Concrete

Resistance in traction is less with values of order 2,1 with 2,7  MPa for a concrete of the type BFC.

The thermal Conductivité usually used is of 1,75  W·m −1 ·K −1 , halfway between metallic materials and wood.

Formulation of a concrete

The choice of the proportions of each component of a concrete in order to obtain the mechanical properties and of implementation wished is called the formulation. Several methods of formulations exist, of which in particular:

• the method Baron;
• the Bolomey method;
• method of Féret;
• method of Faury;
• the Dreux-Gorisse method.

The formulation of a concrete must integrate before all the requirements of the standard NF INTO 206-1, which, according to the environment in which will be set up the concrete, will be more or less constraining with respect to the minimal quantity of cement to insert in the formula as well as the quantity of maximum water tolerated in the formula. In the same way, with each environment given, a resistance guaranteed to 28 days on test-tubes will be required to the producers, being able to justify cement proportionings more or less higher than the recommendation of the standard, and based on the specific experiment to each company, which being dependant as of its raw materials whose density can vary, in particular that of the aggregates.

Other requirements of the NF IN 206-1 impose the use of cement private individuals because of more or less aggressive mediums, as well as the addition of additives conferring of the different properties on cement the paste that it is the time of implementation, plasticity, quantity of entrained air, etc

Classification of the concretes

The concrete used in the building, as in public works includes/understands several categories.

In general the concrete can be classified in four groups, according to its Density ρ:

• very heavy concrete: ρ >: 2500 kg/m;
• heavy concrete (current concrete): ρ between: 1800 and: 2500 kg/m;
• light concrete: ρ = 500 with: 1800 kg/m;
• very light concrete: ρ < 500 kg/m.

The concrete running can also be classified according to the nature of the binders:

• concrete cement;
• concrete silicates (Chaux)
• concrete of gypsum (Gypse);
• concrete asphalts.

When fibers (metal, synthetic or mineral) are added, one distinguishes:

• the reinforced fiber concretes (BRF) which are concretes " classiques" who contain macro-fibers (diameter ~: 1 mm) in voluminal proportion going from: 0,5% with: 2%;
• the concretes fibers with ultra high efficiencies (BFUHP) which are concretes (BUHP) which contain microfibres (diameter >: 50 $\backslash \; driven\; m$ or a mixture of macro-fibers and microfibres.

The concrete can vary according to the nature of the aggregates, the additives, the dyes, the surface treatments and can thus adapt to the requirements of each realization, by its performances and its aspect.

• the current concretes are used, as well in the building as in public works. They have a density of: 2300  kg/m approximately. They can be armed or not, and when they are very solicited in inflection, prestressed.

• the heavy concretes, whose densities can reach: 6000  kg/m is useful, inter alia, for protection against the radioactive rays.
• the light aggregate concretes, whose resistance can be high, are employed in the building, for the platforms offshore oil rig or the bridges.

Various types of aggregates

the aggregates naturels

; Rolled aggregates and aggregates of careers Independently of their mineralogical origin, one classifies the aggregates in two categories which must be in conformity with standard NF INTO 12-620 (aggregates for concretes):

1. the aggregates alluvial, known as rolled, whose form was acquired by erosion. These aggregates are washed to eliminate the argillaceous, harmful particles with the resistance of the concrete and are sifted to obtain various classes of dimension. Although one can find various rocks according to the area of origin, the aggregates used for the concrete are generally siliceous, limestones or silicocalcareous;

2. the aggregates of career are obtained by demolition and crushing, which give them angular forms a phase of précriblage is essential to obtaining clean aggregates. Various phases of crushing lead to obtaining the desired size ranges. The broken up particles show characteristics which depend on a great number of parameters: origin of the rock, regularity of the bench, degree of crushing… The selection of this type of aggregates will have to thus be made with care and after agreement on a sample.

