50 Concrete Engineering International Autumn 2005
member of the Structural Precast Assoc-
iation, supplied:
62 full-height columns in Type B finish
to BS 8110
(1)
, incorporating corbels to
support the structural beams
147 beams spanning 15.6m, with serv-
ice hole penetrations and erection
safety netting sockets
540 pre-finished precast concrete rib
slabs inclusive of lighting conduits and
all interconnecting reinforcement
120 parapet units to Type C architec-
tural finish, with cast-in sockets provid-
ing the main support for the waved ver-
tical louvres
175 precast walls for the main and two
escape staircases
178 precast terracotta-faced cladding
panels from ground to first-floor level
and around all three stair cores to roof
level. These serve both as cladding and
as structural members, assisting the
value engineering of this type of con-
struction.
Conclusion
The building has been carefully positioned
to provide areas of public open space to the
south and west. This creates the opportu-
nity for public art, open-air displays and
exhibitions, or simply space to sit and relax.
All spaces around the site will accommodate
areas of planting and hard/soft landscape
treatment to provide the appropriate setting
for this fine building. The uniformity and
consistency of this system built car park is
currently surpassing many of the project
teams expectations. All initial objectives are
being met and the team is confident that
SCC will complete the project on time and to
a very high quality. I
CAR PARKS
I Reference:
1. BRITISH STANDARDS INSTITUTION. BS 8110-1:
Structural use of concrete. Code of practice for
design and construction. 1997.
Acknowledgements:
Client: Borough Council of Kings Lynn & West Norfolk
Main contractor: Alfred McAlpine Capital Projects
Architect: Pulmann Associates Architects
Engineers and parking consultants: Hill Cannon
Design-and-build: SCC Ltd
Figure 4: Interlocking precast terracotta
panels with cross-flow ventilation apertures
that are a feature throughout the ground floor.
M
ainly built in reinforced concrete,
the structures often suffer prob-
lems such as unintended cracks
and cracks exceeding the maximum permit-
ted width. Water ingress leads to corrosion
of the reinforcing steel, followed by spalling
and carbonation of the concrete, which
leads to the loss of the protective environ-
ment for the reinforcing steel, and so the
structures slowly degrade.
Waterproofing at design stage
Car parks as secondary structures must
have a high degree of usability and low
building and maintenance costs. Therefore,
the design engineer has to aim for an easy -
to-build, cost-effective structure while max-
imising space, usability and durability.
Typically, requirements for new-build car
parks include waterproofing systems that
can, by the use of a mechanically resistant
screed, protect the concrete and provide a
barrier to prevent the ingress of harmful
salts into the structure. Waterproofing and
protective measures should be incorporated
at the design stage in order to achieve high
quality and durability at the lowest cost.
This may lead to a design concept that
defines limits to cracks that are well within
the self-healing capacity of the subsequent
protective screed.
Renovation
The renovation of existing structures usually
aims to restore the damaged concrete and to
provide additional protection by increasing
the concrete cover. Adding a waterproofing
layer to walls and floors can renew water-
proofing systems. Renovation also includes
the renewal of access ramps, installation of
sprinkler systems for fire safety and monitor-
ing to detect toxic exhaust fumes connected
to an automated alarm and ventilation sys-
tem.
Special attention has to be given to all
kinds of joints, especially expansion joints,
which can be subjected to multiple loads
such as movement, vertical loads from cars
and water pressure.
Cracks that might have occurred during
the first period of service may indicate the
need for a joint in the structure and should
therefore not simply be sealed but, after
careful investigation, be turned into an
expansion joint.
The interiors of car parks are very often
connected to the waterproofing system.
With waterproofing slurries based on white
cement applied on the side of negative
water pressure the inside of the building
the interior receives an appealing appear-
ance and does not require an additional
coat of paint.
Shopping centre car park,
Wohlen, Switzerland
Car parks form a part of many modern structures, such as office buildings, hotels, convention centres,
exhibition halls, shopping malls and many more and are very often below ground. They can suffer from
groundwater, mechanical wear and de-icing salts brought in by cars during winter periods.
