SHRINKAGE REDUCING CONCRETE ADMIXTURE

SHRINKAGE REDUCING CONCRETE ADMIXTURE

Shrinkage-reducing concrete admixture promote expansion of the concrete at about the same volume that normal drying shrinkage is contracting it. The net change in length of the hardened concrete is small enough to prevent shrinkage cracks. The typical materials used for shrinkage compensation in concrete are based on calcium sulfo-aluminate or calcium aluminate and calcium oxide.

Shrinkage reducing admixtures when added to concrete during the batching stage, can significantly reduce both the early and long term drying shrinkage. This is achieved by treating the ‘cause’ of drying shrinkage within the capillaries and pores of the cement paste.

This type of admixture should not be confused with shrinkage compensating materials which are normally added at above 5% on cement and function by creating an expansive reaction within the cement paste to treating the ‘effects’ of drying shrinkage.

Fig: Shrinkage in Hardened Concrete

Shrinkage reducing admixtures can be used in situations where shrinkage cracking could lead to durability problems or where large numbers of shrinkage joints are undesirable for economic or technical reasons.

In floor slabs, the joint spacing can be increased from 5- 6m. to 10- 15m.due to the reduced movement of the concrete during drying. The risk of the slab curling at joints and/or edges is also significantly reduced.

Where new concrete is used to strengthen or repair existing structures, shrinkage reducing admixtures can reduce the risk of cracking in what can be a highly restrained environment.

Some losses in properties are typical with the introduction of these anti-shrinkage agents. Any ill-effects on strength are minimized by the use of High Rate Water Reducers, which provide good workability while allowing reduction of the water content.

These admixtures can be used to great advantage in slabs, bridge decks, structures, and repair work where cracking can lead to steel reinforcement corrosion problems, but maintaining effective air entrainment for resistance to freezing and thawing damage can be difficult with shrinkage-reducing admixtures.

Usage rates vary with batch designs and water content but typically range from 8 to 25%. ACI 223 (Standard Practice for the Use of Shrinkage-Compensating Concrete) specifies the best methods for utilization of this admixture.

Materials of Shrinkage Reducing Concrete Admixtures:

Shrinkage reducing admixtures are mainly based on ethylene glycol derivatives.

These organic liquids are totally different to most other admixtures, which are water based solutions. The admixtures are normally 100% active and are water soluble. They have a characteristic odor and a specific gravity of less than 1.00.

Fig: Shrinkage reducing concrete admixtures

Mechanism of Shrinkage Reducing Concrete Admixtures:

The mechanism by which shrinkage reducing admixtures operate is unique. When excess water begins to evaporate from the concrete’s surface after placing, compacting, finishing and curing, an air/water interface or “meniscus” is set up within the capillaries or pores of the cement paste of the concrete. Because water has a very high surface tension, this causes a stress to be exerted on the internal walls of the capillaries or pores where the meniscus has formed. This stress is in the form of an inward pulling force that tends to close up the capillary or pore. Thus the volume of the capillary is reduced leading to shrinkage of the cement paste around the aggregates, leading to an overall reduction in volume.

The shrinkage reducing admixtures operates by interfering with the surface chemistry of the air/water interface within the capillary or pore, reducing surface tension effects and consequently reducing the shrinkage as water evaporates from within the concrete.

TYPES OF CONCRETE ADMIXTURE

TYPES OF CONCRETE ADMIXTURE

Admixtures are defined as the material other than the aggregate, water and cement added to the concrete. The use of suitable admixture can result in the enhancement the performance of concrete.

Types of Admixture

Air Entraining Agents

Air entraining agents are used primarily to increase the resistance of freezing and thawing. They also supply greater resistance to deicing chemicals, improve workability, lower water demands for a particular slump, reduce the amount of fine aggregate needs in the mixture, reduce segregation and bleeding, increase durability, and improve pump ability. Even though air entrainments have no effect on drying shrinkage, they do create a slight loss in the concrete strength.

Water Reducing Agents

A certain amount of the water is added to concrete to make the concrete place able and to ignite the hydration reaction of Portland cement. Over 50% of the water have no useful effect and have direct results in drying shrinkage, durability, and the strength of concrete. Water reducing agents added to the concrete reduce the water demands of the mix, maintain the workability, increase the strength, reduce the cost, reduce bleeding, reduce segregation, reduce honeycombing, reduce cracking and permeability, increase bond strength of concrete to steel, and reduce drying shrinkage. An average of 12-15 % reduction in the total water content will attain this goal.

Set Retarding Agents

These agents are primarily used for delaying the setting time of concrete. Set retards are useful, but are not the solution to poorly designed mixes, inferior materials, or low cement factors. The set retarder must be uniformly dispersed throughout the whole mix. Failure to do so will result in the admixture remaining in voids, causing some areas to remain plastic. This leads to plastic shrinkage.

Accelerating Agents

Accelerating agents are used to shorten the setting time and increase the early strength of the concrete. These agents are mainly used for concrete needing to be used immediately.

Calcium chloride

Calcium chloride is generally used in cold weather to hasten the setting time and produces an early finish of the concrete. Calcium chloride can effect the characteristics of concrete causing temperature rise, increased internal stresses, corrosion of unprotected reinforcement, a decrease in the resistance to freeze, an increase in the attack of sulfates, and an increase in the amount of drying shrinkage between 10 to 50 %.

VISCOSITY MODIFYING ADMIXTURES IN CONCRETE

VISCOSITY MODIFYING ADMIXTURES IN CONCRETE

VISCOSITY MODIFYING CONCRETE ADMIXTURES

Viscosity modifying concrete admixtures (VMAs) is a family of admixtures designed for specific applications. The European Federation for Specialist Construction Chemicals and Concrete Systems cites the following uses:

• Reduce segregation in highly flowable/self compacting concrete.
• Reduce washout in underwater concrete.
• Reduce friction and pressure in pumped concrete.
• Compensating for poor aggregate grading, especially a lack of fines in the sand.
• Reducing powder content in self compacting concrete.
• Compensating for poor aggregate grading, especially a lack of fines in the sand.
• Reducing powder content in self compacting concrete.
• Reduce bleeding in concrete.
• Improve green strength in semi-dry concrete.

Self-consolidating high-strength concrete mixtures are frequently produced using high range water-reducing admixtures in conjunction with viscosity modifying admixtures, such as cellulose ether, welan or diutan gum. Some VMAs are based on inorganic materials such as colloidal silica, which is amorphous with small insoluble, non-diffusible particles, larger than molecules but small enough to remain suspended in water without settling. By ionic interaction of the silica and calcium from the cement a three dimensional gel is formed which increases the viscosity and / or yield points of the paste. This three dimensional structure/gel contributes to the control of the rheology of the mix, improving the uniform distribution and suspension of the aggregate particles and so reducing any tendency to bleeding, segregation and settlement.

Most Viscosity Modifying Admixtures are supplied as a powder blend or are dispersed in a liquid to make dosing easier and improve dosing accuracy. They have little effect on other concrete properties in either the fresh or hardened state but some, if used at high dosage, can affect setting time and or the content stability of entrained air.

STANDARD CODES FOR CONCRETE ADMIXTURES

STANDARD CODES FOR CONCRETE ADMIXTURES

Standard Codes for concrete admixtures:

Admixtures used should conform to the following relevant  standard codes.

In reinforced concrete, the chloride ion of any admixture used should not exceed 2% by weight  of the admixture as determined in accordance with IS:6925 and the total chloride ion in all admixtures used in concrete mi shall not exceed 0.83% by weight of cement. The addition of calcium chloride to concrete containing embedded metal will not be permitted under any circumstance.

Accelerating concrete Admixtures conforming to IS:9103 & 2645 should be used. It is in liquid state with a specific gravity of 1.3 and complying with ASTM C-494 type E. It accelerates the setting and hardening of the concrete mix thereby achieving high early ago strength. Accelerating concrete admixture should be compatible with all types of cement.

