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ACI 212-

Reported by ACI Committee 212

Report on Chemical Admixtures

for Concrete

Report on Chemical Admixtures for Concrete

First Printing

November 2010

ISBN 978-0-87031-402-

American Concrete Institute®

Advancing concrete knowledge

Copyright by the American Concrete Institute, Farmington Hills, MI. All rights reserved. This material

may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other

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concrete

Chapter 1—Introduction, p. 4 1—Introduction

Chapter 2—Definitions, p. 4 2—Definitions

Chapter 3—General information, p. 5 3—Sustainability 3—Admixture benefits 3—Specifications for admixtures 3—Sampling and testing

3—Cost effectiveness 3—Selection and evaluation 3—Proportioning and batching

Chapter 4—Air-entraining admixtures, p. 8 4—Introduction 4—Materials for air entrainment 4—Selection and evaluation 4—Applications 4—Proportioning concrete 4—Effects on fresh and hardening concrete 4—Effects on hardened concrete

ADMIXTURES, THEIR CHARACTERISTICS, AND USAGE

Admixture type Effects and benefits Materials Air-entraining (ASTM C and AASHTO M154)

Improve durability in freezing and thawing, deicer, sulfate, and alkali-reactive environments. Improve workability.

Salts of wood resins, some synthetic detergents, salts of sulfonated lignin, salts of petroleum acids, salts of proteinaceous material, fatty and resinous acids and their salts, tall oils and gum rosin salts, alkylbenzene sulfonates, salts of sulfonated hydrocarbons. Accelerating (ASTM C494/C494M and AASHTO M194, Type C or E)

Accelerate setting and early-strength development.

Calcium chloride (ASTM D98 and AASHTO M144), triethanolamine, sodium thiocyanate, sodium/calcium formate, sodium/calcium nitrite, calcium nitrate, aluminates, silicates. Water-reducing (ASTM C494/C494M and AASHTO M194, Type A)

Reduce water content at least 5%.

Lignosulfonic acids and their salts. Hydroxylated carboxylic acids and their salts. Polysaccharides, melamine polycondensation products, naphthalene polycondensation products, and polycarboxylates. Water-reducing and set- retarding (ASTM C494/C494M and AASHTO M194, Type D)

Reduce water content at least 5%. Delay set time. See water reducer, Type A (retarding component is added).

High-range water-reducing (ASTM C494/C494M and AASHTO M194, Type F or G)

Reduce water content by at least 12 to 40%, increase slump, decrease placing time, increase flowability of concrete, used in self-consolidating concrete (SCC).

Melamine sulfonate polycondensation products, naphthalene sulfonate polycondensation products, and polycarboxylates. Mid-range water-reducing (ASTM C494/C494M, Type A)

Reduce water content by between 5% and 10% without retardation of initial set.

Lignosulfonic acids and their salts. Polycarboxylates. Extended set control (hydration control) (ASTM C494/C494M, Type B or D)

Used to stop or severely retard the cement hydration process. Often used in wash water and in returned concrete for reuse, and can provide medium- to long-term set retardation for long hauls. Retain slump life in a more consistent manner than normal retarding admixtures.

Carboxylic acids. Phosphorus-containing organic acid salts.

Shrinkage-reducing Reduce drying shrinkage. Reductions of 30 to 50% can be achieved. Polyoxyalkylene alkyl ether glycol.

Corrosion-inhibiting (ASTM C1582/C1582M)

Significantly reduce the rate of steel corrosion and extend the time for onset of corrosion.

Amine carboxylates aminoester organic emulsion, calcium nitrite, organic alkyidicarboxylic. Chromates, phosphates, hypophosphites, alkalis, and fluorides. Lithium admixtures to reduce deleterious expansions from alkali-silica reaction

Minimize deleterious expansions from alkali-silica reaction nitrate, lithium carbonate, lithium hydroxide, and lithium nitrite.

Permeability-reducing admixture: non-hydrostatic conditions (PRAN)

Water-repellent surface, reduced water absorption.

Long-chain fatty acid derivatives (stearic, oleic, caprylic capric), soaps and oils (tallows, soya-based), petroleum derivatives (mineral oil, paraffin, bitumen emulsions), and fine particle fillers (silicates, bentonite, talc). Permeability-reducing admixture: hydrostatic conditions (PRAH)

Reduced permeability, increased resistance to water penetration under pressure.

Crystalline hydrophilic polymers (latex, water-soluble, or liquid polymer).

Bonding Increase bond strength. Polyvinyl chloride, polyvinyl acetstyrene copolymers. ate, acrylics, and butadiene-

Coloring Colored concrete. Carbon black, iron oxide, phthalocyanine, raw burnt umber,chromium oxide, and titanium dioxide.

Flocculating Increase interparticle attractias one large flock. on to allow paste to behave Vinyl acetate-maleic anhydride copolymer. Fungicidal, cermicidal, insecticidal

Inhibit or control bacterial, fungal, and insecticidal growth. Polyhalogenated phenols, emulsion, and copper compounds.

Rheology/viscosity-modifying Modify the rheological properties of plastic concrete.

Polyethylene oxides, cellulose ethers (HEC, HPMC), alginates (from seaweed), natural and synthetic gums, and polyacrylamides or polyvinyl alcohol.

Air-detraining Reduce air in concrete mixtother cementing applications, cement slurries, and

Tributyl phosphate, dibutyl phosphate, dibutylphthalate, polydimethylsiloxane, dodecyl (lauryl) alcohol, octyl alcohol, poly- propylene glycols, water-soluble esters of carbonic and boric acids, and lower sulfonate oils.

