Concrete
Concrete’s characteristics
   Resistance
   Workability
   Compactability
Concrete’s components
   Cement
   Aggregates
   Water
   Additives and additions
Test on concrete
   Specimens preparation


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 and dams. 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 test. 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 wide range of products to carry out all kind of test on concrete: we invite you to visit “Concrete” section to browse through the complete products list.

Concrete’s characteristics
Concrete’s components
Test on concrete

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 that 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
Workability
Compactability

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

The breaking procedures of concrete specimens under the compression load too are set by International Standards. A strict respect of standards’ prescriptions is the necessary condition to get test results’ reliability and repeatability.

Concrete resistance to compression can be tested on concrete cubes specimens as well as on 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 is valuated 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.

Different models of flexural testing machines are also available.

Matest offers also the possibility to combine two different frames for compression/flexural test on concrete, compression test on mortar and splitting tensile test. Several devices for compression and flexural machines are available to carry out splitting tensile test on concrete. Beside the compression test, it is possible to verify the compressive strength of the concrete following a non-destructive test or the rebound test and using a standard or a digital concrete test hammer.

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 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 resistance. Being stability and mobility significantly connected to consistency, it is common to check the workability of a concrete mixture by means of consistency test. 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 test are:

• Slump cone test
• Test with Vebè consistometers
• 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 good quality 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 behaviours of the concrete.
Finally, the concrete mixture composition must be studied and evaluated in order to grant the required properties of the concrete mix for a specific project.

Cement
Aggregates
Water
Additives and additions

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 test on concrete, Matest offers also a wide range of products to carry out test on cement. We invite you to enter the “Cement-Mortar” section 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 information about equipment for testing aggregates we invite you to visit our “Aggregates-Rocks” section.

Water
According to Standard ASTM C1602, the water employed can be batch water or even ice during hot weather. Also free moisture on aggregates must be considered when calculating 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 ASTM C1602 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 strength and durability;
• pigments: used for aesthetics to change the colours of concrete. Also mineral filler aggregates and limestone fines can be added to the concrete mixture.

Test on concrete
Several test 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.

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 moulds for cubes, cylinders and beams;
• Vibrating Tables e Poker Vibrators
• Concrete pan mixers.