Analysis on the Causes of Cracks in Bridges

Abstract

In recent years, the transportation infrastructure of our province has developed rapidly, and a large number of concrete bridges have been built in various places. In the process of building and using bridges, reports on the quality of the project due to the occurrence of cracks and even the collapse of the bridge are not uncommon. Concrete cracking can be said to be “frequent onset” and “probable disease”, often plaguing bridge engineering and technical personnel. In fact, many cracks can be overcome and controlled if certain design and construction measures are taken. In order to further strengthen the understanding of cracks in concrete bridges, try to avoid the occurrence of more dangerous cracks in the project. In this paper, we should make a comprehensive analysis and summary of the types and causes of cracks in concrete bridges to facilitate the design and construction to find out the control. The feasible way of cracking can achieve the role of prevention.

Full text

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Analysis on the Causes of Cracks in Bridges
Tingjuan Zhang
Qinghai Luxiang Engineering Supervision Consulting Co., Ltd., Xining, Qinghai,
810001, China
[Abstract] In recent years, the transportation infrastructure of our province has
developed rapidly, and a large number of concrete bridges have been built in various
places. In the process of building and using bridges, reports on the quality of the
project due to the occurrence of cracks and even the collapse of the bridge are not
uncommon. Concrete cracking can be said to be frequent onset and probable
disease , often plaguing bridge engineering and technical personnel. In fact, many
cracks can be overcome and controlled if certain design and construction measures are
taken. In order to further strengthen the understanding of cracks in concrete bridges,
try to avoid the occurrence of more dangerous cracks in the project. In this paper, we
should make a comprehensive analysis and summary of the types and causes of cracks
in concrete bridges to facilitate the design and construction to find out the control.
The feasible way of cracking can achieve the role of prevention.
[Keywords] Bridge engineering; Cracks; Causes; Analysis
1. Introduction
In fact, the causes of cracks in concrete structures are complex and numerous, and
even multiple factors affect each other, but each crack has one or several main causes.
The types of cracks in concrete bridges can be roughly divided into the following
categories:
2. Load-induced Cracks
The cracks generated by concrete bridges under conventional static, dynamic and
secondary stresses are called load cracks. There are two types of direct stress cracks
and secondary stress cracks.
2.1 Direct Stress Cracks
Direct stress cracks are cracks caused by direct stress caused by external loads. The
causes of cracks are:
(1) In the design calculation phase, the structural calculation is not calculated or
partially missed; the calculation model is unreasonable; the structural stress
assumption is inconsistent with the actual force; the load is less calculated or missing;
the internal force and reinforcement calculation are incorrect; the structural safety
factor is not sufficient. Structural design does not consider the possibility of
construction; the design section is insufficient; the reinforcement is less set or

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incorrectly arranged; the structural rigidity is insufficient; the construction is
improperly handled; the design drawings are unclear.
(2) During the construction stage, the construction machinery and materials are
stacked without restrictions; the structural characteristics of the prefabricated structure
are not known, and the body can be turned over, hoisted, transported and installed at
will; the construction sequence is not changed according to the design drawings, and
the structural stress mode is changed without change. Do not check the fatigue
strength of the structure under machine vibration.
(3) During the use phase, heavy vehicles that exceed the design load cross the bridge;
contact and impact by vehicles and ships; strong winds, heavy snow, earthquakes,
explosions, etc.[1]
2.2 Secondary Stress Cracks
The secondary stress crack refers to the secondary stress caused by the external load.
The causes of cracks are:
(1) Under the design external load, the actual working state of the structure is
different from the conventional calculation, and the secondary stress causes the
structure to crack. For example, the design of the arch of the two-hinge arch bridge is
often designed by arranging the X -shaped steel bar and reducing the size of the
section at the same time. The theoretical calculation is that there is no bending
moment, but the hinge can still resist bending and cracks. Causes steel corrosion.
(2) In the bridge structure, it is often necessary to make a groove, open a hole, and set
up a bull s leg. In the conventional calculation, it is difficult to carry out the
simulation calculation with an accurate pattern, and the force-reinforcing steel is
generally set according to experience. Studies have shown that after the
force-receiving member is dug, the force flow will produce a diffraction phenomenon,
which is dense near the hole and generates a large stress concentration. In long-span
pre-stressed continuous beams, it is often necessary to cut the steel bundle according
to the internal force of the section in the span, and the anchor head is set, and the
crack is often seen near the anchoring section. Therefore, if it is handled improperly,
cracks are likely to occur at the corners of these structures or at the sudden change of
the shape of the members and at the cut of the stressed steel bars.
In actual engineering, secondary stress cracks are the most common cause of load
cracks. The secondary stress cracks are mostly tensile, splitting and shearing
properties. The secondary stress crack is also caused by the load, but it is generally
not calculated according to the conventional, but with the continuous improvement of
modern calculation means, the secondary stress crack can also be reasonably checked.
For example, the secondary stresses generated by pre-stressing, creeping, etc., can be

