Concrete Admixtures

Concrete Admixtures

Concrete admixtures are liquids or powders which are added to the concrete during mixing in small quantities, normally based on the cement content. They influence the properties of the fresh and/or hardened concrete chemically and/or physically.

The standard includes the following under “Special Terms” (slightly abbreviated):

Admixtures – definitions and effects

·     Water reducer

Enables the water content of a given concrete mix to be reduced without affecting the consistence, or increases the workability without changing the water content, or achieves both effects.

·     Super plasticizer

Enables the water content of a given concrete mix to be greatly reduced without affecting the consistence, or greatly increases the workability without changing the water content, or achieves both effects.

·     Stabilizer

Reduces mixing water bleeding in the fresh concrete.

·     Air entrainer

Introduces a specific quantity of small, evenly distributed air voids during the mixing process which remain in the concrete after it hardens.

·     Set accelerator

Reduces the time to initial set, with an increase in initial strength.

·     Hardening accelerator

Accelerates the initial strength with or without an effect on the setting time.

·     Retarder

Retards the time to initial set and prolongs the consistence.

·     Water proofer

Reduces the capillary water absorption of the hardened concrete.

·     Retarder/water reducer

Has the combined effects of a water reducer (main effect) and a retarder (additional effect).

·     Retarder/super plasticizer

Has the combined effects of a super plasticizer (main effect) and a retarder

(additional effect).

·     Set accelerator/water reducer

Has the combined effects of a water reducer (main effect) and a set accelerator (additional effect).

Physical requirements for aggregates

Physical requirements for aggregates

Aggregates into categories covering:

·     Resistance to splitting

·     Resistance to wear

·     Resistance to polishing and abrasion

·     Particle density and water absorption

·     Bulk density

·     Durability

Durability

This is mainly associated with the frost and freeze/thaw resistance of coarse aggregates, which must be adequate for the specified purpose and must be verified if necessary.

Alternative aggregates (recycled material)

Large natural gravel and sand deposits are often valuable, non-renewable resources. It is becoming increasingly impossible to obtain and use gravel from these natural areas.

Possible substitutes are:

·     Crushing and processing of old concrete to form concrete granules

·     Reuse of micro fines from concrete wash water installations The suitability of recycled material should preferably be checked in every case.

Practical information of aggregate

Practical information of aggregate

·     Optimum grain shape, crushed/round

Cubic/spherical shapes have proved more suitable than linear forms, which can affect the consistence. Crushed aggregate has a slightly higher water requirement for the same consistence because of its larger specific surface area, but higher concrete compressive and particularly tensile strengths can be obtained due to better interlocking.

·     Predominantly crushed aggregates

The surface of crushed materials from rock, large blocks etc. consists only of broken surfaces, while the surface of crushed round material also includes natural rounded areas.

 Crushed rock material is now mainly used in tunneling, the motto being:

“Extraction point = installation point”.

Quarry sands

These are angular and also longish or flattish depending on the rock. They are not conducive to a good consistence and their water requirement is generally higher.

Harmful contaminants

Loam, humus, marl, clay, gypsum and aggregates containing sulphates, chlorides and alkalis are all potentially harmful and their presence and possible consequences must be clarified.

Standard aggregates

Standard aggregates

In Europe aggregates are defined in standard EN 12620. This standard is very comprehensive and to give more details than in the list below would be outside the scope of this document.

Important terms from the standard (with additional notes)

·     Natural aggregate

Comes from mineral deposits; it only undergoes mechanical preparation and/or washing.

·     Aggregate mix

Aggregate consisting of a mixture of coarse and fine aggregates (sand). An aggregate mix can be produced without prior separation into coarse and fine aggregates or by combining coarse and fine aggregates (sand).

·     Recycled aggregate

Aggregate made from mechanically processed inorganic material previously used as a building material (i.e. concrete).

·     Filler (rock flour)

Aggregate predominantly passing the 0.063 mm sieve, which is added to obtain specific properties.

·     Particle size group

Designation of an aggregate by lower (d) and upper (D) sieve size, ex-pressed as d/D.

·     Fine aggregate (sand)

Designation for smaller size fractions with D not greater than 4 mm. Fine aggregates can be produced by natural breakdown of rock or gravel and/or crushing of rock or gravel, or by the processing of industrially produced minerals.

·     Coarse aggregate

Designation for larger size fractions with D not less than 4 mm and d not less than 2 mm.

·     Naturally formed aggregate 0/8 mm

Designation for natural aggregate of glacial or fluvial origin with D not greater than 8 mm (can also be produced by mixing processed aggregates).

·     Fines

Proportion of an aggregate passing the 0.063 sieve.

·     Granulometric composition

Particle size distribution, expressed as the passing fraction in percent by weight through a defined number of sieves.

Concrete Aggregates

Concrete Aggregates

Gravels, stone and sands form the granular structure, which must have its voids filled as completely as possible by the binder glue. They make up approximately 80% of the weight and 70–75% of the volume. Optimum use of the aggregate size and quality improves the concrete quality. Aggregates can occur naturally (fluvial or glacial); for high quality concrete they are cleaned and graded in industrial facilities by mechanical processes such as mixing together, crushing, screening and washing (mechanical preparation).

Suitable as concrete aggregates are materials which do not interfere with the cement hardening, have a strong enough bond with the hardened cement paste and do not put the resistance of the concrete at risk.

Standard and special aggregates

Standard aggregates	Density2.2–3 kg/dm³	From natural deposits, e.g. river gravel, moraine gravel etc. Material rounded or crushed(e.g. excavated tunnel)
Heavyweight aggregates	Density> 3.0 kg/dm³	Such as barytes, iron ore, steel granulate. For the production of heavy concrete (e.g. radiation shielding concrete)
Lightweight aggregates	Density< 2.0 kg/dm³	Such as expanded clay, pumice, polystyrene. For lightweight concrete, insulating concretes
Hard aggregates	Density> 2.0 kg/dm³	Such as quartz, carborundum; e.g. for the production of granolithic concrete surfacing
Recycled granulates	Density approx. 2.4 kg/dm³	From crushed old concrete etc.