Concrete durability is defined as its ability to resist weathering, chemical attack, abrasion and other deterioration processes. A durable concrete helps the environment by reducing waste and pollution from repair and replacement. Concrete can withstand weathering like freezing and thawing as well as chemicals when properly designed. Factors like mix design, placement, curing and the exposure environment determine the ultimate durability and life of concrete. Special care is needed for severe exposures like seawater to minimize corrosion and chemical attack.

1. DURABILITY OF CONCRETE
Concrete durability has been defined by the American Concrete Institute as its
resistance to weathering action, chemical attack, abrasion and other degradation
processes.
Durability is the ability to last a long time without significant deterioration. A
durable material helps the environment by conserving resources and reducing
wastes and the environmental impacts of repair and replacement. Construction and
demolition waste contribute to solid waste going to landfills. The production of
new building materials depletes natural resources and can produce air and water
pollution The design service life of most buildings is often 30 years, although
buildings often last 50 to 100 years or longer. Most concrete and masonry
buildings are demolished due to obsolescence rather than deterioration. A concrete
shell can be left in place if a building use or function changes or when a building
interior is renovated. Concrete, as a structural material and as the building exterior
skin, has the ability to withstand nature’s normal deteriorating mechanisms as well
as natural disasters.
Durability of concrete may be defined as the ability of concrete to resist weathering
action, chemical attack, and abrasion while maintaining its desired engineering
properties. Different concretes require different degrees of durability depending on
the exposure environment and properties desired. For example, concrete exposed
to tidal seawater will have different requirements than an indoor concrete floor.
Concrete ingredients, their proportioning, interactions between them, placing and
curing practices, and the service environment determine the ultimate durability and
life of concrete.
Seawater Exposure: Concrete has been used in seawater exposures for decades
with excellent performance. However, special care in mix design and material
selection is necessary for these severe environments. A structure exposed to
seawater or seawater spray is most vulnerable in the tidal or splash zone where

2. there are repeated cycles of wetting and drying and/or freezing and thawing.
Sulfates and chlorides in seawater require the use of low permeability concrete to
minimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure
is helpful. Proper concrete cover over reinforcing steel must be provided, and the
water-cementitious ratio should not exceed 0.40.
Chloride Resistance and Steel Corrosion: Chloride present in plain concrete that
does not contain steel is generally not a durability concern. Concrete protects
embedded steel from corrosion through its highly alkaline nature. The high pH
environment in concrete (usually greater than 12.5) causes a passive and
noncorroding protective oxide film to form on steel. However, the presence of
chloride ions from deicers or seawater can destroy or penetrate the film. Once the
chloride corrosion threshold is reached, an electric cell is formed along the steel or
between steel bars and the electrochemical process ofcarrions begins.
The resistance of concrete to chloride is good; however, for severe environments
such as bridge decks, it can be increase by using a low water-cementitious ratio
(about 0.40), at least seven days of moist curing, and supplementary cementitious
materials such as silica fume, to reduce permeability. Increasing the concrete cover
over the steel also helps slow down the migration of chlorides. Other methods of
reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-
coated reinforcing steel, surface treatments, concrete overlays, and cathodic
protection.
Resistance to Alkali-Silica Reaction (ASR): ASR is an expansive reaction
between reactive forms of silica in aggregates and potassium and sodium alkalis,
mostly from cement, but also from aggregates, pozzolans, admixtures, and mixing
water. The reactivity is potentially harmful only when it produces significant
expansion. Indications of the presence of alkali-aggregate reactivity may be a
network of cracks, closed or spalling joints, or movement of portions of a structure.
ASR can be controlled through proper aggregate selection and/or the use of
supplementary cementitious materials (such as fly ash or slag cement) or blended
cements proven by testing to control the reaction.