Les aggregates artificiels

; Industrial by-products, crushed or not The most employed are the crushed crystallized slag and the granulated slag of blast furnaces obtained by water cooling. The apparent bulk density is higher than: 1250  kg/m for the crushed crystallized slag, 800  kg/m ³ for the pellet. These aggregates are used in particular in the road concretes. The various characteristics of the aggregates of slag and their specifications are the subject of standards NF P 18-302 and 18-306.

; Aggregates with high characteristics worked out industrially They are aggregates worked out especially to answer certain employment, in particular very hard aggregates to reinforce the wear resistance of industrial pavements (ferrous aggregates, carborundum…) or refractory aggregates.

; Aggregates reduced by expansion or sintering: These aggregates, very much used in many countries like the USSR or the United States, did not have in France the same development, although they have strength properties, insulation and weight very interesting. Most usual are clay or expanded shale (standard NF P 18-309) and the expanded slag (NF P 18-307). Of a variable density between 400 and 800  kg/m according to the type and the granularity, they make it possible to carry out structural concretes as well as concretes presenting a good thermo isolation. Grains of interesting weights since the concretes carried out have a density ranging between: 1200 and: 2000  kg/m.

; Very light aggregates They are of origin as well vegetable and organics that mineral (wood, expanded polystyrene). Very light - 20 with 100  kg/m - they make it possible to carry out voluminal mass concretes ranging between 300 and 600  kg/m. One thus sees their interest for the concretes of insulation, but also for the realization of light elements: blocks casing, back-up blocks, flagstones, or rechargings on not very resistant floors.

d) The gas concretes (very light concretes) whose densities are lower of 500  kg/m. They are used in the building, to answer the problems of insulation. At the time of his realization one incorporates foaming products to him creating porosities in the concrete.

e) Fiber the concretes, more recent, correspond to very varied uses: decorative pavements, elements, urban furniture.

Standard NF IN 206-1 class concretes according to their characteristic resistance to compression in accordance with table 6.1. In this table fckcyl is the characteristic resistance measured on cylinders 16×32  mm (it is this resistance which corresponds to the characteristic resistance to which it is refers in the Eurocode 2); fckcube is the characteristic resistance measured on cubes 15×15  Misters the underlined values are the values recommended.

Techniques of implementation

The rheological properties of the concrete in a fresh state can make it possible to distinguish various types of concrete:

• vibrated concrete: need a vibration (switches vibrating, vibrating form…) for good set up in the formwork;
• concrete compacted with the roller: very stiff concrete which is set up using a road roller (used mainly for the roadways, the landing strips or the stoppings);
• shotcrete: stiff concrete created by projection on a vertical surface or in overhang (there exist two techniques: projection by wet process and projection by dry roads);
• pumping concrete: fluid concrete which can be conveyed on several hundred meters using a concrete pump;
• concrete car-placing and car-levelling concrete: very fluid concretes which does not require a vibration, laz compaction being carried out by the only gravitating effect.

Study of the composition of the concrete

Many methods of composition of the more or less complicated and clever concrete were elaborate. It will be noted that a study of composition of concrete must always be controlled in experiments and that a study carried out in laboratory must generally be adapted later on to the real conditions of the building site.

A method of composition of the concrete could be regarded as satisfactory if it makes it possible to carry out a concrete fulfilling the following requirements:

The concrete must have, after hardening, a certain compressive strength. The freshly-mixed concrete must easily be able to be implemented with the means and methods used on the building site. The concrete must present a weak withdrawal and a not very important creep. The cost of the concrete must remain low possible. In the past, for the composition of the concrete, one prescribed theoretical proportions of cement, fine aggregate and coarse aggregate. But the development of cements having made considerable progresses, many researchers expressed formulas in connection with required qualities:

• maximum density internal, determining a high resistance;

• good sealing improving durability;
• chemical resistance;
• resistance to the external agents such as freezing, abrasion, the desiccation.

