Ready Mixed Concrete - Introduction

In volume terms, twice as much concrete is used worldwide than all other construction materials combined. It has been used in extreme climates and on the most demanding engineering projects and as a result its physical properties are tried, trusted and well understood by engineers. In Ireland, ready mixed concrete is manufactured to "ISEN 206-1:2002 Concrete - Part 1: Specification, Performance, Production and Conformity" in fully computerised, automated batching plants, so that the end product is delivered to a high standard. When engineers build with concrete, they build with confidence. By varying the mix design, to include various additives, engineers can design-in an extensive range of physical properties to cater for almost any physical, structural or site requirement. Concrete is without doubt, the most flexible of all building material.

The design of the concrete mix will depend on the physical and structural requirements and on the specific conditions in which it is being laid. Ready Mixed concrete is available in a range of ‘workabilities’ to suit the different methods of placement, including pump mix. Concrete will also be specified to have a specific ‘cube strength’ which normally varies between 10N/mm² and 60N/mm². Durability is a key design factor and this is designed relative to the ‘exposure class’ (i.e. for a corrosive or non-corrosive environment) and to the required ‘working life’ of the concrete.

Note: Codes and Standards for all concrete products are available in the Publications section - See Concrete Products Directory, Technical Commentary, P65 - 104

Further Information

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Mix Design

The basic constituents of concrete are cement, aggregates, water and admixtures. These materials combine and harden when water is added, in a chemical reaction known as ‘hydration’. The properties of the hardened concrete vary dramatically depending upon the relative amount and specific characteristics of the constituents. Good ‘proportioning’ of the constituent materials and a detailed knowledge of the physical properties of these materials is therefore critical to achieving an end product which is fit for the purpose for which it is designed.

Concrete consists of a ‘cement water paste’ and ‘aggregates’. The cement water paste is the ‘active ingredient’ and the aggregate is the ‘inactive’ or inert ingredient. The structure of the concrete can be visualised as a mass of aggregate particles suspended in and separated by the cement. The ‘cement water paste’ consists of cement particles suspended in water. The greater the proportion of water to cement (i.e. the ‘water cement ratio’) the greater the distance between the suspended cement particles will be when the water evaporates in the process of hydration. The greater the distance between cement particles the more porous (permeable) the hardened concrete will be and therefore it is clear that the quantity and quality of the paste plays a key role in determining the properties of both the fresh and hardened concrete.

Note:

The ‘consistence’(workability) and ‘cohesiveness’ of fresh concrete is determined by the lubricating effect of the paste, which in turn is governed by the quantity and quality of the latter.
The ‘strength’ of the hardened concrete will be limited by the strength of the paste, since mineral aggregates are practically always far stronger than the paste in which they are suspended.
Permeability (and consequently durability) will be governed by the quality and continuity of the paste, since mineral aggregates are generally practically impermeable.

The most significant factors, which must be taken into consideration in concrete proportioning are the water content, the cement content, the water cement ratio, the consistence of the fresh concrete and admixtures.

Unit Water Content

Usually, two or three of these interrelated factors will be specified, and only those remaining can be adjusted to obtain maximum economy and optimum properties. The unit water content of a mix (i.e. the amount of free water per unit volume of concrete) determines the strength obtained for given cement content, and for given aggregates and mix design, it determines the consistency of the mix. Furthermore, the unit water content is the most important single factor influencing the drying shrinkage of a given mix.

Cement & Cement Content

Cement is composed predominantly of compounds of calcium, silica, alumina and iron which hydrate upon the addition of water to form a strong and durable binder for aggregates in concrete or mortar.

The common cement used in traditional Irish construction has been predominantly Portland Cement Type CEM 1. CEM 1 is a Portland cement (= 95% clinker), which can contain 0-5% of a permitted ‘minor additional constituent’.

CEM II A is defined as Portland composite cement, sub-divided into various sub-types, depending on the type and quantity of main constituents included, which can also contain 6-20% of a permitted ‘minor additional constituent’ ( See Additions)

The strength of concrete increases with increasing cement content for a fixed water content, but a stiff lean mix will have a higher strength than a plastic cement rich mix if the water content of the former is low enough to give a lower W/C ratio than in the rich mix. The cement content of a mix influences the durability of the hardened concrete, and when necessary, a minimum cement content may be specified. The cement content also influences heat development and shrinkage, and a maximum cement content is therefore sometimes specified.