4—Quality assurance 4—Batching 4—Storage

Chapter 5—Accelerating admixtures, p. 12 5—Introduction 5—Materials 5—Selection and evaluation 5—Applications 5—Proportioning concrete 5—Effects on fresh and hardening concrete 5—Effects on hardened concrete 5—Corrosion of metals 5—Quality assurance 5—Batching 5—Storage

Chapter 6—Water-reducing and set-retarding admixtures, p. 16 6—Introduction 6—Materials 6—Selection and evaluation 6—Applications 6—Dosage 6—Proportioning concrete 6—Effects on fresh and hardening concrete 6—Effects on hardened concrete 6—Batching and quality control 6—Storage

Chapter 7—Admixtures for flowing concrete, p. 20 7—Introduction 7—Materials 7—Selection and evaluation 7—Applications 7—Proportioning concrete 7—Effects on fresh and hardening concrete 7—Effects on hardened concrete 7—Quality assurance 7—Storage

Chapter 8—Admixtures for self-consolidating concrete, p. 23 8—Introduction 8—Materials for SCC admixtures 8—Selection and evaluation 8—Proportioning concrete 8—Effects on fresh and hardening concrete 8—Effects on hardened concrete 8—Quality assurance 8—Batching 8—Storage

Chapter 9—Cold weather admixture systems, p. 28 9—Introduction 9—Materials 9—Selection and evaluation 9—Proportioning concrete 9—Batching

9—Trial placement 9—Placing and finishing 9—Effects on fresh and hardening concrete 9—Effects on hardened concrete 9—Quality assurance 9—Cost benefit 9—Storage

Chapter 10—Admixtures for very high-early- strength concrete, p. 30 10—Introduction 10—Materials for very high-early-strength concrete 10—Selection and evaluation 10—Proportioning concrete 10—Effects on fresh and hardening concrete 10—Effects on hardened concrete 10—Quality assurance 10—Batching 10—Storage

Chapter 11—Extended set-control admixtures, p. 33 11—Introduction 11—Materials 11—Selection and evaluation 11—Applications 11—Proportioning concrete 11—Effects on fresh and hardening concrete 11—Effects on hardened concrete 11—Quality assurance 11—Batching 11— Storage

Chapter 12—Shrinkage-reducing admixtures, p. 35 12—Introduction 12—Materials 12—Mode of action 12—Applications 12—Proportioning concrete 12—Effects on fresh and hardening concrete 12—Effects on hardened concrete 12—Quality assurance 12—Storage

Chapter 13—Corrosion-inhibiting admixtures, p. 37 13—Introduction 13—Materials 13—Selection and evaluation 13—Applications 13—Proportioning concrete 13—Effects on fresh and hardening concrete 13—Effects on hardened concrete 13—Quality assurance 13—Storage

Chapter 14—Lithium admixtures to reduce deleterious expansion from alkali-silica reaction, p. 44 14—Introduction

air content —the volume of air voids in cement paste, mortar or concrete, exclusive of pore space in aggregate particles, usually expressed as a percentage of total volume of the paste, mortar, or concrete. concrete, flowing —a cohesive concrete mixture with a slump greater than 7-1/2 in. (190 mm). concrete, high-performance —concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices. concrete, mass —volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat and temperature gradients from hydration of the cementitious materials and attendant volume change due to internal or external restraint. concrete, ready mixed —concrete manufactured for delivery to a purchaser in a fresh state. corrosion inhibitor —a chemical compound, either liquid or powder, usually intermixed in concrete and sometimes applied to concrete, and that effectively decreases corrosion of steel reinforcement. durability —the ability of a material to resist weathering action, chemical attack, abrasion, and other conditions of service. reaction, alkali-silica —a generally deleterious dissolution and swelling of siliceous aggregates in the presence of pore solutions comprised of alkali hydroxides; the reaction products may cause abnormal expansion and cracking of concrete. rheology —the science dealing with deformation and flow of materials. self-consolidating concrete (SCC) —highly flowable, nonsegregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical consolidation. SCC has also been described as self-compacting concrete, self-placing concrete, and self- leveling concrete, which are subsets of SCC. slump —a measure of consistency of freshly mixed concrete, mortar, or stucco equal to the subsidence measured to the nearest 1/4 in. (5 mm) of the molded specimen immediately after removal of the slump cone. slump flow —a measure of the unconfined flow potential of a freshly mixed self-consolidating concrete or grout. The value is equal to the average of two perpendicular diameters of the material measured to the nearest 1/2 in. (12 mm) after it is released from the slump cone and stops flowing. strength, concrete compressive —the measured maximum resistance of a concrete specimen to axial compressive loading; expressed as force per unit cross-sectional areas. strength, early —strength of concrete or mortar usually as developed at various times during the first 72 hours after placement. stress, shear —intensity of internal force (that is, force per unit area) exerted by either of two adjacent parts of a body on the other across an imagined plane of separation; when the forces are parallel to the plane, the stress is called shear stress. viscosity —the property of a material that resists change in the shape or arrangement of its elements during flow, and the measure thereof.

water, adsorbed —water held on the surface of a material by intermolecular forces and having physical properties substantially different from those of absorbed or chemically combined water at the same temperature and pressure. water-cement ratio —the ratio of the mass of water, exclusive only of that absorbed by the aggregate, to the mass of cement in a mixture, stated as a decimal and abbreviated as w / c. water-cementitious material ratio —the ratio of the mass of water, excluding that absorbed by the aggregate, to the mass of cementitious material in a mixture, stated as a decimal and abbreviated as w / cm. water repellent —resistant but not impervious to penetration by water. yield stress —the critical shear stress value below which a viscoplastic material will not flow and once exceeded, flows like a viscous liquid.

CHAPTER 3—GENERAL INFORMATION 3—Sustainability Chemical admixtures can improve sustainability by producing mixture designs that use lower water contents, incorporate higher quantities of supplementary cementitious materials, and lower cement contents by improving cement efficiency. Proper use of chemical admixtures improves concrete performance in the hardened state and, therefore, increases the duration of the concrete life cycle.