The concrete can vary according to the nature of the aggregates, the additives, the dyes, the surface treatments, and can thus adapt to the requirements of each realization, by its performances and its aspect.

The composition of a concrete and the proportioning of its components are strongly influenced by the employment for which is intended the concrete and by the means of implementation used.

Test of mixing

Corrections

a) Consistency: During the test of mixing, it is recommended not to immediately add the quantity of total water E envisaged. It is preferable to add only 95  % of E, to measure consistency, then to add water until obtaining prescribed consistency.

b) cement Proportioning: If cement proportioning indeed carried out is false, one will have to correct it. If it is necessary to add (or remove) a weight ΔC of cement to obtain wished proportioning, one will have to remove (or add) an equivalent absolute sand volume, that is to say a weight ΔC equal to:

If ΔC is important, it will also be necessary to correct the quantity of water.

c) Mechanical resistances: If the mechanical resistances are insufficient, it will be necessary to have recourse to one or more following possibilities:

To increase cement proportioning (beyond 400  kg/m, an increase in proportioning out of cement has nothing any more but one very weak influence on the increase in resistance). To decrease water proportioning without changing granulometry. To correct granulometry and to reduce the quantity of water. To use another type of aggregates. To use an additive and to reduce the quantity of water. To use cement with faster hardening. One will have in any case to always take care that the consistency of the concrete allows implementation a correct

Agglomerated concrete

Reinforced concrete

In an intrinsic way, the cement concrete has an excellent compressive strength. On the other hand, it thus has a low tensile strength with the inflection. Also it is necessary, when a concrete work is designed to undergo requests in traction or inflection (such as for example a floor, a bridge, a beam…), to incorporate in it reinforcements in Steel intended to be opposed and take again the Contrainte S of traction which could put in danger the perenniality of the work. The reinforcements implemented can be is in mild steel (little used to take again pure traction, by definition the mild steel has only one low adherence with the concrete, it thus remains used for the load-carrying members primarily in inflection such as the pylons, the barrels, etc) that is to say in steel high-adherence (steels ha called in the past TOR ). One speaks then about reinforced concrete , composite material developped at the point in 1886 by François Hennebique which used it for construction in 1899 of the first civil reinforced concrete bridge of France, the bridge Camille-of-Hogues with Châtellerault.

Concrete prestressed

Indeed, the concrete has interesting mechanical properties in compression whereas resistance in traction is limited and causes quickly its cracking and its rupture.

They are techniques invented by Eugene Freyssinet in 1928, which consist in tightening (as Ressort S) steels constituting the reinforcements of the concrete, and thus to compress, at rest, this last. Thus, when the structure is requested, these reinforcements lengthen and the concrete tends to decompress without however putting itself in traction, since it was already partly compressed.

According to whether this tension applied to the reinforcements (called cable of prestressed or strand of prestressed) is carried out before the complete catch of the concrete or subsequently to this one, one distinguishes the prestressed by pre-tensioning and the prestressed by posttension .

• In the pre-tensioning (generally used out of building), the reinforcements are put in tension before the catch of the concrete. They are then slackened, thus putting the concrete in compression by simple effect of adherence. This technique does not make it possible to reach values of prestressed as high as in posttension.

• the posttension consists in laying out the cables of prestressed in sheaths incorporated in the concrete. After the catch of the concrete, the cables are tended by means of jacks so as to compress the work at rest. This technique, relatively complex, is generally reserved for the great works (bridges) since it requires the implementation of cumbersome “parts of butt” (devices set up on both sides of the work and allowing the setting in tension of the cables).

Consequently the concrete section is uniformly compressed (according to the position of the cables it appears even a counter-arrow with vacuum). Once subjected to the maximum loading, prestressing out of bottom fiber will be almost cancelled by the charging voltage, whereas in the upper part compression is largely more important than in a traditional reinforced concrete beam.