Additions are finely divided inorganic, pozzolanic or latent hydraulic material that may be added to concrete in order to improve certain properties or to achieve certain properties.

Pulverised Fuel Ash (PFA)

The use of coal for electricity production results in the generation of fly ash of different qualities, some of which have pozzolanic properties. The properties of fly ash suitable for use in concrete are specified in I.S. EN 450-1:2005.

Ground Granulated Blastfurnace Slag (GGBS)

Granulated blastfurnace slag is made by the rapid cooling of a slag obtained by melting iron ore in a blast furnace. It contains at least two-thirds by mass of glassy slag and possess hydraulic properties when suitably activated by a Portland cement. Properties of slag suitable for use in concrete are specified in ISEN 15167:2006 and BS6699:1992.

Cement Codes & Standards

I.S. EN 197-1: 2001	Composition, specification conformity criteria for common cements
B.S. 4027: 1996	Sulphate resisting Portland Cement
I.S. EN 450: 2005	Fly ash for concrete
B.S. 3892-1 : 1997	Pulverised fuel ash for use with Portland cement
ISEN 15167:2006	Ground granulated blastfurnace slag for use with Portland cement

Specification

The specification of cement used in concrete is given in I.S. EN 197-1. The concrete specification I.S. EN 206-1:2002 gives advice on cements suitable for use in concrete in Ireland (see National Annex and Table NA.5(2007).

Aggregates & Aggregate Properties

An aggregate is a granular product obtained by processing natural materials. It may be sand or gravel produced by natural disintegration of rock, or it may be manufactured by passing rock through a series of crushers. Aggregate sizes are designated in terms of lower (d) and upper (D) sieve sizes expressed as d/D.

Codes and Standards for aggregates are covered under I.S. EN 12620 Aggregates for Concrete and NSAI guidance on the use of I.S. EN 1260 is given under S.R. 16. The standard outlines the requirements for aggregates to be used in concrete production. The general requirement for aggregates is that they are clean, hard, durable and always derived from sources of proven quality and consistency. Concreting aggregates may be classified as coarse or fine. A coarse aggregate is the designation given to the larger aggregate sizes with (D) greater than or equal to 4mm and (d) greater than or equal to 2mm.

The gradation of aggregates influences the amount of paste necessary to fill the voids between aggregate particles. Ideal gradations which will give optimum results under all conditions do not exist, but guidance curves or grading envelopes based on experience are a valuable aid as a basis for trial gradations. In general, continuous gradations containing all particle sizes are recommended. Gap gradings, in which one or more size fractions are lacking, have been used with considerable success, but such gradations are very sensitive to fluctuations in grading and use should be strictly controlled. For given aggregates, gradation is usually controlled by varying the percentage of sand in the total aggregate. Good mix design always uses the lowest sand percentage compatible with adequate placeability and consistence (workability), because the minimum water requirement and the maximum strength will result for given aggregates when the largest possible quantity of coarse aggregate is used.

The maximum size of aggregate, influences the unit water requirement, the optimums and percentage and the optimum paste content, all of which decrease with increasing maximum size of aggregate. The largest particle size compatible with placing requirements should, therefore always be used.

The particle shape of aggregates influences the properties of the mix, i.e. angular coarse aggregates require a higher percentage of sand (and consequently more water) than do rounded aggregates, in concretes of equal consistence. However, compensating factors may be involved, and a well shaped, well graded, angular coarse aggregate and a comparable rounded coarse aggregate will usually both produce concrete of about the same compressive strength for the same cement factor, in spite of the higher W/C ratio in the former, because of a natural interlocking action in the angular material. Flat and/or elongated particles should, however, be avoided both in sand and in coarse aggregates, since they tend to give harsh, unworkable mixes. The particle shape and grading of sand has a far more pronounced effect on the water requirement and workability of concrete than has the particle shape of coarse aggregate. Sands, whether natural or manufactured, should always be well graded, and well shaped.

Water Cement Ratio

The relationship between W/C ratio and strength of hardened concrete is expressed by Abrarn's W/C ratio law, which states that, all other factors being equal, the strength of concrete varies as an inverse function of the W/C ratio. This may also be expressed as: the greater the dilution of the paste, the lower the strength of the hardened concrete.