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correctly calculated in many plane finite element programs, but it was difficult 40
years ago. In design, care should be taken to avoid structural abrupt changes (or
sudden changes in the section). When it cannot be avoided, local treatment should be
done, such as rounding at the corners, making a gradual transition at the transition,
and strengthening the structural reinforcement at the corner. For reinforcing bars, for
the larger holes, the retaining angle steel can be placed around.
Load crack characteristics vary from load to load. Such cracks occur mostly in the
tension zone, the shear zone or the severe vibration zone. However, it must be pointed
out that if the compression zone has a peeling or a short crack along the direction of
compression, it is often a sign that the structure reaches the limit of bearing capacity
and is a precursor to structural damage. The reason is often that the section size is too
small.
2.3 Cracks Caused by Different Force Modes of the Structure
According to the different force modes of the structure, the crack characteristics are as
follows:
(1) The center is pulled. The crack penetrates the cross section of the member, the
spacing is substantially equal, and is perpendicular to the direction of force. When a
reinforced bar is used, a secondary crack located near the reinforcing bar occurs
between the cracks.
(2) The center is under pressure. A short, dense parallel crack parallel to the direction
of force is present along the member.
(3) Bent. Near the largest section of the bending moment, cracks perpendicular to the
direction of tension are formed from the edge of the tension zone, and gradually
develop toward the neutral axis. When threaded steel bars are used, shorter secondary
cracks are visible between the cracks. When the structural reinforcement is small, the
crack is small and wide, and the structure may be brittle.
(4) Great eccentricity is under pressure. It is a small eccentric compression member
with a large eccentric compression and a small reinforcement in the tension zone,
similar to a bent member.
(5) Small eccentric pressure. It is a large eccentric compression member with small
eccentric compression and a large amount of reinforcement in the tension zone,
similar to a central compression member.
6) Cut. When the stirrup is too dense, the oblique crushing occurs, and oblique cracks
larger than 45° appear along the abdomen of the beam end; when the stirrups are
appropriate, shear compression occurs, and oblique cracks parallel to each other in the

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direction of 45° appear along the middle and lower ends of the beam end.
(7) Torsion. On the side of one side of the member, a multi- treaty 45° oblique crack
appears first, and the adjacent surface is unfolded in a spiral direction.
(8) Punched. A 45° direction slope is pulled along the four sides of the column head
plate to form a punching surface.
(9) Local compression. There is a plurality of short cracks substantially parallel to the
pressure direction appearing in the local compression zone.
3. Crack Caused by Temperature Changes
Concrete has thermal expansion and contraction properties. The concrete will deform
When the external environment or the internal temperature of the structure changes. If
the deformation is restrained, stress will be generated in the structure. When the stress
exceeds the concrete tensile strength, temperature crack will occur. In some long-span
bridges, the temperature stress can reach or exceed the live load stress. The main
characteristic of temperature cracks that distinguish other cracks is that they will
expand or close with temperature. The main factors causing temperature changes are:
3.1 The Annual Temperature Differences
During the year, the temperature of the four seasons is constantly changing, but the
change is relatively slow. The influence on the bridge structure is mainly caused by
the longitudinal displacement of the bridge. Generally, it can be coordinated by
structural measures such as bridge deck expansion joints, bearing displacement or
setting of flexible piers. Temperature cracks are caused when the displacement is
limited, such as arch bridges and rigid bridges. The annual temperature difference in
China is generally based on the average temperature of January and July. Considering
the creep properties of concrete, the elastic modulus of concrete should be considered
for reduction in the calculation of internal temperature difference.
3.2 The Sunshine
After the side of the bridge deck, main beam or pier is exposed to the sun, the
temperature is significantly higher than other parts, and the temperature gradient is
non-linear. Due to its own restraint, the local tensile stress is large and cracks appear.
Sunshine and sudden cooling below are the most common causes of structural
temperature cracking.

