Paul D. Tennis, Portland Cement Association
The majority of concrete placed in the USA now contains at least one supplementary cementitious material (SCM) such as fly ash, ground granulated blast-furnace slag (GGBFS), or silica fume.(1) Most concrete producers and specifiers are comfortable using these materials. These concrete mixtures could be called binary mixtures, indicating that they contain portland cement and one SCM. Ternary mixtures are simply those mixtures that contain two SCMs in addition to portland cement.
Constituent Materials
There are several options for producing ternary mixtures. Two SCMs can be added with portland cement to the mixture at the batch plant. Alternatively, an SCM can be added with a blended cement. For example, a Type IP cement can be mixed with GGBFS. A third option is for two SCMs to be mixed with portland cement to make a ternary blended cement, which could be classified based on its properties under ASTM C 1157. This final option is relatively common in Canada. For example, a Canadian Type 10E-F/SF cement is one with equivalent (E) performance to a Type 10 cement, with fly ash (F) as the dominant SCM in quantity and silica fume (SF) as the secondary SCM.
Ternary concrete mixtures often produce concretes that may be classified as high performance. This use is increasing as concrete producers become adept at optimizing locally available concreting materials and specifiers become comfortable with the use of ternary concrete mixtures.
When to Consider Ternary Mixtures
The benefits of using SCMs in binary concrete mixtures are now generally wellaccepted. Many SCMs result in increased later-age strengths and lower permeabilities. Consequently, the use of SCMs improves durability, increases resistance to sulfate attack, and reduces alkali-silica reactivity.
Ternary concrete mixtures can provide the same benefits as binary concrete mixtures, and, if properly optimized, may offer additional advantages. Many SCMs decrease the early strength gain of the concrete because they react slower than portland cement. One method for overcoming this slower strength gain is to add a second, more rapidly reacting SCM, such as silica fume. Thus, the potential long-term durability and strength improvements may be obtained with minimal impact on early age strength. This may present an attractive option for specifiers looking to decrease the time before bridge decks can be opened to traffic. However, special care may be needed to prevent early age cracking.
The use of two different SCMs may have synergistic effects as different mechanisms—chemical or physical—may be responsible for the behavior of concrete incorporating SCMs. For example, part of the benefit of combining silica fume and some fly ashes is due to the physical effects of size and shape of the particles allowing more efficient packing, leading to denser or easier-to-finish concrete. Some Class C fly ashes and some GGBFSs have chemistries that make them more reactive, and thus impart additional early strength to the concrete.
Case Study
A high profile example of the use of multiple SCM concrete mixtures is the $200 million reconstruction of Wacker Drive in Chicago.* This bi-level roadway has an ambitious 75 to 100-year design life, and contains not two, but three SCMs: GGBFS, fly ash, and silica fume (a quaternary mixture). In this project, the focus was on maximizing durability for the columns and superstructure since adequate strength was readily achievable. By keeping the total cementitious materials content to 684 lb/cu yd (405 kg/cu m) and the water-cementitious materials ratio to about 0.37, the mix was easier to place and finish than one with a high cement content and low water-cement ratio. The specific proportions of the SCMs were chosen to minimize permeability to chloride ions.
Trial Mixtures
As with all concrete mixtures, ternary concrete mixtures should be tested with the actual brands of concrete components in the proportions that will be used in the field. With a range of possible proportions for each material, ternary mixtures may provide an opportunity for the optimization of several properties.
Specifications
Bridge specifications often limit the maximum amount of SCMs for concrete. For example, fly ash and pozzolans less than 25 percent, GGBFS less than 50 percent, silica fume less than 10 percent, and all SCMs less than 50 percent by mass of total cementitious materials. These limits are subject to debate, as concretes made with higher dosages of particular materials have proved durable in some concrete mixtures in certain environments. Conversely, combinations of certain materials at dosages under these limits have been found to aggravate scaling or may be ineffective. It is, therefore, advisable to confirm behavior by field experience or laboratory testing.
Reference
- Survey of Mineral Admixtures and Blended Cements in Ready Mixed Concrete, Portland Cement Association, Skokie, Illinois, October 2000, 16 pp.
Editor’s Note
This article is the ninth in a series that addresses the benefits of specific materials used in HPC. The benefits of silica fume, lightweight aggregate, different cements, slag cement, fly ash, corrosion inhibitors, chemical admixtures, and air-entrainment were discussed in previous issues of HPC Bridge Views.
*See HPC Bridge Views, Issue No. 19, January/February 2002.