CONCRETE
The concrete is one of the main material in the construction industry.
Concrete is generally used in the structural parts of a building and
largely employed in skyscrapers, bridges, sidewalks, highways and superhighways,
dams and so on. For that reason concrete quality must be accurately
tested for any kind of building realization. Therefore, both fresh concrete
and hardened concrete must be subjected to various tests. Actually several
variables contribute to the definition of the final degree of quality
of the concrete utilized for a structure. The workability, the consistency,
the setting time and the air content are only few examples.
Matest offers you a complete and very wide range of products to carry
out all kind of tests on concrete. We invite you to visit Section C
Concrete, from the home page.
CONCRETE’S CHARACTERISTICS
Concrete is a homogeneous material, composed by a mixture of cement,
water and aggregates, where fine aggregates and coarse aggregates are
coated by the paste formed by cement and water.
When newly mixed, concrete’s main properties are plasticity and
malleability. Once the concrete has hardened, i.e. once the chemical
reaction called hydration has taken place, concrete has gained strength
and durability.
Therefore, the simple and natural concrete’s components, when
combined like in the procedure of concrete making, originate a really
strong and durable material, like concrete is.
Those aspects, together with the concrete low cost, explain the wide
spread use of concrete as building material worldwide.
When planning a realization, engineers have to specify the kind of concrete
which must be used, with reference especially to the class
of resistance, the maximal nominal dimensions of aggregates, the
kind of structure, the climatic and environmental conditions and the
workability.
Resistance
Hardened Concrete can be tested to identify its specific resistance.
Concrete generally has a significantly high compression resistance,
whilst a lower tensile resistance (which is about 10% of the compression
resistance value). For that reason concrete is often reinforced, by
means of the addition of steel or fiber reinforcement.
According to the class of resistance to compression, concrete is generally
classified as:
- Non-structural concrete
- Regular concrete
- High performances concrete
- High resistance concrete
Concrete resistance to compression can be tested on concrete cubes specimens as well as on concrete cylinders specimens. In both cases, concrete specimens must be carefully prepared. Actually they must be grinded and polished by means of a specimen grinding machine. As alternative, sulphur capping equipment can be used to make perfectly flat and parallel the two faces of the cylinder specimen. Otherwise, unbonded capping pads and retainers can be applied on the two flat surfaces of the concrete cylinder specimen.
The quality of the concrete structure valuable from the analysis of the fragments. If the concrete specimen, after being broken on compression, shows a breaking involving all the materials composing the concrete (so the aggregates and the filler looks like to be one only material) we can say it is a quality concrete. Otherwise, if the aggregates inside the concrete specimen remain entire and the breaking involves the filler only, we can definitely say it is not a quality concrete.
Several models of compression machines to test concrete cubes, cylinders and blocks are manufactured by Matest to meet all needs and expectations according to the different International Standards’ requirements. Both prestressed frame and high stability and stiffness frame are available.
Beside the compression tests, it is possible to verify the compressive strenght of the concrete by means of a non-destructive test or the rebound test, by means of a concrete test hammer.
Different models of flexural testing machines are also available.
Several devices for compression and flexural machines are available to carry out splitting tensile tests on concrete.
Matest offers also the possibility to combine two different frames for compression tests on concrete, flexural tests on concrete, compression tests on mortar and splitting cylinders tests.
A system for the determination of the secant compression elastic modulus on concrete is available as well within Matest range of products, being the elastic modulus one of the most important properties to be studied for all kind of structural utilizations.
Workability
The workability represents a key feature in planning a valuable concrete
mixture. The consideration of the workability degree can not be neglected
in no way: according to the different kind of building realization,
the concrete is required to have a specific workability. Engineers must
choose the proper one combining it to the required durability and mechanical
resistance, that means to the hardened concrete’s properties.
Stability, mobility and compactability
are three properties of fresh concrete strictly linked to workability.
Stability is the property of the concrete paste of having and keeping
uniformity distribution of its components. Mobility is the property
of the concrete paste of filling perfectly the mould, in all its parts.
Compactability is the property of the concrete paste of being compacted
in the mould with the correlated removal of the air inside the paste
of fresh concrete. The removal of the air from the concrete paste is
necessary to grant an higher density level, once the concrete has hardened.
With a reduced voids’ content, the concrete mixture will grant,
once hardened, a higher degree of resistence.
Being stability and mobility significantly connected to consistency,
it is common to check the workability of a concrete mixture by means
of consistency tests. However, there is no method to carry out a complete
consistency test, so engineers generally use to choose the proper test
according to different variables linked to the specific realization.
The most common and widespread consistency tests are:
- Slump cone test
- Test with Vebè consistometer
- Compactability index
- Flow test to measure the spread of fresh concrete
Compactability
C185 Compacting Factor Apparatus, C187 K-Slump Tester and C188 Walz Consistometer can be used to determine the degree of compaction of the fresh concrete.
