridge
superstructure due to
reinforcement
corrosion
|
Inspection
and assessment
To manage an asset effectively it is necessary to know its condition at
any given time, how it is likely to change, and the performance required
from the asset in the future. A programmed inspection and maintenance programme
is an essential part of this management and observations need to be recorded
in enough detail to enable repeated problems or progressive deterioration
to be recognised. The frequency of this programme will depend on the current
condition of the structure and the exposure conditions affecting it. For
example, the inspection and maintenance frequency for a coastal port facility
will necessarily be entirely different from that required for a hydroelectric
facility in Otago. Reinforcement corrosion as a result of chloride contamination
is a significant risk in reinforced concrete wharves, and frequent inspections
should be carried out. In contrast, the risk of reinforcement corrosion
in the hydroelectric facility is minor and the inspection frequency can
be relatively long, although other threats to concrete durability such
as abrasion erosion damage due to aggregate bed loads and high velocity
water flows may also need to be considered.
An example of an inspection programme is provided by the Transit New
Zealand bridge inspection policy for state highway bridges which has four
types of inspection, with minimum frequencies:
1. Superficial inspection. Inspection frequency dependent on history
of structure but commonly one year
2. General inspection. Frequency two years
3. Detailed inspection. Frequency six years
4. Special inspections. Frequency varies for special structures or immediately
following earthquake, flood or overloading event.
The scope of inspection required and the personnel qualified to carry
out the inspection are both prescribed by the policy.
A key to the effective management of reinforced concrete structures
is to have suitably qualified and experienced personnel to carry out the
inspections. These inspections must be able to recognise and interpret
minor deterioration which requires routine maintenance, or more serious
deterioration which indicates the durability or structural integrity is
at risk. When the deterioration is considered to be extensive or unusual
then specialist concrete technologists should be engaged to carry out a
detailed condition assessment.
|
Such an assessment will identify the cause and extent of the deterioration
so that technically appropriate remedial options can be selected which
meet service life requirements and the budget constraints of the owner.
A condition assessment is designed to meet the particular requirements
of a structure and the potential remedial options, and will include some
of the following specialist tests:
• Depth of cover concrete over reinforcing steel
• Likely extent and severity of reinforcement corrosion
• Extent and severity of cracking or surface damage
• Crack monitoring
• Carbonation of the cover concrete
• Resistivity of the cover concrete
• Chemical attack
• Chloride ion content of concrete
• Strength of reinforcing steel
• Strength properties of concrete
• Chemical analysis of concrete composition.
Remedial
options
The field of remedial concrete repair to reinstate and protect concrete
affected by reinforcement corrosion has advanced significantly in recent
years. Remedial options can be separated into two general types: conventional
patch repair and electro-chemical repair.
Conventional patch repair is historically the most widespread method
for remedying reinforcement corrosion. Current technology favours cement-based
materials which are compatible with the host concrete and available as
trowellable mortars, free flowing micro-concretes, and spray applied mortars.
These materials are produced as proprietary prebagged products and contain
a variety of polymer modifiers, admixtures and fillers to improve bond,
increase strength, reduce shrinkage, and decrease permeability. Prebagged
products are preferred over site batched repairs because the quality of
the material is guaranteed. Concrete repair systems also often include
a protective coating system to inhibit the ingress of chloride ions, carbon
dioxide and liquid water, yet still allow the concrete to dry.
Where concrete is extensively carbonated or contaminated with chloride
ions, a conventional patch repair system is unlikely to be able to remove
all the affected concrete. When this occurs the remaining affected concrete
may, in the future, cause further reinforcement corrosion to occur. The
length of time before this happens depends on the quality of the concrete,
degree of contamination, and the exposure conditions. Conventional patch
repair may still be a viable option in these circumstances provided the
cost of ongoing repairs is allowed for. In addition, the operational future
of some structures is sometimes uncertain or known to be limited, and a
patch repair solution can provide a limited maintenance-free period which
meets the owner's requirements.
Electro-chemical treatments were developed as an alternative to conventional
patch repair systems. They include cathodic protection, desalination and
realkalisation. Cathodic protection (CP) and desalination are suitable
remedial options for concrete contaminated with chloride ions; the former
operating by imposing a current on the reinforcing steel to overcome the
corrosion current, and the latter relying on the removal of chloride ions
from the concrete under an applied voltage. The impressed current for a
CP system is applied for the lifetime of the structure, whereas the process
of desalination may take 2-12 weeks to complete. Realkalisation aims to
reinstate the alkalinity of carbonated concrete using a carbonate solution
which is drawn into the concrete by a process known as electro-osmosis,
and may take 4-8 days to complete. The electro-chemical repair methods
treat the cause rather than the symptoms of reinforcement corrosion and
should provide an extended maintenance-free life for a structure. They
are also useful where removal of contaminated concrete would cause unacceptable
disruption.
The initial cost of electro-chemical methods is likely to be greater
than for conventional patch repair, and this will often count against their
selection. An alternative approach is to consider the whole of life costs
for the structure. This will allow the cost of the additional repair cycles
which may be necessary if a patch repair option is chosen, and the benefits
of the extended maintenance-free life provided by electro-chemical repair
methods to be accounted for.
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