Concrete is a construction material composed of cement
(commonly Portland cement) as well as other cementitious
materials such as fly ash and slag cement, aggregate (generally
a coarse aggregate such as gravel, limestone, or granite, plus a
fine aggregate such as sand), water, and chemical admixtures.
The word concrete comes from the Latin word "concretus", which
means "hardened" or "hard".
Concrete solidifies and hardens
after mixing with water and placement due to a chemical process
known as hydration. The water reacts with the cement, which
bonds the other components together, eventually creating a
stone-like material. Concrete is used to make pavements,
architectural structures, foundations, motorways/roads,
bridges/overpasses, parking structures, brick/block walls and
footings for gates, fences and poles.
Concrete is used more than any other man-made material in the
world.[1] As of
2006, about 7 cubic kilometres of concrete are made each
year—more than one cubic metre for every person on Earth.[2]
Concrete powers a US $35-billion industry which employs more
than two million workers in the United States alone.
More than 55,000 miles (89,000 km) of highways in America are
paved with this material. The People's Republic of China
currently consumes 40% of the world's cement/concrete
production.
Reinforced concrete and Prestressed concrete
are the most widely used modern kinds of concrete functional
extensions.
History
Many ancient civilizations used forms of concrete using dried
mud, straw, and other materials.
During the Roman Empire, Roman concrete was made from
quicklime, pozzolanic ash/pozzolana, and an aggregate of pumice;
it was very similar to modern Portland cement concrete. The
widespread use of concrete in many Roman structures has ensured
that many survive almost intact to the present day. The Baths of
Caracalla in Rome are just one example of the longevity of
concrete, which allowed the Romans to build this and similar
structures across the Roman Empire. Many Roman aqueducts have
masonry cladding to a concrete core, a technique they used in
structures such as the Pantheon, Rome, the interior dome of
which is unclad concrete.
The secret of concrete was lost for 13 centuries until 1756,
when the British engineer John Smeaton pioneered the use of
hydraulic lime in concrete, using pebbles and powdered brick as
aggregate. Portland cement was first used in concrete in the
early 1840s.
Recently, the use of recycled materials as concrete
ingredients is gaining popularity because of increasingly
stringent environmental legislation. The most conspicuous of
these is fly ash, a by-product of coal-fired power plants. This
has a significant impact by reducing the amount of quarrying and
landfill space required, and, as it acts as a cement
replacement, reduces the amount of cement required to produce a
solid concrete. As cement production creates massive quantities
of carbon dioxide, cement-replacement technology such as this
will play an important role in future attempts to cut carbon
dioxide emissions.
Concrete additives have been used since Roman and Egyptian
times, when it was discovered that adding volcanic ash to the
mix allowed it to set under water. Similarly, the Romans knew
that adding horse hair made concrete less liable to crack while
it hardened, and adding blood made it more frost-resistant[3].
In modern times, researchers have experimented with the
addition of other materials to create concrete with improved
properties, such as higher strength or electrical conductivity.
Composition
There are many types of concrete available, created by
varying the proportions of the main ingredients below.
The mix design depends on the type of structure being
built, how the concrete will be mixed and delivered, and how it
will be placed to form this structure.
Cement
Portland cement is the most common type of cement in general
usage. It is a basic ingredient of concrete, mortar, and
plaster. English engineer Joseph Aspdin patented Portland cement
in 1824; it was named because of its similarity in colour to
Portland limestone, quarried from the English Isle of Portland
and used extensively in London architecture. It consists of a
mixture of oxides of calcium, silicon and aluminium. Portland
cement and similar materials are made by heating limestone (a
source of calcium) with clay, and grinding this product (called
clinker) with a source of sulfate (most commonly gypsum).
Water
Combining water with a cementitious material forms a cement
paste by the process of hydration. The cement paste glues the
aggregate together, fills voids within it, and allows it to flow
more easily.
Less water in the cement paste will yield a stronger, more
durable concrete; more water will give an easier-flowing
concrete with a higher slump.[4]
Impure water used to make concrete can cause problems, when
setting, or in causing premature failure of the structure.
Hydration involves many different reactions, often occurring
at the same time. As the reactions proceed, the products of the
cement hydration process gradually bond together the individual
sand and gravel particles, and other components of the concrete,
to form a solid mass.
Reaction:
- Cement chemist notation: C3S + H2O
→ CSH(gel) + CaOH
- Standard notation: Ca3SiO5 + H2O
→ (CaO)•(SiO2)•(H2O)(gel) + Ca(OH)2
- Balanced: 2Ca3SiO5 + 7H2O
→ 3(CaO)•2(SiO2)•4(H2O)(gel) + 3Ca(OH)2
Aggregates
Fine and coarse aggregates make up the bulk of a concrete
mixture. Sand, natural gravel and crushed stone are mainly used
for this purpose. Recycled aggregates (from construction,
demolition and excavation waste) are increasingly used as
partial replacements of natural aggregates, while a number of
manufactured aggregates, including air-cooled blast furnace slag
and bottom ash are also permitted.
Decorative stones such as quartzite, small river stones or
crushed glass are sometimes added to the surface of concrete for
a decorative "exposed aggregate" finish, popular among landscape
designers.
Reinforcement
Concrete is strong in compression, as the aggregate
efficiently carries the compression load. However, it is weak in
tension as the cement holding the aggregate in place can crack,
allowing the structure to fail. Reinforced concrete solves these
problems by adding either metal reinforcing bars, glass fiber,
or plastic fiber to carry tensile loads.