3. Abrasion Resistance: Concrete is resistant to the abrasive affects of ordinary
weather. Examples of severe abrasion and erosion are particles in rapidly moving
water, floating ice, or areas where steel studs are allowed on tires. Abrasion
resistance is directly related to the strength of the concrete. For areas with severe
abrasion, studies show that concrete with compressive strengths of 12,000 to
19,000 psiwork well.
concrete crack?
Concrete, like most materials, will shrink slightly when it dries out. Common
shrinkage is about 1/16th of an inch in a 10-foot length of concrete. The reason
contractors place joints in concrete pavements and floors is to allow the concrete to
crack in a neat, straight line at the joint, where concrete cracks due to shrinkage are
expected to occur. Control or construction joints are also placed in concrete walls
and other structures.
concrete surfacesspall
Concrete spalling (or flaking) can be prevented. It occurs due to one or more of the
following reasons.
1.) In cold climates subjected to freezing and thawing, concrete surfaces have the
potential to spall if the concrete is not air-entrained.
2.)Too much water in the concrete mix will produce a weaker, more permeable and
less durable concrete. The water-cementitious ratio should be as low as possible
(0.45 or less).
3.) Concrete finishing operations should not begin until the water sheen on the
surface is gone and the excess bleed water on the surface has had a chance to
evaporate. If this excess water is worked into the concrete because finishing
operations have begun too soon, the concrete on the surface will have too high of a
water content and this surface will be weaker and less durable.
High Humidity and Wind-Driven Rain: Concrete is resistant to wind-driven rain
and moist outdoor air in hot and humid climates because it is impermeable to air
infiltration and wind-driven rain. Moisture that enters a building must come

4. through joints between concrete elements. Annual inspection and repair of joints
will minimize this potential. More importantly, if moisture does enter through
joints, it will not damage the concrete. Good practice for all types of wall
construction is to have permeable materials that breathe (are allowed to dry) on at
least one surface and to not encapsulate concrete between two impermeable
surfaces. Concrete will dry out if not covered by impermeable treatments.
Portland cement plaster (stucco) should not be confused with the exterior
insulation finish systems (EIFS) or synthetic stucco systems that have become
popular but may have performance problems, including moisture damage and low
impact-resistance. Synthetic stucco is generally a fraction of the thickness of
portland cement stucco, offering less impact resistance. Due to its composition, it
does not allow the inside of a wall to dry when moisture gets trapped.
inside. Trapped moisture eventually rots insulation, sheathing, and wood framing.
It also corrodes metal framing and metal attachments. There have been fewer
problems with EIFS used over solid bases such as concrete or masonry because
these substrates are very stable and are not subject to rot or corrosion.
Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm
concrete. Using colored pigments in concrete retains the color in concrete long
after paints have faded due to the sun’s effects.
Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some
softer materials are inedible but still provide pathways for insects. Due to its
hardness, vermin and insects will not bore through concrete. Gaps in exterior
insulation to exposethe concretecan provide access for termite inspectors.
Moderate to Severe Exposure Conditions for Concrete: The following are
important exposure conditions and deterioration mechanisms in concrete. Concrete
can withstand these effects when properly designed. The Specifier’s Guide for
Durable Concrete is intended to provide sufficient information to allow the
practitioner to select materials and mix design parameters to achieve durable
concrete in a variety of environments.

5. Resistance to Freezing and Thawing: The most potentially destructive
weathering factor is freezing and thawing while the concrete is wet, particularly in
the presence of deicing chemicals. Deterioration is caused by the freezing of water
and subsequentexpansion in the paste, the aggregate particles, or both.
With the addition of an air entrainment admixture, concrete is highly resistant to
freezing and thawing. During freezing, the water displaced by ice formation in the
paste is accommodated so that it is not disruptive; the microscopic air bubbles in
the paste provide chambers for the water to enter and thus relieve the hydraullic
pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is
more durable than concrete with a high water-cementitious ratio (0.50 or higher).
Air-entrained concrete with a low water-cementitious ratio and an air content of 5
to 8% will withstand a great number of cycles of freezing and thawing without
distress.
Chemical Resistance: Concrete is resistant to most natural environments and
many chemicals. Concrete is virtually the only material used for the construction of
wastewater transportation and treatment facilities because of its ability to resist
corrosion caused by the highly aggressive contaminants in the wastewater stream
as well as the chemicals added to treat these waste products.
However concrete is sometimes exposed to substances that can attack and cause
deterioration. Concrete in chemical manufacturing and storage facilities is
specially prone to chemical attack. The effect of sulfates and chlorides is discussed
below. Acids attack concrete by dissolving the cement paste and calcareous
aggregates. In addition to using concrete with a low permeability, surface
treatments can be used to keep aggressive substances from coming in contact with
concrete. Effects of Substances on Concrete and Guide to Protective Treatments
discuss the effects of hundreds of chemicals on concrete and provide a list of
treatments to help control chemical attack.
Resistance to Sulfate Attack: Excessive amounts of sulfates in soil or water can
attack and destroy a concrete that is not properly designed. Sulfates (for example
calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by

