Chemical Admixtures for Concrete

WATER-REDUCING ADMIXTURES

Özge Andiç Çakır, PhD

Water-Reducers: Definition

An admixture that reduces the amount of mixing water for concrete for a given workability.

In other words; an admixture that increases he consistency of the concrete mix at a given water content without further addition of water.

What happens if water requirement of concrete decreases for given workability?

• Remember the famous Abram’s Law!

• Strength of concrete increases, durability may increase !

Three Ways of Utilizing Water Reducers

Keep strength constant. Is it possible to increase the workability?

Then keep the water/cement ratio constant and introduce a water reducing admixture.

The consistency and thus the workability of concrete containing admixture will be higher than that of control concrete.

So, what happens to strength?

Since W/C ratio of both concretes are equal, strength may not change!

What about Increasing Strength While Keeping the Same Workability?

Add water reducing agent, lower the water/cement ratio by decreasing the amount of water in the mixture.

This will increase strength.

Then it is possible to keep the workability constant compared to control mixture.

No change in any concrete property is desiredIs it possible to save cement?

Cement may be saved by using less cement and water without changing the W/C ratio.

Strength and workability will be similar to control concrete.

Saving cement would be helpful both for economy and decrease the heat of hydration.

The latter is favourable for massive structures and hot weather concreting.

As a Conclusion;

Water reducers are used for various purposes:

Chemistry

• Small amounts of organic materials (e.g. polymerized naphthalene formaldehyde sulfonate salts) were found to increase the fluidity of cement containing compositions.

• Most water reducers as well as most retarders are organic, water soluble products and many formulations of these admixtures are based on the same raw materials, such as lignosulfonates, hydroxycarboxylicacids, carbohydrates, etc.

• Materials such as glucose and hydroxylated polymers have been widely used in North America.

Classification

• Normal water-reducing admixtures,

• The accelerating water-reducing admixtures,

• The retarding water-reducing admixtures,

• The air-entraining water-reducing agents

Normal water-reducing admixtures

They have water reducing characteristics, but they do not significantly effect setting time.

The accelerating water-reducing admixtures

They possess similar water-reducing capability of the ‘normal’ category.

However, they give higher strengths during the earlier hydration period and their setting time is faster.

Fast setting times allow finishing operations to be carried out in a timely manner, particularly at lower temperatures.

The retarding water-reducing admixtures

They possess similar water-reducing capability of the ‘normal’ category.

However, they extend the period of time when the concrete is in the plastic state.

This means that the time available for transport, handling, placing and finishing is lengthened. Favorable for hot-weather concreting.

In fact, the vast majority (about 95%) of materials called retarders are actually retarding water-reducing admixtures.

Both water reducers and retarders are based on the same raw materials.

The retarding water-reducing admixtures

• They possess the ability to entrain microscopic air bubbles into the cement paste,

• They allow a reduction in the water–cement ratio greater than that would be obtained by the air entrainment itself.

Do water reducers always change setting time?

The setting times depend on the type of water reducer used.

In general, all water reducers tend to extend setting times with respect to the control concrete.

This is due to the lower hydration rate of cement during the first hours in the presence of a water reducer.

What about Acceleration of Setting Time?

• Accelerating admixtures slightly accelerate or do not modify setting times,

• Therefore the word accelerating should be employed to indicate early strength development, in place of setting times.

Compressive strength development of concrete containing various types of water-reducing admixture

Water Reduction; What percentage?

• Usually, according to standards the reduction of mixing water by the use of these admixtures must be at least 5%.

• However, commercial water reducers can reduce mixing water up to 10 to 15%.

• Novel chemical admixtures that reduce mixing water more than 12% without any delay in setting time or increase in air concent are called high range water reducers (superplasticisers).

More Chemistry

• The main compounds used in the manufacture of water reducers can be divided into four groups:

• Ca, Na or NH4 salts of lignosulfonic acids.

• Hydroxycarboxylic acids generally as Na, NH4

or triethanolamine salts.

• Carbohydrates.

• Other compounds.

How to Add Water Reducers into Concrete?

1st Method:

Mix dry aggregates and cement

Then add water + admixture.

This procedure generally does not give the best performance in terms of water reduction or increase in workability.

How to Add Water Reducers into Concrete?

2nd Method:

Add the admixture just at the end of the mixing time of aggregates, cement and total gauging water.

This method is best for the admixture to show its effect however there are some practical difficulties in obtaining constant workability of the mix and the uniform dispersion of the admixture.

How to Add Water Reducers into Concrete?

3rd Method:

A reasonable compromise (!) between first two methods

After an initial mixing period of 15 -30 seconds of aggregates, cement and a portion of the mixing water, the admixture dissolved in a part of the gauging water is introduced into the mix, and finally the remainder of the gauging water is added until the required workability is obtained.

Precautions

• Water reducers, like all other admixtures, should be used according to the dosage range recommended by the supplier.

• However, some trial concrete mixes should be made to find out the optimum dosage of the admixture for the specific conditions and raw materials. Particularly when temperature is not within the normal room temperature range, 10 to 30°C, some preliminary tests should be carried out with different dosages of water reducer for the maximum benefit.

Precautions• In some exceptional cases it may be possible that an

overdosage is used. A very long retardation in setting time as well as decrease of early compressive strength can be expected in such cases, particularly in cold weather. Concrete could remain in the plastic state for several days.

• In a concrete mix containing an overdosage of water reducer, if early strengths are not affected there is no risk of low strength at longer ages, provided an abnormal air entrainment has been avoided.

THE EFFECTS OF WATER-REDUCING ADMIXTURES ON THE WATER–CEMENT SYSTEM

Rheological Considerations

It is known that some of the properties of fresh concrete can be considered in terms of the rheological properties of the cement paste contained in the concrete.

A high water–cement ratio concrete will contain a paste content which is more fluid than that of a low water–cement ratio concrete.

A Viscometer

The ‘fluidity’ of the cement paste can be measured in rheological terms by the torque transmitted to a stationary ‘bob’ inside a revolving outer cylinder placed in a water–cement system as shown:

Shear stress – shear rate relationships

The shear stress measured at the stationary bob is plotted against the rate of applied shear.

For pastes of varying water–cement ratios, the results are obtained for readings taken of the shear stress as the shearing rate is increased.

Rheology

• Systems which give linear shear-stress–shear-rate relationships with an intercept on the shear stress axis are said to exhibit plastic flow, and the intercept value is known as the ‘yield stress’.

• The viscosity of the system is given by the slope of the line over its linear portion.

Rheology of Cement Paste

The properties concrete in the plastic state will be a function of many parameters such as aggregate types and shapes, cement contents and characteristics, etc., but it is useful to isolate the effect of the paste rheology where it can be stated that:

• The consistency or fluidity of the concrete will be a function of the viscosity of the cement paste.

• The cohesion of the concrete will be a function of the yield stress of the cement paste.

Effect of Water Reducers on the Rheology of Cement Paste

The effect of water-reducing admixtures on paste viscosity at

different addition levels

The effect of water-reducing admixtures on paste viscosity at

various water– cement ratios

How do they Effect?

In normal cement pastes where particles come into close contact with each other there is a tendency for cement pastes to form large ‘flocs’ due to the attractive Van der Waals forces holding particles together.

Water reducing admixtures are absorbed at the solid-water interfaceand they function by dispersing the cement particles.

How do they Effect?

Cement particles carry charges on their surfaces, which may be positive or negative or both.

The opposing charges on the adjacent particles exert electrostatic attractions causing particles to flocculate.

The molecules of water-reducing agents attach themselves to cement particles and either neutralize these charges or cause all surfaces to carry uniform charges of like sign.

Thus, particles repel each other and remain fully dispersed in the mix.

Effect on Concrete Properties Beneficial Effects

• Improved compressive strength because of reduced w/c ratio,

• Reduce permeability because of reduced w/c ratio

• Improve durability, because...

• Imrpove workability with constant water content. May increase rate of hydration during initial hydration,

• Lower required cement content providing economy and lower HOH

• Facilitate difficult placement and improve consolidation at constant water content..

Effect on Concrete Properties Detrimental Effects

• At constant slump, hydroxylated types generally increase bleeding, lignosulfates reduce bleeding,

• High loss of slump may happen,

• Can retard setting especially high dosages..

THE EFFECTS OF WATER-REDUCING ADMIXTURES ON FRESH CONCRETE PROPERTIES

Water Reduction

• Less mixing water is required in a concrete mix for a given slump when a water-reducing agent is added.

• Water reduction is desirable in a concrete mix, as an increase in strength or a reduction in shrinkage and cement content may result.

• Recommended dosages allow a decrease in mixing water of 5-15% for a specified workability.

Water Reduction

The actual water reduction depends on:(i) type, brand and dosage of admixture; (ii) addition procedure; (iii) workability level and water-cement ratio; (iv) type and brand of cement; (v) cement content; (vi) type of aggregate; (vii)type and amount of SCMs such as slag and fly

ash; (viii) air volume.

Which Type Affects Most?

• Water reduction:

sodium gluconate > glucose and lignosulfonate

• The effect of on water reduction depends on:

- the chemical composition

- concentration of the admixture

• Up to one level, the higher the dosage of the admixture, the higher the water reduction.

Water reduction by water-reducing agents as a function of workability

Water Reduction; Effect of Cement Type

Water reducers are more efficient in reducing the amount of mixing water for concretes with low-alkali and low-C3A cements than those containing high-alkali and high-C3A cements

Water Reduction; Effect of SCMs

• Water reducers are also effective in reducing mixing water requirements for concretes containing fly ash as an addition or a replacement for part of the cement.

• A higher dosage of admixture should be added to concretes containing fly ash in order to obtain the same water reduction as that of plain mixes without fly ash. This could be due to a higher sorption of water reducers by fly ash particles.

Water Reduction; Effect of Admixture Type

• With a combined addition of air-entraining agent and water reducer, increased water reduction is obtained when air volume is increased.

Workability

Workability of concrete is improved when water reducers are incorporated in the mixture at a given water content.

The effectiveness of water reducers on workability;

gluconate > glucose > lignosulfonate

Workability

For a given slump, the admixture-treated concrete with reduced water is generally more workable.

Slump Loss

• Workability of concrete can be measured in terms of the slump.

• Once the required workability of the concrete has been attained there will be progressive loss of workability with time as the hydration proceeds.

• This process continues through the mixing, discharging, handling, placing, vibrating and finishing and workability loss can affect any or all of these steps.

• The loss of workability is generally more for mixes containing water-reducing admixtures.

Slump Loss

In this study, all mixes were designed to initial slump of 10 cm and had a cement content of 300 kg/m3.

An increase in the dosage apparently reduces the slump loss as shown:

How to compensate slump loss

• Water-reducing admixtures may be used to increase the initial workability, so that the initial rate of the slump loss is compensated for.

• This will prolong the time available for the transporting, handling and placing of concrete.

• Even water reducers are used to produce concrete of normal workability, it is generally found that the increased slump loss would cause no problems in normal concrete production.

• In these cases the amount of water required to correct the loss of slump is reduced in the presence of a water-reducing admixture.

Bleeding and Settlement

• Water reducers and retarders affect the rate and capacity of fresh concretes to bleed and settle under the influence of gravity.

• Water reducers based on lignosulfonate and particularly glucose reduce bleeding for a given slump value.

• Conversely, commercial admixtures based on hydroxy acids and their salts increase the rate and capacity of fresh concretes to bleed.

Air Entrainment

Some commercial water reducers at normal dosages cause entrainment of 2-3% air.

At higher dosages even higher values up to 7 to 8%air is entrained, particularly at lower temperatures.

When a certain amount of entrained air is required to improve durability in freezing climates, and the water reducer does not entrain sufficient air for frost resistance, an air-entraining agent must be added.

Finishing Characteristics

• In the presence of water-reducing and retarding admixtures, concrete may remain plastic enough to be vibrated several hours after placing, thus reducing the number of air pockets and cold joints.

SETTING CHARACTERISTICS

Water-Reducing and Retarding Admixtures

• They cause a delay in initial and final setting times.

• The specific retardation with a particular cement can be determined only by trial runs.

• The evaluation of setting times with water reducing and retarding admixtures for concrete should not be based on tests of cement paste. Because, generallycement pastes show longer setting times than those of concrete mixes.

• Many factors may affect the setting time of retarded concrete in the field, the most important of which is the amount of the mixing water and ambient temperature.

Water-Reducing and Retarding Admixtures

• The dosage of admixture recommended by the manufacturer should be used unless specific retardation is required.

• Overdosage may cause excessive retarding effect requiring longer curing times. However, an accidental overdosage of proven admixtures does not damage strength at longer ages, provided that the surface is properly cured and forms are not removed until sufficient strength level has been obtained.

Water Reducers

• Commercial water reducers may contain some chemical products that accelerate cement hydration and counteract the retarding effect of the main water-reducing components, so that the setting time of the concrete is approximately within ±1 h of the control mix.

• A delayed addition of the admixture causes a retarding effect which can be reduced by decreasing the dosage of the water reducer.

Water-Reducing and Accelerating Admixtures

• A certain amount of accelerator to counter the retarding effect of water-reducing components is generally present in these types of admixtures.

• On many jobs, such as conduits and tunnel lining, the level of strength attained within 12 or 24 hr is important for stripping the forms earlier than 1 day after placement.

• Thus acceleration in setting times is needed, as concrete must be strong enough in about 12 hr to allow removal of forms. Also for advancing the finishing operations of concrete slabs, a water-reducing and accelerating admixture can be useful particularly in cold weather.

Heat of Hydration

• Water-reducing agents delay the time of rapid heat evolution in cement hydration.

• A decrease in initial heat release, using retarders or water reducers, can be useful to eliminate some cracking problems, particularly in hot weather.

• On the other hand, water-reducing and accelerating admixtures can be advantageously used to increase the initial heat development in cold weather.

EFFECT OF WATER REDUCERS/RETARDERS ON HARDENED CONCRETE

Permeability

The permeability of concrete is a guide to its durability.

Extreme permeability is usually due to continuous passages in the concrete, due to poor compaction or cracks. This can be minimized by the use of water-reducing admixtures to give increased workability whilst maintaining a constant water–cement ratio.

Water-reducing agents can be used to reduce the water– cement ratio, so ensuring that the permeability is kept to a minimum.

Compressive strength

The compressive strength at 28 days of concrete containing water-reducing admixtures of the lignosulfonate, and hydroxycarboxylic acid types is a function of the water–cement ratio and conforms to Abram’s rule similar to concrete or cement paste which does not contain an admixture.

Compressive strength

Air-entraining water-reducing admixtures require special consideration; the presence of entrained air leads to a reduction in compressive strength, whilst the water reduction results in a compensatory increase in strength.

The effect can be quantified, however, by considering the amount of entrained air in terms of an equivalent volume of water to calculate the (air+water)/cement ratio.

Durability

• The durability of concrete is the ability of the material to maintain its structural integrity, protective capacity, and aesthetic qualities over a prolonged period of time.

• A decrease in the w/c ratio of concrete by the utilization of water reducers will lead to decreased permeability and increased durability.

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