Other techniques of reinforcement

Another option is known as of “reactive powder” with structure Fractale: the grains which compose it have all the same size, and incidentally the property to present the same form to various scales (fractale). The optimal organization of the aggregates within the concrete grants better mechanical properties to him. It is however about a technique always at the experimental stage.

Asphaltic concrete

The bitumen being an oil derivative , the asphaltic concrete is sensitive to the Hydrocarbure S lost by the cars. In the exposed places (service stations) one replaces the bitumen by Goudron. The Tarmacadam of the Aérodrome S is the commercial name of such a concrete of tar (nothing to see with the Macadam, deprived of binder).

Routing of the concrete

The mode, the duration and the conditions of the routing of the concrete are crucial factors in its formulation. They have each one a particular influence on its maneuverability and its quality.

The concrete is transported is by average handbooks (bucket, wheelbarrow…), that is to say, for great quantities, by average mechanics. In this case, it is generally transported since the concrete batching and mixing plant by trucks mixers or “spinning tops” (capacity 7 with 15  m).

Once on the building site, it is transvased either in concrete pouring skips (750 liters with 2  m and at wheel or cuff) which are raised to the crane to be then emptied in the Coffrage, is in a Concrete pump which is coupled with a mast of distribution of the concrete. It can also be projected using a pneumatic compressor. This technique is very useful in order to carry out several repairs on concrete works.

Certain spinning tops are also equipped with a travelator (of ten meters) making it possible in certain cases to do without the means of lifting.

The starting of the time of catch of the concrete is done starting from its malaxation. Transport thus starts this time and must be fastest possible to preserve a maximum of maneuverability of the concrete during its installation.

The temperature during transport is also important. The speed of catch of the concrete is strongly influenced by the room temperature. It is thus possible to use cool water by very hot seasons and of warm water in cold weather, during malaxation. Certain trucks are also insulated

The placement of the concrete

The concrete curing compound is important at the beginning of its catch. It consists in maintaining the concrete in an environment favourable with its catch. It is necessary to avoid any evaporation of the water contained in the concrete (in weather hot and/or windy), which would prevent the chemical reaction of catch from being done and would thus blame the resistance of the concrete.

Also should be avoided the thermal shocks. The exothermic reaction of the concrete, possibly added to an ambient strong heat makes that the concrete could “autocuire”. Contrary it is necessary to protect the concrete from the ambient cold so that the chemical reaction of the concrete starts and that it discusses during a minimum amount of time (until 48h for the concretes with slow catch). In the case of great colds, the formwork are isolated (glass wool or tents heated) and must remain in place until the concrete did its catch.

Aspect and uses

The concrete can be tinted in the mass while incorporating in it of the natural pigments or metallic oxides. It can also be treated using additives to be made damp-proof (it becomes tight then, preventing the capillary gone up). The addition of various materials (textile fibers, shavings, plastics…) allows to modify its physical properties. Its facing which can be smoothed or worked, the cement concrete is sometimes left apparent ( gross of dismantling ) for its spirit minimalist, gross and modern.

Concrete used in coating of large surfaces (esplanades, public places…) is often decontaminated: one proceeds while pulverizing, on the surface of the concrete coldly posed, a decontaminating product which neutralizes its catch. A rinsing with high pressure allows then, after elimination of the milt, to reveal, on the surface, the various constitutive fine gravels.

Moulded or built (i.e. cast in a form: a mould dismountable installed on the building site and dismounted after the catch), the concrete can take all the forms. This technique made it possible the Architecte S to build buildings with curved forms.

In road engineering, the extruded concrete , implemented using Formwork S slipping, makes it possible to carry out low walls of safety, bordurages and devices of reserve on the linear important ones.

Technical data

gray Energy

• Breeze block: 200  kWh /m
• reinforced concrete: : 1850  kWh/m

Strength classes

There exists also class LLC xx/xx for the noncavernous light concretes

C like Concrete and LLC like Light Concrete

Economic importance

In France

• the ready-mixed concrete represents: