It is an inorganic solid material that forms the skeleton of a concrete mix. It consists of several fractions which, together, form a solid matrix and serve as a filler. The standard ČSN EN 12620+A1 ‘Aggregates for concrete’ applies to aggregates for concrete.

It is an inorganic solid material that forms the skeleton of a concrete mix. It consists of several fractions which, together, form a solid matrix and serve as a filler. Aggregates for concrete are governed by ČSN EN 12620+A1 ‘Aggregates for concrete’.

Aggregates are divided into the following groups:

by particle size:

• fine aggregate (sand) – the most commonly used fractions are 0/2 and 0/4 (the fraction indicates the particle size range in mm)
• coarse aggregate – most commonly fractions 4/8, 8/16, 11/22, 16/22
• wide fractions (e.g. 0/22, 0/32, which are referred to as gravel-sand or crushed gravel) are practically never used in standard concrete production, mainly due to their susceptibility to segregation

by type:

• quarried
• quarried and pre-crushed
• crushed

by origin:

• artificial (most commonly lightweight and porous aggregates, e.g. Liapor, formerly expanded clay)
• natural

In standard production, the maximum particle size of the aggregate used is a key consideration for concrete customers, depending on the type of structure and method of processing (thin-walled or solid structures, levelling screed layers, reinforcement density, method of vibration).

It is a hydraulic binder, i.e. a finely ground inorganic substance which, when mixed with water, forms a paste that sets and hardens as a result of hydration processes and reactions. Once set, it retains its strength and stability even when immersed in water. Cement is currently manufactured in accordance with ČSN EN 197-1 ed. 2 ‘Cement – Part 1: Composition, specifications and conformity criteria for cements for general use’.

The basic types of cement are:

• CEM I Portland cement
• CEM II Blended Portland cement
• CEM III Blast furnace cement
• CEM IV Pucolane cement
• CEM V Blended cement

The cement strength classes specified in this standard are:

• 32.5
• 42.5
• 52.5
• Class 22.5, previously listed in the national annex, is missing.

Older designations for cement types (e.g. SPC, PC, VPC) and strength classes (e.g. 250, 325, 425) should no longer be used.

Generally speaking, the higher the strength class of the cement and the fewer the admixtures it contains, the faster it sets and hardens; such cement is suitable, for example, for structures requiring short formwork removal times or for winter concreting. Conversely, lower-grade cements with a high content of additives set and harden more slowly and are suitable, for example, for massive structures or for use in chemically aggressive environments. Furthermore, cements are classified according to the content of their main and supplementary components and according to the increase in early strength. For example, what was previously designated as CEM II/B-S 32.5 is now designated as CEM II/B-S 32.5 N under the current standard. The designation for cements with high early strengths remains the same (e.g. CEM I 42.5 R).

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Are fine-grained solids that influence the properties of concrete in both its fresh and hardened states. In fresh concrete, these primarily affect consistency and workability, whilst in hardened concrete they affect strength, density, durability, resistance to chemically aggressive environments, etc. Examples include ground slag, limestone, stone meal, silica fume (also known as microsilica) and fly ash. The most commonly used types are hard coal or lignite fly ash from electrostatic precipitators in thermal power stations and heating plants. A prerequisite for the use of fly ash is that it complies with ČSN EN 450-1 and 2: Fly ash for concrete. Part 1: Definitions, specifications and conformity criteria, and Part 2: Conformity assessment.

Fly ash is used primarily for the following reasons:

• to save on cement
• to improve the pumpability and workability of concrete
• to slow down the setting and hardening of concrete, and to slow down the release of hydration heat (suitable for massive structures)
• to increase the resistance of hardened concrete to chemically aggressive environments

Generally, it is not advisable to use fly ash in aerated concrete. In exceptional cases, so-called expansive admixtures (sometimes also referred to as additives) are used to prevent concrete shrinkage during hardening; with proper curing (keeping the concrete moist), slight expansion (swelling) of the concrete may even occur. The usual reason for using these admixtures is to prevent the formation of shrinkage cracks, often when filling in various parts of structures.