Cornelius Oesterlee, Vandex International Ltd, Solothurn, Switzerland
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Concrete Engineering International Autumn 2005 51
Case study
The Migros Shopping Centre in Wohlen,
Switzerland features a below-ground park-
ing area that is subjected to limited water
pressure. The waterproofing system that
was chosen for the horizontal areas also
acts as the finishing screed. For a successful
outcome, engineers coordinated the con-
crete design and the waterproofing screed
so that the number and width of cracks in
the concrete did not exceed the self-healing
capacity of the waterproofing screed.
The screed that was used has special
properties that enable it to it fulfil several
functions. Due to the presence of finely
graded quartz sand, the screed has a very
compact structure with very low pore vol-
ume. The specified water/cement ratio
ensures the mechanical and physical prop-
erties. Combined with special chemical addi-
tives a layer of 810 mm is sufficient for
waterproofing against up to seven bars of
positive or negative water pressure. High-
quality, fast-setting cements give a high
compressive strength and, together with the
quartz sand, provide a surface that is highly
resistant to mechanical wear. The fast-set-
ting cement provides a walkable surface
after a few hours, allowing other work to
proceed. Most cementitious screeds, espe-
cially those for waterproofing, have the
capacity to seal cracks that form after appli-
cation. Cracks measuring up to 0.3mm can
be sealed by the reaction of calcium hydrox-
ide with the carbon dioxide in the air to form
insoluble compounds of calcium carbonate.
Cementitious waterproofing
screeds
Waterproofing screeds demonstrate a high
compatibility with concrete in terms of
mechanical and physical properties such as
compressive and bending tensile strength,
Youngs modulus and thermal expansion.
This allows an excellent bond with the pre-
pared concrete surface.
Due to their water vapour permeability,
cementitious waterproofing screeds do not
trap water and humidity inside the host con-
crete. This is contrary, for example, to
polyurethane or other polymer-based coat-
ings, which face the risk of blistering. The
permeability of water vapour also allows the
application of these screeds on the side of
negative water pressure, and this allows
waterproofing to be carried out either after
the concrete structure is completed, or dur-
ing a stage of renovation many years later.
A correct application provides a seamless
seal against water pressure from the outside
and provides protection for the concrete
from de-icing salts and water brought in by
cars. This also applies to intermediate slabs.
Local repairs, if ever necessary, are carried
out easily by removing the damaged screed
and patching with the same material. The
joints between the existing screed and the
patch will be waterproofed using the self-
healing properties described previously.
Application
Prior to application the concrete surface
needs to be prepared. For the best bond the
surface has to be rough and the cement lai-
tance and loose particles have to be
removed. This is best achieved by using a
mechanical grinder, which leaves a rough-
ness of approximately 2mm. Alternatively,
high-pressure water jetting or sand blasting
could be used. The firm mechanical bond is
strong enough to take the stress due to
shrinkage of the screed without cracking. It
is advisable to apply the waterproofing
screed to a newly built structure as late as
possible in order that the shrinkage of the
concrete has reached optimal levels. The
waterproofing screed will then seal potential
cracks easily.
The prepared surface must be damp, but
not wet. The dampness is required to pre-
vent the concrete from absorbing water
from the screed during the process of hydra-
tion which could disturb the specified
water/cement ratio. Excess water would
result in a separating layer and prevent a
proper bond.
Conclusion
Waterproofing car park decks with cementi-
tious screeds combines two main functions:
the screed keeps water out of the structure
and also provides the topcoat to drive on.
As a final coat the seamless screed protects
the concrete structure from damage by
harmful salts carried in by cars. The coatings
provide a high degree of mechanical resist-
ance to wear by tyres. Due to the combined
function, time and cost are reduced since
waterproofing and screeding is done in one
working cycle. I
CAR PARKS
Figure 1: Overview of the finished car park with a cementitious waterproofing screed on the
floor.
Figure 2: Levelling and providing a slope with
steel rails.
Figure 3: Trowelling the screed to a smooth
and dense surface.
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