Chemical admixtures
Chemical admixtures are materials in the form of
powder or fluids that are added to the concrete to give it
certain characteristics not obtainable with plain concrete
mixes. In normal use, admixture dosages are less than 5% by mass
of cement, and are added to the concrete at the time of
batching/mixing.[5]
The most common types of admixtures
[6] are:
- Accelerators speed up the hydration (hardening) of the
concrete. Typical materials used are CaCl2 and
NaCl.
- Retarders slow the hydration of concrete, and are used
in large or difficult pours where partial setting before the
pour is complete is undesirable. A typical retarder is sugar
(C6H12O6).
- Air entrainments add and distribute tiny air bubbles in
the concrete, which will reduce damage during freeze-thaw
cycles thereby increasing the concrete's durability.
However, entrained air is a trade-off with strength, as each
1% of air may result in 5% decrease in compressive strength.
- Plasticizers (water-reducing admixtures) increase the
workability of plastic or "fresh" concrete, allowing it be
placed more easily, with less consolidating effort.
Superplasticizers (high-range water-reducing admixtures) are
a class of plasticizers which have fewer deleterious effects
when used to significantly increase workability.
Alternatively, plasticizers can be used to reduce the water
content of a concrete (and have been called water
reducers due to this application) while maintaining
workability. This improves its strength and durability
characteristics.
- Pigments can be used to change the color of concrete,
for aesthetics.
- Corrosion inhibitors are used to minimize the corrosion
of steel and steel bars in concrete.
- Bonding agents are used to create a bond between old and
new concrete.
- Pumping aids improve pumpability, thicken the paste, and
reduce dewatering – the tendency for the water to separate
out of the paste.
Mineral admixtures and blended
cements
There are inorganic materials that also have pozzolanic or
latent hydraulic properties. These very fine-grained materials
are added to the concrete mix to improve the properties of
concrete (mineral admixtures),[5]
or as a replacement for Portland cement (blended cements).[7]
- Fly ash: A by product of coal fired electric generating
plants, it is used to partially replace Portland cement (by
up to 60% by mass). The properties of fly ash depend on the
type of coal burnt. In general, silicious fly ash is
pozzolanic, while calcareous fly ash has latent hydraulic
properties.[8]
- Ground granulated blast furnace slag (GGBFS or GGBS): A
by product of steel production, is used to partially replace
Portland cement (by up to 80% by mass). It has latent
hydraulic properties.[9]
- Silica fume: A by-product of the production of silicon
and ferrosilicon alloys. Silica fume is similar to fly ash,
but has a particle size 100 times smaller. This results in a
higher surface to volume ratio and a much faster pozzolanic
reaction. Silica fume is used to increase strength and
durability of concrete, but generally requires the use of
superplasticizers for workability.[10]
- High Reactivity Metakaolin (HRM): Metakaolin produces
concrete with strength and durability similar to concrete
made with silica fume. While silica fume is usually dark
gray or black in color, high reactivity metakaolin is
usually bright white in color, making it the preferred
choice for architectural concrete where appearance is
important.
Concrete Production
The processes used vary dramatically, from hand tools to
heavy industry, but result in the concrete being placed where it
cures into a final form.
When initially mixed together, Portland cement and water
rapidly form a gel, formed of tangled chains of interlocking
crystals. These continue to react over time, with the initially
fluid gel often aiding in placement by improving workability. As
the concrete sets, the chains of crystals join up, and form a
rigid structure, gluing the aggregate particles in place. During
curing, more of the cement reacts with the residual water
(Hydration).
This curing process develops physical and chemical
properties. Among other qualities, mechanical strength, low
moisture permeability, and chemical and volumetric stability.
Mixing Concrete
Thorough mixing is essential for the production of uniform,
high quality concrete. Therefore, equipment and methods should
be capable of effectively mixing concrete materials containing
the largest specified aggregate to produce uniform mixtures
of the lowest slump practical for the work. Separate paste
mixing has shown that the mixing of cement and water into a
paste before combining these materials with aggregates can
increase the compressive strength of the resulting concrete.[11]
The paste is generally mixed in a high-speed, shear-type
mixer at a w/cm (water to cement ratio) of 0.30 to 0.45 by mass.
The cement paste premix may include admixtures, e.g.
accelerators or retarders, plasticizers, pigments, or fumed
silica. The latter is added to fill the gaps between the cement
particles. This reduces the particle distance and leads to a
higher final compressive strength and a higher water
impermeability.[12]
The premixed paste is then blended with aggregates and any
remaining batch water, and final mixing is completed in
conventional concrete mixing equipment.[13]
High-Energy Mixed Concrete (HEM concrete) is produced by
means of high-speed mixing of cement, water and sand with net
specific energy consumption at least 5 kilojoules per kilogram
of the mix. It is then added to a plasticizer admixture and
mixed after that with aggregates in conventional concrete mixer.
This paste can be used itself or foamed (expanded) for
lightweight concrete.[14]
Sand effectively dissipates energy in this mixing process. HEM
concrete fast hardens in ordinary and low temperature
conditions, and possesses increased volume of gel, drastically
reducing capillarity in solid and porous materials. It is
recommended for precast concrete in order to reduce quantity of
cement, as well as concrete roof and siding tiles, paving stones
and lightweight concrete block production.
Workability
Workability is the ability of a fresh (plastic)
concrete mix to fill the form/mold properly with the desired
work (vibration) and without reducing the concrete's quality.
Workability depends on water content, aggregate (shape and size
distribution), cementitious content and age (level of
hydration), and can be modified by adding chemical admixtures.
