http://www.cembeton.hu

Hungarian Cement Concrete and Lime Association
Magyar Cement-, Beton- és Mészipari Szövetség

Admixture

Definitions
Reuirements
Admixtures with single effects
Admixtures with multiple effects
Other admixtures
Compatibility of admixtures

According to the Hungarian National Application Document (MSZ 4798) based on the European Concrete Standard (EN 206) admixtures for concrete shall be considered according to the European Concrete Admixture Standard (EN 934-2).

This new European standard is applied in case of several types of concrete, such as concrete created on site, transportable concrete or pre-manufactured concrete. The performance requirements apply to admixtures used in concrete of conventional consistency. It should not be applied to admixtures used for other types of concrete.

Definitions

Performance: The ability of the admixture to be effective for the intended purpose without any adverse effects.

Appropriate dosage: The dosage of the admixture in percentage by weight of the cement, alignes with the manufacturer's instructions, in accordance with the general and additional requirements for admixtures. The correct dosage is within the recommended dosing range.

Recommended dosage range: Product dosage based on on-site experience  within the recommend range by the manufacturer, expressed as a percentage by weight of the cement. Note: Applying the recommended dosage range does not mean that the general and additional requirements for admixtures are met in the full range. Substances used on site should be used to determine compatibility tests (test mixes) to determine the desired result.

Maximum recommended dose: The upper limit of the recommended dosage range.

Reference concrete and mortar: Concrete and mortar according to EN 480-1 to check whether admixtures comply with the general and additional requirements for admixtures.

Admixtures with multiple effects: Admixture that affects several properties of fresh and / or consolidated concrete with more than one main effect of the admixtures with single effects. 

Main effect: An effect of Admixtures with multiple effects, designated by the manufacturer.

Side effect: an effect existing beside ot the Main effect of the Admixture with multiple effects.

Concrete admixture: a material added with to the concrete during mixing, in a quantity less than 5 weight percent of the cement, wih the purpose of changing the properties of the fresh or consolidated mixture.

Admixtures are generally present compared to the others as chemicals with negligible weights and volumes. THese can change or improve the concrete's properties such as:

  • properties of fresh concrete mixtures (eg, plasticity, pumpability),
  • the binding or consolidation process (e.g., accelerating or decelerating),
  • the properties of consolidated concrete (eg increase its frost resistance), etc.
Back to the top

Requirements

There are general and additional requirements established for admixtures.

General requirements
General requirements contain regulation applied to all types of admixtures (steadiness, color, effective ingredients, relative density of liquid admixtures, typical dry matter content, pH, effect on binding at the maximum recommended dosage, total chlorine content, water soluble chloride content, alkali content and corrosion behavior). These requirements assume that the distribution of the admixture in the concrete is proportionate. Particular attention should be paid to the admixture powders with retarding effects

Additional requirements

Additional requirements contains mroe requirements in case of specific admixtures.

Back to the top

Admixtures with single effect

Based on their main effect these admixtures can be classified as the following, and the below additional requirements are placed aginst them:

Plasticizing admixtures

The active ingredient of plasticizing admixtures is generally lingin sulphonate, and they also have a slight delaying effect as well. Plasticizing adminxtures can also have other side effects as formation of air bubbles. The effect and the solidifying atrribute of the mixture with these admixtures also depends on the type of cement used.

The usage of plasticizing admixtures allows to reduce the water content of the concrete miyture by at least 5 %, without affecting consistency. In this case, the compressive strength of the 7 and 28 days old concrete should by higher by 10 % at least. The air content of the fresh mixture can be up to 2 % higher.

Plasticizing admixtures can also increase the shrinking / deflection rate without increasing the water content, or they can also have both of the effects at the same time.

Superplasticizing admixtures

The active agent of superplasticizing admixtures are usually lignin, melamine, naphthalene sulphonate, acrylate and more recently polycarboxylate and mixtures of these. Certain superplasticizing admixtures may cause air bubble formation. With efficient superplasticizing admixtures, it is possible to produce concrete with continuous consistency even from earth-moist concrete. Concrete with the appropriate superplasticizing admixture can be easily applied, will not demix and water escape will not be identified. The effect of superplasticizing admixtures usually deteriorates faster than of Plasticizing admixtures, except for the new types with active ingredient of acrylate and polycarboxylate, so it is advisable to add the superplasticizing admixture only after arrival in case of concrete transported in mixer cars. The concrete prepared with the admixture can't be watered up later.  The effect and the solidifying atrribute of the mixture with these admixtures also depends on the type of cement used.