These are chemical substances added to concrete during mixing. The general purpose of adding admixtures is to improve the properties of fresh or hardened concrete or to achieve entirely new properties (e.g. aerated or self-compacting concrete). The most widespread application is the use of plasticising or superplasticising admixtures, which allow the mixing water content to be reduced, thereby saving cement. The water-cement ratio (w/c = quantity of water/quantity of cement) is an important parameter for the composition of concrete. For a given consistency, concrete with a lower water-cement ratio has higher strength and greater durability. It also follows from the above that increasing the w/c ratio – either by failing to maintain the required consistency at the concrete plant or by diluting the concrete with water on site – has a very negative effect on the concrete.

The dosage of admixtures varies according to requirements, type, effectiveness, etc., and should be verified by a confirmatory test. A number of European standards currently apply to concrete admixtures: ČSN EN 934: Admixtures for concrete, mortar and grout. At present, so-called ‘hyperplasticisers’ based on polycarboxylates – originally used only for the production of self-compacting concrete (SCC) – are being intensively developed and are finding ever wider application.

Self-compacting concretes are highly fluid concretes designed primarily for complex, thin-walled structures with dense reinforcement, where it is not possible to work the concrete using conventional vibration. SCC concrete contains a significantly higher proportion of fine particles (cement, fly ash, fine aggregate) and is most commonly used with a maximum aggregate size of between 8 and 16 mm. For these reasons, it is also suitable for structures where a high-quality finish is required. The production of SCC places increased demands on the consistent quality of the input materials and the inspection of fresh concrete.

Inorganic pigments are used to colour concrete; they have the following properties:

• colour fastness when in contact with cement and when exposed to the elements
• minimal effect on the setting and hardening times of fresh concrete and the strength of hardened concrete

The colour shade achieved in the concrete depends primarily on the pigment dosage, the cement used (both its type and origin), but also on the water-to-cement ratio, the type of aggregate, the method of curing the hardened concrete, etc.

Used in the production of concrete must currently comply with the requirements of ČSN EN 1008: Mixing water for concrete – Specification for sampling, testing and assessment of the suitability of water, including water obtained through recycling at the concrete plant, as mixing water for concrete. The quality of mixing water in concrete production can affect the setting time, the development of concrete strength and the protection of reinforcement against corrosion. If the water used is potable, it is considered suitable for production. Under specified conditions, water from facilities for processing residual concrete (so-called ‘recycling’) is also used. This measure significantly reduces or completely eliminates waste generation during production and has a positive impact on the environment.

Other materials that may be used in the production of concrete, where required, include, in particular, polypropylene fibres and steel wires.

Polypropylene fibres (most commonly 12 mm in length) prevent the formation of shrinkage cracks during the early stages of concrete setting and hardening. Once the concrete has hardened, they have virtually no further effect on its properties. They are most commonly used in fine-grained screeds and waterproof concrete. The dosage ranges from 0.6 to 0.9 kg/m³. The fibres are supplied in soluble sachets and are usually added to a truck-mounted mixer.

Steel wires (known as ‘dispersed reinforcement’) are most commonly used in industrial floors. They fully or partially replace conventional mesh reinforcement and, in hardened concrete, have a particularly beneficial effect on flexural tensile strength. The dosage usually ranges from 20 to 40 kg/m³ and can be added either to a mixer or a truck-mounted mixer. The method and duration of mixing have a significant impact on the homogeneity of the wire-reinforced concrete (a negative effect is the formation of so-called ‘spikes’). The wire content must generally be taken into account in the concrete mix design (an increase in the fine aggregate content, particularly for pumpable concrete).

All the above-mentioned concrete constituents are checked at concrete plants in accordance with the Control and Testing Plan, which is a related document to the Quality Manual. All the listed concrete constituents (except water) are subject to the requirements of Act No. 22/1997 Coll. on technical requirements for products.

May be specified either by a consistency class or, in special cases, by a specific value. Tests are carried out at the time of concrete placement or, in the case of ready-mixed concrete, at the time of delivery. A sample is taken after approximately 0.3 m³ of concrete has been discharged from the truck mixer.

