Cembeton | Concrete

Concrete

The history of concrete
Characteristics and components of concrete
Special characteristic concretes
Components of concrete
The process of concrete manufacture
The standardized indications of concrete
Facts and curiosities about concrete production

Concrete is an artificial stone which is made from at least main components: cement, water and additive. Cement mixed with water becomes cement paste, which fills the voids between fine aggregate grains. As a result of the setting of the cement paste, the mixture becomes a stone-like, solid material. Today the properties of concrete are improved with additives and admixtures. Modern concretes are five-component systems. Concrete is one of the most widely used materials in the world.

The history of concrete

Ancient Romans used so-called Roman cement (aluminium and silicon-rich rubble) to build roads, baths and aqueducts. Concrete appeared first in a technically more sophisticated structure in the dome of the Pantheon. The description of concrete id included Vitruvius Pollio’s book of architecture from 27 AD.

Since the end of the 18th century continuous experiments and researches, mainly in England, France, and later in the USA, have helped concrete to reach its current form. These experiments involved Roman cement, mortar, gypsum, lime, and, of course, cement, which in the meantime received a version which sets underwater as well.

The perfection, composition and utilization of concrete are currently under progress. The architectural challenges of the modern age have brought forward, among others, decorative concrete, glass-concrete and pixel concrete surfaces.

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Characteristics and components of concrete

The popular and frequent utilization of concrete confirms, that it meets structural requirements such as:

mechanical solidity and stability
application safety
fire and noise protection
hygiene, health and environmental requirements
insulation

These properties are durable and guaranteed for the expected lifespan of the structure under specified utilization conditions.

The most important characteristic of concrete is therefore durability, its resistance to the impacts of the environment. A heavy advantage is that concrete fulfils its desired role economically, under acceptable construction and maintenance costs.

Another advantageous property of concrete is its custom formability for a period of time. The workable concrete is called “fresh concrete”, while after it hardens it is called “hardened concrete”.

The characteristics of fresh and hardened concrete are influenced by

main components
their characteristic properties
their dosage rate

The composition of concrete has to be determined in a way that the positive properties of the components strengthen each other during the preparation process. The utilization of incompatible materials should be avoided.

Concrete is classified by various properties, such as:

The density determines if we are talking about lightweight concrete (800-2000 kg/m³), normal-weight concrete (2000-2600 kg/m³), or heavy-weight concrete (> 2600 kg/m³),
We distinguish different consistency classes based on the workability of fresh concrete, which is determined by different methods performed on concrete with consistencies from moderately stiff to liquid,
The basis of the classification by strength is the characteristic value with a 5% risk rate expressed in N/mm2 of the 28-day old compressive strength on cylinders of 150 mm diameter and 300 mm height, and a 150 mm edge length cube. We distinguish 16 separate classes of strength from C8/10 to C100/115. (The first number is always the strength measured on the cylinders, the second is the cube strength).

The composition of concrete is the responsibility of the person or organization which determined it.

The regulated composition of standardized concretes is included in standards or recommendations. The highest class of regulated standard concretes can be C 16/20.
The requirements of designed concrete are set by the designer of the structure (exposure class, strength class). The composition and manufacture is the responsibility of the concrete manufacturer.
The composition of concretes with regulated composition is the responsibility of the designer or the contractor. The responsibility of the manufacture is to comply with the requirements.

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Special characteristic concretes

fibre reinforced concrete
frost resistant and frost- and de-icing agent resistant concrete
watertight concrete
abrasion resistant concrete
concrete resistant to chemical attack
concrete for radiation shielding
concrete resistant to high temperatures and refractory concretes
high strength, high performance concretes (HSCF , HPC)
lightweight aggregate concrete (LWAC)
exposed (architectural) concrete
concrete with restricted shrinkage
rapid or slow hardening concrete
no fines concrete
self compacting concrete (SCC)