Raising the water content or adding chemical admixtures will
increase concrete workability. Excessive water will lead to
increased bleeding (surface water) and/or segregation of
aggregates (when the cement and aggregates start to separate),
with the resulting concrete having reduced quality. The use of
an aggregate with an undesirable gradation can result in a very
harsh mix design with a very low slump, which cannot be readily
made more workable by addition of reasonable amounts of water.
Workability can be measured by the Concrete Slump Test, a
simplistic measure of the plasticity of a fresh batch of
concrete following the ASTM C 143 or EN 12350-2 test standards.
Slump is normally measured by filling an "Abrams cone" with a
sample from a fresh batch of concrete. The cone is placed with
the wide end down onto a level, non-absorptive surface. It is
then filled in three layers of equal volume, with each layer
being tamped with a steel rod in order to consolidate the layer.
When the cone is carefully lifted off, the enclosed material
will slump a certain amount due to gravity. A relatively dry
sample will slump very little, having a slump value of one or
two inches (25 or 50 mm). A relatively wet concrete sample may
slump as much as six or seven inches (150 to 175 mm).
Slump can be increased by adding chemical admixtures such as
mid-range or high-range water reducing agents
(super-plasticizers) without changing the water/cement ratio. It
is bad practice to add excessive water upon delivery to the
jobsite, however in a properly designed mixture it is important
to reasonably achieve the specified slump prior to placement as
design factors such as air content, internal water for
hydration/strength gain, etc. are dependent on placement at
design slump values.
High-flow concrete, like self-consolidating concrete, is
tested by other flow-measuring methods. One of these methods
includes placing the cone on the narrow end and observing how
the mix flows through the cone while it is gradually lifted.
Curing
In all but the least critical applications, care needs to be
taken to properly cure concrete, and achieve best
strength and hardness. This happens after the concrete has been
placed. Cement requires a moist, controlled environment to gain
strength and harden fully. The cement paste hardens over time,
initially setting and becoming rigid though very weak, and
gaining in strength in the days and weeks following. In around 3
weeks, over 90% of the final strength is typically reached
though it may continue to strengthen for decades.[15]
Hydration and hardening of concrete during the first three
days is critical. Abnormally fast drying and shrinkage due to
factors such as evaporation from wind during placement may lead
to increased tensile stresses at a time when it has not yet
gained significant strength, resulting in greater shrinkage
cracking. The early strength of the concrete can be increased by
keeping it damp for a longer period during the curing process.
Minimizing stress prior to curing minimizes cracking. High
early-strength concrete is designed to hydrate faster, often by
increased use of cement which increases shrinkage and cracking.
During this period concrete needs to be in conditions with a
controlled temperature and humid atmosphere. In practice, this
is achieved by spraying or ponding the concrete surface with
water, thereby protecting concrete mass from ill effects of
ambient conditions. The pictures to the right show two of many
ways to achieve this, ponding – submerging setting concrete in
water, and wrapping in plastic to contain the water in the mix.
Properly curing concrete leads to increased strength and
lower permeability, and avoids cracking where the surface dries
out prematurely. Care must also be taken to avoid freezing, or
overheating due to the exothermic setting of cement (the Hoover
Dam used pipes carrying coolant during setting to avoid damaging
overheating). Improper curing can cause scaling, reduced
strength, poor abrasion resistance and cracking.
Properties
Strength
Concrete has relatively high compressive strength, but
significantly lower tensile strength. It is fair to assume that
a concrete sample's tensile strength is about 10%-15% of its
compressive strength.[16]
As a result, without compensating, concrete would almost always
fail from tensile stresses – even when loaded in compression.
The practical implication of this is that concrete elements
subjected to tensile stresses must be reinforced with materials
that are strong in tension.
Reinforced concrete is the most common form of concrete. The
reinforcement is often steel, rebar (mesh, spiral, bars and
other forms). Structural fibers of various materials are
available.
Concrete can also be prestressed (reducing tensile stress)
using internal steel cables (tendons), allowing for beams or
slabs with a longer span than is practical with reinforced
concrete alone. Inspection of concrete structures can be
non-destructive if carried out with equipment such as a Schmidt
hammer, which is used to estimate concrete strength.
The ultimate strength of concrete is influenced by the water-cementitious
ratio (w/cm), the design constituents, and the mixing,
placement and curing methods employed. All things being equal,
concrete with a lower water-cement (cementitious) ratio makes a
stronger concrete than that with a higher ratio. The total
quantity of cementitious materials (Portland cement, slag
cement, pozzolans) can affect strength, water demand, shrinkage,
abrasion resistance and density. All concrete will crack
independent of whether or not it has sufficient compressive
strength. In fact, high Portland cement content mixtures can
actually crack more readily due to increased hydration rate. As
concrete transforms from its plastic state, hydrating to a
solid, the material undergoes shrinkage. Plastic shrinkage
cracks can occur soon after placement but if the evaporation
rate is high they often can actually occur during finishing
operations, for example in hot weather or a breezy day. In very
high strength concrete mixtures (greater than 10,000 psi) the
crushing strength of the aggregate can be a limiting factor to
the ultimate compressive strength. In lean concretes (with a
high water-cement ratio) the crushing strength of the aggregates
is not so significant.
The internal forces in common shapes of structure, such as
arches, vaults, columns and walls are predominantly compressive
forces, with floors and pavements subjected to tensile forces.
Compressive strength is widely used for specification
requirement and quality control of concrete. The engineer knows
his target tensile (flexural) requirements and will express
these in terms of compressive strength.