Note: Inadequate compaction (air void content in non-aerated concrete exceeding 3 per cent) significantly reduces the strength of the concrete, by approximately 6 per cent for every percentage point of air voids. For dry, tampable and moistened concretes, please note the need for rapid processing, typically within 2 to 3 hours depending on the ambient conditions, particularly the temperature; dry concretes are intended for processing at a maximum moistened consistency.

In general, the addition of any water or admixtures upon delivery is prohibited. In exceptional cases, where the manufacturer assumes responsibility, it is possible to add water or admixtures to adjust the consistency to the required value, provided that the limit values specified in the specification are not exceeded and the addition of the admixture is included in the proposed concrete mix design. In any case, any quantity of water or additive added to the truck mixer must be recorded on the delivery note. When additional mixing takes place, the mixing time must not be less than 1 min/m³ and must not be less than 5 minutes after the additive has been added. If a greater quantity of water or additive is added to the truck mixer on site than permitted by the specification, then this batch must be recorded on the delivery note as ‘non-compliant’. The person involved in the process who decided to add water is responsible for the consequences and must be named on the delivery note.

The temperature of fresh concrete at the time of delivery must not be less than +5°C. If a different minimum concrete temperature is required, or if a maximum temperature is specified, this must be stated along with the permitted tolerances. Any requirement for artificial cooling or heating of the concrete prior to delivery must be agreed in advance between the manufacturer and the customer.

The air content in the concrete is determined where aeration is specified. It is set at the minimum value specified in the specification; the upper limit of the air content is the specified minimum value plus 3 per cent absolute.

Nowadays, there is no longer a single universal definition of how to prepare concrete or what the end result should look like. This is because there are so many different types of concrete. Gone are the days when concrete had to be nothing more than a dull, grey, monolithic block of something that would eventually be covered up. A more observant eye may thus spot exposed concrete used as a façade – both smooth and textured – coloured concrete, and it is even possible to imprint a photograph onto concrete. It is also possible to utilise the various functional properties of concrete. For example, there is self-cleaning, impermeable and self-compacting concrete.

Classifying types of concrete can be somewhat problematic, as there are also quite a number of criteria on which to base such a classification.

Possible classifications include:

According to the customer’s requirements

• custom-formulated concrete – the customer specifies the required properties of the concrete
• concrete of a prescribed composition – the customer specifies the required composition of the concrete

By bulk density

• lightweight concrete – concrete which, after drying at 105°C, has a bulk density greater than 800 and less than 2,000 kg/m³. Lightweight concrete is produced either entirely or partly using lightweight aggregate.
• standard concrete – concrete which, after drying at 105°C, has a bulk density of between 2,000 and 2,600 kg/m³.
• heavy concrete – concrete which, after drying at 105°C, has a bulk density greater than 2,600 kg/m³. It is usually produced using heavy aggregate.

According to special properties

• hydraulic x watertight concrete – concrete that is subjected to water pressure on one side over a long period
• frost-resistant concrete – concrete that, after being saturated with water, is capable of withstanding the effects of repeated freezing and thawing
• self-compacting concrete – concrete with an improved composition that does not require compaction by vibration
• high-strength concrete – concrete characterised by high strength, specifically grades C55/67 and above
• fire-resistant concrete – concrete produced from aggregates with the lowest possible feldspar and quartz content. Basalt, diabase or volcanic tuffs are most commonly used. The concrete withstands temperatures of up to 600 °C.
• refractory concrete – concrete that withstands temperatures of up to 1,700 °C, depending on its composition

Based on the final appearance

• exposed concrete – simply put, concrete that is visible
• decorative concrete – concrete whose surface has been treated by engraving, grinding, etching, staining or painting, and is subsequently coated with a protective epoxy layer
• coloured concrete – concrete produced by colouring the cement paste with pigments or by using multi-coloured aggregate
• graphic concrete – concrete with a graphic motif or image imprinted on it
• exposed aggregate concrete – concrete in which the aggregate grains are exposed
• and others

By reinforcement

• plain concrete – concrete without any reinforcement
• reinforced concrete – concrete with embedded steel elements
• fibre-reinforced concrete – concrete with fibres dispersed in all directions
• prestressed concrete – concrete in which the reinforcement is prestressed during manufacture or prestressed subsequently, e.g. by stranding

This applies to concrete and screeds transported in open-top tipper lorries or flatbed lorries.