3—Admixture benefits Chemical admixtures are used singularly or in combination to improve the desired properties of concrete or mortar in the plastic and hardened states. Types and dosages are selected in accordance with climatic conditions for maintaining workability and pumpability, strength, w / cm , air content, setting time, and early and final strengths. Proposed mixtures and admixture choices are often confirmed with successful test placements on site. 3.2 Modification of fresh concrete, mortar, and grout — Admixtures are used to modify and improve properties of fresh concrete, mortar, and grout. Examples are:

Increase the workability without increasing the water content, or decrease the water content without changing the workability;
Increase the slump or slump-flow without increasing the water content;
Retard or accelerate the time of initial setting;
Reduce or prevent settlement, or create a slight expansion;
Modify bleeding characteristics;
Reduce segregation;
Improve finishability;
Improve pumpability;
Modify rheological properties;
Reduce the rate of slump loss; and
Increase placement rate. 3.2 Modification of hardened concrete, mortar, and grout —Admixtures are used to modify properties of hardened concrete, mortar, and grout. Examples are:
Reduce the rate of heat evolution during early cement hydration;
Accelerate the rate of strength development at early ages;
Increase strength (compressive, tensile, or flexural);
Increase resistance to freezing and thawing;
Reduce scaling caused by deicing salts;
Decrease permeability;
Reduce expansion caused by alkali-aggregate reaction;
Increase bond to steel reinforcement and between existing and new concrete;
Improve impact resistance and abrasion resistance;
Inhibit corrosion of embedded metal;
Produce colored concrete or mortar; and
Reduce drying shrinkage and curling.

3—Specifications for admixtures The following standard specifications cover the admixture types that make up the bulk of products covered in this report:

Air-entraining admixtures: ASTM C260 and AASHTO M154;
Water-reducing and set-controlling admixtures: ASTM C494/C494M and AASHTO M194;
Calcium chloride: ASTM D98 and AASHTO M144;
Admixtures for use in producing flowing concrete: ASTM C1017/C1017M; and
Pigments for integrally colored concrete: ASTM C979. ASTM recently approved a Type S (specialty admixture) designation that includes admixtures not covered by other ASTM standards. The results obtained from tests done in accordance with ASTM C494/C494M will ensure that these products give values that closely matched those of the untreated reference concrete and therefore meet all the requirements of the standard.

3—Sampling and testing Admixture samples for testing and evaluation should be obtained by the procedures prescribed for each admixture’s specifications using random sampling from plant production, previously unopened packages or containers, or fresh bulk shipments. Admixtures are tested to determine compliance with specifications; evaluate effects on the properties of concrete made with materials under the anticipated ambient conditions and construction procedures; determine uniformity of the product within or between batches, lots, or containers; or reveal any undesirable effects. The quality-control procedures used by producers of admixtures should ensure product compliance with provisions of ASTM or other applicable specifications, including uniformity. Because a producer’s quality-control test methods can be developed around a particular proprietary product, they may not be applicable for general use or use by consumers. ASTM provides procedures for testing concrete containing admixtures. Producing concrete should be preceded by testing that allows observation and measurement of the performance of the admixture under concrete plant operating conditions in combination with the constituent materials that will be used. Uniformity of results is as important as the average result, with respect to each significant property of the admixture or the concrete.

3—Cost effectiveness Economic evaluation of an admixture should be based on the test results obtained when used with the specified concrete under conditions simulating those expected on the job. The characteristics of the cementitious materials and aggregate; their relative proportions; and the temperature, humidity, and curing conditions influence the test results. When evaluating an admixture, its effect on the volume of a given batch should be taken into account. If the admixture increases the volume of the batch (the yield), the admixture should be considered as a basic ingredient together with the cementitious materials, aggregate, and water. All changes in the composition of a unit volume of concrete should be taken into account when testing the direct effect of the admixture and in estimating its benefits. The cost effectiveness of an admixture should be based on the cost of the concrete in place, rather than the cost of the concrete alone. The cost in place, which includes transporting, placing, and finishing costs, is of greatest interest to the owner. The admixture benefits can allow the use of less-expensive construction methods or allow structural designs that offset the added cost due to its use. For example, novel and economical structural designs have resulted from the use of high-range water-reducing admixtures (HRWRAs). They are essential ingredients of cost-effective, high-performance concrete. Water-reducing and set-retarding admixtures permit placement of large volumes of concrete over extended periods, minimizing the need for forming, placing, and joining separate units. Accelerating admixtures reduce finishing and forming costs. Required physical properties of lightweight concrete may be achieved at a lower density by using air-entraining and water-reducing admixtures.

3—Selection and evaluation Careful attention should be given to the instructions and recommendations provided by the manufacturer of the admixture. An admixture’s effects should be evaluated whenever possible using the specified materials under site conditions. This is particularly important when:

The admixture has not been used previously with the particular combination of materials;
Special types of cementitious materials are specified;
More than one admixture is to be used; or
Mixing and placing is done at temperatures outside recommended temperature ranges for concrete. The use of admixtures also requires a review of the concrete mixture design constituents. Prime concerns are:
Type and amount of cement;
Type and amount of supplementary cementitious materials;
Combined aggregate gradation, water and air content; and
Climatic conditions. On-site testing of the proposed mixture to verify proper workability, finishability, pumpability, and setting time is recommended.

from such causes as sticky valves, buildup of foreign matter in meters or in storage and mixing tanks, or worn pumps.

Components should be protected from dust and temperature extremes, and be kept readily accessible for visual observation and maintenance. Although admixture-batching systems are usually installed and maintained by the admixture producer, plant operators should thoroughly understand the system and be able to adjust it and perform simple maintenance. Plant operators should recalibrate the system on a regular basis, preferably at intervals of not more than 90 days, noting any trends that indicate worn parts needing replacement.

Tanks, conveying lines, and ancillary equipment should be drained and flushed on a regular basis, and calibration tubes should have a water fitting installed to allow the plant operator to flush the tube so that divisions or markings may be clearly seen at all times.