Wired.com reported on April 13, 2007 that a team from the
University of Tehran, competing in a contest sponsored by the
American Concrete Institute, demonstrated several blocks of
concretes with abnormally high compressive strengths between
50,000 and 60,000 PSI at 28 days.[17]
The blocks appeared to use an aggregate of steel fibres and
quartz – a mineral with a compressive strength of 160,000 PSI,
much higher than typical high-strength aggregates such as
granite (15,000-20,000 PSI).
Elasticity
The modulus of elasticity of concrete is a function of the
modulus of elasticity of the aggregates and the cement matrix
and their relative proportions. The modulus of elasticity of
concrete is relatively constant at low stress levels but starts
decreasing at higher stress levels as matrix cracking develops.
The elastic modulus of the hardened paste may be in the order of
10-30 GPa and aggregates about 45 to 85 GPa. The concrete
composite is then in the range of 30 to 50 GPa.
The American Concrete Institute allows the modulus of
elasticity to be calculated using the following equation:[16]
-
(psi)
where
- wc =
weight of concrete (pounds per cubic foot) and where
- f'c =
compressive strength of concrete at 28 days (psi)
This equation is completely empirical and is not based on
theory. Note that the value of Ec found is in
units of psi. For normalweight concrete (defined as concrete
with a wc of 150 pcf) Ec is
permitted to be taken as
.
Expansion and shrinkage
Concrete has a very low coefficient of thermal expansion.
However, if no provision is made for expansion, very large
forces can be created, causing cracks in parts of the structure
not capable of withstanding the force or the repeated cycles of
expansion and contraction.
As concrete matures it continues to shrink, due to the
ongoing reaction taking place in the material, although the rate
of shrinkage falls relatively quickly and keeps reducing over
time (for all practical purposes concrete is usually considered
to not shrink due to hydration any further after 30 years). The
relative shrinkage and expansion of concrete and brickwork
require careful accommodation when the two forms of construction
interface.
Because concrete is continuously shrinking for years after it
is initially placed, it is generally accepted that under thermal
loading it will never expand to its originally placed volume.
Cracking
All concrete structures will crack to some extent. One of the
early designers of reinforced concrete, Robert Maillart,
employed reinforced concrete in a number of arched bridges. His
first bridge was simple, using a large volume of concrete. He
then realized that much of the concrete was very cracked, and
could not be a part of the structure under compressive loads,
yet the structure clearly worked. His later designs simply
removed the cracked areas, leaving slender, beautiful concrete
arches. The Salginatobel Bridge is an example of this.
Concrete cracks due to tensile stress induced by shrinkage or
stresses occurring during setting or use. Various means are used
to overcome this. Fiber reinforced concrete uses fine fibers
distributed throughout the mix or larger metal or other
reinforcement elements to limit the size and extent of cracks.
In many large structures joints or concealed saw-cuts are placed
in the concrete as it sets to make the inevitable cracks occur
where they can be managed and out of sight. Water tanks and
highways are examples of structures requiring crack control.
Shrinkage cracking
Shrinkage cracks occur when concrete members undergo
restrained volumetric changes (shrinkage) as a result of either
drying, autogenous shrinkage or thermal effects. Restraint is
provided either externally (i.e. supports, walls, and other
boundary conditions) or internally (differential drying
shrinkage, reinforcement). Once the tensile strength of the
concrete is exceeded, a crack will develop. The number and width
of shrinkage cracks that develop are influenced by the amount of
shrinkage that occurs, the amount of restraint present and the
amount and spacing of reinforcement provided.
Plastic-shrinkage cracks are immediately apparent, visible
within 0 to 2 days of placement, while drying-shrinkage cracks
develop over time. Autogenous shrinkage also occurs when the
concrete is quite young and results from the volume reduction
resulting from the chemical reaction of the portland cement.
Tension cracking
Concrete members may be put into tension by applied loads.
This is most common in concrete beams where a transversely
applied load will put one surface into compression and the
opposite surface into tension due to induced bending. The
portion of the beam that is in tension may crack. The size and
length of cracks is dependent on the magnitude of the bending
moment and the design of the reinforcing in the beam at the
point under consideration. Reinforced concrete beams are
designed to crack in tension rather than in compression. This is
achieved by providing reinforcing steel which yields before
failure of the concrete in compression occurs and allowing
remediation, repair, or if necessary, evacuation of an unsafe
area.
Creep
Creep is the term used to describe the permanent
movement or deformation of a material in order to relieve
stresses within the material. Concrete which is subjected to
long-duration forces is prone to creep. Short-duration forces
(such as wind or earthquakes) do not cause creep. Creep can
sometimes reduce the amount of cracking that occurs in a
concrete structure or element, but it also must be controlled.
The amount of primary and secondary reinforcing in concrete
structures contributes to a reduction in the amount of
shrinkage, creep and cracking.
Physical properties
The coefficient of thermal expansion of Portland cement
concrete is 0.000008 to 0.000012 (per degree Celsius) (8-12
1/MK).[18] The
density varies, but is around 150 pounds per cubic foot (2400
kg/m³).[19]
Damage modes
Fire
Due to its low thermal conductivity, a layer of concrete is
frequently used for fireproofing of steel structures. However,
concrete itself may be damaged by fire.
Up to about 300 °C, the concrete undergoes normal thermal
expansion. Above that temperature, shrinkage occurs due to water
loss; however, the aggregate continues expanding, which causes
internal stresses. Up to about 500 °C, the major structural
changes are carbonation and coarsening of pores. At 573 °C,
quartz undergoes rapid expansion due to Phase transition, and at
900 °C calcite starts shrinking due to decomposition. At 450-550
°C the cement hydrate decomposes, yielding calcium oxide.