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3.3 Suddenly Cool Down
Heavy rain, cold air, sunsets, etc. can cause a sudden drop in the temperature of the
outer surface of the structure, but a temperature gradient due to a relatively slow
internal temperature change. Sunlight and sudden cooling internal force calculations
can be carried out using design specifications or reference to real bridge data and the
concrete elastic modulus is not considered for reduction
3.4 Hydration Heat
Appeared in the construction process, after the pouring of large-volume concrete
(thickness over 2.0 m), the internal temperature is high due to the hydration of the
cement, and the temperature difference between the inside and the outside is too large,
causing cracks on the surface. In the construction, according to the actual situation, try
to choose the cement type with low hydration heat, limit the amount of cement unit,
reduce the inlet temperature of the aggregate, reduce the temperature difference
between the inside and the outside, and slowly reduce the temperature. If necessary,
use the circulating cooling system for internal heat dissipation, or adopt Thin layers
are continuously cast to accelerate heat dissipation.
3.5 Improper Construction Measures
Steam curing or improper construction measures during winter construction, the
concrete is quenched and heated, the temperature inside and outside is uneven, and
cracks are prone to occur.
3.6 Improper Welding Measures
When the transverse baffle between the prefabricated T-beams is installed, when the
pre-buried steel plate and the leveling steel plate are welded, if the welding measures
are improper, the concrete near the iron piece is prone to burn and crack. When the
pre-stressed member is tensioned by the electro thermal tension method, the
temperature of the pre-stressed steel can be raised to 350 ° C, and the concrete
member is also prone to cracking. The experimental research shows that the strength
of concrete caused by fire and other high temperature burns decreases with the
increase of temperature, and the bond strength between steel and concrete decreases.
The tensile strength decreases by 50% after the concrete temperature reaches 300 °C.
The drop is 60%, and the bond strength between the light-reinforced steel bar and the
concrete is reduced by 80%. Due to the heat, a large amount of free water in the
concrete body can also evaporate rapidly.

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4. Cracks Caused by Shrinkage
4.1 Shrinkage Deformation types
In actual engineering, cracks caused by shrinkage of concrete are the most common.
Among the concrete shrinkage types, plastic shrinkage and shrinkage (dry shrinkage)
are the main causes of concrete volume deformation, and there are also spontaneous
shrinkage and carbonization shrinkage.
4.1.1 Plastic shrinkage
It occurs in the construction process and about 4~5 hours after the concrete is poured.
At this time, the cement hydration reaction is fierce, the molecular chain is gradually
formed, the bleeding and water are rapidly evaporated, the concrete loses water and
shrinks, and the aggregate sinks due to its own weight. When the concrete has not yet
hardened, it is called plastic shrinkage. The plastic shrinkage is produced in a large
order of up to about 1%. If the steel bars are blocked during the sinking process, the
cracks along the direction of the steel bars are formed. At the vertical section of the
component, such as the intersection of the T-beam, the box girder web and the top and
bottom plates, the surface of the surface will be cracked in the direction of the web
due to the unevenness of the solidification before hardening. In order to reduce the
plastic shrinkage of concrete, the water-cement ratio should be controlled during
construction to avoid excessive mixing for a long time. The material should not be too
fast, the vibrating should be dense, and the vertical cross-section should be layered.
4.1.2 Dry shrinkage
After the concrete is hard, as the surface moisture gradually evaporates, the humidity
gradually decreases, and the concrete volume decrease, which is called dry shrinkage.
Because the surface moisture loss of the concrete surface is fast and the internal loss
is slow, the surface shrinkage is large and the internal shrinkage is small. The surface
shrinkage deformation is restrained by the internal concrete, so that the surface
concrete is subjected to tensile force. When the surface concrete is subjected to tensile
force exceeding its tensile strength At this time, shrinkage cracks are generated. The
shrinkage of concrete after hardening is mainly shrinkage. For components with a
large reinforcement ratio (more than 3%), the constraints of steel bars on concrete
shrinkage are obvious, and cracks are likely to occur on concrete surfaces.
4.1.3 Spontaneous shrinkage
Spontaneous shrinkage is the hydration reaction between cement and water during the
hardening process. This shrinkage is independent of the external humidity and can be
positive (i.e. shrinkage, such as ordinary Portland cement concrete) or negative (i.e.
expansion) Such as slag cement concrete and fly ash cement concrete).
4.1.4 Carbonization shrinkage
Shrinkage deformation caused by chemical reaction between carbon dioxide in the