3.7 Storage —Admixtures should be stored in strict accordance with the manufacturer’s recommendations. Most admixtures are not damaged by freezing. The manufacturer’s instructions should be followed regarding the effects of freezing the product. An admixture stored beyond its recommended shelf life should be retested before use.

CHAPTER 4—AIR-ENTRAINING ADMIXTURES

4—Introduction An air-entraining admixture is “an admixture that causes the development of a system of microscopic air bubbles in concrete, mortar, or cement paste during mixing,” usually to increase workability and resistance to freezing and thawing. The entrained air-void system is distinct from air voids physically entrapped in concrete during placement and consolidation. Air-entraining admixtures function by stabilizing the air voids folded into the concrete during the mixing process. Air entrainment should always be required when concrete will be subjected to freezing and thawing and where the use of deicing chemicals is anticipated. Highway pavements, parking structure slabs, bridge decks, garage floors, driveways, curbs, and sidewalks located in cold climates will probably be exposed to such conditions. Specified air content shall be achieved by batching an air- entraining admixture and measuring the air content of fresh concrete with air meters and/or unit weight tests. The resistance of concrete to freezing and thawing is affected by placing, consolidating, finishing, and curing procedures; therefore, acceptable construction practice in these respects should be followed (ACI 201, ACI 302, ACI 304R, ACI 308R).

Extensive laboratory testing and long-term field experience have demonstrated conclusively that portland-cement concrete should contain at least a minimum amount of properly entrained air to resist the action of freezing and thawing (Cordon 1946; Blanks and Cordon 1949; Mather 1990). The process by which air is entrained in concrete and the mechanism by which such air entrainment prevents damage due to freezing and thawing is beyond the scope of this report, but is summarized in various textbooks (Powers 1968; Mindess and Young 1981; Mehta and Monteiro 1993) and in ACI 201. More detailed discussions can be found

in research papers (Cordon 1966; Litvan 1972; MacInnis and Beaudoin 1974; Powers 1975; Whiting and Nagi 1998).

4—Materials for air entrainment Many materials can function as air-entraining admixtures, but those used to create cellular concrete, by creating gas bubbles inside the concrete (ACI 523), such as hydrogen peroxide and powdered aluminum metal, are not acceptable air-entraining admixtures. 4.2 Water-soluble compounds —Water-soluble, air- entraining admixtures are formulated using salts of wood resins, synthetic detergents, salts of petroleum acids, salts of proteinaceous acids, fatty and resinous acids and their salts, and organic salts of sulfonated hydrocarbons. Not every material that fits the preceding description, however, will produce a desirable air-void system. All air-entraining admixtures should meet the requirements of ASTM C260. Most commercial air-entraining admixtures are in liquid form, although a few are powders, flakes, or semisolids. The proprietary name and the net quantity in kilograms (pounds) or liters (gallons) should be indicated on the containers in which the admixture is delivered. 4.2 Solid materials —Solid particles that have a high internal porosity and suitable pore size have been added to concrete and seem to act like air voids. These particles can be hollow plastic spheres, crushed brick, expanded clay or shale, or spheres of suitable diatomaceous earth. Research has indicated that when using inorganic particulate materials, the optimum particle size should be 300 μm to 1 mm (0 to 0 in.) (No. 16 to 50 sieves). The total porosity of the particles should be at least 30% by volume, and the pore- size distribution should be between 0 and 3 μm (0. and 0 in.) (Gibbons 1978; Sommer 1978). Inclusion of such particulates in the proper proportion has produced concrete with excellent resistance to freezing and thawing in laboratory tests using ASTM C666/C666M (Litvan and Sereda 1978; Litvan 1985). Particulate air-entraining admix- tures have the advantage of stability of the air-void system. Once added to the fresh concrete, changes in mixing proce- dure or time; changes in temperature, workability, or finishing procedures; or the addition of other admixtures, such as fly ash, or other cementitious materials such as ground slag, will not change the air content, as may be the case with conventional air-entraining admixtures. 4.2 Entrained air-void systems— Improvements in resistance to freezing and thawing are due to the presence of minute air bubbles dispersed uniformly through the cement- paste portion of the concrete that provide relief from the pressure of freezing water. Because of the bubble’s size, there are literally billions of bubbles in each cubic meter of air- entrained concrete. To provide adequate protection with a relatively low total volume of void space, the bubbles should be small (0 to 0 in. [10 to 100 μm] in diameter). The cement paste in concrete normally is protected against the effects of freezing and thawing if the spacing factor (Powers 1949) does not exceed 0 in. (0 mm), as determined in accordance with ASTM C457/C457M. This is generally achieved when the surface area of the air voids is greater than

600 in. 2 /in. 3 (24 mm 2 /mm 3 ) of air-void volume, and the number of air voids per 1 in. (25 mm) of traverse are 1 times greater than the numerical value of the percentage of air in the concrete (Hover 1994). Many investigators (Tynes 1977; Mather 1979; Schutz 1978; Whiting 1979; Litvan 1983) report that the addition of some HRWRAs to air-entrained concrete increases the spacing factor and decreases the surface areas of the air-void systems beyond the accepted limits. Numerous studies (Kobayashi 1981; Malhotra and Malanka 1979; Philleo 1986), however, indicate that such admixtures do not reduce the freezing-and-thawing resistance of concrete. The air content and the size distribution of air voids produced in air-entrained concrete are influenced by many factors (Backstrom et al. 1958; Mielenz et al. 1958a,b)— most importantly, by the nature and quantity of the air- entraining admixture, the nature and quantity of the constituents of the concrete, the type and duration of mixing employed, the consistency and slump of the concrete, and the kind and degree of consolidation applied in placing the concrete. Admixtures react differently with varying concrete constituents, temperatures, slumps, and cements. Field evaluations are often beneficial because uniformity of air content throughout a project is essential. Field tests should be used to verify the uniform performance of the proposed air-entraining admixture. Therefore, the air-entraining admixture choice on a given project should be based on current information and recent field test data. Air content in hardened concrete is determined either by the linear traverse or point-count technique and generally is slightly lower than values obtained from tests of the fresh concrete (Carlson 1967; Reidenour and Howe 1975). Differences may generally differ by less than two percentage points (Pinto and Hover 2001). Newlon (1971), analyzing field data on cores taken from bridge decks, found that 22 of 26 samples were within 1. percentage points of the air content measured in the fresh concrete. When considerable amounts of entrapped air are present in core specimens; however, air contents, as determined by linear traverse, can be up to 3 percentage points less than those determined by the pressure meter (Amsler et al. 1973). Occasionally, measured air contents in hardened concrete can be as much as twice those measured in as-delivered concrete. Explanations of this phenomenon include the incom- pressibility of very small (0 in. [<50 μm] diameter) air voids (Hover 1989) and the transfer of air between small and large air bubbles (Fagerlund 1990). Attempts to reproduce this phenomenon under controlled laboratory and field conditions have not been successful (Ozyildirim 1991). Air content readings should be verified on site by unit weight (density) tests. The use of air meter buckets to determine density, regardless of nominal maximum size of coarse aggregate, is preferable to not performing density tests. Concrete suppliers should determine the relationship between density and air content for various concrete mixtures, as explained in the last sentence of Section 4.8. Also, concrete cylinders should be weighed after demolding and before capping, and the weight recorded on strength test reports. This provides useful information relative to cylinder manufacture, density, yield, air content, and strength.

4—Selection and evaluation To improve resistance to freezing and thawing, intentionally entrained air should have certain characteristics as outlined in Section 4.2. An admixture that meets the requirements of ASTM C260 will produce a desirable air-void system when recommended air contents are achieved. Improvements in resistance to freezing and thawing are due to the presence of minute air bubbles uniformly dispersed throughout the concrete. ASTM C260 also sets limits on the effects of any given air-entraining admixture on bleeding, time of setting, compressive and flexural strength, resistance to freezing and thawing, and length change on drying of a hardened concrete mixture in comparison with a similar concrete mixture that contains a standard reference air-entraining admixture, such as neutralized thermoplastic resin. Acceptance testing should follow ASTM C231/C231M. ASTM C457/C457M can be used to determine the actual parameters of the air-void system in hardened concrete to provide greater assurance that satis- factory resistance to freezing and thawing will be obtained.

4—Applications Air-entrained concrete should be used wherever concrete contains absorbed moisture and is exposed to freezing and thawing, especially when deicing chemicals are used. Because air entrainment also improves the workability of concrete, it is particularly effective in lean mixtures and in various kinds of lightweight-aggregate concrete. Air entrainment is used in insulating and fill concrete (ACI 523), structural lightweight concrete, and normalweight concrete. Blisters and delaminations can occur when normal-weight concrete with an air content above 3% receives a hard- troweled finish (ACI 302; Bimel 1998). No general agreement exists on the benefits of using air- entraining admixtures in the manufacture of concrete block (Farmer 1945; Kennedy and Brickett 1986; Kuenning and Carlson 1956), and air entrainment is used in some areas and is specified in other areas. Air-entraining admixtures, however, are marketed specifically for zero- and low-slump concrete to produce a stable air-void system with proper bubble size and spacing. Air entrainment is desirable in wet-process shotcrete for the same purposes as in conventional concrete. The process of pumping, spraying, and impinging on a surface limits the air content of in-place shotcrete to approximately 4%, in spite of higher air contents before pumping (Morgan 1991). In dry-process shotcrete, using air-entraining admixtures is questionable because there is no mixing to develop an acceptable air-void system (Segebrecht et al. 1989). Nevertheless, air-entrained dry-process shotcrete exhibits excellent durability when exposed to freezing and thawing (Litvin and Shideler 1966; Gebler 1992). ACI 506R recom- mends the air-entraining admixture be introduced to the mixture at the nozzle in combination with the mixing water.

4—Proportioning concrete 4.5 Air entrainment changes the properties of fresh concrete. Mixture designs should be proportioned in accordance with ACI 211 or ACI 211. Air-entrained

caused by the increased shearing action imposed by the screw that moves the concrete from the hopper into the pump cylinder. The type and degree of consolidation used in placing concrete can reduce the air content. Vibration applied to air- entrained concrete removes air as long as the vibration is continued (Backstrom et al. 1958); however, laboratory tests have shown that concrete’s resistance to freezing and thawing is not reduced by moderate amounts of vibration. Stark (1986), however, has shown that extended vibration, particularly at high frequencies, can significantly reduce this resistance and can disrupt the smaller bubbles.

4—Effects on hardened concrete 4.7 Freezing-and-thawing cycles —Properly proportioned air-entrained concrete is resistant to freezing-and-thawing cycles after achieving 4000 psi (27 MPa). 4.7 Compressive strength— Air entrainment reduces the compressive strength of hardened concrete, particularly with moderate to high cementitious material contents. Reduction is approximately 5% for each percent of entrained air, however, the rate of reduction of strength increases with higher amounts of air. Adding entrained air reduces the water content required to achieve the specified slump. The result in w / cm can partially offset the reduction of strength. This is particularly true of lean cementitious content concretes and/or concretes that contain a large maximum- size aggregate. In these cases, air entrainment may cause only a small decrease in strength or possibly a slight increase in strength. 4.7 Flexural strength —Air-entrained concrete with the same w / cm shows a slight decrease in flexural strength (Mailvaganam 1984; Departments of the Army and the Air Force 1987). 4.7 Permeability —Air-entrained concrete with a w / cm of 0 or less will reduce permeability by intersecting internal capillaries that can provide ingress for liquids (Lukas 1981).

4—Quality assurance 4.8 Benefits of air entrainment —The full benefits of air- entrained concrete can only be realized when the mixture design provides consistent air contents that should be checked and verified in accordance with the recommendations of ACI 311 and ACI 311. Proper air content in the hardened concrete is the key requirement. Air losses due to pumping, handling, transportation, and consolidating are not detected by air content tests performed at the mixer (ACI 309R). For control purposes, samples for determining air content should be taken directly from concrete at the point of deposit (ACI 301). The pumping process should eliminate freefall by a loop or slight curve in the line. When air loss between delivery and point of deposit becomes consistent, air content tests may be made at the point of delivery. Proper air content in the hardened state is the goal. 4.8 Air control tests —Air content tests of concrete should be made at regular intervals or whenever there is a reason to suspect a change in air content. The properties of the concrete-making materials, the proportioning of the concrete mixture, and all aspects of mixing, handling, and

placing should be maintained as constantly as possible to ensure that the air content will be uniform and within the range specified for the work. Too much air may reduce strength without a commensurate improvement in durability, whereas too little air will fail to provide desired durability and workability. 4.8 Test methods —In general, air content tests are taken each time compressive and/or flexural test specimens are taken and at any other time so designated by the purchaser. Unit weight tests are recommended in addition to air meter tests to ensure that all the entrained air is recorded. Sometimes air meters do not detect the smaller entrained air bubbles. A chart indicating air contents versus unit weights can be estab- lished by the testing agency and be kept on site to quickly detect deviations from the specified air content envelope. 4.8 ASTM standards —There are three standard ASTM methods for measuring the air content of fresh concrete:

The gravimetric method, ASTM C138/C138M;
The volumetric method, ASTM C173/C173M; and
The pressure method, ASTM C231. The pressure method, however, is not applicable to light- weight concrete. The Chace Air Indicator (Grieb 1958), which is an adaptation of the volumetric method, has not been standardized and should not be used to determine compliance with specification limits. The ASTM methods measure only air volume and not the air-void characteristics. While the spacing factor and other significant parameters of the air-void system in hardened concrete have traditionally been determined only by microscopic methods such as those described in ASTM C457/C457M, methods have been developed (Wojakowski 2003; Whiting l993) that determine air-void parameters of fresh concrete.

4—Batching 4.9 Uniformity —To achieve the greatest uniformity between batches of a concrete mixture, water-soluble, air- entraining admixtures should be added to the mixture in the form of solutions rather than solids. Generally, only small quantities of air-entraining admixtures—approximately 0% of active ingredients by mass of cementitious materials—are required to entrain the desired amount of air. If the admixture is in solid or semisolid forms, a solution should be prepared before use, following the recommendations of the manufacturer. 4.9 Dosages —The dosage required to achieve the desired air content should be determined by trial mixtures, starting from the manufacturer’s recommendations or from experience. For any given set of conditions and materials, the amount of air entrained is roughly proportional to the amount of admixture used. Proper air content problems are particularly acute during hot weather.

4—Storage Air-entraining admixtures should be stored in strict accordance with the manufacturer’s recommendations. Although most admixtures usually are not damaged by freezing, the manufacturer’s instructions should be followed regarding the effects of freezing the product. An admixture stored beyond its recommended shelf life should be retested before use.

CHAPTER 5—ACCELERATING ADMIXTURES

5—Introduction An accelerating admixture is “an admixture that causes an increase in the rate of hydration of the hydraulic cement and thus shortens the time of setting, increases the rate of strength development, or both” (American Concrete Institute 2010). Accelerating admixtures purchased for the use in concrete should meet the requirements of ASTM C494/C494M for Type C, accelerating admixtures, or Type E, water-reducing and accelerating admixtures.

Accelerating admixtures are used to decrease setting time and increase early strength gain, particularly in cold weather, to expedite the start of finishing operations, reduce finishing time, and reduce the time required for proper curing and protection; and to increase the early-strength level to permit earlier form removal and decrease the overall construction time. Accelerators are normally used in conjunction with other recommended practices (ACI 306R) to counteract the effects of low temperatures. Quick-setting admixtures permit more efficient plugging of leaks against hydrostatic pressure and produce rapid setting of concrete placed by shotcreting.

Certain accelerating admixtures used in combination with high-range water-reducing admixtures significantly lower the freezing point of concrete and allow concrete placement to 20°F (–7°C). These admixtures and their effects are discussed in detail in Chapter 9, Cold Weather Admixture Systems.

5—Materials Accelerating admixtures can be divided into four groups: those that contain soluble inorganic salts; those that contain soluble organic compounds; quick-setting admixtures; and miscellaneous solid admixtures. Water-reducing and accelerating admixtures are often formulated with one or more of the compounds listed in Section 6.2 to produce the required water reduction.

5.2 Soluble inorganic salts —Studies (Edwards and Angstadt 1966; Rosskopf et al. 1975) have shown that a variety of soluble inorganic salts, including chlorides, bromides, fluorides, carbonates, thiocyanates, nitrites, nitrates, thiosulfates, silicates, aluminates, and alkali hydroxides, decrease the setting time of portland cement. Of these salts, calcium chloride is the most widely used because it is the most cost-effective. Research by numerous investigators has shown that inorganic accelerating admixtures act primarily by accelerating the hydration of tricalcium silicate (Ramach- andran and Malhotra 1984). Calcium chloride should meet the requirements of ASTM D98. Forms of calcium chloride and their equivalent masses are shown in Table 5.