Calcium carbonate decomposes at about 600 °C. Rehydration of the
calcium oxide on cooling of the structure causes expansion,
which can cause damage to material which withstood fire without
falling apart. Concrete in buildings that experienced a fire and
were left standing for several years shows extensive degree of
carbonation.
Concrete exposed to up to 100 °C is normally considered as
healthy. The parts of a concrete structure that is exposed to
temperatures above approximately 300 °C (dependent of
water/cement ratio) will most likely get a pink color. Over
approximately 600 °C the concrete will turn light grey, and over
approximately 1000 °C it turns yellow-brown.[20]
One rule of thumb is to consider all pink colored concrete as
damaged that should be removed.
Fire will expose the concrete to gases and liquids that can
be harmful to the concrete, among other salts and acids that
occur when gasses produced by fire come into contact with water.
Aggregate expansion
Various types of aggregate undergo chemical reactions in
concrete, leading to damaging expansive phenomena. The most
common are those containing reactive silica, that can react (in
the presence of water) with the alkalis in concrete (K2O
and Na2O, coming principally from cement). Among the
more reactive mineral components of some aggregates are opal,
chalcedony, flint and strained quartz. Following the reaction
(Alkali Silica Reaction or ASR), an expansive gel forms, that
creates extensive cracks and damage on structural members. On
the surface of concrete pavements the ASR can cause pop-outs,
i.e. the expulsion of small cones (up to 3 cm about in diameter)
in correspondence of aggregate particles. When some aggregates
containing dolomite are used, a dedolomitization reaction occurs
where the magnesium carbonate compound reacts with hydroxyl ions
and yields magnesium hydroxide and a carbonate ion. The
resulting expansion may cause destruction of the material. Far
less common are pop-outs caused by the presence of pyrite, an
iron sulfide that generates expansion by forming iron oxide and
ettringite. Other reactions and recrystallizations, e.g.
hydration of clay minerals in some aggregates, may lead to
destructive expansion as well.
Sea water effects
Concrete exposed to sea water is susceptible to its corrosive
effects. The effects are more pronounced above the tidal zone
than where the concrete is permanently submerged. In the
submerged zone, magnesium and hydrogen carbonate ions
precipitate a layer of brucite, about 30 micrometers thick, on
which a slower deposition of calcium carbonate as aragonite
occurs. These layers somewhat protect the concrete from other
processes, which include attack by magnesium, chloride and
sulfate ions and carbonation. Above the water surface,
mechanical damage may occur by erosion by waves themselves or
sand and gravel they carry, and by crystallization of salts from
water soaking into the concrete pores and then drying up.
Pozzolanic cements and cements using more than 60% of slag as
aggregate are more resistant to sea water than pure Portland
cement.
Bacterial corrosion
Bacteria themselves do not have noticeable effect on
concrete. However, anaerobic bacteria (Thiobacillus) in
untreated sewage tend to produce hydrogen sulfide, which is then
oxidized by aerobic bacteria present in biofilm on the concrete
surface above the water level to sulfuric acid which dissolves
the carbonates in the cured cement and causes strength loss.
Concrete floors lying on ground that contains pyrite are also at
risk. Using limestone as the aggregate makes the concrete more
resistant to acids, and the sewage may be pretreated by ways
increasing pH or oxidizing or precipitating the sulfides in
order to inhibit the activity of sulfide utilizing bacteria.
Chemical damage
Carbonation
Carbon dioxide from air can react with the calcium hydroxide
in concrete to form calcium carbonate. This process is called
carbonation, which is essentially the reversal of the chemical
process of calcination of lime taking place in a cement kiln.
Carbonation of concrete is a slow and continuous process
progressing from the outer surface inward, but slows down with
increasing diffusion depth. Carbonation has two effects: it
increases mechanical strength of concrete, but it also decreases
alkalinity, which is essential for corrosion prevention of the
reinforcement steel. Below a pH of 10, the steel's thin layer of
surface passivation dissolves and corrosion is promoted. For the
latter reason, carbonation is an unwanted process in concrete
chemistry. Carbonation can be tested by applying Phenolphthalein
solution, a pH indicator, over a fresh fracture surface, which
indicates non-carbonated and thus alkaline areas with a violet
color.
Chlorides
Chlorides, particularly calcium chloride, have been used to
shorten the setting time of concrete.[21]
However, calcium chloride and (to a lesser extent) sodium
chloride have been shown to leach calcium hydroxide and cause
chemical changes in Portland cement, leading to loss of
strength,[22] as
well as attacking the steel reinforcement present in most
concrete.
Sulphates
Sulphates in solution in contact with concrete can cause
chemical changes to the cement, which can cause significant
microstructural effects leading to the weakening of the cement
binder.
Leaching
Leaching is a self healing of cracks with chemical process in
concrete.
Physical damage
Damage can occur during the casting and de-shuttering
processes. For instance, the corners of beams can be damaged
during the removal of shuttering because they are less
effectively compacted by means of vibration (improved by using
form-vibrators). Other physical damage can be caused by the use
of steel shuttering without base plates. The steel shuttering
pinches the top surface of a concrete slab due to the weight of
the next slab being constructed.
Types of concrete
Mix Design
Modern concrete mix designs can be complex. The design of a
concrete, or the way the weights of the components of a concrete
is determined, is specified by the American Concrete Institute,
the specifications of the project, and the building code where
the project is located.