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atmosphere and hydrates of cement. Carbonization shrinkage occurs only at a
humidity of about 50% and accelerates as the concentration of carbon dioxide
increases. Carbonization shrinkage is generally not calculated.
The characteristics of concrete shrinkage cracks are that most of them are surface
cracks, the crack width is thin, and they are crisscrossed and criss-shaped, and the
shape has no regularity.
4.2 Main Factors of Concrete Shrinkage Cracks
4.2.1 Cement type, Label and Dosage
The slag cement, quick-hardening cement and low-heat cement concrete have higher
shrinkage, and the ordinary cement, volcanic ash cement and bauxite cement concrete
have lower shrinkage. In addition, the lower the cement number, the larger the unit
volume, and the greater the grinding degree, the larger the concrete shrinks and the
longer the shrinkage time occurs. For example, in order to increase the strength of
concrete, the practice of forcibly increasing the amount of cement is often used during
construction, and as a result, the shrinkage stress is significantly increased.
4.2.2 Aggregate Varieties
Among the aggregates, quartz, limestone, dolomite, granite and feldspar have lower
water absorption and lower shrinkage; while sandstone, slate and amphibolite have
higher water absorption and higher shrinkage. In addition, the large particle size
shrinkage is small, and the water content is large and the shrinkage is larger.
4.2.3 Water-cement Ratio
It is that the greater the water consumption, the higher the water-cement ratio and the
greater the concrete shrinkage.
4.2.4 External Admixture
It is that the better the water retention of the external admixture, the smaller the
concrete shrinkage.
4.2.5 Maintenance Methods
Good maintenance accelerates the hydration reaction of concrete and achieves higher
concrete strength. The higher the humidity during maintenance, the lower the
temperature, and the longer the curing time, the smaller the concrete shrinks. The
steam curing method is smaller than the natural curing method.
4.2.6 The External Environment
When the humidity in the atmosphere is small, the air is dry, the temperature is high,
and the wind speed is large, the concrete moisture evaporates quickly and the concrete
shrinks faster.

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4.2.7 Vibration Mode and Time
The mechanical vibrating method is less contractile than the manual tamping method.
The vibration time should be determined according to the mechanical properties,
generally 5~15s/time. The time is too short, the vibration is not dense, and the
strength of the concrete is insufficient or uneven; the time is too long, causing
delamination, the coarse aggregate sinks into the bottom layer, the fine aggregate
remains in the upper layer, the strength is uneven, and the upper layer is prone to
shrinkage cracks.
For cracks caused by temperature and shrinkage, the addition of structural steel bars
can significantly improve the crack resistance of concrete, especially thin-walled
structures (wall thickness 20~60cm). The structural reinforcement should preferably
use small-diameter s -pitch arrangements (@10~@15cm).
The full-section structure reinforcement ratio should not be lower than 0.3%,
generally 0.3%~0.5%.
5. Cracks Caused by Ground Deformation
Due to the uneven vertical settlement or horizontal displacement of the foundation,
additional stress is generated in the structure, which exceeds the tensile strength of the
concrete structure, resulting in structural cracking. The main reasons for uneven
settlement of the foundation are:
5.1 The Accuracy of Geological Survey Is Not Enough and the Test
Data Is Not Allowed
Designing and constructing without fully grasping the geological conditions is the
main reason for the uneven settlement of the foundation. For example, in the hilly
area or the mountainous area bridge, the drilling distance is too far during the survey,
and the foundation rock surface is undulating and large, and the survey report cannot
fully reflect the actual geological conditions.
5.2 The Differences in Ground Quality Is Too Large
The bridges built in the mountain valleys have great changes in the geology and
slopes of the river ditch. There are even weak foundations in the river ditch, and the
foundation soils are unevenly settled due to different compressibility.
5.3 The Structural Load Difference Is Too Large
Under the condition that the geological conditions are relatively consistent, when the
difference of the foundation loads of each part is too large, uneven settlement may
occur. For example, the middle of the high-filled box-shaped culvert is larger than the