Dry consistency

Contains no added water, only water from the aggregate. It is intended for immediate remixing at the placement site using the site’s own mixer to a consistency of S1 at most. Due to the moisture content, the working time is the same as for moist concrete and screeds, i.e. 2–3 hours from mixing, depending on the ambient conditions, particularly the temperature.

Moist and compactable consistency

With added water – intended for direct processing of the mix. These mixes require thorough compaction. If the concrete cannot be compacted, add sufficient water and mix thoroughly until the concrete can be worked effectively. When collecting further batches, inform the dispatcher of the requirement for a different consistency to avoid the need for dilution. Insufficient compaction results in a reduction in strength of approximately 5 per cent for every one per cent of air voids. Similarly, adding too much water leads to a reduction in the final strength of the concrete.

When placing concrete and screed, it is essential to prevent contamination (for example, by soil or snow) or the absorption of water from the concrete. The temperature of the substrate must be above 0°C. The final properties of the hardened concrete are significantly influenced by the curing of the structure. During the early stages of the concrete’s life, it is necessary to prevent water from evaporating from the surface (due to high temperatures or wind) and to protect it from frost or vibrations.

The successful execution of concrete works during the winter months is complicated, in particular, by the following reasons:

• at temperatures below +5°C, the hydration of cement slows down significantly, and at temperatures below 0°C, it practically comes to a halt. This significantly slows down the development of the concrete’s strength
• When water changes to a solid state, its volume increases by 9 per cent. If it freezes within the porous structure of concrete that does not yet have sufficient strength, this leads to an irreversible deterioration in its mechanical properties, or even complete failure. The minimum strength of concrete capable of withstanding a single freeze is stated as 5 MPa
• During the hardening of concrete, heat of hydration is released; if the surface of the structure is cooled by cold air, cracks will form.

For these reasons, it is essential to adhere to the principles set out in the following text when carrying out concrete works in winter. According to ČSN EN 206-1+A2, the temperature of fresh concrete at the time of delivery must not be lower than +5 °C. This requirement is met with a sufficient margin at concrete plants operating year-round. As further measures regarding the composition of the concrete, we note in particular:

• the use of concrete produced from cement with high early strength (in particular class CEM I 42.5 R or, where applicable, Portland blended cement class 42.5 R) without the addition of (fly ash)
• the use of higher concrete strength classes, at least C16/20 (B20), but preferably C20/25 (B25) to C25/30 (B30)
• the use of concrete containing superplasticisers to accelerate hardening

The measures outlined will help ensure that the concrete achieves the strength required as soon as possible, so that it does not need to be protected against frost. You may consult the concrete plant supervisor or a process engineer regarding the selection of a suitable concrete mix.

Further measures relate to the placement and curing of concrete and are based on the applicable ČSN EN 13670 standard:

• the implementation specification shall specify the ambient temperatures at which measures must be planned to protect the concrete against harmful effects
• there must be no ice or snow in the formwork; at the time of concreting, the surface temperature of the working joint must be above 0°C. Concreting on a frozen subgrade must not be permitted unless special working procedures are followed.
• Until the concrete has sufficient strength to withstand the effects of frost, the soil, rock, formwork or parts of the structure in contact with the concrete being placed must be at a temperature that will not cause the concrete to freeze
• If the ambient temperature is low or the weather forecast indicates that the external temperature will be low at the time of concrete placement or during the curing period, preliminary measures must be taken to protect the concrete against frost damage
• During the early stages of curing, the concrete must be cured and protected against freezing; the surface temperature of the concrete must not fall below 0°C until the compressive strength of the concrete surface reaches at least 5 MPa

It follows from the points mentioned above that concrete requires at least a minimum level of protection once it has been placed on site: the structure must be covered and insulated to prevent the loss of hydration heat, to protect it from rain and snow, and to ensure uniform cooling (i.e. thinner sections should be insulated more thoroughly). Nor can so-called ‘anti-freeze’ additives generally be recommended; whilst these prevent water in the concrete from freezing, they do not accelerate hardening and therefore often fail to meet a key requirement of concrete customers: the ability to continue construction even at sub-zero temperatures.

