CPD Specifier May 2015 issue September 2015 | Page 47

Structural Repair

Testing key to car park
repair strategies
The Structural Concrete Alliance explains how
repair and maintenance strategies for car parks
can be tailored to meet the specific requirements
of each area of the structure.

Since steel in an electrolyte (concrete) forms a half cell, with a relative
potential that can be measured using a high impedance digital voltmeter,
this can be compared against a reference cell. The voltage will be more
negative in areas that have a higher chance of corrosion activity. Typically,
a grid is marked on the concrete area to be tested and readings recorded
at the node points, with the data often converted into a colour contour
map.

Many of the UK’s concrete multi-storey car parks have fallen into disrepair,
with corrosion problems resulting in the need for extensive repair and
maintenance.
Since corrosion initiation will often be in areas where permeable concrete,
low cover and more severe exposure conditions coincide on a particular
element, structures such as car parks can often be subdivided into
different exposure categories. Then, by undertaking comprehensive
corrosion testing, it is possible to gain an understanding of the exposure
variability and ascertain the corrosion risk. This enables a repair and
maintenance strategy to be developed that provides the best technical and
commercial solution for the structure.

Once testing is complete, all of the test data should be mapped onto a
single drawing, showing the visual/delamination survey data alongside
the half-cell potential data and supported by cover readings, carbonation
depths and chloride contents at the bar depth. This enables the user to
quantify the areas that are likely to be ‘at risk’ from corrosion and provide
the basis for a repair and maintenance strategy.
Structural Concrete Alliance members are skilled in the repair of parking
structures and the use of corrosion control systems to manage the
corrosion activity in accordance with European Standards.

TEST METHODS
Deterioration of car parks is mainly caused by chloride based de-icing
salts being used in the car park or brought into the structure on vehicle
tyres. These chloride salts allow corrosion of the embedded reinforcing
steel and, ultimately, result in significant spalling of the concrete, and
restricted load-bearing capability.

CASE STUDIES

Walkden Street, Mansfield
Walkden Street multi-storey car park is a reinforced concrete ‘wafflepot’ structure built in 1964. It comprises 6 parking decks split over three
levels above a retail space that had lain vacant for a number of years
having been vacated by a major retailer. The waterproof membrane
applied to the reinforced concrete top deck had degraded and failed in
many areas allowing the ingress of de-icing salts through the original
daywork and failed movement joints in the concrete deck, resulting in
a significant volume of spalling and corrosion damage to the concrete
soffits below.

Furthermore, the form of construction can also be a problem. For
example, where precast items are used, movement of the structure may
reduce the adequate bearing surfaces and potentially lead to failure over
time.
The first stage of testing will usually involve a delamination or visual
survey. However, such a survey is generally limited to the areas where
visual indications of damage are seen, such as cracking, rust staining and
concrete spalling/delamination. Since sheet delaminations and corrosion
can occur with no outward visible signs of damage, careful hammer
testing should also be undertaken. The results from visual surveys can
also provide information as to where other, more scientific methods could
be used.

Following a competitive tender process, Makers Construction was
awarded the contract to carry out the repairs and re-waterproofing
works. Prior to commencement, a detailed hammer survey was carried
out which identified a significant volume of delaminated concrete as a
result of reinforcement corrosion.

The depth of carbonation and the degree of chloride contamination
in each area should also be evaluated. Carbonation will occur where
atmospheric carbon dioxide (CO2) penetrates into the concrete surface
and reduces the protective alkalinity of the concrete cover. The level of
carbonation can be determined on site using phenolphthalein indicator,
sprayed onto a freshly broken concrete sample, as outlined in BS EN
14630: 2006. A number of sample’s should be taken over the structure to
obtain a distribution or carbonation depths and these compared against
the cover depths around the structure to find areas at risk.

Following removal of the defective concrete and thorough preparation
of the exposed reinforcing bars, a full programme of concrete repairs
were carried out using the Sika Monotop system with some 5m3 of
repair having to be reinstated. A migratory corrosion inhibitor was also
applied along the daywork joints for added protection.
The top deck was completely re-waterproofed with the Sika Pronto deck
waterproofing system to ensure that the previously leaking daywork
joints could not fail again. The system was applied to some 4,600m2 of
deck area including the up and down access ramps between the upper
decks.

Chloride testing can also be used to determine the level of chloride
contamination at the depth of the reinforcement and to provide information
as to the most likely mode of entry. The level of chloride determined
should only be used as a guide to risk and these results used in
conjunction with other results, for instance from half-cell potential testing.

The Mall, Maidstone
Repair works to the 1000-space, multi-storey car park at The Mall,
Chequers in Maidstone, Kent were undertaken when insufficient
concrete-cover to the steel reinforcement had hastened corrosion of
the bars, resulting in expansion of the steel and causing spalling of the
concrete.

Concrete cover to steel reinforcement is the first line o