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load on both sides and the settlement in the middle is larger than the two sides, and
the box culvert may crack.
5.4 There Are Big Differences in the Types of Structural Foundations
In the same bridge, if different foundations are used, such as expanding the
foundation and pile foundation, or using the pile foundation at the same time, but the
difference between the pile diameter or the pile length is large, or when the enlarged
foundation is used at the same time, but the difference in the base elevation is large,
the uneven settlement of the foundation may be caused.
5.5 The Basis for the Construction of Phases
When new bridges are built near the original bridge foundation, such as the left and
right half-bridges of the highways built in phases, the new foundation bridge loads or
foundation treatment will cause the foundation soil to re-consolidate, which may
cause large settlement of the original bridge foundation.
5.6 The Foundation Frost Heaving
At a temperature below zero, the foundation soil with a higher water content expands
due to freezing; once the temperature rises, the frozen soil melts and the foundation
sinks. Therefore, freezing or melting of the foundation can cause uneven settlement.
5.7 The Ground Quality Is Poor
When the bridge foundation is placed in bad geology such as landslides, caves or
active faults, it may cause uneven settlement.
5.8 After the Completion of the Bridge, the Original Foundation
Conditions Will Change
After the natural foundation and artificial foundation are immersed in water, especially the
special foundation soil such as plain fill, loess and expansive soil, the strength of the soil is
reduced with water and the compression deformation is increased. In the soft soil foundation,
the groundwater level is reduced due to artificial pumping or dry season, the foundation soil
layer is re-consolidated and sinking, and the buoyancy of the foundation is reduced, the
negative frictional resistance is increased, and the foundation is increased. Some bridge
foundations are buried too shallow, and they may be displaced by flooding and scouring.
Changes in ground loading conditions, such as the accumulation of a large number of waste,
sand and gravel in the vicinity of the bridge due to landslides, landslides, etc., the soil layer in
the bridge site may be compressed and deformed again. Therefore, changes in the original
foundation conditions during use may cause uneven settlement.[2]

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For arch bridges and other structures that generate horizontal thrust, the geological conditions
are not enough, the design is unreasonable, and the original geological conditions are
destroyed during construction, which is the main cause of horizontal displacement cracks.
6. Cracks Caused by Steel Corrosion
Due to the poor quality of the concrete or the insufficient thickness of the protective
layer, the concrete protective layer is carbonized by carbon dioxide to the surface of
the steel bar, which reduces the alkalinity of the concrete around the steel bar, or the
chloride ion content around the steel bar is high due to the intervention of chloride,
which can cause oxidation of the steel surface. Membrane damage, iron ions in the
steel bar and oxygen and water invading into the concrete distribute the rust and
corrosion reaction, and the volume of the rust iron hydroxide is about 2 to 4 times
larger than that of the original, thereby causing expansion stress on the surrounding
concrete, resulting in cracking and peeling of the protective layer concrete. Cracks are
formed along the longitudinal direction of the steel bar, and rust is infiltrated into the
concrete surface. Due to rust, the effective cross-sectional area of the steel bar is
reduced, the reinforcing force of the steel bar and the concrete is weakened, the
structural bearing capacity is reduced, and other forms of cracks are induced, which
intensifies the corrosion of the steel bar and causes structural damage.
To prevent corrosion of steel bars, the crack width should be controlled according to
the specifications and sufficient thickness of the protective layer should be used in
design (of course, the protective layer should not be too thick, otherwise the effective
height of the member should be reduced, and the crack width will be increased when
the force is applied); Control the water-cement ratio of concrete, strengthen vibrating,
ensure the compactness of concrete, prevent oxygen intrusion, and strictly control the
amount of admixture containing chlorine salt. Especially in coastal areas or other
areas with corrosive air and groundwater, it should be cautious.
7. Cracks Caused by Frost Heaving
When the atmospheric temperature is below zero, the water-saturated concrete freezes,
the free water turns into ice, and the volume expands by 9%, so the concrete produces
expansion stress; at the same time, the super-cooled water in the concrete gel hole (the
freezing temperature is below -78 °C) The migration and redistribution in the
microstructure causes osmotic pressure, which increases the expansion force in the
concrete, reduces the strength of the concrete, and causes cracks to appear. In
particular, the concrete is most severely frozen when it is initially set, and the concrete
strength loss after ageing can reach 30% to 50%. Frost heave cracks along the pipe
direction may occur if the insulation is not applied to the pre-stressed tunnel during
winter construction.