5.2 Soluble organic compounds —The most common organic accelerating admixtures in this class are triethanol- amine and calcium formate, which are commonly used to offset the retarding effects of water-reducing admixtures or to provide noncorrosive acceleration. The effectiveness of calcium formate depends on the tricalcium aluminate-to- sulfur trioxide ratio (C 3 A/SO 3 ) of the cement (Gebler 1983). Cements that are under-sulfated (C 3 A/SO 3 > 4) provide the best potential for calcium formate to accelerate the early-age

strength of concrete. The production of ettringite is greater in mixtures containing calcium formate (Bensted 1978). Accelerating properties have been reported for calcium salts of carboxylic acid, including acetate (Washa and Withey 1953), propionate (Arber and Vivian 1961), and butyrate (RILEM 1968). Salts of the higher homologs, however, are retarders (RILEM 1968). Studies (Ramachandran 1973, 1976a) indicate that trietha- nolamine accelerates the hydration of tricalcium aluminate but retards hydration of tricalcium silicate. Thus, triethanolamine can act as a retarder of cement at high dosages or low tempera- tures. A number of other organic compounds have been found to accelerate the setting of portland cement when a low w / cm is used. Organic compounds reported as accelerating admixtures include urea (RILEM 1968), oxalic acid (Bash and Rakimbaev 1969; Djabarov 1970), lactic acid (Bash and Rakimbaev 1969; Lieber and Richartz 1972), various cyclic compounds (Lieber and Richartz 1972; Wilson 1927), and condensation compounds of amines and formaldehyde (Rosskopf et al. 1975; Kossivas 1971). Retardation can occur if high dosages are used, because like triethanolamine, such compounds will retard the hydration of tricalcium silicate. 5.2 Quick-setting admixtures —Quick-setting admixtures are used to produce quick-setting mortar or concrete suitable for shotcreting and sealing leaks against hydrostatic pressure. These admixtures are believed to act by promoting the flash setting of tricalcium aluminate (Schutz 1977). Ferric salts, sodium fluoride, aluminum chloride, sodium aluminate, and potassium carbonate are reported to produce quick-setting (Mahar et al. 1975) mortars, but many proprietary formulations are mixtures of accelerating admixtures. Quick-setting admixtures for shotcrete, employed extensively in both the dry and wet processes (ACI 506R), are a specific class of quick-setting admixtures, traditionally based on soluble aluminates, carbonates, and silicates. These materials are caustic, hazardous, and require special handling (refer to material safety data sheets from the manu- facturer). Newer, neutral-pH, chloride-free proprietary admix- tures, based on specific sugar-acid compounds, are available to overcome these deficiencies. Generally, the wet-process shotcrete mixture quickly stiffens and reaches a rapid initial

Table 5—Calcium chloride: amount introduced

Calcium chloride by mass of cement, %

Solid form, %

Liquid form, 29% solution‡ Amount of chloride ion Dihydrate* Anhydrous† added, %

L/100 kg (qt/100 lb) 0 0 0 0 (0) 0. 0 1 0 0 (1) 0. 1 1 1 1 (1) 0. 1 2 1 1 (1) 1. 2 2 2 2 (2) 1. *Commercial flake products generally have an assay of 77 to 80% calcium chloride, which is often close to dihydrate. †Commercial anhydrous calcium chloride generally has an assay of 94 to 97% calcium chloride. Remaining solids are usually chlorides of magnesium, sodium, or potassium, or combinations thereof. Thus, the chloride content, assuming the material is 100% calcium chloride, introduces very little error. ‡A 29% solution often is the concentration of commercially used liquid forms of calcium chloride and is made of dissolving 0 kg (1 lb) dihydrate to make 0 L (1 qt) of solution.

verify setting time; early strength gain; and the standard mixture design qualities such as satisfactory workability, pumpability, or, for slabs, finishing characteristics. 5.5 Dosages —Accelerating mixture designs are often prepared with varying dosages. This allows the contractors to base their daily mixture design selections on the actual temperatures and wind conditions, as well as the setting characteristics of the concrete being supplied that day. Often the mixture design on rapid cycle projects has a higher dosage of accelerator in the first half of the slab since that area must set so that the contractor can walk on it in the late morning or early afternoon without marring the surface. In other projects, the accelerator dosage is increased in the last portion of slab concrete to minimize finishing over time.

5—Effects on fresh and hardening concrete 5.6 Time of setting —Initial and final setting times are reduced by an amount dependent on the dosage of accelerator used, the temperature of the concrete, the ambient temperature, and the characteristics of other materials used in the concrete. Many accelerators have a greater accelerating effect at 32 to 41°F (0 to 5°C) than at 77°F (25°C). High dosages of accelerating admixtures can cause very rapid setting in hot weather. Excessive dosage rates of certain organic compounds may result in extended times of setting. Similarly, at high dosages (6% by mass of cement), calcium nitrate begins to show retarding properties (Murakami and Tanaka 1969), whereas ferric chloride retards at additions of 2 to 3% by mass, but accelerates at 5% (Rosskopf et al. 1975). With quick-setting admixtures, setting times as short as 15 to 30 seconds can be attained. Prepackaged mortar formulations are available that have an initial time of setting of 1 to 4 minutes and a final setting time of 3 to 10 minutes They are used to seal leaks in below-grade structures, for patching, and for emergency repair. The ultimate strength of such mortar is much lower than if no accelerating admixture had been added. 5.6 Air entrainment —When an accelerating admixture is used, different dosages of air-entraining admixture may be required to produce the required air content. In some cases, however, larger bubble sizes and higher spacing factors are obtained, possibly reducing the beneficial effects of entrained air. Concrete containing a specific admixture may be evaluated to ascertain air-void parameters using ASTM C457/C457M or resistance to freezing and thawing using ASTM C666/C666M. 5.6 Freezing and thawing —Concrete must achieve a compressive strength of 500 psi (3 MPa) before freezing. Concrete exposed to freezing and thawing in service must achieve a compressive strength of 4000 psi (27 MPa) prior to being exposed to freezing-and-thawing cycles. Properly proportioned mixture designs with accelerators can achieve this strength more quickly.