6. reacting with hydrated compounds in the hardened cement paste. These reactions
can induce sufficient pressure to cause disintegration of the concrete.
Like natural rock such as limestone, porous concrete (generally with a high water-
cementitious ratio) is susceptible to weathering caused by salt crystallization.
Examples of salts known to cause weathering of concrete include sodium
carbonate and sodium sulfate.
Sulfate attack and salt crystallization are more severe at locations where the
concrete is exposed to wetting and drying cycles, than continuously wet cycles. For
the best defense against external sulfate attack, design concrete with a low water to
cementitious material ratio (around 0.40) and use cements specially formulated for
sulfate environments
Sulfate attack in concrete and mortar
Sulfate attack can be ‘external’ or ‘internal’.
External: due to penetration of sulfates in solution, in groundwater for example,
into the concrete from outside.
External sulfate attack
This is the more common type and typically occurs where water containing
dissolved sulfate penetrates the concrete. A fairly well-defined reaction front can
often be seen in polished sections; ahead of the front the concrete is normal, or
near normal. Behind the reaction front, the composition and microstructure of the
concrete will have changed. These changes may vary in type or severity but
commonly include:
· Extensive cracking
· Expansion
· Loss of bond between the cement paste and aggregate

7. · Alteration of paste composition, with monosulfate phaseconverting to ettringite
and, in later stages, gypsum formation The necessary additional calcium is
provided by the calcium hydroxide and calcium silicate hydrate in the cement paste
The effect of these changes is an overall loss of concrete strength.
The above effects are typical of attack by solutions of sodium sulfate or potassium
sulfate. Solutions containing magnesium sulfate are generally more aggressive, for
the same concentration. This is because magnesium also takes part in the reactions,
replacing calcium in the solid phases with the formation of brucite (magnesium
hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates
mainly as gypsum.
Other sources of sulfate which can cause sulfate attack include:
· Seawater
· Oxidation of sulfide minerals in clay adjacent to the concrete – this can produce
sulfuric acid which reacts with the concrete
· Bacterial action in sewers – anaerobic bacterial produce sulfur dioxide which
dissolves in water and then oxidizes to form sulfuric acid
· In masonry, sulfates present in bricks and can be gradually released over a long
period of time, causing sulfate attack of mortar, especially where sulfates are
concentrated due to moisture movement
· Internal sulfate attack
Occurs where a source of sulfate is incorporated into the concrete when mixed.
Examples include the use of sulfate-rich aggregate, excess of added gypsum in the
cement or contamination. Proper screening and testing procedures should generally
avoid internal sulfate attack.
· Delayed ettringite formation

8. Delayed ettringite formation (DEF) is a special case of internal sulfate attack.
Delayed ettringite formation has been a significant problem in many countries. It
occurs in concrete which has been cured at elevated temperatures, for example,
where steam curing has been used. It was originally identified in steam-cured
concrete railway sleepers (railroad ties). It can also occur in large concrete pours
where the heat of hydration has resulted in high temperatures within the concrete.
DEF causes expansion of the concrete due to ettringite formation within the paste
and can cause serious damage to concrete structures. DEF is not usually due to
excess sulfate in the cement, or from sources other than the cement in the concrete.
Although excess sulfate in the cement would be likely to increase expansion due to
DEF, it can occurat normal levels of cement sulfate.
A key point in understanding DEF is that ettringite is destroyed by heating above
about 70 C.
A definition of delayed ettringite formation
DEF occurs if the ettringite which normally forms during hydration is
decomposed, then subsequently re-forms in the hardened concrete.
Damage to the concrete occurs when the ettringite crystals exert an expansive force
within the concrete as they grow.
In normal concrete, the total amount of ettringite which forms is evidently limited
by the sulfate contributed by the cement initially. It follows that the quantity of
ettringite which forms is relatively small. Ettringite crystals form widely-dispersed
throughout the paste. If expansion causes cracking, ettringite may subsequently
form in the cracks but this does not mean the ettringite in the cracks caused the
cracks initially.
Conditions necessary for DEF to occur are:

9. · High temperature (>65-70 C approx.), usually during curing but not necessarily
· Water: intermittent or permanent saturation aftercuring
· Commonly associated with alkali-silica reaction (ASR)
In laboratory tests, limestone coarse aggregate has been found to reduce expansion.
DEF usually occurs in concrete which has either been steam cured, or which
reached a high temperature during curing as a result of the exothermic reaction of
cement hydration.
As the curing temperature of concrete increases, ettringite normally persists up to
about 70 C. Above this temperature it decomposes. In mature concrete,
monosulfate is usually the main sulfate-containing hydrate phase and this persists
up to about 100 C. DEF could occur in concrete which was heated externally, eg:
from fire.
An ettringite molecule contains 32 molecules of water; ettringite formation
therefore requires wet conditions
DEF and ASR appear to be closely linked; in one study (Diamond and Ong, 1994)
a mortar made using limestone aggregate was cured at 95 C. Subsequent ettringite
formation within the paste was scarce and expansion was minimal. However, if
aggregate susceptible to ASR was used instead of limestone, ettringite formation
and expansion were both much greater. This, and other studies, suggests that ASR
is, or can be, a precursorfor DEF expansion.
The effect of cement composition on DEF is not well understood. Some factors
correlate strongly but the causes are not clear. In laboratory tests, DEF expansion
was shown to correlate positively with cement-related factors, including:

10. a. high sulfate
b. high alkali
c. high MgO
d. cement fineness
e. high C3A
f. high C3S
DEF is still by no means fully understood. Forfurther reading on this subject, try:
The resistance to deformation that makes concrete a useful material means also
that volume changes of the concrete itself can have important implications in use.
Any potential growth or shrinkage may lead to complications, externally because
of structural interaction with other components or internally when the concrete is
reinforced. There may even be distress if either the cement paste or the aggregate
changes dimension, with tensile stresses set up in one component and compressive
stresses in the other. Cracks may be produced when the relatively low tensile
strength of the concreteor its constituent materials is exceeded.
Cracking not only impairs the ability of a structure to carry its design load but may
also affect its durability and damage its appearance. In addition, shrinkage and
creep may increase deflections in one member of a structure, adversely affecting
the stability of the whole. These factors have to be considered in design. Volume
change of concrete is not usually associated with changes that occur before the
hardened state is attained. Quality and durability, on the other hand, are dependent
on what occurs from the time the concretemix has been placed in the mold.
Settlement and Bleeding
Concrete is said to be in a plastic state before it begins to set. The aggregate is
dispersed by the cement paste and the particles in the paste are dispersed in the
water. After placing, there is a period of settlement when the particles come closer
together; most of this settlement usually occurs within an hour or so of placement.
Total volume change may, in extreme cases, amount to 1 per cent or more, but it is