The design begins by determining the "durability"
requirements of the concrete. These requirments take into
consideration the weather conditions (freeze-thaw) that the
concrete will be exposed to in service, and the required design
strength, or f'c, at twenty eight (28) days after placement. The
compressive strength of a concrete, fc, is determined by taking
standard molded, standard-cured, 4"x8" or 6"x12", cylinder
samples.
Many factors need to be taken into account, from the cost of
the various additives and aggregates, to the trade offs between,
the "slump" for easy mixing and placement and ultimate
performance. These factors are also specified by the Americam
Concrete Institute, project specifications, and the local
building code where the project is located.
A mix is then designed using cement (Portland or other
cementitious material), coarse and fine aggregates, water and
chemical admixtures. The method of mixing will also be
specified, as well as conditions that it may be used in.
This allows a user of the concrete to be confident that the
structure will perform properly.
Various types of concrete have been developed for specialist
application and have become known by these names.
Regular concrete
Regular concrete is the lay term describing concrete
that is produced by following the mixing instructions that are
commonly published on packets of cement, typically using sand or
other common material as the aggregate, and often mixed in
improvised containers. This concrete can be produced to yield a
varying strength from about 10 MPa (1450 psi) to about 40 MPa
(5800 psi), depending on the purpose, ranging from blinding to
structural concrete respectively. Many types of pre-mixed
concrete are available which include powdered cement mixed with
an aggregate, needing only water.
Typically, a batch of concrete can be made by using 1 part
Portland cement, 2 parts dry sand, 3 parts dry stone, 1/2 part
water. The parts are in terms of weight – not volume. For
example, 1-cubic-foot (0.028 m3) of concrete would be
made using 22 lb (10.0 kg) cement, 10 lb (4.5 kg) water, 41 lb
(19 kg) dry sand, 70 lb (32 kg) dry stone (1/2" to 3/4" stone).
This would make 1-cubic-foot (0.028 m3) of concrete
and would weigh about 143 lb (65 kg). The sand should be mortar
or brick sand (washed and filtered if possible) and the stone
should be washed if possible. Organic materials (leaves, twigs,
etc) should be removed from the sand and stone to ensure the
highest strength.
High-strength concrete
High-strength concrete has a compressive strength
generally greater than 6,000 pounds per square inch (40 MPa =
5800 psi). High-strength concrete is made by lowering the
water-cement (W/C) ratio to 0.35 or lower. Often silica fume is
added to prevent the formation of free calcium hydroxide
crystals in the cement matrix, which might reduce the strength
at the cement-aggregate bond.
Low W/C ratios and the use of silica fume make concrete mixes
significantly less workable, which is particularly likely to be
a problem in high-strength concrete applications where dense
rebar cages are likely to be used. To compensate for the reduced
workability, superplasticizers are commonly added to
high-strength mixtures. Aggregate must be selected carefully for
high-strength mixes, as weaker aggregates may not be strong
enough to resist the loads imposed on the concrete and cause
failure to start in the aggregate rather than in the matrix or
at a void, as normally occurs in regular concrete.
In some applications of high-strength concrete the design
criterion is the elastic modulus rather than the ultimate
compressive strength.
High-performance concrete
High-performance concrete (HPC) and
Ultra-high-performance concrete are relatively new terms used to
describe concrete that conforms to a set of standards above
those of the most common applications, but not limited to
strength. While all high-strength concrete is also
high-performance, not all high-performance concrete is
high-strength. Notable concrete-mixtures are: Ductal, concrete
mixed with titanium oxide, ... Some examples of such standards
currently used in relation to HPC are:
- Ease of placement
- Compaction without segregation
- Early age strength
- Long-term mechanical properties
- Permeability
- Density
- Heat of hydration
- Toughness
- Volume stability
- Long life in severe environments
- Depending on its implementation, environmental
[23]
Self-consolidating concretes
During the 1980s a number of countries including Japan,
Sweden and France developed concretes that are
self-compacting, known as self-consolidating concrete in the
United States. This self-consolidating concrete (SCCs) is
characterized by:
- extreme fluidity as measured by flow, typically
between 650-750 mm on a flow table, rather than slump(height)
- no need for vibrators to compact the concrete
- placement being easier.
- no bleed water, or aggregate segregation
- Increased Liquid Head Pressure, Can be detrimental to
Safety and workmanship
SCC can save up to 50% in labor costs due to 80% faster
pouring and reduced wear and tear on formwork.
As of 2005, self-consolidating concretes account for 10-15%
of concrete sales in some European countries. In the US precast
concrete industry, SCC represents over 75% of concrete
production. 38 departments of transportation in the US accept
the use of SCC for road and bridge projects.
This emerging technology is made possible by the use of
polycarboxylates plasticizer instead of older naphthalene based
polymers, and viscosity modifiers to address aggregate
segregation.
Shotcrete
Shotcrete (also known by the trade name Gunite)
uses compressed air to shoot concrete onto (or into) a frame or
structure. Shotcrete is frequently used against vertical soil or
rock surfaces, as it eliminates the need for formwork. It is
sometimes used for rock support, especially in tunneling.
Shotcrete is also used for applications where seepage is an
issue to limit the amount of water entering a construction site
due to a high water table or other subterranean sources. This
type of concrete is often used as a quick fix for weathering for
loose soil types in construction zones.
There are two application methods for shotcrete.
- dry-mix – the dry mixture of cement and
aggregates is filled into the machine and conveyed with
compressed air through the hoses. The water needed for the
hydration is added at the nozzle.