Retarding concrete Admixtures conforming to IS:9103. It is in liquid state with a specific gravity of 1.22 and complying with ASTM C-494 type B&D, CRD-C87 type B & D, BS:5075part 1. It should be added to the concrete mix during the mixing process along with water and aggregates. No extension of normal mixing time is necessary. It will delay the initial and final setting time and helps to spread the heat of hydration over a longer period of time. It gives a highly workable concrete with a low W/C ratio. It must be compatible with all types of cement depending on requirement.

Air-entraining Admixtures conforming to IS:9103.

Water-reducing admixtures conforming to IS:9103.

Integral waterproofing admixtures conforming to IS:2645. It is used as an excellent cement admixture in all types of concrete/ plaster mortars, pointing mortars, masonry works, guniting works and pressure grouting works. It improves  resistance of the surface to weathering and chemical attacks. It should not be toxic so as to use for waterproofing in water tanks, reservoirs, tanks, etc.

FACTORS AFFECTING CONCRETE ADMIXTURES PERFORMANCE

FACTORS AFFECTING CONCRETE ADMIXTURES PERFORMANCE

The various factors which affect the performance of concrete admixtures are:

1. Type of super-plasticizer:

The admixture will be more effective if molecular weight of the super-plasticizer is high.

2. Dosage:

The quantity of admixture should be optimum. Excess of admixture may cause segregation or bleeding. It may also cause excessive retardation. The optimum does should be estimated by trials.

3. Compatibility with Cement:

All admixtures may not produce same results with different cements. Therefore before using any admixture, its compatibility with cement has to be established. Properties of cement like fineness, chemical-composition, C3A content etc. affect the performance of admixture. Therefore, trials have to be made before finalizing an optimum does of admixture.

4. Mix Design:

All constituents of mix affect the performance of the super-plasticizer as given below:

• Water: more water in the mix improves the physical interaction and dispersion of admixtures.
• Coarse aggregate: proportioning and grading of coarse aggregates influence the performance of concrete admixture.
• Fine aggregate: proportioning, grading and silt content also influence the performance of concrete admixture.
• Cement: its fineness, C3A content influence the performance of admixture. Higher C3A reduces efficiency of admixture.
• Other admixture: presence of other admixtures also influences the performance of concrete admixtures.

Therefore, proper trials before actual use are very vital for effectiveness of admixture.

Other factors admixture performance:

Certain other factors like temperature and humidity at the time of concreting also affect the performance of the concrete admixtures.

Drum mixtures are considered ideal for mixing admixtures, instead we should use pan or compulsive shaft mixes.

RETARDING WATER-REDUCING CONCRETE ADMIXTURE

RETARDING WATER-REDUCING CONCRETE ADMIXTURE

Retarding water-reducing admixtures are chemicals that slow down the initial reaction between cement and water by reducing the rate of water penetration to the cement and slowing down the growth of the hydration products. The concrete therefore stays workable longer than it would otherwise.

The length of time during which a concrete remains workable depends on its temperature, consistence class, and water-cement ratio, and on the amount of retarder used. Although the occasions justifying the use of retarders in the UK and US are limited, these admixtures may be helpful when one or more of the following conditions apply:

• In warm weather, when the ambient temperature is higher than about 20^oC, to prevent early stiffening (going-off) and loss of workability, which would otherwise make placing and finishing difficult.
• When a large pour of concrete will take several hours and must be considered without already placed concrete hardening before subsequent concrete is merged with it (i.e. without a cold joint).
• When the complexity of slip-forming demands a slow rate of rise.
• When there is a delay of half an hour or more between mixing and placing –for example, when ready mixed concrete is being used and when there may be traffic delays and/or long hauls. This can be seriously aggravated during hot weather, especially if the concrete has a high cement content.

The amount of retardation can be varied – usually upto about four to six hours – by altering the dosage, but longer delays can be obtained for special purposes.

While the early strength of concrete is reduced by using a retarder, which may affect formwork striking times, the 7 and 28 day strength are not likely to be significantly affected.

Retarded concrete needs careful proportioning to minimize bleeding due to the longer period during which the concrete remains fresh.

