Michael A. Caldarone, Construction Technology Laboratories, Inc.
Chemical admixtures have been incorporated into concrete mix proportions for many years in order to attain performance properties. Most high performance concrete (HPC) mixes contain at least one type of chemical admixture. In girder construction, high compressive strength is the property frequently required. For concrete compressive strengths above 5000 psi (34 MPa), chemical admixtures are usually necessary to achieve a cost-effective mix. In bridge decks, low chloride permeability is required. Chemical admixtures are used to facilitate placing and finishing. AASHTO M 194 (ASTM C 494) classifies the most commonly used chemical admixtures as Types A through G, based on the admixture’s effect on lowering water demand or influencing setting time.
Water-Reducing Admixtures
Water-reducing admixtures can be used to increase slump while maintaining a constant water-cementitious materials ratio (w/cm), or maintaining slump while lowering the w/cm. In general, increasing the amount of water in a concrete mix while maintaining a constant cementitious materials content results in an increase in slump of the fresh concrete and a decrease in compressive strength in the hardened concrete. Water-reducing admixtures can be used to produce concrete with higher slump without the usual strength reduction associated with an increase in water content.
Water-reducing admixtures are distinguished by the reduction in the amount of water necessary to make workable concrete. Today, most high-strength concrete requires a high range water-reducer (HRWR). Classified as Type F in AASHTO M 194, HRWRs typically reduce water demand by 12 to 30 percent. HRWRs can be added to concrete with low-to-normal slump to produce a high-slump, flowing concrete. They also can be added to concrete with low slump and very low w/cm to produce high-strength concrete. In addition to increasing compressive strength, HRWRs can increase concrete’s durability by decreasing permeability.
Conventional water-reducing admixtures, classified as Type A in AASHTO M 194 typically reduce water demand by 5 to 10 percent. Conventional water reducers differ chemically from HRWRs and can cost about half as much or even less. They are often used in combination with HRWRs to reduce overall cost and enhance performance. Varying the dosages of different admixtures allows fine-tuning of slump, slump retention, and setting time. In projects where closely spaced or congested reinforcement makes concrete placement difficult, HRWRs can help concrete to flow around these obstructions without segregation.*
Retarding Admixtures
Retarding admixtures, classified as Type B in AASHTO M 194, are admixtures that decrease the rate at which concrete sets. High temperatures can result in early stiffening and rapid slump loss. When concrete sets too rapidly, placing and finishing can be very difficult. Retarding admixtures are often used when concrete is to be placed in hot weather, or when concrete is placed under difficult circumstances, or conveyed over unusually long distances. Retarding admixtures that also comply with the water-reduction requirements of AASHTO M 194 are classified as Type D, while high range water-reducing admixtures that comply with the set retarding requirements of AASHTO M 194 are classified as Type G.
Accelerating Admixtures
Accelerating admixtures, classified as Type C in AASHTO M 194, are admixtures that increase the rate at which concrete sets. Accelerating admixtures are often used during cold-weather placement, or in other cases where high early strength or faster setting time is desirable. Although calcium chloride is historically the most effective accelerating admixture, it must not be used in reinforced or prestressed concrete, concrete with embedded aluminum, concrete subject to alkali-aggregate reaction, concrete exposed to sulfates, or in mass concrete applications. Accelerating admixtures that also comply with the water-reduction requirements of AASHTO M 194 are classified as Type E.
Air-Entraining Admixtures
Although not considered chemical admixtures by definition, air-entraining admixtures can be very important in an HPC mix. Air-entraining admixtures, covered in AASHTO M 154 (ASTM C 260) introduce stable microscopic air bubbles into the concrete mix. Air entrainment can significantly improve the freeze-thaw durability of concrete subjected to saturated conditions and deicers and is essential in bridge decks exposed to freeze-thaw conditions. Air-entraining admixtures generally reduce water demand slightly and increase workability. Because the introduction of entrained air can reduce compressive strength, it should be used very cautiously in high-strength concrete.
Testing
The availability of chemical admixtures allows dramatic improvements in concrete performance. However, extra care and attention are required during the specification process and mix proportioning. Various combinations and dosages of chemical admixtures result in numerous ways to achieve a particular set of properties in a cost-effective manner. It’s important to understand not only the effect of each component in the concrete mix, but also how all the components interact. For these reasons, mix verification testing is very important and essential for HPC.
Further Information
For further information, contact the author at 847-972-3158 or [email protected]
*See HPC Bridge Views, Issue No. 18, November/December 2001 for an article on self-consolidating concrete.