- wet-mix – the mixes are prepared with all
necessary water for hydration. The mixes are pumped through
the hoses. At the nozzle compressed air is added for
spraying.
For both methods additives such as accelerators and fiber
reinforcement may be used.[24]
Pervious concrete
Pervious concrete contains a network of holes or
voids, to allow air or water to move through the concrete.
This allows water to drain naturally through it, and can both
remove the normal surface-water drainage infrastructure, and
allow replenishment of groundwater when conventional concrete
does not.
It is formed by leaving out some or all of the fine aggregate
(fines), the remaining large aggregate then is bound by a
relatively small amount of Portland Cement. When set, typically
between 15% and 25% of the concrete volume are voids, allowing
water to drain at around 5 gal/ft²/ min or 200 L/m²/min) through
the concrete.
Installation
Pervious is installed by being poured into forms, then
screeded off, to level (not smooth) the surface, then packed or
tamped into place. Due to the low water content and air
permeability, within 5-15 minutes of tamping, the concrete must
be covered with a 6-mil poly plastic, or it will dry out
prematurely and not properly hydrate and cure.
Characteristics
Pervious can significantly reduce noise, by allowing air to
be squeezed between vehicle tires and the roadway to escape.
This product cannot be used on major U.S. state highways
currently due to the high psi ratings required by most states.
Pervious has been tested up to 4500psi so far.
Cellular concrete
Aerated concrete produced by the addition of an air
entraining agent to the concrete (or a lightweight aggregate
like expanded clay pellets or cork granules and vermiculite) is
sometimes called Cellular concrete.
Cork-cement composites
Waste Cork granules are obtained during production of bottle
stoppers from the treated bark of Cork oak.[25]
These granules have a density of about 300 kg/m³, lower than
most lightweight aggregates used for making lightweight
concrete. Cork granules do not significantly influence cement
hydration, but cork dust may.[26]
Cork cement composites have several advantages over standard
concrete, such as lower thermal conductivities, lower densities
and good energy absorption characteristics. These composites can
be made of density from 400 to 1500 kg/m³, compressive strength
from 1 to 26 MPa, and flexural strength from 0.5 to 4.0 MPa.
Roller-compacted concrete
Roller-compacted concrete, sometimes called
rollcrete, is a low-cement-content stiff concrete placed
using techniques borrowed from earthmoving and paving work. The
concrete is placed on the surface to be covered, and is
compacted in place using large heavy rollers typically used in
earthwork. The concrete mix achieves a high density and cures
over time into a strong monolithic block.[27]
Roller-compacted concrete is typically used for concrete
pavement, but has also been used to build concrete dams, as the
low cement content causes less heat to be generated while curing
than typical for conventionally placed massive concrete pours
Glass concrete
The use of recycled glass as aggregate in concrete has become
popular in modern times, with large scale research being carried
out at Columbia University in New York. This greatly enhances
the aesthetic appeal of the concrete. Recent research findings
have shown that concrete made with recycled glass aggregates
have shown better long term strength and better thermal
insulation due to its better thermal properties of the glass
aggregates. [28]
Asphalt concrete
Strictly speaking, asphalt is a form of concrete as
well, with bituminous materials replacing cement as the
binder.
Rapid strength concrete
This type of concrete is able to develop high resistance
within few hours after being manufactured. This feature has
advantages such as removing the formwork early and to move
forward in the building process at record time, repair road
surfaces that become fully operational in just a few hours.
Rubberized concrete
While "rubberized asphalt concrete" is common, rubberized
Portland cement concrete ("rubberized PCC") is still undergoing
experimental tests, as of 2007[29]
[30]
[31]
[32].
Polymer concrete
Polymer concrete is concrete which uses polymers to bind the
aggregate. Polymer concrete can gain a lot of strength in a
short amount of time. For example, a polymer mix may reach 5000
psi in only four hours. Polymer concrete is generally more
expensive than conventional concretes.
Geopolymer or green
concrete
Geopolymer concrete is a greener alternative to
ordinary Portland cement made from inorganic aluminosilicate
(Al-Si) polymer compounds that can utilise 100% recycled
industrial waste (e.g. fly ash and slag) as the manufacturing
inputs resulting in up to 80% lower carbon dioxide emissions.
Greater chemical and thermal resistance, and better mechanical
properties, are said to be achieved by the manufacturer at both
atmospheric and extreme conditions.[33]
Similar concretes have not only been used in Ancient Rome
(see Roman concrete) as mentioned but also in the former Soviet
Union in the 1950s and 1960s. Buildings in the Ukraine are still
standing after 45 years so that this kind of formulation has a
sound track record.[34]
Limecrete
Limecrete or lime concrete is concrete where cement is
replaced by lime.[35]
Refractory Cement
High-temperature applications, such as masonry ovens and the
like, generally require the use of a refractory cement;
concretes based on Portland cement can be damaged or destroyed
by elevated temperatures, but refractory concretes are better
able to withstand such conditions.
Concrete testing
Engineers usually specify the required compressive strength
of concrete, which is normally given as the 28 day compressive
strength in megapascals (MPa) or pounds per square inch (psi).
Twenty eight days is a long wait to determine if desired
strengths are going to be obtained, so three-day and seven-day
strengths can be useful to predict the ultimate 28-day
compressive strength of the concrete. A 25% strength gain
between 7 and 28 days is often observed with 100% OPC (ordinary
Portland cement) mixtures, and up to 40% strength gain can be
realized with the inclusion of pozzolans and supplementary
cementitious materials (SCMs) such as fly ash and/or slag
cement. Strength gain depends on the type of mixture, its
constituents, the use of standard curing, proper testing and
care of cylinders in transport, etc. It is imperative to
accurately test the fundamental properties of concrete in its
fresh, plastic state.