On one hand, superplasticizing admixtures allow a reduction of the water content of the mixture by at least 12%  without affecting consistency. In this case, the compressive strength of the 1 day old concrete should by higher by 40 % at least, and in case of the 40 day old concrete by 2 %. The air content of the fresh mixture can be up to 2 % higher.

Superplasticizing admixtures can also increase the shrinking rate with at least 120 mm and the deflection rate with at least 160 mm (and this effect can disappear only after 20 minutes), without increasing the water content, or they can also have both of the effects at the same time. The compressive strength can only be reduced by up to 10% at 28 days of age. The air content of fresh concrete can be up to 2% higher.

Superplasticizing admixtures can have both effects at the same time.

Stabilizing admixtures

Concrete created with stabilizing admixtures will not demix and water escape will not be identified - this can be reduced by 50 % in case of the mixing water. The compressive strength can only be reduced by up to 20% at 28 days of age. The air content of fresh concrete can be up to 2% higher.

Air-entraining admixtures

Air-entraining admixtures create small-sized, uniformly distributed and defined air bubbles during mixing with an average 50 mm in diameter and not greater than 300 mm. By providing an expansion space for the increasing volume of freezing water and by preventing water-repelling by interrupting the capillaries of concrete, air bubbles greatly improve the frost resistance of the concrete and the resistance to freeze-thawed salt.

However, the frost resistance of the concrete and its ability to resist melting salting does not only depend on the air bubble system but also on the water / cement factor. Therefore, the intake of air bubbles into fresh concrete alone is not enough. In practice, the air-entraining admixtures are usually combined with superplasticizing admixtures.

The air-entraining admixtures improve the workability of the mixture according to the "ball bearing principle" but at the same time they reduce the compressive strength of the solidified concrete. This can be avoided when combined with superplasticizing admixtures. The air bubble system created by the air bubble forming admixtures depends on many factors, so its use requires great care.

The air content of fresh concrete may be up to 2.5% higher, but may be 4-6% overall. The longitudinal factor of the solidified concrete is 0.200 mm, its compressive strength must not fall by more than 25%.

Bind-accelerating admixtures (antifreeze)

The binding process involves the solidification of cement paste until the silica gel is formed from the supersaturated solution. Chloride-containing (water-soluble chlorine content> 0.10% by weight) bind-accelerating admixtures promote the oxidation of steel inserts, therefore those not be used in reinforced concrete and stretched concrete structures. The inhibitors include materials containing anti-oxidation chemicals. The inhibitors - nitrites, chromates, bicromates - are water-soluble, can be removed from the concrete and thus the inhibitory effect can cease.

This group also includes some of the chemicals called "antifreeze" admixtures, but these are essentially not bind-accelerating admixtures (unless they have such side effects), their role is merely solving the blockage of C3A grains.

Bind-accelerating admixtures bring the time of transition from plastic to solid state at least 30 minutes earlier at 20 degrees Celsius, and at least 40% earlier of the a time at 5 degrees Celsius. The compressive strength of the solidified concrrete can only be reduced by up to 20% at 28 days of age, and this should be kept at 90 days of age. The air content of fresh concrete can be up to 2% higher.

Consolidation-accelerating admixtures (antifreeze)

During the solidification process, formation of solid compounds from the gel system is taking place and they gradually become to a crystalline structure. Chloride-containing (water-soluble chlorine content> 0.10% by weight) consolidation-accelerating admixtures promote the oxidation of steel inserts, therefore those not be used in reinforced concrete and stretched concrete structures. The inhibitors include materials containing anti-oxidation chemicals. The inhibitors - nitrites, chromates, bicromates - are water-soluble, can be removed from the concrete and thus the inhibitory effect can cease.

This group also includes some of the chemicals called "antifreeze" admixtures, but these are essentially not bind-accelerating admixtures (unless they have such side effects), their role is merely solving the blockage of C3A grains.

Consolidation-accelerating admixtures accelerate the consolidation time of the concrete wihout changing the binding time.  The compressive strength of the consolidated concrrete can should be higher with at least 20 %, and should not be decreased by more than 10% at 28 days of age. The compressive strength of the consolidated concrete at 5 degrees Celsius at 48 hours age should be at least 30% higher. The air content of fresh concrete can be up to 2% higher.

Retarding admixtures

The active ingredient of these admixtures are generally lignin sulphonate, sugar, starch and phosphate, and mixtures of these. They can delay the binding of the concrete by 4 to 40 hours and they also slow early consolidation. The strength of the concrete is usually reaching the strength of the concrete without this admixture at the age of 7 days, and then exceeds the strength of it. This concrete tends to have more shrinkage cracks as well. Therefore, it is important to have professional and thorough aftercare when using these admixtures.