Consistence of Fresh Concrete

The required consistence (workability) of a mix is established by the requirements of placing, and the lowest consistence compatible with these requirements should always be used. On the other hand, it is essential that consistyence which is too low for the case in question be avoided, as their use will lead to incomplete compaction. Consistence is generally expressed in terms of slump, although this is in reality a measure of consistency only. When the slump test is used, an evaluation of consistence may be made by noting the way in which the cone reacts when tapped on the side.

Admixtures

Admixtures are sometimes used in order to improve certain qualities in a concrete mix. The most frequently used are: air entraining agents, plasticizers, and retarders.

Air Entraining Agents: Entrained air reduces the water requirement of concrete mixes and improves the consistence and cohesiveness of fresh concrete. It also improves the frost resistance and general durability of hardened concrete, and its use is therefore recommended for all concrete which will be exposed to destructive weathering agencies.

Plasticizers: The water content can be reduced while maintaining the same cement content and consistence; this results in a reduced water / cement ratio by about 10%. and therefore increased strength and improved durability.

Retarders: The length of time concrete remains workable depends on its temperature, consistence class, water / cement ratio and on the amount of retarder used. While the early strength of concrete is reduced by using a retarder which may affect formwork striking times, the 7 and 28 day strengths are not likely to be significantly affected.

Note: Trial mixes using the cement, aggregates and admixtures in question, in the proportions to be used in the field, should always be made in order to evaluate the effect of the admixture.

IS EN 206-1 New Terminology

On the 1st December 2003, the Irish Standard for Concrete IS326 Part 2 (1) was replaced by the new European Standard IS EN 206 -1. Below are some of the new terms which were introduced as part of the standard. Section 3 of the standard has a fuller list of terms and definitions

Additions: This is the term for constituent materials, such as fly ash, ground granulated blastfurnace slag, silica fume etc., that are added to the concrete mixer.
Chloride Class: The way of expressing the maximum chloride content of concrete. For example, a chloride class of Cl 0,40 means a maximum chloride content of 0.40% by mass of cement.
Comma: IS EN 206-1 uses a ‘comma’ where we would expect to see a decimal point. Where a comma has been used in a class notation (e.g. Cl 0,40) the comma has been retained.
Compressive Strength Class: A more complex way of expressing the ‘grade’ of concrete using letters (‘C’ for normal weight and heavy weight concrete and ‘LC’ for light weight concrete) followed by the minimum characteristic strength of a 150mm diameter by 300mm cylinder, a slash, and the minimum characteristic cube strength e.g. C40/50
Concrete: A specifier specifies ‘concrete’ and a producer designs a ‘mix’ that satisfies the all the specified requirements for the concrete.
Conformity: Tests and procedures undertaken by the producer to verify the claims made on the delivery ticket. This replaces the compliance testing procedures in IS 326 or BS 5328.
Consistence = Workability
Consistence Class: A recommended alternative to specifying consistence by a target value.
Designed Concrete: Called designed mix in IS 326
Established Suitability: The concept of established suitability allows materials and procedures to be used on a national basis that are not currently covered by European standards, but have a satisfactory history of local use.
Execution: Workmanship
Fly Ash: Pulverised-fuel ash (pfa)
Identity Testing: Acceptance of testing in all but name. It ‘identifies’ whether a particular batch or batches of concrete come from a conforming population.
Intended Working Life: Period of time that a properly maintained structure is required to be serviceable and durable.
Minimum Cover to Reinforcement: Cover to reinforcement assumed to be achieved when determining the concrete quality.
Mix: A composition that satisfies all the requirements specified for the concrete. Different producers may have different mixes, all of which satisfy the concrete specification.
Nominal Cover: Cover to reinforcement shown on the drawings equal to the minimum cover plus a tolerance (margin) for fixing precision.
Prescribed Concrete: Called ‘prescribed mix’ in IS 326 (see ‘Concrete’ and ‘Mix’).
Recycled Aggregates: Aggregates resulting from the reprocessing of inorganic material previously used in construction. A sub-set of this is ‘recycled concrete aggregate’, which is mostly crushed concrete.
Specification: Final compilation of documented technical requirements, in terms of performance or composition, given to the producer by the specifier.
Specifier: Term reserved for the person or body who passes the specification to the producer.
Standardised Prescribed Concrete: Called ‘Standard mix’ in IS 326 (see ‘Concrete’ and ‘Mix’. The new term correctly identifies the type of concrete and avoids the misunderstanding caused when ‘standard’ is taken to mean ‘normal’.
User: Person or body using fresh concrete

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