CONCRETE’S COMPONENTS
The concrete is a material obtained from a mixture of cement,
aggregates, water and,
in some cases, additives and additions.
The key to get a valuable concrete mixture is a careful proportioning
of the ingredients. Particular attention must be paid to the ratio of
water to cement. Actually, considering two kind of concrete prepared
with the same cement content, the mixture with a lower quantity of water
will be the most resistant one. However, with a low quantity of water,
the resulting concrete will be dry and so hardly-workable. For this
reason, the right water dosage becomes the key of a valuable concrete.
Sometimes, some chemicals additives are added to the mixture in order
to grant more fluidity without increasing the water proportion.
The quantity of water is relevant also in the curing process
of the concrete. An intense vaporisation caused by a high level
of water, may produce some small cracks inside the concrete structure.
However, some chemicals additives may be added also in this case to
prevent this problem and so to prevent future unproper behaviors of
the concrete.
Finally, the concrete mixture composition must be studied and evaluated
in order to grant the specific properties of the concrete mix for a
specific project.
Cement
The cement is an inorganic material finely grinded, derived from the
shattering of stones. When melted with water the cement becomes like
a paste, that is the reason why the cement is considered an hydraulic
binder.
The most common type of cement used for concrete, mortar and plaster
is the Portland Cement, a cement produced by pulverizing clinckers mainly
of calcium silicates. Anyway, several kind of cement exist, each one
with its specific properties. When projecting a structure, engineers
have to choose the right type of cement, to get the higher performances
from the concrete to be used in the project realization.
When preparing the concrete mixture, mixing different kind of cements,
or mixing cements with different resistance class or different origin,
is not allowed.
Besides the products used to carry out tests on concrete, Matest offers
also a wide range of products to carry out tests on cement. We invite
you to enter Section E Cement for more
info.
Aggregates
Aggregates are considered to be the main material used in the construction
field. Aggregates are largely employed especially for mixture of concrete,
representing really the concrete’s framework.
For more info about aggregates we invite you to visit our overview as
well as our Section A Aggregates, for
all products to test aggregates.
Water
According to Standard ASTM C 1602, the water employed can be batch water
or even ice, during hot weather. Also free moisture on aggregates must
be considered in the calculation of the proportion of water/cement ratio.
It is only important to be sure that any harmful substance is not contained
in the moisture and on aggregates. Non-potable water and water from
concrete production operations may be used, provided that they meet
standard ASTM C 1602 requirements. Actually the use of recycled water
in mixing concrete has been recently increased.
One important aspect concerning the choice of water for concrete mixture
is the presence of chloride, sulfates, solids and alkalis, which must
be under some settled limits. Actually impurities in excess may negatively
influence some concrete characteristics like setting time, concrete
strength and concrete durability.
Additives and additions
The use of additives in concrete mixture is allowed according to specific
dosages for specific mixtures. They are used for example to grant more
fluidity to the concrete paste without increasing the water proportion
or to prevent the rising of small cracks, inside the concrete structure,
caused by a high level of water in the concrete mixture. These are few
examples, in any case additives must not contain any kind of dangerous
substances which could affect the concrete’s durability.
Additions are inorganic materials finely grinded, used to improve concrete
properties and to achieve different properties according to the project
involved.
Some of the additions are nearly inert; others are pozzolanic.
Additions mainly used in the concrete mixtures are:
- fly ash: used to partially replace Portland cement;
- ground granulated blastfurnace slag: used to increase the concrete durability;
- metakaolin: it accelerates the chemical reaction called hydration and also the pozzolanic reaction. It exhibits an immediate filler effect;
- silica fume: similar to fly ash but really smaller. It is used to improve the concrete strenght and the concrete durability;
- pigments: used for aesthetics to change the colors of concrete Also mineral filler aggregates and limestone fines can be added to the concrete mixture.
TESTS ON CONCRETE
Several tests have to be carried out on concrete components, on fresh
concrete mixture and on hardened concrete as well.
Some of them must be effected during works, others once works have taken
place.
Even preparation and maintenance of concrete specimens which have to
be tested under a compression load or a tensile stress must strictly
refer to International Standards, which require specific procedures
for the
extraction of concrete samples but also procedures for preparing
concrete specimens in laboratory or in situ.
Concrete specimens preparation
A proper preparation of concrete specimens is an essential part of the
concrete testing.
Here below you will find some equipments offered by Matest for your
sampling preparation:
- Moulds: plastic cube and cylinder moulds; polystyrene cube moulds; steel cube, cylinders and beam moulds
- Vibrating Tables and Poker Vibrators
- Concrete pan mixers
