By Brian Miller
Achemical admixture is usually defined as a nonpozzolanic (does not require calcium hydroxide to react) admixture in the form of a liquid, suspension or water-soluble solid. An admixture is a material other than water, aggregates, hydraulic cementitious material or fiber reinforcement that is used as an ingredient of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that is added to the batch before or during mixing.
General Use of Chemical Admixtures
Chemical admixtures have been used in concrete construction for many years and for several reasons. The primary reason is to improve or control concrete’s plastic, setting or hardened properties. For example, admixtures are used to improve workability, develop high early strength, increase ultimate strength, compensate for temperatures, and improve resistance to corrosion, shrinkage or freezing and thawing. Admixtures are also used to overcome certain emergencies during placement, such as unexpected delays or equipment failures.
Today, advances in admixture technology enhance concrete’s applications, altering and/or improving placement techniques. One example is self-consolidating concrete, more commonly known as SCC. Many precast operations use SCC to save on consolidation costs, to flow through heavily reinforced sections and to cast monolithic structures that usually would require multiple pours. This type of concrete is possible due to advances in admixtures and does not require external energy in order to be placed and consolidated.
Overall project costs may be reduced as a result of using admixtures. This may be realized through labor savings on placement and consolidation efforts, the reduction of cement content, and reduction of warranty issues and call backs. No matter what the task, admixtures have become an accepted part of today’s precasting practice. Understanding and utilizing them is vital to the precast industry.
How do they work?
To achieve the desired results, all admixtures affect the cement interactions with water, air and other cement particles or alter the paste structure. Admixtures are comprised of chemicals, fillers and water. Surprising as it sounds, in some cases, admixtures contain up to 70 percent water. This is not a bad thing. In fact, water is required to develop the appropriate concentration, distribution and delivery of the essential chemicals. In some cases, enough water must be included to dissolve a certain chemical and ensure its distribution to the cement grains effectively during mixing.
Since admixtures contain some amount of water, this water needs to be accounted for when adjusting the mixing water to maintain a specified water-cementitious (w/c) ratio. For some admixtures this amount is minimal. A water reducer may add only approximately 1 or 2 pounds of water to a 600-pound batch of cement. That would not pose any serious changes unless you are mixing SCC. Other admixtures, such as calcium nitrite-based corrosion inhibitors, can contain much greater amounts of water, as high as 70 percent. This water must be deducted from the mixing water in order to maintain the desired w/c ratio. Note that automated mixing water adjustment systems with moisture probes adjust for aggregate moisture content but not water in admixtures. Consult with your admixture supplier to learn the water content of the admixtures you use.
General suggestions and precautions
The following are some general suggestions and precautions that should be observed when using chemical admixtures.
1. In general, chemical admixtures should meet the appropriate ASTM requirements, if they exist, for the type of admixture being used. ASTM C 494, “Standard Specification for Chemical Admixtures for Concrete,” covers a wide range of admixture types. Table 1 contains a summary of the classification system found in ASTM C 494. Air-entraining admixtures must meet the requirements of ASTM C 260, “Standard Specification for Air-Entraining Admixtures for Concrete.” At the time this article was written, ASTM was developing a corrosion-inhibiting admixture specification. Other admixtures such as mid-range water reducers, shrinkage and ASR controlling admixtures do not have ASTM specifications.
2. Ensure proper storage and accurate dosing systems for admixtures. Most admixtures are concentrated and a small overdose may cause a batch of concrete to be out of spec.
3. Suppliers should be able to provide guidelines for use, MSDS and technical information sheets. Most suppliers will also assist with design of mixture proportions and admixture dosages. This may reduce liability for the precaster and the time required to qualify a particular mixture.
4. Mixture redesign and trial batches are recommended when materials or mixture proportions are changed.
5. Some factors may affect admixture dosage and/or performance:
– Batching procedure
– Mixing time
– Mixing action, or type of mixer (Note: admixtures may perform differently when mixed in a laboratory mixer as compared to a ready-mix truck)
– Individual materials (cement, aggregates, etc.)
– W/c ratio
– Other admixture interactions
– Mixture proportions
IMPORTANT NOTE: Admixtures do not compensate for poor quality materials, poor mixture proportions, or poor concreting and placement practices. For further information about good concreting practices, refer to NPCA’s “Quality Control Manual for Precast Concrete Plants.”
Air-entraining admixtures were discovered accidentally when it was observed that concrete containing cement ground with beef tallow, which was added as a grinding aid, proved more durable than concrete containing cement ground without it. Tallow is now known to be a natural air-entraining agent. Air-entraining admixtures are used to create stable systems of microscopic air bubbles within the paste portion of concrete. Air-entrained concrete has several advantages over non air-entrained concrete, including increased resistance to freezing and thawing, scaling and reduced bleeding. Air entrainment also improves workability, placement and cohesion of the concrete.
Entrapped air, sometimes referred to as accidental air, is usually generated during the mixing or placing operations. This type of air is not considered to be air entrainment and does not provide the benefits mentioned above. Entrapped air differs from entrained air in shape, size and distribution of the air bubbles. Entrapped air usually has irregularly shaped, large-diameter bubbles, many of which can be seen with the naked eye. Bug holes or pockmarks are examples of entrapped air at the concrete surface.