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Components of concrete

Aggregate
- Normal aggregate
- Sand, gravel, sandy gravel
- Crushed stone
- Heavy aggregate e.g.: barite, hermatite
- Light aggregate e.g.: expanded material, pumice
- Demolished building debris (recycled aggregate)
Cement
- Portland cement
- Heterogeneous/composite Portland cement
- Blast furnace cement
- Pozzolanic cement
- Composite cement
Mixin water
- Drinking water
- Wash water (with concrete containing aggregates forming air bubbles, heavy concrete ³C55/67; it is FORBIDDEN to use with lightweight concrete)
Admixtures
- Plasticizing and superplasticizing admixtures
- Water retaining admixtures
- Air entraining admixtures
- Set and hardening admixtures
- Set retarding admixtures
- Water resisting admixtures
- Anti-freeze admixtures
Additives
- Inert additives
  - Limestone
  - Quartz
  - Threads
  - Pigments
- Pozzolanic additives
  - Fly ash
  - Silica fume
  - Tuff
- Hydraulic additives
  - Blast furnace
  - Hydraulic lime

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The process of concrete manufacture

The manufacture of concrete starts with the putting together of the components. After proper dosage, the components are blended in a mixer. Based on the location of the mixing we distinguish site-mixed concrete (manufactured on the site of application) or ready-mixed concrete. Fresh concrete is transported and worked into the formwork or applied to the surface. The concrete is worked in with compression, after which its surface is evened. Post-treatment, which is basically the preservation of moisture content of the fresh concrete, is an essential process of concreting.

Concrete worked in between steel reinforcements in the formwork is called reinforced concrete. Hardened concrete can be a construction structure worked in on site, or prepared from precast elements. The elements of concrete product manufacture (paving, curbstone) are usually prepared without steel reinforcement.

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The standardized indications of concrete in accordance with the standards and regulations in effect in Hungary

MSZ 4798

C25/30-XC1-24-F2-Cl 0;10-CEM I 42;5 N-100 and MSZ 4798

C25/30 – compressive strength class (C8/10 – C100/115; LC8/9 – LC80/88 - lightweight concrete; in case of bulk density higher than 2600 kg/m³ - heavy concrete)

XC1 – exposure/environmental class

No risk of corrosion: X0; XN(H); X0b(H); X0v(H)
Risk of corrosion by carbonization: XC1; XC2; XC3; XC4
Risk of corrosion caused by chloridesm different from those in seawater: XD1; XD2; XD3
Risk of corrosion due to chlorides from seawater: XS1; XS2; XS3
Freeze/thaw effect with or without ice melting admixtures:XF1; XF2; XF2(H); XF3; XF3(H); XF4; XF4(H)
Chemical corrosion caused by soil or groundwater: XA1; XA2; XA3
Chemical corrosion caused by other aggressive waters and liquids: XA4(H); XA5(H); XA6(H)
Wearing risk: XK1(H); XK2(H); XK3(H); XK4(H)
Water wear risk: XV1(H); XV2(H); XV3(H)

24 – maximum aggregate size (8-63)

F2 – consistency class

Flow: F1 – F6
Slump: S1 –S5
Compactability: C0 – C4
Viscosity (related to the slump area): VS1 – VS2
Slump flow: SF1 – SF3
Viscosity (Funnel outflow): VF1 – VF2
Flow capability (L-box): PL1 – PL2
Flow capability (J-ring): PJ1 – PJ2
Stability against Fractioning with sieve: SR1 – SR2

Cl 0;10 –the largest allowd chloride content of conrete (Cl 1;00; Cl 0;40; Cl 0;20; Cl 0;10)

CEM I 42;5 N –the quality of applied cement

100 years - the domestic practical durability of concrete

MSZ 4798 – standard reference number

The name of the additive may also be given (if it is not sandy gravel): e.g. basalt, andesite, limestone, dolomite, rhubarb, crushed stone, barite, swollen clay gravel, etc.

In case of lightweight concretes the density class also has to be added: D 1;0; D 1;2; D 1,4; D 1,6; D 1,8; D 2;0.

e-ÚT 07.01.14

C20/25-16/KK vz5 f50

C20/25 – compressive strength class (C8/10 – C100/115)
16 – maximum aggregate size (4-32)
KK- consistency degree (FN - damp; KK – moderately ductile; K - ductile; F - liquid)
vz5 – watertightness degree (vz2; vz4; vz5; vz6;vz8)
f50 – frost resitance degree (f25; f50; f100; f150)