Retarding admixtures are delaying the start of transitioning from plastic to solid state with at least 90 minutes, but the end can not exceed 360 minutes. The compressive strength of the consolidated concrete can should be higher with at least 20 %, and should not be decreased by more than 10% at 28 days of age.  The compressive strength of the consolidated concrete at 7 days age can be decreased by up to 20%, and by up to 10% at the age of 28 days. The air content of fresh concrete can be up to 2% higher.

Sealing admixtures

Sealing admixtures should be applied to concrete structures that are to be protected against capillary absorbing moisture. These admixtures increase the resistance to freeze and melting salt as well as improve the waterproofness of the concrete. THey are designed to protect building elements from rain, surface water, absorbing moisture, leaking or flowing water.

Sealing admixtures reduce the capillary water absorption of the solidified concrete by at least 50% by 7 days after 7 days of post-treatment. Capillary water absorption after 90 days post-treatment for 28 days should also be less than 50% by weight. The compressive strength of the solidified concrete can be reduced by up to 15% by 28 days of age. The air content of fresh concrete can be up to 2% higher.

Lap tetejére

Admixtures with multiple effects

Based on their main and side effects, these admixtures can be classified as the following, and the below additional requirements are placed aginst them:

Plasticizing admixture with retarding side effect

Admixtures with retarding side effect create the combined effect of the plasticizing admixtures (main effect) and the retarding admixtures (side effect). The compressive strength of the consolidated concrete should not be reduced at the age of 28 days. The transition of the mixture from plastic to solid state is initiated at least 90 minutes later in time but the end can not be more than 360 minutes. These admixtures allow to reduce the water content of the concrete mix by at least 5% without affecting consistency. The air content of fresh concrete can be up to 2% higher.

superplasticizing admixture with retarding side-effect

Admixtures with retarding side effect create the combined effect of the superplasticizing  admixtures (main effect) and the retarding admixtures (side effect).

Given a same consistency, the compressive strength of the solidified concrete should not be less by up to 15 % at the age of 7 days, and it should be 15% higher at the age of 28 days. The transition of the mixture from plastic to solid state is initiated at least 90 minutes later in time but the end can not be more than 360 minutes. These admixtures allow to reduce the water content of the concrete mixture at a rate of at least 12%. The air content of fresh concrete can be up to 2% higher.

Given a same cement/water ratio, the compressive strength of the consolidated concrete can be smaller by up to 10% in 28 days age. These admixtures can also increase the shrinking / deflection rate without increasing the water content, and this effect can only be discontinued after 60 minutes. The air content of fresh concrete can be up to 2% higher.

Plasticizing admixture with bind-acceleraing side effect

Admixtures with bind-accelerating side effect create the combined effect of the plasticizing admixtures (main effect) and the bind-accelerating admixtures (side effect). The compressive strength of the consolidated concrete should not be reduced at the age of 28 days. The transition of the mixture from plastic to solid state is started at least 30 minutes earlier at 20 degrees Celscuis and at least 40 % sooner at 5 degrees Celsius. These admixtures allow to reduce the water content of the concrete mix by at least 5% without affecting consistency. The air content of fresh concrete can be up to 2% higher.

Lap tetejére

Other admixtures

In addition to the listed above, there are additional admixtures used with concrete as well. Use them always according to the instructions given by the manufacturer in the area of use they propose.

Such concrete admixtures - without limitation - are intended to protect against corrosion, improve surface quality, reduce shrinkage, molding, foaming, undercoating, etc. This area is developing dynamically so it can be expected that new admixtures appear. 

Admixtures can also be used with mortars. The most important mortar admixtures are: plasticizing, air-entraining, sealing, delaying, accelerating, injection aiding, transport mortar additives, etc.

 

Lap tetejére

Compatibility of admixtures

Regarding compatibility, one should distuingish compatibility and mixability of the admixtures from each other. Combinability means that admixtures can be both added to the concrete combined but separated from each other,  even though they may not be mixed together. Mixable admixtures may also be meet in liquid (or powder) form, mixed without precipitation, in any order or in combination with the concrete. Admixtures are compatible with each other if, together with the manufacturer's statement, they do not detract from each other's effects or produce additional effects that would not be known separately. Use only the same manufacturer's admixtures in a concrete.

CEMKUT Betonujsag.hu Cembeton Update Beton.hu

Hírlevelek