Comparatively, entrained air bubbles typically have diameters in the range of 0.002 – 0.050 inches and are spherical in shape. Entrained air also is a stable and structured system of millions of microscopic bubbles per cubic yard of concrete.
How it works
Air-entraining admixtures are comprised of surfactants, which alter the surface tension of water and allow for the creation of microscopic air bubble systems during agitation. These surfactants contain long molecular chains with a hydrophilic (loves water) component at one end and a hydrophobic (repels water) component on the other. Each end aligns with water and air, respectively. Air bubbles are generated from air in the mix and are essentially covered by a sheath of air entrainer, which allows them to become stable. These sheaths repel one another, which prevents coalescence of the bubbles and allows for better distribution. Air-entraining admixtures are typically made from various materials:
– Salts of fatty acids typically derived from animal fats
– Alkali salts of wood resins (neutralized vinsol resin)
– Alkali salts of sulfated and sulfonated organic compounds
Effects on concrete
The primary benefit of air entrainment in concrete is increased resistance to freezing and thawing. The small microscopic bubbles provide space for expansion when moisture within the concrete pore structure freezes. Otherwise, this expansion would apply stress to the surrounding areas of the pores, which results in damage to the concrete. This is typically observed as scaling of the concrete’s finished surface. In severe cases, this degradation can completely destroy the concrete’s integrity.
It should be noted that good resistance to freezing and thawing requires more than just air entrainment. Guidelines for concrete mixture proportioning for freeze-thaw durability can be found in ACI 318 Building Code.
Air entrainment also improves the workability of concrete. The microscopic air bubbles act like little ball bearings in the mixture, thereby increasing workability and allowing the concrete to be placed with less effort. A rule of thumb is that for every 0.5 percent to 1.0 percent of entrained air, the slump may increase by as much as 1 inch.
Suspension of solid particles is improved with air entrainers. This reduces the degree of sedimentation (sinking of aggregates and paste) that occurs, thereby reducing the amount of bleed water that comes to the surface. Fewer bleed channels mean less permeability of the concrete and therefore improved resistance to scaling and frost damage. With better suspension of the solid particles, segregation during transport will also be reduced.
It is equally important to discuss the adverse effects of air entrainers. Air entrainment will reduce the compressive strength of concrete. This is typically offset by a reduction in the water-cementitious ratio as compared to the same mixture proportions without air entrainment. Another rule of thumb is that a 1 percent increase in air entrainment will reduce compressive strength by approximately 5 percent. The term “air content” refers to the total of both entrapped and entrained air in concrete. Typically, air contents greater than 9 percent are not desired primarily due to this strength loss effect. Greater air-contents also produce very cohesive or sticky mixtures, which are more difficult to finish.
Air-entraining admixtures are dosed based on the weight of cement, usually by fluid ounce per hundred pounds of cement (cwt). Air entrainers are usually added with the sand or mix water in small dosages; however, the manufacturer’s recommendations should be followed.
The effectiveness of air-entraining admixtures in developing an air-entrainment system is influenced by several factors. Table 2 contains a summary of factors that can influence air entrainment and, therefore, the dosage of air-entraining admixture. As each of the factors listed in Table 2 are increased, the effect on air entrainment is noted. Typically the air-entrainer dosage will need to be increased when a decrease in air entrainment is expected and vise-versa.
It should be noted that certain types of air-entraining admixtures may not be compatible with other admixture chemistries. For example, synthetic detergent air entrainers may not be compatible with certain high-range water reducers. Consult with your admixture supplier for best results on admixture interaction.
The air content of fresh concrete can be tested with three methods: gravimetric, volumetric and pressure. The gravimetric method is described in ASTM C 138, “Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete,” and is based on the comparison of the measured density of the compacted concrete containing air to the calculated density of the concrete without air, the mixture proportions being the same.
The volumetric method is described in ASTM C 173, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method.” The method is based on the differences in volume between a consolidated concrete sample with air and the same sample after the air is expelled. The volumetric method is commonly used for testing concrete containing lightweight aggregates.
The pressure method is described in ASTM C 231, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method.” The method is based on Boyle’s Law, which describes the relationship of applied pressure and a given volume of air at a constant temperature. This is the most widely used method since no calculations are required and mixture proportions need not be known. This method should not be used for lightweight or highly porous aggregates.
The size and distribution of the air bubbles are very important to achieve desired results. The air-void system parameters of hardened concrete can also be determined. This is typically done by removing a cross section of concrete, polishing it and performing a point-count using a microscope. This procedure should be performed in accordance with ASTM C 457, “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.” Table 3 presents some minimum limits for key parameters. Note that concrete must have an air content of at least 3 percent to be classified as air-entrained; however, this is typically too low for protection against freezing and thawing.
Freeze-thaw durable concrete typically contains a total air content of between 5 percent and 7 percent. Rapid freezing and thawing durability can be tested in accordance with ASTM C 666, “Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing,” which results in a relative durability factor. This can be used to compare concrete mixtures. Scaling resistance can be tested in accordance with ASTM C 672, “Standard Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals.” This test typically is used to evaluate concrete mixtures that will be exposed to deicing salts.
Air entrainment is one of the important factors in making durable concrete. However, it is also one of the most challenging parameters to achieve. Air entrainment parameters of concrete and slump are directly related, which means one will increase with the other. With proper trial batching and good concreting practices, desired air-void parameters can be accomplished.