Volume 10, Number 2
September 1999 Copyright
1999, CRITERIUM ENGINEERS
The Engineering Advisor is intended to enhance your
knowledge of technical issues relating to buildings. For additional information on any subject, please feel free to
call us. Our commitment is to provide
you with timely, accurate information.
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PARKING GARAGES |
The conditions of a parking garage may seem like an
afterthought to some property managers and investors. When asked about maintenance costs, one
representative for a national cost estimating company replied: "Parking garages don't require maintenance. They're just steel and
concrete." It is true that the
mechanical systems generally pale in complexity to the main building(s). It is the parking garage, however, that
suffers most the effects of weather and poor maintenance and often becomes the
area of greatest deferred maintenance and cost. |
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Types of Structures |
There are generally three types of parking structures: Precast Concrete (Prestressed) Structures The advantage of precast is
a high degree of quality control.
When the precast units are prestressed, the potential for cracking is
reduced. Steel-Framed Structures typically consist of a structural steel superstructure with a
cast-in-place slab. Steel beams can
be left exposed or encased in concrete for added strength and/or fire
protection. Cast-in-Place (CIP), Conventionally Reinforced Concrete
Structures account for the largest percentage (approximately 36%)
of structural systems found in the U.S.
Because of their span limitations and susceptibility to cracking, CIP
structures are not built as frequently today. One variant, Cast-in-Place Post-Tension Concrete Structures
is among the most durable of all the types of parking structures. Underground parking structures are typically found in
urban areas because of space constraints.
The structures are usually made of CIP concrete or, steel frame with
poured concrete floors. As a rule, if
habitable living space is found above a parking structure, the parking
structure has to be fire-rated or equipped with an automatic sprinkler
system. |
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Problems with Garages |
All types of garages are exposed to moisture problems
wherein water penetrates the concrete and causes rust on the rebar, which in
turn expands and causes cracking and spalling of the concrete. Such corrosion is caused by two
mechanisms. "Carbonation"
is a natural process whereby carbon dioxide (CO2) in the air
interacts with moisture in the hydrated cement minerals, causing gaps and
voids. On a regional basis, chlorides
(deicing salts) brought into the garage on tires and the undersides of cars
permeate the concrete deck. Both
these mechanisms lower the pH and lead to water entry and rust. While corrosion due to water infiltration
is a major problem with all types of parking structures, each parking
structure type has specific problems. Precast Concrete The T-shaped units or planks used in
precast structures are not monolithic; they contain many joints. Precast units are subject to flexure. A "rocking" effect can lead to
cracks and separation in the connections and joints. As cracks develop, water has an opening to
penetrate the slab, migrate down the sides of the beams, and collect on the
ledger beams. If a topping is used on
the precast concrete, the problem is magnified. Other concrete-related problems with precast parking structures
include spalls to columns, beams, and corbels; cracks in precast membranes;
and joint nosing failures at expansion joints. Operationally, plowing with steel blades can also cause great
damage. Steel-Framed One of the problems associated with
steel-framed structures is differential movement between the steel members
and the concrete slab. When moisture
and corrosion set in, the slab can heave from the beam, which causes cracking
in the concrete. Cracks can also
develop in areas subjected to negative bending moments, where tensile
stresses are present in the top fibers of the concrete slab. When cracks develop in any slab, moisture
is given a path of entry into the concrete.
If the cracks are deep enough and the water ponds on top from
insufficient drainage, moisture can pass completely through the slab. Water entry can cause corrosion to the
rebar and also to the metal deck if it is still in place.
If a corrugated metal deck is left in place below the concrete deck,
the metal deck should be perforated so any moisture that permeates the
deck has a way to escape. Cast-in-Place Random shrinkage cracks are very common in
cast-in-place, conventionally reinforced concrete structures. When cracks fully penetrate the slab,
moisture can enter the top and pass completely through to the bottom of the
slab. Over time, this causes
deterioration to the top and bottom steel, which eventually spalls the
concrete deck and the concrete soffit.
Inadequate rebar cover (less than ¾ inch) is another source of
problems for cast-in-place structures.
It does not take much for the contaminants to permeate the concrete
and cause corrosion of the rebar. If
the slab contains low points and has insufficient drainage, the water can
collect and eventually cause problems. |
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Inspection & Repair |
During a standard Property Condition Assessment, we
inspect parking structures using a sounding device or chain drag (to identify
subsurface delaminated areas) and a flashlight. With these simple tools, we look for evidence of corrosion,
rust, spalling, cracking, and other visible defects. For a more detailed garage inspection, we
may also use a masonry hammer and a pactometer to locate the rebar. Our report will then include a spall
layout plan, a summary spreadsheet with quantity matrix, a repair cost estimate,
and a projected future expenditure analysis.
Additional testing may be required, including: · Photography
(microscopic analysis to see interior cracking); this can be accomplished by
taking core samples. · Carbonation analysis
to detect the presence and depth of carbonation, and chloride analysis to
locate the sources of chloride. Dealing with the symptoms without pinpointing the cause
can result in inappropriate repair.
Repairs to structures with delaminated concrete usually include removing
all the delaminated concrete, cleaning the corroded rebar, and installing a
corrosion inhibitor on the bar in addition to placing a high early strength
modified cementitious material in the patch.
All deteriorated expansion joints and control joints should be
repaired by completely replacing the sealant material. |
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Maintenance |
All decks should be washed in the spring and fall of
each year to remove chloride-containing salts, locate potential areas of
water infiltration in and around drains and expansion and control joints, and
identify delaminations. On a yearly
basis, the deck of the parking garage should be sounded to determine if any
delaminating concrete is present. If
delaminated concrete or leaking control/expansion joints are encountered,
they should be repaired immediately.
Corrosion and/or water infiltration of the slab can greatly increase
the repair cost. All caulked joints
should be checked regularly and completely recaulked approximately every ten
years. |
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Typical Costs |
Because the procedures to repair parking garages are so
repetitive, the more square footage or lineal footage of repair, the lower
the price will be. This factor, along
with the location, accessibility, and economic parameters, will determine the
cost. Preventive maintenance is
generally more cost-effective than waiting to make repairs. In our experience, maintenance costs of
$35 to over $100 per space per year can be expected, depending on the type of
structure and climate.
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CRITERIUM ENGINEERS 22 Monument Square Portland, ME 04101 1-800-242-1969 |