Summer measures at concrete plants are taken depending on the climatic conditions, especially when the ambient temperature rises above +30 °C for several consecutive days, or in the event of intense sunlight, low relative humidity or strong wind. In common concrete practice, the recommended upper limit for the temperature of fresh concrete is considered to be 30 °C. If this is exceeded, it is advisable to take special technological measures or, by agreement of all construction participants, move the concreting to more favorable climatic conditions. At high outdoor temperatures, the temperature of fresh concrete also reaches values ​​of around 30 °C. During transport and especially during pumping, the temperature of fresh concrete may further increase due to friction of the concrete mixture against the walls of the pipe. Higher concrete temperature accelerates cement hydration, shortens the workability time of the concrete mixture, increases the risk of loss of consistency and promotes the evaporation of water from fresh concrete.

The greatest risk of concreting at high temperatures is the rapid evaporation of water, which can cause plastic shrinkage cracks, increased shrinkage and subsequent deterioration of the mechanical properties and durability of the concrete structure. For this reason, it is necessary to pay increased attention to the production, placement and subsequent treatment of the concrete mixture.

According to the standards ČSN EN 206+A2, ČSN P 73 2404 and ČSN EN 13670, concrete must be produced, transported, placed and treated in such a way that its required properties are maintained. Although these standards do not specify the maximum permissible temperature of fresh concrete, they require the adoption of appropriate technological measures in adverse climatic conditions, in particular:

• limiting the overheating of aggregates by appropriate storage or sprinkling it with water
• minimizing the time between production and placement of the concrete mix
• using a higher consistency level (S4, S5) when pumping concrete
• using suitable superplasticizing additives or additives that extend workability (retarder) - consultation required before concreting
• strictly avoiding additional addition of water on the construction site
• immediately starting concrete treatment after completion of surface treatments
• using sprinkling, geotextiles, foil or suitable spray-on paraffin-based treatment agents
• ensuring a sufficiently long treatment time with regard to climatic conditions, cement used and the nature of the structure

For concreting carried out on ŘSD ČR construction sites at ambient temperatures above 30 °C, TKP 18 requires the processing of technological concreting procedure and subsequent treatment of the concrete. This procedure must be approved by the investor or the construction site's technical supervision (TDS) before concreting begins and its use must be recorded in the construction log. The first hours after placing are the most critical period in terms of concrete quality. It is necessary to prevent premature drying of the surface, because the resulting plastic shrinkage cracks cannot be removed later.

With expected daily temperatures exceeding 30 °C, we recommend planning concreting sufficiently in advance and consulting the dispatcher and technologist of the concrete plant regarding their preparation.

The guaranteed workability time of fresh concrete is 90 minutes (at +1 °C to +25 °C). The guaranteed workability time of concrete outside standard climatic conditions is 45 minutes. The processing time of the fresh concrete mixture begins with mixing cement with water (the time is always indicated on the DL) and lasts for the entire period of placing - according to Art. VIII – Product properties and documentation - point 8.

For larger concreting works, we recommend starting work in the early morning hours and preparing all means for immediate concrete treatment before starting concreting.

The customer of the concrete mix is ​​obliged to ensure the conditions for proper placement, compaction and subsequent treatment of fresh concrete. Failure to comply with these principles may lead to the formation of plastic shrinkage cracks, increased shrinkage, reduced durability of the structure and other defects, even though the concrete mix was produced in accordance with the requirements of ČSN EN 206+A2 [N1]. In the event of extreme climatic conditions, when it will not be possible to ensure the delivery of a concrete mix with the required properties even after taking normal technological measures, the customer will be informed in a timely manner. If the concreting continues despite this warning, the customer assumes responsibility for the conditions of placement and treatment of the concrete mix on the construction site. In such a case, ZAPA beton, a.s. is not liable for defects resulting from failure to comply with the technological principles of concreting at high temperatures.