Concrete has relatively high compressive strength, but significantly lower tensile strength,
and as such is usually reinforced with materials that are strong in tension. The elasticity
of concrete is relatively constant at low stress levels but starts decreasing at higher
stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal
expansion, and as it matures concrete shrinks. All concrete structures will crack to some
extent, due to shrinkage and tension. Concrete which is subjected to long-duration forces
is prone to creep. Tests can be made to ensure the properties
of concrete correspond to specifications for the application. The density of concrete varies,
but is around 2,400 kilograms per cubic metre. 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. Structural fibers of various materials
are available. Concrete can also be prestressed using internal steel cables, allowing for
beams or slabs with a longer span than is practical with reinforced concrete alone.
Inspection of existing concrete structures can be non-destructive if carried out with
equipment such as a Schmidt hammer, which is sometimes used to estimate relative concrete
strengths in the field. The ultimate strength of concrete is influenced
by the water-cementitious ratio, the design constituents, and the mixing, placement and
curing methods employed. All things being equal, concrete with a lower water-cement
ratio makes a stronger concrete than that with a higher ratio. The total quantity of
cementitious materials 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 the
crushing strength of the aggregate can be a limiting factor to the ultimate compressive
strength. In lean concretes 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. Engineers know their target tensile 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 340 and 410 MPa at 28 days. The blocks appeared to use an aggregate of
steel fibres and quartz – a mineral with a compressive strength of 1100 MPa, much higher
than typical high-strength aggregates such as granite. Reactive Powder Concrete, also
known as Ultra-High Performance Concrete, can be even stronger, with strengths of up
to 800 MPa. These are made by eliminating large aggregate completely, carefully controlling
the size of the fine aggregates to ensure the best possible packing, and incorporating
steel fibers into the matrix. Reactive Powder Concretes may also make use of silica fume
as a fine aggregate. Commercial Reactive Powder Concretes are available in the 170–210 MPa
strength range.
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:
where weight of concrete and where
compressive strength of concrete at 28 days This equation is completely empirical and
is not based on theory. Note that the value of Ec found is in units of psi. For normal
weight concrete Ec is permitted to be taken as .
The publication used by structural bridge engineers is the AASHTO Load and Resistance
Factor Design Manual, or "LRFD." From the LRFD, section 5.4.2.4, Ec is determined by:
where correction factor for aggregate source
weight of concrete, where and specified compressive strength of concrete
at 28 days For normal weight concrete Ec may 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. The coefficient of thermal expansion of Portland cement concrete
is 0.000008 to 0.000012(8-12 1/MK). 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. 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.
Due to its low thermal conductivity, a layer of concrete is frequently used for fireproofing
of steel structures. 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 as a
result of either drying, autogenous shrinkage or thermal effects. Restraint is provided
either externally or internally. 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.These are minor indications and have no real structural
impact on the concrete member. 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 permanent movement or deformation
of a material in order to relieve stresses within the material. Concrete that is subjected
to long-duration forces is prone to creep. Short-duration forces 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.
Concrete testing
Engineers usually specify the required compressive strength of concrete, which is normally given
as the 28 day compressive strength in megapascals or pounds per square inch. 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 mixtures, and between 25% and 40% strength gain can be realized with the inclusion of
pozzolans and supplementary cementitious materials 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
by certified technicians, and care of cylinders in transport. For practical immediate considerations,
it is incumbent 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.
Additional samples may be field cured 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" properties of concrete prior
to, and during placement. As these properties affect the hardened compressive strength and
durability of concrete, the properties of workability, temperature, density and age
are monitored to ensure the production and placement of 'quality' concrete. Depending
on project location, 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 and placed, it is imperative that concrete technicians
performing concrete tests are certified to do so according to these standards. Structural
design, concrete material design and properties are often specified in accordance with national/regional
design codes such as American Concrete Institute. Compressive strength tests are conducted by
certified technicians using an instrumented, hydraulic ram which has been annually calibrated
with instruments traceable to the Cement and Concrete Reference Laboratory of the National
Institute of Standards and Technology in the U.S., or regional equivalents internationally.
Standardized form factors are 6" by 12" or 4" by 8" cylindrical samples, with some laboratories
opting to utilize cubic samples. These samples are compressed 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 standard cylindrical specimen. These destructive tests
are not to be equated with nondestructive testing using a rebound hammer or probe systems
which are hand-held indicators, for relative strength of the top few millimeters, of comparative
concretes in the field. Testing fraud in New York City
American Standard Testing and Consulting Laboratories, company president Alan Fortich, and five other
executives admitted filing false documents on "thousands" of New York City construction
projects — including the Yankee Stadium, the Second Avenue Subway, and Memorial Sloan-Kettering
Cancer Center — over 10 years. ASTCL had replaced Testwell Inc., another firm indicted
for faking concrete tests, in 2008. See also
Segregation in concrete - particle segregation in concrete applications
Creep and shrinkage of concrete References