5—Effects on hardened concrete 5.7 Discoloration of flatwork —Discoloration of concrete flatwork has been associated with the use of calcium chloride (Greening and Landgren 1966). Two major types of mottling discoloration can result from the interaction

between cement alkalis and calcium chloride. The first type has light spots on a dark background and is characteristic of mixtures in which the ratio of cement alkalis to calcium chloride is relatively low. The second consists of dark spots on a light background, and is characteristic of mixtures in which the ratio of cement alkalis to chlorides is relatively high. The magnitude and permanence of discoloration increase as the calcium chloride concentration increases from 0 to 2% by mass of cement. Discoloration can be aggravated by high rates of evaporation during curing and by improper placement of vapor barriers (the use of a sheet membrane for curing that is not kept flat on the surface). Using a continuous fog spray during placement or a proper curing compound can help alleviate this problem. 5.7 Strength development —Many accelerators increase compressive strength substantially at early ages. In some cases, later strength may be reduced slightly. Strength gains up to 200%/day can be achieved with varying dosages of many accelerators. These early strength levels are very dependent on the strength potential of the selected cement. Flexural strength of 400 psi (2 MPa) has been achieved in 4 hours with certain accelerators, cements, and high-range water-reducing admixtures. Certain accelerators with high- range water-reducing admixtures have easily achieved 4000 psi (27 MPa) in 12 hours in the laboratory and in the field. The percentage increase in flexural strength is usually less than that of the compressive strength (Ramachandran 1976b). The effects of other accelerating admixtures on strength development are not completely known, although other calcium salts behave similarly. Because accelerated strength development depends on accelerated hydration, heat of hydration also develops faster, but there is no appreciable effect on total heat generation. Quick-setting admixtures, such as carbonates, silicates, and aluminates, may decrease concrete strengths and ultimate strengths as early as 1 day (Mailvaganam 1984). Quick-setting mixtures of portland cement and calcium-aluminate cement behave similarly. Organic compounds such as triethanolamine and calcium formate appear to be sensitive in their accelerating action to the particular concrete mixture to which they are added, and to the ambient temperature. Preliminary laboratory tests should be followed by field tests to assure that the proposed mixture design easily achieves the target strength levels at early and late ages without presenting placing or finishing problems due to too quick setting characteristics. 5.7 Volume change —Accelerating admixtures can increase the volume changes that occur under moist curing and drying conditions. Calcium chloride can increase creep and drying shrinkage of concrete (Shideler 1942). Mather (1964) offered an alternative hypothesis to the presumed association of the use of calcium chloride with increased drying shrinkage. Bruere et al. (1971) observed that such volume changes depend on the length of curing before beginning measurements, the length of the drying or loading periods, and the composition of the cement used. They also noted that changes in the rate of deformation are greater than changes in the total deformation. Berger et al. (1967)

suggested that the influence of calcium chloride in drying shrinkage can result from changes in the size distribution of capillary pores due to calcium chloride’s effect on hydration of the cement. Drying shrinkage and swelling in water are higher for mixtures containing both portland calcium-alumi- nate cements and calcium chloride, and their durability may be adversely affected by using an accelerating admixture (Feret and Venuat 1957). Excessive shrinkage and subsequent cracking can significantly decrease durability characteristics. Therefore, shrinkage tests on the proposed mixtures should be required. 5.7 Freezing and thawing —Properly proportioned mixtures with the proper air content and w / cm and many accelerators provide very satisfactory long-term resistance to freezing and thawing. 5.7 Chemical attack —Resistance to sulfate attack is decreased when conventional portland-cement concrete mixtures contain calcium chloride (USBR 1975), but when used with Type V cement to mitigate the effects of cold weather, it is not harmful (Mather 1992). The expansion produced by alkali-silica reaction (ASR) is greater when calcium chloride is used (USBR 1975). Nonchloride admixtures may increase expansion.

5—Corrosion of metals One of the major disadvantages of calcium chloride is that it induces corrosion of metals embedded in concrete when in the presence of sufficient moisture and oxygen. ACI 318 lists the chloride limits for concrete in new construction that should be used to determine the maximum permissible water-soluble chloride-ion content for concrete in various types of construction (shown in Table 5). Table 5 shows the acid-soluble and water-soluble chloride limits for new construction reported by ACI 222R. Gaynor (1985) discusses the calculation of chloride contents for comparison with these limits, as seen in Table 5. The user should evaluate the chloride levels from all ingredients. Other factors such as moisture and oxygen are always necessary for electrochemical corrosion. Many nonchloride- based accelerators are available. The use of calcium chloride will aggravate the effects of poor-quality concrete construction, particularly when the concrete is exposed to chlorides during service. When good construction practices are followed, the limits listed in Table 5 have shown to be highly effective in limiting corrosion. The user should determine whether a nonchloride accelerator would be a better choice in the particular type of construction. The chlorides contributed by all ingredients should then be determined. The potential for in- service corrosion should be evaluated accordingly. Background chloride contents in concrete are naturally- occurring chlorides in the concrete materials. When back- ground chloride or when the chloride content is found to be excessive, the Soxhlet Method should be conducted for final acceptance. The Soxhlet Method for aggregates is detailed in ASTM C1524 and the Soxhlet Method for concrete is detailed in ACI 222. If the concrete or mortar fails the acid-soluble test according to ASTM C1152/C1152M, then the water-soluble test must be conducted according to ASTM C1218/C1218M.

If the results from the water soluble test fail, then the Soxhlet method for water-soluble (extractable) chloride may be conducted. Admixtures based on calcium nitrate or thiocyanates have been proven effective in accelerating initial set and early strength gain. The fact that an accelerating admixture does not contain significant amounts of chloride, however, does not necessarily render it noncorrosive; for example, Manns and Eichler (1982) report that thiocyanates may promote corrosion. Nmai and Corbo (1989), however, found that the threshold level for initiation of corrosion by sodium thiocyanate lies between 0 and 1% by mass of cement, and

Table 5—Maximum chloride-ion concentration for corrosion protection (ACI 318-08)

Type of member

Maximum water-soluble chloride ion in concrete, percent by mass of cement Prestressed concrete 0. Reinforced concrete exposed to chloride in service 0. Reinforced concrete that will be dry or protected from moisture in service 1. Other reinforced concrete construction 0.

Table 5—Chloride limits for new construction (ACI 222R)