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Temperatures below zero and concrete water saturation are necessary for frost heaving
damage. When the concrete has many voids and strong water absorption; the
aggregate contains too much impurities such as mud; the concrete water-cement ratio
is too large, the vibration is not dense; the weak curing causes the concrete to be
frozen early, etc., which may lead to concrete frost heaving cracks. . In the winter
construction, the electric heating method, the warm shed method, the underground
heat storage method, the steam heating method, and the antifreeze in the concrete
mixing water (but the chlorine salt should not be used) can ensure the concrete
hardens under low temperature or negative temperature.
8. Cracks Caused by Building Material Quality
Concrete is mainly composed of cement, sand, aggregate, mixing water and admixture.
Unqualified materials used to dispose of concrete may cause cracks in the structure.
8.1 Cement
Concrete is mainly composed of cement, sand, aggregate, mixing water and admixture.
Unqualified materials used to dispose of concrete may cause cracks in the structure.
(1) The cement stability is unqualified, and the free calcium oxide content in the
cement exceeds the standard. Calcium oxide hydrates very slowly during the
coagulation process, and continues to hydrate after the cement concrete is coagulated,
which can destroy the hardened cement stone and reduce the tensile strength of the
concrete.
(2) When the cement is out of strength, the cement is damp or expired, which may
cause insufficient concrete strength, which may cause cracking of the concrete.
(3) When the cement contains a high amount of alkali (for example, more than 0.6%)
and uses an alkali-containing aggregate, it may cause an alkali aggregate reaction.
8.2 Sand and Stone Aggregate
Particle size, gradation, and impurity content of sand. The particle size of the
sandstone is too small, the gradation is poor, and the void ratio is large, which will
lead to an increase in the amount of cement and mixing water, affecting the strength
of the concrete, and increasing the shrinkage of the concrete. If the ultrafine sand
exceeding the regulations is used, the consequences are more serious. The high
content of mica in sand and gravel will weaken the bond between cement and
aggregate and reduce the strength of concrete. The high mud content in sand and
gravel will not only increase the amount of cement and mixing water, but also reduce
concrete strength, frost resistance and impermeability. Excessive organic matter and
light materials in sand and gravel will delay the hardening process of cement and