Concrete is typically sampled while being placed, with
testing protocols requiring that test samples be cured under
laboratory conditions (standard cured). Additional samples may
be field cured (non-standard) for the purpose of early
'stripping' strengths, that is, form removal, evaluation of
curing, etc. but the standard cured cylinders comprise
acceptance criteria. Concrete tests can measure the "plastic" (unhydrated)
properties of concrete prior to, and during placement. As these
properties affect the hardened compressive strength and
durability of concrete (resistance to freeze-thaw), the
properties of workability (slump/flow), temperature, density and
age are monitored to ensure the production and placement of
'quality' concrete. Tests are performed per ASTM International,
European Committee for Standardization or Canadian Standards
Association. As measurement of quality must represent the
potential of concrete material delivered, placed and properly
cured, it is imperative that concrete technicians performing
concrete tests are certified to do so according to these
standards. Structural design, material design and properties are
often specified in accordance with national/regional design
codes such as American Concrete Institute.
Compressive-strength tests are conducted using an
instrumented hydraulic ram to compress a cylindrical or cubic
sample to failure. Tensile strength tests are conducted either
by three-point bending of a prismatic beam specimen or by
compression along the sides of a cylindrical specimen.
Concrete recycling
Concrete recycling is an increasingly common method of
disposing of concrete structures. Concrete debris was once
routinely shipped to landfills for disposal, but recycling is
increasing due to improved environmental awareness, governmental
laws, and economic benefits.
Concrete, which must be free of trash, wood, paper and other
such materials, is collected from demolition sites and put
through a crushing machine, often along with asphalt, bricks,
and rocks.
Reinforced concrete contains rebar and other metallic
reinforcements, which are removed with magnets and recycled
elsewhere. The remaining aggregate chunks are sorted by size.
Larger chunks may go through the crusher again. Smaller pieces
of concrete are used as gravel for new construction projects.
Aggregate base gravel is laid down as the lowest layer in a
road, with fresh concrete or asphalt placed over it. Crushed
recycled concrete can sometimes be used as the dry aggregate for
brand new concrete if it is free of contaminants, though the use
of recycled concrete limits strength and is not allowed in many
jurisdictions. On March 3, 1983, a government funded research
team (the VIRL research.codep) approximated that almost 17% of
worldwide landfill was by-products of concrete based waste.
Recycling concrete provides environmental benefits,
conserving landfill space and use as aggregate reduces the need
for gravel mining.
Use of concrete in infrastructure
Mass concrete structures
These include gravity dams such as the Itaipu, Hoover Dam and
the Three Gorges Dam and large breakwaters. Concrete that is
poured all at once in one block (so that there are no weak
points where the concrete is "welded" together) is used for
tornado shelters.
Concrete Textures
When one thinks of concrete, oftentimes the image of a dull,
gray concrete wall comes to mind. Nevertheless, with the use of
formliner, concrete can be cast and molded into different
textures. Sound/retaining walls, bridges, office buildings and
more serve as the optimal canvases for concrete art.
For example, the Pima Freeway/Loop 101 retaining and sound
walls in Scottsdale, Arizona, feature desert flora and fauna, a
67-foot lizard and 40-foot cacti along the 8-mile stretch. The
project, titled "The Path Most Traveled," is one example of how
concrete can be shaped using elastomeric formliner.
Reinforced concrete structures
Reinforced concrete contains steel reinforcing that is
designed and placed in structural members at specific positions
to cater for all the stress conditions that the member is
required to accommodate.
Prestressed concrete structures
Prestressed concrete is a form of reinforced concrete
which builds in compressive stresses during construction to
oppose those found when in use. This can greatly reduce the
weight of beams or slabs, by better distributing the stresses in
the structure to make optimal use of the reinforcement.
For example a horizontal beam will tend to sag down. If the
reinforcement along the bottom of the beam is prestressed, it
can counteract this.
In pre-tensioned concrete, the prestressing is achieved by
using steel or polymer tendons or bars that are subjected to a
tensile force prior to casting, or for post-tensioned concrete,
after casting.
Concrete Paving to Lower City
Temperatures
Using light-colored concrete has proven effective in
reflecting up to 50% more light than asphalt and reducing
ambient temperature.[36]
A low albedo value, characteristic of black asphalt, absorbs a
large percentage of solar heat and contributes to the warming of
cities. By paving with light colored concrete, in addition to
replacing asphalt with light-colored concrete, communities can
lower their average temperature.[37]
Many U.S. cities show that pavement comprise approximately
30-40% of their surface area.[36]
This directly impacts the temperature of the city, as
demonstrated by the urban-heat-island effect. In addition to
decreasing the overall temperature of parking lots and large
paved areas by paving with light-colour concrete, there are
supplemental benefits. One example is 10-30% improved night time
visibility.[36]
The potential of energy saving within an area is also high. With
lower temperatures, the demand for air conditioning decreases,
saving vast amounts of energy.
Atlanta has tried to mitigate the heat-island effect. City
officials noted that when using heat-reflecting concrete, their
average city temperature decreased by 6 °F.[38]
New York City offers another example. The Design Trust for
Public Space in New York City found that by slightly raising the
albedo value in their city, beneficial effects such as energy
savings could be achieved. It was concluded that this could be
accomplished by the replacement of black asphalt with light-colored
concrete.[37]
