Low Cover - It's Implications and Effective Solutions
The major factor in determining the durability of reinforced
concrete structures and elements lies in the effectiveness, that
is the depth and quality, of the cover concrete. All too often
our interest in the outermost surface regions concerns the texture,
appearance and functional competence rather than its significance
to the overall durability of the structure.
It is important to realise the key function that this relatively
thin “skin” of concrete cover plays is protecting the reinforcing
steel from corrosion by maintaining an alkali environment and
preventing the ingress of chloride ions and the fuels for corrosion.
Whilst concrete cover can be disregarded when calculating the
structural performance of reinforced concrete elements, failure
to give due regard to achieving adequate cover can be potentially
disastrous to contractors, precast concrete manufacturers and
ultimately the client. If left untreated, low or inadequate cover
will lead to a legacy of expensive maintenance bills with the
likelihood of the design life of the structure being drastically
reduced and the possibility of premature demolition.
Faced with these possibilities, clients are
not prepared to take the risk, and with quality control low cover
checks on-site becoming increasingly stringent, incidents of precast
elements being rejected and the requirement for demolition of
localised sections of new construction are becoming all too frequent.
The causes of low cover are numerous, and whilst
mistakes do happen on-site, problems can sometimes be traced back
to the design process. With the desire to make modern concrete
structures more intricate and aesthetically pleasing, the design
becomes more complicated, and often insufficient consideration
is given to the practicality of turning the concept into reality.
In many cases, designers do not always picture the construction
sequence or appreciate the difficulties on-site, and consequently
the requirements for the steel reinforcement are based on structural
needs rather than the ease of assembly and the subsequent encasement
in concrete. This is a common problem on highway and marine structures,
where the sheer density of reinforcement provides strong challenges
for the contractor. In these instances sufficient consideration
is not given to the mix design of the concrete to enable it to
be poured into confined spaces with congested steel.
Another frequent cause of low cover is poor workmanship, with
wrong or inadequate spacers being used or inadequate fixing of
the formwork. Furthermore, extreme care must be taken when concrete
is being poured and compacted to avoid displacement of the reinforcement.
It must be remembered that if the concrete fails to meet its specified
characteristic strength, the cover will be inadequate to maintain
the required durability requirements. Poor compaction and honeycombing
will again reduce the effectiveness of the concrete cover and
will require remedial action. Therefore, it is absolutely imperative
to maintain quality control on-site at all times to avoid such
potentially costly mistakes.
Once a problem with inadequate cover has been
identified it is important that steps are taken to rectify the
problem. If structures are left untreated, the lack of protection
offered to the reinforcement will lead to premature de-passivation
of the steel and corrosion, necessitating expensive maintenance
procedures. A particularly common form of degradation on structures
with low concrete cover is carbonation, caused by carbon dioxide
in the atmosphere reducing the alkalinity of concrete, whilst
chloride ingress from road salts or sea water is the major threat
on highway and marine structures. Inadequate cover, for whatever
reason, will not only accelerate the effects of carbonation but
will also allow more rapid ingress of chlorides, moisture and
oxygen to promote corrosion.
In such cases, a consulting engineer will need
to be brought in to assess the extent of the problem by determining
the quantity and quality of the cover on the structure. The implications
on the design in terms of structural and durability requirements
for the particular exposure conditions can then be assessed and
consideration given to possible remedial measures.
Demolition does appear to be a drastic measure,
but once a client finds a major defect within a new structure
the first suggestion is often to demolish the sections which do
not meet the specification, or reject the item in the case of
precast concrete elements. However, this is not always practical,
as it may be some time before the problem is found or the costs
in terms of delay to the project are unacceptable.
One alternative option is partial recasting,
which involves removing the concrete back to behind the level
of the reinforcement using the high pressure water technique,
to prevent damage to the adjacent structure. The formwork is then
repositioned to achieve the desired cover and the concrete recast.
However, depending on when the problem is discovered, there may
be problems with accessing the area to carry out such remedial
work.
It may be practicable to increase the cover
by building out the face of the concrete with a proprietary polymer
modified render, although it is important to ensure adequate key
by removing surface laitance and achieving a rough surface using
wet grit blasting techniques. Depending upon the protective properties
of the render, it may be possible to reduce the thickness of the
cover whilst still providing the same degree of cover.
By far the easiest and most effective way of
reinstating cover is to use a protective coating, which not only
reinstates cover but provides additional aesthetic and protective
properties. However, care must be taken when selecting a coating
system to fully assess the film thickness required to provide
the necessary cover. In addition, the coating must be compatible
with the substrate and its surroundings, and the product’s expected
life span should be considered. It is also imperative that the
system is backed up by relevant independent test data and approvals.
There is a wide range of generic polymer coatings
available on the market, both waterborne or solvent based and
single or twin pack, all with a successful track record in the
protection of reinforced concrete structures. However, advances
in polymer, cement, fibre and microsilica technology in the development
of one of Flexcrete’s cement based coatings - Cementitious Coating
851 - have added a new dimension to structural protection. For
example, Taylor Woodrow Technology has assessed the chloride ion
diffusion resistance of Cementitious Coating 851 for the past
nineteen years, and to date, no steady state flux of chloride
ions has been detected. It is resistant to water at 10 bar pressure
and equivalent to 100mm of good quality concrete. Being cement
based, it chemically reacts with the substrate to form an integral
part and will have a design life equivalent to that of the concrete
to which it is applied. 851 can be applied to green concrete by
spray techniques, exhibits minimal hazard during application and
is non-toxic when cured.
The benefits of such technology have been clearly
illustrated in some of the world’s most demanding marine applications.
For example, when slip forming caissons for a major Hong Kong
port, concerns were raised about the adequacy of the cover in
resisting the ingress of chlorides into the concrete as they were
due to be submerged into the south China seas within 7 days of
casting. In this instance, 851 was applied to the green concrete
as it emerged from the slipforms and also formed part of the curing
system. The long-term durability of the caissons was ensured in
a particularly aggressive environment where adjacent concrete
structures were showing signs of distress from chloride attack
after only a few years in service.
Even the most technically advanced and best
designed concrete is not free from the scourge of low cover. On
a prestigious concrete platform which forms part of North Sea
Oil production facility where the concrete was specially produced
to the highest standards, low cover was found on the gravity feeds
tanks. 851 was subsequently used to reinstate the required degree
of protection, primarily because of its excellent waterproofing
properties.
Even in the relatively controlled environment
of a precast concrete yard, problems of low cover still occur
and the cost of rejecting a major element such as a tunnel segment
would be astronomical. This was the case on a major tunnelling
project in a marine environment, and a sceptical consultant had
to be convinced that 851 would ensure the 120 year design life
required. After surveying the wealth of existing independent test
data available and carrying out further tests to confirm its resistance
to scratch and impact damage in particular, the product was finally
approved and specified for use.
851 has also been used on several major bridge
projects in Scandinavia, Hong Kong and the UK. Once again it was
the product’s ability to resist the ingress of chloride ions and
offer protection from carbonation that saved the contractor from
expensive remedial measures.
Thus, many international consultants can conclude that a high
performance cementitious coating such as 851 is an ideal solution
to non-conformance with specification. Not only does it reinstate
cover, it also provides structures with additional protection
against freeze/thaw cycles, de-icing salts, water and chloride
ion penetration, thus ensuring that the life span of the structure
is both achieved and extended.
|