11. not of great significance because the concrete is in a plastic or semiplastic state and
no appreciable stresses can result from these changes. During settlement, water
often appears at the surface, having exuded from the plastic mass. This
phenomenon is called bleeding.
Accumulation of water at the top of a mass of concrete is often undesirable; for
example, when concrete is placed continuously in a deep form, the upper part can
gain progressively more water as the filling of the form progresses, leading to
relatively poor quality at the top. On the other hand, the accumulation of some
water at the surface is not always undesirable because surface water is required to
prevent plastic shrinkage and to lubricate the tools used for finishing the surface.
Again, an excess of surface water may lead to a thin layer of slurry on the finished
surface and a weak susceptible layer on the surface of the concrete. Care must be
taken that finishing does not begin before the bleeding period is over.
Settlement may give rise to structural flaws. A layer of water may be left under
horizontal reinforcing bars so that half the area of contact between the steel and
concrete is lost. This problem can be eliminated by proper vibration or revibration
of the plastic concrete, care being taken not to touch reinforcing. It must not be
overlooked, however, that settlement and bleeding do result in a reduction of water
content. If not offset by one of the undesirable features discussed, the effect is
beneficial to strength, permeability and volume stability.
Plastic Shrinkage
When the evaporation rate exceeds the rate of bleeding and the free settlement
period is ended, a hydrostatic tension begins to develop throughout the mass owing
to the formation of menisci at the water surfaces in the capillaries. This results in
vertical as well as lateral compressive forces and may be manifested in a slab by
pattern cracking. It is called plastic shrinkage cracking. Remedial measures may
involve sun shades and windbreaks, application of water sprays or application of a
curing compound to arrest evaporation.

12. Nature of Hydrated Portland Cementand Mechanismof Volume Change
Following hydration and hardening, cement consists of a mixture of several
compounds, all chemically combined with water in different ways. The compound
that has the greatest influence on the characteristics of hydrated cement, including
shrinkage, is calcium silicate, which has a large internal surface area of 25 to 50
thousand square yards per pound. This internal surface is composed of the walls of
the tiny pores and fissures within the physical dimensions of the specimen. (It is
the character of this surface that makes hydrated cement an effective cementing
agent and provides the versatility of concrete in forming bodies of high strength
and almost any desired shape. When surfaces are very close to each other there is a
mutual gravitation-like attraction that forms a strong “weld.” When the internal
surface area is high the many strong welds develop the strength and rigidity of the
body.)
Thus concrete is not a solid inert mass but a vast number of small pores or
capillaries that in total can account for up to 50 per cent of the volume of the
concrete. During curing the pores and capillaries are usually full of water and no
stresses exist. As drying takes place, three mechanisms cause shrinkage:
1. The unstable nature of newly-formed calcium silicate hydrate results in
shrinkage as drying occurs; the exact nature of this mechanism is not clearly
understood but it is permanent and irreversible;
2. Compressive stresses are set up in the concrete because of the development of
menisci in the capillaries as drying progresses;

13. 3. Energy changes occurat the surface of calcium silicate as the water evaporates.
These mechanisms (phenomena) acting separately or in combination cause initial
drying shrinkage of the concrete. Part of it, 30 per cent or more, is irreversible.
Autogenously Volume Changes andExpansive Cements
Before volume changes resulting from drying or wetting of hardened concrete are
discussed, autogenously volume changes should be mentioned because they occur
where little or no change in total moisture content is possible and are of particular
importance in the interior of mass concrete. Two opposing effects can be produced.
As reaction between water and the unhydrated cement proceeds, the actual volume
of the solid increases. This causes stresses through the set structure and results in
expansion. At later ages, the water available for the reaction will decrease,
resulting in self-desiccation of the cement paste and a shrinkage ranging from
0.001 to more than 0.015 per cent.
The increase in volume of some constituents during their formation has been used
as the basis for developing expansive cements. Some, specially prepared, undergo
relatively large expansions at early age so that if used in concrete that is restrained
they develop compressive stresses. Later, when drying occurs, the resulting
shrinkage that would have developed is partly or completely offset, and
compressive stresses no longer exist in the concrete.
Volume Changes due to Moisture Changes
Although the mechanism of volume change that occurs during moisture change is
not fully understood, much has been learned to provide useful information for
engineering purposes. When concrete is dried, the first water to be removed causes
no change in volume. This is considered to be free water held in rather large
“pores.” With continued drying, shrinkage becomes quite large and at equilibrium
in 50 per cent RH values in excess of 0.10 per cent have been recorded for some
concretes. The above behaviour is somewhat similar to that of wood (in a