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reduce the strength of concrete, especially early strength. Sulfide in sand and gravel
can chemically react with tricalcium aluminate in cement and expand in volume by
2.5 times.
8.3 Mixing Water and Admixture
When the content of impurities such as chloride in the mixing water or the admixture
is high, the steel corrosion is greatly affected. Mixing concrete with seawater or
alkaline spring water, or using an alkali-containing admixture, may have an effect on
the alkali aggregate reaction.[3]
9. Cracks Caused by Construction Process Quality
In the process of concrete structure pouring, component making, demoulding,
transportation, stacking, assembling and hoisting, if the construction process is
unreasonable and the construction quality is inferior, it is easy to produce vertical,
horizontal, oblique, vertical, horizontal, Surface, deep and penetrating cracks,
especially elongated thin-walled structures, are more likely to occur. The location and
direction of the crack and the width of the crack vary depending on the cause. The
more common ones are:
(1) The concrete protective layer is too thick, or the upper layer of steel bars that have
been tied up is stepped on, so that the protective layer of the stressed tendon that bears
the negative bending moment is thickened, resulting in the effective height of the
member being reduced, forming a crack perpendicular to the stressed reinforcing bar.
(2) The vibrating concrete is not dense and uneven, and there are honeycombs,
pockmarks and voids, which lead to the origin of steel corrosion or other load cracks.
(3) The concrete is poured too fast, and the concrete has low fluidity. Before the
hardening, the concrete is not enough. After hardening, the sedimentation is too large.
It is easy to crack after several hours of pouring, and plastic shrinkage cracks.
(4) The concrete mixing, transportation time is too long, so that the water evaporation
too much, causing the concrete slump is too low, resulting in irregular shrinkage
cracks in the concrete volume.
(5) The concrete is sharply dried during the initial maintenance, resulting in irregular
shrinkage cracks on the surface of the concrete in contact with the atmosphere.
(6) When pumping concrete, in order to ensure the fluidity of the concrete, increase
the amount of water and cement, or increase the water-cement ratio for other reasons,
resulting in an increase in the shrinkage of concrete when it hardens, causing irregular
cracks in the concrete volume.

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(7) When the concrete is layered or sectioned, the joints are not handled well, and
cracks are likely to occur between the new and old concrete and the construction
joints. For example, when concrete is poured in layers, the post-casting concrete
cannot be poured before the initial pouring of the pre-concrete concrete due to power
failure, rain, etc., causing horizontal cracks between the layers; when using the staged
cast-in-place, the first pouring concrete contact surface is chiseled. The cleaning is not
good, the adhesion between the new and old concrete is small, or the post-concrete
concrete is not in place, causing the concrete to shrink and causing cracks.
(8) The concrete is frozen in the early stage, causing cracks on the surface of the
component, or partial peeling, or an empty drum phenomenon after demolding.
(9)The rigidity of the formwork during construction is insufficient. When the concrete
is poured, the formwork is deformed due to the lateral pressure, and cracks are formed
consistent with the deformation of the formwork.
(10) During the construction, the demoulding is too early, and the strength of the
concrete is insufficient, so that the components are cracked under the action of
self-weight or construction load.
(11) Before the construction, the bracket is not compacted or the bracket stiffness is
insufficient. After the concrete is poured, the bracket will not sink evenly, resulting in
cracks in the concrete.
(12) When the components of assembled structure are transported and stacked, the
supporting mat is not on a vertical line, or the cantilever is too long, or it is violently
bumped during transportation; the hanging point is improperly placed during lifting,
and the lateral stiffness of the T beam is small. Components, laterally unreliable
reinforcement measures, etc., may cause cracks.
(13) The installation sequence is incorrect, and the consequences of the consequences
are insufficient, resulting in cracks. For example, when the reinforced concrete
continuous beam is fully casted in the cast-in-place construction, if the reinforced
concrete wall guardrail is poured at the same time as the main beam, the wall
guardrail often has cracks after the disassembly; after the dismantling and then
pouring the guardrail, the crack is not easy to appear.
(14) Poor construction quality control. Any combination of concrete mix ratio, water,
sand and cement materials are not accurately measured, resulting in insufficient
concrete strength and other properties (workability, compactness), resulting in
structural cracking.

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10. Conclusion
From the completion to the use of a bridge, it involves design, construction,
supervision, operation management and other aspects. It can be seen from the above
that the design of the omission, poor construction, and poor supervision can cause
cracks in the concrete bridge. Therefore, design, construction and supervision in strict
accordance with relevant national norms and technical standards are the premise and
basis for ensuring the safety and durability of the structure. In the operation and
management process, it is also a very important link to further strengthen inspection
and management and timely discover and deal with problems.
References
[1] Dawen Peng, Guofen Li, Xiaoguang Huang. Bridge Engineering[M].Beijing:
People s Communications Press, 2007. (in Chinese)
[2] Shunquan Qin. Bridge Construction Control[M]. Beijing:People s Communi-
cations Publishing House, 2007. (in Chinese)
[3] Wang Yuli. Bridge Construction Technology and Quality Control[M].Beijing:
China Water Resources and Hydropower Press, 2006. (in Chinese)