Causes and Prevention of Bridge Deck Cracking

Michael F. Praul, P.E.

Construction & Materials Engineer

FHWA-Maine Division

Concrete is a material that, by its very nature, will crack during its
service life. For bridge decks this is especially significant as these
cracks allow accelerated ingress of chlorides and the subsequent
corrosion of the reinforcing steel and deck deterioration. Although
there has been little research to document the degree of cracking
occurring, there has been a general perception among highway agencies
that deck cracking, particularly early-age deck cracking, is on the
increase. Our goal needs to be to minimize the occurrence of cracks and
properly treat the cracking that does occur.

There are many causes of deck cracking; causes that span the range of
design, materials, and construction issues. It is important to realize
that cracking may often be the result of the cumulative effects of many
different causes. While agencies may address some of the causes of
cracking, either through their design practices, and materials and
construction specifications, most do not have a concerted program aimed
at reducing deck cracking.

The following is a list of some of the causes of bridge deck cracking
along with what can be done to mitigate the degree of cracking
experienced:

-   Placement of Deck Reinforcing: Designers generally place transverse
    reinforcing on top of longitudinal reinforcing in the top mat to
    maximize the efficiency of the section. Switching this results in a
    section less prone to propagation of transverse cracking.

-   Concrete Water Content: Regardless of the w/c ratio, concretes with
    high water content are more likely to experience drying shrinkage.
    Aiming for lower total mix water can mitigate the problem.

-   Aggregates: The size, shape, and texture can lead to a need for more
    water (see above) in the mix. Also, harder aggregates provide
    resistance to shrinkage cracking.

-   Cement Content: High cement content also leads to more water (see
    above). Additionally, the amount of heat generated can lead to
    thermal cracking. Concrete with a high cement content may also have
    a higher modulus with can contribute to long-term crack growth.

-   Use of Pozzolans: The growth of HPC has led to an increase in use of
    pozzolanic materials. These can make concrete more sensitive to
    construction practices, particularly curing, and these concretes may
    be more prone to plastic shrinkage cracking. Timely curing is
    extremely important for these concretes.

-   Placement Conditions: The potential for plastic shrinkage cracking
    is increased when placing decks in conditions resulting in high
    surface evaporation rates. Mitigating measures include restrictions
    on placement conditions or possibly the use of fogging equipment.

-   Temperature Gradient: During either cold or hot weather, a thermal
    gradient may develop between the deck and the underlying
    superstructure. If this gradient is too severe, cracking can occur.
    During cold weather, a constant temperature environment must be
    provided by heating the girders as well as providing appropriate
    curing conditions for the deck. During hot weather, placements
    should begin in the early morning hours which will allow the girders
    to heat up due to ambient conditions as the deck concrete is
    generating its heat of hydration.

-   Placement Sequence: With the use of retarders, it is becoming common
    practice to place a multiple-span deck in a single, continuous
    placement. Concrete that is not kept plastic during the placement
    may crack as the placement proceeds and the camber rolls out of the
    beams. Returning to placing by moment zone or assuring concrete
    plasticity can alleviate this problem.

-   Vibration: Inadequate consolidation results in areas of crack
    initiation and propagation. It is important to have enough vibrators
    being used during the placement and to assure they are used
    properly.

-   Curing: Curing concrete as soon as practical after finishing, 15-20
    minutes, with wet burlap or blankets for as long as possible will
    help reduce the chances for early-age cracking due to surface
    evaporation and will also allow the concrete to fully more develop
    its strength.

-   Concrete Girder/Deck Age Differential: If long-span prestressed
    concrete girders are cast well in advance of the deck placement,
    they may experience creep and drying shrinkage at an earlier age
    than the deck. While not always practical, if possible the girders
    should be cast as close to the time they are needed for the project.

-   Overlay Preparation: Concrete overlays have demonstrated a marked
    tendency to crack along the bond line with concrete parapet. This
    area should be scarified prior to overlay placement to assure a good
    bond.

If cracking does occur, most agencies are treating smaller cracks with
high molecular weight methacrylate and larger cracks with epoxy. If
epoxy is used to repair cracks in substrate concrete in preparation for
a deck overlay, sand should be cast over top of the epoxy. This will
offset the concrete bond-breaker effect of the epoxy.