14. qualitative manner). Shrinkage values for neat cement paste have been observed in
excess of 0.40 per cent; the difference of this value from that of concrete is due to
various restraints. A large portion of concrete is made up of relatively inert
aggregate (from 3 to 7 times the weight of cement) and this, together with
reinforcement, reduces shrinkage. In addition to internal restraints, some restraint
arises from non-uniform shrinkage within the concrete member itself. Moisture
loss takes place at the surface so that a moisture gradient is established. The
resultant differential shrinkage is associated with internal stresses, tensile near the
surface and compressive in the core, and may result in warping or cracking.
If concrete that has been allowed to dry in air at 50 per cent RH is subsequently
placed in water, it will swell. Not all initial shrinkage obtained on drying is
recovered, however, even after prolonged storage. For the usual range of concretes
the irreversible part of shrinkage is about 30 to 60 per cent of total drying
shrinkage, the lower value being more common. Because shrinkage has such an
influence on the performance of concrete structures much work has been carried
out to obtain information on the factors affecting it.
Effectof Cementand Water Contents on Shrinkage
Water content is probably the largest single factor influencing the shrinkage of
paste and concrete. Typical shrinkage values for concrete specimens with a 5 to 1
aggregate-cement ratio are 0.04, 0.06, 0.075 and 0.085 per cent for water-cement
ratios of 0.4, 0.5, 0.6 and 0.7, respectively. One of the reasons is that the density
and composition of calcium silicate formed at different water-cement ratios may be
slightly different. In general, a higher cement content increases the shrinkage of
concrete; the relative shrinkages of neat paste, mortar and concrete may be of the
order of about 5, 2 and 1. For given materials, however, and a uniform water
content, the shrinkage of concrete varies little for a wide range of cement contents;
a richer mix will have a lower water-cement ratio and these factors offset each
other.

15. Properties of Cement
Fineness of cement seems to be a factor in shrinkage and particles coarser than No.
200 sieve, which react with water very slowly, have a restraining effect similar to
that of aggregate. Thus, high-early-strength cement, which is finely ground,
shrinks about 10 per cent more than normal cement. Low-heat and portland-
pozzolan cements shrink a further 20 and 35 per cent, respectively. This is believed
to be caused by larger quantities of calcium silicate, the shrinking component,
present in them.
Type and Gradation of Aggregate
As stated previously, the drying shrinkage of concrete is a fraction of that of neat
cement because the aggregate particles not only dilute the paste but reinforce it
against contraction. It has been shown that when readily compressible aggregate is
used concrete will shrink as much as neat cement, and that expanded shale leads to
shrinkage one-third more than that of ordinary aggregate. Steel aggregate on the
other hand, leads to shrinkage one-third less than that of ordinary concrete. In
general terms the elastic properties of aggregate determine the degree of restraint
offered. The size and grading of aggregate do not, by themselves, influence the
magnitude of shrinkage, but an aggregate incorporating larger sizes permits the use
of a mix with less cement and hence a lower shrinkage. Increasing the maximum
aggregate size and thereby the aggregate content by 20 per cent of the total volume
of the concrete will ensure a substantial decrease in shrinkage.
The shrinkage of aggregates themselves may be of considerable importance in
determining the shrinkage of concrete; some fine-grained sandstones, slate, basalt,
trap rock and aggregates containing clay show large shrinkage. In general,
concretes low in shrinkage often contain quartz, limestone, granite or feldspar.
Various harmful effects of abnormal shrinkage of concretes, caused by the
aggregate and observed in actual structures, have included excessive cracking,
large deflection of reinforced beams and slabs and some spalling. It is essential that
any new source of aggregate be tested to ascertain whether its use in concrete will

16. cause excessive shrinkage to develop. Any shrinkage in excess of 0.08 per cent is
taken to indicate an undesirable aggregate.
Effectof Admixtures
As can be predicted from the effect of water-cement ratio on shrinkage, admixtures
that increase the water requirement of concrete increase shrinkage and those that
decrease the water requirement decrease it. Calcium chloride in the amount often
added as an accelerator – 2 per cent by weight of the amount of cement – may
increase drying shrinkage by as much as 50 per cent.
The over-all effect of the use of air entrained concrete is not to increase shrinkage.
Some admixtures, if used in somewhat larger than normal doses, do increase
shrinkage greatly and care must be exercised in the proportioning.
Rate of Drying
The size of the specimen and conditions of exposure are important in assessing the
relevance of the shrinkage problem. Drying of ordinary concrete exposed to an
environment maintained at 50 per cent RH will affect moisture content to a depth
of 3 in. in one month. Continued exposure to these conditions would be a
significant factor in small concrete members but would be of no importance in
massive elements.
CarbonationShrinkage
Another mechanism that will result in shrinkage of concrete is the reaction
between carbon dioxide and hydrated cement. Maximum shrinkage occurs when
the concrete is at equilibrium in a 50 per cent RH environment. This shrinkage
combined with drying shrinkage results in excessive crazing of exposed surfaces
such as concrete floors when CO2 levels are high, a condition often found on
winter construction projects.

17. Carbonation during the curing of concrete products is sometimes used to encourage
shrinkage and thus reduce shrinkage stresses when these units are incorporated into
a structure. Carbonation also reduces permeability, presumably due to deposition
of the reaction products in the pores and capillaries.
Creepof Concrete
Creep of concrete resulting from the action of a sustained stress is a gradual
increase in strain with time; it can be of the same order of magnitude as drying
shrinkage. As defined, creep does not include any immediate elastic strains caused
by loading or any shrinkage or swelling caused by moisture changes. When a
concrete structural element is dried under load the creep that occurs is one to two
times as large as it would be under constant moisture conditions. Adding normal
drying shrinkage to this and considering the fact that creep can be several times as
large as the elastic strain on loading, it may be seen that these factors can cause
considerable deflection and that they are of great importance in structural
mechanics.
If a sustained load is removed, the strain decreases immediately by an amount
equal to the elastic strain at the given age; this is generally lower than the elastic
strain on loading since the elastic modulus has increased in the intervening period.
This instantaneous recovery is followed by a gradual decrease in strain, called
creep recovery. This recovery is not complete because creep is not simply a
reversible phenomenon.
It is now believed that the major portion of creep is due to removal of water from
between the sheets of a calcium silicate crystallite and to a possible rearrangement
of bonds between the surfaces of the individual crystallites.

18. Factors Influencing Creep:-
Concrete that exhibits high shrinkage generally also shows a high creep, but how
the two phenomena are connected is still not understood. Evidence suggests that
they are closely related. When hydrated cement is completely dried, little or no
creep occurs; for a given concrete the lower the relative humidity, the higher the
creep.
Strength of concrete has a considerable influence on creep and within a wide range
creep is inversely proportional to the strength of concrete at the time of application
of load. From this it follows that creep is closely related to the water-cement ratio.
There is no doubt also that the modulus of elasticity of aggregate controls the
amount of creep that can be realized and concretes made with different aggregates
exhibit creep of varying magnitudes.
Experiments have shown that creep continues for a very long time; detectable
changes have been found after as long as 30 years. The rate decreases
continuously, however, and it is generally assumed that creep tends to a limiting
value. It has been estimated that 75 per cent of 20-year creep occurs during the first
year.
Effects of Creep
Creep of plain concrete does not by itself affect strength, although under very high
stresses creep hastens the approach of the limiting strain at which failure takes
place. The influence of creep on the ultimate strength of a simply supported,
reinforced concrete beam subjected to a sustained load is insignificant, but
deflection increases considerably and may in many cases be a critical consideration
in design. Another instance of the adverse effects of creep is its influence on the
stability of the structure through increase in deformation and consequent transfer of
load to other components. Thus, even when creep does not affect the ultimate

19. strength of the component in which it takes place, its effect may be extremely
serious as far as the performance of the structure as a whole is concerned.
The loss of prestress due to creep is well known and accounted for the failure of all
early attempts at prestressing. Only with the introduction of high tensile steel did
prestressing become a successful operation. The effects of creep may thus be
harmful. On the whole, however, creep unlike shrinkage is beneficial in relieving
stress concentrations and has contributed to the success of concrete as a structural
material.