According to the American Concrete Institute (ACI), an admixture is defined as “a material other than water, aggregates, cementitious materials and fiber reinforcement, 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 its mixing.”
The most common types of chemical admixtures are water-reducing and set-controlling chemicals. Water-reducing and set-controlling admixtures offer precasters real advantages in improving their production capabilities. Water-reducing admixtures are used to produce concrete of higher strength, obtain specified strength with lower cement content, and increase the slump of a given mixture without an increase in water content. Water-reducing admixtures also improve the properties of concrete containing aggregates that are harsh or poorly graded, or both. Or they may be used in concrete placed under difficult conditions. Water-reducing admixtures can be very beneficial in producing high-strength concrete with a low water-cement ratio.
Types of water-reducing and set-controlling admixtures
The American Society of Testing and Materials (ASTM) specification C494 covers eight types of chemical admixtures:
Type A – Water-reducing admixtures
Type B – Retarding admixtures
Type C – Accelerating admixtures
Type D – Water-reducing and retarding admixtures
Type E – Water-reducing and accelerating admixtures
Type F – Water-reducing, high-range admixtures
Type G – Water-reducing, high-range and retarding admixtures
Type S – Specific performance admixtures
Each of these admixture types is defined by the range at which it decreases water. Types A, D and E must reduce the water content by at least 5%. Types F and G are high-range water-reducing (HRWR) admixtures and are required to reduce the water content of a concrete mix by at least 12%. This may, in some circumstances, decrease the water content of a mix by 30% or more. Types B and C have no water-reducing requirements.
There is also a class of water reducers called mid-range water-reducing (MRWR) admixtures that reduce the water content from about 6% to more than 12% while maintaining slump and avoiding excessive retardation. Generally, this class of water-reducing admixtures falls into either the Type A or F category. Mid-range water reducers were developed to fill in the gap between Type A admixtures capable of producing a 5-in. slump and Type F admixtures capable of producing a 12-in. slump. They have proven very effective with the use of fly ash and other cement substitutes.
Type A – Water-Reducing Admixtures
Type A water reducers typically decrease the water content of a concrete mix by 5 to 10%. With all admixtures, the results and dosage rates will vary with the cement and other materials used. A typical dosage rate for Type A is between 2 and 6 fl oz per 100 lbs of cementitious materials (130 to 390 mm per 100 kg). Most Type A water reducers are composed of organic materials that act as set retardants. Other ingredients are added during manufacture to provide “normal” setting time. Excessive dosage rates will retard the setting time of concrete. Dosage rates should conform to the manufacturer’s recommendations and be tested in trial batches.
Type B – Retarding, and Type D – Water-Reducing and Retarding Admixtures
Retarding admixtures cause a decrease in the hydration rate of hydraulic cement and an increase in the setting time of concrete. Retarders are used to offset the effect of high temperature and improve the workability of concrete in warmer temperatures. Benefits of retarders include reduced cold joints and better finish in hot weather. While retarding admixtures can be beneficial, they are not a substitute for good hot-weather concreting procedures.
There are some retarders that can stop or significantly slow the hydration of portland cement. These are known as hydration stabilizers. Hydration stabilizers are used primarily in the ready-mixed industry to control the set time of concrete wash water, unused or returned concrete, and for long hauls. Hydration stabilizers may be beneficial in concrete that is steam cured. Precasters who use ready-mixed concrete should check with their suppliers to see what admixtures are available.
Type C – Accelerating, and Type E – Water-Reducing and Accelerating Admixtures
Accelerators are used to shorten setting time and to increase early strength development. Precasters can benefit by using accelerators, as they reduce bleeding and allow earlier finishing. Accelerators also increase early strength, which can protect the concrete from freezing, and they allow for faster removal of forms. Accelerators are available in chloride and non-chloride compounds. The use of chloride-based accelerators in reinforced concrete subjected to weathering should be avoided. Accelerators do not act as antifreeze for concrete. Good cold-weather concrete practices must be followed.
Type F – High-Range Water-Reducing, and Type G –
High-Range Water-Reducing and Retarding Admixtures
Type F and G admixtures are known as high-range water-reducing (HRWR) admixtures or superplasticizers. They are capable of producing large water reductions or great flowability without causing undue set retardation or entrainment of air in cementitious mixtures. HRWRs must reduce the water content of a concrete mix by at least 12% and may reduce it by more than 30% from a control mix.
The composition of HRWRs has seen a great change during the last several years. In the past, the most common HRWRs consisted of products commonly referred to as melamine (MSFC) or naphthalene (NSFC) based chemicals. A problem with MSFC- and NSFC-based products is they have limited slump-life capabilities. In the past several years, melamine- and naphthalene-based high-range water reducers have been replaced by a new class of chemicals called “polycarboxylates” or PCs. PCs consist of comb-shaped molecules that provide water reduction without affecting setting time. They are highly engineered and provide a wide range of slump-life capabilities. They are also very efficient in producing self-consolidating concrete (SCC).
High-range water reducers are very efficient at interacting with fine-grained materials such as cement, fly ash, slag and silica fume. The ability to disperse these finely ground particles creates strength that is higher than one would expect from just the decreased water-cement ratio.
Plasticizers – ASTM C1017
ASTM C1017 is a subset of ASTM C494, Types F & G. It is a special class that requires the admixture to produce a slump in excess of 7.5 in. These products are often referred to as superplasticizers. They may be beneficial to precasters in improving workability, increasing strength and decreasing permeability. Most products meeting ASTM C494 Type F or G also meet ASTM C1017 Type 1 or Type 2.
Water-reducing and set-controlling admixtures have proven themselves to be useful tools in the precast industry. They can significantly reduce the water-cement ratio (w/c) while decreasing the use of cement, altering the set time to help in production, improving workability and decreasing permeability. Their use does not, however, preclude the use of good concrete practices such as curing. The type of admixture and dosage recommended varies by manufacturer and must be considered with other materials used. Discuss the specifics of your operations with your admixture supplier and use trial batches to ensure that you are getting the properties you want in the most economical manner.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services and Sustainability.