CONCRETE CHEMICALS AND APPLICATIONS

CONCRETE CHEMICALS AND APPLICATIONS

Concrete Chemicals (Admixtures) and Applications

Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as workability, curing temperature range, set time or color. Some admixtures have been in use for a very long time in concrete construction, such as calcium chloride to provide a cold-weather setting concrete.

Based on their functions, admixtures can be classified into the following five major categories:

• Retarding admixtures
• Accelerating admixtures
• Super plasticizers
• Water reducing admixtures
• Air-entraining admixtures

Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction, damp proofing and coloring. The following paragraphs provides details on the above-mentioned categories of concrete admixtures.

Retarding Admixtures

Retarding admixtures slow down the hydration of cement, lengthening set time. Retarders are beneficially used in hot weather conditions in order to overcome accelerating effects of higher temperatures and large masses of concrete on concrete setting time. Because most retarders also act as water reducers, they are frequently called water-reducing retarders. As per chemical admixture classification by ASTM-ASTM C 494, type B is simply a retarding admixture, while type D is both retarding and water reducing, resulting in concrete with greater compressive strength because of the lower water-cement ratio.

Retarding admixtures consists of both organic and inorganic agents. Organic retardants include unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydrocarboxylic acids, and carbohydrates. Inorganic retardants include oxides of lead and zinc, phosphates, magnesium salts, fluorates and borates. As an example of a retardant’s effects on concrete properties, lignosulfate acids and hydroxylated carboxylic acids slow the initial setting time by at least an hour and no more than three hours when used at 65 to 100 degrees Fahrenheit. The concrete contractor, however, need not memorize these chemical-specific results. Given the specific job requirements and goals, the concrete supplier should offer appropriate admixtures and concrete mixes from which to choose.

Accelerating admixtures

Accelerators shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used accelerators cause an increase in the drying shrinkage of concrete.

Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain. It should meet the requirements of ASTM D 98. Excessive amounts of calcium chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and corrosion of reinforcement. In colder climates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete, which may ruin the concrete.

Super plasticizers

Super plasticizers, also known as plasticizers, include water-reducing admixtures. Compared to what is commonly referred to as a “water reducer” or “mid-range water reducer”, super plasticizers are “high-range water reducers”. High range water reducers are admixtures that allow large water reduction or greater flowability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially slowing set time or increasing air entrainment.

Each type of super plasticizer has defined ranges for the required quantities of concrete mix ingredients, along with the corresponding effects. They can maintain a specific consistency and workability at a greatly reduced amount of water. Dosages needed vary by the particular concrete mix and type of super plasticizer used. They can also produce a high strength concrete. As with most types of admixtures, super plasticizers can affect other concrete properties as well. The specific effects, however, should be found from the manufacturer or concrete supplier.

Water reducing admixtures

Water reducing admixtures require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content. They can have the side effect of changing initial set time. Water reducers are mostly used for hot weather concrete placing and to aid pumping. A water-reducer plasticizer, however, is a hygroscopic powder, which can entrain air into the concrete mix via its effect on water’s surface tension, thereby also, obtaining some of the benefits of air-entrainment (see below).

Air-entraining admixtures

Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing the water-cement ratio via improved workability (due to the air-entraining agent itself) or through the use of other appropriate admixtures. As always, admixtures should only be combined in a concrete mix by a competent professional because some of them can interact in undesirable ways.

Bonding admixtures

Bonding admixtures including addition of compounds and materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers, can be used to assist in bonding new / fresh concrete with old / set concrete.

Coloring agents have become more commonly used, especially for patios and walkways. Most are surface applied and often have the additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete. Integrally colored concrete is also available.

Waterproofing and damp proofing admixtures

Water proofing and damp proofing admixtures including soaps, butyl stearate, mineral oil and asphalt emulsions, are used to decrease the amount of water penetration into the larger pores of concrete. “Antifreeze” admixtures typically are accelerators used in very high doses, with a corresponding high price, to achieve a very fast set-time, though they do not have properties to protect against freezing on their own. However, in general, these are not used for residential work.